Signi®cance of substrate temperature on the properties of ¯ash evaporated CuIn 0.75 Ga 0.25 Se 2 thin ®lms E. Ahmed a , R.D. Tomlinson c , R.D. Pilkington c , A.E. Hill c , W. Ahmed b, * , Nasar Ali b , I.U. Hassan b a Department of Physics, Bahauddin Zakariya University, Multan, Pakistan b Department of Chemistry and Materials, Manchester Metropolitan University, John Dalton Building, Chester Street, Manchester, M1 5GD, UK c Department of Physics, University of Salford, Salford, M5 4WT, UK Received 9 December 1997; accepted 15 May 1998 Abstract The substrate temperature is an important parameter in thin ®lm deposition regardless of the deposition technique being used to deposit thin ®lms. In this paper the effects of the substrate temperature on the properties of CuIn 0.75 Ga 0.25 Se 2 (CIGS) thin ®lms are reported. Structure, surface morphology and optical properties of CIGS thin ®lms deposited at various substrate temperatures have been investigated using a number of analysis techniques. X-ray diffraction (XRD) analysis at room temperature shows that CIGS ®lms exhibit a strong k112l preferred orientation. As expected, at higher substrate temperatures the ®lms displayed a higher degree of crystallinity. The k112l peak was also enhanced and other CIGS peaks appeared simultaneously. These results were supported by experimental work using Raman spectroscopy. The Raman spectra of the as-grown CIGS thin ®lms show only the A 1 mode peak. The intensity of this peak was enhanced at higher deposition temperatures. Scanning electron microscopy (SEM) results revealed very small grains separated by larger voids in ®lms fabricated at room temperature. When the substrate temperature was increased the average grain size also increased together with a reduction in the number and size of the voids. The deposition temperature also had a signi®cant in¯uence on the transmission spectra. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Deposition process; Evaporation; Solar cells 1. Introduction Copper based ternary and multinary compounds such as Cu(In,Ga)Se 2 have signi®cant potential in the preparation of high ef®ciency solar cells for space and terrestrial applica- tions [1,2]. These continue to attract an increasing amount of interest for devices based on absorber layers of these materials. Solar cells made from these materials have ef®- ciencies in excess of 17% [3]. However, a number of proces- sing dif®culties still exist which prevent large scale industrial applications [4]. A clearer fundamental under- standing of the structural and electro-optical properties of copper based materials such as CuIn 12x Ga x Se 2 thin ®lms is essential in order to overcome some of the problems encountered. In addition, it is also equally important to study and understand the effects of process parameters on the ®lm properties so that ®lms with optimum performance characteristics can be deposited. Major advantages of copper based semiconducting compounds compared to other photovoltaic materials arise due to their unique combination of properties [5]. The chal- copyrite forms of these compounds have large absorption coef®cient, moderate surface recombination velocities, direct band gap and superior radiation resistance [6]. Thin ®lm heterojunction devices based on CIS absorber layers have achieved greater success in terms of ef®ciencies. However, there is a fundamental limitation due to the small band gap value (approximately 1.02 eV for CIS) compared to the theoretical optimum value of 1.5 eV. This prevents devices based on these materials from reach- ing their maximum expected ef®ciencies [7]. An attractive feature of the CIGS thin ®lms is that its band gap can be tailored to ful®l the precise photovoltaic device require- ments. For example, by altering the In/Ga ratio the band gap can be varied from 1.02 eV (for CuInSe 2 ) to 1.68 eV (for CuGaSe 2 ) [8]. Therefore, a composition of CuIn 0.75- Ga 0.25 Se 2 is highly desirable. This paper presents results obtained from a study on the effects of the substrate temperature on the properties of Thin Solid Films 335 (1998) 54±58 0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-6090(98)00885-2 * Corresponding author. Tel.: 144 161 2471423; fax: 144 161 2471438; e-mail: w.ahmed@mmu.ac.uk.