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p-Si heterojunction diodes
Accepted Manuscript Colloidal synthesis and electrical behaviour of n-ZnGdO/p-Si heterojunction diodes Siva Chidambaram, Ganga Gnanasekaran, G. Mohan Kumar, Baraneedharan Pari, Karthikeyan Balasubramanian, Sivakumar Muthusamy PII: DOI: Reference:
S0021-9797(15)00382-3 http://dx.doi.org/10.1016/j.jcis.2015.04.018 YJCIS 20392
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
11 January 2015 10 April 2015
Please cite this article as: S. Chidambaram, G. Gnanasekaran, G. Mohan Kumar, B. Pari, K. Balasubramanian, S. Muthusamy, Colloidal synthesis and electrical behaviour of n-ZnGdO/p-Si heterojunction diodes, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.04.018
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Colloidal synthesis and electrical behaviour of n-ZnGdO/p-Si heterojunction diodes Siva Chidambarama, Ganga Gnanasekarana, G. Mohan Kumarb, Baraneedharan Paria, Karthikeyan Balasubramanianc*, Sivakumar Muthusamy*a,d a
Division of Nanoscience and Technology, dDepartment of Chemistry, Anna University - BIT Campus, Thiruchirappalli-620024, India. b
Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul, Republic of Korea.
c
Department of Physics, National Institute of Technology, Thiruchirappalli-620015 *Corresponding authors: Tel: +91-43-240-7959 Email: [email protected]; [email protected]
Abstract: Studies on manoeuvring the optoelectronic characteristics of a semiconducting nanostructure are of recent specific interest for a wide range of photonic applications. In this regard, the optical and electrical characteristics of ZnO nanostructures have been tuned and studied systematically using Gd ions. The structural and morphological characteristics of the solution processed ZnGdO nanostructures were studied in detail using the results of X-ray diffraction and microscopic measurements. The absorption band edge in ZnO was noted to shift towards the lower wavelength values on Gd substitution, suggesting an increase in its energy band gap. The blue emissivity from ZnO complexes was also noted to improve as a function of Gd composition in ZnO. The potential of ZnGdO nanostructures for optoelectronic functions was evaluated by fabricating heterojunction diodes based on n-ZnGdO/p-Si. The diode characteristics revealed an improved electrical conductivity and rectifying behavior from the fabricated architectures upon Gd substitution and photon illumination. The findings are correlated with the increased charge carrier concentration and defect states existing within ZnGdO species, through appropriate mechanisms. Keywords: ZnGdO; p-Si; heterojunction; optical band gap; nanostructures.
1
Introduction: Oxide nanostructures are nowadays being targeted as viable candidates for potential applications in transparent electrodes, luminescent devices, sensors, photovoltaics, etc. The primary interest beneath such systems is usually attributed to their unique physicochemical and optoelectronic characteristics [1, 2]. Zinc oxide (ZnO) is one of such compound semiconductor that is being extensively investigated for photonics related applications. The most prominent ways to fabricate ZnO based systems include vapour deposition, laser ablation, solid state reactions and solution synthesis primarily [3-10]. These diversified material processing methodologies are already proven to serve as efficient gateways for the fabrication of tailored ZnO nanostructures. The methodologies also favour the authentic tuning of their material characteristics, as a function of their particle/grain size. Till date, ZnO has been realized in form of zero-dimensional quantum dots, one-dimensional nanowires, two-dimensional sheets and three-dimensional bulk nanocrystallites [11-13]. For electronic and optoelectronic applications, controlling the particle size is said to directly influence its electronic energy levels [14]. Likewise, substitution of foreign elements into a host lattice is considered as a promising option to induce new functions in the corresponding material system [15]. In case of ZnO, such studies were found to report their structural, optical, magnetic and electrical characteristics to be greatly influenced [16-18]. Recently, rare earth (RE) materials are being looked upon as promising substituent elements that can activate and enrich the optoelectronic functions in ZnO for possible applications in displays and other optoelectronics devices [19-21]. The rich energy levels available in the trivalent RE materials and their temperature-independent luminescence (visible and near-infrared region) are of added importance [22-24]. So, from such intriguing aspects a significant attempt has been made in the present case to investigate the influence of Gd ions on the optical and electrical behaviour of ZnO by means of chemical doping. The significance of the adopted experimental methodology favours the large-scale production of nanomaterials with uniform size at low growth temperatures. This strategy also ensures the feasibility to control the stoichiometric composition of precursors and maintain a high homogeneity at the molecular level, for the efficient substitution of Gd3+ ions in the wurtzite matrix and the realization of customized nanostructures. So, by employing a simple strategy, the meaningful role of Gd ions in ZnO has 2
