Effects of heavy nitrogen doping in III-V semiconductors - How well does the conventional wisdom hold for the dilute nitrogen "III-V-N alloys"?

Although heavily N doped III-V semiconductors are frequently referred to as dilute nitrogen III-V-nitride alloys in general, it is important to realize that there are some subtle but important differences among them. In the indirect gap semiconductor GaP, since even a single N impurity can have a bound state, as far as the absorption near the band gap is concerned, it is difficult to describe GaP : N as an alloy. The N induced bound states (a hierarchy of impurity complexes) can in fact give rise to rather strong absorption below the indirect band gap, which effectively reduced the energy of the "absorption edge", but the position of the absorption edge cannot be defined in a conventional way. In the direct gap semiconductor GaAs, a single N impurity does not form a bound state but instead has a resonant state above the conduction band edge. In this aspect, GaAs:N is similar to the situation in a conventional alloy, say, GaAs : P, except that the perturbation of N to the host is much stronger than that of P. However, because in reality N incorporation is typically in a random manner and a cluster as small as one N pair can generate bound states, the permissible region for GaAs : N to behave as a regular alloy is in fact rather limited, i.e., only when the N concentration is low enough so that the N pair and cluster states do not significantly interact with the host. In this study, a precise tracking of the evolution of the host and N induced impurity states will be offered for the two prototype systems, GaAs:N and GaP:N, with x varying from as low as ∼10 ^-5 to ∼10^-2. Such a study gives valuable insight to the underlying physics of the material evolution, sets up a bench-mark for testing the theoretical modeling of this type of system, and serves to enhance our understanding of the behavior of isoelectronic impurities in semiconductors in general. The dissimilarity between GaP : N and GaAs : N indicates that seeking a unified model for all the isoelectronic doping systems is unrealistic. Our study also indicates that for strongly perturbed systems like GaP:N and GaAs:N, certain materials properties could be less well defined or not uniquely defined, as compared to those in other conventional semiconductor alloys.