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GaSe interface
Accepted Manuscript Band offsets and optical conduction in the CdSe/GaSe interface T.S. Kayed, A.F. Qasrawi, Khaled A. Elsayed PII:
S1567-1739(16)30081-5
DOI:
10.1016/j.cap.2016.04.010
Reference:
CAP 4202
To appear in:
Current Applied Physics
Received Date: 30 January 2016 Revised Date:
14 April 2016
Accepted Date: 14 April 2016
Please cite this article as: T.S. Kayed, A.F. Qasrawi, K.A. Elsayed, Band offsets and optical conduction in the CdSe/GaSe interface, Current Applied Physics (2016), doi: 10.1016/j.cap.2016.04.010. 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.
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Band offsets and optical conduction in the CdSe/GaSe interface
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T. S. Kayed 1, A. F. Qasrawi 2, 3,*, Khaled A. Elsayed 1 1 Department of Basic Sciences and Humanities, College of Engineering, University of Dammam, Dammam, Saudi Arabia 2 Department of Physics, Arab-American University, Jenin, West Bank, Palestinian Authority 3 Group of Physics, Faculty of Engineering, Atilim University, 06836 Ankara, Turkey Abstract
In this work, the design and characterization of CdSe/GaSe heterojunction is considered. The
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CdSe/GaSe thin film interface was prepared by the physical vapor deposition technique. Systematic structural and optical analysis were performed to explore the crystalline nature, the optical band
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gaps, the conduction and valence band offsets, the dielectric spectra, and the frequency dependent optical conductivity at terahertz frequencies. The X-ray diffraction analysis revealed a polycrystalline interface that is mostly dominated by the hexagonal CdSe oriented in the (002) direction. It was also found that the CdSe/GaSe interface exhibits conduction and valence band
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offsets of 1.35 and 1.23/1.14 eV, respectively. The dielectric spectra displayed two dielectric resonance peaks at 530 and 445 THz.
Moreover, the computational fittings of the optical
conductivity of the interface revealed a free carrier scattering time of 0.41 (fs) for a free carrier
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density of 7.0 × 1018 (cm −3 ) . The field effect mobility for the CdSe/GaSe interface was found to be
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5.22 ( cm 2 / Vs ). The remarkable features of this device having large band offsets and qualitative optical conduction dominated by a scattering time in the order of femtoseconds in addition to the dielectric property nominate the device to be used in optoelectronic technology.
Keywords: Optical materials; Coating; Dielectric properties; Optical properties ----------------------------------------------------------------------------------------------------------------------*
Corresponding author: Tel.: +970-599379412, Fax: +97042510810/817. Email: [email protected], [email protected]
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1. Introduction
Due to its large photosensitivity and wide optoelectronic applicability, CdSe based optoelectronic devices have attracted the attention of the research and quality control centers. Recently, a largearea, flexible, high-speed analog and digital colloidal CdSe nanocrystal integrated circuits operating
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at low voltages were reported. The circuits involved amplifiers with 7 kHz bandwidth, ring oscillators with ~ 10 µs stage delays, and NAND and NOR logic gates [1]. In addition to that, CdSe films deposited on glass substrates coated with fluorine-doped tin oxide (FTO), exhibited an electronic quality of the FTO/n-CdSe/Au Schottky diodes. The films had rectification factor of
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102.9, reverse saturation current of ~ 372 nA, and threshold voltage of ~ 0.15 V. The potential
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barrier observed for Au/n-CdSe interface was found to be ~ 1.10 eV [2]. Moreover, an optimal magnetic-ion quantum dot system for implementation in a single-ion-based spin memory is identified [3].
Another promising Se based compound that is known to have some novel optoelectronic properties is the GaSe. GaSe thin films are reported to exhibit nonlinear optical properties. Studies on these
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films using the multi-photon microscopy, revealed both second- and third-harmonic generation [4]. In addition to that, tunable micro-cavities with few monolayers of GaSe films have been
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observed to reflect a significant modification of the spectral and temporal properties of photoluminescence (PL) spectra. The spectra are emitted in narrow and wavelength-tunable cavity
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modes with quality factors up to 7400. In that process a 10-fold PL lifetime shortening is achieved [5]. Moreover, the optical analysis of the MgO/GaSe heterojunction [6] have also displayed an enhanced absorbing ability of the GaSe below 2.90 eV with an energy band gap shift from 2.10 eV for the GaSe substrate to 1.90 eV for the GaSe/MgO heterojunction. This interface behaves like a tunneling-type device with a depletion region width of 670 nm and 116 nm when forward and reverse biased, respectively. The GaSe/MgO heterojunction exhibited resonance–antiresonance behavior and negative capacitance characteristics near 1.0 GHz. The features of this device appear to be suitable for the use as a band reject filter in visible light optoelectronics and in microwave
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device sectors [6]. In another work, layers of GaSe are used for the fabrication of a thin film field effect transistor (FET). The FET displayed a typical n-and p-type conductance transistor with good ON/OFF ratio and electron high differential mobility [7]. It is also reported that the 2D GaSe crystals exhibit characteristics that demonstrated the potential of the GaSe usage in electronic and
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optoelectronic applications [8]. In light of the above reported information we were motivated to fabricate a double layer thin film FET that joins these two smart materials together to reveal wider range of applications in
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optoelectronics. Particularly, in this work we will focus on the structural and optical properties of CdSe/GaSe interface as a thin film transistor suitable for visible light communications and/or
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terahertz sensing. The FETs deposited onto glass substrates will be characterized by means of ultraviolet-visible light spectrophotometric analysis to discover the valence and conduction band offsets, the dielectric spectra, the optical conductivity, and the FET electrical parameters through
2. Experimental Details
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optical conductivity modeling.
