MOCVD HgCdTe heterostructures for uncooled infrared photodetectors

Abstract

Recent progress at VIGO/MUT (Military University of Technology) MOCVD Laboratory in the growth of Hg_1-xCd_xTe (HgCdTe) multilayer heterostructures on GaAs/CdTe substrates for uncooled infrared photodetectors is presented. The optimum conditions for the growth of single layers and complex multilayer heterostructures have been established. One of the crucial stages of HgCdTe epitaxy is CdTe nucleation on GaAs substrate. Successful composite substrates have been obtained with suitable substrate preparation, liner and susceptor treatment, proper control of background fluxes and appropriate nucleation conditions. The other critical stage is the interdiffused multilayer process (IMP). The growth of device-quality HgCdTe heterostructures requires complete homogenization of CdTe-HgTe pairs preserving at the same time suitable sharpness of composition and doping profiles. This requires for IMP pairs to be very thin and grown in a short time. Arsenic and iodine have been used for acceptor and donor doping. Suitable growth conditions and post growth anneal is essential for stable and reproducible doping. <i>In situ</i> anneal seems to be sufficient for iodine doping at any required level. In contrast, efficient As doping with near 100% activation requires <i>ex situ</i> anneal at near saturated mercury vapors. As a result we are able to grow multilayer fully doped (100) and (111) heterostructures for various infrared devices including photoconductors, photoelectromagnetic and photovoltaic detectors. The present generation of uncooled long wavelength infrared devices is based on multijunction photovoltaic devices. Near-BLIP performance is possible at &#8776; 230 K with optical immersion. These devices are especially promising as 7.8-9.5-&#956;m detectors, indicating the potential for achieving detectivities above 10⁹ cmHz^1/2/W.