Growth of long wavelength infrared MCT emitters on conductive substrates

Abstract

Uncooled operation of Auger suppressed fully doped mercury cadmium telluride (MCT) devices designed by Ashley and Elliott1 and grown by metalorganic vapor phase epitaxy (MOVPE) by Maxey et al.2 has been demonstrated. These devices also demonstrate efficient negative luminescent emission in the long wavelength infrared (LWIR) spectra.3 However, to operate a large area device (>1 cm2) requires a large current (∼10 A), and consequently, it is critical that the series resistance is minimized. To increase optical efficiency, deep optical concentrators are needed. Similar InSb molecular 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 transmission of the 6 m IR emission. A suitable high conductivity substrate for MCT emitter devices is required to have a sheet resistivity of <1 /□. The conventional MCT epitaxy substrates are CdZnTe and GaAs. High conductivity cadmium zinc telluride (CZT) was not found to be commercially available. Although high conductivity, n-type GaAs is available, the maximum doping is limited by the degree of free carrier absorption in the LWIR which would reduce the potential emitter efficiency. This paper describes a novel investigation into achieving working LW emitter devices with deep mesas in which the current is carried by the GaAs substrate. The key issue which 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 results 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.