CdSe and GaSe thin films were evaporated onto glass substrates from the source materials which
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are CdSe (Alfa Aeser 1306-24-7 with maximum obtainable purity of 99.999%) metal basis and Ga2Se3 (Alfa Aeser 12024-24-7 with maximum obtainable purity of 99.99%) single crystals. The
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evaporation was achieved in a VCM 600 physical vapor deposition system at vacuum pressure of 10-5 mbar. A thickness monitor attached to the system controlled the thicknesses of the films. The hexagonal nature of crystallization of the CdSe films and the amorphous nature of the GaSe films were reported in our previous works [6,9]. The X-ray diffraction patterns for the CdSe/GaSe interface being deposited onto Al film and onto glass substrate (geometrical design is shown in Fig. 1 (a) ) were recorded with the help of Ultima IV type III X-ray diffractometer. The optical transmittance and reflectance were recorded using Evolution 300 spectrophotometer equipped with pike VEE MAX II variable angle reflectance accessory. 3
ACCEPTED MANUSCRIPT 3. Results and Discussion Based on our previous studies of InSe/CdSe heterojunction [9], some very interesting characteristics were observed for the interface and were promising for the use in thin film field effect transistor
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technology [9]. In this work we are aiming to explore and discuss the properties of the CdSe/GaSe interface as well. It was previously mentioned that the CdSe layer exhibits p-type of conduction [9] whereas the GaSe layer exhibits n-type of conduction [6]. The thickness of each of the CdSe and
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GaSe films was 1.2 µm whereas the thickness ( d ) of the CdSe/GaSe interface was 2.4 µm . To reveal detailed information about the crystalline nature of the CdSe/GaSe interface, we have
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recorded the X-ray diffraction patterns for CdSe/GaSe interface for films deposited onto glass and compared them to those deposited onto Al substrates. The X-ray diffraction patterns which are displayed in Fig. 1 (a) reveal patterns that are indexed in accordance with the PDF card numbers of 01-070-2554 and 00-008-0459. The cards refer the main peak to the hexagonal CdSe being best
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oriented in the (002) direction. The corresponding lattice constants are a = 4.2985 A° and c = 7.01520 Ao . Most of the other minor peaks are also assigned to CdSe polycrystals. The test of the possibility of finding polycrystalline GaSe was also tested and only two peaks located at 23.06 o and
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at 41.18o could be found in the pdf cards of GaSe. On the other hand, the figure also display more sharp intensive peak for the Al/CdSe/GaSe than for the glass/CdSe/GaSe films. The most intensive
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peak which is enlarged in Fig. 1 (b) also reflects larger full wave half length broadening ( β ). The calculated grain sizes ( D ) in accordance with Scherrer equation ( D = 0.94λ /( β cos(θ )) ) and the degree of orientation ( f = (( I max / ∑ I hkl ) − I phkl ) /(1 −I phkl ) ) [10] are found to be 26 nm and 0.71 and 23 nm and 0.76, for CdSe/GaSe interface grown onto glass and Al substrates, respectively. Thus, in accordance with these numerical data, the appearance of the more sharp intensive peak for the Al/CdSe/GaSe than that for the glass/CdSe/GaSe films can be ascribed to the improvement of the crystalinity in the films when grown onto Al substrate. The crystalline nature improves as a
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result of the lattice matching between the hexagonal closed pack unit cell of Al and the hexagonal CdSe polycrystal. At the Al/CdSe interface, the hexagonal structure of alternating layers of CdSe is shifted so its atoms are aligned to the gaps of the Al as a preceding layer. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer leading to a higher
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orientation values. Fig. 1 (c) displays the transmittance (T) spectrum for the CdSe/GaSe interface being recorded in the incident light wavelength range of 300-1100 nm. This spectrum exhibits an increasing trend of
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variation in the region of 462-624 nm. While in the region of 625-680 nm, the T spectrum is λ invariant, in the region of 625-800 nm, it sharply re-increases when λ increases. For larger λ
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values the T spectrum continues decreasing and tends to remain constant above 1000 nm. On the other hand, the reflectance ( R ) spectrum which is also shown in Fig. 1 (c) appears to increase with increasing incident light wavelength up 560 nm where it reaches a local maximum. Similarly, another broadened peak appears at 670 nm. In the wavelength range of 820-1100 nm, the
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reflectance steadily increases with increasing wavelength. Apart from the reflectivity which was recorded at normal incidence that is displayed in Fig.1 (c), the effect of the angle of incidence of light ( α ) on the interface reflectivity is shown in the inset of Fig. 1 (d). As the inset shows, at a
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particular wavelength of 600 nm, increasing the angle of incidence of light increases the total reflectivity. The maximum reflectivity is obtained for an angle of incidence of 65o. For higher α
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values, R % sharply falls reaching a value of 2.9% at 80o. The curve which is displayed in the inset represent an application guidance for optoelectronic applications. The holistic picture of the optical spectra becomes more clear when the absorbance ( A ) spectrum is considered. The absorbance spectrum that was calculated from the relation A% = 100 − T − R (R of normal incidence) is illustrated in Fig. 1 (d). The absorbance of the CdSe/GaSe interface in the incident photon energy range of 4.0-2.6 eV (corresponding to 310-478 nm) is almost steady and has very high (95%) value. When the incident photon energy values fall below 2.6 eV, the absorbance sharply falls reaching a value of 40% at 1.8 eV (corresponding to 690 nm). The relation A% = 78.97 E − 107.80 can present
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the changes of the absorbance with the incident photon energy. The slope of the A − E variation ( A% = 144.28 E − 218.24 ) becomes more pronounced down to 1.5 eV (corresponding to 828 nm). Below 1.5 eV, the absorbance spectrum increases with decreasing incident light energy following the linear relation, A = −76.96 E + 126.97 , and it then exhibits a maximum at 1.3 eV (corresponding
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to 996 nm). The variations in the absorbance of the CdSe/GaSe interface are comparable with those of the InSe/CdSe interface [9]. However, the absorption peak that is detected at 1.3 eV for the CdSe/GaSe was not previously observed in the InSe/CdSe absorbance spectrum. Nor it was
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observed in the MgO/GaSe absorbance spectrum [6]. The increase in the absorbance values with decreasing light energy in the low energy region of 1.70-1.1 eV was also seen in the GaSe
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absorbance spectrum [6]. This absorption peak that is seen at 1.3 eV could be ascribed to the interband transitions in the GaSe as well as in the CdSe. While the CdSe is reported to exhibit inter-band transitions due to the instantaneous generation of electrons during the decay of the Plasmon in short separation times [11], the inter-band transitions in GaSe below 2.0 eV is ascribed to the electronic transitions from the p -like to the s -like orbits [12]. The top of the valence band in the GaSe is
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located in the pz − like orbit that originates from Se atoms. It is also mentioned that the p x and p y like orbits reserve deep locations in the valence band of the GaSe. The electronic transitions from
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these two orbits to the pz reveal an energy value of 3.2 eV [12]. The latter value explains the reasons that are behind the existence of high absorbance above 3.0 eV in GaSe as appears in Fig. 1
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(c) of Ref. 6.
