Synthesis and characterization of vertically aligned ZnO nanorods with controlled aspect ratio

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 48, June 2010, pp Synthesis and characterization of vertically aligned ZnO nanorods with controlled aspect ratio R N Gayen, R Bhar & A K Pal* Department of Instrumentation Science, USIC Building, Jadavpur University, Calcutta * msakp2002@yahoo.co.in Received 30 December 2009; revised 1 April 2010; accepted 30 April 2010 Vertically aligned zinc oxide (ZnO) nanorods with controlled aspect ratio have been deposited by hybrid wet chemical route onto glass substrates with pre-deposited ZnO seeds produced by sputtering a ZnO target in argon plasma. Films have been characterized by measuring microstructural and optical properties. The photoluminescence (PL) spectra have been found to be dominated by the presence of peaks at ~ 394 nm (~3.15 ev) due to the free exciton emission and at ~470 nm (~2.64 ev) due to transitions from deep donor states arising out of oxygen vacancy (V O ) located ~0.06 ev below the conduction band to the valance band. Bonding environment has been obtained from Raman and Fourier transformed infrared (FTIR) studies. Keywords: ZnO nanorods, Microstructure, Optical properties, Photoluminescence 1 Introduction ZnO nanostructures with various morphologies have been widely investigated in the past few years 1-11 due to their fundamental and technological importance. This is basically due to its extraordinary properties like wide-direct-band gap, negative electron affinity, high mechanical strength, high thermal stability, oxidation resistance in harsh environments and large exciton binding energy. Recently, the synthesis of ZnO nanotubes 2, wellaligned ZnO nanorods 3,8,9, nanowires 4,7 and ZnO nanoneedle arrays 5 have been studied. Numerous efforts have also been employed in controlling the sizes and shapes of ZnO nanocrystals, because it provides a better mode for investigating the dependence of electronic and optical properties on the size confinement and dimensionality 12,13. Over the past few years, 1D ZnO nanostructures have been synthesized with various methods including vapourliquid-solid growth, vapour-solid growth, oxideassisted growth, carbothermal reactions, templatebased synthesis and hydrothermal synthesis 6,11, Among all the structures, vertically aligned ZnO nanorods would offer unique opportunity to modulate the physical properties by changing the aspect ratio of the ZnO nanorods. The deposition of ZnO nanorods on the seeds by the hybrid technique adopted here is essentially a solution growth process. Thus, increased time of deposition would render the length and diameter of the ZnO nanorods increase simultaneously. This hybrid technique, unlike other techniques adopted so far, would facilitate the formation of ZnO nanorods with controlled aspect ratios just by varying the deposition time for growing ZnO nanorods from the bath. In this paper, aligned ZnO nanorods with controlled aspect ratio deposited by hybrid wet chemical route on glass substrate having pre-deposited ZnO seeds obtained by sputtering have been studied. The films are characterized by measuring microstructural, optical, photoluminescence, Fourier transformed infrared (FTIR) and Raman spectroscopy. 2 Experimental Details ZnO seed crystallites were deposited at room temperature onto glass substrates by high pressure sputtering at ~30 Pa argon pressure. A 100 mm diameter ZnO target was sputtered in argon plasma for 35 min. The base pressure was better than 10 4 Pa. In this high pressure sputtering regime, particles ejected from the target would undergo multiple collisions with the argon gas atom and get fragmented into finer particles before growing by rapid condensation on the substrate to form ultra-fine particles. The vertically aligned ZnO nanorods were deposited by wet chemical route onto the above glass substrates with pre-deposited ZnO seed particles. These substrates were then transferred to a chemical bath containing zinc nitrate [Zn(NO 3 ) 2.6H 2 O, 0.01 M]