been studied systematically. In this regard, the structural and optical characteristics of the processed nanomaterials were also studied in detail to personify the importance of Gd substitution in ZnO. Likewise, on considering the need for efficient semiconducting photodiodes in prominent photonic applications, n-ZnGdO/p-Si based heterojunction diodes were also investigated. The developed heterostructures were collectively studied for their texture, morphology and other electronic properties. Experiment: Synthesis of ZnO and ZnGdO nanostructures: Undoped ZnO nanostructures were initially prepared through precipitating an aqueous solution made up of 0.05 M zinc nitrate using 0.1 M of sodium hydroxide stock solutions. The resulting precipitates were aged for 12 h and dried overnight at 60 °C. The obtained products were further calcined at 600 °C for 3 h, prior to their material property studies. Similar procedures were involved in the preparation of ZnGdO nanostructures, by using gadolinium nitrate as the substituent source (1 and 3%). The processed materials are hereafter referred as ZnO, Zn0.99Gd0.01O and Zn0.97Gd0.03O, respectively. Fabrication of n-ZnGdO/p-Si heterostructures: Initially, 1 mg of the processed ZnO nanomaterial was dispersed in isopropanol media to prepare a colloidal dispersion (Fig. S1, ESI). The dispersion was used to establish the n-region of the photodiodes on Si wafers; through a conventional spray technique (Fig. S2, ESI). The native oxides on p-type Si wafers were earlier removed using HF solution (2%) and the wafers were also cleansed by ultrasonic treatment using trichloroethylene-acetone and isopropyl acetate. During spray deposition, the wafers were held at 120 °C and the nozzle to wafer distance was around 3-4 cm. The solution flow rate was held at 0.5 ml/min and compressed air (5 Pa) was used as the carrier gas in all the experiments. The contacts for the fabricated diodes were established using silver paste. A similar procedure was followed for the construction of ZnGdO/Si based heterostructures. The fabricated diodes namely ZnO/p-Si, Zn0.99Gd0.01O/p-Si and Zn0.97Gd0.03O/p-Si are here after referred as D1, D2 and D3, respectively. Characterization: The structural properties of the chemically processed samples were investigated using an XPERT-Pro X-ray diffractometer. The absorbance and emission spectrum 3
were obtained at room temperature using JASCO 7800 and Flouromax (HORIBA JobinYvon) type spectrophotometers. The morphologies of ZnO and ZnGdO specimens were examined using a Shimadzu SFT 3500 atomic force microscope and a JEOL TEM-2100F transmission electron microscope. The diode characteristics were obtained using Keithley-6517B semiconductor parameter analyzer. The photo response studies were carried out at A.M 1.5 conditions. Results and discussion: The mechanism behind the formation of ZnO and ZnGdO systems is summarized through the equations given below. The interaction between zinc and hydroxyl ions present in the solvent might have resulted with the formation of ZnO micelles, which later evolves as a corresponding nanostructure. Similarly, in reactions involving gadolinium nitrate, the Gd ions collectively interacts with the zinc and hydroxyl ions present in the solvent to evolve as a ZnGdO complex. Also, such interactions ensure the potential capability of Gd ions in getting well substituted within the evolving ZnO micellar structures, during efficient substitution reactions. Zn(NO3)2+H2O Zn2+ + 2(NO3)-+H2O
-----
(1)
Gd(NO3)2+H2O Gd3++ 2(NO3)-+H2O
-----
(2)
NaOH + H2O Na+ + OH- + H2O
-----
(3)
Zn2+ + OH- Zn(OH)2 ZnO + H2O
-----
(4)
Zn2+ + xGd3+ + yOH- ZnGdx(OH)y
-----
(5)
ZnGdx(OH)y ZnGdxO0.5y+ 0.5yH2O, y = 3x + 2
-----
(6)
The structural characteristics on solution processed nanostructures were initially studied using X-ray diffraction measurements. The diffraction patterns shown in Fig. 1 corresponds to that of undoped and ZnGdO materials. This is in consistent with the JCPDS data (089-1397) and could be correlated with the hexagonal phase of ZnO. Interestingly, the (101) peak position was noted to shift slightly towards the higher angles on increase in Gd substitution in the wurtzite system (Fig. S3, ESI). This behaviour could be reasoned for the successful substitution of Gd3+ ions into 4
ZnO matrix. Likewise, the absence of characteristics corresponding to metallic Gd or its oxide forms illustrates the phase purity in the solution processed systems. Additionally, the lattice constant values “a” and “c” were calculated for the undoped and ZnGdO materials using the relations given below, v = √3a2C/2 dhkl = [(4(h2+hk+k2)/3a2)+(l2/c2)]-1
----- (7) ----- (8)
The variation in lattice parameter values and lattice strain developed in ZnO systems on Gd substitution has been graphically illustrated in the supplementary information (Fig. S4 and S5). The volume of unit cell in undoped ZnO and ZnGdO was additionally estimated to be around 47.7, 47.3 and 47.2 Å, respectively. Here, the decreasing trend might be reasoned with the difference in ionic radii between the host and substituent elements (0.074 and 0.094 nm), correspondingly [25]. The particulate-like physical morphology of the undoped and Zn1-xGdxO nanocrystallites were examined through atomic force microscopy (AFM). Figure 2a-c shows the three-dimensional AFM images that illustrate the nanostructures to possess a uniform particle size distribution with almost identical characteristics. Here, the substitution of Gd ions into the wurtzite matrix could be noted to have a minimal influence on their morphological evolution or shape of the final materials. The diameter of Zn1-xGdxO particulates was also found to be in consistent with that of the undoped ZnO specimens. However, the density of particle clusters was noted to increase, suggesting the trace element Gd to make more nanostructures agglomerate together. The morphology and phase purity of the Zn1-xGdxO nanostructures were additionally examined using TEM analysis, whose results are shown in Fig. 2d. The electron microscopic results were found to confirm the findings of AFM analysis, where the particulates could be observed to be cluster free with uniform structure. The statistical diameter of the particles was found to be less than 20 nm. The selected area electron diffraction (SAED) pattern is shown at the inset of Fig. 2d. The d-spacing values calculated from the observable SAED pattern was also found to be in consistent with those of hexagonal ZnO structure. The pattern also illustrates the co-existence of both single-crystalline and polycrystalline nature in the specimens [26]. 5
