The carrier recombination dynamics in MBE grown silicon films, doped with Sb or In, are investigated by the transient grating method under strong pumping conditions with 35 PS laser pulses. The temporal decay of the induced gratings gives carrier lifetimes of the order 10"o-10-8 S for doping concentrations in the range lO''-IOz0 cm-3. A complementary theoretical evaluation of the lifetimes shows the importance of the different kinds of band-toband Auger recombination increases with increasing doping concentration for Sb-doped Si. For In-doped (10'7-10'8 cm-3) Si the SRH recombination via In centres dominates. The measured lifetimes agree well with the theoretical values if it is assumed that, at the doping levels, internal stress increases the light absorption coefficient for 1 = 0.53 pm by a factor of up to 2 greater than t h e pure silicon value.
Time-resolved photoconductivity and decay measurements in optoelectronic switches made from silicon-on-sapphire films of different thicknesses have been used to obtain the distributions of carrier drift mobility and lifetime as functions of the distance x from the substrate. It is shown that the evaluation of the true lifetime τ is complicated by the carrier diffusion in the case of thick samples (≫3 μm) and by back surface recombination in the case of thin samples (≤0.3 μm). For the lifetime, we have found the relation τ(ns)=0.4+0.4x (μm), and the rear surface recombination velocity has been determined to be of the magnitude s≂104 cm/s.
A number of articles have recently been published (1-4) where microwave signals are switched and controlled by means of a ps photoconductor illuminated by a mode-locked laser pulse. The advantages of such an approach over conventional electronic circuitry include higher speeds, excellent timing and high power capability. Here we describe a simple arrangement which makes it possible to both switch an arbitrary number of cycles from a microwave generator (case 1), as well as to shape microwave pulses from a dc bias source (case 2), with easily adjustable frequency and duration. To that end we use a ps photoconductor with a PEPS (5), which controls the pulse produced.
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