2 386 INDIAN J PURE & APPL PHYS, VOL 48, JUNE 2010 and sodium hydroxide (NaOH, 0.4 M) and water. The solution was heated to ~343 K with constant stirring and the solution started becoming turbid resembling a colloidal solution. As soon as the above temperature was attained (after 5 min or so), the growth of ZnO nanorods was initiated. The deposition process was continued for different durations (varying between 30 and 90 min) for obtaining ZnO nanorods with different aspect ratios. After deposition, the films were taken out of the bath, washed with deionized water and dried in an air-oven at 353 K. Microstructural, optical, PL, Raman and FTIR studies were performed on the above films for obtaining information on the dependence of physical properties on the aspect ratio of the ZnO nanorods deposited as above. Scanning electron microscopy (SEM) images were recorded by using a FEI Quanta 200 microscope and X-ray diffraction (XRD) studies were carried out by using a Rigaku MiniFlex XRD (0.154 nm Cu K α line) to obtain the micro-structural information. Optical studies were performed by measuring the transmittance and the absorbance in the wavelength region λ= nm at room temperature using a spectrophotometer (Hitachi-U3410). The spectra were recorded with a resolution of λ ~ 0.07 nm along with a photometric accuracy of ± 0.3% for transmittance measurements. The atomic force microscope (AFM) used for this experiment was a NanoSurf Easy Scan 2 AFM. Typical scan area for the AFM studies was ~5 µm 5 µm. Raman spectra were recorded using Renishaw invia micro-raman spectrometer using 514 nm argon laser. FTIR spectra were recorded in the range of cm 1 in the reflection mode by using a Nicolet TM -380 FTIR. 3 Results and Discussion 3.1 Microstructural studies ZnO nanorods were deposited on glass substrates having pre-deposited ZnO seeds obtained by sputtering. AFM picture of a representative glass substrate with ZnO seeds deposited by high pressure (~30 Pa) sputtering technique is shown in Fig. 1. The seeds are well distributed on the substrate surface and the surface roughness was estimated as ~2.6 nm. As the ZnO seeds were deposited at a fixed deposition condition, the number density of the seeds would remain invariant as far as practicable, the number density of the ZnO nanorods would also be same for the films deposited at different times of deposition. But, the deposition of ZnO nanorods on the seeds is Fig. 1 AFM picture of ZnO seed crystals deposited on glass substrate essentially a solution growth process; increased time of deposition would render the length and diameter of the ZnO nanorods increase simultaneously. Proportionality of the increase in diameter and length of the ZnO nanorods with time were not the same as is evident from the SEM pictures shown in Fig. 2. This would mean that the films deposited at higher duration would consist of longer as well as thicker ZnO nanorods making the film more compact. Thus, this hybrid technique adopted here would facilitate the formation of ZnO nanorods with controlled aspect ratios just by varying the deposition time for growing ZnO nanorods from the bath. Figure 2(a-d) shows the plane view SEM micrographs of four representatives vertically aligned ZnO nanorods deposited onto the above pre-deposited ZnO seed particles at different deposition times. Films deposited at lower duration contained ZnO nanorods with smaller length while the films deposited for longer duration had significantly larger length (inset of Fig. 2a). Heights and widths of the nanorods were estimated from the lateral view (insets of Fig. 2a-c) of the SEM picture taken at a tilt angle of 30. TEM picture and the corresponding diffraction pattern (inset of Fig. 2d) of a ZnO nanorod were obtained from a film deposited at significantly lower duration to enable the electron beam to penetrate the sample. The electron diffraction pattern indicated preferential orientation in (002) direction of hexagonal ZnO. Average length and diameter of the nanorods were ascertained by measuring the same for a large number of nanorods constituting the films. The values of the length and the diameter obtained as above are given in Table 1. Both the length and the diameter showed an increase for films deposited at higher deposition time

