- Email: [email protected]
Effect of zinc doping and temperature on the properties of sprayed CuInS2 thin films
Materials Science in Semiconductor Processing 15 (2012) 522–530
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of zinc doping and temperature on the properties of sprayed CuInS2 thin films C. Mahendran a, N. Suriyanarayanan b,n a b
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts & Science, Coimbatore 641020, Tamil Nadu, India Department of Physics, Government College of Technology, Coimbatore 641013, Tamil Nadu, India
a r t i c l e in f o
abstract
Available online 18 April 2012
Copper indium disulfide (CuInS2) is an efficient absorber material for photovoltaic and solar cell applications. The structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified by suitably doping CuInS2 thin films with dopants such as Zn, Sn, Bi, Cd, Na, N, O, P and As. In this work Zn (0.01 M) doped CuInS2 thin films are (Cu/In¼ 1.25) deposited on to glass substrates in the temperature range 300–400 1C. It is observed that the film growth temperature, ion ratio (Cu/In¼ 1.25) and Zn-doping affect structural, optical, photoluminescence and electrical properties of sprayed CuInS2 thin films. As the XRD patterns depict, Zn-doping facilitates the growth of CuInS2 thin films along (112) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Zn in the films. The optical studies show, about 90% of light transmission occurs in the IR regions; hence Zn-doped CuInS2 can be used as an IR transmitter. The absorption coefficient (a) in the UV–visible region is found to be in the order of 104–105 cm 1 which is the optimum value for an efficient absorber. The optical band gap energies increase with increase of temperatures (1.66–1.78 eV). SEM photographs reveal crystalline and amorphous nature of the films at various temperature ranges. Photoluminescence study shows that well defined broad Blue and Green band emissions are exhibited by Zn-doped CuInS2 thin films. All the films present low resistivity (r) values and exhibit semiconducting nature. An evolution of p-type to n-type conductivity is obtained in the temperature range 325–350 1C. Hence, Zn species can be used as a donor and acceptor impurity in CuInS2 thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Thin films Optical properties X-ray diffraction Luminescence Spray pyrolysis
1. Introduction Chalcopyrite semiconductors CuInE2 (E¼S, Se, or Te) are currently investigated for applications in photovoltaic, solar cells [1] and LED devices [2]. These applications are based on a p–n junction. Controlled doping of dopants on these materials appears desirable, where the aimed free carrier concentrations are in the range of 1016–1018 cm 3.
n
Corresponding author. E-mail address: [email protected] (N. Suriyanarayanan).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.04.001
Further, intrinsic doping of chalcopyrite semiconductors is difficult, owing to the following reasons: (a) 12 types of possible intrinsic defects can compensate extrinsic donors (or) acceptors [3]; (b) depending on site occupation, acceptors and donors can be formed by the same dopant; (c) low dissociation temperature of chalcopyrite crystals hampers diffusion doping; (d) secondary phases combining host and doping elements can form. These possible effects limit the number of elements for extrinsic doping of chalcopyrites. Moreover, it appears that the applied doping technique is also very important in determining their properties. Among the ternary compound semiconductors
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
CuInS2 has the potential for high conversion efficiency due to its direct band gap of 1.5 eV [4], which very well matches with the solar spectrum and its high absorption coefficient of approximately 105 cm 1 [5]. No toxic component is included in this compound semiconductor. A variety of methods has been employed to deposit CIS films, including a rapid thermal process [6], single source thermal evaporation [7], co-evaporation from elemental sources [8], sulfurization of metallic precursors [9], chemical vapor deposition [10], sputtering [11,12], electrodeposition [13], and double source thermal evaporation [14]. Among them, spray pyrolysis is an attractive, low-cost method and the application of that enables the deposition of the films of larger area with good uniformity [14]. For controlling a conduction type and obtaining low resistivity, several impurities doped CuInS2 thin films have been studied. Doping of iron during the crystal growth of CuInS2 by chemical vapor transport is investigated [15,16]. Electrical properties of cadmium and zinc doped CuInS2 crystal grown by the Bridgman technique is studied by Mittleman and Singh [17]. Ueng and Hwang [18] showed that the crystals annealed in Cd and zinc vapors exhibited shallow activation energies 0.02 and 0.025 eV, respectively. They also reported the electrical and photoluminescence properties of phosphorous and zinc doped CuInS2 crystals [18]. The study of p-type doping was done using V group elements such as N, P and As in CuInS2, which increase the Madelung energy [19] resulting in instabilities of ionic charge distribution in p-type doped CuInS2 crystals [20]. The difference in the electronic structures and compensation mechanism between n-type Zn and Cd-doped CuInS2 crystals is carried out [21]. The effects of Ti-incorporation in CuInS2 solar cells have been investigated. Solar cells made from Ti containing materials show an increase in open circuit voltage and increase in efficiency compared to the Ti free devices [22]. The influence of Al-doping in CuInS2 by spray pyrolysis on different substrates is analyzed [23]. A single phase of Sb-doped CuInS2 can be prepared by annealing the films above 400 1C in Ar (or) N2 environment [24]. The effect of Na doping on the properties of CuInS2 thin films has been investigated [25]. The effect of the doping element Sn in CuInS2 is succeeded [26] by annealing the Sn-doped films in vacuum. After annealing, the Sn-doped CuInS2 exhibits n-type conductivity [27]. In our previous paper, we presented the effect of Sb-doping on structural, optical and photoluminescence properties of CuInS2 thin films by spray pyrolysis [28]. But, the open circuit voltage of solar cell based CuInS2 thin films can be improved by controlled doping of small amounts of Zn [29]. Hence, this paper presents the structural, optical, photoluminescence and electrical properties of Zn-doped CuInS2 thin films prepared by spray pyrolysis. 2. Materials and methods Zn-doped CuInS2 thin films are deposited by spray pyrolysis onto glass substrates from aqueous solutions of CuCl2, InCl3, SC(NH2)2 and ZnCl2 using compressed air as the carrier gas. At first, aqueous solutions (0.1 M) of the salts are prepared, and then they are mixed with appropriate portions in order to have copper to indium molar
523
ratio (Cu/In¼ 1.25) and (CuþIn)/S fixed to 1 in the solution. The copper (II) chloride and indium (III) chloride are mixed and then thiourea solution is added. The resulting solution is doped with zinc chloride (ZnCl2) of (0.01 M). The solutions are prepared by dissolving in deionized water. Then the solution is sprayed using spray rates of 2 ml/min in air onto glass substrates (2.5 2.5 cm2) heated at different temperatures from 300 to 400 1C. The X-ray diffraction (XRD) patterns of sprayed films are recorded using the XPERT-PRO Gonio scan diffractometer with CuKa radiation. The phases are identified using JCPDS data files. The optical transmittance spectra are recorded in the wavelength range of 300–1100 nm using the double beam Beckman Ratio Recording spectrophotometer. The surface morphology of the film is investigated using a Jeol, JSM-6390, JM-Spot size 35. The compositional analysis is carried out using the energy dispersive X-ray spectroscopy (EDAX). Photoluminescence (PL) spectra of the film are recorded using a Cary Eclipse instrument in fluorescence emission scan mode with excitation wavelength of 485 nm. The electrical resistivity and conductivity studies are carried out by the four-probe method. 3. Result and discussions 3.1. Structural analysis The XRD patterns of Zn-doped polycrystalline CuInS2 thin films grown in the temperature range 300–400 1C are shown in Fig. 1. Characteristic peaks corresponding to (220) and (312) planes of CuInS2 phase with preferential orientation along (112) are recorded at 300–350 1C. The Zn concentrations and the temperature have strong effects on the formation of polycrystalline CuInS2. It is also observed that, the (112) peak intensity decreases with increasing temperatures [28,35] which are due to increase in defects and disorders developed in the films. However, few minor peaks of secondary phases with lower intensities such as In6S7 along (104), (116), and (301), Cu along (111) and Zn, Cu2S, CuIn6S7 (or) Cu/In/S/Zn alloys are formed, which could be responsible for additional reflections [30]. The sum of internal origins obeying the thermodynamics of solid solutions, to defect chemistry and the thermal gradient plays an important role in the formation of minor phases [31]. The higher mobility of Cu þ , Zn þ and its migration towards the surface layers are mainly responsible for additional Cu and Zn phases [32]. On further increase in temperature to 375 1C and beyond, the film becomes amorphous. The maximum peak intensity at (112) plane of Zn-doped CuInS2 is observed at 300 1C, which shows better crystallinity. Hence, at 300 1C the incorporation of Zn leads to an improvement of crystallinity by occupying sulfur, Cu and interstitial sites without affecting structure [17]. Further, an increase in temperature leads to saturation of sulfur, Cu and interstitial sites by Zn gradually and the formation of other crystallographic structures by secondary phases contributes the degradation of crystallinity [33,34]. The Zn-doped CuInS2 thin films are composed of compactly packed nano-sized crystals of grain size in the range of 3–5 nm, calculated
524
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
Fig. 1. XRD of sprayed Zn-doped CuInS2 film prepared at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
Fig. 2. EDAX spectrum of sprayed Zn-doped CuInS2 films prepared at 300 1C.