To investigate the change in the energy band gap of CdSe as a result of the CdSe/GaSe interfacing, the absorption coefficient α = A / d was calculated. The (αE ) 2 − E plots that are presented in Fig. 2 and its inset indicate the domination of the direct allowed transitions type of energy band gap in the high and low absorption regions, respectively. The E − axis intercept of the dashed lines that appear in the figure and its inset, relate to the energy band gaps ( E g ) of 1.97 and 1.64 eV being dominant in the incident photon range of 2.08-2.50 eV and 1.65-1.90 eV, respectively. The energy
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band gap of the CdSe is 1.85 eV [9] and that of GaSe is 2.10 eV [6]. The energy band gap differences between that of CdSe and those of CdSe/GaSe interface are ∆Eg1=0.12 eV and
∆Eg2 =0.21 eV, respectively. Since the electron affinity ( qχ ) of CdSe is 4.58 eV [9, 13] and that of GaSe is 3.23 eV [6, 14], then the conduction band offset (∆Ec = (q χ CdSe − q χ GaSe ) = 1.35 eV. This
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leads to a respective valence band offset of ∆Ev1 = ∆Eg1 − ∆Ec =1.23 eV and ∆Ev2 of 1.14 eV. The values of the conduction and valence band offsets are relatively high when compared to those of InSe/CdSe interface [9] as 0.03 and 0.36 eV and of CdSe/CdTe interface [15] as 0.54 and 0.28 eV,
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respectively. The values of ∆Ev reported here are also higher than those reported as 0.75 eV for the
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CdSe/ZnTe heterojunction [16]. The difference between the theoretically calculated literature data and the currently reported is ascribed to the low crystal symmetry (as detected by experiments compared to theoretical assumptions), and/or significantly larger lattice mismatches, possible dipole corrections and orientation dependencies may have to be considered on the theoretical side. On the experimental side, such conditions could lead to a stronger confinement of the charge carriers at the
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interfaces. This confinement make the excitons effect more apparent. This means that the employed theoretical model should had been justified to correctly fit up with experimental data [16]. The
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values of the conduction and valence band offsets are also presented by the energy band diagram as an inset in Fig. 3 (a). The presented offset values for the CdSe/GaSe interface are promising as they
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guarantee the success of this interface when used in thin film transistors, optoelectronic devices, and as quantum dots [9, 15-17].
Fig. 3 (a) and (b) displays the real ( ε r ) and imaginary ( ε i ) parts of the dielectric constant spectra for the CdSe/GaSe interface being calculated using the previously reported method [9] from the reflectance spectrum that appeared in Fig. 1 (c). As Fig. 3 (a) shows, except for the minor and major resonance peaks that appear at 530 and 445 THz, the real dielectric constant increases with decreasing incident light wave frequency. The ε r − F inverse variation becomes more pronounced below 365 THz. The relation ε r = − 0.219 F + 81.82 can describe the behavior of the dielectric 7
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constant. The resonance peaks, which appeared at 530 and 445 THz, correspond to oscillator energy values of 2.20 and 1.85 eV, respectively. While the latter number coincides with the energy band gap of CdSe film, the earlier is close to that of GaSe film. In addition, the shoulder which appeared at 416.6 THz near the major resonance peak (445 THz) correspond to an energy value of 1.71 eV is On the other hand, the ε i
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probably due to the extended tail states that arises from the interfacing.
spectra that are presented in Fig. 3 (b) exhibit values that are two orders of magnitude lower than that of ε r indicating that the quality factor of the electromagnetic resonance at the interface is large
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enough to employ the CdSe/GaSe interface in optoelectronic devices technology. As an example, the quality factor values for the interface at 530, 445 and 300 THz are 115, 175 and 300,
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respectively.
The importance of the dielectric spectra from the point of view of applications in thin film transistor technology and other related devices is better presented by the optical conductivity and its relative parameters. The optical conductivity calculated from the equation, σ (w = 2π F) = ε i .w / (4π ) [9], is
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presented in Fig. 3 (c). The figure reflects a very slow varying trend of optical conductivity with incident light frequency in the range of 1000-530 THz. Below 530 THz; the conductivity sharply decreases with decreasing frequency. Here, the tendency of σ ( w) to remain constant arises from the
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inability of the electric dipoles at the interface to follow the light signal above 530 THz. Contrarily, below 530 THz, the optical conductivity sharply falls by one order of magnitude as the frequency
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reaches 360 THz indicating the enhanced polarizeability of the electron hole pairs at the p-n junction interface with the incident light signal [18]. The modeling of the optical conductivity, σ ( w) , in accordance with the Drude model, which takes into account the negative contribution of the free carriers (n) to the optical conductivity, starts from the frequency (w) dependent optical conductivity which is given by,
σ ( w) =
ne 2τ m * (1 − iwτ )
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(1)
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In the above equation, m * = 0.138 mo is the reduced carrier effective mass of the interface being calculated from the values of the effective mass of CdSe and of GaSe that are reported as 0.45mo [9] and as 0.20mo [19], respectively. Here, n is the number of free carriers, e is the electronic charge, i stands for complex number presentation and τ is the carrier scattering time or relaxation
negative and represents the free carrier contribution [18].
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time. According to the above equation, the real part of the frequency dependent conductivity is
As seen from Fig. 3 (c), the optical conductivity data are reproduced by equation 1 with the values
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of n = 7.0 × 1018 (cm −3 ) and τ = 0.41 fs . In addition, the value of the field effect mobility,
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µ = eτ / m* , for the CdSe/GaSe interface turns out to be 5.22 ( cm 2 / Vs ). While, the computational parameters found here are close to those we have reported for InSe/CdSe interface [9], the importance of the current study appears in the conduction and valence band offsets. Although the mobility values are suitable for the thin film transistor production technology, efforts must be spent
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to lower the free carrier density in order to prevent high current generations and to improve the free carrier mobility at the interface. We believe that the reduction of the free carrier density that is mainly generated from the polycrystalline CdSe could cause a backward tunneling current through
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the device [6].
4. Conclusions
In this study we have discussed the features of CdSe/GaSe as a new promising design being suitable for optoelectronic device applications. The physical parameters which were evaluated by means of the structural and optical analysis techniques have shown that, the interface could exhibit better crystalline nature when grown onto aluminum substrate and could also exhibit a conduction and valence band offsets that can be used for the design of thin film transistors (TFT). While the
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dielectric spectra indicated two resonance frequencies at 530 and 445 THz, the optical conductivity computational fittings displayed parameters that are comparable with those used in TFT technology.
