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Preparation and optical characterization of β-MnO2 nano thin films for application in heterojunction photodiodes
Accepted Manuscript Title: Preparation and optical characterization of -MnO2 nano thin films for application in heterojunction photodiodes Author: M.M. Makhlouf PII: DOI: Reference:
S0924-4247(17)32027-7 https://doi.org/10.1016/j.sna.2018.06.003 SNA 10808
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
12-11-2017 15-4-2018 4-6-2018
Please cite this article as: Makhlouf MM, Preparation and optical characterization of -MnO2 nano thin films for application in heterojunction photodiodes, Sensors and Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and optical characterization of β-MnO2 nano thin films for application in heterojunction photodiodes M. M. Makhlouf
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Department of Physics, Turabah University College, Taif University, Turabah 21995, Saudi
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Arabi
Email: [email protected]; [email protected]; Tele: +966 533776359.
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Graphical Abstract
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Highlights
Deposited films of MnO2 have amorphous structure and become nanocrystalline with β phase of tetragonal crystal under annealing temperature.
The linear and nonlinear optical constants β-MnO2 thin films were measured.
Dispersion parameters of oscillator model.
The optical energy gap decreases with annealing temperature.
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β-MnO2 thin films were calculated based on single
Abstract
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The present work shows that manganese dioxide (β-MnO2) nano films can be applied as a
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potential candidate for solar energy conversion. β-MnO2 nano thin films were successfully
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grown on glass and quartz substrates using thermal evaporation technique. The structural
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properties of as-deposited and annealed thin films were analyzed using both X-ray diffraction
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and field emission scanning electron microscopy techniques. The spectroscopic characterizations of β-MnO2 nano thin films with different film thickness were studied using
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spectrophotometric technique in the wavelength of 200 - 2400 nm. The optical constants of these films were calculated and showed variation with film thickness and annealing
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temperature. Wemple -DiDomenico oscillator model was applied in the non-absorbing region of refractive index spectrum in order to evaluate the dispersion parameters. The nonlinear
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optical parameters were calculated using the linear optical parameters. The annealing temperature considered to be a good tool for enhancement of the nonlinear optical parameters of β-MnO2 nano films. Unraveling the correlated optical properties with the photoresponse characterizations of β-MnO2 nano films opens the avenue to design and fabricate heterojunction photodiodes based on β-MnO2 nano-films. The photoresponsivity,
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photoconductivity and specific detectivity of fabricated Au/β-MnO2/p-Si/Al photodiodes were studied.
Keywords: MnO2; Thin films; Structural properties; Optical properties; Photodiodes.
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Corresponding author: M.M. Makhlouf
1. Introduction
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Manganese dioxide (MnO2) is a transitional material of interesting characterizations.
It has long been widely used for many applications due to the low cost of its raw material, easy preparation, high intercalation voltage, nontoxicity and environmental merit [1]. MnO2
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catalysts, [4] and magnetoelectronic devices [5].
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is used as a cathode electrode for most dry batteries [2], electrochemical supercapacitors [3],
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MnO2 exists in different structural formation polymorphs, such as hollandite (α),
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pyrolusite (β), γ types and various controlled morphologies, depending on the different ways
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in which the basic structure units (namely, the MnO6 octahedron) are linked [6, 7]. The structural formation of these various materials was described by Wells [8] as well as by
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Burns and Burns [9]. These studies described the various octahedral crystal structures that consist of oxygen atoms with manganese atoms in the center and the type of linking these
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octahedral together. For most MnO2 materials, the structure consists of parallel chains of variant edge-linked manganese oxygen octahedral. The pyrolusite (β-MnO2), in the work
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reported here, consists of single chains connected by corner sharing to other single chains. There are numerous reports on physical and chemical characterizations for several
MnO2 polymorphs [10 - 23]. Preisler [10] studied the electrical properties of MnO2 and the obtained results showed that the electrical conductivity of MnO2 was ranged from 10-6 to l0-3 Ω cm-1. Wiley and Knight [11] studied the electrical conductivity of the bulk β-MnO2
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prepared by pyrolysis of Mn(I1) nitrate. They measured the thermal activation energy of βMnO2 and it exhibited in the range of 0.19 - 0.096 eV at -55 to 190 oC, respectively. Brenet [12] discussed the semiconducting behavior of β-MnO2 based on band model theory. Euler et al. [13, 14] studied the electrical properties of β-MnO2 including the Hall effect, thermal e.m.f., and magnetic characterizations. They confirmed that both solid and powder of β-
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MnO2 revealed n-type semiconducting phenomenon. The measured Hall mobility was 0.16 -
0.38 cm2 V-1s-1 for the powder β-MnO2, and 0.56 - 300 cm2 V-1s-1 for the solid β-MnO2
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samples [14]. Brenet et al. [15] reported the influence of metal dopants (Li+, Th4+ and Cr3+) on the semiconducting properties of β-MnO2 and showed increasing in the electrical conductivity with the dopant. Parodi [16] was the first one documented the infrared (FT-IR)
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spectrum of β-MnO2, but he could not analysis the obtained results. Even today, numerous
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literatures on the FT-IR studies of MnO2 polymorphs were achieved by various workers [16 -
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18]. Valetta et al. [19] studied the effect of various impurities on the spectroscopic properties of manganese oxides. The FT-IR characteristics of various number of manganese oxides,
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such as pyrolusite, romanechite, birnessite, ramsdellite nsutite, hollandite, and cryptomelane, were reported [20]. Faber [18] studied the FT-IR spectra of many kinds of manganese
forms of MnO2.
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dioxides chemically produced and reported that ramsdellite showed polymorphs of α and β
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Recently, nanostructure materials have large attention not only for their unique
phenomena significance but also for many Hi-Tech applications derived from their magical
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features, superior to the corresponding bulk counterparts [21, 22]. Nanostructured materials have been comprehensively studied as new window for producing the next generation of nano-electronic and nano-optoelectronic devices. Therefore, intensive efforts have been directed towards the preparation of MnO2 nanostructures with various structural formation such as rods, wires and tubes [23 - 25]. MnO2 thin films were grown using spray pyrolysis
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method [26]; the structural analysis revealed the cubical nature of the film with average crystallite size 26.5 nm. The photoluminescence (PL) study showed intensity increment with increasing substrate temperature possessing blue and green fluorescence. The optical band gap obtained from PL and UV-Vis spectra was found to be ~ 2.33 eV. Iqbal et al. [27] prepared the Mn doped ZnO nanorods by using solvothermal method. The characterization of
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the doped ZnO exhibits the improvement of spectroscopic and photoluminescence properties. Additionally, the absorption spectrum is shifted in red-shift side by 22 nm and the optical
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band gap of doped ZnO reduced. The obtained results makes Mn doped ZnO as potential for optolectronic and photovoltaic devices.
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Furthermore, many methods were achieved to prepare MnO2 thin films for use in the
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photovoltaics and supercapacitors applications [28 - 31]. Prasetio et al.[28] prepared and
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characterized dye sensitized solar cell (DSSC) based on heterojunction TiO2/MnO2 thin film
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as an electrode. Addition of MnO2 to TiO2 semiconductor reduced energy gap of the pure TiO2 from 3.29 to 2.81 eV. The efficiency of DSSC was improved from 0.0065 % for pure
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TiO2 to 0.0108 % for TiO2/MnO2(6%) composite film. Pang et al. [29] prepared dual-planar electrode of MnO2 thin film for electrochemical capacitor. It exhibited an excellent capacitive
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behavior in mild Na2SO4 aqueous electrolyte. MnO2 thin film electrochemical capacitor is a good candidate in applications of pulsed power sources and load-leveling functions in several
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electronics. A nanostructured MnO2 thin film was electrodeposited on carbon nanotubes (CNTs) thin film to form hybrid CNT/MnO2 thin film electrode for supercapacitors [30]. The obtained results
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showed that the CNT/MnO2 film electrode is higher capacitance compared with a pure CNT electrode. The nanostructured MnO2 layer increased the surface area and increase reactions with the cations in the electrolyte which causes increasing in specific capacitance. A nanocomposite of MnO2/Au film was deposited on ITO substrate using the potentiostatic method [31]. The thin films of thickness (50 ~ 100 nm) contain MnO2 nanoparticles. The thin film electrode doped 0.8% of Au exhibited an excellent specific capacitance of 1230 Fcm-3 at a scan rate of 20mVs-1. 5
Although there were numerous reports on the characterizations of different MnO2 crystal structure, these studies seem to be inconclusive for use the nanostructure of β-MnO2 in the photovoltaics even now. Herein, the present study reports the structural, optical properties and spectral dispersion parameters of β-MnO2 nano thin films prepared by thermal
of heterojunction photodiodes based on active nano thin layer of β-MnO2.
2.1 Preparation of thin films and measurement techniques
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2. Experimental details
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evaporation technique. This study applied β-MnO2 nano thin films for use towards fabrication
The pyrolusite (β-MnO2) fine powder was procured from Sigma-Aldrich Co. and used in
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the present study without any further purification. β-MnO2 nano thin films were grown by
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thermal evaporation technique using a high vacuum coating unit (Edwards E306 A, England).
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The vacuum pressure was below 1.5×10-5 mbar during the thermal deposition process. The
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nano thin films were grown on clean optical flat quartz and glass substrates for optical and
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structural measurements, respectively. The β-MnO2 was carried out was vaporized using boat-shaped tungsten filament. Deposition rate and film thickness were measured during the
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evaporation process by using a quartz crystal thickness monitor (Model TM-350 MAXTEK, Inc. USA) attached to the coating system. The thickness of deposited films of β-MnO2 were
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45, 86 and 100 nm with deposition rate 1.5 Å/sec. The X-ray diffraction analysis was carried out using Philips X-ray diffraction system
(model X/ Pert Pro.) with monochromatic CuKα radiation of λ=1.5418 Å. The morphologies
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of the resulting β-MnO2 films were characterized by a field emission scanning electron microscopy, FESEM, (model FEI QUANTA 250 FEG) provided with an energy dispersive X-ray analysis (EDX) to fulfill element microanalysis. The surface morphology of asdeposited and annealed thin films was investigated after coating these films with thin layer of gold. Images were carried out at 30 kV and magnification of 80, 000. 6
The absorbance, transmittance and reflectance of β-MnO2 thin films with different thicknesses were measured using double-beam spectrophotometer (JASCO model V-570 UV-Vis-NIR) in the wavelength range 200 - 2400 nm. These measurements were repeated again after annealing temperature for the film of thickness 85 nm at 473 K for 2h. The experimental errors for calculation of the transmittance and reflectance are considered to be
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1.5 ± 0.5 % and 1 ± 0.5% , respectively.
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2.2 Fabrication of Au/β-MnO2/p-Si/Al photodiodes
The heterojunction photodiodes of Au/β-MnO2/p-Si/Al were fabricated using thermal evaporation technique. The p-type Si wafer is a single crystal wafer with orientation (1 0 0)
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parallel to the surface, the hole concentration of p-type Si single crystal wafer is 1.6×1023 m-3
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and its thickness is 400 μm. β-MnO2 thin layer was grown on the Si wafer and its film
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thickness was 86 nm and 45 nm for photodiode1 and photodiode2, respectively. The back contact electrode as Ohmic metal electrode was made by depositing a relatively thick film of
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pure Al metal (120 nm) on the p-type Si wafer substrate. Then, a top contact of mesh Au metal (40 nm) was deposited on β-MnO2 layer as Schottky electrode, which has a high
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contact resistance with β-MnO2 layer. The photoactive area of the junction is 1.2 cm2. The current-voltage, I-V, characteristics of the heterojunction device were measured in dark and
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under illumination conditions at ambient temperature by using Keithly 6517B electrometer.
