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Analysis of PV deposited ZnTe thin films through Urbach tail and photoluminescence spectroscopy
Journal of Luminescence 194 (2018) 257–263
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Analysis of PV deposited ZnTe thin films through Urbach tail and photoluminescence spectroscopy
MARK
⁎
Rashmitha Keshav, Meghavarsha Padiyar, Meghana N, Mahesha MG Department of Physics, Manipal Institute of Technology, Manipal University, 576104, India
A R T I C L E I N F O
A B S T R A C T
Keywords: II – VI compound Thin films Solar cell materials Photoluminescence Roosbroeck–Shockley relation
ZnTe thin films have been grown by thermal evaporation on glass substrates at room temperature at a residual pressure about 10−6 Torr and characterized for their structural and optical properties. Crystallite size and strain have been estimated from X-ray diffractogram by William – Hall method. Urbach tail, which originates near band edge due to structural disorder, has been evaluated along with optical bad gap from the absorbance spectra. Photoluminescence spectra has been recorded to get information on various excited levels. Correlation between optical absorbance and photoluminescence spectra has been established with Roosbroeck–Shockley relation. Finally effect of annealing on structural and optical properties along with photoluminescence spectra has been studied in detail.
1. Introduction II – VI compound semiconductors find extensive application in optoelectronic devices such as light emitting diodes, photodetectors, nonlinear optical devices and solar cells. Zinc telluride (ZnTe) is an important member of II – VI compound chalcogenides which has direct bandgap of about 2.26 eV [1]. ZnTe is a well suited material for IR detector, thin film transistors, phosphor in x-ray imaging and light emitting diodes in visible region [2–4]. In cadmium telluride (CdTe) based solar cells, achievement of high efficiency has been stalled due to challenge in formation of a stable ohmic back contact and in recent years, application of ZnTe as back contact or buffer layer has been explored by several research groups [5–7]. The defect states like vacancies, ad atoms, interstitials and anti-sites are having crucial role in deciding the electrical properties of the materials and hence, determining the properties of hetero structure formed with other materials. In the case of materials deposited in the thin film form by different deposition techniques, preparation method and preparation parameters dictate these defect states. There are reports available on ZnTe films deposited on glass substrate by various techniques [8–10] In the process of evaluation of a material for device application in the thin film form, it is inevitable to characterize various defects. Optical spectroscopy is the best suited technique to probe different defect states and hence, in the present paper, we report a study carried out to investigate the defect states in thermally evaporated ZnTe thin films using Urbach tail and photoluminescence (PL) spectroscopy. Apart from this, associated studies such as x-ray diffractogram (XRD), scanning electron microscopy
⁎
(SEM) and energy dispersive spectroscopy (EDS) have been carried out to get structural, morphological and compositional information which are correlated with spectroscopic results. 2. Experimental ZnTe thin films have been deposited on chemically cleaned glass substrate by thermal evaporation technique at residual pressure 10−6 Torr by taking 99.99% pure compound as source material. Deposition rate and the thickness have been monitored with digital thickness monitor operating at 6 MHz frequency. Films of about 300 nm thickness have been obtained by depositing the material at constant rate of 0.2 nm/s. Deposited films have been annealed at 473 K for about half an hour under air ambience. Structural study of pristine and annealed films has been carried out by recording XRD with Rigaku Miniflex – 600 X-ray diffractometer with Cu Kα (0.154 nm) at 40 kV, 15 mA in scanning range 20° to 70° at scan speed of 2°/min. Morphological changes induced in the films due to annealing has been analyzed by SEM model Zeiss SEM EVO18. Shimadzu UV-1800 PC spectrophotometer has been used to record absorption spectra in the wavelengths ranged from 400 to 1100 nm. The photoluminescence spectra has been recorded using Agilent Cary Eclise WinFLR photoluminescence spectrometer by using 480 nm wavelength as exciting source.
Corresponding author. E-mail address: [email protected] (M. MG).
http://dx.doi.org/10.1016/j.jlumin.2017.10.047 Received 5 September 2017; Received in revised form 11 October 2017; Accepted 16 October 2017 Available online 17 October 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 257–263
R. Keshav et al.
Fig. 1. X-ray diffractogram of ZnTe thin films (Inset: Enlarged plots near peaks). Fig. 2. Williamson – Hall (W – H) analysis of X-ray diffractogram of ZnTe thin films.
