P-type doping of double layer mercury cadmium telluride for junction formation

Abstract

Extrinsic p-type doping of long wavelength infrared-HgCdTe double layer heterostructure for p-on-n device application requires good control of the p-type dopant, regardless of the doping technique. The approach is to place the electrical junction ahead of the compositional interface, thus avoiding quantum efficiency reduction. This research addresses the As and P doping of HgCdTe by an implant/diffusion process. The data demonstrates an enhanced atomic diffusion process for As and P from an ion implanted source, within the single phase domain, with a diffusion rate orders of magnitude higher than the rate under Hg-saturated conditions at the same temperature. This work also reveals a new phenomenon, namely, a transition in the enhanced diffusion of both As and P from an exponential to a Gaussian redistribution. This transition is controlled by temperature at a given P_Hg. Gaussian diffusion dominates at high temperatures, T >400°C. The diffusion coefficient of the Gaussian mechanism decreases as the P_Hg increases, from D_p ~2 x 10^-11 cm²/s at P_Hg ~0.02 atm to D_p ~3 x 10^-14 cm²/ s under Hg-saturated conditions (quartz ampoule) at 440°C. The difference in the diffusion coefficients between open tube and closed tube (quartz ampoule), under nominally Hg-saturated conditions, indicates that P_Hg is undersaturated regardless of the Hg-source proximity. The deviation of P_Hg from saturation is estimated from the annealing furnace temperature profile up to a maximum of 50%. Variation of the diffusion coefficient close to Hg saturation appears to be sharply dependent on the actual P_Hg value (example: D_p ~1 x 10^-12 cm²/s in open-tube anneal vs D_p ~3 x 10^-14 cm²/s in closed tube at nominally the same temperature, T = 440°C). Comparative anneals of As and P showed faster diffusion rates for P than for As in both mechanisms.