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The power intensity of light illumination is 10 mW/cm2 provided by tungsten filament lamp.
3. Results and discussion 3.1 Structural properties Fig.1 represents XRD analysis and the compositional stoichiometry of energy dispersive X-ray (EDX) of as-deposited and annealed thin films. The inset of Fig.1(a) represents the compositional stoichiometry analysis of the as-deposited thin film 7
confirmed by energy dispersive analysis of X-ray (EDX). Quantification of the EDX spectrum showed that the film material consists of Mn4+ and O2− with ratios of 87.24% and 12.76%, respectively; which suggests that the as-deposited film had a chemical formula of MnO2. The structural formation was carried out using XRD and FESEM for as-deposited and
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annealed thin films on the glass substrates. MnO2 can exist in different polymorphism under environmental condition. The XRD technique was used to identify the phase form of MnO2
of
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in powder form, as-deposited and annealed films condition. Fig.1(a) illustrates XRD patterns MnO2 powder. It is observed that XRD pattern exhibited many diffraction peaks
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indicating that MnO2 powder is a polycrystalline structure. All the diffraction peaks of MnO2 powder were well indexed to the tetragonal crystal system structure (JCPDS file No. 44-
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0141) with lattice constants of a = b = 9.785 Å, c = 2.863 Å and a = b = c = 90o. This
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confirmed that the powder is a pure pyrolusite (β-MnO2) [32, 33]. Fig. 1(b) shows XRD
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pattern of as-deposited film showed amorphous structure based on the hump between ca. 2θ =
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15° and 35. Upon annealing temperature at 473 K for 2 h, it was clear that XRD pattern of the as-deposited MnO2 film exhibited a broad peak superimposed on its a diffraction peaks
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observed at 28.56o, 37.31o, 43.61 o and 56.55 o corresponded to the (1 1 0), (1 0 1), (1 1 1) and (2 1 1), reflection directions, respectively. These diffraction peaks are attributable to the
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nanocrystalline structure of annealed thin film (Fig. 2(c)). The XRD pattern for annealed MnO2 film matched with (JCPDS file No. 44-0141) confirming the presence of β-MnO2
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phase with tetragonal crystal system [32, 33]. The crystallite sizes were determined using Scherrer's equation [34] and they were calculated in the range of ca. 11 - 52 nm. The surface morphology of the β-MnO2 thin films was investigated by FESEM. Fig. 2 shows FESEM images of the as-deposited and annealed β-MnO2 thin films. It can be seen that the as-deposited film showed the amorphous structure (Fig. 2(a)) and this character was
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confirmed by the XRD pattern as in Fig. 1(b). Fig. 2(b) shows the structure of thermally annealed β-MnO2 film that contains spherically shaped of nanocrystals with different crystallite sizes in the range of ca. 14 - 58 nm harmonious with the calculated results from Xray diffraction patterns. Then, the results of FESEM showed consistent with those obtained by XRD technique and confirmed that the structure of annealed film of β-MnO2 is
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nanocrystallites spread in amorphous medium. 3.2. Linear optical characteristics
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The spectrophotometric measurements for the β-MnO2 thin films with different
thickness in the spectral range UV-Vis-NIR were investigated. Fig.3(a) illustrates the
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absorption spectrum as a function of wavelength, λ, for β-MnO2 thin films with different
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thickness (45, 86 and 100 nm). The thin film has relatively high absorption spectrum in the UV region and it decreases rapidly with increasing wavelength until it approaches constant
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value of 0.2 at λ ≥ 460 nm. The absorbance spectrum increases with increasing the film thickness. Fig.3(b) shows the annealing temperature changed the value of absorbance and
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shifted the absorption edge towards high wavelengths (red shift). At λ ≥ 600 nm, the absorbance was not affected by the annealing temperature. The absorption band of MnO2
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nanostructures in UV region may be assigned to the transitions between valence and conduction bands [34] while the optical absorption in the visible region may be attributed to
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d–d transitions of Mn ions [35]. Fig. 4 shows the spectral distribution of transmittance, T(λ), and reflectance, R(λ),
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measured in the wavelength of 200 - 2400 nm for as-deposited and annealed β-MnO2 thin films and with different film thicknesses. There are two regions in the spectrum of T(λ) depending on the wavelength of incident light: region (I) is in the wavelength range (200 < λ < 500 nm). The sum of T(λ) and R(λ) is less than unity thus implying an absorption region. The region (II) is for the wavelength (500 < λ < 2400 nm); T(λ) is much greater than R(λ)
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and their total sum is approximately equal to unity [T(λ) + R(λ) ≈ 1]. Therefore, all films were transparent and could not be absorbed the light (non-absorbing region). The T(λ) and R(λ) are influenced with the film thickness and annealing temperature. Study of optical constants play an important role in improvement the optical features of semiconductor materials towards application in photovoltaic devices. The optical constants
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of MnO2 thin films grown on quartz substrates can be deduced in terms of the measured
values of T(λ) and R(λ) after introducing refine the measured data due to experimental errors
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from absorption and reflection of light on the quartz substrate [36, 37]. The optical constants
such as refractive index, n, and the extinction coefficient, k, can be determined by using
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program of modified bi-variant search [38] based on minimizing and simultaneously, 2
R R( n, k ) R 2
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(1) (2)
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2
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T T( n,k ) T ,
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where T(n,k) and R(n,k) are the computed values of transmittance and reflectance using
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Murmann’s exact equations [38 - 40] via a computer program throughout the whole range of measurements (200 - 2400 nm); T and R are the measured values of transmittance and reflectance, respectively. The variances of T and R were calculated and compared to 2
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seek their simultaneous minimization. The corresponding values of n and k represent the
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solution.
The spectral behavior of refractive index, n, and absorption index, k, are in Fig. 5.
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The refractive index of β-MnO2 thin films shows the normal dispersion behavior at wavelength of 900 - 2400 nm, whereas the anomalous dispersion behavior at λ < 900 nm. At lower wavelengths region (anomalous dispersion), the spectral curve of n showed one band with peak at 868 nm, which locates in visible region. This region can be demonstrated by adopting multioscillator model [38]. At higher wavelengths region (normal dispersion),
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the n decreases when the wavelength of light increases, and then at higher wavelength of light, the refractive index tends to be constant. This region can be explained by single oscillator model [41]. Fig.5(a) shows the refractive index increases with increasing the thickness of β-MnO2 film, while annealing temperature decreases the refractive index over the whole spectral range except 316 – 575 nm spectral (Fig.5(b)). The spectral behavior of
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absorption index, k, for the thin film is in Fig. 5. The value of k decreases rapidly when the wavelength increases until it reaches a constant value of about (0.1 - 0.2) due to the
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amorphous structure of β-MnO2 thin films. Both annealing temperature and decreasing of the
film thickness induce the absorption edge of spectrum to move slightly to the higher
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wavelengths (red shift).
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The incident photon energy dependence of refractive index of optical materials can be
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analyzed based on the single-oscillator model [41, 42] to obtain several optical dispersion
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constants, such as the dispersion energy, the single oscillator energy, the dielectric constant at infinity, the lattice dielectric constant and the carrier concentration to effective mass ratio.
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Wemple and DiDomenico (WDD) [39, 41] proposed a dispersion relationship, that describes the dispersion energy parameters which measures the average strength of the interband
Eo 1 (hv)2 Ed Eo Ed
(3)
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(n2 1)1
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optical transitions according to the relation:
where Ed is the dispersion energy, Eo is the oscillator energy, and hv is the incident photon energy. By plotting (n2-1)-1 vs. (hv)2 as shown in Fig. 6. Eo and Ed can be calculated from
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the slope of straight line (= ( Eo Ed ) 1 ) and intercept with ordinate axis (= Eo Ed ). The values of Ed and Eo are listed in Table1. Further analysis of the (n2-1)-1 versus (hv)2 allows to determining the static refractive index ( no at hv = 0), then equation (3) becomes: no (
Ed 1)1 / 2 Eo
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The value of the static refractive index, no, for all investigated films are listed in Table 1. The values of the dielectric constant at infinity, ε∞, was determined from the intersection of the straight line with the (n2-1)-1, since ε∞ = n2 at hv = 0. The real part of the dielectric constant, r , in non-absorbing region can be deduced by the following relation [39, 40]:
e2 N 2 4 oc 2 m*
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r n2 k 2 L
(5)
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where εo is the permittivity of free space, e is the charge of the electron, and c is the speed of light in space. The relation between r and 2 in the normal dispersion region is shown in
Fig.7. The intersection of resultant straight line with the n2-axis determines the lattice
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dielectric constant, L , and the value of the carrier concentration to effective mass ratio,
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N m* , was calculated according to:
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N 4 o c 2 ( slope) m* e2
(6)
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The obtained values of L and N/m* are listed in Table 1. The observed curvature in the curves at high-energy values is a result of interactions of incident photons with thermal
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vibrations of the lattice. The values of all dispersion parameters are listed in Table 1. It is
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clear that both of ε∞ and εL have certain values. Moreover, εL > ε∞ for all β-MnO2 nano thin films. This trend was observed for other metal oxide [42] and was attributed to the contribution of a small concentration of free carriers.
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The absorption coefficient, α, is related to extinction coefficient, k, through the well-
known relation α =4πk/λ. The absorption coefficient spectra of β-MnO2 thin films with different thickness as a function of wavelength of incident light are depicted in Fig.8. The films show high absorption spectra decreases with decreasing the wavelength. All different thin films have α > 105 cm-1 in UV-region of spectra. Annealing temperature and film
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thickness slightly change the intensity of α spectrum and also move the absorption edge towards the lower or higher wavelengths of spectra. Investigation of the absorption coefficient spectrum edge is one of the important well-known method for determining the optically the type of band transitions and provides the values of optical band gap for different materials. Tauc [42, 43] proposed the relationship between the α and hv applied at the edge of
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absorption band as follows:
hv B(hv Eg )r
(7)
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where B is a constant depend on the type of material, r = 1/2 or 2 for direct or indirect
allowed transitions, respectively and r = 3/2 or 3/2 for direct or indirect forbidden transitions,
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respectively. The relation between (αhν)1/r and photon energy (hν) for absorption edge was
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drawn and discussed for different possible values of r. The best linear fit of the results was
for different types of films that exhibits portion of straight line in the
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(αhν)1/2 versus hν
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observed for r = 2 which indicates to indirect allowed transition. Fig.8(a, b) shows the plot
region of absorption edge. The intersection of this line with the hv-axis provides the value of
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the energy band gap, as shown in the inset of Fig.8. The values of indirect allowed energy gap, Eg, for the films with different thickness are listed in Table 1. Annealing temperature
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slightly decreases the optical energy gap of β-MnO2 because the annealing temperature
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removes traps and defect centres through the incorporation of oxygen or increased localized states. Stevels [42, 44] proposed for the metal oxides that the shift of absorption edge to low energy side of spectra (red shift) is due to the electronic transitions from non-bridging
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oxygen, which has a less-tightly bound electron than bridging oxygen. Hence, the value of energy gap decreases with increasing the number of non-bridging oxygen [45]. It could be observed that the values of energy gap result in the present work are compatible with the values reported in the literatures [33, 46]. The values of band gap of β-MnO2 thin films of different thickness in the present work (1.82 -1.98 eV) closely matches the energy gaps of 13
values 1.65 and 2.6 eV for tetragonal [33] and hexagonal [46] MnO2 nanostructures, respectively. In many amorphous materials, the spectrum of absorption coefficient appears as a clear short tail at the wavelengths behind the value of energy gap. This tail is called Urbach tail which undergoes an empirical formula proposed by Urbach [47]. The origin of Urbach tail assigned
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to the phonon-assisted indirect electronic transitions. Tauc [43, 48] reported that the
exponential behaviour of Urbach tail is due to transitions between localized states and would
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vary depending on the type and nature of material. Whereas Davis and Mott [48] assumed that the value of Urbach energy will be approximately the same for most amorphous semiconductors. The Urbach tail can be analysed by the relation [47]:
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hv Eu
(8)
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o exp
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where αo is a pure experimental factor and Eu is Urbach energy. The value of Eu was
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calculated by taking the reciprocals of slope of the linear part of the ln α vs. hν at values of
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photon energy are less than the energy gap. The Urbach energies of the different films thickness are listed in Table 1. It was found that the value of Eu increases with increasing the
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film thickness, while Eu decreased with annealing temperature. 3.3 Nonlinear optical characteristics
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When light with high-intensity passes through the optical material, it causes nonlinear
optical behaviour. Many of metal oxides semiconductors have large values of non-linear
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refractive index, non-linear optical susceptibility and non-linear absorption coefficient. They are promising materials for many nonlinear optoelectronic applications. When nonlinear optical material is exposed to the electric field of the incident light, the
nonlinearity
phenomenon exhibits in the polarization, p, of the material which is represented by a power series expansion in terms of the electric field, E, by the following relation [49]
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P (1) E ( 2) E 2 (3) E 3 ....
