Journal of ELECTRONIC MATERIALS

DOI: 10.1007/s11664-014-3229-8 Ó 2014 TMS

Investigations on the Structure, Morphology, and Optoelectronic Properties of Chemically Deposited ZnSe Thin Films: The Effect of Solution pH K. DEEPA,1,2,3 A. C. DHANYA,2 and T. L. REMADEVI1,2 1.—Department of Physics, Pazhassi Raja NSS College, Kannur, Mattannur 670702, Kerala, India. 2.—School of Pure and Applied Physics, Kannur University, Kerala 670327, India. 3.—e-mail: [email protected]

Nanocrystalline ZnSe films were prepared by the chemical bath deposition technique by varying the pH of the reaction bath from 9.2 to 10.6, and the physical properties of the annealed films were investigated. The x-ray diffraction profiles of the samples showed hexagonal structure of ZnSe nanocrystallites except for films deposited at pH 10.6. Strain was compressive, but became tensile with increase in pH. Topographical studies by atomic force microscopy showed a smooth surface of the samples deposited at higher pH and therefore low surface roughness. Optical studies revealed homogeneous grain shape and size. The thickness, grain size, and bandgap of the samples showed consistent results. The photoluminescence emission peaks occurred at the same wavelength for different pH values. The physical properties of the annealed samples seem to make them suitable for application as buffer layers in thin-film solar cells. Key words: ZnSe, pH, AFM, PL, XRD

INTRODUCTION Zinc selenide is a II–VI semiconductor having distinctive properties, such as a direct, wide energy gap (2.7 eV), high refractive index, and low optical absorbance in the visible–infrared (Vis–IR) regions. It is a prominent material for red, blue, and green light emitters, laser screens, ultrasonic transducers, photovoltaic detectors and converters, as well as for buffer layers in thin-film solar cells (TFSCs).1–4 Such buffer layers play a vital role in TFSCs, being employed between an absorber layer and a transparent conducting oxide layer, since they ensure the appropriate interface charge and reduce the lattice mismatch between the two layers as well as the chemical modification that results from chemical species in the sensitive surface of the absorber layer and junction regions.5,6 Currently, CdS is extensively used as a buffer layer for higher efficiency in copper–indium–gallium–(di)selenide (CIGS)-based

(Received November 15, 2013; accepted April 29, 2014)

solar cells. Although chemical-bath-deposited CdS buffer layers provide the above-mentioned advantages for outstanding TFSC performance, light with wavelength shorter than 520 nm cannot be transmitted through the absorber layer due to its relatively narrow bandgap energy of 2.4 eV. This results in a drop of the quantum efficiency compared with the theoretical value. Furthermore, due to its toxic nature, attention has recently been focused on developing cadmium-free, ecofriendly buffer layers.4 ZnSe is one of the substitutes offering better lattice parameter conformity, nontoxicity, and a good conduction band that may transfer high-energy photons to the absorber layer of the solar cell.4 Several methods such as physical vapor deposition,7 metalorganic chemical vapor deposition,8 pulsed laser deposition,9 molecular beam epitaxy,2 spray pyrolysis,10 sol–gel deposition,11 successive ionic layer deposition and reaction (SILAR),12 chemical bath deposition (CBD),13–17 etc. have been used to grow ZnSe thin films. In the present investigation, ZnSe thin films were synthesized by the CBD technique using solutions

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with various pH values. Features such as the low temperature requirement, inexpensive equipment, and ease of deposition on any size and shape of substrate make this chemical route ideal for industrial adaptation. Many investigations have presented ZnSe thin-film deposition by a CBD route, but only a handful of these reports considered the effect of solution pH on the film properties.3,18 Considering all these facts, the effects of solution pH on the structural, morphological, optical, and photoluminescence (PL) properties are highlighted herein. EXPERIMENTAL PROCEDURES

Sodium selenosulfate solution (0.2 M) was prepared by refluxing 2 g selenium powder and 6.2 g sodium sulfite in 200 ml deionized water for 8 h at temperature of 70°C. As this solution is relatively unstable, it must be freshly prepared prior to the thin-film deposition process. This solution was then diluted to obtain 0.05 M solution and used to prepare the reaction bath. The precleaned glass substrates were kept inclined at an angle of approximately 60° to the walls of the beaker containing the reaction solution. The beaker was sealed to avoid solution evaporation. All samples reported herein correspond to 90 min deposition time at bath temperature of 70°C. After deposition, the substrates were removed