Acknowledgement:
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This work was funded by the Deanship of Scientific Research at the University of Dammam in Saudi Arabia under project number 2015228.
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References
[1] F. S. Stinner, Y. Lai, D. B. Straus, B. T. Diroll, D. K. Kim, C. B. Murray, C. R. Kagan, Flexible
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high-speed CdSe nanocrystal integrated circuits, Nano Lett. 15 (2015) 7155–7160. [2] O. I. Olusola, O. K. Echendu, I. M. Dharmadasa, Development of CdSe thin films for application in electronic devices, J. Mater. Sci. - Mater. Electron. 26 (2015) 1066-1076. [3] J. Kobak, T. Smoleński, M. Goryca, M. Papaj, K. Gietka, A. Bogucki, M. Koperski, J. G. Rousset, J. Suffczyński, E. Janik, M. Nawrocki, A. Golnik, P. Kossacki, W. Pacuski, Designing quantum dots for solotronics, Nat. Commun. 5 (2014) 1-8.
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[4] L. Karvonen, A. Säynätjoki, S. Mehravar, R. D. Rodriguez, S. Hartmann, D. R. T. Zahn, S. Honkanen, R. A. Norwood, N. Peyghambarian, K. Kieu, H. Lipsanen, J. Riikonen, Investigation of second- and third-harmonic generation in few-layer gallium selenide by multiphoton
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microscopy, Sci. Rep. 5 (2015) 1-8.
[5] S. Schwarz, S. Dufferwiel, P. M. Walker, F. Withers, A. A. P. Trichet, M. Sich, F. Li, E. A.
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Chekhovich, D. N. Borisenko, N. N. Kolesnikov, K. S. Novoselov, M. S. Skolnick, J. M. Smith, D. N. Krizhanovskii, A. I. Tartakovskii, Two-dimensional metal–chalcogenide films in tunable optical microcavities, Nano Lett. 14 (2014) 7003–7008. [6] R. R. N. Kmail, A. F. Qasrawi, Physical design and dynamical analysis of resonant– antiresonant Ag/MgO/GaSe/Al optoelectronic microwave devices, J. Electron. Mater. 44 (2015) 4191-4198. [7] D. J. Late, B. Liu, J. Luo, A. Yan, H. S. S. R. Matte, M. Grayson, C. N. R. Rao, V. P. Dravid, GaS and GaSe ultrathin layer transistors, Adv. Mater. 24 (2012) 3549–3554. [8] X. Li, M. W. Lin, A. A. Puretzky, J. C. Idrobo, C. Ma, M. Chi, M. Yoon, C. M. Rouleau, I. I. Kravchenko, D. B. Geohegan, K. Xiao, Controlled vapor phase growth of single crystalline twodimensional GaSe crystals with high photoresponse, Sci. Rep. 4 (2014) 1-9. 10
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[9] A. F. Qasrawi, S. Rabaa, Optical interactions in the InSe/CdSe interface, Phys. Stat. Sol. (b), in press DOI 10.1002/pssb.201552726. [10] M. Thirumoorthia, J. Thomas Joseph Prakash, Effect of F doping on physical properties of (211) oriented SnO2 thin films prepared by jet nebulizer spray pyrolysis technique, Superlattices and Microstructures, 89 (2016) 378–389
interfacial charge-transfer transition, Sci. 349 (2015) 632-635.
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[11] K. Wu, J. Chen, J. R. McBride,T. Lian, Efficient hot-electron transfer by a plasmon-induced
[12] S. Lei, L. Ge, Z. Liu, S. Najmaei, G. Shi, G. You, J. Lou, R. Vajtai, P. M. Ajayan, Synthesis
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and photoresponse of large GaSe atomic layers, Nano Lett. 13 (2013) 2777−2781.
[13] D. U. Kim, C. M. Hangarter, R. Debnath, J. Y. Ha, C. R. Beauchamp, M. D. Widstrom, J. E. Guyer, N. Nguyen, B. Y. Yoo, D. Josell, Backcontact CdSe/CdTe windowless solar cells, Sol.
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Energy Mater. Sol. Cells 109 (2013) 246-253.
[14] Q. A. Acton, Ions-Advances in research and application, GA: Scholarly Editions, Atlanta, 2012.
[15] A. V. Antanovich, A. V. Prudnikau, D. Melnikau, Y. P. Rakovich, A. Chuvilin, U. Woggon, A. W. Achtstein, M. V. Artemyev, Colloidal synthesis and optical properties of type-II CdSe–CdTe
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and inverted CdTe–CdSe core–wing heteronanoplatelets, Nanoscale 7 (2015) 8084-8092. [16] D. Mourad, J. P. Richters, L. Gérard, R. André, J. Bleuse, H. Mariette, Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: a combined experimental and theoretical approach, Phys. Rev. B. 86 (2012) 195308.
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[17] V. Devi, M. Kumar, R. J. Choudhary, D. M. Phase, R. Kumar, B. C. Joshi, Band offset studies in pulse laser deposited Zn12xCdxO/ZnO hetero-junctions, J. Appl. Phys. 117 (2015) 225305.
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[18] M. Dresselhaus, Optical properties of solids, American Scientific Publishers, New York, 1980. [19] Y. Sasaki, C. Hamaguchi, J. Nakai, Electroabsorption and electroluminescence of GaSe, Jap. J. App. Phys. 14 (4) (1975).
List of Figure captions
Fig. 1. (a) the XRD patterns for the Al/CdSe/GaSe and glass/CdSe/GaSe substrates, (b) The enlargement of the main peak showing the broadening in the peaks, (c) the transmittance and the
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reflectance and (d) the absorbance spectra of the CdSe/GaSe interface. The inset of (d) show the variation of reflectance with angle of incidence.
Fig. 2. the (αE ) 2 − E variation for the CdSe/GaSe interface. The inset shows the low region of absorption.
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Fig. 3. (a) the real and (b) the imaginary parts of the dielectric constant spectra. (c) Represents the optical conductivity of the CdSe/GaSe interface. The inset of (a) displays the energy band diagram
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for the interface.