from Debye–Scherrer relation: D¼
0:9l b cos y
ð1Þ
D is the grain size in nm, l is the wavelength of ˚ b is the full-width at half-maximum of CuKa ¼1.5404 A, the peak in radian and y is the Bragg angle. The undoped films deposited in the temperature range 300–400 1C have shown polycrystalline growth along (112) preferred plane with chalcopyrite structure [35]. But, at 375 1C and beyond the film becomes amorphous. Further, the XRD patterns show the formation of binary sulfides containing metal
chlorides and thio-carbamide [36–38]. At higher temperatures (375–400 1C) oxidation contributes to the growth of In2O3 [35]. The average grain size is found to be in the range of 26–125 nm. 3.2. EDAX analysis EDAX results confirm the presence of Cu, In, S and Zn in the doped CuInS2 films (Fig. 2) and the calculated percentage of elemental compositions is given in Table 1. Better stoichiometry of the films are observed when oxygen is taken into account in the relative compositions. All the
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
525
Table 1 Elemental compositions of Zn-doped CuInS2 thin films deposited at various substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C. S. no.
Temperature (1C)
Cu (at%)
In (at%)
S (at%)
Zn (at%)
O (at%)
1 2 3 4 5
300 325 350 375 400
18.29 17.54 16.94 16.21 15.94
21.94 21.24 20.86 19.76 19.00
43.74 42.63 42.00 41.36 41.02
14.03 13.86 13.02 12.86 11.96
2.00 4.73 7.18 9.81 12.08
Zn-doped films contain oxygen at all deposition temperatures. There is a slight decrease in the concentrations of Cu, In, S and Zn from the films as the temperature increases from 300 to 400 1C. Oxygen is not only chemisorbed in all the films [39], but also an oxygen containing phase has been formed during the film growth. InOCl and/or ZnOCl could be formed very easily due to the thermal decomposition product of (NH4)2InCl5 H2O and/or (NH4)2ZnCl5 H2O, originally formed in the spray solution [40]. Without any chemical reaction, oxygen probably diffuses along grain boundaries of the films. It does not enter into crystallites but, is bonded at the surface and grain boundaries [41]. Hence, the properties of the films are not altered by oxygen content. The doped Zn in low concentration increases the oxygen content as the temperature increases (maximum of 12% is observed) and make the film with a better stoichiometry with reduced crystallographic defects [42]. But, better crystalline quality and purity of the films can be improved by increasing the substrate temperature above 400 1C in a flowing hydrogen atmosphere [41].
3.3. Optical properties The optical transmission spectra of the Zn-doped CuInS2 thin films in the wavelength range 300–1100 nm are shown in Fig. 3. About 50% of light transmission is observed in the UV–visible region when the substrate temperature is at 300 1C and the transmission increases as the substrate temperature increases from 325 to 400 1C. Nearly 80% transmission of light is measured in the UV– visible region when the substrate temperature is in the range of 325–400 1C. Improved optical transmission property is observed in the IR regions than the films obtained from vacuum and thermal evaporation methods [43,44] (which are not reported by other researchers). Nearly 70% of light transmission occurs in the IR regions when the substrate temperature is at 300 1C and it increases and reaches 90% as the temperature increases from 325 to 350 1C. Hence, in this temperature range Zn-doped CuInS2 thin films can be used as a good IR transmitter. It is attributed to the Zn atoms that occupy sulfur, Cu and other vacancies created by substrate temperatures. Further, the Zn atoms are localized near the surface as well as in the volume of the material [43]. The decrease in film thickness (Table 2) due to the rearrangement of atoms in the films may also be the reason for better transmittance. But at a temperature of 375 1C and
Fig. 3. Transmittance spectra of Zn-doped CuInS2 film sprayed at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C. Table 2 Variation of band gap energies (Eg) and thickness of the film with temperatures. Temperature (1C)
Thickness (nm)
Eg (eV)
300 325 350 375 400
992 978 969 967 952
1.66 1.70 1.72 1.75 1.78
beyond, the transmittance slightly decreases due to the amorphous nature of the film. The undoped films show similar behavior and 70% of light is transmitted by all the films, deposited in the temperature range 300–400 1C. No significant effect of light transmission is noticed as the temperature increases from 300 to 400 1C. Further, the reduction in film thickness from 300 to 400 1C has no effect on the optical transmittance [35]. The absorption coefficient (a) is calculated from the following relation [45]:
a¼
2:303 log 1=T t
ð2Þ
t is the thickness of the film and T is the transmittance. The dependence of the absorption coefficient (a) versus the photon energy hn of the Zn-doped CuInS2 thin film grown in the temperature range 300–400 1C is shown in Fig. 4(a). The films have relatively high absorption coefficient in the range 5 104–3.5 105 cm 1 in the UV–visible spectral region. Higher value of a is obtained when the substrate temperature is at 300 1C (a ¼3.5 105 cm 1) and its value decreases as the temperature increases from 325 to 400 1C. The absorption co-efficient value is found to be in the order of 104–105 cm 1. Hence, Zn-doped CuInS2 thin films grown in the temperature range 300–400 1C can be used as an efficient absorber for solar cells and photovoltaic applications in the UV–visible region. But in the IR regions, a slight absorption is noticed, when the film is at 300 1C and the
526
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
Fig. 4. (a) Absorption spectra of Zn-doped CuInS2 film sprayed at different substrate temperatures and (b) variation of optical band gap energy values (Eg) of sprayed Zn-doped CuInS2 films prepared at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
absorption is quite low in the temperature range 325–350 1C (90% of the transmission is observed as shown in Fig. 3). Hence, in this temperature range the Zn-doped CuInS2 thin films can be used as an efficient IR transmitter. The undoped samples show a value in the order of 105–106 cm 1, which is found to be quite higher and the spectral dependence of a at such higher values may drastically affect the solar conversion efficiency of solar cells [35].The absorption coefficient (a) for the direct band gap semiconductors is related by the
equation: ðahnÞ2 ¼ AðhnEg Þ
ð3Þ
A is a constant, Eg is the band gap energy and hn is the photon energy. A plot of (ahn)2 against hn for the Zn-doped CuInS2 films grown in the temperature range 300–400 1C is presented in Fig. 4(b). It is observed that the band gap energy (Eg) slightly increases from 1.66 to 1.78 eV as the substrate temperature increases (Table 2). From the XRD
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
527
Fig. 5. (a–e) SEM micrographs of sprayed Zn-doped CuInS2 films prepared at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
patterns, it is noticed that the crystalline growth occurs in the temperature range 300–350 1C. The small increase in (1.66–1.73 eV, Table 2) Eg at this temperature range is attributed to the development of defects and disorders in the films at such higher temperatures. These defects and disorders can be minimized and a better crystalline quality and purity can be obtained using hydrogen as the carrier
gas or postannealing in a hydrogen flowing atmosphere [41]. But, at a temperature of 375 1C and beyond the film becomes amorphous leading to further deterioration of crystalline properties, which gives rise to defect states and thus seems as a smearing of the absorption edge. For undoped samples, it is observed that the Eg values and thickness of the film decreases as the substrate
528
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
temperature increases from 300 to 400 1C [35]. The reduction in Eg values from 1.66 to 1.58 eV is due to the presence of unsaturated defects and localized disordered regions which increase the density of localized states in the band gap [46].
