Growth of long wavelength infrared MCT emitters on conductive substrates

Uncooled operation of Auger suppressed fully doped mercury cadmium tellurid e (MCT) devices designed by Ashley and Elliott(1) and grown by metalorganic vapor phase epitaxy (MOVPE) by Maxey et al.(2) has been demonstrated. Thes e devices also demonstrate efficient negative luminescent emission in the l ong wavelength infrared (LWIR) spectra.(3) However, to operate a large area device(>1 cm(2)) requires a large current (similar to 10 A), and consequen tly, it is critical that the series resistance is minimized. To increase op tical efficiency, deep optical concentrators are needed. Similar InSb molec ular beam epitaxy (MBE) devices utilize a highly doped InSb substrate which allows a conduction path into the substrate with reduced series resistance and acts as an optical window (due to Moss-Burstein shift) allowing transm ission of the 6 mum IR emission. A suitable high conductivity substrate for MCT emitter devices is required to have a sheet resistivity of <1 <Omega>/ square. The conventional MCT epitaxy substrates are CdZnTe and GaAs. High c onductivity cadmium zinc telluridle (CZT) was not found to be commercially available. Although high conductivity, n-type GaAs is available, the maximu m doping is limited by the degree of free carrier absorption in the LWIR wh ich would reduce the potential emitter efficiency. This paper describes a n ovel investigation into achieving working LW emitter devices with deep mesa s in which the current is carried by the GaAs substrate. The key issue whic h had to be addressed was obtaining conduction between the II/VI and III/V materials. A variety of interface designs was investigated but the best res ults were achieved by minimizing the band-gap of the interfacial II/VI MCT and optimizing the properties of the top region of the GaAs substrate.