3. Results and discussion originate due to several reasons like the mismatch in thermal properties of film and substrate, imperfections including vacancy and interstitials. Lattice mismatch or difference between the thermal coefficient of film and substrate can induce uniform strain which causes a shift in the peak position. Broadening of the X-ray peak affirms point defects or poor crystallinity of the films. Hence it is suitable to adopt William-Hall (W – H) method to estimate crystallite size and internal strain [14]. In W – H method, it is assumed that broadening of X-ray peak due to size and strain is an additive component to total integral breadth of a Bragg peak. The W – H relation is given by [15],
3.1. Structural analysis XRD of pristine and annealed ZnTe thin films are depicted in Fig. 1. Soft analysis of XRD pattern reveals that the peak intensity has increased and peak position has shifted to lower angle after annealing. A detailed analysis has been done to index the peaks and to extract the parameters that assess the structural properties of the film and listed in Table 1. Detailed study has revealed that films have cubic structure with strong (111) orientation belonging to F-43m (216) space group. II – VI group chalcogenides normally found in cubic zinc blende or hexagonal wurtzite type structure or even in the mixed phase [5,11–13]. Thin film preparation technique and growth parameters have profound influence on the structure. Substrate temperature and growth rate are two important parameters among them. A published report revealed that low deposition rate and/or high substrate temperature favors hexagonal phase which is attributed to reduced vapor concentration reaching the substrate; exhibit cubic structure otherwise [13]. Crystallite size, micro-strain and dislocation density have also been estimated from XRD. From Scherrer formula, crystallite size (D) is given by,
D=
0.9λ β cos θ
β cos θ =
0.9λ +(4ε sin θ) D
(2)
where ε is the strain in the film. Eq. (2) assumes uniform deformation model according to which the strain is uniform in all crystallographic direction, thus considering the isotropic property of the crystal. Plot of β cosθ vs. 4 sin θ (Fig. 2) shows positive slope for both the films with correlation coefficient about 0.96. The strain may be due to thermal mismatch between films and substrate in addition to the contribution from defects. Crystallite size obtained from intercept of W – H plot is slightly higher than the value obtained in Scherrer formula. SEM micrographs have shown uniform growth of the films over considerably large area (Fig. 3). However there is difference in the elemental composition of the films compared with the source material as depicted by EDS in Fig. 4. Estimated elemental composition of as-deposited and annealed films is listed in Table 2, which shows that both the films are Zn rich. The difference in the composition of films deposited by thermal evaporation is mainly due to difference in the vapor pressure.
(1)
where λ is the wavelength of the X-ray source, β is FWHM of the peak and θ is Bragg angle. Crystallite size estimation shows that films are nano-crystalline (Table 1). Even though no much change in the crystallite size has been observed, there is change in the strain and dislocation density of the films. These quantities, namely, lattice strain ε = βcosθ/4 and dislocation density δ = 1/D 2 have also been tabulated in Table 1. In case of thin films deposited on the substrate, strain may Table 1 Analysis of XRD data of ZnTe thin films. Sample
As-deposited Annealed
2θ (°)
26.02 25.96
d (nm)
0.342 0.343
hkl
(111) (111)
a=b=c (nm)
0.592 0.594
Cell volume
0.207 0.209
Crystallite size D(nm)
Micro-strain ε (×10−4)
Scherrer formula
W-H plot
βcosθ/4
W-H plot
4.43 3.94
8.15 6.60
7.8 8.8
11.3 15.9
258
Dislocation density (D values estimated from W – H plot are used) Δ (m−2)
1.50 × 1016 2.29 × 1016
Journal of Luminescence 194 (2018) 257–263
R. Keshav et al.
Fig. 3. SEM micrographs of (a) As-deposited, (b) Annealed ZnTe thin films and elemental mapping of Zn and Te for (c), (d) As-deposited (e), (f) Annealed ZnTe thin films.
where h is Planck's constant, ν is the frequency of incident radiation C is a constant that depends on the transition probability, Eg is the energy band gap and n depends on the characteristic of the transition (n = ½ for direct band gap and n = 2 for direct band gap). Confirmation of direct band gap type can be done by linearity in the plot of (αhν)2 vs. hν which is evident from Fig. 6a. Pristine films have band gap about 2.20 eV as estimated from extrapolation of linearity in the graph and annealing resulted in slight reduction in the bad gap value. This manifestation of minor change in the optical band gap can be credited to combined effect of annealing induced change in the crystallite size and strain in the film [17].