The optical band gap (Eg) of undoped ZnO and ZnGdO nanostructures were studied using UV-vis absorbance spectra. The spectra shown in Figure 3 were actually recorded through a diffused reflectance mode, over a wavelength of 250-700 nm. Here, the characteristic absorption edge corresponding to that of typical ZnO was observed around 360-370 nm for undoped ZnO. On Gd substitution, this edge was noted to blue shift (as a function of Gd composition). The Eg of the nanostructures were evaluated by means of Kubelka-Munk theory, as shown at the inset of Fig. 3. The estimated values were noted to blue shift from 3.28 eV to 3.32 and 3.38 eV, in accordance with their corresponding band edges. Generally, Eg of a nanomaterial relies on a number of factors such as carrier concentration, grain size, lattice strain, size induced quantum confinement, orbital hybridization resulting from doping, etc. [27]. In our case, the observed blue shift in Eg values and the decreasing absorption intensities (ultraviolet region) could be correlated with the increased substitution of Gd ions into the wurtzite matrix of ZnO. Here, we believe the incorporation of Gd ions to have established new occupied electronic states in ZnO (around its Fermi level), whose origin could be rooted to the rich electronic states (4f) available within the RE entity [22-25]. And this increased charge carries could have shifted the Fermi level towards the conduction band, thereby resulting with observed shift in Eg. Generally, the variation in Eg values as a function of charge carrier concentration is expressed as, ∆Eg = (h2/8m*)(3ne/π)2/3
----- (9)
where h is Planck’s constant, m* is the effective mass and ne is the electron concentration [28]. According to this equation, the substitution of Gd3+ ions might have increased the net electron concentration in the material, thereby influencing the band gap values. The room temperature emission spectra of undoped ZnO and ZnGdO specimens are shown in Fig. 4. Here, a weak ultraviolet emission could be observed from the undoped and Zn0.99Gd0.01O systems, along with a broad blue emission in all the specimens. The corresponding emissions are commonly referred as near band edge (NBE) emissions in ZnO and are believed to result from the phenomenon of annihilation of excitation [29, 30]. They are also indirectly related with the particle size [31]. Interestingly, the intensity of blue emission in undoped ZnO was noted to get improved and broader, while the composition of Gd ions in the host matrix was raised. This behaviour could be correlated with the improved interaction of free excitons and the LO phonon 6
replicas in ZnO [32, 33]. Likewise, the incorporation of Gd ions could have also introduced new impurity levels within the host matrix. This in turn might have enriched the optical transitions between free and bound states (such as shallow donor and valence band or the corresponding donor-acceptor pairs), thereby enhancing the observed luminescence [34]. Similarly, the origin of emissions over 450-500 nm could be rooted to the electron transitions between ionized oxygen vacancies and valence band [35, 36]. A weak green emission was also observed around 560 nm in all the samples, whose corresponding intensity was noted to vary as a function of Gd composition (similar to NBE). The origin of green emission could be reasoned with several factors, as the conflictions arise due to attribution of deep levels or trapped state emissions. In our case, the occurrence of green emissions also emphasizes the existence of intrinsic defects such as oxygen vacancies (VO), zinc vacancies (VZn), zinc interstitials (Zni) or oxygen interstitials (Oi) associated within the solution processed materials [34]. Additionally, such emissions also characterize the radiative recombination taking place between photo-generated hole and electron occupying the VO or the transitions from conduction band to the deeply trapped holes [37, 38]. We would also like to emphasize that, the improved blue luminescence in ZnGdO nanomaterials could be efficiently capitalized for electronic and biological applications. Moreover, as ZnO is a biocompatible material their visible emissions can also be capitalized for bio-labelling related applications. Three heterojunction diodes (D1, D2 and D3) were fabricated to evaluate the electronic properties of Gd substituted ZnO systems (refer experimental section) and the schematic representation of a typical diode structure is shown in Fig. 5a. The room temperature I-V characteristics of the diodes were studied under dark and light (1.5 AM) conditions. From the results shown in Fig. 5b it is clear that all the devices tend to exhibit a good rectifying behaviour and a significant increment in the current values on Gd substitution. The dark current was also noted to improve considerably upon illumination. This aspect could be seen as clear evidence that illustrates the decrease in resistance values on Gd substitution in ZnO. The semi log I-V plots of the same (Fig. S6, ESI) signify an exponential increase in the current values at low voltages, substantiating the formation of depletion region across the diodes. The ideality factor (n) of the diodes D1, D2 and D3 were additionally estimated from the slope of the straight line region (forward bias) using Fig. S6. Generally, n is expressed as, 7
n = q/kT(dV/d(lnI))
----- (10)
In a typical photodiode, the tunnelling current and charge carrier generation-recombination current is said to be predominant if its n value varies from 1 to 2 or lies exactly at 2. And if n>2, then the leakage current is said to be dominant [39]. In our case, n was found to be greater than 2 in all cases, which could be attributed to the existence of surface defects existing on the oxides and along the contact area between nanostructures within the device (as already inferred from the emission studies). According to Anderson model, the conduction band offset (∆Ec) and valance band offset (∆Ev) in a typical n-ZnO/p-Si made semiconducting diode could be expressed as given below. ∆Ec =χZnO - χSi ∆Ev = EgZnO - EgSi + ∆Ec