3.4. Surface morphology
Fig. 6. Photoluminescence spectra of sprayed Zn-doped CuInS2 films excited by 485 nm at various substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
The SEM photographs of Zn-doped CuInS2 thin films grown in the temperature range 300–400 1C are presented in Fig. 5(a–e). Large number of small crystals of size about 100–140 nm are formed at 300 1C (Fig. 5a). Nearly 50% of light is transmitted by the crystals in the UV–visible region (Fig. 3), and about 60% of transmission is noticed in the IR regions. The crystal size increases from 200 to 500 nm as the temperature of the substrate increases from 325 to 350 1C (Fig. 5(b) and (c)), the scattering of light is low and 90% transmission of light occurs in this temperature range. Hence, crystals grown in the temperature range 325–350 1C can be used as an infra-red transmitter (Fig. 3). Further increase in temperature to 375 1C (Fig. 5(d) and (e)) and beyond leads to amorphous nature of the films. The surface contains large number of pores, cracks and becomes brittle and friable due to the increase in temperature of the substrate. The undoped CuInS2 samples show crystallites of 100–700 nm and about 70% of light transmission is noticed in the temperature range 300–400 1C [35].
3.5. Photoluminescence (PL)
Fig. 7. (a) Variations of the resistivity of Zn-doped CuInS2 thin films with different substrate temperatures and (b) variations of the conductivity of Zn-doped CuInS2 thin films with different substrate temperatures.
Fig. 6(a–e) shows the emission spectra of Zn-doped CuInS2 thin films grown in the temperature range 300– 400 1C. Photoluminescence spectra have been recorded at room temperature with an excitation wavelength of 485 nm for all the samples. A strong and broad emission peak is observed in the wavelength range 400–600 nm (Blue and Green Band emission) centered at 500 nm (maximum emission) when the substrate temperature is 300 1C. This feature corresponds to donor–acceptor pair transition (DAP) between a sulfur vacancy and an indium vacancy (or) Cu (or) Zn on an indium site [15,47–49] (or) band–band gap luminescence of Zn doped CuInS2 crystals deposited on the substrate [50]. On further increase in temperature from 325 to 400 1C, there is a decrease in emission of intensities and the peak at which the maximum emission occurs slightly shifts towards shorter wavelength region. Another very small emission peak around at 650 nm is probably related to secondary separate phases of CuIn6S7 [51]. Better photoluminescence property is obtained by Zndoped CuInS2 thin films than undoped and Sb-doped films [28,35]. All emissions are associated with defects emerging during the growth of crystallites and are related to deformation of crystallinity due to dislocations and vacancies [52]. The undoped films present more number of emission peaks in the visible and IR regions, which reveal large number of defect states or secondary phases generated during its growth than the doped ones [35].
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
529
3.6. Electrical properties
References
The variations of resistivity (r) and conductivity (s) for the Zn-doped CuInS2 thin films grown in the temperature range 300–400 1C are shown in Fig. 7(a), and (b), respectively. The incorporation of Zn in CuInS2 materials can occur in three different ways: (1) occupying the sulfur site to make an acceptor, (2) occupying the copper site to make a donor and (3) occupying the interstitial site to make a donor, and donors ZnCu and Zni would be compensated by acceptor ZnS [32]. From Fig. 7(a), it is observed that the r value decreases and the lowest r obtained at 325 1C is 5.875 10–3 O cm with significant p-type conductivity (Fig. 7(b)). It is attributed to the Zn atom occupying the sulfur site to make an acceptor which explains the origin of p-type conductivity in temperature range 300–325 1C [33,34]. On further increase in temperature from 325 to 350 1C, an increase in r is noticed. It may be due to the scattering of electrons by thermally generated defects or due to localization of the impurity states and the compensation of Cu vacancy defects, and interstitial sites by Zn-donor states generated by the Zn-doping [53]. Further, the introduced Zn atoms with low amount (0.01 M) cannot compensate all the donors and acceptors; hence n-type conductivity is obtained in this temperature range 325– 350 1C. An evolution of p-type conductivity to n-type conductivity is obtained in this temperature range (i.e.) 325–350 1C. Further, increase in temperature from 375 to 400 1C leads to a slight decrease in r due to change in crystalline to amorphous phase. The undoped films present high resistivity values.