3.2. Optical analysis Typical optical absorbance and transmittance spectra for as-deposited and annealed films are shown in Fig. 5. From the plot, it is evident that absorbance of the films has increased after annealing. Signature of interference fringes in the transmittance spectra indicates that the films are uniform. Tauc relation is the most reliable way to determine the electron transition type and the optical band gap from absorption coefficient (α) with the function of wavelength (λ) [16]:
αhν = C (hν − Eg )n
(3)
259
Journal of Luminescence 194 (2018) 257–263
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suitability of the materials in thin film form. Defect induced local states can be characterized by studying the spectral dependence of absorption coefficient at lower photon energy regime which is less than band gap energy. This low energy region, so called Urbach tail, characterizes the slope of the exponential edge [18]
E α = α 0exp ⎛ ⎞ E ⎝ u⎠ ⎜
⎟
(4)
where α 0 is a temperature dependent constant and Eu is Urbach energy. Eu signifies the width of the tail of localized states or width of the exponential absorption edge. Urbach energy can be estimated from inverse of the slope of the plot Ln(α ) vs. hν and the estimated value for the as-deposited film is observed to be 0.078 eV (Fig. 6b). Further analysis has revealed that the Urbach energy has increased from 0.078eV to 0.084 eV after annealing. This variation could be due to change in crystallite size, strain and defect density as revealed by XRD. Air annealing always favors incorporation of oxygen into films, mostly in the form of oxides, which in turn can degrade the film [19]. This particular effect may lead to slight change in the crystallite size also. Shift in the Urbach energy and Tauc gap have followed the same trend implying strong correlation between them. This correlation suggests that the transitions across the band gap and the transitions responsible for the Urbach edge are the electronic transitions which are not distinct except for the differences in the relevant densities of states. Such strong association between these two parameters is recently reported elsewhere also [20]. The steepness parameter, σ can be calculated from following relation [18]: Fig. 4. Energy dispersive spectra of (a) As-deposited, (b) Annealed ZnTe thin films.
σ= Table 2 Elemental composition of ZnTe thin films obtained from EDS. Element
Zn L Te L
As-deposited films Atomic%
Weight%
Atomic%
38.03 61.97
54.50 45.50
37.83 62.17
54.29 45.71
(5)
where kB is Boltzmann constant and T is absolute temperature. Eq. (5) characterizes the broadening/shrinkage of optical absorption edges due to electron – phonon or exciton – phonon interactions. The increase in the Urbach energy or decrease in the steepness parameter from 0.33 to 0.30 upon annealing can be assigned to increase in the localized states initiated from the non-radiative recombination centers.
Annealed films
Weight%
kB T Eu
3.3. Photo-luminescence study Defect states in thin film semiconductors have major role in deciding the electrical properties of the film. Hence characterization of these defect states is very essential for the assessment of device
For as-deposited films, photo-luminescence (PL) spectra along with de-convoluted peaks is depicted in Fig. 7a. Non-symmetry in the most intense peak indicates the convolution of multiple peaks.
Fig. 5. (a) Absorbance spectra and (b) Transmittance spectra of ZnTe thin films in the wavelength range 400–1100 nm.
260
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R. Keshav et al.
Fig. 6. (a) Plot of (αhν)2 vs. hν for ZnTe films (b) Plot of Ln(α) vs. hν for ZnTe films.
(Fig. 7b). In our present investigation, for annealed films, we have not observed any considerable change in effective sample thickness; but a marginal increase in carrier energy is observed. Improved crystallinity of the sample which is depicted in XRD may be the possible reason for this change. This improvement in carrier energy may lead to reduction in the carrier life time and, hence making the films suitable for photodetector application at higher frequency. Peaks P1 and P2 are attributed to deep and shallow band transitions whereas P3 may be due to defect. Comparison of the PL spectra of as-deposited and annealed films is represented in Fig. 8. Peak P1 position has not changed much after annealing, however the broadening has been observed in the width of the peak. Peak P2 has shown significant red shift and, also broadening in the width. Broadening of the emission peak is the indication of change in the lattice environment. And the change in the composition or doping can lead to shift in the peak position. Interstitial, vacancy and anti-site are commonly observed defect states which play a major role in deciding the electrical and opto-electronic properties of these films. Peak P3 may be due to such defect states. P4 is the additional peak observed in the annealed films which could be due to oxidized states in the films [22]. The CIE plot for both as-deposited and annealed films is represented in Fig. 9 which shows significant shift in the emission color. This indicates that air annealing or incorporation of oxygen can be used as luminescence tuning tool.
Fig. 7. (a) Photoluminescence spectra of as-deposited ZnTe thin films showing deconvoluted peaks (b) Comparison of experimental and calculated PL intensity in 450–550 nm range.