3 GAYEN et al.: CHARACTERIZATION OF ZnO NANORODS 387 Fig. 2 SEM pictures of films of ZnO nanorod deposited on glass substrates with pre-deposited ZnO seed for different duration: (a) 90 min (Inset: cross-sectional view), (b) 60 min (Inset: cross-sectional view), (c) 45 min (Inset: cross-sectional view) and (d) 30 min (Inset: TEM image and diffraction pattern of a nanorod) Table 1 Thickness, refractive index, band gap, є, ω p and carrier concentration of ZnO nanorods Sample name and deposition time Length of nanorod in nm (from SEM) Diameter of nanorod in nm (from SEM) Aspect ratio (from SEM) Refractive index (n) at 600 nm Band gap (ev) є ω p ( ) (sec 1 ) N (cm 3 ) ZnO-1 (30 min) ZnO-2 (45 min) ZnO-3 (60 min) ZnO-4 (90 min) (Fig. 3a). One may clearly observe that the aspect ratio could be modulated (inset of Fig. 3a) by this deposition technique. The corresponding XRD traces of the films deposited at different durations indicated (Fig. 3b) the evolution of oriented and aligned ZnO nanorods with presence of strong reflections from (002) planes along with another peak with very small intensity arising for reflections from (004) planes of ZnO nanorods. Thus, the ZnO nanorods grew preferentially in (002) direction perpendicularly to the glass substrate (Fig. 3b). It may also be noted here that the intensity of the peak for reflections from (002) planes increased significantly for films deposited at higher deposition time i.e. for films

4 388 INDIAN J PURE & APPL PHYS, VOL 48, JUNE Optical studies The transmittance (T r ) spectra of the ZnO nanorod films as recorded by a spectrophotometer are shown in Fig. 3c. It may be observed that the films deposited at lower deposition time i.e. having lower aspect ratio are more transparent than those deposited at higher deposition temperature. Transparency decreased from 90% for films deposited for 30 min to 40% when the films were deposited for 90 min. This may basically be due to enhanced scattering effect in films deposited at longer duration. It may be worthwhile to mention that no interference fringes were seen in the transmission spectra for all the films although the thickness was quite sufficient for generating the fringes. This absence of the interference fringes also affirms the significant contribution due to scattering from the nanorod surfaces. This scattering would necessarily be increased for films grown at higher deposition time as the gaps between the nanorods would decrease and surface participating in scattering would increase with the increase in diameter and length, respectively. Now, the optical absorption coefficient (α) may be expressed as : α = (A/hν)(hν E g ) m (1) where, A is a constant which is different for different transitions indicated by different values of m and E g is the corresponding band gap. This Eq. (1) may be rewritten as: [d(lnαhν)/d (hν)] = m/(hν E g ) (2) Fig. 3 (a) Variation of length and diameter of ZnO nanorods with deposition time (inset shows the variation of aspect ratio with deposition time); (b) XRD traces of films of ZnO nanorod deposited on glass substrate with ZnO seed for different duration: (1) 30 min, (2) 45 min, (3) 60 min and (4) 90 min and (c) transmission spectra for some representative films of ZnO nanorod deposited on glass substrate with pre-deposited ZnO seed for different duration [Inset: expanded view of the (002) peak] having higher aspect ratio and full width at half maxima (FWHM) decreased as the aspect ratio increased (inset of Fig. 