[1] H.W. Schock, A. Shah. Stephens Associates (Public), Proceedings of the European Conference on Photovoltaic Solar Energy Conversion, Barcelona, Spain, 1997, pp. 2000. [2] S. Chichibu, S. Shirakata, A. Isomura, H. Nakanishi, Japanese Journal of Applied Physics 36 (1997) 1703. [3] H.Y. Ueng, H.L. Wang, Materials Science and Engineering B 12 (1992) 261. [4] B. Tell, J.L. Shay, H.M. Kasper, Physical Review B 4 (8) (1971) 2463. [5] R. Scheer, K. Diesner, H.-J. Lewerenz, Thin Solid Films 168 (1995) 130. ¨ [6] K. Siemer, J. Klaer, I. Luck, J. Bruns, R. Klenk, D. Braunig, Solar Energy Materials and Solar Cells 67 (2001) 159–166. [7] L.L. Kasmerski, G.A. Sanbon, Journal of Applied Physics 48 (1977) 3178. [8] R. Scheer, T. Walter, H.W. Schock, M.L. Fearheiley, H.J. Lewerenz, Applied Physics Letters 63 (1993) 3294. [9] Y. Ogawa, A. Jager-waldau, Y. Hashimoto, K. Ito, Japanese Journal of Applied Physics 33 (1994) L1775. [10] H.L. Hwang, C.Y. Sun, C.S. Fang, S.D. Chang, C.H. Cheng, M.H. Yang, et al., Journal of Crystal Growth 55 (1981) 116. [11] H.L. Hwang, C.L. Cheng, L.M. Liu, C.Y. Sun, Thin Solid Films 67 (1980) 83. ¨ sterreicher, W. Kriegseis, [12] Y.B. He, A. Polity, H.R. Alves, I. O D. Pfisterer, et al., Thin Solid Films 403–404 (2002) 62. [13] G. Hodes, T. Engelhard, D. Cahen, Thin Solid Films 128 (1985) 93. [14] H. Onagawa, K. Miyashita, Japanese Journal of Applied Physics 23 (1984) 965. [15] G. Brandt, A. Ranber, J. Schneider, Solid State Communications 12 (1983) 481. [16] J.J.M. Binsma, L.J. Giling, J. Bloem, Journal of Luminescence 27 (1982) 35. [17] S.D. Mittleman, R. Singh, Solid State Communications 22 (1981) 659. [18] H.Y. Ueng, H.L. Hwang, Journal of Physics and Chemistry of Solids 51 (1990) 11. [19] T. Yamamoto, V. Luck, R. Scheer, Applied Surface Science 159–160 (2000) 350. [20] T. Yamamoto, H.K. Yoshida, Japanese Journal of Applied Physics 35 (1996) L1562. [21] T. Yamamoto, I.V. Luck, R. Scheer, Applied Surface Science 159–160 (2000) 350. [22] Bjorn Marsen, Lars Steinkopf, Abhishek Singh, Helena Wilhelm, Iver Lauermann, Thomas Unold, et al., Solar Energy Materials and Solar Cells 94 (2010) 1730–1733. [23] N. Kamoun Allouche, N. Jebbari, C. Guasch, N. Kamoun Turki, Journal of Alloys and Compounds 501 (2010) 85–88. [24] M. Ben Rabeh, N. Chaglabou, M. Kanzari, Chalcogenide Letters 6 (4) (2009) 155–161. [25] M. Zribi, M. Kanzari, B. Rezig, European Physical Journal: Applied Physics 29 (2005) 203. [26] M. Ben Rabeh, M. Zribi, M. Kanzari, B. Rezig, Materials Letters 59 (2005) 3164. [27] M. Ben Rabeh, M. Kanzari, B. Rezig, Thin Solid Films 515 (2007) 5943. [28] N. Suriyanarayanan, C. Mahendran, Materials Science and Engineering B 176 (2011) 417–424. [29] T. Enzenhofer, T. Unold, R. Scheer, Schock H-H. Material Research Society 2005. Spring Proceedings, San Francisco (CA), Spring Meeting, MRS, 2005, F11.3.1. pp. [30] R. Caballero, C. Guillen, Solar Energy Materials and Solar Cells 86 (2005) 1–10. [31] M. Kanzari, M. Abaab, B. Rezig, M. Brunel, Materials Research Bulletin 32 (1997) 1009. [32] R. Scheer, K. Diesner, H.L. Lewerenz, Thin Solid Films 268 (1995) 130. [33] L.L. Kazmerski, G.A. Sanborn, Journal of Applied Physics 48 (1977) 3178. [34] P. Migliorato, J.L. Shay, Journal of Applied Physics 46 (1975) 1777. [35] C. Mahendran, N. Suriyanarayanan, Physica B 405 (2010) 2009–2013. [36] B.J. Brown, C.W. Bates, Thin Solid Films 188 (1990) 301. [37] M. Krunks, J. Madarasz, L. Hiltunen, R. Mannonen, E. Mellikov, L. Niinisto, Acta Chemica Scandinavica 51 (1997) 294. [38] M. Krunks, E. Mellikov, O. Bijakina, Physica Scripta T69 (1997) 189. [39] R.S. Berg, R.D. Nasby, Journal of Vacuum Science and Technology 15 (1978) 359. [40] M. Krunks, T. Leskela, L. Ninisto, Japanese Journal of Applied Physics 39 (2000) 181.
4. Conclusions The (112) oriented Zn-doped polycrystalline CuInS2 thin films (at Cu/In ¼1.25) are deposited onto glass substrates in the temperature range 300–400 1C. The increase in temperature and Zn doping affects the structure of the films. The Zn-doped polycrystalline CuInS2 thin film growth occurs in the temperature range 300– 350 1C. Beyond 375 1C, the film becomes amorphous. The EDAX result confirms the presence of Cu, In, S and Zn in the film. The optical absorption coefficient (a) in the UV– visible region is found to be in the range of 104– 105 cm 1 and highest a value (3 105 cm 1) is obtained when the film is at 300 1C. In the IR regions the Zn-doped (0.01 M) CuInS2 thin films can be used as a good IR transmitter. The optical band gap energies (Eg) slightly increase with increase in temperatures (Eg ¼1.66– 1.78 eV). The surface morphology reveals that an increase in temperature up to 350 1C facilitates the growth of Zn-doped polycrystalline CuInS2 films. The PL spectra exhibit two peak emissions, one is a broad spectral emission centered at 500 nm (Blue and Green band) in the visible region and another is centered at 610 nm (IR region). Resistivity studies reveal the semiconducting nature of the films. An evolution of p-type to n-type conductivity is obtained in temperature range 325–350 1C which requires further investigations.
530
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
[41] R. Scheer, M. Alt, I. Luck, H.J. Lewerenz, Solar Energy Materials and Solar Cells 9 (1993) 423. [42] S.S. Lin, J.L. Huang, D.F. Lii, Surface and Coatings Technology 176 (2004) 173. [43] M. Ben Rabeh, M. Kanzari, B. Rezig, Acta Physica Polonica A 115 (2009) 699–703. [44] M. Ben Rabeh, N. Chaglabou, M. Kanzari, B. Rezig, Physics Procedia 2 (2009) 745–750. [45] M. Ortega-Lopez, A. Morales-Acevedo, Thin Solid Films 330 (1998) 96. [46] EI-Zahed, Thin Solid Films 11 (2003) 19–27. [47] J. Van Gheluwe, J. Versluys, D. Poelman, J. Verschraegen, M. Burgelman, P. Clauws, Thin Solid Films 511–512 (2006) 304–308.
