Dynamics of arsenic diffusion in metalorganic chemical vapor deposition HgCdTe on GaAs/Si substrates

Abstract

This paper describes the behavior of arsenic when diffused from an ion implanted source in Hg1−xCdxTe (x≂0.19–0.23) grown by metalorganic chemical vapor deposition (MOCVD) on GaAs/Si substrates. The results are compared with the diffusion of arsenic from a grown source in the same material. Diffusion mechanisms of arsenic were studied from chemical analysis (secondary ion mass spectroscopy) and theoretical modeling, combined with techniques for radiation damage study [transmission electron microscopy (TEM)] for revealing the defect structure of material [chemical defect etching-etch pit density (EPD), TEM], and for electrical junction determination (electron beam induced current). The main experimental observation was that arsenic redistributes during thermal anneal by a multicomponent mechanism in which the tail-components differ depending on the EPD of the material. We propose a model that explains the four components observed: (1) a surface retarded diffusion component; (2) an atomic vacancy-based component in which arsenic starts on Te sublattice, well described by the theoretical Gaussian solution to Fick’s second law of diffusion with a constant diffusion coefficient (D(400 °C)≂3×10−14 cm2/s and D(450 °C)≂2×10−13 cm2/s ); (3) an atomic vacancy-based component in which arsenic starts on metal sublattice and undergoes a diffusion process enhanced by a chemical nonequilibrium dependent on EPD of the material; and (4) a short circuit diffusion component. The same model applies to diffusion from a grown source. To control the electrical junction of a photovoltaic device, the concentration of arsenic in the tail components should be reduced to values lower than the n-type background. We show that the primary requirement to obtain this control is reducing the dislocation density of the material. Proper choice of implant/anneal conditions helps in reducing the tail of the arsenic profiles for a given EPD in the material. No diffusion tails were observed in MOCVD material with EPD in the low 106/cm2 range.