- Email: [email protected]
Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation
Materials Science in Semiconductor Processing 19 (2014) 17–23
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation Qeemat Gul a,n, M. Zakria b, Taj Muhammad Khan b, Arshad Mahmood b, Amjid Iqbal b a b
International Islamic University, Islamabad, Pakistan National Institute of Lasers and Optronics (NILOP), Nilore, Islamabad, Pakistan
a r t i c l e i n f o
abstract
Available online 18 December 2013
The present paper reports on a systematic study of the Cu doping effect on the optical, electrical and structural properties of ZnTe:Cu (Cu ¼ 0, 6, 8, and 10 at%) thin films. Polycrystalline Cu-doped ZnTe thin films were deposited on glass substrates at room temperature by thermal evaporation. A detailed characterization of the Cu-doped ZnTe films were performed by X-ray diffraction (XRD), Spectrophotometry, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. XRD of the as-deposited Cu-doped ZnTe films belong to single-phase cubic structure of ZnTe with preferential orientation along (111) planes revealed minor effect of Cu content. The interference pattern in optical transmission spectra was analyzed to determine energy band gap, refractive index, extinction coefficient and thickness of the films. Wemple–DiDomenico and Tauc's relation were used for the determination and comparison of optical band gap values. The formation of ZnTe and Cu-doped ZnTe phase was confirmed by FT-IR. AC conductivity in a frequency range of 0–7 MHz has been studied for investigation of the carriers hoping dynamics in the films. Raman spectra indicated merely typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194 cm 1 because the excitation energy is well above of the optical band-gap of the material and exhibited a blue-shift from 194 to 203 cm 1 with Cu which could be associated to the substitution of Zn atom with Cu at the lattice sites. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Zn1 xCuxTe XRD Band gap FT-IR Raman spectroscopy AC conductivity
1. Introduction During the past few decades, II–VI compound semiconductors have engrossed considerable attention because of their wide use in the fabrication of solar cells and optoelectronic devices [1]. Among the wide band gap II–VI semiconductor materials, ZnTe (band gap 2.26 eV at room temperature) is fantastic material used in IR detectors, solar cells as well as in the field of light emitting devices (LED) in visible and near UV optical range [2–4].
n Corresponding author at: International Islamic University (IIU) Islamabad, 44000, Pakistan. Tel.: þ92 345 4514101. E-mail address: [email protected] (Q. Gul).
1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.11.033
Moreover, ZnTe have been used as optorefractive materials for optical data processing ray detectors and non-polarized memory switching [5,6]. One of the useful application of ZnTe is high efficiency stable electrical back contacts and a small valance band discontinuity for CdTe based solar cells, which have required low resistive p-type ZnTe. To finely tune the structural, optical and electrical properties of ZnTe thin films, it is required to add some proper element as dopant in ZnTe. Cu is transition element and can be doped degenerately into ZnTe, which leads to decrease in its electrical resistance. Furthermore, Cu can be easily substituted into ZnTe lattice sites due to its abundant electronic shell structure and close ionic radius of Cu þ 2 (73 pm) with Zn þ 2 (74 pm) [7–10]. Several researchers have employed various deposition techniques
18
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
like co-evaporation, electro-deposition, RF and dc sputtering, hot wall evaporation, diffusion, ion-exchange process, etc. [11–16]. However, very little information is available on the Cu-doped ZnTe thin films prepared by vacuum thermal evaporation technique. In regards to this, an effort has been made to prepare thin films of ZnTe:Cu (Cu¼ 0, 6, 8, and 10 at%) by vacuum thermal evaporation technique which is more common, economical and conventionally simple process for the deposition of Cu-doped ZnTe films and accompanied materials of this group for the fabrication of solar cell. In the present work the effect of Cu concentration in ZnTe regarding structural, optical, electrical and vibrational properties have been investigated with the aim to explore an efficient material for solar cells and other optoelectronics devices. To our knowledge, this article describes elegantly on the Spectrophotometry, FT-IR and presents a detailed analysis of Raman spectroscopy for ZnTe:Cu films prepared by thermal deposition technique.
Fig. 1. XRD pattern of as-deposited ZnTe:Cu (0, 6, 8 and 10% Cu) thin films.
3. Results and discussion 3.1. Structural properties
2. Experimental work For the deposition of ZnTe:Cu thin films, targets were prepared by a conventional solid state reaction route method. The desired amount of highly pure (99.999%) ZnTe powder and Cu (0, 6, 8, and 10 at%) metal were taken and mixed for 2 h by pestle and mortar. The different compounds have been taken as follows: Wt. (grams) of ZnTe¼ΜW(1 Χ) with MW¼193.01 (ZnTe). Wt. (grams) of Cu¼atomic mass (AM) Χ with AM ¼63.55 (Cu), and X¼0.0, 0.06, 0.08, and 0.1. As these weights were being very large, we reduced them in the same proportion. Before vacuum, the targets and ultrasonically cleaned substrates were placed inside the chamber. The target material is placed in the molybdenum boat and thermally deposited on the rotating glass substrates at room temperature to achieve uniform thickness of films. The vacuum was 2 10 5 Torr during deposition of the films. The deposition processes were carried out for 24 min and repeated for all the samples while keeping the same deposition conditions. The structural properties of deposited films were analyzed by X-ray diffraction technique using Bruker D-8 X-ray Diffractometer with CuKα-radiation (λ¼1.5405 Å). The optical transmittance measurements of as-deposited ZnTe:Cu thin films relative to an uncovered glass substrate were recorded in the wavelength range of interest from 400 to 160 nm, using a UV–vis–NIR spectrophotometer model U4001 of Hitachi. The optical phonon modes and crystalline phases in the ZnTe:Cu samples were assessed by employing the Raman spectroscopy (model MST-4000A; DONGWOO OPTRON Co., Ltd.). He–Cd laser source of wavelength (λ) 442 nm and beam diameter 2.00 mm was used as excitation wavelength. The spectrum was taken by using a DM320 monochromator and ANDOR DV 401A-BV CCD software. Chemical bonds were analyzed by NICOLET 6700 Fourier Transform Infrared spectroscopy (FT-IR).