(9)
where χ(1) is the linear susceptibility, χ(2) is the second order susceptibility which describes a second harmonic susceptibility and χ(3) is the non-linear optical susceptibility which describes third harmonic generation. The χ(3) is used to determine the possibility of applying the material in optical switching or not. Tichy and Ticha [50] proposed a relation to calculate χ(3)
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that combines Miller’s generalized rule [38] with dispersion parameters calculated based on WDD model [41] as follows: Eo Ed A 2 2 4 ( Eo (hv) )
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( 3)
(10)
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where A is a quantity assumed to be frequency independent and nearly the same for most
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materials (1.7×10-10 esu). Figs. 9(a, b) show the variation of χ(3) as a function of photon
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energy for different β-MnO2 thin films. It was observed that the value of χ(3) increases with
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increasing hv and it is significance affected by increasing film thickness and annealing temperature at 473K. The nonlinear refractive index, n2, can be computed in terms of the non-
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12 (3) n2 no
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linear susceptibility and static linear refractive index by the relation [42, 50]:
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where no is the static refractive index ( n0 ) calculated previously from Fig.6 and listed in Table 1 for the β-MnO2 thin films. Figs. 9(c, d) illustrate the spectral distribution of n2 for the different film thicknesses of MnO2. It can be observed that n2 abruptly decreases when the
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wavelength increases. Annealing temperature and film thickness have slight effect on the value of n2. Additionally, there is another important nonlinear parameter called the non-linear absorption coefficient ( c ) [42, 45]:
c
K c E1pl 2 F
(12)
n2 Eg3 15
where Kc is constant (3100 cmGW-1), Ep is Kane energy parameter, Eg is the optical band gap of material and F is a function that exhibits the dispersion of c with respect to the photon energy (hν). This function depends on the band structure of the material and it can be calculated using the following relation [42, 51]:
2h / E 1 F 2h / E
3/ 2
g
(12)
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g
The incident photon energy range should satisfy the two-photons absorption condition Eg 2
h Eg ). Fig.10 shows the plot of c versus hν and it can be observed that c
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(
increases with increasing hv until it reaches to the maximum value at peak of 1.05 eV. After
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that, c decreases significantly with increasing hv until it reaches hv = 1.7 eV that undergoes
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the two-photon absorption condition. Annealing temperature increases the value of c while
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the value of c decreases with increasing the film thickness. Therefore, it can be concluded
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that the annealing process is considered a desirable effect for enhancement the nonlinear
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optical properties of the MnO2 films. Furthermore, increasing the film thickness decreases the
c and increases χ(3) and n2. The increasing values of nonlinear optical constants (χ(3), n2 and
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c ) of MnO2 are due to bound oxygen bonds and free ions that have high polarizability in
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oxide matrix that result in the short and long Mn-O bonds. This difference in the bond length causes polarizability between long and short bonds that leading to increase the values of nonlinear optical parameters.
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3.4. Photoresponse characteristics of Au/β-MnO2/p-Si/Al heterojunction photodiodes The Schematic diagram of Au/β-MnO2/p-Si/Al heterojunction photodiode is shown in Fg.11. We decided to design and compare Au/β-MnO2 (86 nm)/p-Si/Al photodiode1 with Au/βMnO2 (45 nm)/p-Si/Al photodiode2. The current-voltage (I-V) characteristics of heterojunction photodiodes in dark state and under illumination light intensity of 10 mW/cm2 are 16
illustrated in Fig.12. The dark and illuminated I-V curves of the Au/β-MnO2(86 nm)/p-Si/Al photodiode1 at ambient temperature show that a distinct kind of S-shape was observed in the I-V plot under illumination, which was not observed in the dark I-V plot as shown in Fig. 12(a). A similar S-shape behavior of I-V characteristic under illumination, and not in the dark state, was observed and reported by A. Iefanova et al [52]. The absence of short circuit
IP T
current and open circuit voltage of this diode under illumination in this plot indicates an
insufficient build-in electric field between the Au grid and the Al electrode, which results in
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poor collection of photogenerated charges at reverse bias voltage [52]. Fig. 12(b) shows the I-V curves of the Au/β-MnO2 (45 nm)/p-Si/Al photodiode2, the photocurrent in reverse
U
bias voltage increases strongly with illumination and this could be due to the increase in the numbers of photo-generated charge carriers at the interface states. The short circuit current
A
N
and open circuit voltage of the photodiode were found to be 70 μA and 0.64 V, respectively.
M
These obtained values led to poor photovoltaic parameters of this device to be a solar cell comparable with other inorganic solar cells. Furthermore, they are enough to potential for However, both photodiodes are sensitive to UV-Visible light,
ED
photodiode applications.
especially the second photodiode of β-MnO2(45nm)/p-Si and it can be utilized as a
PT
photosensor applications.
The photodiodes are fabricated to achieve a spectral or a rapid time response, while
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solar cells are designed to convert the solar energy to the photo-current or photo-voltage [53]. Generally, we decided to calculate some important photovoltaic parameters for the
A
heterojunction photodiodes. The first one is the responsivity, which measures the outputinput gain of a detector system and in case of a photodectector, photoresponsivity measures the output photoelectrical per input optical power and can be directly determined from the photocurrent density, Jph, divided by the light intensity power, P. The photoresponsivity, R,
17
and photoconductivity, S, of the photodiode can be obtained in accordance with the following relations [54, 55] as: R
J ph
(13)
P
L V
(14)
IP T
SR
where L is the thickness (400 μm) of illuminated active layer of the diode and V is the applied
SC R
potential. As we can see from Fig. 13(a), the values of R and S of the photodiodes as a function of applied reverse voltage. In both of two photodiodes, the values of R increased with increasing reverse bias potential, while the values of S decreased with increasing reverse
U
bias potential for photodiode2 but S is still constant with low values in photodiode1. The
N
maximum values of photoresponsivity for both photodiodes are ≈ 0.15 A/W at 2 V reverse
A
bias. Another important parameter is specific detectivity, D*, used to characterize
M
performance for a photodetectors. D* is commonly expressed in Jones units (cm Hz0.5 W-1) as
D*
ED
follows [55, 56]:
R 2eJ d
(15)
PT
where Jd is the dark current density. Fig.13(b) shows D* as a function of applied potential bias
CC E
and it can observed that, the D* of the photodiode1 has values of 1.0×108 - 1.7×108 cm Hz0.5 W-1 at low reverse potential bias 0 – 1.1 V. These values increase rapidly at potential bias >1.1 V and reach 9×108 cm Hz0.5 W-1, while the photodiode2 is more stable and the values
A
of D* are almost fixed at ≈ 5 ± 0.5 ×108 cm Hz0.5 W-1 along the whole range of reverse potential bias 0 – 2 V. Comparable of Au/β-MnO2(86 nm)/p-Si/Al photodiode1 with Au/βMnO2(45 nm)/p-Si/Al photodiode2 showed that the photovoltaic properties of photodiode2 was better than first device in both I-V characteristics and photovoltaic properties. Jph of
18
heterojunction photodiode is maximized at the thickness layer of β-MnO2 ≈ 45 nm, but it decreased at the thicker active layer ≈ 86 nm, indicating the thickness dependence on Jph. The present results of photovoltaic parameters were comparable with the pervious works [54 - 59] that studied the photoresponse properties of different heterojunction photodiodes. Makhlouf et al. [57] showed that photoresponsivity of PbO2/p-Si heterojunction photodiode
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reached to 0.19 A/W at 2 V while, Park et al. [54] designed n-ZnO/p-Si photodiode and the
device was measured under red laser illumination of the wavelength 670 nm. The
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photoresponsivity was 0.28 A/W. The heterojunction photodiode of InGaN/GaN was
fabricated and characterized. It was found that the photodiode exhibited a photoresponsivity
U
around 1.76 A/W at potential bias 3 V and under light illumination of the wavelength 380 nm
N
[58]. Ge/Si heterojunction photodiode operating in the IR spectral region was fabricated and
A
the photoresponsivity was 0.23 A/W at 2 V reverse bias under illumination IR light of
M
wavelength 1550 nm [59]. The photoresponse properties of MnO2 thin films open the door for the use thereof as potential photosensitizing material in different photovoltaic devices
ED
with several hybrid constructions (MnO2 film / inorganic, MnO2 film / inorganic, ….etc), especially that the heterojunction photodiodes based on active thin layer of MnO2 in the
PT
present work have acceptable values of photovoltaic parameters, which may attract
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considerable attention as a promising candidate in solar cells in the future.
Conclusion
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The nano thin films of β-MnO2 were grown on substrates using the thermal evaporation technique. FESEM and XRD techniques were used to investigate the structural formation of β-MnO2 nano thin films. The results confirmed that XRD patterns showed that the MnO2 powder has polycrystalline structure with β phase and tetragonal crystal system. The asdeposited films showed amorphous structure, while the annealed film revealed
19
nanocrystalline of β-MnO2 with tetragonal structure. The crystallite size was found in the range of ca. 14 - 58 nm for these films. The optical constants of β-MnO2 films with different thickness were calculated from the absolute values of transmittance and reflectance. The optical studies of β-MnO2 films revealed that the annealing temperature and different thickness change the optical spectra in
1
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UV-Vis-NIR regions for these films. The absorption coefficient of β-MnO2 films is > 105 cmin UV region of spectra. The electronic transition observed near the absorption edge is
SC R
indirect allowed transition with optical energy band gaps (Eg) of 1.91 and 1.98 eV for the
films of thickness 46 and 100 nm, respectively. Furthermore, the Eg of as-deposited and annealed film (86 nm) are 1.98 and 1.82 eV, respectively. Dispersion parameters like
U
oscillation energy (Eo), dispersion energy (Ed), dielectric constant at high frequency (ɛ∞), and
N
the lattice dielectric constant (ɛL) were calculated. The nonlinear refractive index (n2), third-
A
order nonlinear susceptibility (χ(3)) and nonlinear absorption coefficient (βc) were determined.
M
It was found that χ(3), n2 and β increased when both photon energy and annealing temperature
ED
increased. The annealing temperature is a suitable tool for improvement the nonlinear optical properties of β-MnO2 nano thin films.