Deconvolution resulted in three different peaks P1, P2 and P3 (Table 3) with χ2 = 0.986. Literatures show that strong coupling exists between absorbance and PL spectra of materials. The intrinsic relationship between absorption and PL is given by Roosbroeck–Shockley relation (RSR) [21]:
IPL (hν )∝
4. Conclusion ZnTe thin films grown by thermal evaporation have exhibited cubic structure with preferential (111) orientation. Films are slightly Zn rich and have not shown any considerable change in composition upon annealing. However, strain in the films increased due to annealing along with marginal change in crystallite size. Minor change in the band gap energy and Urbach energy can be assigned to combined effect of crystallite size and strain. Normalized PL spectra in 450–550 nm range has shown good agreement with normalized intensity calculated
(hν )2×(1−exp[−α (hν ) deff ]) exp (hν / kTc )−1
(6)
where deff is the effective sample thickness, defines depth of the volume contribution to PL, and kTc is the carrier energy which are the two fitting parameters. For pristine samples, these fitting parameters were observed to be 0.28 ± 0.01 µm and 7.3 ± 0.1 × 10–21 J respectively Table 3 Results of deconvolution of PL spectra of ZnTe thin films. Peak →
P1
Sample
λ (nm)
FWHM
λ (nm)
FWHM
λ (nm)
FWHM
λ (nm)
FWHM
As-deposited Annealed
491 492
4 38
496 503
6 12
605 609
9 14
– 742
– 10
P2
P3
261
χ2
P4
0.986 0.983
Journal of Luminescence 194 (2018) 257–263
R. Keshav et al.
Fig. 8. (a) Comparison of photoluminescence spectra of as-deposited and annealed ZnTe thin films, (b), (c), (d) Comparison of different deconvoluted peaks of as-deposited and annealed ZnTe thin films.
References [1] O.I. Olusola, M.I. Madugu, N.A. Abdul-Manaf, I.M. Dharmadasa, Growth and characterisation of n- and p-type ZnTe thin films for applications in electronic devices, Curr. Appl. Phys. 16 (2016) 120–130. [2] Q. Gul, M. Zakria, T.M. Khan, A. Mahmood, A. Iqbal, Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation, Mater. Sci. Semicond. Process. 19 (2014) 17–23. [3] G. Lastra, A. Olivas, J.I. Mejía, M.A. Quevedo-López, Thin-films and transistors of pZnTe, Solid-State Electron. 116 (2016) 56–59. [4] Z.T. Kang, C.J. Summers, H. Menkara, B.K. Wagner, R. Durst, Y. Diawara, G. Mednikova, T. Thorson, ZnTe:O phosphor development for x-ray imaging applications, Appl. Phys. Lett. 88 (2006) 1119041–1119043. [5] S. Ulicna, P.J.M. Isherwood, P.M. Kaminski, J.M. Walls, J. Li, C.A. Wolden, Development of ZnTe as a back contact material for thin film cadmium telluride solar cells, Vacuum 139 (2017) 159–163. [6] C.A. Wolden, A. Ali, J. Li, et al., The roles of ZnTe buffer layers on CdTe solar cell performance, Sol. Energy Mater. Sol. Cells 147 (2016) 203–210. [7] C.A. Wolden, A. Abbas, J. Li, D.R. Diercks, D.M. Meysing, T.R. Ohno, J.D. Beach, T.M. Barnes, J.M. Walls, solar energy materials & solar cells the roles of ZnTe buffer layers on CdTe solar cell performance, Solar Energy Mater. Sol. Cells 147 (2016) 203–210. [8] M. Ahonen, M. Pessa, A study of ZnTe films grown on glass substrates using an atomic layer evaporation method, Thin Solid Films 65 (1980) 301–307. [9] B. Ghosh, D. Ghosh, S. Hussain, R. Bhar, A.K. Pal, Growth of ZnTe films by pulsed laser deposition technique, J. Alloy. Compd. 541 (2012) 104–110. [10] K.C. Bhahada, B. Tripathi, N.K. Acharya, P.K. Kulriya, Y.K. Vijay, Formation of ZnTe by stacked elemental layer method, Appl. Surf. Sci. 255 (2008) 2143–2148. [11] B.B. Wang, M.K. Zhu, N. Hu, L.J. Li, Raman scattering and photoluminescence of zinc telluride nano powders at room temperature, J. Lumin. 131 (2011) 2550–2554. [12] G.K. Rao, K.V. Bangera, G.K. Shivakumar, Influence of substrate temperature and post deposition annealing on the properties of vacuum deposited ZnSe thin films, Mater. Sci. Semicond. Process. 16 (2013) 269–273. [13] K.N. Shreekanthan, K.V. Bangera, G.K. Shivakumar, M.G. Mahesha, Structure and properties of vacuum deposited cadmium telluride thinfilms, Indian J. Pure Appl. Phys. 44 (2006) 705–708. [14] V.K. Ashith, G.K. Rao, A study of microstructural properties and quantum size effect in SILAR deposited nano-crystalline CdS thin films, Thin Solid Films 616 (2016) 197–203. [15] Z. Khorsand, A.A. Majid, W.H. Abrishami, M.E. Yousefi, Ramin, X-ray analysis of
Fig. 9. CIE chromaticity diagram for as-deposited (spot a) and annealed (spot b) ZnTe films.
