JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS - SYMPOSIA, Vol. 1, No. 3, 2009, p. 202 - 205 Investigation of meta- and in-stability effect in thin film silicon materials using intensity and temperature dependence of photoconductivity M. GÜNEŞ * , E. TURAN, G. YILMAZ Department of Physics, Faculty of Sciences and Arts, Mugla University, Kotekli Yerleskesi, Mugla Turkey. Thin film silicon materials in the form of a-Si:H, μc-Si:H, and a-SiGe:H alloy deposited by plasma enhanced chemical vapour deposition have been investigated using intensity and temperature dependence of photoconductivity both in the annealed and degraded states. In the annealed state, a-Si:H shows lower defect density and the exponent γ is closer to unity. Alloying a-Si:H with Ge introduces new defect distribution around midgap and indicating two different exponent γ as temperature decreases. μc-Si:H has higher μτ-products in the annealed state. After light induced degradation, a-SiGe:H alloy exhibits higher degradation than a-Si:H. However, both show similar exponent γ after light soaking at all temperatures. Even tough μc-Si:H thin film showed almost insignificant meta-stability (Staebler-Wronski effect (SWE)), atmospheric aging substantially reduces its dark and photo transport properties. Such type of instability is not present in amorphous forms. Long term aging creates two distinctly different defect bands indicating a thermal quenching effect in temperature dependence of photoconductivity, which is not present in short term aging. (Received Mart 25, 2008; accepted May 21, 2009) Keywords: Amorphous silicon, Amorphous silicon-germanium, Microcrystalline silicon, Staebler-Wronski effect, Instability, low temperature photoconductivity. 1. Introduction Thin film silicon materials in the form of hydrogenated amorphous silicon (a-Si:H), hydrogenated amorphous silicon-germanium (a-SiGe:H) and hydrogenated microcrystalline silicon (μc-Si:H) are the main candidates of absorber layers in multi-junction thin film silicon solar cells [1,2]. One of the major drawbacks of amorphous silicon solar cell technology has been the Staebler-Wronski effect (SWE) discovered in amorphous silicon [3]. It was later reported to be significant in its alloys such as a-SiGe:H [4]. In contrast, single junction solar cells fabricated using microcrystalline silicon absorber layers exhibited insignificant SWE after prolonged light exposure [5]. Because of this observation, μc-Si:H has become an important attractive material in large area photovoltaics. However, this material also suffers from another major problem of atmospheric aging as thin films are exposed to air after deposition [6,7]. Even tough the first aging effect in microcrystalline silicon was reported by Veprek et al in 1983 [6], not many studies have been reported until the last decade. Due to its heterogeneous structure, microcrystalline silicon has a mixture of crystalline and amorphous phases in different ratios. A recent more detailed investigation of instability in μc-Si:H indicated different classification of the material such as Type I and Type II [7]. There are only several preliminary investigations of instability reported in literature [8-10]. Therefore, understanding of the problem is not complete yet. In this paper, intensity and temperature dependence of photoconductivity have been used to study meta- and in-stability effects in a-Si:H, a-SiGe.H and μc-Si:H thin films, which are the main absorber layers of multi-junction thin film silicon solar cells. 2. Experimental details Undoped hydrogenated amorphous silicon (a-Si:H), amorphous silicon-germanium (a-SiGe:H) and microcrystalline silicon (μc-Si:H) thin films were prepared using RF plasma enhanced chemical vapour deposition method [11,12]. a-SiGe:H sample was prepared with 20% germanium. Microcrystalline silicon film has I C RS of 0.40 as determined from Raman measurements. Dark and steady-state photoconductivity measurements were carried out between 450K and 77K in a JANIS VPF 475 liquid nitrogen cryostat with turbo molecular pumping station by applying a dc voltage to coplanar contacts of sample in the Ohmic region. Monochromatic interference filters with appropriate wavelength were used to provide homogeneous volume absorbed light with generation rates between 10 16 and 10 21 cm -3 s -1 . Samples were annealed in dark at 450K in high vacuum for several hours. Light soaking was performed in high vacuum without exposing the samples to air after annealing process under approximately 5 suns of water filtered white light at controlled soaking temperatures.