Role of Surface on the Persistent Photoconductivity in Porous Silicon and Boron Doped a-Si:H
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0910-A12-01
Role of Surface on the Persistent Photoconductivity in Porous Silicon and Boron Doped aSi:H* S. C. Agarwal, Abhishek Kumar, and N P Mandal Department of Physics, IIT Kanpur, Kanpur, Uttar pradesh, 208016, India ABSTRACT The effect of ambient conditions on light soaking (LS) in porous silicon (PS) is studied. In vacuum, LS on porous silicon increases dark current (DC) while in presence of water vapor it decreases DC. Interestingly, LS gives a higher DC in vacuum as well as in presence of water vapor for boron doped hydrogenated amorphous silicon a-Si:H(B) samples. A thin layer of polystyrene polymer almost eliminates the light induced degradation of porous silicon (PS) layers but can do so only to a smaller extent in a-Si:H(B). This shows that the effect of LS in PS is different than the effect of LS in a-Si:H(B) and that the surface plays a more important role in PS than in a-Si:H. INTRODUCTION Nanocrystalline porous silicon (PS) [1,2] and a-Si:H are promising materials for device applications. However, both degrade upon exposure to visible light [3-6]. This photo-induced degradation has been of major concern. The cause for this is not yet fully known but it is generally believed that photo-structural changes are caused by hydrogen movement [4,6]. The objective of this study is to see the role played by the surface in the degradation process by measuring the effect of light soaking on PS and boron doped a-Si:H for different ambient conditions. We find that coating by polystyrene arrests the photo-induced degradation in PS almost fully. On the other hand, for the case of a-Si:H(B), the polystyrene coating decreases the photo-induced degradation but does not remove it completely. The importance of the surface in the two cases is compared and accounted for. EXPERIMENTAL Free standing PS layers were prepared by the electrochemical anodization of boron doped (100) crystalline silicon wafers of resistivity ~1 Ω-cm [4]. The boron doped a-Si:H thin films were deposited on the Corning 7059 substrates through plasma enhanced chemical vapor deposition method [7]. The r.f. power density was ~ 13.5 mW/cm2 and the substrate temperature was 200 0C. The electrical transport measurements were done using silver paint or evaporated nichrome as electrodes in a coplanar geometry with about 0.5 mm separation. A bias of 100 V was applied for the dark current (DC) measurement. All samples were annealed at 190 oC in the dark for 1 hour in vacuum (10-6 Torr), to remove the effect of previous light exposure, if any, and then were cooled slowly (rate ~ 0.5 K/min) to room temperature (state A) [4]. Light soaking (LS) was done through heat filtered white light from a 360W tungsten halogen lamp, kept at a distance of ~15cm from the sample. A beaker containing 2 cm column of water was used as a heat filter during LS. For the polymer coating, polystyrene was dissolved in toluene and the samples were dipped in. They were shaken ultrasonically and left in air for about 30 min, till the toluene evaporated. To study the effect of light soa
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