on the basis of absorbance spectra. Observation suggests that carrier life time may decrease upon annealing for ZnTe thin films which is essential for photodetector device suitability at high frequency regime. Luminescence property of ZnTe films can be fine-tuned by annealing as evident from CIE plot which could be due to incorporation of oxygen in the films. Acknowledgement The authors are grateful to VGST, Govt. of Karnataka State India (VGST/K-FIST(L1)/GRD-377/2014-15) for financial assistance. 262
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[19] O.D. Melo, S. Larramendi, G. Contreras-Puente, M. Behar, S. Rodríguez-López, D.G. Trabada, M. Hernández-Vélez, ZnO nanosheet network formation by ZnTe oxidation in humid argon atmosphere annealing, Mater. Lett. 81 (2012) 202–204. [20] L. Trichy, H. Ticha, Correlation between photo-induced red shift of the optical band gap and the slope of Urbach edge in amorphous and glassy As2S3, Mater. Lett. 164 (2016) 232–234. [21] B. Ullrich, A.K. Singh, M. Bhowmick, P. Barik, D. Ariza-Flores, H. Xi, J.W. Tomm, Photoluminescence lineshape of ZnO, AIP Adv. 4 (2014) 123001(1)–123001(4). [22] R.S. Zeferino, M.B. Flores, U. Pal, Photoluminescence and Raman scattering in Agdoped ZnO nanoparticles, J. Appl. Phys. 109 (2011) 14308-1–14308-6.
ZnO nanoparticles by Williamson-Hall and size-strain plot methods, Solid State Sci. 13 (2011) 251–256. [16] D. Sarkar, G. Sanjeev, M.G. Mahesha, Influence of electron beam irradiation on structural and optical properties of thermally evaporated GeTe thin films, Radiat. Phys. Chem. 98 (2014) 64–68. [17] M.Z. Ansari, N. Khare, Effect of intrinsic strain on the optical band gap of single phase nanostructured Cu2ZnSnS4, Mater. Sci. Semicond. Process. 63 (2017) 220–226. [18] M. Hossain, H. Kabir, M.M. Rahman, et al., Understanding the shrinkage of optical absorption edges of nanostructured Cd-Zn sulphide films for photothermal applications, Appl. Surf. Sci. 392 (2017) 854–862.
263
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Analysis of PV deposited ZnTe thin films through Urbach tail and photoluminescence spectroscopy
MARK
⁎
Rashmitha Keshav, Meghavarsha Padiyar, Meghana N, Mahesha MG Department of Physics, Manipal Institute of Technology, Manipal University, 576104, India
A R T I C L E I N F O
A B S T R A C T
Keywords: II – VI compound Thin films Solar cell materials Photoluminescence Roosbroeck–Shockley relation
ZnTe thin films have been grown by thermal evaporation on glass substrates at room temperature at a residual pressure about 10−6 Torr and characterized for their structural and optical properties. Crystallite size and strain have been estimated from X-ray diffractogram by William – Hall method. Urbach tail, which originates near band edge due to structural disorder, has been evaluated along with optical bad gap from the absorbance spectra. Photoluminescence spectra has been recorded to get information on various excited levels. Correlation between optical absorbance and photoluminescence spectra has been established with Roosbroeck–Shockley relation. Finally effect of annealing on structural and optical properties along with photoluminescence spectra has been studied in detail.
