Assessment of electrical inhomogeneity of undoped and doped Hg_1−xCd_xTe MOVPE (IMP) layers by variable magnetic field hall profile measurements

Abstract

A Hall profiling technique in which measurements are made at 77K as a function of magnetic field (0.01–1T) and layer thickness (by chemical thinning) has been developed for the characterization of Hg_1−xCd_xTe epitaxial layers. The technique has been applied to undoped and lightly In doped MOVPE interdiffusion multilayer process (IMP) layers grown on (100) GaAs and CdTe with x values between 0.19 and 0.30 in order to investigate the parameters controlling the electrical homogeneity. Results show unpassivated layers with x>0.25 are uniformly p-type with 77K hole concentrations consistent with Hg vacancy concentrations for the growth conditions while layers with x<0.25 are anomalous, being either inhomogeneous or uniformly n-type depending on x. The inhomogeneous layers are mainly n on p layer structures, in which the junction depth, Xj, but not the carrier concentration is found to be a function of x and can be comparable to layer thickness at x=0.20. Carrier levels in the n and p-type regions are consistent with the net background impurity donor and Hg vacancy concentrations respectively, suggesting that the n-type properties occur through post growth Hg in-diffusion filling in the vacancies. This model was partly confirmed by capping layers with CdTe, which significantly reduced the extent of the n-type region, and by ex-situ annealing experiments to simulate the inhomogeneity. Results show the low temperature process responsible for the type conversion in MOVPE CMT is highly x dependent, suggesting a variation in the electrical “Hg diffusion coefficient” of over an order of magnitude between 0.20 and 0.30 at 200°C. Carrier and mobility profiles of both as-grown and annealed n-p structures are abrupt and consistent with an interstitial Hg diffusion mechanism.