HgTe/HgCdTe Superlattices Grown on CdTe/Si by Molecular Beam Epitaxy for Infrared Detection

Journal of Electronic Materials, Jun 2004 by Selamet, Y, Zhou, Y D, Zhao, J, Chang, Y, Et al

This paper describes our preliminary studies on the growth and characterization of HgTe/CdTe SLs with cutoff wavelength in the MWIR region. Diffusion of Au was employed here as a quick method to evaluate the possibility of photovoltaic p-n junction fabrication while ensuring low process temperatures during the device fabrication in order to prevent interdiffusion of the SL layers. Further rigorous work is required to optimize the photovoltaic junction design and fabrication to exploit the advantages of higher effective masses in the growth direction.

MATERIAL GROWTH AND CHARACTERIZATION

The HgTe/CdTe SLs were grown on CdTe/Si substrates. Both single SL layers and device structures incorporating SLs were studied. Although HgTe/ CdTe SL materials have been studied in the past, reports on SL-based structures and devices are very few. An IR detector structure incorporating carrier exclusion (at the bottom n^sup ^ layer/absorber interface) and carrier extraction (at the top p^sup ^ layer/absorber interface) is shown in Fig. 1. The figure shows a top undoped layer that is later converted to a p region by Au diffusion. The excluding and extracting layers are made of the HgCdTe alloy with Cd mole fractions of x = 0.35-0.4. The layer between the absorber layer (slightly ?-type HgTe/CdTe SL) and the bottom excluding n layer is an undoped, widegap HgCdTe buffer to reduce the possibility of a doping tail in the active absorber layer. Attention must also be given to the interface between the SL layer and HgCdTe alloy layers in order to smooth out barriers caused by the energy band discontinuities. The SL growth started with a HgTe layer and ended with another HgTe growth to avoid extra CdTe barriers that hinder carrier transportation. The uppermost CdTe cap layer acts as a device passivant.

Increasing the operating temperature of an IR detector by depleting the carriers in the absorber region through the use of a higher operating temperature (HOT) architecture requires that the absorber layer have a low carrier concentration and be composed of high-quality material. It should be thinner than the diffusion length, yet thick enough for IR absorption. The n^sup ^ and p^sup ^ layers should have a wider bandgap than the absorber layer. Also, the n^sup ^ exclusion layer should be thick enough, at least three times the minority carrier-diffusion length, to inhibit minority carrier injection into the undoped layer from this layer.

Before starting the SL growth, the growth rates of HgTe and CdTe were calibrated using in-situ ellipsometry and ex-situ step profiling. The calibration data were used to control the thicknesses of the HgTe wells and CdTe barriers.

During the SL growth, the Te^sub 2^ and CdTe cell shutters were opened sequentially under a fixed Hg flux, which results in HgTe well layers and barrier layers that are close to pure CdTe. According to previously reported results4 for (211) HgTe/CdTe SLs, the barrier layers are of approximately 90-95% Cd mole fraction.