1. Introduction II – VI compound semiconductors find extensive application in optoelectronic devices such as light emitting diodes, photodetectors, nonlinear optical devices and solar cells. Zinc telluride (ZnTe) is an important member of II – VI compound chalcogenides which has direct bandgap of about 2.26 eV [1]. ZnTe is a well suited material for IR detector, thin film transistors, phosphor in x-ray imaging and light emitting diodes in visible region [2–4]. In cadmium telluride (CdTe) based solar cells, achievement of high efficiency has been stalled due to challenge in formation of a stable ohmic back contact and in recent years, application of ZnTe as back contact or buffer layer has been explored by several research groups [5–7]. The defect states like vacancies, ad atoms, interstitials and anti-sites are having crucial role in deciding the electrical properties of the materials and hence, determining the properties of hetero structure formed with other materials. In the case of materials deposited in the thin film form by different deposition techniques, preparation method and preparation parameters dictate these defect states. There are reports available on ZnTe films deposited on glass substrate by various techniques [8–10] In the process of evaluation of a material for device application in the thin film form, it is inevitable to characterize various defects. Optical spectroscopy is the best suited technique to probe different defect states and hence, in the present paper, we report a study carried out to investigate the defect states in thermally evaporated ZnTe thin films using Urbach tail and photoluminescence (PL) spectroscopy. Apart from this, associated studies such as x-ray diffractogram (XRD), scanning electron microscopy
⁎
(SEM) and energy dispersive spectroscopy (EDS) have been carried out to get structural, morphological and compositional information which are correlated with spectroscopic results. 2. Experimental ZnTe thin films have been deposited on chemically cleaned glass substrate by thermal evaporation technique at residual pressure 10−6 Torr by taking 99.99% pure compound as source material. Deposition rate and the thickness have been monitored with digital thickness monitor operating at 6 MHz frequency. Films of about 300 nm thickness have been obtained by depositing the material at constant rate of 0.2 nm/s. Deposited films have been annealed at 473 K for about half an hour under air ambience. Structural study of pristine and annealed films has been carried out by recording XRD with Rigaku Miniflex – 600 X-ray diffractometer with Cu Kα (0.154 nm) at 40 kV, 15 mA in scanning range 20° to 70° at scan speed of 2°/min. Morphological changes induced in the films due to annealing has been analyzed by SEM model Zeiss SEM EVO18. Shimadzu UV-1800 PC spectrophotometer has been used to record absorption spectra in the wavelengths ranged from 400 to 1100 nm. The photoluminescence spectra has been recorded using Agilent Cary Eclise WinFLR photoluminescence spectrometer by using 480 nm wavelength as exciting source.
Corresponding author. E-mail address: [email protected] (M. MG).
http://dx.doi.org/10.1016/j.jlumin.2017.10.047 Received 5 September 2017; Received in revised form 11 October 2017; Accepted 16 October 2017 Available online 17 October 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 257–263
R. Keshav et al.
Fig. 1. X-ray diffractogram of ZnTe thin films (Inset: Enlarged plots near peaks). Fig. 2. Williamson – Hall (W – H) analysis of X-ray diffractogram of ZnTe thin films.
3. Results and discussion originate due to several reasons like the mismatch in thermal properties of film and substrate, imperfections including vacancy and interstitials. Lattice mismatch or difference between the thermal coefficient of film and substrate can induce uniform strain which causes a shift in the peak position. Broadening of the X-ray peak affirms point defects or poor crystallinity of the films. Hence it is suitable to adopt William-Hall (W – H) method to estimate crystallite size and internal strain [14]. In W – H method, it is assumed that broadening of X-ray peak due to size and strain is an additive component to total integral breadth of a Bragg peak. The W – H relation is given by [15],
3.1. Structural analysis XRD of pristine and annealed ZnTe thin films are depicted in Fig. 1. Soft analysis of XRD pattern reveals that the peak intensity has increased and peak position has shifted to lower angle after annealing. A detailed analysis has been done to index the peaks and to extract the parameters that assess the structural properties of the film and listed in Table 1. Detailed study has revealed that films have cubic structure with strong (111) orientation belonging to F-43m (216) space group. II – VI group chalcogenides normally found in cubic zinc blende or hexagonal wurtzite type structure or even in the mixed phase [5,11–13]. Thin film preparation technique and growth parameters have profound influence on the structure. Substrate temperature and growth rate are two important parameters among them. A published report revealed that low deposition rate and/or high substrate temperature favors hexagonal phase which is attributed to reduced vapor concentration reaching the substrate; exhibit cubic structure otherwise [13]. Crystallite size, micro-strain and dislocation density have also been estimated from XRD. From Scherrer formula, crystallite size (D) is given by,
D=
0.9λ β cos θ
β cos θ =
0.9λ +(4ε sin θ) D
(2)
where ε is the strain in the film. Eq. (2) assumes uniform deformation model according to which the strain is uniform in all crystallographic direction, thus considering the isotropic property of the crystal. Plot of β cosθ vs. 4 sin θ (Fig. 2) shows positive slope for both the films with correlation coefficient about 0.96. The strain may be due to thermal mismatch between films and substrate in addition to the contribution from defects. Crystallite size obtained from intercept of W – H plot is slightly higher than the value obtained in Scherrer formula. SEM micrographs have shown uniform growth of the films over considerably large area (Fig. 3). However there is difference in the elemental composition of the films compared with the source material as depicted by EDS in Fig. 4. Estimated elemental composition of as-deposited and annealed films is listed in Table 2, which shows that both the films are Zn rich. The difference in the composition of films deposited by thermal evaporation is mainly due to difference in the vapor pressure.