Fig. 1 shows the XRD profile of un-doped ZnTe and doped ZnTe thin films with different amount of Cu contents. All diffraction peaks in the XRD pattern for the asdeposited films belongs to zinc blende (cubic) structure of ZnTe with preferred orientation along the (111) plane. Doping of Cu into ZnTe with different concentrations does not significantly affect the crystal structure of ZnTe and hence no secondary phase were appeared for Cu. In-fact, Zn2 þ (74 pm) has close ionic radius with Cu2 þ (73 pm), which implies that Cu can easily penetrate into ZnTe to substitute Zn position in the crystal lattice resulting that Cu-doping does not change the crystal structure of ZnTe and single phase has been formed [16–18]. FENG et. al. also observed the cubic structure of ZnTe:Cu films deposited by thermal evaporation for 7 at% of Cu [11]. When Cu doping concentration in ZnTe was increased, the sharpness and diffraction intensity of (111) plane also increased. From the above results, it is quite evident that Cu doping enhanced the crystallization quality of deposited films. Actually the doping atoms replace the Zn sites in the host material and the Zn shifts to another point in the lattice result in an expansion of the crystallite size and therefore crystallinity has been improved. However, the crystallization quality dropped when Cu doping concentration was increased beyond 6.0 at%. In addition, the location of the (111) peak shifts to a little higher angle with Cu doping content increased from 0 to 10.0 at% as shown in Fig. 1. The slight shift in the peak stems from stress increase due to Cu interstitials and systematic incorporation of the Cu2 þ ions into the Zn2 þ lattice site without changing its cubic crystal structure [19]. The full width at half maximum (FWHM) of the diffraction peaks for Cu-doped ZnTe thin films are decreasing with Cu contents, which illustrates that Cu doping can considerably influence ZnTe crystallinity. The FWHM is inversely proportional to the crystallite size and can be related to the polycrystalline nature of thin films [20]. The effect of Cu-dopant concentration on the crystallite size of
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
19
Table 1 Calculated parameters for pure and Cu-doped ZnTe thin films. Compositional formula
Thickness (nm)
Crystallite size (nm)
Peak angle [12Th]
FWHM (Radian)
d-spacing [Å]
(h k l)
Tauc's Band gap (eV)
Band gap by Wemple– DiDomenico expression (eV)
Lattice constant a [Å]
Refractive index
ZnTe Zn0.94Cu0.06Te Zn0.92Cu0.08Te Zn0.90Cu0.10Te
276.33 267.40 218.78 292.13
64.66 66.31 67.42 67.71
25.380 25.420 25.420 25.460
0.178 0.149 0.149 0.119
3.506 3.501 3.501 3.495
C(111) C(111) C(111) C(111)
2.24 2.0 1.84 1.64
2.14 1.80 1.74 1.56
6.07 6.06 6.06 6.05
2.39 2.47 3.03 3.20
the films was determined by Scherrer's formula [21] D¼
0:9λ ðFWHMÞ cos θ
ð1Þ
Where λ is the wavelength of the X-ray (λ¼1.5406 Å), FWHM in radian, D is the crystallite size and θ is half of the angle between the incident and reflected X-ray beams. All the calculated values of as-deposited ZnTe and Cu-doped ZnTe are summarized in Table 1. These results imply that the crystallite size of polycrystalline thin films increases with increasing Cu contents as shown in Fig. 2. 3.2. Optical properties Optical transmission spectra of as-deposited films were performed by measuring transmittance of the films in the wavelength range 400–1600 nm. Fig. 3 shows the transmission (T) vs. wavelength (λ) traces of pure ZnTe and Cudoped ZnTe thin films. By neglecting the exciton effects, for direct band gap materials the absorption coefficient (α) and band gap energy (Eg) is described by Tauc's relation [22]. αhν ¼ AðEg hνÞ1=2
ð2Þ
Where “A” is a characteristic parameter (independent of photon energy) for respective transitions and (hυ) is photon energy. Consequently the values of band gap energy Eg can be found by extrapolating the linear portion of (αhυ)2 ¼f(hυ) curves to (αhυ)2 ¼0 as shown in Fig. 4. Satisfactory fit is obtained for (αhυ)2 vs. (hυ) representing the presence of a direct band gap materials. The band gaps of undoped and 10% Cu-doped ZnTe films were respectively determined as 2.24 eV and 1.64 eV by extrapolating the linear portion of these plots at (αhυ)2 ¼0, which results in the domination of direct allowed transition in the undoped and Cu-doped ZnTe films. The calculated band gap energies of ZnTe:Cu thin films are 2.24 eV, 2.0 eV, 1.84 eV and 1.64 eV for Cu (0, 6, 8, and 10 at%) contents respectively as shown in Fig. 4(a). This continuous change in the band gap of ZnTe by introduction of Cu indicates solid solution formation. The decrease in the band gap energy with increase of transition metal (Cu) doped II–IV compound semiconductors has been observed and can be best understand in terms of sp– d spin exchange interaction between band electrons and the localized “d” electrons of the transition metal ions
Fig. 2. Crystallite concentration.
sizes
of
ZnTe:Cu
thin
films
at
different
Cu
substituting the cations [21,22]. The relation of extinction co-efficient (k) vs. wavelength (λ) can be calculated by using the equation, k ¼ αλ=4π
ð3Þ
It is deduced from Fig. 5(b) that the extinction coefficient (k) increases with increase of Cu contents in ZnTe thin films. The refractive index and film thickness were determined using Swanepoel's method (method of envelopes around the maxima and minima of the transmission interference spectra). By this method the refractive index (n) at a specific wavelength (λ) can be calculated by the expression [23] h i 1 n ¼ NðN2 n2s Þ2 ð4Þ With ð1 þ n2s Þ TM Tm þ2ns N¼ 2 TMTm
ð5Þ
(ns) is the substrate refractive index (ns ¼ 1.51 for glass) TM and Tm are transmission maxima and minima which corresponds to respective wavelengths as shown in Fig. 6. Moreover, the film thickness (t) can be determined by the relation t¼M
λ1 λ2 2ðn2 λ1 n1 λ2 Þ
Where M ¼1 for two adjacent maxima or minima.
