MOCVD based zinc diffusion process for planar InP/InGaAs avalanche photodiode fabrication

We investigated the diffusion of zinc into InP/InGaAs avalanche photodiode structures using dimethylzinc in an MOCVD reactor. Diffusion profiles were measured by secondary ion mass spectrometry and compared with cleaved cross-sections imaged by scanning electron microscopy, in order to accurately target the diffusion depth for device fabrication. The dependence of the diffusion depth on the diffusion temperature, partial pressures of dimethylzinc and phosphine, and diffusion time is reported. Diffused devices exhibit, in some cases, a step increase in dark current at or near the punch-through voltage. We show that the dark current above the punch-through voltage is proportional to the junction area and originates in the bulk of the material. The dependence of this bulk dark current contribution on the diffusion process parameters has been studied in detail, and a reduction of three orders of magnitude was achieved.