Secondary ion mass spectrometry and electrical characterization of Zn diffusion in n-type InP

Zn diffusion in InP has been investigated in two configurations: diffusion from an external source into uniformly n-doped substrates and diffusion between layers in n-p-n-p-n structures grown by metalorganic chemical vapor deposition (MOCVD). Alternating layers of p-type InP (0.5 μm, 4×1017<[Zn]<2×1018 cm−3) and n-type InP (0.5 μm, 1016<[Si]<3×1019 cm−3) were grown by low-pressure MOCVD at T=625 °C. The distribution of Si and Zn was determined by secondary ion mass spectrometry, using implanted standards to calibrate the data. For undoped spacer layers (n∼1016 cm−3) the diffusion profiles depended strongly on the Zn-doping level; little out-diffusion of Zn was observed for [Zn]=4×1017 cm−3, but for [Zn]>1018 cm−3, the Zn completely diffused across the spacer layers during growth (1–2 h). For doped spacer layers, the doping level (Si) had a dramatic effect on the Zn diffusion profiles. The total Zn diffusion across the grown dopant interface was not substantially affected, but an accumulation of Zn took place in the Si-doped layers, with the formation of Zn spikes, for which the increase in Zn level, as compared to that in the Zn-doped layer (∼1018 cm−3) was similar to [Si]. No diffusion of Si across the grown dopant interface was detected. Electrochemical C-V profiling indicated that the Si and Zn were electrically active. The results have been explained in terms of a model in which mobile Zn species diffusing into the Si-doped layers are immobilized by the formation of Zn-donor pairs. The model is shown to be consistent with diffusion profiles obtained for diffusion of Zn into n-InP from an external ZnGaCdIn source. The effect of this diffusion on as-grown junctions, is to displace the location of the junction. These results can have important consequences for device fabrication.