Probing hydrogenated amorphous silicon surface states by spectroscopic and real-time second-harmonic generation

The second harmonic generation (SHG) signal from hydrogenated amorphous silicon (a-Si:H) thin films is measured in situ and in real-time during film growth. Polarization and spectral dependences of the SHG radiation are investigated for as-deposited films and after subsequent mol. oxygen dosing in order to confirm the sensitivity of SHG to a-Si:H surface states. On the basis of these expts., we conclude the SHG radiation is partly generated at the surface (a-Si:H/vacuum interface) and the microscopic origin of the SHG signal appears to be dangling and surface Si-Si bonds. This has been supported by simulations of the nonlinear surface response for the different polarization configurations using an excitonic line shape model with two resonances. Furthermore, real-time measurements during film growth, up to a film thickness of 412 nm, demonstrate the potential of the technique to monitor surface states during the deposition process of a-Si:H films