----- (11) ----- (12)
Here, after assuming the band gap values of ZnO and Si to be 3.4 and 1.12 eV, the values of ∆Ec and ∆Ev were estimated to be around 0.4 and 2.55 eV, respectively. Since the valance band discontinuity (∆Ev) tends to be much larger than the conduction band discontinuity (∆Ec) in the present architecture, the electrons in conduction band of ZnO tends to act as the majority charge carriers in ZnO/Si made heterojunctions (rather than holes in the valance band) [28, 40]. Likewise, the increase in band gap values of ZnO (on Gd substitution) suggests an upward shift in its corresponding Fermi level, which results with the corresponding lowering of χZnO values (schematically illustrated in Fig. 6). These variations could have further lead to a continuous decrement in the ∆Ec values on increased Gd substitution, thereby favouring the energetic transfer of charge carriers to take place. And this could be the possible mechanism behind the increased current conductivity in D2 and D3, while compared to D1. Conclusion: Zn1-xGdxO nanostructures have been established through a facile surfactant free wet chemical approach by systematically doping ZnO with Gd ions. The structural characteristic of the hexagonal system was studied to be greatly influenced by the substitution of Gd3+ ions, possibly due to their higher ionic radius. The morphological studies revealed hardly any difference in the 8
physical appearance of ZnGdO nanostructures. The absorbance measurements demonstrate the potential to tune the optical band gap of ZnO towards the lower wavelength regions using Gd ions. The luminescence spectra strongly suggest the existence of surface defects in the solution processed nanostructures. The electrical characteristics evaluated through fabricating n-ZnGdO/p-Si made heterojunction diodes revealed their potential to exhibit superior diode behaviour/conductivity on Gd substitution. References:
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LIST OF FIGURES: Figure 1: XRD patterns of undoped ZnO and ZnGdO nanocrystallites revealing the significant shift in (101) peak position towards the higher angles on increased Gd substitution in the host matrix. Figure 2: AFM/TEM analyses revealing the morphological appearance of particulate like (a) ZnO, (b) Zn0.99Gd0.01O and (c, d) Zn0.97Gd0.03O nanocrystallites. The AFM analyses were carried out over 2 × 2 µm scale. Figure 3: UV-vis absorbance spectra of undoped ZnO and ZnGdO nanocrystallites with the inset revealing the Tauc’s plot. Figure 4: Room temperature emission spectra of undoped ZnO and ZnGdO nanocrystallites. Figure 5: (a) Schematic illustration of the typical device structure and (b) I-V characteristics of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions. Figure 6: Schematic representation illustrating the shift in Fermi level in ZnO on Gd doping and the corresponding electron transfer mechanism across the suggested heterostructures. LIST OF SUPPLEMENTARY FIGURES: Figure S1: Colloidal dispersion of (a) ZnO and (b) ZnGdO systems in isopropanol media. Figure S2: Spray set up involved in the deposition of thin films. Figure S3: The shift in (101) peak position towards the higher angles on increased Gd substitution in ZnO. Figure S4: The variation in lattice parameters as a function of Gd substitution. Figure S5: The variation in lattice strain as a function of Gd substitution. Figure S6: Semi-log IV plots of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions. Table ST1: The rectification ratio (forward to reverse current ratio) of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions. 12
Figure 1: XRD patterns of undoped ZnO and ZnGdO nanocrystallites revealing the significant shift in (101) peak position towards the higher angles on increased Gd substitution in the host matrix.
13
Fiigurre 22: A AFM M/T TEM M aanaalyses rrevealiingg thee m morpphoologgicaal apppeearaance of paartiicullatee likke ((a) ZnO O, (b) Znn0.999Gdd0.011O aandd (c, d)) Znn0.977Gdd0.033O nannoccrysstalllites. T Thee AF FM M annalyysess weere carrried oout ooveer 2 × 2 µ m sscalle.
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11
Intensity (a.u.)
(a) (b)
10
9
8
(c)
7 3.8
3.6
3.4
3.2
Energy (eV)
3.0
2.8
2.6
250 300 350 400 450 500 550 600 650 700 Wavelength (nm)
Figure 3: UV-vis absorbance spectra of undoped ZnO and ZnGdO nanocrystallites with the inset revealing the Tauc’s plot.
15
Figure 4: Room temperature emission spectra of undoped ZnO and ZnGdO nanocrystallites.
16
Figure 5: (a) Schematic illustration of the typical device structure and (b) I-V characteristics of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions.
17
F Figu uree 6: Scchem mattic rreppresenttatioon iilluustraatinng tthe upw warrd sshifft inn Feerm mi leevell inn ZnnO on Gdd doppinng aand thee coorreespoonddingg eleectrronn traansffer meechaanissm acrrosss thhe suugggestted hetteroostrructturees.
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GRAPHICAL ABSTRACT
0.008
Current (A)
0.006
0.004
0.002
0.000
-4
-2
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Voltage (V)
2
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HIGHLIGHTS • The optical and electrical properties of ZnO nanostructures have been systematically maneuvered by means of Gd substitution. • n-ZnGdO/p-Si based heterojunction diodes were fabricated through a conventional spray technique, using colloidal dispersions. • Diodes based on Gd substituted ZnO nanostructures revealed an improved rectifying behavior with superior conductivity values under dark and illuminated conditions.