[48] J. Eberhardt, H. Metzner, R. Goldhahn, F. Hudert, U. Reislohner, C. Hulsen, et al., Thin Solid Films 480–481 (2005) 415. [49] M. Nanu, J. Schoonman, A. Goossens, Thin Solid Films 451–452 (2004) 193. [50] N. Tsujii, Y. Imanaka, T. Takamasu, H. Kitazawa, G. Kido, Journal of Alloys and Compounds 408–412 (2006) 791. [51] N. Khemiri, M. Kanzari, Journal of Materials Science 44 (2009) 4743. [52] T. Onishi, K. Abe, Y. Miyoshi, K. Wakita, N. Sato, K. Mochizuki, Journal of Physics and Chemistry of Solids 66 (2005) 1947–1949. [53] Tetsuya Yamamoto, Ilka V. Luck, Roland Scheer, Applied Surface Science 159–160 (2000) 350–354.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Effect of zinc doping and temperature on the properties of sprayed CuInS2 thin films C. Mahendran a, N. Suriyanarayanan b,n a b
Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts & Science, Coimbatore 641020, Tamil Nadu, India Department of Physics, Government College of Technology, Coimbatore 641013, Tamil Nadu, India
a r t i c l e in f o
abstract
Available online 18 April 2012
Copper indium disulfide (CuInS2) is an efficient absorber material for photovoltaic and solar cell applications. The structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified by suitably doping CuInS2 thin films with dopants such as Zn, Sn, Bi, Cd, Na, N, O, P and As. In this work Zn (0.01 M) doped CuInS2 thin films are (Cu/In¼ 1.25) deposited on to glass substrates in the temperature range 300–400 1C. It is observed that the film growth temperature, ion ratio (Cu/In¼ 1.25) and Zn-doping affect structural, optical, photoluminescence and electrical properties of sprayed CuInS2 thin films. As the XRD patterns depict, Zn-doping facilitates the growth of CuInS2 thin films along (112) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Zn in the films. The optical studies show, about 90% of light transmission occurs in the IR regions; hence Zn-doped CuInS2 can be used as an IR transmitter. The absorption coefficient (a) in the UV–visible region is found to be in the order of 104–105 cm 1 which is the optimum value for an efficient absorber. The optical band gap energies increase with increase of temperatures (1.66–1.78 eV). SEM photographs reveal crystalline and amorphous nature of the films at various temperature ranges. Photoluminescence study shows that well defined broad Blue and Green band emissions are exhibited by Zn-doped CuInS2 thin films. All the films present low resistivity (r) values and exhibit semiconducting nature. An evolution of p-type to n-type conductivity is obtained in the temperature range 325–350 1C. Hence, Zn species can be used as a donor and acceptor impurity in CuInS2 thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Thin films Optical properties X-ray diffraction Luminescence Spray pyrolysis
1. Introduction Chalcopyrite semiconductors CuInE2 (E¼S, Se, or Te) are currently investigated for applications in photovoltaic, solar cells [1] and LED devices [2]. These applications are based on a p–n junction. Controlled doping of dopants on these materials appears desirable, where the aimed free carrier concentrations are in the range of 1016–1018 cm 3.
n
Corresponding author. E-mail address: [email protected] (N. Suriyanarayanan).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.04.001
Further, intrinsic doping of chalcopyrite semiconductors is difficult, owing to the following reasons: (a) 12 types of possible intrinsic defects can compensate extrinsic donors (or) acceptors [3]; (b) depending on site occupation, acceptors and donors can be formed by the same dopant; (c) low dissociation temperature of chalcopyrite crystals hampers diffusion doping; (d) secondary phases combining host and doping elements can form. These possible effects limit the number of elements for extrinsic doping of chalcopyrites. Moreover, it appears that the applied doping technique is also very important in determining their properties. Among the ternary compound semiconductors
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
CuInS2 has the potential for high conversion efficiency due to its direct band gap of 1.5 eV [4], which very well matches with the solar spectrum and its high absorption coefficient of approximately 105 cm 1 [5]. No toxic component is included in this compound semiconductor. A variety of methods has been employed to deposit CIS films, including a rapid thermal process [6], single source thermal evaporation [7], co-evaporation from elemental sources [8], sulfurization of metallic precursors [9], chemical vapor deposition [10], sputtering [11,12], electrodeposition [13], and double source thermal evaporation [14]. Among them, spray pyrolysis is an attractive, low-cost method and the application of that enables the deposition of the films of larger area with good uniformity [14]. For controlling a conduction type and obtaining low resistivity, several impurities doped CuInS2 thin films have been studied. Doping of iron during the crystal growth of CuInS2 by chemical vapor transport is investigated [15,16]. Electrical properties of cadmium and zinc doped CuInS2 crystal grown by the Bridgman technique is studied by Mittleman and Singh [17]. Ueng and Hwang [18] showed that the crystals annealed in Cd and zinc vapors exhibited shallow activation energies 0.02 and 0.025 eV, respectively. They also reported the electrical and photoluminescence properties of phosphorous and zinc doped CuInS2 crystals [18]. The study of p-type doping was done using V group elements such as N, P and As in CuInS2, which increase the Madelung energy [19] resulting in instabilities of ionic charge distribution in p-type doped CuInS2 crystals [20]. The difference in the electronic structures and compensation mechanism between n-type Zn and Cd-doped CuInS2 crystals is carried out [21]. The effects of Ti-incorporation in CuInS2 solar cells have been investigated. Solar cells made from Ti containing materials show an increase in open circuit voltage and increase in efficiency compared to the Ti free devices [22]. The influence of Al-doping in CuInS2 by spray pyrolysis on different substrates is analyzed [23]. A single phase of Sb-doped CuInS2 can be prepared by annealing the films above 400 1C in Ar (or) N2 environment [24]. The effect of Na doping on the properties of CuInS2 thin films has been investigated [25]. The effect of the doping element Sn in CuInS2 is succeeded [26] by annealing the Sn-doped films in vacuum. After annealing, the Sn-doped CuInS2 exhibits n-type conductivity [27]. In our previous paper, we presented the effect of Sb-doping on structural, optical and photoluminescence properties of CuInS2 thin films by spray pyrolysis [28]. But, the open circuit voltage of solar cell based CuInS2 thin films can be improved by controlled doping of small amounts of Zn [29]. Hence, this paper presents the structural, optical, photoluminescence and electrical properties of Zn-doped CuInS2 thin films prepared by spray pyrolysis. 2. Materials and methods Zn-doped CuInS2 thin films are deposited by spray pyrolysis onto glass substrates from aqueous solutions of CuCl2, InCl3, SC(NH2)2 and ZnCl2 using compressed air as the carrier gas. At first, aqueous solutions (0.1 M) of the salts are prepared, and then they are mixed with appropriate portions in order to have copper to indium molar
523
ratio (Cu/In¼ 1.25) and (CuþIn)/S fixed to 1 in the solution. The copper (II) chloride and indium (III) chloride are mixed and then thiourea solution is added. The resulting solution is doped with zinc chloride (ZnCl2) of (0.01 M). The solutions are prepared by dissolving in deionized water. Then the solution is sprayed using spray rates of 2 ml/min in air onto glass substrates (2.5 2.5 cm2) heated at different temperatures from 300 to 400 1C. The X-ray diffraction (XRD) patterns of sprayed films are recorded using the XPERT-PRO Gonio scan diffractometer with CuKa radiation. The phases are identified using JCPDS data files. The optical transmittance spectra are recorded in the wavelength range of 300–1100 nm using the double beam Beckman Ratio Recording spectrophotometer. The surface morphology of the film is investigated using a Jeol, JSM-6390, JM-Spot size 35. The compositional analysis is carried out using the energy dispersive X-ray spectroscopy (EDAX). Photoluminescence (PL) spectra of the film are recorded using a Cary Eclipse instrument in fluorescence emission scan mode with excitation wavelength of 485 nm. The electrical resistivity and conductivity studies are carried out by the four-probe method. 3. Result and discussions 3.1. Structural analysis The XRD patterns of Zn-doped polycrystalline CuInS2 thin films grown in the temperature range 300–400 1C are shown in Fig. 1. Characteristic peaks corresponding to (220) and (312) planes of CuInS2 phase with preferential orientation along (112) are recorded at 300–350 1C. The Zn concentrations and the temperature have strong effects on the formation of polycrystalline CuInS2. It is also observed that, the (112) peak intensity decreases with increasing temperatures [28,35] which are due to increase in defects and disorders developed in the films. However, few minor peaks of secondary phases with lower intensities such as In6S7 along (104), (116), and (301), Cu along (111) and Zn, Cu2S, CuIn6S7 (or) Cu/In/S/Zn alloys are formed, which could be responsible for additional reflections [30]. The sum of internal origins obeying the thermodynamics of solid solutions, to defect chemistry and the thermal gradient plays an important role in the formation of minor phases [31]. The higher mobility of Cu þ , Zn þ and its migration towards the surface layers are mainly responsible for additional Cu and Zn phases [32]. On further increase in temperature to 375 1C and beyond, the film becomes amorphous. The maximum peak intensity at (112) plane of Zn-doped CuInS2 is observed at 300 1C, which shows better crystallinity. Hence, at 300 1C the incorporation of Zn leads to an improvement of crystallinity by occupying sulfur, Cu and interstitial sites without affecting structure [17]. Further, an increase in temperature leads to saturation of sulfur, Cu and interstitial sites by Zn gradually and the formation of other crystallographic structures by secondary phases contributes the degradation of crystallinity [33,34]. The Zn-doped CuInS2 thin films are composed of compactly packed nano-sized crystals of grain size in the range of 3–5 nm, calculated
524
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
Fig. 1. XRD of sprayed Zn-doped CuInS2 film prepared at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
Fig. 2. EDAX spectrum of sprayed Zn-doped CuInS2 films prepared at 300 1C.