ð6Þ
20
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
Fig. 3. Optical transmission spectra of ZnTe:Cu thin films.
Fig. 4. (a) Optical band gap spectra of ZnTe:Cu films and (b) band gap vs. Cu-doping concentration.
due to decrease in crystallite boundaries, increase in packing density and improved crystallinity of thin films. The dispersion of refractive index “n(hν)” has been fitted by the Wemple–DiDomenico expression (single oscillator model) [22]. 1 n2 ðhνÞ 1
Fig. 5. Extinction co-efficient (K) vs. wavelength for each sample (In-set plot is the K vs. Cu concentrations).
The refractive index (n) vs. wavelength profile is indicated by Fig. 7. The value of refractive index of Cu-doped ZnTe thin film is higher than that of the ZnTe films. This is
¼
E2o – ðhνÞ2 E0 Ed
ð7Þ
Where Eo is oscillator energy, Ed is oscillator strength and hν is photon energy. The graphs are plotted between (n2 1) 1 and f[(hυ)2] as shown in Fig.8. The values of Ed and Eo have been calculated from the slope (i.e. (EdEo) 1) and intercept (Eo/Ed) of straight line fitted with the ordinate axis. For most of semiconductors it was found that Eo ¼2Ego, where Ego is the band gap energy [23,24]. The values of optical band gaps calculated from Wemple–DiDomenico expression are in close agreement with those determined from optical transmission spectra, both are given in Table 1.
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
Fig. 6. Enveloping of transmission curve for ZnTe:Cu (Cu ¼0%).
21
Fig. 8. Dispersion curve by Wemple–DiDomenico expression for all samples.
3.4. Raman analysis Room temperature Raman spectra was taken with 442 nm excitation wavelength using He–Cd laser source. Raman spectra are illustrated in Fig. 10. The spectra show merely the typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194 cm 1 because the excitation energy is well above the optical band-gap of the material [26]. The fundamental LO mode exhibited blue-shift from 194 to 203 cm 1 with Cu and could be associated to the substitution of Zn atom with Cu at the lattice sites. To better understand, the Raman frequency of the longitudinal and transverse optical phonons (LO, TO) according to classical theory is given by the following approximation [27]: Fig. 7. Refractive index (n) vs. wavelength (λ).
3.3. FT-IR analysis Analyzing chemical nature of the deposited films (bonding characteristic), IR transmittance spectra of both materials (pure and Cu-doped) were evaluated by Fourier Transform Infrared spectroscopy (FT-IR). The spectra of ZnTe and Cu-doped ZnTe thin films were recorded in the range of 400–3500 cm 1 as shown in Fig. 9. From the spectra, six frequency bands (marked as 1: 761.233 cm 1, 2: 878.683 cm 1, 3: 1412.852 cm 1, 4: 1591.745 cm 1, 5: 2856.247 cm 1, and 6: 2923.526 cm 1) are well evident. Absorption peak at 761.233 cm 1 corresponds to ZnTe while the peak at 878.683 cm 1 is assigned to substrate (Si–Te) dandling bond. The minute absorption peaks at 1412.852 cm 1 and 1591.745 cm 1, are related to Cu bonds with Tellurium. For the higher percentage composition of Cu, the peaks at 2856.247 cm 1 and 2923.526 cm 1 appeared and attributed to (O–H) molecule groups [25]. Because of strong absorption behavior of Cu for oxygen from the atmosphere, moisture on the surface of thin films could be expected which result in the absorption peaks due to OH molecules.
ωLO ffiωTO ffi
sffiffiffiffiffiffi K Mi
ð8Þ
where, Mi is the mass of the extrinsic impurity atom and K is the force constant, if Mi is mass of the impurity atom such that Mi oM (mass of the host atom), the vibrational frequency will be higher, such that ωi 4ω and because of this higher energy of the impurity atom than the optical phonon energy, no energy would be transferred to the lattice vibration. Nevertheless, in the case if Mi 4M, then ωi oω and the energy could be transferred to the lattice vibration. According to this approximation, the assignment may not be strange, and the local vibrational phonon modes of Cu substituting ZnTe should be located at higher wave numbers than those of pure ZnTe because of the smaller mass of Cu. The intensity of Raman mode gives information about interface and crystallinity. Furthermore, spatial correlation model is employed to explain and get insight of the broadening and asymmetry of the first-order laser Raman scattering optical phonon mode induced by the dopant potential fluctuation. Spatial correlation model is based on finite phonon mode correlations and related to q-vector relaxation induced by material disorder in the microstructure to account quantitatively for the observed findings and related detailed analysis. These results are well consisted with the previous reported data so far and
22
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
Fig. 9. FT-IR spectra of (a) un-doped, (b) 6% Cu, (c) 8% Cu and (d) 10% Cu-doped ZnTe thin films.
Fig. 10. Raman spectra of ZnTe:Cu thin films.
Fig. 11. AC conductivity (s) vs. frequency (f) for various Cu doping concentration of ZnTe:Cu films.