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- The CdSe/GaSe interface is prepared by the physical vapor deposition technique - The Structural properties are investigated by the X-ray diffraction technique - The optical transmittance, reflectance, absorbance and conductance are explored - The conduction and valence band offsets are determined - The dielectric spectra is studied. - The optical conductivity parameters are computed by Lorentz approach
S1567-1739(16)30081-5
DOI:
10.1016/j.cap.2016.04.010
Reference:
CAP 4202
To appear in:
Current Applied Physics
Received Date: 30 January 2016 Revised Date:
14 April 2016
Accepted Date: 14 April 2016
Please cite this article as: T.S. Kayed, A.F. Qasrawi, K.A. Elsayed, Band offsets and optical conduction in the CdSe/GaSe interface, Current Applied Physics (2016), doi: 10.1016/j.cap.2016.04.010. 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.
ACCEPTED MANUSCRIPT
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Band offsets and optical conduction in the CdSe/GaSe interface
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T. S. Kayed 1, A. F. Qasrawi 2, 3,*, Khaled A. Elsayed 1 1 Department of Basic Sciences and Humanities, College of Engineering, University of Dammam, Dammam, Saudi Arabia 2 Department of Physics, Arab-American University, Jenin, West Bank, Palestinian Authority 3 Group of Physics, Faculty of Engineering, Atilim University, 06836 Ankara, Turkey Abstract
In this work, the design and characterization of CdSe/GaSe heterojunction is considered. The
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CdSe/GaSe thin film interface was prepared by the physical vapor deposition technique. Systematic structural and optical analysis were performed to explore the crystalline nature, the optical band
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gaps, the conduction and valence band offsets, the dielectric spectra, and the frequency dependent optical conductivity at terahertz frequencies. The X-ray diffraction analysis revealed a polycrystalline interface that is mostly dominated by the hexagonal CdSe oriented in the (002) direction. It was also found that the CdSe/GaSe interface exhibits conduction and valence band
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offsets of 1.35 and 1.23/1.14 eV, respectively. The dielectric spectra displayed two dielectric resonance peaks at 530 and 445 THz.
Moreover, the computational fittings of the optical
conductivity of the interface revealed a free carrier scattering time of 0.41 (fs) for a free carrier
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density of 7.0 × 1018 (cm −3 ) . The field effect mobility for the CdSe/GaSe interface was found to be
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5.22 ( cm 2 / Vs ). The remarkable features of this device having large band offsets and qualitative optical conduction dominated by a scattering time in the order of femtoseconds in addition to the dielectric property nominate the device to be used in optoelectronic technology.
Keywords: Optical materials; Coating; Dielectric properties; Optical properties ----------------------------------------------------------------------------------------------------------------------*
Corresponding author: Tel.: +970-599379412, Fax: +97042510810/817. Email: [email protected], [email protected]
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1. Introduction
Due to its large photosensitivity and wide optoelectronic applicability, CdSe based optoelectronic devices have attracted the attention of the research and quality control centers. Recently, a largearea, flexible, high-speed analog and digital colloidal CdSe nanocrystal integrated circuits operating
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at low voltages were reported. The circuits involved amplifiers with 7 kHz bandwidth, ring oscillators with ~ 10 µs stage delays, and NAND and NOR logic gates [1]. In addition to that, CdSe films deposited on glass substrates coated with fluorine-doped tin oxide (FTO), exhibited an electronic quality of the FTO/n-CdSe/Au Schottky diodes. The films had rectification factor of
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102.9, reverse saturation current of ~ 372 nA, and threshold voltage of ~ 0.15 V. The potential
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barrier observed for Au/n-CdSe interface was found to be ~ 1.10 eV [2]. Moreover, an optimal magnetic-ion quantum dot system for implementation in a single-ion-based spin memory is identified [3].
Another promising Se based compound that is known to have some novel optoelectronic properties is the GaSe. GaSe thin films are reported to exhibit nonlinear optical properties. Studies on these
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films using the multi-photon microscopy, revealed both second- and third-harmonic generation [4]. In addition to that, tunable micro-cavities with few monolayers of GaSe films have been
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observed to reflect a significant modification of the spectral and temporal properties of photoluminescence (PL) spectra. The spectra are emitted in narrow and wavelength-tunable cavity
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modes with quality factors up to 7400. In that process a 10-fold PL lifetime shortening is achieved [5]. Moreover, the optical analysis of the MgO/GaSe heterojunction [6] have also displayed an enhanced absorbing ability of the GaSe below 2.90 eV with an energy band gap shift from 2.10 eV for the GaSe substrate to 1.90 eV for the GaSe/MgO heterojunction. This interface behaves like a tunneling-type device with a depletion region width of 670 nm and 116 nm when forward and reverse biased, respectively. The GaSe/MgO heterojunction exhibited resonance–antiresonance behavior and negative capacitance characteristics near 1.0 GHz. The features of this device appear to be suitable for the use as a band reject filter in visible light optoelectronics and in microwave
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device sectors [6]. In another work, layers of GaSe are used for the fabrication of a thin film field effect transistor (FET). The FET displayed a typical n-and p-type conductance transistor with good ON/OFF ratio and electron high differential mobility [7]. It is also reported that the 2D GaSe crystals exhibit characteristics that demonstrated the potential of the GaSe usage in electronic and
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optoelectronic applications [8]. In light of the above reported information we were motivated to fabricate a double layer thin film FET that joins these two smart materials together to reveal wider range of applications in
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optoelectronics. Particularly, in this work we will focus on the structural and optical properties of CdSe/GaSe interface as a thin film transistor suitable for visible light communications and/or
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terahertz sensing. The FETs deposited onto glass substrates will be characterized by means of ultraviolet-visible light spectrophotometric analysis to discover the valence and conduction band offsets, the dielectric spectra, the optical conductivity, and the FET electrical parameters through
2. Experimental Details
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optical conductivity modeling.
