Excitation Energy Dependence of Photoinduced Absorption in Intrinsic a-Si:H
- PDF / 285,072 Bytes
- 5 Pages / 414.72 x 648 pts Page_size
- 93 Downloads / 178 Views
ABSTRACT We have studied subgap absorption of intrinsic a-Si:H induced by below- and above-gap photoexcitation. We find very similar photoinduced subgap absorption spectra when excited with 2.4 eV or 1.2 eV light. Both spectra exhibit a power-law dependence on laser intensity AT - 1", where a is 0.5 and 0.7 for 2.4 and 1.2 eV excitation energy, respectively. This behavior indicates a change in the recombination mechanism as a function of excitation energy. The PA spectrum excited at 1.2 eV shows a strong dependence on bias illumination. Bias illumination bleaches the absorption according to a power-law as AT = c(Ebias)-I•, where P3 is approximately 0.85 and independent of probe energy and bias energy. The parameter c(Ebias) increases superlinearly with bias illumination energy for Ebias > 1.7 eV. INTRODUCTION Photomodulation (PM) is a powerful technique to investigate electronic states within the gap of amorphous semiconductors. In this technique the sample is photoexcited with a (pump) laser and induced changes are monitored in the absorption of (probe) light from an external light source. When the subgap absorption is enhanced or decreased under photoexcitation the process is called either photoinduced absorption (PA) or photoinduced bleaching (PB), respectively. In previous studies excitation energies, Eex, well above the bandgap energy were employed, and photoinduced absorption was found over the whole below-gap energy range. Though featureless, the spectrum was attributed to at least two contributions,' 2 and correlations to the photoluminescence spectrum were found.3 In the studies presented here we examine subgap absorption induced by below-gap photoexcitation. In photoluminescence, below-gap excitation has yielded much information on the distribution of states within the gap, and evidence has been found4 for two-step excitation processes of charge carriers through the deep defects. EXPERIMENTAL The intrinsic sample used in theses studies was prepared by glow discharge deposition on a sapphire substrate. The sample, whose thickness was -7 gm, was mounted inside a cryostat and illuminated from the film side. For above- and below-gap excitation the unfocused laser lines of an argon laser (514.5 nm, 30 mW/cm 2 ) and a Nd:YAG laser (1064 nm, 2.5 W/cm 2), respectively, were used. The modulation frequency of the exciting light was in all cases around 130 Hz. The photoinduced changes in absorption that were in phase with the laser excitation were probed with a focused tungsten halogen light or an ir-glowbar. The probe light was dispersed by a single grating monochromator and detected with Si, Ge and InSb solid-state detectors covering the spectral range from 0.3 to 1.7 eV. The measured spectra contained interference fringes due to internal reflections that were spectrally averaged. For the measurements under bias illumination, the light of a second tungsten halogen lamp was focused onto the sample. In all cases the low energy part of the tungsten spectrum was blocked with a heat absorbing filter (shortpass at appro
Data Loading...
