Electron Drift Mobility in a-Si:H Prepared by Hot-Wire Deposition
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ABSTRACT We have measured the electron drift mobility in a-Si:H prepared by hot wire (HW) deposition using photocarrier time-of-flight. Initial work has shown that light-soaked HW material can have much better ambipolar diffusion lengths than the plasma-deposited material following extended light soaking. In a sample with about 2% H-concentration in the intrinsic layer, we find that the electron drift mobility is quite different from that of a-Si:H alloys prepared by normal glowdischarge CVD, even allowing for the reduced bandgap of the hot wire material. This result challenges the principle that the bandgap of optimized amorphous silicon based material is sufficient to predict the electron drift mobility.
INTRODUCTION Amorphous silicon-based materials with modified bandgap are of considerable scientific and technical interest. Amorphous silicon-germanium alloys (a-Si,_,Ge,:H) have received particulary intensive investigation as narrow bandgap materials. There are many reports on the photogenerated carrier transport in this reduced bandgap material [1-5]. The general findings are that electron drift mobility declines with germanium alloying and that hole drift mobility is relatively unaffected by the alloying. The underlying physics is generally interpreted with the exponential bandtail multiple trapping model; the decline of electron mobility is attributed primarily to the broadening of the bandtail width So with alloying. The remaining two parameters, the microscopic mobility #0 and the attempt-to-escape frequency v are apparently relatively unchanged with alloying. An extensive analysis was done by Wang, et al [3] on the electron transport in silicon-germanium alloys. Wang was able to fit essentially all the electron drift mobility data from different laboratories around the world using only the bandtail broadening effect, and assuming a bandgapindependent attempt-to-escape frequency v = 5x 1011 Hz and a microscopic mobility /t 0 = I cm /Vs,. The conclusion was that vand o change little with alloying, the electron drift mobility of a material is determined adequately by the broadening of the bandtail which accompanies narrowing of the bandgap. The bandgap of a-Si:H can also be varied solely by varying the concentration of hydrogen; less hydrogen reduces the bandgap. Maruyama, et al, have discussed how germanium alloying and hydrogen concentration determine the bandgap in a-SiGe:H alloys [6]. a-Si:H material produced by the "hot-wire" (HW) deposition technique combines reduced hydrogen content and bandgap with the remarkable property that the material is apparently more stable under light exposure than 437 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
conventional plasma deposited a-Si:H [7-8]. This property is clearly of potential importance in designing solar cells using a-Si based materials. In the present paper we present a first account of the electron drift mobility in HW a-Si:H. The data fairly clearly violate the empirical rule of Wang, et al,[3] governing the effects of bandgap
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