Increase of temperature and crystallinity during electrical switching in microcrystalline silicon
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A4.25.1
Increase of temperature and crystallinity during electrical switching in microcrystalline silicon Jian Hu, Paul Stradins, Howard M. Branz, and Qi Wang National Renewable Energy Laboratory, Golden, CO 80401, USA J.R. Weinberg-Wolf, E.C.T. Harley, Chris Lawyer, Brittany Huie, and Daxing Han Department of Physics & Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
ABSTRACT We investigate electrical stressing and switching in hydrogenated microcrystalline silicon (µc-Si:H) by thermal, and optical and electrical measurements of Cr/µc-Si:H/metal thin-film structures. Boron-doped microcrystalline Si films of 30-50 nm thick are deposited by hotwire chemical vapor deposition (HWCVD) on Cr-coated glass at 160°C and contacted with Ag or Al . Switching in devices of size 5 to 30 µm is stimulated by a current-ramp from 10 nA to 50 mA. We find that the voltage across the µc-Si:H devices initially increases logarithmically with current, then saturates at 2~3 V, and finally drops to a low value of 1 to 1.5 V. This drop indicates a permanent decrease of device resistance to below 1 kΩ. During current stressing, the surface temperature increases with the bias current, and the surface reflectivity changes. After switching, a small increase in crystalline fraction can be observed by micro-Raman scattering measurements. The observations suggest electrothermal processes which cause changes in microstructure of the µc-Si bulk during current stress.
INTRODUCTION Hydrogenated microcrystalline silicon (µc-Si:H) has been extensively studied for applications in thin-film electronic devices. Thin-film solar cells based on µc-Si:H are more stable against long-time light exposure than hydrogenated amorphous silicon (a-Si:H) cells and are used as the low-gap partner in µc-Si:H/a-Si:H tandem cells [1,2]. Microcrystalline silicon is also used as an active layer in thin film transistors (TFTs) because of higher mobilities and greater stability compared to a-Si:H TFTs [3]. The optical and electric properties of µc-Si:H prepared at 240 °C by hot-wire CVD (HWCVD) have been studied in our laboratories using Raman and photoluminescence spectroscopy [4,5]. We have extensively studied switching phenomena induced by high electric field or current density in metal/a-Si:H/metal devices and metal/µc-Si:H/metal structures [6-8]. We have found that the switching or breakdown in µc-Si:H devices generally requires lower applied voltage than comparable a-Si:H switches, and they have different area scaling behaviour [6]. In this paper, we report on the electrical, thermal, and microstructural changes accompanying switching in metal/µc-Si:H(p)/metal thin film structures.
A4.25.2
DEVICE STRUCTURES AND TESTING Boron-doped µc-Si:H thin films are deposited by hot-wire CVD on glass substrate coated by Cr at 160°C. In this process, a W wire heated to about 2000°C is used to decompose silane, H2 and trimethylboron (3.1% in helium) precursor gases. The flow rates for those gases are 3, 45 and 3 sccm, respectively. Thickness of µc-Si:H(p) i
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