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According to the obtained results of optical properties of MnO2 nano films, we decided to design and fabricate two heterojunction photodiodes based on β-MnO2 nano films
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with different thicknesses of β-MnO2 active layer with thickness 45 and 86 nm. The photoresponsivity (R), photoconductivity (S) and specific detectivity (D*) of Au/β-MnO2/p-
A
Si/Al photodiodes were studied. The present values of photoresponse parameters of Au/ βMnO2/p-Si/Al heterojunction photodiodes were comparable to the pervious works [54 - 59] that studied the photoresponse parameters of different heterojunction photodiodes.
20
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ED
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PT
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ED
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[51] M. Sheik-Bahae, D.J. Hagan, E.W. Van Stryland, Phys. Rev. Lett. 65, 96 (1990). [52] A. Iefanova, N. Adhikari, A. Dubey, D. Khatiwada, Q. Qiao, AIP Advances 6, 085312
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[53] R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Company, New York, 1996. [54] C.H. Park, J.Y. Lee, S. Im, T.G. Kim, Nucl. Instrum. Methods Phys. Res. B 206, 432 (2003)
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[55] R. C. Jones, J. Opt. Soc. Am. 50, 1058 (1960) [56] E.Y. Choi, S.H. Eom, C.E. Song, S.Y. Nam, J. Lee, H.Y. Woo, I.H. Jung, S.C. Yoon, C. Lee, Org. Electron. (2017), doi: 10.1016/j.orgel.2017.04.016. [57] M.M. Makhlouf, M.M. EL-Nahass, M.H. Zeyada, Mater. Sci. in Semicond. Process. 58, 68 (2017)
Chang, Solid-State Electron. 47, 879 (2003)
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M
A
N
U
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[59] Z. Zhou, J. He, R. Wang, C. Li, J. Yu, Opt. Commun. 283, 3404 (2010)
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[58] Y -K. Su, S -J. Chang, Y -Z. Chiou, T -Y. Tsai, J. Gong, Y -C. Lin, S -H. Liu, C -S.
24
IP T
A
CC E
PT
ED
M
A
N
U
SC R
M. M. Makhlouf: He was born in Demiatta, Egypt, in 1976. He received the MSc. and Ph.D. degrees in physics from Faculty of science, Mansoura University, Damietta Campus, Egypt in 2003 and 2007, respectively. He is associate professor in Physics department, Turabah University College, Taif university, Saudi Arabia. His current research focuses on materials science, nano thin films, inorganic-organic semiconductors, photovoltaics and photosensor devices.
25
Caption figures Fig. 1: XRD patterns of β-MnO2: (a) as-received powder , (b) pristine film, (c) annealed thin at 473 K for 2 h and the inset shows EDX spectrum of β-MnO2 film Fig. 2: FESEM images of nanostructures β-MnO2 thin films of thickness 86 nm: (a) asdeposited and (b) annealed at 473 K for 2 h. Fig. 3: Absorbance of β-MnO2 films of (a) different thicknesses and (b) as-deposited and
IP T
annealed Fig. 4: Spectral behavior of transmittance, T, and reflectance, R, of β-MnO2 films (a) different thicknesses and (b) as-deposited and annealed
SC R
Fig. 5: Refractive, n, and absorption, k, indices of β-MnO2 films of (a) different thicknesses and (b) as-deposited and annealed Fig. 6: Plot of (n2-1)-1 versus of (hν)2. Fig. 7: Plot of n2 versus λ2.
U
Fig. 8: Variation of absorption coefficient, α, of β-MnO2 films and the inset shows
N
plot of (αhν)1/2 as a function of the incident photon energy for different films refractive index, n2, of β-MnO2 films.
A
Fig. 9: The third order nonlinear susceptibility, χ(3), spectra and the spectral nonlinear
and (b) as-deposited and annealed
M
Fig. 10: The nonlinear absorption coefficient, βc, of β-MnO2 films of (a) different thicknesses
ED
Fig. 11: Schematic diagram of Au/β-MnO2/p-Si/Al heterojunction photodiode Fig. 12: Dark and illuminated I-V characteristic plots of (a) Au/β-MnO2(86 nm)/p-Si/Al
PT
photodiode1 and (b) Au/β-MnO2(45 nm)/p-Si/Al photodiode2 Fig. 13: The responsivity, R, and photoconductivity sensitivity, S, and the specific
A
CC E
detectivity, D*, of two photodiodes
26
400
(c)
(2 1 1)
80
(b)
70
Mn
60
C ounts
300 200
Energy 0.40 1.09 1.78 3.82
Mass% 12.76 10.01 68.50 8.73
EDX
40 30 20
O Mn
10
100
Element O Mn Mn Mn
50
0
0
1
Mn
2
3
4
5
6
7
8
9
10 11
0
(a)
10
20
N 30
PT
A
CC E
Fig. 1
ED
(2)
o
27
40
(5 2 1)
(1 1 1)
A M
0
(3 3 0) (4 2 0)
(2 2 0)
20
(2 0 0)
(1 1 0)
40
U
(1 0 1)
60
(2 1 1)
Energy, keV
(1 1 0)
Intensity, counts/s
0 400
IP T
100
SC R
200
(1 1 1)
(1 0 1)
(1 1 0)
300
50
60
A ED
PT
CC E (b)
IP T
SC R
U
N
A
M
(a)
Fig. 2
28
2.0
Film thickness 45 nm 86 nm 100 nm
IP T
1.0
0.5
(a) 300
400
500
600
U
0.0 200
SC R
Absorbance
1.5
800
M
A
N
, nm
700
ED PT
1.0
A
CC E
Absorbance
1.5
Thickness = 86 nm As-deposited film Annealed film
0.5
(b)
0.0 200
300
400
500
, nm Fig. 3 29
600
700
800
100
(a)
80
Film thickness 45 nm 86 nm 100 nm
R% & T%
70 60
T
50
IP T
90
R
40
SC R
30 20 10
500
1000
U
0 1500
A
N
, nm
2000
(b)
ED
90 80
PT
50
Thickness = 86 nm As-deposited film Annealed film
T
CC E
R% & T%
70 60
M
100
R
40 30
A
20 10
0 500
1000
1500
, nm
Fig. 4 30
2000
4
Film thickness 45 nm 86 nm 100 nm
(a)
n
3
2
IP T
1 0.8
k
0.6
SC R
0.4
0.2
0.0
, nm
4
(b)
A M
n
PT
CC E
0.5
ED
2
0.6
2000
Thickness = 86 nm As-deposited film Annealed film
3
1 0.7
1500
U
1000
N
500
k
0.4 0.3
A
0.2 0.1 0.0
500
1000
1500
, nm
Fig. 5
31
2000
0.5
Film thickness 45 nm 86 nm 100 nm
0.3
IP T
2
(n -1)
-1
0.4
SC R
0.2
(a) 0.1 0.5
2
(h) , (eV)
1.0
1.5
2
N
U
0.0
M
A
0.5
PT
0.3
CC E
2
(n -1)
-1
ED
0.4
As-deposited film Annealed film
0.2
A
(b)
0.1
0.0
0.5
1.0 2
2
(h) , (eV)
Fig. 6
32
1.5
7
Film thickness 45 nm 86 nm 100 nm
6
4
SC R
n
2
IP T
5
3
(a) 1
2
3
2
4
5
6
7
2
N
0
U
2
M
A
, (m)
ED
7
n
2
CC E
5
PT
6
Thickness = 86 nm As-deposited film Annealed film
A
4
3
(b) 2 0
1
2
3 2
, (m)
Fig. 7 33
4 2
5
6
7
5
6x10
2000 0.5 0.5
-1
5
4x10
5
3x10
1500
1000
IP T
(h) , (eV/cm)
5
5x10
, cm
Film thickness 45 nm 86 nm 100 nm
500 5
0 0
5
1x10
1
2
3
4
h, eV
(a) 0
6
7
1500
A
N
h, eV
1000
5
U
500
SC R
2x10
2000
5
0.5
(h) , (eV/cm)
0.5
-1
5
CC E
, cm
1000
2x10
5
1x10
A
1500
PT
5
Thickness = 86 nm As-deposited film Annealed film
ED
4x10
3x10
M
5
5x10
500
0 0
1
2
(b)
3
h, eV
4
5
6
7
0 500
, nm
Fig. 8 34
1000
1500
-10
8.0x10
-10
6.0x10
IP T
(esu)
Thickness = 86 nm As-deposited film Annealed film
Film thickness 45 nm 86 nm 100 nm
-9
1.0x10
(3)
-10
4.0x10
(b)
(a) 0.0 0.4
0.6
0.8
1.0
1.2
1.4
0.4
0.6
h, eV
0.8
SC R
-10
2.0x10
1.0
1.2
1.4
Film thickness 45 nm 86 nm 100 nm
-9
M
1.0x10
-10
-10
CC E
-10
PT
6.0x10
-10
Thickness = 86 nm As-deposited Annealed
ED
n2 (esu)
8.0x10
4.0x10
A
N
U
h, eV
2.0x10
0.0
1000
A
(d)
(c)
1500
1000
2000
, nm
Fig. 9
35
1500
, nm
2000
Film thickness 45 nm 86 nm 100 nm
(a)
20
10
IP T
, cm GW-1
15
0 1.0
1.1
1.2
1.3
1.4
1.5
h, eV
1.6
1.7
1.8
U
0.9
Thickness = 86 nm Pristine film Annealed film
PT
CC E
, cm.GW
-1
15
10
M
(b)
ED
20
A
N
0.8
SC R
5
A
5
0
0.9
1.0
1.1
1.2
1.3
h, eV
Fig. 10
36
1.4
1.5
1.6
1.7
IP T
SC R
Incident light
U
Au grid
To the measuring system
N
β-MnO2 film
CC E
PT
ED
M
A
P-Si wafer
A
Fig.11
37
Al electrode
.
0.0004
Dark state Illumination state
0.0002
IP T
0.0001
0.0000
SC R
Current, A
(a)
Photodiode1 MnO2(86 nm)/p-Si
0.0003
-0.0001
-0.0003 -2
-1
0
U
-0.0002
1
0.0004
Photodiode2 MnO2(45 nm)/p-Si
Dark state Illumination state
PT
0.0000
CC E
Current, A
0.0002
0.0001
(b)
ED
0.0003
M
A
N
Potential bias, V
2
-0.0001
A
-0.0002
-0.0003 -2
-1
0
Potential bias, V
Fig.12
38
1
2
-4
2.5x10
-1 -4
-4
1.5x10
IP T
0.10
-4
1.0x10 0.05
(a)
0.0
0.00 1
2
9
M
Photodiode1 Photodiode2 8
ED
8.0x10
8
8
A
CC E
4.0x10
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*
Detectivity (D ), cm Hz
0.5
W
-1
1.0x10
A
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Potential bias, V
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0
6.0x10
8
2.0x10
0.0
(b)
0.0
-5
5.0x10
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Rsponsitivity (R), A W
-1
2.0x10
-1
0.15
Photodiode1 Photodiode2
Photoconductivity (S), A W V
Photodiode1 Photodiode2
0.5
1.0
1.5
Reverse bias potential, V
Fig. 13 39
2.0
Caption Table Table 1: The optical and dispersion parameters of MnO2 nano thin films with different thickness.
Eg (eV)
EU (meV)
Eo (eV)
Ed (eV)
no
ε∞
εL
N/m* (Kg-1m-3)
As-deposited (45 nm)
1.91
94
1.59
4.30
1.95
3.79
4.09
1.09 × 1056
As-deposited (85 nm)
1.98
121
1.63
4.97
2.03
4.13
As-deposited (100 nm)
1.98
121
1.62
5.32
2.13
4.55
Annealed (85 nm)
1.82
98
1.65
5.47
2.15
4.64
A
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ED
M
A
N 40
1.13 × 1056
4.57
1.16 × 1056
4.85
3.3 × 1056
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MnO2 thin film
S0924-4247(17)32027-7 https://doi.org/10.1016/j.sna.2018.06.003 SNA 10808
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
12-11-2017 15-4-2018 4-6-2018
Please cite this article as: Makhlouf MM, Preparation and optical characterization of -MnO2 nano thin films for application in heterojunction photodiodes, Sensors and Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and optical characterization of β-MnO2 nano thin films for application in heterojunction photodiodes M. M. Makhlouf
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Department of Physics, Turabah University College, Taif University, Turabah 21995, Saudi
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Arabi
Email: [email protected]; [email protected]; Tele: +966 533776359.
