Electronic transport properties of microcrystalline silicon thin films prepared by VHF-PECVD

Abstract

Steady-state photocarrier grating (SSPG) and steady-state photoconductivity, σ_ph, experiments have been carried out to investigate the electronic transport properties of undoped hydrogenated microcrystalline silicon (μc-Si : H) films prepared with very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Material with different crystalline volume fractions was obtained by variation of the silane concentration (SC) in the process gas mixture. Pure amorphous silicon material was investigated for comparison. The ambipolar diffusion length, L_amb, which is dominated by the minority carrier properties, is obtained both from the best fit to the experimental photocurrents ratio, β, versus grating period (Λ), and from the “Balberg plot” for the generation rates between 10¹⁹ and 10²¹ cm⁻³ s⁻¹. L_amb increases from 86 nm with increasing SC and peaks around 200 nm for the SC=5.6% and decreases again for higher SCs. L_amb values obtained from the intercept of the Balberg plot result in a small difference of around 5% for most of the samples. Minority carrier mobility-lifetime (μτ)-products are much lower than those of majority carriers, however, both majority and minority carrier μτ-products in microcrystalline silicon are higher than those of undoped hydrogenated amorphous silicon. The grating quality factor (γ₀) changes from 0.70 to 1.0 indicating almost negligible surface roughness present in the samples.