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S0021-9797(15)00382-3 http://dx.doi.org/10.1016/j.jcis.2015.04.018 YJCIS 20392
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
11 January 2015 10 April 2015
Please cite this article as: S. Chidambaram, G. Gnanasekaran, G. Mohan Kumar, B. Pari, K. Balasubramanian, S. Muthusamy, Colloidal synthesis and electrical behaviour of n-ZnGdO/p-Si heterojunction diodes, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.04.018
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.
Colloidal synthesis and electrical behaviour of n-ZnGdO/p-Si heterojunction diodes Siva Chidambarama, Ganga Gnanasekarana, G. Mohan Kumarb, Baraneedharan Paria, Karthikeyan Balasubramanianc*, Sivakumar Muthusamy*a,d a
Division of Nanoscience and Technology, dDepartment of Chemistry, Anna University - BIT Campus, Thiruchirappalli-620024, India. b
Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul, Republic of Korea.
c
Department of Physics, National Institute of Technology, Thiruchirappalli-620015 *Corresponding authors: Tel: +91-43-240-7959 Email: [email protected]; [email protected]
Abstract: Studies on manoeuvring the optoelectronic characteristics of a semiconducting nanostructure are of recent specific interest for a wide range of photonic applications. In this regard, the optical and electrical characteristics of ZnO nanostructures have been tuned and studied systematically using Gd ions. The structural and morphological characteristics of the solution processed ZnGdO nanostructures were studied in detail using the results of X-ray diffraction and microscopic measurements. The absorption band edge in ZnO was noted to shift towards the lower wavelength values on Gd substitution, suggesting an increase in its energy band gap. The blue emissivity from ZnO complexes was also noted to improve as a function of Gd composition in ZnO. The potential of ZnGdO nanostructures for optoelectronic functions was evaluated by fabricating heterojunction diodes based on n-ZnGdO/p-Si. The diode characteristics revealed an improved electrical conductivity and rectifying behavior from the fabricated architectures upon Gd substitution and photon illumination. The findings are correlated with the increased charge carrier concentration and defect states existing within ZnGdO species, through appropriate mechanisms. Keywords: ZnGdO; p-Si; heterojunction; optical band gap; nanostructures.
1
Introduction: Oxide nanostructures are nowadays being targeted as viable candidates for potential applications in transparent electrodes, luminescent devices, sensors, photovoltaics, etc. The primary interest beneath such systems is usually attributed to their unique physicochemical and optoelectronic characteristics [1, 2]. Zinc oxide (ZnO) is one of such compound semiconductor that is being extensively investigated for photonics related applications. The most prominent ways to fabricate ZnO based systems include vapour deposition, laser ablation, solid state reactions and solution synthesis primarily [3-10]. These diversified material processing methodologies are already proven to serve as efficient gateways for the fabrication of tailored ZnO nanostructures. The methodologies also favour the authentic tuning of their material characteristics, as a function of their particle/grain size. Till date, ZnO has been realized in form of zero-dimensional quantum dots, one-dimensional nanowires, two-dimensional sheets and three-dimensional bulk nanocrystallites [11-13]. For electronic and optoelectronic applications, controlling the particle size is said to directly influence its electronic energy levels [14]. Likewise, substitution of foreign elements into a host lattice is considered as a promising option to induce new functions in the corresponding material system [15]. In case of ZnO, such studies were found to report their structural, optical, magnetic and electrical characteristics to be greatly influenced [16-18]. Recently, rare earth (RE) materials are being looked upon as promising substituent elements that can activate and enrich the optoelectronic functions in ZnO for possible applications in displays and other optoelectronics devices [19-21]. The rich energy levels available in the trivalent RE materials and their temperature-independent luminescence (visible and near-infrared region) are of added importance [22-24]. So, from such intriguing aspects a significant attempt has been made in the present case to investigate the influence of Gd ions on the optical and electrical behaviour of ZnO by means of chemical doping. The significance of the adopted experimental methodology favours the large-scale production of nanomaterials with uniform size at low growth temperatures. This strategy also ensures the feasibility to control the stoichiometric composition of precursors and maintain a high homogeneity at the molecular level, for the efficient substitution of Gd3+ ions in the wurtzite matrix and the realization of customized nanostructures. So, by employing a simple strategy, the meaningful role of Gd ions in ZnO has 2
