Optical Studies of Electronic States, Vibrations and Relaxation Processes in Amorphous Systems

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Studies will be made of electron states in the gap (in the bulk and at interfaces), carrier transport and recombinatiion processes in a-Si:H and related materials (undoped and variously doped films,and multilayer structures). CW and time-resolved photomodulation spectroscopy in a broad spectral range and time domain from femtoseconds to seconds will be used. Photogenerated carriers will be followed as they dribble down in energy through band states, different traps in the gap, and eventually recombine. In the femtosecond range, the pump and probe technique for studying energy, momentum and coherence relaxations of excited electronic states will be employed. In the picosecond range, a new technique, optical time-of-flight method will be used, for studying the carrier transport in a-Si:H at very short times, using the a-Si:H/a-SiNx:H multilayer structures. In the ranges fromnanoseconds up, studies will be made of the evolution of the carrier densities both in energy and time from which carrier transport between specific states in the gap and carrier recombination can be obtained. Another new method, cw photomodulation spectroscopy of vibrations, for identifying the bonding structures with which a certain electronic state in the gap is correlated will be used.