CdSe and GaSe thin films were evaporated onto glass substrates from the source materials which
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are CdSe (Alfa Aeser 1306-24-7 with maximum obtainable purity of 99.999%) metal basis and Ga2Se3 (Alfa Aeser 12024-24-7 with maximum obtainable purity of 99.99%) single crystals. The
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evaporation was achieved in a VCM 600 physical vapor deposition system at vacuum pressure of 10-5 mbar. A thickness monitor attached to the system controlled the thicknesses of the films. The hexagonal nature of crystallization of the CdSe films and the amorphous nature of the GaSe films were reported in our previous works [6,9]. The X-ray diffraction patterns for the CdSe/GaSe interface being deposited onto Al film and onto glass substrate (geometrical design is shown in Fig. 1 (a) ) were recorded with the help of Ultima IV type III X-ray diffractometer. The optical transmittance and reflectance were recorded using Evolution 300 spectrophotometer equipped with pike VEE MAX II variable angle reflectance accessory. 3
ACCEPTED MANUSCRIPT 3. Results and Discussion Based on our previous studies of InSe/CdSe heterojunction [9], some very interesting characteristics were observed for the interface and were promising for the use in thin film field effect transistor
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technology [9]. In this work we are aiming to explore and discuss the properties of the CdSe/GaSe interface as well. It was previously mentioned that the CdSe layer exhibits p-type of conduction [9] whereas the GaSe layer exhibits n-type of conduction [6]. The thickness of each of the CdSe and
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GaSe films was 1.2 µm whereas the thickness ( d ) of the CdSe/GaSe interface was 2.4 µm . To reveal detailed information about the crystalline nature of the CdSe/GaSe interface, we have
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recorded the X-ray diffraction patterns for CdSe/GaSe interface for films deposited onto glass and compared them to those deposited onto Al substrates. The X-ray diffraction patterns which are displayed in Fig. 1 (a) reveal patterns that are indexed in accordance with the PDF card numbers of 01-070-2554 and 00-008-0459. The cards refer the main peak to the hexagonal CdSe being best
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oriented in the (002) direction. The corresponding lattice constants are a = 4.2985 A° and c = 7.01520 Ao . Most of the other minor peaks are also assigned to CdSe polycrystals. The test of the possibility of finding polycrystalline GaSe was also tested and only two peaks located at 23.06 o and
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at 41.18o could be found in the pdf cards of GaSe. On the other hand, the figure also display more sharp intensive peak for the Al/CdSe/GaSe than for the glass/CdSe/GaSe films. The most intensive
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peak which is enlarged in Fig. 1 (b) also reflects larger full wave half length broadening ( β ). The calculated grain sizes ( D ) in accordance with Scherrer equation ( D = 0.94λ /( β cos(θ )) ) and the degree of orientation ( f = (( I max / ∑ I hkl ) − I phkl ) /(1 −I phkl ) ) [10] are found to be 26 nm and 0.71 and 23 nm and 0.76, for CdSe/GaSe interface grown onto glass and Al substrates, respectively. Thus, in accordance with these numerical data, the appearance of the more sharp intensive peak for the Al/CdSe/GaSe than that for the glass/CdSe/GaSe films can be ascribed to the improvement of the crystalinity in the films when grown onto Al substrate. The crystalline nature improves as a
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result of the lattice matching between the hexagonal closed pack unit cell of Al and the hexagonal CdSe polycrystal. At the Al/CdSe interface, the hexagonal structure of alternating layers of CdSe is shifted so its atoms are aligned to the gaps of the Al as a preceding layer. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer leading to a higher
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orientation values. Fig. 1 (c) displays the transmittance (T) spectrum for the CdSe/GaSe interface being recorded in the incident light wavelength range of 300-1100 nm. This spectrum exhibits an increasing trend of
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variation in the region of 462-624 nm. While in the region of 625-680 nm, the T spectrum is λ invariant, in the region of 625-800 nm, it sharply re-increases when λ increases. For larger λ
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values the T spectrum continues decreasing and tends to remain constant above 1000 nm. On the other hand, the reflectance ( R ) spectrum which is also shown in Fig. 1 (c) appears to increase with increasing incident light wavelength up 560 nm where it reaches a local maximum. Similarly, another broadened peak appears at 670 nm. In the wavelength range of 820-1100 nm, the
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reflectance steadily increases with increasing wavelength. Apart from the reflectivity which was recorded at normal incidence that is displayed in Fig.1 (c), the effect of the angle of incidence of light ( α ) on the interface reflectivity is shown in the inset of Fig. 1 (d). As the inset shows, at a
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particular wavelength of 600 nm, increasing the angle of incidence of light increases the total reflectivity. The maximum reflectivity is obtained for an angle of incidence of 65o. For higher α
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values, R % sharply falls reaching a value of 2.9% at 80o. The curve which is displayed in the inset represent an application guidance for optoelectronic applications. The holistic picture of the optical spectra becomes more clear when the absorbance ( A ) spectrum is considered. The absorbance spectrum that was calculated from the relation A% = 100 − T − R (R of normal incidence) is illustrated in Fig. 1 (d). The absorbance of the CdSe/GaSe interface in the incident photon energy range of 4.0-2.6 eV (corresponding to 310-478 nm) is almost steady and has very high (95%) value. When the incident photon energy values fall below 2.6 eV, the absorbance sharply falls reaching a value of 40% at 1.8 eV (corresponding to 690 nm). The relation A% = 78.97 E − 107.80 can present
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the changes of the absorbance with the incident photon energy. The slope of the A − E variation ( A% = 144.28 E − 218.24 ) becomes more pronounced down to 1.5 eV (corresponding to 828 nm). Below 1.5 eV, the absorbance spectrum increases with decreasing incident light energy following the linear relation, A = −76.96 E + 126.97 , and it then exhibits a maximum at 1.3 eV (corresponding
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to 996 nm). The variations in the absorbance of the CdSe/GaSe interface are comparable with those of the InSe/CdSe interface [9]. However, the absorption peak that is detected at 1.3 eV for the CdSe/GaSe was not previously observed in the InSe/CdSe absorbance spectrum. Nor it was
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observed in the MgO/GaSe absorbance spectrum [6]. The increase in the absorbance values with decreasing light energy in the low energy region of 1.70-1.1 eV was also seen in the GaSe
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absorbance spectrum [6]. This absorption peak that is seen at 1.3 eV could be ascribed to the interband transitions in the GaSe as well as in the CdSe. While the CdSe is reported to exhibit inter-band transitions due to the instantaneous generation of electrons during the decay of the Plasmon in short separation times [11], the inter-band transitions in GaSe below 2.0 eV is ascribed to the electronic transitions from the p -like to the s -like orbits [12]. The top of the valence band in the GaSe is
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located in the pz − like orbit that originates from Se atoms. It is also mentioned that the p x and p y like orbits reserve deep locations in the valence band of the GaSe. The electronic transitions from
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these two orbits to the pz reveal an energy value of 3.2 eV [12]. The latter value explains the reasons that are behind the existence of high absorbance above 3.0 eV in GaSe as appears in Fig. 1
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(c) of Ref. 6.