supported our XRD analysis for the claim of cubic ZnTe thin film [26,28]. 3.5. AC conductivity In the context of electrical conduction phenomenon, for characterizing the hopping dynamics of the charge carriers, the ac conductivity in most of the materials due to localized states is expressed by Jonscher's power law [29]: sðacÞ ¼ so þ Aωn
ð9Þ
Where “so” is the frequency-independent dc part of conductivity and found to be thermally activated with Arrhenius form. The pre-factor “A” is temperature dependent constant and the value of “n” depends on frequency and temperature. It is shown from Fig. 11 that all results of Cudoped ZnTe thin films have one threshold frequency f, separating the entire variation into two regions: (i) low frequency region, f of1, in which the conductivity is almost frequency independent called so. (ii) High frequency region, f 4f1, the conductivity increases linearly with the frequency. The values of n (slope) are determined
by taking logarithmic on both sides of Eq. (9). For f4f1 region, the values of exponent “n” lying between 0.05 and 0.64 implies that slope of the curve is less than one. This suggests that the conduction mechanism in this region is corresponding to the translational hopping motion with successful hops of carriers [30]. It is revealed from Fig. 11 that the AC conductivity increases as Cu concentration increases at room temperature, which is attributed to the crystallite sizes effect and increases in carrier/electrons concentration. As Cu content increases in the ZnTe films, the crystallite size increases as shown in XRD analysis, consequently the crystallite boundaries density decreases. This leads to less accumulated charge at the interfaces and results in the decrease in resistance [31]. 4. Conclusion In conclusion, thermal evaporation is a low-cost, simple and economical technique for the preparation of composite polycrystalline thin films even at room temperature. The deposited ZnTe:Cu films showed cubic structures and
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
phase singularity with no impurity peaks of Cu, Te as separate phase. The optical constants (n, k), energy band gap, crystallite size, FWHM and lattice parameter were tuned with Cu incorporation. The energy band gap exhibited a red-shift from 2.24 to 1.64 eV as Cu concentration varied from 0.0 to 0.10 while the refractive index was increased and the dispersion followed single oscillator model. A comparison of the energy band gap determined by Wemple–DiDomenico expression and Tauc's relation are well consisted. For higher Cu concentration, FT-IR results showed Cu–Te bond and single phase chemical nature for low Cu contents. The crystallinity and interface quality of the films could be considerably enhanced with Cu as observed from the Raman and XRD results. Optical interference properties are used to enhance the Raman scattering from thin films and substrate interfaces which could be used as to measure the film quality. The variation of conductivity showed two regions of conduction, low frequency region corresponds to DC conduction and high frequency region due to successful translational hoping motion corresponding to exponent “n” value ranging between 0.05 and 0.64. Acknowledgments The authors wish to thank Photonics division (NILOP) particularly Nano devices group for helping and providing the required facilities for this research work. References [1] S.V. Borse, S.D. Chavhan, Ramphal Sharma, J. Alloys Compd. 436 (2007) 407–414. [2] J. Camacho, A. Cantarero, J. Appl. Phys. 92 (2002) 10. [3] P.V. Meyers, Sol. Cell 27 (1989) 91.
23
[4] U.M.K. Shahed, Z. Shimin, J. Electrochem. Soc. 142 (1995) 2539. [5] H. Zhou, A. Zebib, S. Trivedi, W.M.B. Duval, J. Cryst. Growth 167 (1996) 534. [6] T. Ota, K. Takashi, Solid State Electron. 16 (1973) 1089. [7] D.L. Baetzner, R. Wendt, A. Romeo, H. Zogg, A.N. Tiwari, Thin Solid Films 463 (2000) 361–362. [8] D. Rioux, D.W. Niles, H. Hochst, J. Appl. Phys. 73 (12) (1993) 8381. [9] N. Amin, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 41 (2002) 2834. [10] N.B. Chaure, J.P. Nair, R. Jayakrishan, V. Ganesan, R.K. Pandey, Thin Solid Films 78 (1998) 324. [11] L. Feng, D. Mao, J. Tang, R.T. Collins, J.U. Trefny, J. Electron. Mater. 25 (1996) 1442. [12] T.A. Gessert, A.R. Mason, P. Sheldon, A.B. Swartzlander, D. Niles, T. J. Coutts, J. Vac. Sci. Technol. A 14 (3) (1996) 806. [13] T.A. Gessert, T.J. Coutts, Photovoltaic Program Review Proceedings, 1993, p. 345. [14] I.S. Athwal, R.K. Bedi, J. Appl. Phys. 64 (11) (1988) 6345. [15] S.I. Koboyashi, N. Saito, Jpn. J. Appl. Phys. 19 (1980) 1199. [16] Akram K.S. Aqili, Asghari Maqsood, Appl. Surf. Sci. 167 (2001) 73–80. [17] V.S. John, T. Mahalingam, Solid State Electron. 49 (2005) 3–7. [18] J. Zhang, L. Feng, W. Cai, J. Zheng, Y. Cai, B. Li, L. Wu, Y. Shao, Thin Solid Films 414 (2002) 113–118. [19] W. Wang, G. Xia, J. Zheng, L. Feng, R. Hao, J. Mater. Sci. Mater. Electron. 18 (2007) 427–431. [20] M. Maniv, A. Zangvil, J. Appl. Phys. 49 (1978) 2787. [21] B.D. Cullity, Elements of X-ray Diffractions, Addison-Wesley, Reading, MA, 1978, 102. [22] G.I. Rusu, M. Diciu, C. Pirghie, E.M. Popa, Appl. Surf. Sci. 253 (2007) 9500–9505. [23] C.B. Fitzgerald, Appl. Surf. Sci. 247 (2005) 493–496. [24] E. Popa, G.I. Rusu, Phys. Low-Dim. Struct. 7 (2003) 43–54. [25] D.P. Padiyan, A. Marikini, K.R. Murli, Mater. Chem. Phys. 78 (2002) 51–58. [26] L. Feng, D. Mao, J. Tang, R.T. Collins, J.U. Trefny, J Electron. Mater. 25 (1996) 1422–1427. [27] Z.W. Dong, C.F. Zhang, H. Deng, G.J. You, S.X. Qian, Mater. Chem. Phys. 9 (2006) 160–163. [28] W. Szuszkiewicz, J.F. Morhange, E. Dynowska, E. Janik, W. H. Zaleszczyk, A. Presz, J.Z. Domagala, W. Caliebe, G. Karczewski, T. Wojtowicz, Mater. Sci.—Pol. 26 (2008) 1053–1059. [29] S. Mahboob, G. Prasad, G.S. Kumar, Bull. Mater. Sci. 29 (2006) 347. [30] M. Ram, S. Chakrabarti, J. Alloys Compd. 462 (2008) 214–219. [31] Ala'eddin A. Saif, P. Poopalan, J. Mater. Sci. Technol. 27 (9) (2011) 802–808.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation Qeemat Gul a,n, M. Zakria b, Taj Muhammad Khan b, Arshad Mahmood b, Amjid Iqbal b a b
International Islamic University, Islamabad, Pakistan National Institute of Lasers and Optronics (NILOP), Nilore, Islamabad, Pakistan