Film Synthesis ZnSe thin films were deposited onto microscopic glass substrates by the CBD technique using zinc acetate as cationic precursor and sodium selenosulfate as chalcogenizing agent. In a typical synthesis process, the reaction solution was obtained by mixing 30 ml 0.5 M zinc acetate solution with 10 ml 80% hydrazine hydrate (serving as a complexing agent) and stirring thoroughly using a magnetic stirrer. This was followed by addition of 0.5 M ammonium acetate solution, which acts as a buffer. Keeping all other deposition parameters fixed, the pH of the alkaline solution bath was varied between 9.2 and 10.6 by varying the volume of ammonia solution added. Finally, 30 ml 0.05 M sodium selenosulfate solution was added and stirred continuously to obtain a clear, homogeneous solution.

Fig. 2. Variation of lattice constants with solution pH.

Fig. 1. XRD patterns of ZnSe films deposited from solutions with pH of (a) 9.2, (b) 9.7, (c) 10.1, and (d) 10.6.

Investigations on the Structure, Morphology, and Optoelectronic Properties of Chemically Deposited ZnSe Thin Films: The Effect of Solution pH Table I. Strain and grain size of ZnSe samples deposited at different solution pH values pH

Grain Size, D (nm)

Strain, e

9.2 9.7 10.1 10.6

23.2 25.6 27.7 21.4

0.045 0.0075 0.018 0.012

morphology was investigated using atomic force microscopy (AFM, Ntegra Prima; NT-MDT, Russia) in noncontact mode. Fig. 3. Variation of c/a ratio with solution pH.

RESULTS AND DISCUSSION The as-deposited films were highly uniform and well adhered to the substrate. The thickness of the films deposited from solutions with pH 9.2, 9.7, 10.1, and 10.6 as determined by the gravimetric technique was 320 nm, 400 nm, 540 nm, and 128 nm, respectively. The effect of solution pH on the thickness of various thin films has been widely studied by different groups.19–22 The thickness of the films was found to be strongly affected by the variation in solution pH. Crystallographic Structure

Fig. 4. W–H plot for ZnSe samples deposited at solution pH of (a) 9.2, (b) 9.7, (c) 10.1, and (d) 10.6.

from the bath. The thin film deposited onto the side of the substrate closer to the wall of the beaker was retained for various measurements, whereas that on the other side was removed using a cotton swab moistened with dilute hydrochloric acid, followed by rinsing with deionized water for a few minutes to remove any ZnSe particles loosely adhered to the surface, finally being dried in air. Further samples were annealed in air at 373 K for 20 min. Film Characterization The structure and crystallinity of the films were determined using a Bruker AXS-8 Advance x-ray diffractometer (XRD) with Cu Ka radiation (wave˚ ). Layer thickness was estimated by length 1.5406 A the gravimetric method using a high-precision balance. Optical characterization of the films was done using a CARY 5000 ultraviolet (UV)–Vis spectrophotometer. Transmittance measurements were carried out in the wavelength range from 300 nm to 900 nm. Room-temperature PL emission spectra of as-prepared and annealed samples were recorded using an F-2500 FL spectrophotometer. The surface

The pH value of the chemical bath plays a vital role in determining the crystal structure and stoichiometry of the deposited film. XRD measurements were performed to follow the change of layer crystallinity induced by the pH of the chemical bath. Figure 1 shows the XRD patterns of the annealed samples. The films deposited from the solution bath with pH 9.2, 9.7, and 10.1 were found to exhibit hexagonal structure with lattice constants ˚ and c = 6.55 A ˚ , whereas the solution pH a = 3.996 A of 10.6 resulted in a film with mixed hexagonal and cubic phases [Joint Committee on Powder Diffraction Standards (JCPDS) file 15-0105]. The different peaks in the diffractogram were indexed, and the corresponding values of interplanar spacing d were compared with the standard JCPDS file (80-0008) for ZnSe. It was observed from the diffractograms that all identified peaks corresponded to ZnSe, with no additional peaks corresponding to Zn or Se being present. The predominant orientation was along (002) for all samples, irrespective of the solution pH, though there was a variation in the corresponding peak intensity and full-width at half-maximum (FWHM). These observations corroborate the fact that there is a variation in crystallinity, grain size, and strain in the films. As the pH of the solution was increased from 9.2 to 10.1, the intensity of the prominent reflection along (002) increased, but a further increase in pH to 10.6 led to a decrement of the peak intensity. The values of the lattice

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Fig. 5. AFM images of ZnSe films deposited from solutions with pH of (a) 9.2, (b) 9.7, (c) 10.1, and (d) 10.6.

