Possible Origin of Large Response Times and Ambipolar Diffusion Lengths in Hot-Wire-Cvd Silicon Films
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Ilnstituto Superior T6cnico, Physics Department, P-1096 Lisbon, PORTUGAL Instituto Superior Tdcnico, Department of Materials Engineering, P-1096 Lisbon, PORTUGAL Instituto de Engenharia de Sistemas e Computadores, P-1000 Lisbon, PORTUGAL
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ABSTRACT We measured the response time zR and the ambipolar diffusion length Lamb in amorphous (a-Si:H) and microcrystalline silicon films (gc-Si:H) prepared by hot-wire chemical vapor deposition (HW-CVD). The response times in the amorphous and microcrystalline HW films were larger by factors of 100 and 1000, respectively, than in standard PE-CVD a-Si:H films (1-2 Jgs). The ambipolar diffusion length of the HW-CVD films was about twice as large as in conventional glow-discharge films. Strong doping of microcrystalline HW films with trimethylboron (TMB) led to a reduction of the response time. The results hint to a positive effect of the compact microstructure of HW-CVD films. We suggest the dark conductivity activation energy, Eact, and response time, rR, to be used as suitable parameters to describe optoelectronic film properties. INTRODUCTION Hot-wire chemical vapor deposition (HW-CVD) has been employed recently by a number of researchers since HW films with low hydrogen content showed improved stability to light soaking which is crucial for solar cell applications [1]. The NREL group reported an efficiency of about 7 % in a HW-deposited amorphous silicon based solar cell where the intrinsic layer had an ambipolar diffusion length of 150-180 nm [2]. Application of amorphous HW films in thin film transistor (TFT) structures has been reported by two groups recently [3,4]. Also the realization of thin film solar cells based on entirely microcrystalline 14W films with an efficiency of 3.15 % has been reported [5]. In an earlier study we have discussed response times in microcrystalline silicon films prepared by different CVD methods [6]. Here we present the response time rR and the ambipolar diffusion length Lamb in two HW-CVD sample series: in the first series the hydrogen dilution of the source gas was varied [7], and in the second one the doping level was varied to obtain compensated and p-type gc-Si:H films [8]. SAMPLE PREPARATION AND FILM CHARACTERIZATION The films were deposited by HW-CVD in a turbomolecular-pumped UHV-quality deposition system. Silane (SiH 4 ) and hydrogen (H2) were used as the reaction gases. In addition, for p-type doping, 2% .trimethylboron (B(CH 3 )3 , TMB) diluted in H 2 was used. The sample was clamped to the grounded upper substrate holder which is heated to the deposition temperature,
799 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
Tsub. Tungsten filaments were placed 3 cm from the substrate and were resistively heated to the temperature Tfi. The dark conductivity, ad, was measured between 110 0(2 and room temperature. Raman spectra were measured in backscattering geometry using a SPEX micro-Raman spectrometer. The power of the 514.5 nm line of an Ar+ laser, which was focused onto the sample through a microscope, was
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