a r t i c l e i n f o
abstract
Available online 18 December 2013
The present paper reports on a systematic study of the Cu doping effect on the optical, electrical and structural properties of ZnTe:Cu (Cu ¼ 0, 6, 8, and 10 at%) thin films. Polycrystalline Cu-doped ZnTe thin films were deposited on glass substrates at room temperature by thermal evaporation. A detailed characterization of the Cu-doped ZnTe films were performed by X-ray diffraction (XRD), Spectrophotometry, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. XRD of the as-deposited Cu-doped ZnTe films belong to single-phase cubic structure of ZnTe with preferential orientation along (111) planes revealed minor effect of Cu content. The interference pattern in optical transmission spectra was analyzed to determine energy band gap, refractive index, extinction coefficient and thickness of the films. Wemple–DiDomenico and Tauc's relation were used for the determination and comparison of optical band gap values. The formation of ZnTe and Cu-doped ZnTe phase was confirmed by FT-IR. AC conductivity in a frequency range of 0–7 MHz has been studied for investigation of the carriers hoping dynamics in the films. Raman spectra indicated merely typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194 cm 1 because the excitation energy is well above of the optical band-gap of the material and exhibited a blue-shift from 194 to 203 cm 1 with Cu which could be associated to the substitution of Zn atom with Cu at the lattice sites. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Zn1 xCuxTe XRD Band gap FT-IR Raman spectroscopy AC conductivity
1. Introduction During the past few decades, II–VI compound semiconductors have engrossed considerable attention because of their wide use in the fabrication of solar cells and optoelectronic devices [1]. Among the wide band gap II–VI semiconductor materials, ZnTe (band gap 2.26 eV at room temperature) is fantastic material used in IR detectors, solar cells as well as in the field of light emitting devices (LED) in visible and near UV optical range [2–4].
n Corresponding author at: International Islamic University (IIU) Islamabad, 44000, Pakistan. Tel.: þ92 345 4514101. E-mail address: [email protected] (Q. Gul).
1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.11.033
Moreover, ZnTe have been used as optorefractive materials for optical data processing ray detectors and non-polarized memory switching [5,6]. One of the useful application of ZnTe is high efficiency stable electrical back contacts and a small valance band discontinuity for CdTe based solar cells, which have required low resistive p-type ZnTe. To finely tune the structural, optical and electrical properties of ZnTe thin films, it is required to add some proper element as dopant in ZnTe. Cu is transition element and can be doped degenerately into ZnTe, which leads to decrease in its electrical resistance. Furthermore, Cu can be easily substituted into ZnTe lattice sites due to its abundant electronic shell structure and close ionic radius of Cu þ 2 (73 pm) with Zn þ 2 (74 pm) [7–10]. Several researchers have employed various deposition techniques
18
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
like co-evaporation, electro-deposition, RF and dc sputtering, hot wall evaporation, diffusion, ion-exchange process, etc. [11–16]. However, very little information is available on the Cu-doped ZnTe thin films prepared by vacuum thermal evaporation technique. In regards to this, an effort has been made to prepare thin films of ZnTe:Cu (Cu¼ 0, 6, 8, and 10 at%) by vacuum thermal evaporation technique which is more common, economical and conventionally simple process for the deposition of Cu-doped ZnTe films and accompanied materials of this group for the fabrication of solar cell. In the present work the effect of Cu concentration in ZnTe regarding structural, optical, electrical and vibrational properties have been investigated with the aim to explore an efficient material for solar cells and other optoelectronics devices. To our knowledge, this article describes elegantly on the Spectrophotometry, FT-IR and presents a detailed analysis of Raman spectroscopy for ZnTe:Cu films prepared by thermal deposition technique.
Fig. 1. XRD pattern of as-deposited ZnTe:Cu (0, 6, 8 and 10% Cu) thin films.
3. Results and discussion 3.1. Structural properties
2. Experimental work For the deposition of ZnTe:Cu thin films, targets were prepared by a conventional solid state reaction route method. The desired amount of highly pure (99.999%) ZnTe powder and Cu (0, 6, 8, and 10 at%) metal were taken and mixed for 2 h by pestle and mortar. The different compounds have been taken as follows: Wt. (grams) of ZnTe¼ΜW(1 Χ) with MW¼193.01 (ZnTe). Wt. (grams) of Cu¼atomic mass (AM) Χ with AM ¼63.55 (Cu), and X¼0.0, 0.06, 0.08, and 0.1. As these weights were being very large, we reduced them in the same proportion. Before vacuum, the targets and ultrasonically cleaned substrates were placed inside the chamber. The target material is placed in the molybdenum boat and thermally deposited on the rotating glass substrates at room temperature to achieve uniform thickness of films. The vacuum was 2 10 5 Torr during deposition of the films. The deposition processes were carried out for 24 min and repeated for all the samples while keeping the same deposition conditions. The structural properties of deposited films were analyzed by X-ray diffraction technique using Bruker D-8 X-ray Diffractometer with CuKα-radiation (λ¼1.5405 Å). The optical transmittance measurements of as-deposited ZnTe:Cu thin films relative to an uncovered glass substrate were recorded in the wavelength range of interest from 400 to 160 nm, using a UV–vis–NIR spectrophotometer model U4001 of Hitachi. The optical phonon modes and crystalline phases in the ZnTe:Cu samples were assessed by employing the Raman spectroscopy (model MST-4000A; DONGWOO OPTRON Co., Ltd.). He–Cd laser source of wavelength (λ) 442 nm and beam diameter 2.00 mm was used as excitation wavelength. The spectrum was taken by using a DM320 monochromator and ANDOR DV 401A-BV CCD software. Chemical bonds were analyzed by NICOLET 6700 Fourier Transform Infrared spectroscopy (FT-IR).