from Debye–Scherrer relation: D¼
0:9l b cos y
ð1Þ
D is the grain size in nm, l is the wavelength of ˚ b is the full-width at half-maximum of CuKa ¼1.5404 A, the peak in radian and y is the Bragg angle. The undoped films deposited in the temperature range 300–400 1C have shown polycrystalline growth along (112) preferred plane with chalcopyrite structure [35]. But, at 375 1C and beyond the film becomes amorphous. Further, the XRD patterns show the formation of binary sulfides containing metal
chlorides and thio-carbamide [36–38]. At higher temperatures (375–400 1C) oxidation contributes to the growth of In2O3 [35]. The average grain size is found to be in the range of 26–125 nm. 3.2. EDAX analysis EDAX results confirm the presence of Cu, In, S and Zn in the doped CuInS2 films (Fig. 2) and the calculated percentage of elemental compositions is given in Table 1. Better stoichiometry of the films are observed when oxygen is taken into account in the relative compositions. All the
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
525
Table 1 Elemental compositions of Zn-doped CuInS2 thin films deposited at various substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C. S. no.
Temperature (1C)
Cu (at%)
In (at%)
S (at%)
Zn (at%)
O (at%)
1 2 3 4 5
300 325 350 375 400
18.29 17.54 16.94 16.21 15.94
21.94 21.24 20.86 19.76 19.00
43.74 42.63 42.00 41.36 41.02
14.03 13.86 13.02 12.86 11.96
2.00 4.73 7.18 9.81 12.08
Zn-doped films contain oxygen at all deposition temperatures. There is a slight decrease in the concentrations of Cu, In, S and Zn from the films as the temperature increases from 300 to 400 1C. Oxygen is not only chemisorbed in all the films [39], but also an oxygen containing phase has been formed during the film growth. InOCl and/or ZnOCl could be formed very easily due to the thermal decomposition product of (NH4)2InCl5 H2O and/or (NH4)2ZnCl5 H2O, originally formed in the spray solution [40]. Without any chemical reaction, oxygen probably diffuses along grain boundaries of the films. It does not enter into crystallites but, is bonded at the surface and grain boundaries [41]. Hence, the properties of the films are not altered by oxygen content. The doped Zn in low concentration increases the oxygen content as the temperature increases (maximum of 12% is observed) and make the film with a better stoichiometry with reduced crystallographic defects [42]. But, better crystalline quality and purity of the films can be improved by increasing the substrate temperature above 400 1C in a flowing hydrogen atmosphere [41].
3.3. Optical properties The optical transmission spectra of the Zn-doped CuInS2 thin films in the wavelength range 300–1100 nm are shown in Fig. 3. About 50% of light transmission is observed in the UV–visible region when the substrate temperature is at 300 1C and the transmission increases as the substrate temperature increases from 325 to 400 1C. Nearly 80% transmission of light is measured in the UV– visible region when the substrate temperature is in the range of 325–400 1C. Improved optical transmission property is observed in the IR regions than the films obtained from vacuum and thermal evaporation methods [43,44] (which are not reported by other researchers). Nearly 70% of light transmission occurs in the IR regions when the substrate temperature is at 300 1C and it increases and reaches 90% as the temperature increases from 325 to 350 1C. Hence, in this temperature range Zn-doped CuInS2 thin films can be used as a good IR transmitter. It is attributed to the Zn atoms that occupy sulfur, Cu and other vacancies created by substrate temperatures. Further, the Zn atoms are localized near the surface as well as in the volume of the material [43]. The decrease in film thickness (Table 2) due to the rearrangement of atoms in the films may also be the reason for better transmittance. But at a temperature of 375 1C and
Fig. 3. Transmittance spectra of Zn-doped CuInS2 film sprayed at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C. Table 2 Variation of band gap energies (Eg) and thickness of the film with temperatures. Temperature (1C)
Thickness (nm)
Eg (eV)
300 325 350 375 400
992 978 969 967 952
1.66 1.70 1.72 1.75 1.78
beyond, the transmittance slightly decreases due to the amorphous nature of the film. The undoped films show similar behavior and 70% of light is transmitted by all the films, deposited in the temperature range 300–400 1C. No significant effect of light transmission is noticed as the temperature increases from 300 to 400 1C. Further, the reduction in film thickness from 300 to 400 1C has no effect on the optical transmittance [35]. The absorption coefficient (a) is calculated from the following relation [45]:
a¼
2:303 log 1=T t
ð2Þ
t is the thickness of the film and T is the transmittance. The dependence of the absorption coefficient (a) versus the photon energy hn of the Zn-doped CuInS2 thin film grown in the temperature range 300–400 1C is shown in Fig. 4(a). The films have relatively high absorption coefficient in the range 5 104–3.5 105 cm 1 in the UV–visible spectral region. Higher value of a is obtained when the substrate temperature is at 300 1C (a ¼3.5 105 cm 1) and its value decreases as the temperature increases from 325 to 400 1C. The absorption co-efficient value is found to be in the order of 104–105 cm 1. Hence, Zn-doped CuInS2 thin films grown in the temperature range 300–400 1C can be used as an efficient absorber for solar cells and photovoltaic applications in the UV–visible region. But in the IR regions, a slight absorption is noticed, when the film is at 300 1C and the
526
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
Fig. 4. (a) Absorption spectra of Zn-doped CuInS2 film sprayed at different substrate temperatures and (b) variation of optical band gap energy values (Eg) of sprayed Zn-doped CuInS2 films prepared at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
absorption is quite low in the temperature range 325–350 1C (90% of the transmission is observed as shown in Fig. 3). Hence, in this temperature range the Zn-doped CuInS2 thin films can be used as an efficient IR transmitter. The undoped samples show a value in the order of 105–106 cm 1, which is found to be quite higher and the spectral dependence of a at such higher values may drastically affect the solar conversion efficiency of solar cells [35].The absorption coefficient (a) for the direct band gap semiconductors is related by the
equation: ðahnÞ2 ¼ AðhnEg Þ
ð3Þ
A is a constant, Eg is the band gap energy and hn is the photon energy. A plot of (ahn)2 against hn for the Zn-doped CuInS2 films grown in the temperature range 300–400 1C is presented in Fig. 4(b). It is observed that the band gap energy (Eg) slightly increases from 1.66 to 1.78 eV as the substrate temperature increases (Table 2). From the XRD
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
527
Fig. 5. (a–e) SEM micrographs of sprayed Zn-doped CuInS2 films prepared at different substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
patterns, it is noticed that the crystalline growth occurs in the temperature range 300–350 1C. The small increase in (1.66–1.73 eV, Table 2) Eg at this temperature range is attributed to the development of defects and disorders in the films at such higher temperatures. These defects and disorders can be minimized and a better crystalline quality and purity can be obtained using hydrogen as the carrier
gas or postannealing in a hydrogen flowing atmosphere [41]. But, at a temperature of 375 1C and beyond the film becomes amorphous leading to further deterioration of crystalline properties, which gives rise to defect states and thus seems as a smearing of the absorption edge. For undoped samples, it is observed that the Eg values and thickness of the film decreases as the substrate
528
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
temperature increases from 300 to 400 1C [35]. The reduction in Eg values from 1.66 to 1.58 eV is due to the presence of unsaturated defects and localized disordered regions which increase the density of localized states in the band gap [46].
