Nonlinear optical defect absorption in hydrogenated amorphous silicon

By using a Nd:YAG-pumped optical parametric oscillator (OPO) as excitation light source (8 ns pulse width), the dynamic range of photothermal deflection spectroscopy (PDS) is increased by a factor up to 1000. This enables the study of nonlinear defect absorption in intrinsic, phosphorus and boron doped a-Si:H. To probe nonlinear absorption of defects, the intensity of the fundamental emission of the Nd:YAG (1064 nm) has been varied over three orders of magnitude. For intensities greater than 10²⁴ photons/(cm²s), a significant increase of α by a factor 1.4 in p-, 1.2 in i-, and 1.15 in n-a-Si:H is detected. From electron spin resonance (ESR) and transient photoconductivity experiments on intrinsic a-Si:H, a bleaching of the defect density by electron excitation into the conduction band is measured. A hole-like signature in the ESR spectra indicates that electrons from the valence band are excited into the defect band as well. The data are interpreted based on a model for IR-induced nonlinear optical effects.