Rf Photoreflectance Characterization Of Binary And Quasi-Binary Substrates And Antimonide-Based Tpv Devices

Both starting substrates and complete TPV device structures have been characterized using a radio-frequency(RF) photoreflectance technique, in which a Nd-YAG pulsed laser is used to excite excess carriers, and the short-pulse response and photoconductivity decay are monitored with an inductively-coupled non-contacting RF probe. The initial exponential transient decay, indicative of bulk recombination and surface recombination mechanisms as demonstrated previously for doubly-capped sample structures, is approximately 30-40 ns for GaSb substrates, with the decay constant increasing with increasing optical er;citation (similar to Shockley-Read-Hall (SRH) high injection behavior). In the InGaAsSb quasi-binary substrates two distinct decays are observed, an initial decay transient of 15-20 ns which is independent of optical intensity and a subsequent decay of 30-60 ns which decreases with increasing optical intensity. This latter dependence on optical intensity was observed with doubly-capped epitaxial layers and is indicative of radiative recombination.Similar measurements on quaternary device structures indicate that both the pulse amplitude and initial decay are reduced significantly without a front-surface capping layer that reduces surface recombination velocity. With reduction of the front surface recombination velocity, initial decays of 20-25 ns were obtained under open-circuit conditions.These results indicate that the RF photoreflectance technique can be useful in characterizing and qualifying starting substrates and can be used to qualify epitaxial structures as well, particularly when doubly-capped standards are available for initial understanding of recombination processes in the material systems being investigated.