been studied systematically. In this regard, the structural and optical characteristics of the processed nanomaterials were also studied in detail to personify the importance of Gd substitution in ZnO. Likewise, on considering the need for efficient semiconducting photodiodes in prominent photonic applications, n-ZnGdO/p-Si based heterojunction diodes were also investigated. The developed heterostructures were collectively studied for their texture, morphology and other electronic properties. Experiment: Synthesis of ZnO and ZnGdO nanostructures: Undoped ZnO nanostructures were initially prepared through precipitating an aqueous solution made up of 0.05 M zinc nitrate using 0.1 M of sodium hydroxide stock solutions. The resulting precipitates were aged for 12 h and dried overnight at 60 °C. The obtained products were further calcined at 600 °C for 3 h, prior to their material property studies. Similar procedures were involved in the preparation of ZnGdO nanostructures, by using gadolinium nitrate as the substituent source (1 and 3%). The processed materials are hereafter referred as ZnO, Zn0.99Gd0.01O and Zn0.97Gd0.03O, respectively. Fabrication of n-ZnGdO/p-Si heterostructures: Initially, 1 mg of the processed ZnO nanomaterial was dispersed in isopropanol media to prepare a colloidal dispersion (Fig. S1, ESI). The dispersion was used to establish the n-region of the photodiodes on Si wafers; through a conventional spray technique (Fig. S2, ESI). The native oxides on p-type Si wafers were earlier removed using HF solution (2%) and the wafers were also cleansed by ultrasonic treatment using trichloroethylene-acetone and isopropyl acetate. During spray deposition, the wafers were held at 120 °C and the nozzle to wafer distance was around 3-4 cm. The solution flow rate was held at 0.5 ml/min and compressed air (5 Pa) was used as the carrier gas in all the experiments. The contacts for the fabricated diodes were established using silver paste. A similar procedure was followed for the construction of ZnGdO/Si based heterostructures. The fabricated diodes namely ZnO/p-Si, Zn0.99Gd0.01O/p-Si and Zn0.97Gd0.03O/p-Si are here after referred as D1, D2 and D3, respectively. Characterization: The structural properties of the chemically processed samples were investigated using an XPERT-Pro X-ray diffractometer. The absorbance and emission spectrum 3
were obtained at room temperature using JASCO 7800 and Flouromax (HORIBA JobinYvon) type spectrophotometers. The morphologies of ZnO and ZnGdO specimens were examined using a Shimadzu SFT 3500 atomic force microscope and a JEOL TEM-2100F transmission electron microscope. The diode characteristics were obtained using Keithley-6517B semiconductor parameter analyzer. The photo response studies were carried out at A.M 1.5 conditions. Results and discussion: The mechanism behind the formation of ZnO and ZnGdO systems is summarized through the equations given below. The interaction between zinc and hydroxyl ions present in the solvent might have resulted with the formation of ZnO micelles, which later evolves as a corresponding nanostructure. Similarly, in reactions involving gadolinium nitrate, the Gd ions collectively interacts with the zinc and hydroxyl ions present in the solvent to evolve as a ZnGdO complex. Also, such interactions ensure the potential capability of Gd ions in getting well substituted within the evolving ZnO micellar structures, during efficient substitution reactions. Zn(NO3)2+H2O Zn2+ + 2(NO3)-+H2O
-----
(1)
Gd(NO3)2+H2O Gd3++ 2(NO3)-+H2O
-----
(2)
NaOH + H2O Na+ + OH- + H2O
-----
(3)
Zn2+ + OH- Zn(OH)2 ZnO + H2O
-----
(4)
Zn2+ + xGd3+ + yOH- ZnGdx(OH)y
-----
(5)
ZnGdx(OH)y ZnGdxO0.5y+ 0.5yH2O, y = 3x + 2
-----
(6)
The structural characteristics on solution processed nanostructures were initially studied using X-ray diffraction measurements. The diffraction patterns shown in Fig. 1 corresponds to that of undoped and ZnGdO materials. This is in consistent with the JCPDS data (089-1397) and could be correlated with the hexagonal phase of ZnO. Interestingly, the (101) peak position was noted to shift slightly towards the higher angles on increase in Gd substitution in the wurtzite system (Fig. S3, ESI). This behaviour could be reasoned for the successful substitution of Gd3+ ions into 4
ZnO matrix. Likewise, the absence of characteristics corresponding to metallic Gd or its oxide forms illustrates the phase purity in the solution processed systems. Additionally, the lattice constant values “a” and “c” were calculated for the undoped and ZnGdO materials using the relations given below, v = √3a2C/2 dhkl = [(4(h2+hk+k2)/3a2)+(l2/c2)]-1
----- (7) ----- (8)
The variation in lattice parameter values and lattice strain developed in ZnO systems on Gd substitution has been graphically illustrated in the supplementary information (Fig. S4 and S5). The volume of unit cell in undoped ZnO and ZnGdO was additionally estimated to be around 47.7, 47.3 and 47.2 Å, respectively. Here, the decreasing trend might be reasoned with the difference in ionic radii between the host and substituent elements (0.074 and 0.094 nm), correspondingly [25]. The particulate-like physical morphology of the undoped and Zn1-xGdxO nanocrystallites were examined through atomic force microscopy (AFM). Figure 2a-c shows the three-dimensional AFM images that illustrate the nanostructures to possess a uniform particle size distribution with almost identical characteristics. Here, the substitution of Gd ions into the wurtzite matrix could be noted to have a minimal influence on their morphological evolution or shape of the final materials. The diameter of Zn1-xGdxO particulates was also found to be in consistent with that of the undoped ZnO specimens. However, the density of particle clusters was noted to increase, suggesting the trace element Gd to make more nanostructures agglomerate together. The morphology and phase purity of the Zn1-xGdxO nanostructures were additionally examined using TEM analysis, whose results are shown in Fig. 2d. The electron microscopic results were found to confirm the findings of AFM analysis, where the particulates could be observed to be cluster free with uniform structure. The statistical diameter of the particles was found to be less than 20 nm. The selected area electron diffraction (SAED) pattern is shown at the inset of Fig. 2d. The d-spacing values calculated from the observable SAED pattern was also found to be in consistent with those of hexagonal ZnO structure. The pattern also illustrates the co-existence of both single-crystalline and polycrystalline nature in the specimens [26]. 5