3b). Eq. (2) indicates that a plot of d[ln(αhν)]/d[hν] versus hν will show a divergence at hν = E g, from which a rough estimate of E g may be obtained and as such by using Eq. (1), the value of m can easily be evaluated from the slope of the plot of ln(αhν) versus ln(hν E g ). Inset of Fig. 4a shows the plot of ln(αhν) versus ln(hν E g ) for a representative film of ZnO nanorod deposited for 30 min. The value of m obtained from the slope was ~0.49 which indicated allowed direct transition to be present in the film deposited here. The band gap was determined by extrapolating the linear portion of the plot of (αhν) 2 versus hν (Fig. 4a), which indicated a value of E g ~3.2 ev for films deposited at 30 min duration. This value agrees well with the values (~3.33 ev) obtained by other researchers for ZnO nanorods 20. Values of the band gap obtained as above for all the

5 GAYEN et al.: CHARACTERIZATION OF ZnO NANORODS 389 to obtain the optical parameters. This problem could be overcome by simultaneous computation of the optical parameters of transparent thin film along with its thickness by using the same KK approach as discussed below 23. Following KK model 21, for semiconductor materials, the real part of the refractive index, n(λ) may be related to the optical absorption coefficient α(λ), for a given wavelength λ as: Fig. 4 Plots of (αhν) 2 versus hν for ZnO nanorod deposited on glass substrate with pre-deposited ZnO seed at different deposition time: (a) 30 min (Inset shows a plot of ln (αhν) versus ln(hν E g ) for the same film), (b) 45 min, (c) 60 min and (d) 90 min films studied here are shown in Fig. 4(b-d) and are presented in Table 1. It may be noted that the transmittance spectra of the ZnO nanorod films recorded here (Fig. 3c) do not indicate interference fringes needed for the determination of the thickness. This would render the determination of the optical constants difficult and uncertain. Apart from ellipsometric studies, simultaneous determination of the thickness (d) of the thin film and obtaining information about its complex refractive index (η=n+ik, where n = refractive index and k = extinction coefficient) is quite difficult experimentally as in most practical cases, like the one under consideration in this study. The Kramers- Kronig (KK) model would seem to overcome this impasse elegantly 21. Apart from the fact that KK model uses only a single transmittance spectrum, it does not require any dispersion relation for the wavelength dependent refractive index, thus making it a superior choice for calculating the optical constants of the thin film. The only difficulty regarding the KK theory is that, to obtain the best results, the transmittance spectra over a wide range of wavelength are necessary, but, even measurements in a moderate range of wavelength would also yield quite useful results. Xue et al. 22 modified the KK theory successfully to obtain the spectral dependence of the real part of the refractive index and the extinction coefficient and hence the real and imaginary parts of the complex dielectric constant for transparent ZnO:Al thin films with different Al doping concentration. Thickness of the films were measured separately and taken as input 1 α( ψ) dψ n( λ ) = 1+ 2 π 1 λ ψ (3) 2 where, ψ is the running variable for the wavelength in the wavelength range [0, ]. Henceforth, the wavelength dependent extinction coefficient k(λ) may be related to the absorption coefficient as: α( λ) λ k( λ ) = 4π (4) Hence, by knowing the values of the absorption coefficient over the whole wavelength range, one may use Eqs (3) and (4) to evaluate the complex refractive index of the thin film and from thereon, the complex dielectric constants (ε 1 + iε 2 ) of the thin film can be estimated. But, the main problem lies in knowing the optical absorption coefficient value over the wavelength range [0, ]. From the optical absorption studies using an UV-Vis-NIR spectrophotometer, it is only