Fig. 1 shows the XRD profile of un-doped ZnTe and doped ZnTe thin films with different amount of Cu contents. All diffraction peaks in the XRD pattern for the asdeposited films belongs to zinc blende (cubic) structure of ZnTe with preferred orientation along the (111) plane. Doping of Cu into ZnTe with different concentrations does not significantly affect the crystal structure of ZnTe and hence no secondary phase were appeared for Cu. In-fact, Zn2 þ (74 pm) has close ionic radius with Cu2 þ (73 pm), which implies that Cu can easily penetrate into ZnTe to substitute Zn position in the crystal lattice resulting that Cu-doping does not change the crystal structure of ZnTe and single phase has been formed [16–18]. FENG et. al. also observed the cubic structure of ZnTe:Cu films deposited by thermal evaporation for 7 at% of Cu [11]. When Cu doping concentration in ZnTe was increased, the sharpness and diffraction intensity of (111) plane also increased. From the above results, it is quite evident that Cu doping enhanced the crystallization quality of deposited films. Actually the doping atoms replace the Zn sites in the host material and the Zn shifts to another point in the lattice result in an expansion of the crystallite size and therefore crystallinity has been improved. However, the crystallization quality dropped when Cu doping concentration was increased beyond 6.0 at%. In addition, the location of the (111) peak shifts to a little higher angle with Cu doping content increased from 0 to 10.0 at% as shown in Fig. 1. The slight shift in the peak stems from stress increase due to Cu interstitials and systematic incorporation of the Cu2 þ ions into the Zn2 þ lattice site without changing its cubic crystal structure [19]. The full width at half maximum (FWHM) of the diffraction peaks for Cu-doped ZnTe thin films are decreasing with Cu contents, which illustrates that Cu doping can considerably influence ZnTe crystallinity. The FWHM is inversely proportional to the crystallite size and can be related to the polycrystalline nature of thin films [20]. The effect of Cu-dopant concentration on the crystallite size of
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
19
Table 1 Calculated parameters for pure and Cu-doped ZnTe thin films. Compositional formula
Thickness (nm)
Crystallite size (nm)
Peak angle [12Th]
FWHM (Radian)
d-spacing [Å]
(h k l)
Tauc's Band gap (eV)
Band gap by Wemple– DiDomenico expression (eV)
Lattice constant a [Å]
Refractive index
ZnTe Zn0.94Cu0.06Te Zn0.92Cu0.08Te Zn0.90Cu0.10Te
276.33 267.40 218.78 292.13
64.66 66.31 67.42 67.71
25.380 25.420 25.420 25.460
0.178 0.149 0.149 0.119
3.506 3.501 3.501 3.495
C(111) C(111) C(111) C(111)
2.24 2.0 1.84 1.64
2.14 1.80 1.74 1.56
6.07 6.06 6.06 6.05
2.39 2.47 3.03 3.20
the films was determined by Scherrer's formula [21] D¼
0:9λ ðFWHMÞ cos θ
ð1Þ
Where λ is the wavelength of the X-ray (λ¼1.5406 Å), FWHM in radian, D is the crystallite size and θ is half of the angle between the incident and reflected X-ray beams. All the calculated values of as-deposited ZnTe and Cu-doped ZnTe are summarized in Table 1. These results imply that the crystallite size of polycrystalline thin films increases with increasing Cu contents as shown in Fig. 2. 3.2. Optical properties Optical transmission spectra of as-deposited films were performed by measuring transmittance of the films in the wavelength range 400–1600 nm. Fig. 3 shows the transmission (T) vs. wavelength (λ) traces of pure ZnTe and Cudoped ZnTe thin films. By neglecting the exciton effects, for direct band gap materials the absorption coefficient (α) and band gap energy (Eg) is described by Tauc's relation [22]. αhν ¼ AðEg hνÞ1=2
ð2Þ
Where “A” is a characteristic parameter (independent of photon energy) for respective transitions and (hυ) is photon energy. Consequently the values of band gap energy Eg can be found by extrapolating the linear portion of (αhυ)2 ¼f(hυ) curves to (αhυ)2 ¼0 as shown in Fig. 4. Satisfactory fit is obtained for (αhυ)2 vs. (hυ) representing the presence of a direct band gap materials. The band gaps of undoped and 10% Cu-doped ZnTe films were respectively determined as 2.24 eV and 1.64 eV by extrapolating the linear portion of these plots at (αhυ)2 ¼0, which results in the domination of direct allowed transition in the undoped and Cu-doped ZnTe films. The calculated band gap energies of ZnTe:Cu thin films are 2.24 eV, 2.0 eV, 1.84 eV and 1.64 eV for Cu (0, 6, 8, and 10 at%) contents respectively as shown in Fig. 4(a). This continuous change in the band gap of ZnTe by introduction of Cu indicates solid solution formation. The decrease in the band gap energy with increase of transition metal (Cu) doped II–IV compound semiconductors has been observed and can be best understand in terms of sp– d spin exchange interaction between band electrons and the localized “d” electrons of the transition metal ions
Fig. 2. Crystallite concentration.
sizes
of
ZnTe:Cu
thin
films
at
different
Cu
substituting the cations [21,22]. The relation of extinction co-efficient (k) vs. wavelength (λ) can be calculated by using the equation, k ¼ αλ=4π
ð3Þ
It is deduced from Fig. 5(b) that the extinction coefficient (k) increases with increase of Cu contents in ZnTe thin films. The refractive index and film thickness were determined using Swanepoel's method (method of envelopes around the maxima and minima of the transmission interference spectra). By this method the refractive index (n) at a specific wavelength (λ) can be calculated by the expression [23] h i 1 n ¼ NðN2 n2s Þ2 ð4Þ With ð1 þ n2s Þ TM Tm þ2ns N¼ 2 TMTm
ð5Þ
(ns) is the substrate refractive index (ns ¼ 1.51 for glass) TM and Tm are transmission maxima and minima which corresponds to respective wavelengths as shown in Fig. 6. Moreover, the film thickness (t) can be determined by the relation t¼M
λ1 λ2 2ðn2 λ1 n1 λ2 Þ
Where M ¼1 for two adjacent maxima or minima.