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Graphical Abstract
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Highlights
Deposited films of MnO2 have amorphous structure and become nanocrystalline with β phase of tetragonal crystal under annealing temperature.
The linear and nonlinear optical constants β-MnO2 thin films were measured.
Dispersion parameters of oscillator model.
The optical energy gap decreases with annealing temperature.
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β-MnO2 thin films were calculated based on single
Abstract
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The present work shows that manganese dioxide (β-MnO2) nano films can be applied as a
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potential candidate for solar energy conversion. β-MnO2 nano thin films were successfully
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grown on glass and quartz substrates using thermal evaporation technique. The structural
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properties of as-deposited and annealed thin films were analyzed using both X-ray diffraction
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and field emission scanning electron microscopy techniques. The spectroscopic characterizations of β-MnO2 nano thin films with different film thickness were studied using
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spectrophotometric technique in the wavelength of 200 - 2400 nm. The optical constants of these films were calculated and showed variation with film thickness and annealing
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temperature. Wemple -DiDomenico oscillator model was applied in the non-absorbing region of refractive index spectrum in order to evaluate the dispersion parameters. The nonlinear
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optical parameters were calculated using the linear optical parameters. The annealing temperature considered to be a good tool for enhancement of the nonlinear optical parameters of β-MnO2 nano films. Unraveling the correlated optical properties with the photoresponse characterizations of β-MnO2 nano films opens the avenue to design and fabricate heterojunction photodiodes based on β-MnO2 nano-films. The photoresponsivity,
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photoconductivity and specific detectivity of fabricated Au/β-MnO2/p-Si/Al photodiodes were studied.
Keywords: MnO2; Thin films; Structural properties; Optical properties; Photodiodes.
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Corresponding author: M.M. Makhlouf
1. Introduction
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Manganese dioxide (MnO2) is a transitional material of interesting characterizations.
It has long been widely used for many applications due to the low cost of its raw material, easy preparation, high intercalation voltage, nontoxicity and environmental merit [1]. MnO2
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catalysts, [4] and magnetoelectronic devices [5].
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is used as a cathode electrode for most dry batteries [2], electrochemical supercapacitors [3],
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MnO2 exists in different structural formation polymorphs, such as hollandite (α),
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pyrolusite (β), γ types and various controlled morphologies, depending on the different ways
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in which the basic structure units (namely, the MnO6 octahedron) are linked [6, 7]. The structural formation of these various materials was described by Wells [8] as well as by
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Burns and Burns [9]. These studies described the various octahedral crystal structures that consist of oxygen atoms with manganese atoms in the center and the type of linking these
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octahedral together. For most MnO2 materials, the structure consists of parallel chains of variant edge-linked manganese oxygen octahedral. The pyrolusite (β-MnO2), in the work
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reported here, consists of single chains connected by corner sharing to other single chains. There are numerous reports on physical and chemical characterizations for several
MnO2 polymorphs [10 - 23]. Preisler [10] studied the electrical properties of MnO2 and the obtained results showed that the electrical conductivity of MnO2 was ranged from 10-6 to l0-3 Ω cm-1. Wiley and Knight [11] studied the electrical conductivity of the bulk β-MnO2
3
prepared by pyrolysis of Mn(I1) nitrate. They measured the thermal activation energy of βMnO2 and it exhibited in the range of 0.19 - 0.096 eV at -55 to 190 oC, respectively. Brenet [12] discussed the semiconducting behavior of β-MnO2 based on band model theory. Euler et al. [13, 14] studied the electrical properties of β-MnO2 including the Hall effect, thermal e.m.f., and magnetic characterizations. They confirmed that both solid and powder of β-
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MnO2 revealed n-type semiconducting phenomenon. The measured Hall mobility was 0.16 -
0.38 cm2 V-1s-1 for the powder β-MnO2, and 0.56 - 300 cm2 V-1s-1 for the solid β-MnO2
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samples [14]. Brenet et al. [15] reported the influence of metal dopants (Li+, Th4+ and Cr3+) on the semiconducting properties of β-MnO2 and showed increasing in the electrical conductivity with the dopant. Parodi [16] was the first one documented the infrared (FT-IR)
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spectrum of β-MnO2, but he could not analysis the obtained results. Even today, numerous
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literatures on the FT-IR studies of MnO2 polymorphs were achieved by various workers [16 -
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18]. Valetta et al. [19] studied the effect of various impurities on the spectroscopic properties of manganese oxides. The FT-IR characteristics of various number of manganese oxides,
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such as pyrolusite, romanechite, birnessite, ramsdellite nsutite, hollandite, and cryptomelane, were reported [20]. Faber [18] studied the FT-IR spectra of many kinds of manganese
forms of MnO2.
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dioxides chemically produced and reported that ramsdellite showed polymorphs of α and β
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Recently, nanostructure materials have large attention not only for their unique
phenomena significance but also for many Hi-Tech applications derived from their magical
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features, superior to the corresponding bulk counterparts [21, 22]. Nanostructured materials have been comprehensively studied as new window for producing the next generation of nano-electronic and nano-optoelectronic devices. Therefore, intensive efforts have been directed towards the preparation of MnO2 nanostructures with various structural formation such as rods, wires and tubes [23 - 25]. MnO2 thin films were grown using spray pyrolysis
4
method [26]; the structural analysis revealed the cubical nature of the film with average crystallite size 26.5 nm. The photoluminescence (PL) study showed intensity increment with increasing substrate temperature possessing blue and green fluorescence. The optical band gap obtained from PL and UV-Vis spectra was found to be ~ 2.33 eV. Iqbal et al. [27] prepared the Mn doped ZnO nanorods by using solvothermal method. The characterization of
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the doped ZnO exhibits the improvement of spectroscopic and photoluminescence properties. Additionally, the absorption spectrum is shifted in red-shift side by 22 nm and the optical
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band gap of doped ZnO reduced. The obtained results makes Mn doped ZnO as potential for optolectronic and photovoltaic devices.
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Furthermore, many methods were achieved to prepare MnO2 thin films for use in the
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photovoltaics and supercapacitors applications [28 - 31]. Prasetio et al.[28] prepared and
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characterized dye sensitized solar cell (DSSC) based on heterojunction TiO2/MnO2 thin film
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as an electrode. Addition of MnO2 to TiO2 semiconductor reduced energy gap of the pure TiO2 from 3.29 to 2.81 eV. The efficiency of DSSC was improved from 0.0065 % for pure
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TiO2 to 0.0108 % for TiO2/MnO2(6%) composite film. Pang et al. [29] prepared dual-planar electrode of MnO2 thin film for electrochemical capacitor. It exhibited an excellent capacitive
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behavior in mild Na2SO4 aqueous electrolyte. MnO2 thin film electrochemical capacitor is a good candidate in applications of pulsed power sources and load-leveling functions in several
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electronics. A nanostructured MnO2 thin film was electrodeposited on carbon nanotubes (CNTs) thin film to form hybrid CNT/MnO2 thin film electrode for supercapacitors [30]. The obtained results
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showed that the CNT/MnO2 film electrode is higher capacitance compared with a pure CNT electrode. The nanostructured MnO2 layer increased the surface area and increase reactions with the cations in the electrolyte which causes increasing in specific capacitance. A nanocomposite of MnO2/Au film was deposited on ITO substrate using the potentiostatic method [31]. The thin films of thickness (50 ~ 100 nm) contain MnO2 nanoparticles. The thin film electrode doped 0.8% of Au exhibited an excellent specific capacitance of 1230 Fcm-3 at a scan rate of 20mVs-1. 5
Although there were numerous reports on the characterizations of different MnO2 crystal structure, these studies seem to be inconclusive for use the nanostructure of β-MnO2 in the photovoltaics even now. Herein, the present study reports the structural, optical properties and spectral dispersion parameters of β-MnO2 nano thin films prepared by thermal
of heterojunction photodiodes based on active nano thin layer of β-MnO2.
2.1 Preparation of thin films and measurement techniques
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2. Experimental details
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evaporation technique. This study applied β-MnO2 nano thin films for use towards fabrication
The pyrolusite (β-MnO2) fine powder was procured from Sigma-Aldrich Co. and used in
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the present study without any further purification. β-MnO2 nano thin films were grown by
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thermal evaporation technique using a high vacuum coating unit (Edwards E306 A, England).
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The vacuum pressure was below 1.5×10-5 mbar during the thermal deposition process. The
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nano thin films were grown on clean optical flat quartz and glass substrates for optical and
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structural measurements, respectively. The β-MnO2 was carried out was vaporized using boat-shaped tungsten filament. Deposition rate and film thickness were measured during the
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evaporation process by using a quartz crystal thickness monitor (Model TM-350 MAXTEK, Inc. USA) attached to the coating system. The thickness of deposited films of β-MnO2 were
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45, 86 and 100 nm with deposition rate 1.5 Å/sec. The X-ray diffraction analysis was carried out using Philips X-ray diffraction system
(model X/ Pert Pro.) with monochromatic CuKα radiation of λ=1.5418 Å. The morphologies
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of the resulting β-MnO2 films were characterized by a field emission scanning electron microscopy, FESEM, (model FEI QUANTA 250 FEG) provided with an energy dispersive X-ray analysis (EDX) to fulfill element microanalysis. The surface morphology of asdeposited and annealed thin films was investigated after coating these films with thin layer of gold. Images were carried out at 30 kV and magnification of 80, 000. 6
The absorbance, transmittance and reflectance of β-MnO2 thin films with different thicknesses were measured using double-beam spectrophotometer (JASCO model V-570 UV-Vis-NIR) in the wavelength range 200 - 2400 nm. These measurements were repeated again after annealing temperature for the film of thickness 85 nm at 473 K for 2h. The experimental errors for calculation of the transmittance and reflectance are considered to be
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1.5 ± 0.5 % and 1 ± 0.5% , respectively.
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2.2 Fabrication of Au/β-MnO2/p-Si/Al photodiodes
The heterojunction photodiodes of Au/β-MnO2/p-Si/Al were fabricated using thermal evaporation technique. The p-type Si wafer is a single crystal wafer with orientation (1 0 0)
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parallel to the surface, the hole concentration of p-type Si single crystal wafer is 1.6×1023 m-3
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and its thickness is 400 μm. β-MnO2 thin layer was grown on the Si wafer and its film
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thickness was 86 nm and 45 nm for photodiode1 and photodiode2, respectively. The back contact electrode as Ohmic metal electrode was made by depositing a relatively thick film of
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pure Al metal (120 nm) on the p-type Si wafer substrate. Then, a top contact of mesh Au metal (40 nm) was deposited on β-MnO2 layer as Schottky electrode, which has a high
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contact resistance with β-MnO2 layer. The photoactive area of the junction is 1.2 cm2. The current-voltage, I-V, characteristics of the heterojunction device were measured in dark and
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under illumination conditions at ambient temperature by using Keithly 6517B electrometer.
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The power intensity of light illumination is 10 mW/cm2 provided by tungsten filament lamp.