Fig. 6. Variation of root-mean-square roughness of the films with solution pH.

Fig. 7. Transmittance of ZnSe films deposited from solutions with pH of (a) 9.2, (b) 9.7, (c) 10.1, and (d) 10.6.

constants a and c for the deposited films were calculated using the equation18

c/a is presented in Fig. 3. The presence of strain in the films is inevitable irrespective of the deposition technique used. The strain can be uniform or nonuniform. In the case of uniform strain, the interplanar lattice spacing d shifts to lower or higher values depending upon the nature of the strain (tensile or compressive). Nonuniform strain changes from one region to another within the same grain. The presence of nonuniform strain is manifested by


l2 1 4 2 2 þ 2: ¼ h þ hk þ k d2 3a2 c

(1)

The variations of a and c with solution pH are depicted in Fig. 2. The corresponding variation in

Investigations on the Structure, Morphology, and Optoelectronic Properties of Chemically Deposited ZnSe Thin Films: The Effect of Solution pH

broadening of the x-ray diffraction lines.23 Information on the strain and the particle size can be obtained from the FWHM of the diffraction peaks. The FWHM (b) can be expressed as a linear combination of the contributions from the strain (e) and particle size (D) by the following relation24: b cos h 1 e sin h ¼ þ : k D k

(2)

Figure 4 presents a William–Hall (W–H) plot for the ZnSe samples. The slope of this plot gives the amount of residual strain. A negative slope value indicates compressive, whereas a positive value indicates tensile strain. Here the strain is compressive for pH 9.2, but for all other films the strain is tensile. The reciprocal of the intercept on the y-axis gives the particle size. The variation of the strain and grain size of the ZnSe samples with solution pH is presented in Table I. Surface Morphology

Fig. 8. Tauc plot of ZnSe films deposited from solutions with pH of (a) 9.2, (b) 9.7, (c) 10.1, and (d) 10.6.

Surface topography studies of the as-deposited ZnSe thin films were carried out using AFM. Threedimensional (3D) AFM images (Fig. 5) of typical as-deposited ZnSe thin films revealed variation in the surface roughness. This surface roughness variation clearly supports the effect of pH on the chemical synthesis process. The AFM image of the film deposited at pH 9.2 reveals the formation of conical shaped grains, which gradually evolved to almost spherical shaped grains at pH 10.1. An increase in pH to 10.6 led to deposition of needlelike grains. As depicted in Fig. 6, the root-meansquare values of surface roughness (Ra) of the ZnSe films initially increased with increase in solution pH. The film surface roughness was maximum for pH 10.1, whereas the film surface was comparatively smoother for films deposited at pH 10.6. These findings substantiate the variation in thickness and grain size of the films with solution pH.

Fig. 9. PL spectra of ZnSe films deposited from solutions with pH of (a) 9.2, (b) 9.7, (c) 10.1, and (d) 10.6.

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Optical Studies Transmission measurements were performed at normal incidence over the spectral range from 300 nm to 800 nm. The transmission spectra of the ZnSe thin films deposited at different pH values are shown in Fig. 7. The broad cutoff towards short wavelengths indicates the onset of intrinsic interband absorption in ZnSe. An increase of the transmission values over the whole spectral range is observed with increasing pH value. In the transparency region, transmission was highest for pH of 10.6 and varied from 64% to 79%. For pH of 10.1, the transmission in the visible region was between 54% and 75%. When the pH was decreased to 9.7 and 9.2, the maximum transmission reduced to 63%. However, there was no detectable variation of the shortwavelength absorption edge with the pH. The increase in transmittance with increasing pH up to 10.1 can be attributed to the enhanced crystallinity and larger grain size of the films. Although the crystallinity decreased slightly at pH 10.6, the transmission increased since the film thickness was much lower than for the films deposited at other pH values. Furthermore, at this pH value, the deposited film had a comparatively smooth surface, as revealed by the AFM studies. High pH leads to more OH ions in the solution, which tend to combine readily with zinc to form Zn(OH)2 in the bath without leaving many zinc ions for deposition on the substrate. Hence, the growth rate will be low (at pH 10.6), and the films are thinner and have lower absorption. It can be seen that the films exhibit a steep optical absorption feature, indicating good homogeneity of grain shape and size as well as low defect density near the band edge. Based on the optical transmission measurements, the square of the absorption coefficient (a) is plotted as a function of photon energy (ht) in Fig. 8. The following equation can be applied for a direct interband transition: ðahmÞ2 ¼ Cðhm  Eg Þ;