ð6Þ
20
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
Fig. 3. Optical transmission spectra of ZnTe:Cu thin films.
Fig. 4. (a) Optical band gap spectra of ZnTe:Cu films and (b) band gap vs. Cu-doping concentration.
due to decrease in crystallite boundaries, increase in packing density and improved crystallinity of thin films. The dispersion of refractive index “n(hν)” has been fitted by the Wemple–DiDomenico expression (single oscillator model) [22]. 1 n2 ðhνÞ 1
Fig. 5. Extinction co-efficient (K) vs. wavelength for each sample (In-set plot is the K vs. Cu concentrations).
The refractive index (n) vs. wavelength profile is indicated by Fig. 7. The value of refractive index of Cu-doped ZnTe thin film is higher than that of the ZnTe films. This is
¼
E2o – ðhνÞ2 E0 Ed
ð7Þ
Where Eo is oscillator energy, Ed is oscillator strength and hν is photon energy. The graphs are plotted between (n2 1) 1 and f[(hυ)2] as shown in Fig.8. The values of Ed and Eo have been calculated from the slope (i.e. (EdEo) 1) and intercept (Eo/Ed) of straight line fitted with the ordinate axis. For most of semiconductors it was found that Eo ¼2Ego, where Ego is the band gap energy [23,24]. The values of optical band gaps calculated from Wemple–DiDomenico expression are in close agreement with those determined from optical transmission spectra, both are given in Table 1.
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
Fig. 6. Enveloping of transmission curve for ZnTe:Cu (Cu ¼0%).
21
Fig. 8. Dispersion curve by Wemple–DiDomenico expression for all samples.
3.4. Raman analysis Room temperature Raman spectra was taken with 442 nm excitation wavelength using He–Cd laser source. Raman spectra are illustrated in Fig. 10. The spectra show merely the typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194 cm 1 because the excitation energy is well above the optical band-gap of the material [26]. The fundamental LO mode exhibited blue-shift from 194 to 203 cm 1 with Cu and could be associated to the substitution of Zn atom with Cu at the lattice sites. To better understand, the Raman frequency of the longitudinal and transverse optical phonons (LO, TO) according to classical theory is given by the following approximation [27]: Fig. 7. Refractive index (n) vs. wavelength (λ).
3.3. FT-IR analysis Analyzing chemical nature of the deposited films (bonding characteristic), IR transmittance spectra of both materials (pure and Cu-doped) were evaluated by Fourier Transform Infrared spectroscopy (FT-IR). The spectra of ZnTe and Cu-doped ZnTe thin films were recorded in the range of 400–3500 cm 1 as shown in Fig. 9. From the spectra, six frequency bands (marked as 1: 761.233 cm 1, 2: 878.683 cm 1, 3: 1412.852 cm 1, 4: 1591.745 cm 1, 5: 2856.247 cm 1, and 6: 2923.526 cm 1) are well evident. Absorption peak at 761.233 cm 1 corresponds to ZnTe while the peak at 878.683 cm 1 is assigned to substrate (Si–Te) dandling bond. The minute absorption peaks at 1412.852 cm 1 and 1591.745 cm 1, are related to Cu bonds with Tellurium. For the higher percentage composition of Cu, the peaks at 2856.247 cm 1 and 2923.526 cm 1 appeared and attributed to (O–H) molecule groups [25]. Because of strong absorption behavior of Cu for oxygen from the atmosphere, moisture on the surface of thin films could be expected which result in the absorption peaks due to OH molecules.
ωLO ffiωTO ffi
sffiffiffiffiffiffi K Mi
ð8Þ
where, Mi is the mass of the extrinsic impurity atom and K is the force constant, if Mi is mass of the impurity atom such that Mi oM (mass of the host atom), the vibrational frequency will be higher, such that ωi 4ω and because of this higher energy of the impurity atom than the optical phonon energy, no energy would be transferred to the lattice vibration. Nevertheless, in the case if Mi 4M, then ωi oω and the energy could be transferred to the lattice vibration. According to this approximation, the assignment may not be strange, and the local vibrational phonon modes of Cu substituting ZnTe should be located at higher wave numbers than those of pure ZnTe because of the smaller mass of Cu. The intensity of Raman mode gives information about interface and crystallinity. Furthermore, spatial correlation model is employed to explain and get insight of the broadening and asymmetry of the first-order laser Raman scattering optical phonon mode induced by the dopant potential fluctuation. Spatial correlation model is based on finite phonon mode correlations and related to q-vector relaxation induced by material disorder in the microstructure to account quantitatively for the observed findings and related detailed analysis. These results are well consisted with the previous reported data so far and
22
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
Fig. 9. FT-IR spectra of (a) un-doped, (b) 6% Cu, (c) 8% Cu and (d) 10% Cu-doped ZnTe thin films.
Fig. 10. Raman spectra of ZnTe:Cu thin films.
Fig. 11. AC conductivity (s) vs. frequency (f) for various Cu doping concentration of ZnTe:Cu films.