possible to estimate α in a finite optical range using the relation: T = exp[ α( ψ ) d] (5) Therefore, the crux of the problem lies in accurate estimation of the film thickness in order to derive meaningful information about the optical constants of the thin film. Using the KK relations along with the average length of the nanorods taken to be the thickness of the films, the values for the real part of refractive index (n) and the extinction coefficient (k) are calculated and hence the real and complex parts of the dielectric constant (ε 1 and ε 2, respectively) are estimated by using the relations: ε = λ λ (6) n ( ) k ( ) ε 2 = 2 n( λ) k( λ ) (7)

6 390 INDIAN J PURE & APPL PHYS, VOL 48, JUNE 2010 The values of n and k obtained as above are shown in Figs 5 (a, b), respectively. The films containing ZnO nanorods with higher aspect ratios had higher refractive index and extinction coefficient. Figure 5 (c, d) shows the variation of ε 1 and ε 2 with λ while Fig. 5e depicts the variation of ε 1 with inverse of the square of the frequency (1/ω 2 ) of incident radiation. In fact the variation of ε 1 with incident photon energy depends on the plasma frequency ω p. In the infrared region the reflectivity of the semiconductor shows 24 anomalous dispersion as the incident photon energy approaches the corresponding value of the plasma wavelength. It can be shown that 21,23 for ωτ<< 1 and n 2 >> k 2 : ε 1 = ε (ε ω p 2 )/ω 2 (8) where, τ is the relaxation time and ε is the limiting value of the high frequency dielectric constant. Equation (8) suggests that the values of ω p and ε could be determined from the slope and the intercept of the linear portion of the ε 1 versus 1/ω 2 plots shown in Fig. 5e for four representative films of ZnO nanorod deposited at different durations. The values of ε determined as above (Table 1) varied between 4.04 and 3.91 which compares favorably with the bulk value 25,26 ~3.75. The values of plasma frequency, ω p, were seen to vary (Table 1) between 6.35 and s 1 for the films deposited with increasing Fig. 5 Variation of: (a) n, (b) k, (c) ε 1 and (d) ε 2 with wavelength (λ ) for films of ZnO nanorod deposited on glass substrates with pre-deposited ZnO seed for different duration and (e) Plots of ε 1 vs. 1/ω 2 for different films

7 GAYEN et al.: CHARACTERIZATION OF ZnO NANORODS 391 deposition time i.e. with increasing aspect ratio of the nanorods 27. In a semiconductor the carrier concentration (N) varies according to the square of plasma frequency ω p and we have 23,24,28 : ω p 2 = (4 πne 2 )/(m e * ε ) (9) Thus, if N is known, the effective mass of charge carriers, m e *, could be determined from ω p and vice versa. Now, we know that the band gap (E g ) may be related to the effective mass as 29 : 1/ m* = 1 + p 2 /2m E g (10) where m*= m e */m, m is the free electron mass and p = ħg, G being the smallest reciprocal lattice vector. Also, p is related to a as p ~ ħ/a, a being the lattice constant. Thus, using the experimental values of E g and the lattice constant (a), the value of m e * was estimated as ~0.183 m e. Using the above values of ω p and m e *, the value of carrier concentration as estimated from Eq. (9) is given in Table 1. It was found that carrier concentration ( cm 3 ) in ZnO nanorods with lower aspect ratio was higher than that for films with higher aspect ratio ( cm 3 ). 3.3 Photoluminescence studies Photoluminescence (PL) measurements were recorded at 300 K by using a 300 W xenon arc lamp as the emission source. A Hamamatsu photo multiplier along with a 1/4 m monochromator was used as the detecting system. The spectra were recorded with excitation at 258 nm radiations for detecting PL peaks in the range nm. PL spectra for the ZnO nanorods having different aspect ratios are shown in Fig. 6 which are dominated by the presence of a strong PL peaks at ~ 394 nm (~3.15 ev) followed by a low intensity peak at ~470 nm (~2.64 ev). It is known that undoped ZnO, having