3.4. Surface morphology
Fig. 6. Photoluminescence spectra of sprayed Zn-doped CuInS2 films excited by 485 nm at various substrate temperatures: (a) 300 1C; (b) 325 1C; (c) 350 1C; (d) 375 1C; (e) 400 1C.
The SEM photographs of Zn-doped CuInS2 thin films grown in the temperature range 300–400 1C are presented in Fig. 5(a–e). Large number of small crystals of size about 100–140 nm are formed at 300 1C (Fig. 5a). Nearly 50% of light is transmitted by the crystals in the UV–visible region (Fig. 3), and about 60% of transmission is noticed in the IR regions. The crystal size increases from 200 to 500 nm as the temperature of the substrate increases from 325 to 350 1C (Fig. 5(b) and (c)), the scattering of light is low and 90% transmission of light occurs in this temperature range. Hence, crystals grown in the temperature range 325–350 1C can be used as an infra-red transmitter (Fig. 3). Further increase in temperature to 375 1C (Fig. 5(d) and (e)) and beyond leads to amorphous nature of the films. The surface contains large number of pores, cracks and becomes brittle and friable due to the increase in temperature of the substrate. The undoped CuInS2 samples show crystallites of 100–700 nm and about 70% of light transmission is noticed in the temperature range 300–400 1C [35].
3.5. Photoluminescence (PL)
Fig. 7. (a) Variations of the resistivity of Zn-doped CuInS2 thin films with different substrate temperatures and (b) variations of the conductivity of Zn-doped CuInS2 thin films with different substrate temperatures.
Fig. 6(a–e) shows the emission spectra of Zn-doped CuInS2 thin films grown in the temperature range 300– 400 1C. Photoluminescence spectra have been recorded at room temperature with an excitation wavelength of 485 nm for all the samples. A strong and broad emission peak is observed in the wavelength range 400–600 nm (Blue and Green Band emission) centered at 500 nm (maximum emission) when the substrate temperature is 300 1C. This feature corresponds to donor–acceptor pair transition (DAP) between a sulfur vacancy and an indium vacancy (or) Cu (or) Zn on an indium site [15,47–49] (or) band–band gap luminescence of Zn doped CuInS2 crystals deposited on the substrate [50]. On further increase in temperature from 325 to 400 1C, there is a decrease in emission of intensities and the peak at which the maximum emission occurs slightly shifts towards shorter wavelength region. Another very small emission peak around at 650 nm is probably related to secondary separate phases of CuIn6S7 [51]. Better photoluminescence property is obtained by Zndoped CuInS2 thin films than undoped and Sb-doped films [28,35]. All emissions are associated with defects emerging during the growth of crystallites and are related to deformation of crystallinity due to dislocations and vacancies [52]. The undoped films present more number of emission peaks in the visible and IR regions, which reveal large number of defect states or secondary phases generated during its growth than the doped ones [35].
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
529
3.6. Electrical properties
References
The variations of resistivity (r) and conductivity (s) for the Zn-doped CuInS2 thin films grown in the temperature range 300–400 1C are shown in Fig. 7(a), and (b), respectively. The incorporation of Zn in CuInS2 materials can occur in three different ways: (1) occupying the sulfur site to make an acceptor, (2) occupying the copper site to make a donor and (3) occupying the interstitial site to make a donor, and donors ZnCu and Zni would be compensated by acceptor ZnS [32]. From Fig. 7(a), it is observed that the r value decreases and the lowest r obtained at 325 1C is 5.875 10–3 O cm with significant p-type conductivity (Fig. 7(b)). It is attributed to the Zn atom occupying the sulfur site to make an acceptor which explains the origin of p-type conductivity in temperature range 300–325 1C [33,34]. On further increase in temperature from 325 to 350 1C, an increase in r is noticed. It may be due to the scattering of electrons by thermally generated defects or due to localization of the impurity states and the compensation of Cu vacancy defects, and interstitial sites by Zn-donor states generated by the Zn-doping [53]. Further, the introduced Zn atoms with low amount (0.01 M) cannot compensate all the donors and acceptors; hence n-type conductivity is obtained in this temperature range 325– 350 1C. An evolution of p-type conductivity to n-type conductivity is obtained in this temperature range (i.e.) 325–350 1C. Further, increase in temperature from 375 to 400 1C leads to a slight decrease in r due to change in crystalline to amorphous phase. The undoped films present high resistivity values.