The optical band gap (Eg) of undoped ZnO and ZnGdO nanostructures were studied using UV-vis absorbance spectra. The spectra shown in Figure 3 were actually recorded through a diffused reflectance mode, over a wavelength of 250-700 nm. Here, the characteristic absorption edge corresponding to that of typical ZnO was observed around 360-370 nm for undoped ZnO. On Gd substitution, this edge was noted to blue shift (as a function of Gd composition). The Eg of the nanostructures were evaluated by means of Kubelka-Munk theory, as shown at the inset of Fig. 3. The estimated values were noted to blue shift from 3.28 eV to 3.32 and 3.38 eV, in accordance with their corresponding band edges. Generally, Eg of a nanomaterial relies on a number of factors such as carrier concentration, grain size, lattice strain, size induced quantum confinement, orbital hybridization resulting from doping, etc. [27]. In our case, the observed blue shift in Eg values and the decreasing absorption intensities (ultraviolet region) could be correlated with the increased substitution of Gd ions into the wurtzite matrix of ZnO. Here, we believe the incorporation of Gd ions to have established new occupied electronic states in ZnO (around its Fermi level), whose origin could be rooted to the rich electronic states (4f) available within the RE entity [22-25]. And this increased charge carries could have shifted the Fermi level towards the conduction band, thereby resulting with observed shift in Eg. Generally, the variation in Eg values as a function of charge carrier concentration is expressed as, ∆Eg = (h2/8m*)(3ne/π)2/3
----- (9)
where h is Planck’s constant, m* is the effective mass and ne is the electron concentration [28]. According to this equation, the substitution of Gd3+ ions might have increased the net electron concentration in the material, thereby influencing the band gap values. The room temperature emission spectra of undoped ZnO and ZnGdO specimens are shown in Fig. 4. Here, a weak ultraviolet emission could be observed from the undoped and Zn0.99Gd0.01O systems, along with a broad blue emission in all the specimens. The corresponding emissions are commonly referred as near band edge (NBE) emissions in ZnO and are believed to result from the phenomenon of annihilation of excitation [29, 30]. They are also indirectly related with the particle size [31]. Interestingly, the intensity of blue emission in undoped ZnO was noted to get improved and broader, while the composition of Gd ions in the host matrix was raised. This behaviour could be correlated with the improved interaction of free excitons and the LO phonon 6
replicas in ZnO [32, 33]. Likewise, the incorporation of Gd ions could have also introduced new impurity levels within the host matrix. This in turn might have enriched the optical transitions between free and bound states (such as shallow donor and valence band or the corresponding donor-acceptor pairs), thereby enhancing the observed luminescence [34]. Similarly, the origin of emissions over 450-500 nm could be rooted to the electron transitions between ionized oxygen vacancies and valence band [35, 36]. A weak green emission was also observed around 560 nm in all the samples, whose corresponding intensity was noted to vary as a function of Gd composition (similar to NBE). The origin of green emission could be reasoned with several factors, as the conflictions arise due to attribution of deep levels or trapped state emissions. In our case, the occurrence of green emissions also emphasizes the existence of intrinsic defects such as oxygen vacancies (VO), zinc vacancies (VZn), zinc interstitials (Zni) or oxygen interstitials (Oi) associated within the solution processed materials [34]. Additionally, such emissions also characterize the radiative recombination taking place between photo-generated hole and electron occupying the VO or the transitions from conduction band to the deeply trapped holes [37, 38]. We would also like to emphasize that, the improved blue luminescence in ZnGdO nanomaterials could be efficiently capitalized for electronic and biological applications. Moreover, as ZnO is a biocompatible material their visible emissions can also be capitalized for bio-labelling related applications. Three heterojunction diodes (D1, D2 and D3) were fabricated to evaluate the electronic properties of Gd substituted ZnO systems (refer experimental section) and the schematic representation of a typical diode structure is shown in Fig. 5a. The room temperature I-V characteristics of the diodes were studied under dark and light (1.5 AM) conditions. From the results shown in Fig. 5b it is clear that all the devices tend to exhibit a good rectifying behaviour and a significant increment in the current values on Gd substitution. The dark current was also noted to improve considerably upon illumination. This aspect could be seen as clear evidence that illustrates the decrease in resistance values on Gd substitution in ZnO. The semi log I-V plots of the same (Fig. S6, ESI) signify an exponential increase in the current values at low voltages, substantiating the formation of depletion region across the diodes. The ideality factor (n) of the diodes D1, D2 and D3 were additionally estimated from the slope of the straight line region (forward bias) using Fig. S6. Generally, n is expressed as, 7
n = q/kT(dV/d(lnI))
----- (10)
In a typical photodiode, the tunnelling current and charge carrier generation-recombination current is said to be predominant if its n value varies from 1 to 2 or lies exactly at 2. And if n>2, then the leakage current is said to be dominant [39]. In our case, n was found to be greater than 2 in all cases, which could be attributed to the existence of surface defects existing on the oxides and along the contact area between nanostructures within the device (as already inferred from the emission studies). According to Anderson model, the conduction band offset (∆Ec) and valance band offset (∆Ev) in a typical n-ZnO/p-Si made semiconducting diode could be expressed as given below. ∆Ec =χZnO - χSi ∆Ev = EgZnO - EgSi + ∆Ec