3. Results and discussion 3.1 Structural properties Fig.1 represents XRD analysis and the compositional stoichiometry of energy dispersive X-ray (EDX) of as-deposited and annealed thin films. The inset of Fig.1(a) represents the compositional stoichiometry analysis of the as-deposited thin film 7
confirmed by energy dispersive analysis of X-ray (EDX). Quantification of the EDX spectrum showed that the film material consists of Mn4+ and O2− with ratios of 87.24% and 12.76%, respectively; which suggests that the as-deposited film had a chemical formula of MnO2. The structural formation was carried out using XRD and FESEM for as-deposited and
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annealed thin films on the glass substrates. MnO2 can exist in different polymorphism under environmental condition. The XRD technique was used to identify the phase form of MnO2
of
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in powder form, as-deposited and annealed films condition. Fig.1(a) illustrates XRD patterns MnO2 powder. It is observed that XRD pattern exhibited many diffraction peaks
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indicating that MnO2 powder is a polycrystalline structure. All the diffraction peaks of MnO2 powder were well indexed to the tetragonal crystal system structure (JCPDS file No. 44-
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0141) with lattice constants of a = b = 9.785 Å, c = 2.863 Å and a = b = c = 90o. This
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confirmed that the powder is a pure pyrolusite (β-MnO2) [32, 33]. Fig. 1(b) shows XRD
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pattern of as-deposited film showed amorphous structure based on the hump between ca. 2θ =
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15° and 35. Upon annealing temperature at 473 K for 2 h, it was clear that XRD pattern of the as-deposited MnO2 film exhibited a broad peak superimposed on its a diffraction peaks
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observed at 28.56o, 37.31o, 43.61 o and 56.55 o corresponded to the (1 1 0), (1 0 1), (1 1 1) and (2 1 1), reflection directions, respectively. These diffraction peaks are attributable to the
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nanocrystalline structure of annealed thin film (Fig. 2(c)). The XRD pattern for annealed MnO2 film matched with (JCPDS file No. 44-0141) confirming the presence of β-MnO2
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phase with tetragonal crystal system [32, 33]. The crystallite sizes were determined using Scherrer's equation [34] and they were calculated in the range of ca. 11 - 52 nm. The surface morphology of the β-MnO2 thin films was investigated by FESEM. Fig. 2 shows FESEM images of the as-deposited and annealed β-MnO2 thin films. It can be seen that the as-deposited film showed the amorphous structure (Fig. 2(a)) and this character was
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confirmed by the XRD pattern as in Fig. 1(b). Fig. 2(b) shows the structure of thermally annealed β-MnO2 film that contains spherically shaped of nanocrystals with different crystallite sizes in the range of ca. 14 - 58 nm harmonious with the calculated results from Xray diffraction patterns. Then, the results of FESEM showed consistent with those obtained by XRD technique and confirmed that the structure of annealed film of β-MnO2 is
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nanocrystallites spread in amorphous medium. 3.2. Linear optical characteristics
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The spectrophotometric measurements for the β-MnO2 thin films with different
thickness in the spectral range UV-Vis-NIR were investigated. Fig.3(a) illustrates the
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absorption spectrum as a function of wavelength, λ, for β-MnO2 thin films with different
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thickness (45, 86 and 100 nm). The thin film has relatively high absorption spectrum in the UV region and it decreases rapidly with increasing wavelength until it approaches constant
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value of 0.2 at λ ≥ 460 nm. The absorbance spectrum increases with increasing the film thickness. Fig.3(b) shows the annealing temperature changed the value of absorbance and
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shifted the absorption edge towards high wavelengths (red shift). At λ ≥ 600 nm, the absorbance was not affected by the annealing temperature. The absorption band of MnO2
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nanostructures in UV region may be assigned to the transitions between valence and conduction bands [34] while the optical absorption in the visible region may be attributed to
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d–d transitions of Mn ions [35]. Fig. 4 shows the spectral distribution of transmittance, T(λ), and reflectance, R(λ),
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measured in the wavelength of 200 - 2400 nm for as-deposited and annealed β-MnO2 thin films and with different film thicknesses. There are two regions in the spectrum of T(λ) depending on the wavelength of incident light: region (I) is in the wavelength range (200 < λ < 500 nm). The sum of T(λ) and R(λ) is less than unity thus implying an absorption region. The region (II) is for the wavelength (500 < λ < 2400 nm); T(λ) is much greater than R(λ)
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and their total sum is approximately equal to unity [T(λ) + R(λ) ≈ 1]. Therefore, all films were transparent and could not be absorbed the light (non-absorbing region). The T(λ) and R(λ) are influenced with the film thickness and annealing temperature. Study of optical constants play an important role in improvement the optical features of semiconductor materials towards application in photovoltaic devices. The optical constants
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of MnO2 thin films grown on quartz substrates can be deduced in terms of the measured
values of T(λ) and R(λ) after introducing refine the measured data due to experimental errors
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from absorption and reflection of light on the quartz substrate [36, 37]. The optical constants
such as refractive index, n, and the extinction coefficient, k, can be determined by using
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program of modified bi-variant search [38] based on minimizing and simultaneously, 2
R R( n, k ) R 2
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(1) (2)
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2
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T T( n,k ) T ,
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where T(n,k) and R(n,k) are the computed values of transmittance and reflectance using
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Murmann’s exact equations [38 - 40] via a computer program throughout the whole range of measurements (200 - 2400 nm); T and R are the measured values of transmittance and reflectance, respectively. The variances of T and R were calculated and compared to 2
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seek their simultaneous minimization. The corresponding values of n and k represent the
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solution.
The spectral behavior of refractive index, n, and absorption index, k, are in Fig. 5.
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The refractive index of β-MnO2 thin films shows the normal dispersion behavior at wavelength of 900 - 2400 nm, whereas the anomalous dispersion behavior at λ < 900 nm. At lower wavelengths region (anomalous dispersion), the spectral curve of n showed one band with peak at 868 nm, which locates in visible region. This region can be demonstrated by adopting multioscillator model [38]. At higher wavelengths region (normal dispersion),
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the n decreases when the wavelength of light increases, and then at higher wavelength of light, the refractive index tends to be constant. This region can be explained by single oscillator model [41]. Fig.5(a) shows the refractive index increases with increasing the thickness of β-MnO2 film, while annealing temperature decreases the refractive index over the whole spectral range except 316 – 575 nm spectral (Fig.5(b)). The spectral behavior of
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absorption index, k, for the thin film is in Fig. 5. The value of k decreases rapidly when the wavelength increases until it reaches a constant value of about (0.1 - 0.2) due to the
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amorphous structure of β-MnO2 thin films. Both annealing temperature and decreasing of the
film thickness induce the absorption edge of spectrum to move slightly to the higher
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wavelengths (red shift).
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The incident photon energy dependence of refractive index of optical materials can be
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analyzed based on the single-oscillator model [41, 42] to obtain several optical dispersion
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constants, such as the dispersion energy, the single oscillator energy, the dielectric constant at infinity, the lattice dielectric constant and the carrier concentration to effective mass ratio.
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Wemple and DiDomenico (WDD) [39, 41] proposed a dispersion relationship, that describes the dispersion energy parameters which measures the average strength of the interband
Eo 1 (hv)2 Ed Eo Ed
(3)
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(n2 1)1
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optical transitions according to the relation:
where Ed is the dispersion energy, Eo is the oscillator energy, and hv is the incident photon energy. By plotting (n2-1)-1 vs. (hv)2 as shown in Fig. 6. Eo and Ed can be calculated from
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the slope of straight line (= ( Eo Ed ) 1 ) and intercept with ordinate axis (= Eo Ed ). The values of Ed and Eo are listed in Table1. Further analysis of the (n2-1)-1 versus (hv)2 allows to determining the static refractive index ( no at hv = 0), then equation (3) becomes: no (
Ed 1)1 / 2 Eo
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The value of the static refractive index, no, for all investigated films are listed in Table 1. The values of the dielectric constant at infinity, ε∞, was determined from the intersection of the straight line with the (n2-1)-1, since ε∞ = n2 at hv = 0. The real part of the dielectric constant, r , in non-absorbing region can be deduced by the following relation [39, 40]:
e2 N 2 4 oc 2 m*
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r n2 k 2 L
(5)
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where εo is the permittivity of free space, e is the charge of the electron, and c is the speed of light in space. The relation between r and 2 in the normal dispersion region is shown in
Fig.7. The intersection of resultant straight line with the n2-axis determines the lattice
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dielectric constant, L , and the value of the carrier concentration to effective mass ratio,
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N m* , was calculated according to:
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N 4 o c 2 ( slope) m* e2
(6)
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The obtained values of L and N/m* are listed in Table 1. The observed curvature in the curves at high-energy values is a result of interactions of incident photons with thermal
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vibrations of the lattice. The values of all dispersion parameters are listed in Table 1. It is
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clear that both of ε∞ and εL have certain values. Moreover, εL > ε∞ for all β-MnO2 nano thin films. This trend was observed for other metal oxide [42] and was attributed to the contribution of a small concentration of free carriers.
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The absorption coefficient, α, is related to extinction coefficient, k, through the well-
known relation α =4πk/λ. The absorption coefficient spectra of β-MnO2 thin films with different thickness as a function of wavelength of incident light are depicted in Fig.8. The films show high absorption spectra decreases with decreasing the wavelength. All different thin films have α > 105 cm-1 in UV-region of spectra. Annealing temperature and film
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thickness slightly change the intensity of α spectrum and also move the absorption edge towards the lower or higher wavelengths of spectra. Investigation of the absorption coefficient spectrum edge is one of the important well-known method for determining the optically the type of band transitions and provides the values of optical band gap for different materials. Tauc [42, 43] proposed the relationship between the α and hv applied at the edge of
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absorption band as follows:
hv B(hv Eg )r
(7)
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where B is a constant depend on the type of material, r = 1/2 or 2 for direct or indirect
allowed transitions, respectively and r = 3/2 or 3/2 for direct or indirect forbidden transitions,
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respectively. The relation between (αhν)1/r and photon energy (hν) for absorption edge was
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drawn and discussed for different possible values of r. The best linear fit of the results was
for different types of films that exhibits portion of straight line in the
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(αhν)1/2 versus hν
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observed for r = 2 which indicates to indirect allowed transition. Fig.8(a, b) shows the plot
region of absorption edge. The intersection of this line with the hv-axis provides the value of
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the energy band gap, as shown in the inset of Fig.8. The values of indirect allowed energy gap, Eg, for the films with different thickness are listed in Table 1. Annealing temperature
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slightly decreases the optical energy gap of β-MnO2 because the annealing temperature
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removes traps and defect centres through the incorporation of oxygen or increased localized states. Stevels [42, 44] proposed for the metal oxides that the shift of absorption edge to low energy side of spectra (red shift) is due to the electronic transitions from non-bridging
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oxygen, which has a less-tightly bound electron than bridging oxygen. Hence, the value of energy gap decreases with increasing the number of non-bridging oxygen [45]. It could be observed that the values of energy gap result in the present work are compatible with the values reported in the literatures [33, 46]. The values of band gap of β-MnO2 thin films of different thickness in the present work (1.82 -1.98 eV) closely matches the energy gaps of 13
values 1.65 and 2.6 eV for tetragonal [33] and hexagonal [46] MnO2 nanostructures, respectively. In many amorphous materials, the spectrum of absorption coefficient appears as a clear short tail at the wavelengths behind the value of energy gap. This tail is called Urbach tail which undergoes an empirical formula proposed by Urbach [47]. The origin of Urbach tail assigned
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to the phonon-assisted indirect electronic transitions. Tauc [43, 48] reported that the
exponential behaviour of Urbach tail is due to transitions between localized states and would
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vary depending on the type and nature of material. Whereas Davis and Mott [48] assumed that the value of Urbach energy will be approximately the same for most amorphous semiconductors. The Urbach tail can be analysed by the relation [47]:
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hv Eu
(8)
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o exp
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where αo is a pure experimental factor and Eu is Urbach energy. The value of Eu was
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calculated by taking the reciprocals of slope of the linear part of the ln α vs. hν at values of
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photon energy are less than the energy gap. The Urbach energies of the different films thickness are listed in Table 1. It was found that the value of Eu increases with increasing the
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film thickness, while Eu decreased with annealing temperature. 3.3 Nonlinear optical characteristics
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When light with high-intensity passes through the optical material, it causes nonlinear
optical behaviour. Many of metal oxides semiconductors have large values of non-linear
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refractive index, non-linear optical susceptibility and non-linear absorption coefficient. They are promising materials for many nonlinear optoelectronic applications. When nonlinear optical material is exposed to the electric field of the incident light, the
nonlinearity
phenomenon exhibits in the polarization, p, of the material which is represented by a power series expansion in terms of the electric field, E, by the following relation [49]
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P (1) E ( 2) E 2 (3) E 3 ....