large exciton binding energy (60 mev), is known to exhibit PL emission peaks due to donor bound exciton (D o X) and free exciton 30,31. As the band gap energy was found to be ~3.2 ev, the observed peak at ~ 394 nm (~3.15 ev) may be ascribed to the free exciton emission. The peak at ~470 nm (~2.64 ev) may arise due to transitions from deep donor states arising out of oxygen vacancy (V O ), located ~0.06 ev below the conduction band 32,33 to the valance band. Fig. 6 PL spectra of ZnO nanorods deposited at four different duration Fig. 7 Raman spectra for ZnO nanorod deposited on glass substrate with predoposited ZnO seed for different duration: (a) 30 min, (b) 45 min, (c) 60 min and (d) 90 min 3.4 Raman studies Typical Raman spectra of representative films of ZnO nanorods grown at different duration on the as-deposited ZnO seeds are shown in Fig. 7. The spectra are dominated by the presence of a strong peak located at ~438 cm 1 followed by peaks at ~ 332 cm 1 in the low wavelength number region and at ~577 cm 1 in the higher wave number region. It may be noted here that the peak position remained invariant for films with different aspect ratios while the intensity of the peak at ~438 cm 1 increased significantly with the increase in aspect ratio of the ZnO nanorods constituting the films. The peak is also asymmetric and asymmetry became enhanced for films containing ZnO nanorods with higher aspect ratio. Group theory predicts that for ZnO, belonging to the C 4 6v space group, would indicate the phonon modes near the center of Brillouin zone (Γ point): Γ = A 1 + 2B 1 +E 1 + 2E 2, where A 1, E 1 and 2E 2 are Raman active modes while 2B 1 is the forbidden mode

8 392 INDIAN J PURE & APPL PHYS, VOL 48, JUNE 2010 Fig. 8 FTIR spectra for ZnO nanorod deposited on glass substrate with predeposited ZnO seed for different duration: (a) 30 min (b) 45 min (c) 60 min and (d) 90 min of ZnO 34. Thus, the peak at ~438 cm 1 may be identified as E 2 optical mode of ZnO and the broadening asymmetry of this peak could be ascribed to ZnO Raman active branches, a characteristic feature of ZnO nanocrystalline form. This observation is in conformity with those observed by Wu et al. 35 on the ZnO nanowires prepared via sol-gel template route. Gao et al. 36 also reported similar observation from their studies on ZnO nanorods deposited by solution deposition method on GaN wafer. Umar et al. 37 reported the presence of an optical phonon E 2 mode at 437 cm 1 for the needle-shaped ZnO nanowires prepared by thermal evaporation of metallic zinc powder followed by oxidation under flow of oxygen in a tube furnace. This result also supports our Raman spectra for the ZnO nanorods. The broad and asymmetric nature of this peak is also typical of the Raman active mode specially observed in wurtzite structure. The origin of the peak at ~ 577 cm 1 could be ascribed to the longitudinal A 1 (LO) modes and may be due to the presence of Zn interstitials in the films. The peak at ~332 cm 1 is due to the second order Raman arising from the E 2 (high) E 2 (low) multiphoton scattering process. This observation is in conformity with that observed by Gao et al FTIR studies Figure 8 (a-d) shows the FTIR spectra of four representative ZnO nanorod films deposited at different deposition times. One may observe the existence distinct characteristic absorption peaks at 478 cm 1 for Zn-O stretching modes. The absorption in ~782 cm 1 with a shoulder at ~568 cm 1 is because of the presence of stretching modes related to ZnO nanorods. It may be noted here that Wu et al. 38 and Kleinwechter et al. 39 observed stretching modes for ZnO at ~ cm 1. Broad absorption peaks at ~3380 cm 1 and ~1762 cm 1 could be assigned to the O-H stretching vibrations of the absorbed water on the ZnO surface. 4 Conclusions Vertically aligned zinc oxide (ZnO) nanorods with controlled aspect ratio were deposited by hybrid wet chemical route onto glass substrates with predeposited ZnO seeds obtained by sputtering a ZnO target in argon plasma. Strong peak for reflections from (002) planes of ZnO was observed in the XRD traces. Intensity of the (002) peak increased significantly with increased aspect ratio of the ZnO nanorods. The films containing ZnO nano-rods with higher aspect ratios had higher refractive index and

