Cluster Engineering for Photoconductive Switches

Abstract

Molecular beam epitaxy (MBE) of arsenides such as GaAs or AlGaAs with typical group III and As fluxes, but with a substrate temperature in the range of 200°C to 300°C, results in the incorporation of excess As in the epilayer;¹ annealing at temperatures of 600°C or higher causes the excess As to precipitate.^2–5 The final average size and corresponding density of the As clusters is controlled by the temperature and duration of the anneal,^6–7 while the amount of excess As in the epilayer is controlled by the substrate temperature during MBE.⁸ This composite material, consisting of semi-metallic As clusters in a semiconductor matrix, exhibits very interesting electrical and optical properties. The composite is semi-insulating due to the internal Schottky barriers associated with the As clusters.^9,10 In addition, the composite exhibits reasonable mobilities and in some cases sub-picosecond lifetimes, making it an attractive material as a high-speed photoconductor.^11–15 The lifetime of photogenerated carriers is very dependent on the spacing of the As clusters and can be tuned from less than 200 fs to over 10 ps with anneal.¹⁶ The lifetime varies as the square of the average spacing between precipitates, which indicates the lifetime may be controlled by diffusion of carriers to the As precipitates where they recombine. In addition, when the composite is used as a photoconductive switch to generate and detect freely propagating bursts of electromagnetic radiation, the radiated intensity increases with either substrate growth temperature¹⁷ or with anneal temperature, indicating an increase in carrier mobilites. In this paper we present details of the control of the lifetime in these composites and use of the material to launch electromagnetic pulses. In addition, we introduce a technique to form composites using ion-implantation of metals—such as Fe and Ni—into GaAs and a subsequent anneal to nucleate clusters.