The possibility of phase manipulation and temporal tailoring of ultrashort laser pulses enables new opportunities for optimal processing of materials. Phase-manipulated ultrafast laser pulses allow adapting the laser energy delivery rate to the material properties for optimal processing laying the groundwork for adaptive optimization in materials structuring. Different materials respond with specific reaction pathways to the sudden energy input depending on the efficiency of electron generation and on the ability to release the energy into the lattice. The sequential energy delivery with judiciously chosen pulse trains may induce softening of the material during the initial steps of excitation and change the energy coupling for the subsequent steps. We show that this can result in lower stress, cleaner structures, and allow for a materialdependent optimization process.

The steady state photoconductivity up of n-type, p-type and intrinsic a-Si:H has been studied up to photocarrier generation rates of G=5×1027cm-3s-1. In the 20ppm B2H6/SiH4 p-type sample photoconduction switches from holes at low G to electrons at high G. The electron photoconduction at high G is increased by n-type and decreased by p-type doping. This is explained by the charge state of the dominant electron recombination centers. The σp(G) curves of doped and intrinsic a-Si:H merge at the highest G used.

We have studied the dependence of the photoconductivity σp on photocarrier generation rate G in intrinsic a-Si:H at 300K between G=1012cm−3s−1 and 1028cm−3s−1. Below a certain value Go, we find σo =AGγ with γ=0.9±0.05 and the values of A vary considerably with defect concentration Nd which signifies monomolecular recombination through defects. Above Go the recombination is bimolecular, γ=0.5±0.02 and A=(6±3)×10−15 Ω−1cm1/2s1/2 is indpendent of Nd. The transition value Go is about 3×1020cm−3s−1 for high quality annealed a-Si:H and increases with Nd. A simulation of σp(G) assuming conduction in and recombination from extended states fits our experiments within a capture coefficient Ct=(6±2)×10−9cm3s−1 of carriers to their opposite tail states. Our Ct is close to the value (5±2)×10−9cm3s−1 obtained from optical measurements but higher than (0.5±0.1)×10−9cm3s−1 determined from photoelectric studies. Below T=150K our model calculations overestimate σp because the tunneling transitions, becoming important for recombination and conduction, are neglected.

An undoped and a compensated a-Si:H sample have been degraded by 17–34 ns laser pulses and by steady light (CW) at 78K and 300K. The light-induced defect concentration N is monitored by the increase in subgap absorption a. For the same change in a pulses degrade the photoconductivity σp more than CW light and more strongly for exposures at 78K than at 300K. The lack of correlation between σp and N suggests that light soaking causes additional damage besides an increase in N. This additional effect is more pronounced for pulse and low temperature exposures.

Amorphous Si:H and Si1−xGex:H films were prepared by mixing electron beam evaporated silicon with a molecular beam of germanium from a Knudsen cell and with a beam of ionized hydrogen produced by a 0–3 keV ion source. Aluminum Schottky barriers on two types of samples: (1) a-Si1−xGex:H with.15<×<.85 and (2) modulated structures of 50 × 100 Å layers of a-Si:H/a-Si.8Ge.2:H (10-5 Torr PH hydrogen) were investigated. Barrier height was found to depend on the Ge concentration and possible Fermi-level pinning due to the dangling bond deep level. The modulated structures showed a negative resistance region and a barrier height determined only by the composition of the first layer.