(1)
where λ is the wavelength of the X-ray source, β is FWHM of the peak and θ is Bragg angle. Crystallite size estimation shows that films are nano-crystalline (Table 1). Even though no much change in the crystallite size has been observed, there is change in the strain and dislocation density of the films. These quantities, namely, lattice strain ε = βcosθ/4 and dislocation density δ = 1/D 2 have also been tabulated in Table 1. In case of thin films deposited on the substrate, strain may Table 1 Analysis of XRD data of ZnTe thin films. Sample
As-deposited Annealed
2θ (°)
26.02 25.96
d (nm)
0.342 0.343
hkl
(111) (111)
a=b=c (nm)
0.592 0.594
Cell volume
0.207 0.209
Crystallite size D(nm)
Micro-strain ε (×10−4)
Scherrer formula
W-H plot
βcosθ/4
W-H plot
4.43 3.94
8.15 6.60
7.8 8.8
11.3 15.9
258
Dislocation density (D values estimated from W – H plot are used) Δ (m−2)
1.50 × 1016 2.29 × 1016
Journal of Luminescence 194 (2018) 257–263
R. Keshav et al.
Fig. 3. SEM micrographs of (a) As-deposited, (b) Annealed ZnTe thin films and elemental mapping of Zn and Te for (c), (d) As-deposited (e), (f) Annealed ZnTe thin films.
where h is Planck's constant, ν is the frequency of incident radiation C is a constant that depends on the transition probability, Eg is the energy band gap and n depends on the characteristic of the transition (n = ½ for direct band gap and n = 2 for direct band gap). Confirmation of direct band gap type can be done by linearity in the plot of (αhν)2 vs. hν which is evident from Fig. 6a. Pristine films have band gap about 2.20 eV as estimated from extrapolation of linearity in the graph and annealing resulted in slight reduction in the bad gap value. This manifestation of minor change in the optical band gap can be credited to combined effect of annealing induced change in the crystallite size and strain in the film [17].
3.2. Optical analysis Typical optical absorbance and transmittance spectra for as-deposited and annealed films are shown in Fig. 5. From the plot, it is evident that absorbance of the films has increased after annealing. Signature of interference fringes in the transmittance spectra indicates that the films are uniform. Tauc relation is the most reliable way to determine the electron transition type and the optical band gap from absorption coefficient (α) with the function of wavelength (λ) [16]:
αhν = C (hν − Eg )n
(3)
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suitability of the materials in thin film form. Defect induced local states can be characterized by studying the spectral dependence of absorption coefficient at lower photon energy regime which is less than band gap energy. This low energy region, so called Urbach tail, characterizes the slope of the exponential edge [18]
E α = α 0exp ⎛ ⎞ E ⎝ u⎠ ⎜
⎟
(4)
where α 0 is a temperature dependent constant and Eu is Urbach energy. Eu signifies the width of the tail of localized states or width of the exponential absorption edge. Urbach energy can be estimated from inverse of the slope of the plot Ln(α ) vs. hν and the estimated value for the as-deposited film is observed to be 0.078 eV (Fig. 6b). Further analysis has revealed that the Urbach energy has increased from 0.078eV to 0.084 eV after annealing. This variation could be due to change in crystallite size, strain and defect density as revealed by XRD. Air annealing always favors incorporation of oxygen into films, mostly in the form of oxides, which in turn can degrade the film [19]. This particular effect may lead to slight change in the crystallite size also. Shift in the Urbach energy and Tauc gap have followed the same trend implying strong correlation between them. This correlation suggests that the transitions across the band gap and the transitions responsible for the Urbach edge are the electronic transitions which are not distinct except for the differences in the relevant densities of states. Such strong association between these two parameters is recently reported elsewhere also [20]. The steepness parameter, σ can be calculated from following relation [18]: Fig. 4. Energy dispersive spectra of (a) As-deposited, (b) Annealed ZnTe thin films.