(9)
where χ(1) is the linear susceptibility, χ(2) is the second order susceptibility which describes a second harmonic susceptibility and χ(3) is the non-linear optical susceptibility which describes third harmonic generation. The χ(3) is used to determine the possibility of applying the material in optical switching or not. Tichy and Ticha [50] proposed a relation to calculate χ(3)
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that combines Miller’s generalized rule [38] with dispersion parameters calculated based on WDD model [41] as follows: Eo Ed A 2 2 4 ( Eo (hv) )
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( 3)
(10)
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where A is a quantity assumed to be frequency independent and nearly the same for most
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materials (1.7×10-10 esu). Figs. 9(a, b) show the variation of χ(3) as a function of photon
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energy for different β-MnO2 thin films. It was observed that the value of χ(3) increases with
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increasing hv and it is significance affected by increasing film thickness and annealing temperature at 473K. The nonlinear refractive index, n2, can be computed in terms of the non-
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12 (3) n2 no
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linear susceptibility and static linear refractive index by the relation [42, 50]:
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where no is the static refractive index ( n0 ) calculated previously from Fig.6 and listed in Table 1 for the β-MnO2 thin films. Figs. 9(c, d) illustrate the spectral distribution of n2 for the different film thicknesses of MnO2. It can be observed that n2 abruptly decreases when the
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wavelength increases. Annealing temperature and film thickness have slight effect on the value of n2. Additionally, there is another important nonlinear parameter called the non-linear absorption coefficient ( c ) [42, 45]:
c
K c E1pl 2 F
(12)
n2 Eg3 15
where Kc is constant (3100 cmGW-1), Ep is Kane energy parameter, Eg is the optical band gap of material and F is a function that exhibits the dispersion of c with respect to the photon energy (hν). This function depends on the band structure of the material and it can be calculated using the following relation [42, 51]:
2h / E 1 F 2h / E
3/ 2
g
(12)
5
IP T
g
The incident photon energy range should satisfy the two-photons absorption condition Eg 2
h Eg ). Fig.10 shows the plot of c versus hν and it can be observed that c
SC R
(
increases with increasing hv until it reaches to the maximum value at peak of 1.05 eV. After
U
that, c decreases significantly with increasing hv until it reaches hv = 1.7 eV that undergoes
N
the two-photon absorption condition. Annealing temperature increases the value of c while
A
the value of c decreases with increasing the film thickness. Therefore, it can be concluded
M
that the annealing process is considered a desirable effect for enhancement the nonlinear
ED
optical properties of the MnO2 films. Furthermore, increasing the film thickness decreases the
c and increases χ(3) and n2. The increasing values of nonlinear optical constants (χ(3), n2 and
PT
c ) of MnO2 are due to bound oxygen bonds and free ions that have high polarizability in
CC E
oxide matrix that result in the short and long Mn-O bonds. This difference in the bond length causes polarizability between long and short bonds that leading to increase the values of nonlinear optical parameters.
A
3.4. Photoresponse characteristics of Au/β-MnO2/p-Si/Al heterojunction photodiodes The Schematic diagram of Au/β-MnO2/p-Si/Al heterojunction photodiode is shown in Fg.11. We decided to design and compare Au/β-MnO2 (86 nm)/p-Si/Al photodiode1 with Au/βMnO2 (45 nm)/p-Si/Al photodiode2. The current-voltage (I-V) characteristics of heterojunction photodiodes in dark state and under illumination light intensity of 10 mW/cm2 are 16
illustrated in Fig.12. The dark and illuminated I-V curves of the Au/β-MnO2(86 nm)/p-Si/Al photodiode1 at ambient temperature show that a distinct kind of S-shape was observed in the I-V plot under illumination, which was not observed in the dark I-V plot as shown in Fig. 12(a). A similar S-shape behavior of I-V characteristic under illumination, and not in the dark state, was observed and reported by A. Iefanova et al [52]. The absence of short circuit
IP T
current and open circuit voltage of this diode under illumination in this plot indicates an
insufficient build-in electric field between the Au grid and the Al electrode, which results in
SC R
poor collection of photogenerated charges at reverse bias voltage [52]. Fig. 12(b) shows the I-V curves of the Au/β-MnO2 (45 nm)/p-Si/Al photodiode2, the photocurrent in reverse
U
bias voltage increases strongly with illumination and this could be due to the increase in the numbers of photo-generated charge carriers at the interface states. The short circuit current
A
N
and open circuit voltage of the photodiode were found to be 70 μA and 0.64 V, respectively.
M
These obtained values led to poor photovoltaic parameters of this device to be a solar cell comparable with other inorganic solar cells. Furthermore, they are enough to potential for However, both photodiodes are sensitive to UV-Visible light,
ED
photodiode applications.
especially the second photodiode of β-MnO2(45nm)/p-Si and it can be utilized as a
PT
photosensor applications.
The photodiodes are fabricated to achieve a spectral or a rapid time response, while
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solar cells are designed to convert the solar energy to the photo-current or photo-voltage [53]. Generally, we decided to calculate some important photovoltaic parameters for the
A
heterojunction photodiodes. The first one is the responsivity, which measures the outputinput gain of a detector system and in case of a photodectector, photoresponsivity measures the output photoelectrical per input optical power and can be directly determined from the photocurrent density, Jph, divided by the light intensity power, P. The photoresponsivity, R,
17
and photoconductivity, S, of the photodiode can be obtained in accordance with the following relations [54, 55] as: R
J ph
(13)
P
L V
(14)
IP T
SR
where L is the thickness (400 μm) of illuminated active layer of the diode and V is the applied
SC R
potential. As we can see from Fig. 13(a), the values of R and S of the photodiodes as a function of applied reverse voltage. In both of two photodiodes, the values of R increased with increasing reverse bias potential, while the values of S decreased with increasing reverse
U
bias potential for photodiode2 but S is still constant with low values in photodiode1. The
N
maximum values of photoresponsivity for both photodiodes are ≈ 0.15 A/W at 2 V reverse
A
bias. Another important parameter is specific detectivity, D*, used to characterize
M
performance for a photodetectors. D* is commonly expressed in Jones units (cm Hz0.5 W-1) as
D*
ED
follows [55, 56]:
R 2eJ d
(15)
PT
where Jd is the dark current density. Fig.13(b) shows D* as a function of applied potential bias
CC E
and it can observed that, the D* of the photodiode1 has values of 1.0×108 - 1.7×108 cm Hz0.5 W-1 at low reverse potential bias 0 – 1.1 V. These values increase rapidly at potential bias >1.1 V and reach 9×108 cm Hz0.5 W-1, while the photodiode2 is more stable and the values
A
of D* are almost fixed at ≈ 5 ± 0.5 ×108 cm Hz0.5 W-1 along the whole range of reverse potential bias 0 – 2 V. Comparable of Au/β-MnO2(86 nm)/p-Si/Al photodiode1 with Au/βMnO2(45 nm)/p-Si/Al photodiode2 showed that the photovoltaic properties of photodiode2 was better than first device in both I-V characteristics and photovoltaic properties. Jph of
18
heterojunction photodiode is maximized at the thickness layer of β-MnO2 ≈ 45 nm, but it decreased at the thicker active layer ≈ 86 nm, indicating the thickness dependence on Jph. The present results of photovoltaic parameters were comparable with the pervious works [54 - 59] that studied the photoresponse properties of different heterojunction photodiodes. Makhlouf et al. [57] showed that photoresponsivity of PbO2/p-Si heterojunction photodiode
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reached to 0.19 A/W at 2 V while, Park et al. [54] designed n-ZnO/p-Si photodiode and the
device was measured under red laser illumination of the wavelength 670 nm. The
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photoresponsivity was 0.28 A/W. The heterojunction photodiode of InGaN/GaN was
fabricated and characterized. It was found that the photodiode exhibited a photoresponsivity
U
around 1.76 A/W at potential bias 3 V and under light illumination of the wavelength 380 nm
N
[58]. Ge/Si heterojunction photodiode operating in the IR spectral region was fabricated and
A
the photoresponsivity was 0.23 A/W at 2 V reverse bias under illumination IR light of
M
wavelength 1550 nm [59]. The photoresponse properties of MnO2 thin films open the door for the use thereof as potential photosensitizing material in different photovoltaic devices
ED
with several hybrid constructions (MnO2 film / inorganic, MnO2 film / inorganic, ….etc), especially that the heterojunction photodiodes based on active thin layer of MnO2 in the
PT
present work have acceptable values of photovoltaic parameters, which may attract
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considerable attention as a promising candidate in solar cells in the future.
Conclusion
A
The nano thin films of β-MnO2 were grown on substrates using the thermal evaporation technique. FESEM and XRD techniques were used to investigate the structural formation of β-MnO2 nano thin films. The results confirmed that XRD patterns showed that the MnO2 powder has polycrystalline structure with β phase and tetragonal crystal system. The asdeposited films showed amorphous structure, while the annealed film revealed
19
nanocrystalline of β-MnO2 with tetragonal structure. The crystallite size was found in the range of ca. 14 - 58 nm for these films. The optical constants of β-MnO2 films with different thickness were calculated from the absolute values of transmittance and reflectance. The optical studies of β-MnO2 films revealed that the annealing temperature and different thickness change the optical spectra in
1
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UV-Vis-NIR regions for these films. The absorption coefficient of β-MnO2 films is > 105 cmin UV region of spectra. The electronic transition observed near the absorption edge is
SC R
indirect allowed transition with optical energy band gaps (Eg) of 1.91 and 1.98 eV for the
films of thickness 46 and 100 nm, respectively. Furthermore, the Eg of as-deposited and annealed film (86 nm) are 1.98 and 1.82 eV, respectively. Dispersion parameters like
U
oscillation energy (Eo), dispersion energy (Ed), dielectric constant at high frequency (ɛ∞), and
N
the lattice dielectric constant (ɛL) were calculated. The nonlinear refractive index (n2), third-
A
order nonlinear susceptibility (χ(3)) and nonlinear absorption coefficient (βc) were determined.
M
It was found that χ(3), n2 and β increased when both photon energy and annealing temperature
ED
increased. The annealing temperature is a suitable tool for improvement the nonlinear optical properties of β-MnO2 nano thin films.
PT
According to the obtained results of optical properties of MnO2 nano films, we decided to design and fabricate two heterojunction photodiodes based on β-MnO2 nano films
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with different thicknesses of β-MnO2 active layer with thickness 45 and 86 nm. The photoresponsivity (R), photoconductivity (S) and specific detectivity (D*) of Au/β-MnO2/p-
A
Si/Al photodiodes were studied. The present values of photoresponse parameters of Au/ βMnO2/p-Si/Al heterojunction photodiodes were comparable to the pervious works [54 - 59] that studied the photoresponse parameters of different heterojunction photodiodes.