To investigate the change in the energy band gap of CdSe as a result of the CdSe/GaSe interfacing, the absorption coefficient α = A / d was calculated. The (αE ) 2 − E plots that are presented in Fig. 2 and its inset indicate the domination of the direct allowed transitions type of energy band gap in the high and low absorption regions, respectively. The E − axis intercept of the dashed lines that appear in the figure and its inset, relate to the energy band gaps ( E g ) of 1.97 and 1.64 eV being dominant in the incident photon range of 2.08-2.50 eV and 1.65-1.90 eV, respectively. The energy
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band gap of the CdSe is 1.85 eV [9] and that of GaSe is 2.10 eV [6]. The energy band gap differences between that of CdSe and those of CdSe/GaSe interface are ∆Eg1=0.12 eV and
∆Eg2 =0.21 eV, respectively. Since the electron affinity ( qχ ) of CdSe is 4.58 eV [9, 13] and that of GaSe is 3.23 eV [6, 14], then the conduction band offset (∆Ec = (q χ CdSe − q χ GaSe ) = 1.35 eV. This
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leads to a respective valence band offset of ∆Ev1 = ∆Eg1 − ∆Ec =1.23 eV and ∆Ev2 of 1.14 eV. The values of the conduction and valence band offsets are relatively high when compared to those of InSe/CdSe interface [9] as 0.03 and 0.36 eV and of CdSe/CdTe interface [15] as 0.54 and 0.28 eV,
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respectively. The values of ∆Ev reported here are also higher than those reported as 0.75 eV for the
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CdSe/ZnTe heterojunction [16]. The difference between the theoretically calculated literature data and the currently reported is ascribed to the low crystal symmetry (as detected by experiments compared to theoretical assumptions), and/or significantly larger lattice mismatches, possible dipole corrections and orientation dependencies may have to be considered on the theoretical side. On the experimental side, such conditions could lead to a stronger confinement of the charge carriers at the
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interfaces. This confinement make the excitons effect more apparent. This means that the employed theoretical model should had been justified to correctly fit up with experimental data [16]. The
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values of the conduction and valence band offsets are also presented by the energy band diagram as an inset in Fig. 3 (a). The presented offset values for the CdSe/GaSe interface are promising as they
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guarantee the success of this interface when used in thin film transistors, optoelectronic devices, and as quantum dots [9, 15-17].
Fig. 3 (a) and (b) displays the real ( ε r ) and imaginary ( ε i ) parts of the dielectric constant spectra for the CdSe/GaSe interface being calculated using the previously reported method [9] from the reflectance spectrum that appeared in Fig. 1 (c). As Fig. 3 (a) shows, except for the minor and major resonance peaks that appear at 530 and 445 THz, the real dielectric constant increases with decreasing incident light wave frequency. The ε r − F inverse variation becomes more pronounced below 365 THz. The relation ε r = − 0.219 F + 81.82 can describe the behavior of the dielectric 7
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constant. The resonance peaks, which appeared at 530 and 445 THz, correspond to oscillator energy values of 2.20 and 1.85 eV, respectively. While the latter number coincides with the energy band gap of CdSe film, the earlier is close to that of GaSe film. In addition, the shoulder which appeared at 416.6 THz near the major resonance peak (445 THz) correspond to an energy value of 1.71 eV is On the other hand, the ε i
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probably due to the extended tail states that arises from the interfacing.
spectra that are presented in Fig. 3 (b) exhibit values that are two orders of magnitude lower than that of ε r indicating that the quality factor of the electromagnetic resonance at the interface is large
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enough to employ the CdSe/GaSe interface in optoelectronic devices technology. As an example, the quality factor values for the interface at 530, 445 and 300 THz are 115, 175 and 300,
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respectively.
The importance of the dielectric spectra from the point of view of applications in thin film transistor technology and other related devices is better presented by the optical conductivity and its relative parameters. The optical conductivity calculated from the equation, σ (w = 2π F) = ε i .w / (4π ) [9], is
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presented in Fig. 3 (c). The figure reflects a very slow varying trend of optical conductivity with incident light frequency in the range of 1000-530 THz. Below 530 THz; the conductivity sharply decreases with decreasing frequency. Here, the tendency of σ ( w) to remain constant arises from the
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inability of the electric dipoles at the interface to follow the light signal above 530 THz. Contrarily, below 530 THz, the optical conductivity sharply falls by one order of magnitude as the frequency
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reaches 360 THz indicating the enhanced polarizeability of the electron hole pairs at the p-n junction interface with the incident light signal [18]. The modeling of the optical conductivity, σ ( w) , in accordance with the Drude model, which takes into account the negative contribution of the free carriers (n) to the optical conductivity, starts from the frequency (w) dependent optical conductivity which is given by,
σ ( w) =
ne 2τ m * (1 − iwτ )
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(1)
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In the above equation, m * = 0.138 mo is the reduced carrier effective mass of the interface being calculated from the values of the effective mass of CdSe and of GaSe that are reported as 0.45mo [9] and as 0.20mo [19], respectively. Here, n is the number of free carriers, e is the electronic charge, i stands for complex number presentation and τ is the carrier scattering time or relaxation
negative and represents the free carrier contribution [18].
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time. According to the above equation, the real part of the frequency dependent conductivity is
As seen from Fig. 3 (c), the optical conductivity data are reproduced by equation 1 with the values
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of n = 7.0 × 1018 (cm −3 ) and τ = 0.41 fs . In addition, the value of the field effect mobility,
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µ = eτ / m* , for the CdSe/GaSe interface turns out to be 5.22 ( cm 2 / Vs ). While, the computational parameters found here are close to those we have reported for InSe/CdSe interface [9], the importance of the current study appears in the conduction and valence band offsets. Although the mobility values are suitable for the thin film transistor production technology, efforts must be spent
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to lower the free carrier density in order to prevent high current generations and to improve the free carrier mobility at the interface. We believe that the reduction of the free carrier density that is mainly generated from the polycrystalline CdSe could cause a backward tunneling current through
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the device [6].
4. Conclusions
In this study we have discussed the features of CdSe/GaSe as a new promising design being suitable for optoelectronic device applications. The physical parameters which were evaluated by means of the structural and optical analysis techniques have shown that, the interface could exhibit better crystalline nature when grown onto aluminum substrate and could also exhibit a conduction and valence band offsets that can be used for the design of thin film transistors (TFT). While the
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dielectric spectra indicated two resonance frequencies at 530 and 445 THz, the optical conductivity computational fittings displayed parameters that are comparable with those used in TFT technology.