----- (11) ----- (12)
Here, after assuming the band gap values of ZnO and Si to be 3.4 and 1.12 eV, the values of ∆Ec and ∆Ev were estimated to be around 0.4 and 2.55 eV, respectively. Since the valance band discontinuity (∆Ev) tends to be much larger than the conduction band discontinuity (∆Ec) in the present architecture, the electrons in conduction band of ZnO tends to act as the majority charge carriers in ZnO/Si made heterojunctions (rather than holes in the valance band) [28, 40]. Likewise, the increase in band gap values of ZnO (on Gd substitution) suggests an upward shift in its corresponding Fermi level, which results with the corresponding lowering of χZnO values (schematically illustrated in Fig. 6). These variations could have further lead to a continuous decrement in the ∆Ec values on increased Gd substitution, thereby favouring the energetic transfer of charge carriers to take place. And this could be the possible mechanism behind the increased current conductivity in D2 and D3, while compared to D1. Conclusion: Zn1-xGdxO nanostructures have been established through a facile surfactant free wet chemical approach by systematically doping ZnO with Gd ions. The structural characteristic of the hexagonal system was studied to be greatly influenced by the substitution of Gd3+ ions, possibly due to their higher ionic radius. The morphological studies revealed hardly any difference in the 8
physical appearance of ZnGdO nanostructures. The absorbance measurements demonstrate the potential to tune the optical band gap of ZnO towards the lower wavelength regions using Gd ions. The luminescence spectra strongly suggest the existence of surface defects in the solution processed nanostructures. The electrical characteristics evaluated through fabricating n-ZnGdO/p-Si made heterojunction diodes revealed their potential to exhibit superior diode behaviour/conductivity on Gd substitution. References:
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LIST OF FIGURES: Figure 1: XRD patterns of undoped ZnO and ZnGdO nanocrystallites revealing the significant shift in (101) peak position towards the higher angles on increased Gd substitution in the host matrix. Figure 2: AFM/TEM analyses revealing the morphological appearance of particulate like (a) ZnO, (b) Zn0.99Gd0.01O and (c, d) Zn0.97Gd0.03O nanocrystallites. The AFM analyses were carried out over 2 × 2 µm scale. Figure 3: UV-vis absorbance spectra of undoped ZnO and ZnGdO nanocrystallites with the inset revealing the Tauc’s plot. Figure 4: Room temperature emission spectra of undoped ZnO and ZnGdO nanocrystallites. Figure 5: (a) Schematic illustration of the typical device structure and (b) I-V characteristics of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions. Figure 6: Schematic representation illustrating the shift in Fermi level in ZnO on Gd doping and the corresponding electron transfer mechanism across the suggested heterostructures. LIST OF SUPPLEMENTARY FIGURES: Figure S1: Colloidal dispersion of (a) ZnO and (b) ZnGdO systems in isopropanol media. Figure S2: Spray set up involved in the deposition of thin films. Figure S3: The shift in (101) peak position towards the higher angles on increased Gd substitution in ZnO. Figure S4: The variation in lattice parameters as a function of Gd substitution. Figure S5: The variation in lattice strain as a function of Gd substitution. Figure S6: Semi-log IV plots of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions. Table ST1: The rectification ratio (forward to reverse current ratio) of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions. 12
Figure 1: XRD patterns of undoped ZnO and ZnGdO nanocrystallites revealing the significant shift in (101) peak position towards the higher angles on increased Gd substitution in the host matrix.
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Fiigurre 22: A AFM M/T TEM M aanaalyses rrevealiingg thee m morpphoologgicaal apppeearaance of paartiicullatee likke ((a) ZnO O, (b) Znn0.999Gdd0.011O aandd (c, d)) Znn0.977Gdd0.033O nannoccrysstalllites. T Thee AF FM M annalyysess weere carrried oout ooveer 2 × 2 µ m sscalle.
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11
Intensity (a.u.)
(a) (b)
10
9
8
(c)
7 3.8
3.6
3.4
3.2
Energy (eV)
3.0
2.8
2.6
250 300 350 400 450 500 550 600 650 700 Wavelength (nm)
Figure 3: UV-vis absorbance spectra of undoped ZnO and ZnGdO nanocrystallites with the inset revealing the Tauc’s plot.
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Figure 4: Room temperature emission spectra of undoped ZnO and ZnGdO nanocrystallites.
16
Figure 5: (a) Schematic illustration of the typical device structure and (b) I-V characteristics of n-ZnO/p-Si; n-Zn0.99Gd0.01O/p-Si and n-Zn0.97Gd0.03O/p-Si made heterojunctions.
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F Figu uree 6: Scchem mattic rreppresenttatioon iilluustraatinng tthe upw warrd sshifft inn Feerm mi leevell inn ZnnO on Gdd doppinng aand thee coorreespoonddingg eleectrronn traansffer meechaanissm acrrosss thhe suugggestted hetteroostrructturees.
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GRAPHICAL ABSTRACT
0.008
Current (A)
0.006
0.004
0.002
0.000
-4
-2
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Voltage (V)
2
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HIGHLIGHTS • The optical and electrical properties of ZnO nanostructures have been systematically maneuvered by means of Gd substitution. • n-ZnGdO/p-Si based heterojunction diodes were fabricated through a conventional spray technique, using colloidal dispersions. • Diodes based on Gd substituted ZnO nanostructures revealed an improved rectifying behavior with superior conductivity values under dark and illuminated conditions.
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