(3)

where C is a constant and Eg is the direct bandgap. The bandgap energy can be obtained by extrapolating the linear portion of the plot to the energy axis. The calculated values of the bandgap energy for the films deposited at the different solution pH values are 2.71 eV, 2.68 eV, 2.65 eV, and 2.67 eV, respectively, in close agreement with values reported for ZnSe thin films obtained by CBD.13–17 The variation in the bandgap is in accordance with the film thickness, crystallinity, and grain size for the different pH values. Photoluminescence The PL spectra of the samples recorded at room temperature using an excitation wavelength of 425 nm are depicted in Fig. 9. The PL spectra peak at around 2.54 eV, which is less than the bandgap

energy. Self-activated centers arising from complexes of zinc vacancies and shallow donors (selenium interstitials) would occur at around 2.5 eV.25 The PL emission from intrinsic ZnSe has been attributed to the presence of native defects such as zinc and selenium vacancies or interstitials, which are likely to be introduced during the growth process.26,27 The peak position remains the same irrespective of the solution pH used. The visible luminescence exhibited by the ZnSe films prepared at various pH values shows them to be potential candidates for device applications. CONCLUSIONS Detailed investigations on nanocrystalline ZnSe thin films deposited at various solution pH values by CBD highlight the possibility of their use as good buffer layers in TFSCs. The variation was achieved only by changing the amount of ammonia added to the bath. At higher pH values, the nanocrystallites were of mixed phase. The optical and PL properties of the samples confirm their potential for use in photovoltaic devices. ACKNOWLEDGEMENTS One of the authors acknowledges the UGC for financial assistance under FDP. The authors are grateful to STIC-CUSAT for technical support. REFERENCES 1. A.R. Balu, V.S. Nagarethinam, M.G. Syed Basheer Ahmed, A. Thayumanavan, and K.R. Murali, Mater. Sci. Eng. B 171, 93 (2010). 2. C.W. Huang, H.M. Weng, Y.U. Jiang, and H.Y. Heng, Vacuum 83, 313 (2009). 3. C. Mehta, G.S.S. Saini, J.M. Abbas, and S.K. Tripathi, Appl. Surf. Sci. 256, 608 (2009). 4. L. Chen, D. Zhang, G. Zhai, and J. Zhang, Mater. Chem. Phys. 120, 456 (2010). 5. W. Wang, S.Y. Han, S.J. Sung, D.H. Kim, and C.H. Chang, Phys. Chem. Chem. Phys. 14, 11154 (2012). 6. S.W. Shin, S.R. Kang, J.H. Yun, A.V. Moholkar, J.H. Moon, J.Y. Lee, and J.H. Kim, Sol. Energy Mater. Sol. Cells 95, 856 (2011). 7. G.I. Rusu, V. Ciupina, M.E. Popa, G. Prodan, G.G. Rusu, and C. Baban, J. Non-Cryst. Solids 352, 1525 (2006). 8. M.J. Bevan, H.D. Shih, J.A. Dodge, A.J. Syllaios, and F. Weirauch, J. Electron. Mater. 27, 769 (1998). 9. T. Zhang, N. Xu, Y. Shen, W. Hu, J. Wu, J. Sun, and Z. Ying, J. Electron. Mater. 36, 75 (2007). 10. M. Oztas and M. Bedir, Mater. Lett. 61, 343 (2007). 11. J. Hai-qing, C.H. Jun, and Y. Xi, Trans. Nonferr. Met. Soc. China 16, 266 (2006). 12. R.B. Kale and C.D. Lokhande, Mater. Res. Bull. 39, 1829 (2004). 13. P.P. Hankare, P.A. Chate, P.A. Chavan, and D.J. Sathe, J. Alloys Compd. 461, 623 (2008). 14. R.B. Kale and C.D. Lokhande, Appl. Surf. Sci. 252, 929 (2005). 15. R.B. Kale, C.D. Lokhande, R.S. Mane, and S.H. Han, Appl. Surf. Sci. 252, 5768 (2006). 16. C.D. Lokhande, P.S. Patilet, A. Ennaoui, and H. Tributsch, Appl. Surf. Sci. 123, 294 (1998). 17. L. Chen, D. Zhang, G. Zhai, and J. Zhang, Mater. Chem. Phys. 120, 56 (2010). 18. J. Mazher, A.K. Srivastav, R.V. Nandedkar, and R.K. Pandey, Nanotechnology 15, 572 (2004). 19. A. Karipera, E. Guneria, F. Godeb, C. Gumus, and T. Ozpozan, Mater. Chem. Phys. 129, 183 (2011).

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