20
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ED
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PT
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ED
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PT
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[40] A.S. Radwan, M.M. Makhlouf, E. Abdel-Latif, Dyes Pigm. 134, 516 (2016)
[43] J. Tauc, Amorphous and Liquid Semiconductors, Plenum Press, New York, 1974.
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[51] M. Sheik-Bahae, D.J. Hagan, E.W. Van Stryland, Phys. Rev. Lett. 65, 96 (1990). [52] A. Iefanova, N. Adhikari, A. Dubey, D. Khatiwada, Q. Qiao, AIP Advances 6, 085312
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[53] R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Company, New York, 1996. [54] C.H. Park, J.Y. Lee, S. Im, T.G. Kim, Nucl. Instrum. Methods Phys. Res. B 206, 432 (2003)
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[55] R. C. Jones, J. Opt. Soc. Am. 50, 1058 (1960) [56] E.Y. Choi, S.H. Eom, C.E. Song, S.Y. Nam, J. Lee, H.Y. Woo, I.H. Jung, S.C. Yoon, C. Lee, Org. Electron. (2017), doi: 10.1016/j.orgel.2017.04.016. [57] M.M. Makhlouf, M.M. EL-Nahass, M.H. Zeyada, Mater. Sci. in Semicond. Process. 58, 68 (2017)
Chang, Solid-State Electron. 47, 879 (2003)
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PT
ED
M
A
N
U
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[59] Z. Zhou, J. He, R. Wang, C. Li, J. Yu, Opt. Commun. 283, 3404 (2010)
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[58] Y -K. Su, S -J. Chang, Y -Z. Chiou, T -Y. Tsai, J. Gong, Y -C. Lin, S -H. Liu, C -S.
24
IP T
A
CC E
PT
ED
M
A
N
U
SC R
M. M. Makhlouf: He was born in Demiatta, Egypt, in 1976. He received the MSc. and Ph.D. degrees in physics from Faculty of science, Mansoura University, Damietta Campus, Egypt in 2003 and 2007, respectively. He is associate professor in Physics department, Turabah University College, Taif university, Saudi Arabia. His current research focuses on materials science, nano thin films, inorganic-organic semiconductors, photovoltaics and photosensor devices.
25
Caption figures Fig. 1: XRD patterns of β-MnO2: (a) as-received powder , (b) pristine film, (c) annealed thin at 473 K for 2 h and the inset shows EDX spectrum of β-MnO2 film Fig. 2: FESEM images of nanostructures β-MnO2 thin films of thickness 86 nm: (a) asdeposited and (b) annealed at 473 K for 2 h. Fig. 3: Absorbance of β-MnO2 films of (a) different thicknesses and (b) as-deposited and
IP T
annealed Fig. 4: Spectral behavior of transmittance, T, and reflectance, R, of β-MnO2 films (a) different thicknesses and (b) as-deposited and annealed
SC R
Fig. 5: Refractive, n, and absorption, k, indices of β-MnO2 films of (a) different thicknesses and (b) as-deposited and annealed Fig. 6: Plot of (n2-1)-1 versus of (hν)2. Fig. 7: Plot of n2 versus λ2.
U
Fig. 8: Variation of absorption coefficient, α, of β-MnO2 films and the inset shows
N
plot of (αhν)1/2 as a function of the incident photon energy for different films refractive index, n2, of β-MnO2 films.
A
Fig. 9: The third order nonlinear susceptibility, χ(3), spectra and the spectral nonlinear
and (b) as-deposited and annealed
M
Fig. 10: The nonlinear absorption coefficient, βc, of β-MnO2 films of (a) different thicknesses
ED
Fig. 11: Schematic diagram of Au/β-MnO2/p-Si/Al heterojunction photodiode Fig. 12: Dark and illuminated I-V characteristic plots of (a) Au/β-MnO2(86 nm)/p-Si/Al
PT
photodiode1 and (b) Au/β-MnO2(45 nm)/p-Si/Al photodiode2 Fig. 13: The responsivity, R, and photoconductivity sensitivity, S, and the specific
A
CC E
detectivity, D*, of two photodiodes
26
400
(c)
(2 1 1)
80
(b)
70
Mn
60
C ounts
300 200
Energy 0.40 1.09 1.78 3.82
Mass% 12.76 10.01 68.50 8.73
EDX
40 30 20
O Mn
10
100
Element O Mn Mn Mn
50
0
0
1
Mn
2
3
4
5
6
7
8
9
10 11
0
(a)
10
20
N 30
PT
A
CC E
Fig. 1
ED
(2)
o
27
40
(5 2 1)
(1 1 1)
A M
0
(3 3 0) (4 2 0)
(2 2 0)
20
(2 0 0)
(1 1 0)
40
U
(1 0 1)
60
(2 1 1)
Energy, keV
(1 1 0)
Intensity, counts/s
0 400
IP T
100
SC R
200
(1 1 1)
(1 0 1)
(1 1 0)
300
50
60
A ED
PT
CC E (b)
IP T
SC R
U
N
A
M
(a)
Fig. 2
28
2.0
Film thickness 45 nm 86 nm 100 nm
IP T
1.0
0.5
(a) 300
400
500
600
U
0.0 200
SC R
Absorbance
1.5
800
M
A
N
, nm
700
ED PT
1.0
A
CC E
Absorbance
1.5
Thickness = 86 nm As-deposited film Annealed film
0.5
(b)
0.0 200
300
400
500
, nm Fig. 3 29
600
700
800
100
(a)
80
Film thickness 45 nm 86 nm 100 nm
R% & T%
70 60
T
50
IP T
90
R
40
SC R
30 20 10
500
1000
U
0 1500
A
N
, nm
2000
(b)
ED
90 80
PT
50
Thickness = 86 nm As-deposited film Annealed film
T
CC E
R% & T%
70 60
M
100
R
40 30
A
20 10
0 500
1000
1500
, nm
Fig. 4 30
2000
4
Film thickness 45 nm 86 nm 100 nm
(a)
n
3
2
IP T
1 0.8
k
0.6
SC R
0.4
0.2
0.0
, nm
4
(b)
A M
n
PT
CC E
0.5
ED
2
0.6
2000
Thickness = 86 nm As-deposited film Annealed film
3
1 0.7
1500
U
1000
N
500
k
0.4 0.3
A
0.2 0.1 0.0
500
1000
1500
, nm
Fig. 5
31
2000
0.5
Film thickness 45 nm 86 nm 100 nm
0.3
IP T
2
(n -1)
-1
0.4
SC R
0.2
(a) 0.1 0.5
2
(h) , (eV)
1.0
1.5
2
N
U
0.0
M
A
0.5
PT
0.3
CC E
2
(n -1)
-1
ED
0.4
As-deposited film Annealed film
0.2
A
(b)
0.1
0.0
0.5
1.0 2
2
(h) , (eV)
Fig. 6
32
1.5
7
Film thickness 45 nm 86 nm 100 nm
6
4
SC R
n
2
IP T
5
3
(a) 1
2
3
2
4
5
6
7
2
N
0
U
2
M
A
, (m)
ED
7
n
2
CC E
5
PT
6
Thickness = 86 nm As-deposited film Annealed film
A
4
3
(b) 2 0
1
2
3 2
, (m)
Fig. 7 33
4 2
5
6
7
5
6x10
2000 0.5 0.5
-1
5
4x10
5
3x10
1500
1000
IP T
(h) , (eV/cm)
5
5x10
, cm
Film thickness 45 nm 86 nm 100 nm
500 5
0 0
5
1x10
1
2
3
4
h, eV
(a) 0
6
7
1500
A
N
h, eV
1000
5
U
500
SC R
2x10
2000
5
0.5
(h) , (eV/cm)
0.5
-1
5
CC E
, cm
1000
2x10
5
1x10
A
1500
PT
5
Thickness = 86 nm As-deposited film Annealed film
ED
4x10
3x10
M
5
5x10
500
0 0
1
2
(b)
3
h, eV
4
5
6
7
0 500
, nm
Fig. 8 34
1000
1500
-10
8.0x10
-10
6.0x10
IP T
(esu)
Thickness = 86 nm As-deposited film Annealed film
Film thickness 45 nm 86 nm 100 nm
-9
1.0x10
(3)
-10
4.0x10
(b)
(a) 0.0 0.4
0.6
0.8
1.0
1.2
1.4
0.4
0.6
h, eV
0.8
SC R
-10
2.0x10
1.0
1.2
1.4
Film thickness 45 nm 86 nm 100 nm
-9
M
1.0x10
-10
-10
CC E
-10
PT
6.0x10
-10
Thickness = 86 nm As-deposited Annealed
ED
n2 (esu)
8.0x10
4.0x10
A
N
U
h, eV
2.0x10
0.0
1000
A
(d)
(c)
1500
1000
2000
, nm
Fig. 9
35
1500
, nm
2000
Film thickness 45 nm 86 nm 100 nm
(a)
20
10
IP T
, cm GW-1
15
0 1.0
1.1
1.2
1.3
1.4
1.5
h, eV
1.6
1.7
1.8
U
0.9
Thickness = 86 nm Pristine film Annealed film
PT
CC E
, cm.GW
-1
15
10
M
(b)
ED
20
A
N
0.8
SC R
5
A
5
0
0.9
1.0
1.1
1.2
1.3
h, eV
Fig. 10
36
1.4
1.5
1.6
1.7
IP T
SC R
Incident light
U
Au grid
To the measuring system
N
β-MnO2 film
CC E
PT
ED
M
A
P-Si wafer
A
Fig.11
37
Al electrode
.
0.0004
Dark state Illumination state
0.0002
IP T
0.0001
0.0000
SC R
Current, A
(a)
Photodiode1 MnO2(86 nm)/p-Si
0.0003
-0.0001
-0.0003 -2
-1
0
U
-0.0002
1
0.0004
Photodiode2 MnO2(45 nm)/p-Si
Dark state Illumination state
PT
0.0000
CC E
Current, A
0.0002
0.0001
(b)
ED
0.0003
M
A
N
Potential bias, V
2
-0.0001
A
-0.0002
-0.0003 -2
-1
0
Potential bias, V
Fig.12
38
1
2
-4
2.5x10
-1 -4
-4
1.5x10
IP T
0.10
-4
1.0x10 0.05
(a)
0.0
0.00 1
2
9
M
Photodiode1 Photodiode2 8
ED
8.0x10
8
8
A
CC E
4.0x10
PT
*
Detectivity (D ), cm Hz
0.5
W
-1
1.0x10
A
N
Potential bias, V
U
0
6.0x10
8
2.0x10
0.0
(b)
0.0
-5
5.0x10
SC R
Rsponsitivity (R), A W
-1
2.0x10
-1
0.15
Photodiode1 Photodiode2
Photoconductivity (S), A W V
Photodiode1 Photodiode2
0.5
1.0
1.5
Reverse bias potential, V
Fig. 13 39
2.0
Caption Table Table 1: The optical and dispersion parameters of MnO2 nano thin films with different thickness.
Eg (eV)
EU (meV)
Eo (eV)
Ed (eV)
no
ε∞
εL
N/m* (Kg-1m-3)
As-deposited (45 nm)
1.91
94
1.59
4.30
1.95
3.79
4.09
1.09 × 1056
As-deposited (85 nm)
1.98
121
1.63
4.97
2.03
4.13
As-deposited (100 nm)
1.98
121
1.62
5.32
2.13
4.55
Annealed (85 nm)
1.82
98
1.65
5.47
2.15
4.64
A
CC E
PT
ED
M
A
N 40
1.13 × 1056
4.57
1.16 × 1056
4.85
3.3 × 1056
IP T 4.18
SC R
U
MnO2 thin film