supported our XRD analysis for the claim of cubic ZnTe thin film [26,28]. 3.5. AC conductivity In the context of electrical conduction phenomenon, for characterizing the hopping dynamics of the charge carriers, the ac conductivity in most of the materials due to localized states is expressed by Jonscher's power law [29]: sðacÞ ¼ so þ Aωn
ð9Þ
Where “so” is the frequency-independent dc part of conductivity and found to be thermally activated with Arrhenius form. The pre-factor “A” is temperature dependent constant and the value of “n” depends on frequency and temperature. It is shown from Fig. 11 that all results of Cudoped ZnTe thin films have one threshold frequency f, separating the entire variation into two regions: (i) low frequency region, f of1, in which the conductivity is almost frequency independent called so. (ii) High frequency region, f 4f1, the conductivity increases linearly with the frequency. The values of n (slope) are determined
by taking logarithmic on both sides of Eq. (9). For f4f1 region, the values of exponent “n” lying between 0.05 and 0.64 implies that slope of the curve is less than one. This suggests that the conduction mechanism in this region is corresponding to the translational hopping motion with successful hops of carriers [30]. It is revealed from Fig. 11 that the AC conductivity increases as Cu concentration increases at room temperature, which is attributed to the crystallite sizes effect and increases in carrier/electrons concentration. As Cu content increases in the ZnTe films, the crystallite size increases as shown in XRD analysis, consequently the crystallite boundaries density decreases. This leads to less accumulated charge at the interfaces and results in the decrease in resistance [31]. 4. Conclusion In conclusion, thermal evaporation is a low-cost, simple and economical technique for the preparation of composite polycrystalline thin films even at room temperature. The deposited ZnTe:Cu films showed cubic structures and
Q. Gul et al. / Materials Science in Semiconductor Processing 19 (2014) 17–23
phase singularity with no impurity peaks of Cu, Te as separate phase. The optical constants (n, k), energy band gap, crystallite size, FWHM and lattice parameter were tuned with Cu incorporation. The energy band gap exhibited a red-shift from 2.24 to 1.64 eV as Cu concentration varied from 0.0 to 0.10 while the refractive index was increased and the dispersion followed single oscillator model. A comparison of the energy band gap determined by Wemple–DiDomenico expression and Tauc's relation are well consisted. For higher Cu concentration, FT-IR results showed Cu–Te bond and single phase chemical nature for low Cu contents. The crystallinity and interface quality of the films could be considerably enhanced with Cu as observed from the Raman and XRD results. Optical interference properties are used to enhance the Raman scattering from thin films and substrate interfaces which could be used as to measure the film quality. The variation of conductivity showed two regions of conduction, low frequency region corresponds to DC conduction and high frequency region due to successful translational hoping motion corresponding to exponent “n” value ranging between 0.05 and 0.64. Acknowledgments The authors wish to thank Photonics division (NILOP) particularly Nano devices group for helping and providing the required facilities for this research work. References [1] S.V. Borse, S.D. Chavhan, Ramphal Sharma, J. Alloys Compd. 436 (2007) 407–414. [2] J. Camacho, A. Cantarero, J. Appl. Phys. 92 (2002) 10. [3] P.V. Meyers, Sol. Cell 27 (1989) 91.
23
[4] U.M.K. Shahed, Z. Shimin, J. Electrochem. Soc. 142 (1995) 2539. [5] H. Zhou, A. Zebib, S. Trivedi, W.M.B. Duval, J. Cryst. Growth 167 (1996) 534. [6] T. Ota, K. Takashi, Solid State Electron. 16 (1973) 1089. [7] D.L. Baetzner, R. Wendt, A. Romeo, H. Zogg, A.N. Tiwari, Thin Solid Films 463 (2000) 361–362. [8] D. Rioux, D.W. Niles, H. Hochst, J. Appl. Phys. 73 (12) (1993) 8381. [9] N. Amin, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 41 (2002) 2834. [10] N.B. Chaure, J.P. Nair, R. Jayakrishan, V. Ganesan, R.K. Pandey, Thin Solid Films 78 (1998) 324. [11] L. Feng, D. Mao, J. Tang, R.T. Collins, J.U. Trefny, J. Electron. Mater. 25 (1996) 1442. [12] T.A. Gessert, A.R. Mason, P. Sheldon, A.B. Swartzlander, D. Niles, T. J. Coutts, J. Vac. Sci. Technol. A 14 (3) (1996) 806. [13] T.A. Gessert, T.J. Coutts, Photovoltaic Program Review Proceedings, 1993, p. 345. [14] I.S. Athwal, R.K. Bedi, J. Appl. Phys. 64 (11) (1988) 6345. [15] S.I. Koboyashi, N. Saito, Jpn. J. Appl. Phys. 19 (1980) 1199. [16] Akram K.S. Aqili, Asghari Maqsood, Appl. Surf. Sci. 167 (2001) 73–80. [17] V.S. John, T. Mahalingam, Solid State Electron. 49 (2005) 3–7. [18] J. Zhang, L. Feng, W. Cai, J. Zheng, Y. Cai, B. Li, L. Wu, Y. Shao, Thin Solid Films 414 (2002) 113–118. [19] W. Wang, G. Xia, J. Zheng, L. Feng, R. Hao, J. Mater. Sci. Mater. Electron. 18 (2007) 427–431. [20] M. Maniv, A. Zangvil, J. Appl. Phys. 49 (1978) 2787. [21] B.D. Cullity, Elements of X-ray Diffractions, Addison-Wesley, Reading, MA, 1978, 102. [22] G.I. Rusu, M. Diciu, C. Pirghie, E.M. Popa, Appl. Surf. Sci. 253 (2007) 9500–9505. [23] C.B. Fitzgerald, Appl. Surf. Sci. 247 (2005) 493–496. [24] E. Popa, G.I. Rusu, Phys. Low-Dim. Struct. 7 (2003) 43–54. [25] D.P. Padiyan, A. Marikini, K.R. Murli, Mater. Chem. Phys. 78 (2002) 51–58. [26] L. Feng, D. Mao, J. Tang, R.T. Collins, J.U. Trefny, J Electron. Mater. 25 (1996) 1422–1427. [27] Z.W. Dong, C.F. Zhang, H. Deng, G.J. You, S.X. Qian, Mater. Chem. Phys. 9 (2006) 160–163. [28] W. Szuszkiewicz, J.F. Morhange, E. Dynowska, E. Janik, W. H. Zaleszczyk, A. Presz, J.Z. Domagala, W. Caliebe, G. Karczewski, T. Wojtowicz, Mater. Sci.—Pol. 26 (2008) 1053–1059. [29] S. Mahboob, G. Prasad, G.S. Kumar, Bull. Mater. Sci. 29 (2006) 347. [30] M. Ram, S. Chakrabarti, J. Alloys Compd. 462 (2008) 214–219. [31] Ala'eddin A. Saif, P. Poopalan, J. Mater. Sci. Technol. 27 (9) (2011) 802–808.