Acknowledgement:
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This work was funded by the Deanship of Scientific Research at the University of Dammam in Saudi Arabia under project number 2015228.
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References
[1] F. S. Stinner, Y. Lai, D. B. Straus, B. T. Diroll, D. K. Kim, C. B. Murray, C. R. Kagan, Flexible
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high-speed CdSe nanocrystal integrated circuits, Nano Lett. 15 (2015) 7155–7160. [2] O. I. Olusola, O. K. Echendu, I. M. Dharmadasa, Development of CdSe thin films for application in electronic devices, J. Mater. Sci. - Mater. Electron. 26 (2015) 1066-1076. [3] J. Kobak, T. Smoleński, M. Goryca, M. Papaj, K. Gietka, A. Bogucki, M. Koperski, J. G. Rousset, J. Suffczyński, E. Janik, M. Nawrocki, A. Golnik, P. Kossacki, W. Pacuski, Designing quantum dots for solotronics, Nat. Commun. 5 (2014) 1-8.
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[4] L. Karvonen, A. Säynätjoki, S. Mehravar, R. D. Rodriguez, S. Hartmann, D. R. T. Zahn, S. Honkanen, R. A. Norwood, N. Peyghambarian, K. Kieu, H. Lipsanen, J. Riikonen, Investigation of second- and third-harmonic generation in few-layer gallium selenide by multiphoton
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microscopy, Sci. Rep. 5 (2015) 1-8.
[5] S. Schwarz, S. Dufferwiel, P. M. Walker, F. Withers, A. A. P. Trichet, M. Sich, F. Li, E. A.
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Chekhovich, D. N. Borisenko, N. N. Kolesnikov, K. S. Novoselov, M. S. Skolnick, J. M. Smith, D. N. Krizhanovskii, A. I. Tartakovskii, Two-dimensional metal–chalcogenide films in tunable optical microcavities, Nano Lett. 14 (2014) 7003–7008. [6] R. R. N. Kmail, A. F. Qasrawi, Physical design and dynamical analysis of resonant– antiresonant Ag/MgO/GaSe/Al optoelectronic microwave devices, J. Electron. Mater. 44 (2015) 4191-4198. [7] D. J. Late, B. Liu, J. Luo, A. Yan, H. S. S. R. Matte, M. Grayson, C. N. R. Rao, V. P. Dravid, GaS and GaSe ultrathin layer transistors, Adv. Mater. 24 (2012) 3549–3554. [8] X. Li, M. W. Lin, A. A. Puretzky, J. C. Idrobo, C. Ma, M. Chi, M. Yoon, C. M. Rouleau, I. I. Kravchenko, D. B. Geohegan, K. Xiao, Controlled vapor phase growth of single crystalline twodimensional GaSe crystals with high photoresponse, Sci. Rep. 4 (2014) 1-9. 10
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[9] A. F. Qasrawi, S. Rabaa, Optical interactions in the InSe/CdSe interface, Phys. Stat. Sol. (b), in press DOI 10.1002/pssb.201552726. [10] M. Thirumoorthia, J. Thomas Joseph Prakash, Effect of F doping on physical properties of (211) oriented SnO2 thin films prepared by jet nebulizer spray pyrolysis technique, Superlattices and Microstructures, 89 (2016) 378–389
interfacial charge-transfer transition, Sci. 349 (2015) 632-635.
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[11] K. Wu, J. Chen, J. R. McBride,T. Lian, Efficient hot-electron transfer by a plasmon-induced
[12] S. Lei, L. Ge, Z. Liu, S. Najmaei, G. Shi, G. You, J. Lou, R. Vajtai, P. M. Ajayan, Synthesis
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and photoresponse of large GaSe atomic layers, Nano Lett. 13 (2013) 2777−2781.
[13] D. U. Kim, C. M. Hangarter, R. Debnath, J. Y. Ha, C. R. Beauchamp, M. D. Widstrom, J. E. Guyer, N. Nguyen, B. Y. Yoo, D. Josell, Backcontact CdSe/CdTe windowless solar cells, Sol.
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Energy Mater. Sol. Cells 109 (2013) 246-253.
[14] Q. A. Acton, Ions-Advances in research and application, GA: Scholarly Editions, Atlanta, 2012.
[15] A. V. Antanovich, A. V. Prudnikau, D. Melnikau, Y. P. Rakovich, A. Chuvilin, U. Woggon, A. W. Achtstein, M. V. Artemyev, Colloidal synthesis and optical properties of type-II CdSe–CdTe
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and inverted CdTe–CdSe core–wing heteronanoplatelets, Nanoscale 7 (2015) 8084-8092. [16] D. Mourad, J. P. Richters, L. Gérard, R. André, J. Bleuse, H. Mariette, Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: a combined experimental and theoretical approach, Phys. Rev. B. 86 (2012) 195308.
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[17] V. Devi, M. Kumar, R. J. Choudhary, D. M. Phase, R. Kumar, B. C. Joshi, Band offset studies in pulse laser deposited Zn12xCdxO/ZnO hetero-junctions, J. Appl. Phys. 117 (2015) 225305.
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[18] M. Dresselhaus, Optical properties of solids, American Scientific Publishers, New York, 1980. [19] Y. Sasaki, C. Hamaguchi, J. Nakai, Electroabsorption and electroluminescence of GaSe, Jap. J. App. Phys. 14 (4) (1975).
List of Figure captions
Fig. 1. (a) the XRD patterns for the Al/CdSe/GaSe and glass/CdSe/GaSe substrates, (b) The enlargement of the main peak showing the broadening in the peaks, (c) the transmittance and the
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reflectance and (d) the absorbance spectra of the CdSe/GaSe interface. The inset of (d) show the variation of reflectance with angle of incidence.
Fig. 2. the (αE ) 2 − E variation for the CdSe/GaSe interface. The inset shows the low region of absorption.
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Fig. 3. (a) the real and (b) the imaginary parts of the dielectric constant spectra. (c) Represents the optical conductivity of the CdSe/GaSe interface. The inset of (a) displays the energy band diagram
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for the interface.
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- The CdSe/GaSe interface is prepared by the physical vapor deposition technique - The Structural properties are investigated by the X-ray diffraction technique - The optical transmittance, reflectance, absorbance and conductance are explored - The conduction and valence band offsets are determined - The dielectric spectra is studied. - The optical conductivity parameters are computed by Lorentz approach