[1] H.W. Schock, A. Shah. Stephens Associates (Public), Proceedings of the European Conference on Photovoltaic Solar Energy Conversion, Barcelona, Spain, 1997, pp. 2000. [2] S. Chichibu, S. Shirakata, A. Isomura, H. Nakanishi, Japanese Journal of Applied Physics 36 (1997) 1703. [3] H.Y. Ueng, H.L. Wang, Materials Science and Engineering B 12 (1992) 261. [4] B. Tell, J.L. Shay, H.M. Kasper, Physical Review B 4 (8) (1971) 2463. [5] R. Scheer, K. Diesner, H.-J. Lewerenz, Thin Solid Films 168 (1995) 130. ¨ [6] K. Siemer, J. Klaer, I. Luck, J. Bruns, R. Klenk, D. Braunig, Solar Energy Materials and Solar Cells 67 (2001) 159–166. [7] L.L. Kasmerski, G.A. Sanbon, Journal of Applied Physics 48 (1977) 3178. [8] R. Scheer, T. Walter, H.W. Schock, M.L. Fearheiley, H.J. Lewerenz, Applied Physics Letters 63 (1993) 3294. [9] Y. Ogawa, A. Jager-waldau, Y. Hashimoto, K. Ito, Japanese Journal of Applied Physics 33 (1994) L1775. [10] H.L. Hwang, C.Y. Sun, C.S. Fang, S.D. Chang, C.H. Cheng, M.H. Yang, et al., Journal of Crystal Growth 55 (1981) 116. [11] H.L. Hwang, C.L. Cheng, L.M. Liu, C.Y. Sun, Thin Solid Films 67 (1980) 83. ¨ sterreicher, W. Kriegseis, [12] Y.B. He, A. Polity, H.R. Alves, I. O D. Pfisterer, et al., Thin Solid Films 403–404 (2002) 62. [13] G. Hodes, T. Engelhard, D. Cahen, Thin Solid Films 128 (1985) 93. [14] H. Onagawa, K. Miyashita, Japanese Journal of Applied Physics 23 (1984) 965. [15] G. Brandt, A. Ranber, J. Schneider, Solid State Communications 12 (1983) 481. [16] J.J.M. Binsma, L.J. Giling, J. Bloem, Journal of Luminescence 27 (1982) 35. [17] S.D. Mittleman, R. Singh, Solid State Communications 22 (1981) 659. [18] H.Y. Ueng, H.L. Hwang, Journal of Physics and Chemistry of Solids 51 (1990) 11. [19] T. Yamamoto, V. Luck, R. Scheer, Applied Surface Science 159–160 (2000) 350. [20] T. Yamamoto, H.K. Yoshida, Japanese Journal of Applied Physics 35 (1996) L1562. [21] T. Yamamoto, I.V. Luck, R. Scheer, Applied Surface Science 159–160 (2000) 350. [22] Bjorn Marsen, Lars Steinkopf, Abhishek Singh, Helena Wilhelm, Iver Lauermann, Thomas Unold, et al., Solar Energy Materials and Solar Cells 94 (2010) 1730–1733. [23] N. Kamoun Allouche, N. Jebbari, C. Guasch, N. Kamoun Turki, Journal of Alloys and Compounds 501 (2010) 85–88. [24] M. Ben Rabeh, N. Chaglabou, M. Kanzari, Chalcogenide Letters 6 (4) (2009) 155–161. [25] M. Zribi, M. Kanzari, B. Rezig, European Physical Journal: Applied Physics 29 (2005) 203. [26] M. Ben Rabeh, M. Zribi, M. Kanzari, B. Rezig, Materials Letters 59 (2005) 3164. [27] M. Ben Rabeh, M. Kanzari, B. Rezig, Thin Solid Films 515 (2007) 5943. [28] N. Suriyanarayanan, C. Mahendran, Materials Science and Engineering B 176 (2011) 417–424. [29] T. Enzenhofer, T. Unold, R. Scheer, Schock H-H. Material Research Society 2005. Spring Proceedings, San Francisco (CA), Spring Meeting, MRS, 2005, F11.3.1. pp. [30] R. Caballero, C. Guillen, Solar Energy Materials and Solar Cells 86 (2005) 1–10. [31] M. Kanzari, M. Abaab, B. Rezig, M. Brunel, Materials Research Bulletin 32 (1997) 1009. [32] R. Scheer, K. Diesner, H.L. Lewerenz, Thin Solid Films 268 (1995) 130. [33] L.L. Kazmerski, G.A. Sanborn, Journal of Applied Physics 48 (1977) 3178. [34] P. Migliorato, J.L. Shay, Journal of Applied Physics 46 (1975) 1777. [35] C. Mahendran, N. Suriyanarayanan, Physica B 405 (2010) 2009–2013. [36] B.J. Brown, C.W. Bates, Thin Solid Films 188 (1990) 301. [37] M. Krunks, J. Madarasz, L. Hiltunen, R. Mannonen, E. Mellikov, L. Niinisto, Acta Chemica Scandinavica 51 (1997) 294. [38] M. Krunks, E. Mellikov, O. Bijakina, Physica Scripta T69 (1997) 189. [39] R.S. Berg, R.D. Nasby, Journal of Vacuum Science and Technology 15 (1978) 359. [40] M. Krunks, T. Leskela, L. Ninisto, Japanese Journal of Applied Physics 39 (2000) 181.
4. Conclusions The (112) oriented Zn-doped polycrystalline CuInS2 thin films (at Cu/In ¼1.25) are deposited onto glass substrates in the temperature range 300–400 1C. The increase in temperature and Zn doping affects the structure of the films. The Zn-doped polycrystalline CuInS2 thin film growth occurs in the temperature range 300– 350 1C. Beyond 375 1C, the film becomes amorphous. The EDAX result confirms the presence of Cu, In, S and Zn in the film. The optical absorption coefficient (a) in the UV– visible region is found to be in the range of 104– 105 cm 1 and highest a value (3 105 cm 1) is obtained when the film is at 300 1C. In the IR regions the Zn-doped (0.01 M) CuInS2 thin films can be used as a good IR transmitter. The optical band gap energies (Eg) slightly increase with increase in temperatures (Eg ¼1.66– 1.78 eV). The surface morphology reveals that an increase in temperature up to 350 1C facilitates the growth of Zn-doped polycrystalline CuInS2 films. The PL spectra exhibit two peak emissions, one is a broad spectral emission centered at 500 nm (Blue and Green band) in the visible region and another is centered at 610 nm (IR region). Resistivity studies reveal the semiconducting nature of the films. An evolution of p-type to n-type conductivity is obtained in temperature range 325–350 1C which requires further investigations.
530
C. Mahendran, N. Suriyanarayanan / Materials Science in Semiconductor Processing 15 (2012) 522–530
[41] R. Scheer, M. Alt, I. Luck, H.J. Lewerenz, Solar Energy Materials and Solar Cells 9 (1993) 423. [42] S.S. Lin, J.L. Huang, D.F. Lii, Surface and Coatings Technology 176 (2004) 173. [43] M. Ben Rabeh, M. Kanzari, B. Rezig, Acta Physica Polonica A 115 (2009) 699–703. [44] M. Ben Rabeh, N. Chaglabou, M. Kanzari, B. Rezig, Physics Procedia 2 (2009) 745–750. [45] M. Ortega-Lopez, A. Morales-Acevedo, Thin Solid Films 330 (1998) 96. [46] EI-Zahed, Thin Solid Films 11 (2003) 19–27. [47] J. Van Gheluwe, J. Versluys, D. Poelman, J. Verschraegen, M. Burgelman, P. Clauws, Thin Solid Films 511–512 (2006) 304–308.
[48] J. Eberhardt, H. Metzner, R. Goldhahn, F. Hudert, U. Reislohner, C. Hulsen, et al., Thin Solid Films 480–481 (2005) 415. [49] M. Nanu, J. Schoonman, A. Goossens, Thin Solid Films 451–452 (2004) 193. [50] N. Tsujii, Y. Imanaka, T. Takamasu, H. Kitazawa, G. Kido, Journal of Alloys and Compounds 408–412 (2006) 791. [51] N. Khemiri, M. Kanzari, Journal of Materials Science 44 (2009) 4743. [52] T. Onishi, K. Abe, Y. Miyoshi, K. Wakita, N. Sato, K. Mochizuki, Journal of Physics and Chemistry of Solids 66 (2005) 1947–1949. [53] Tetsuya Yamamoto, Ilka V. Luck, Roland Scheer, Applied Surface Science 159–160 (2000) 350–354.