9 GAYEN et al.: CHARACTERIZATION OF ZnO NANORODS 393 extinction coefficient. The values of ε varied between 4.04 and 3.91 while the values of plasma frequency, ω p, varied between 6.35 and s 1 for the films deposited with increasing deposition time i.e. with increasing aspect ratio of the nanorods. The PL peak at ~ 394 nm (~3.15 ev) may be ascribed to the free exciton emission. The peak at ~470 nm (~2.64 ev) may arise due to transitions from deep donor states arising out of oxygen vacancy (V O ), located ~0.06 ev below the conduction band to the valance band. The Raman spectra were dominated by the presence of a strong peak located at ~438 cm 1 arising due to E 2 mode of ZnO followed by peaks at ~332 cm 1 for second order Raman vibration and 379 cm 1 for A1 mode of ZnO. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peaks at ~478 cm 1 for Zn-O stretching mode. Acknowledgement The authors wish to thank Dr Anat Rajaram, CLRI, Chennai, India for his kind help in recording the SEM micrographs. One of the authors (R N G) wishes to thank the University Grants Commission, Government of India for financial assistance. References 1 Shao S, Jia P, Liu S & Bai W, Mater Lett, 62 (2008) Wei A, Sun X W, Xu C X, et al., Nanotechnology, 17 (2006) Lim Y S, Park J W, Hong S T & Kim J, J Mater Sci Eng B, 129 (2006) Lee C J, Lee T J, Lyu S C, et al. Appl Phys Lett, 81 (2002) Lin C C, Lin W H, Hsiao C Y, et al., J Phys D Appl Phys, 41 (2008) Li Z, Huang X, Liu J, et al., Mater Lett, 62 (2008) Wan Q, Li Q H, Chen Y J, et al., Appl Phys Lett, 84 (2004) Ye Z Z, Yang F, Lu Y F, et al., Solid State Commun, 142 (2007) Liao L, Zhang W F, Lu H B, et al., Nanotechnology, 18 (2007) Huang Y, Yu K & Zhu Z, Current Appl Phys, 7 (2007) Park S H, Kim S H & Han S W, Nanotechnology, 18 (2007) Yin L W, Bando Y, Zhan J H, et al., Adv Mater, 17 (2005) Sun Y, Fuge G M, Fox N A, et al., Adv Mater, 17 (2005) Wagner R S & Ellis W C, Appl Phys Lett, 4 (1964) Wu Y &Yang P, J Am Chem Soc, 123 (2001) Liao L, Liu D H, Li J C, et al., Appl Surf Sci, 240 (2005) Bhattacharya D, Chaudhuri S, Pal A K & Bhattacharyya S K, Vacuum, 43 (1992) Manifacier J C, Gasiot J & Fillard J P, J Phys E, 9 (1976) Bhattacharya D, Chaudhuri S, Pal A K & Bhattacharya S K, Vacuum, 43 (1992) Guo M, Yang C Y, Zhang M, et al., Electrochim Acta, 53 (2008) Pankove J I, Optical Processes in semiconductors (Prentice- Hall, Inc., NJ, USA), 1971, p Xue S W, Zu X T, Zheng W G, et al., Physica B, 381 (2006) Bhattacharyya S R, Gayen R N, Paul R & Pal A K, Thin Solid Films, 517 (2009) Manifacier J C, Murcia M D, Fillard J P & Vicario E, Thin Solid Films, 41 (1977) Look D C, Reynolds D C, Sizelove J R, et al., Solid State Commun, 105 (1998) Bornstein L, Numerical Data and Functional Relationships in Science and Technology (New Series Group III, 22a, Springer Verlag, Berlin), 1986, p Liao S C, Lin H -F, Hung S & Hu C, J Vac Sci Technol B, 24 (2006) Ferry D K, Semiconductors (McMillan, New York), 1995, Ch Bhattacharya D, Chaudhuri S & Pal A K, Vacuum, 46 (1995) Park W I, Jun Y H, Jung S W & Yi G C, Appl Phys Lett, 82 (2003) Yang Y, Tay B K, Sun X W, et al., Appl Phys Lett, 91 (2007) Look D C, Mater Sci Engg B, 80 (2001) Auret F D, Goodman S A, Legodi M J, et al., Appl Phys Lett, 80 (2002) Calleja J M & Cardona M, Phys Rev B, 16 (1977) Wu G S, Xie T, Yuan X Y, et al., Solid State Commun, 134 (2005) Gao H, Yan F, Li J, et al., J Phys D Appl Phys, 40 (2007) Umar A, Kim S H, Kim J H, et al., Mater Lett, 61 (2007) Wu L, Wu Y, Pan X & Kong F, Optical Mater, 28 (2006) Kleinwechter H, Janzen C, Knipping J, et al., J Mater Sci, 37 (2002) 4349.