σ= Table 2 Elemental composition of ZnTe thin films obtained from EDS. Element
Zn L Te L
As-deposited films Atomic%
Weight%
Atomic%
38.03 61.97
54.50 45.50
37.83 62.17
54.29 45.71
(5)
where kB is Boltzmann constant and T is absolute temperature. Eq. (5) characterizes the broadening/shrinkage of optical absorption edges due to electron – phonon or exciton – phonon interactions. The increase in the Urbach energy or decrease in the steepness parameter from 0.33 to 0.30 upon annealing can be assigned to increase in the localized states initiated from the non-radiative recombination centers.
Annealed films
Weight%
kB T Eu
3.3. Photo-luminescence study Defect states in thin film semiconductors have major role in deciding the electrical properties of the film. Hence characterization of these defect states is very essential for the assessment of device
For as-deposited films, photo-luminescence (PL) spectra along with de-convoluted peaks is depicted in Fig. 7a. Non-symmetry in the most intense peak indicates the convolution of multiple peaks.
Fig. 5. (a) Absorbance spectra and (b) Transmittance spectra of ZnTe thin films in the wavelength range 400–1100 nm.
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Fig. 6. (a) Plot of (αhν)2 vs. hν for ZnTe films (b) Plot of Ln(α) vs. hν for ZnTe films.
(Fig. 7b). In our present investigation, for annealed films, we have not observed any considerable change in effective sample thickness; but a marginal increase in carrier energy is observed. Improved crystallinity of the sample which is depicted in XRD may be the possible reason for this change. This improvement in carrier energy may lead to reduction in the carrier life time and, hence making the films suitable for photodetector application at higher frequency. Peaks P1 and P2 are attributed to deep and shallow band transitions whereas P3 may be due to defect. Comparison of the PL spectra of as-deposited and annealed films is represented in Fig. 8. Peak P1 position has not changed much after annealing, however the broadening has been observed in the width of the peak. Peak P2 has shown significant red shift and, also broadening in the width. Broadening of the emission peak is the indication of change in the lattice environment. And the change in the composition or doping can lead to shift in the peak position. Interstitial, vacancy and anti-site are commonly observed defect states which play a major role in deciding the electrical and opto-electronic properties of these films. Peak P3 may be due to such defect states. P4 is the additional peak observed in the annealed films which could be due to oxidized states in the films [22]. The CIE plot for both as-deposited and annealed films is represented in Fig. 9 which shows significant shift in the emission color. This indicates that air annealing or incorporation of oxygen can be used as luminescence tuning tool.
Fig. 7. (a) Photoluminescence spectra of as-deposited ZnTe thin films showing deconvoluted peaks (b) Comparison of experimental and calculated PL intensity in 450–550 nm range.
Deconvolution resulted in three different peaks P1, P2 and P3 (Table 3) with χ2 = 0.986. Literatures show that strong coupling exists between absorbance and PL spectra of materials. The intrinsic relationship between absorption and PL is given by Roosbroeck–Shockley relation (RSR) [21]:
IPL (hν )∝
4. Conclusion ZnTe thin films grown by thermal evaporation have exhibited cubic structure with preferential (111) orientation. Films are slightly Zn rich and have not shown any considerable change in composition upon annealing. However, strain in the films increased due to annealing along with marginal change in crystallite size. Minor change in the band gap energy and Urbach energy can be assigned to combined effect of crystallite size and strain. Normalized PL spectra in 450–550 nm range has shown good agreement with normalized intensity calculated
(hν )2×(1−exp[−α (hν ) deff ]) exp (hν / kTc )−1
(6)
where deff is the effective sample thickness, defines depth of the volume contribution to PL, and kTc is the carrier energy which are the two fitting parameters. For pristine samples, these fitting parameters were observed to be 0.28 ± 0.01 µm and 7.3 ± 0.1 × 10–21 J respectively Table 3 Results of deconvolution of PL spectra of ZnTe thin films. Peak →
P1
Sample
λ (nm)
FWHM
λ (nm)
FWHM
λ (nm)
FWHM
λ (nm)
FWHM
As-deposited Annealed
491 492
4 38
496 503
6 12
605 609
9 14
– 742
– 10
P2
P3
261
χ2
P4
0.986 0.983
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Fig. 8. (a) Comparison of photoluminescence spectra of as-deposited and annealed ZnTe thin films, (b), (c), (d) Comparison of different deconvoluted peaks of as-deposited and annealed ZnTe thin films.
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Fig. 9. CIE chromaticity diagram for as-deposited (spot a) and annealed (spot b) ZnTe films.
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