Dilute nonisovalent (II-VI)-(III-V) semiconductor alloys: Monodoping, codoping, and cluster doping in ZnSe-GaAs

A dilute nonisovalent semiconductor alloy, made of a III-V semiconductor component (GaAs) mixed with a II-VI semiconductor (ZnSe), can be viewed as the doping of a host semiconductor with a lower (higher) valent cation and a higher (lower) valent anion. We have investigated different doping types, i.e., monodoping, triatomic codoping, and cluster doping, in the ZnSe-GaAs system using ab initio pseudopotential plane-wave calculations. We find the following: (i) The acceptor dopant clusters are stabilized in a chemical potential range different from that of the donor dopant clusters. This explains the experimental observation that a nonisovalent alloy has a distinct carrier polarity. (ii) Cluster doping, e.g., (Zn-Se-4)(3+) or (Se-Zn-4)(3-) in GaAs, is predicted to be stable at extreme chemical potential limits, and also to contribute free carriers. (iii) Triatomic codoping is predicted to be thermodynamically unstable. (iv) Cluster doping produces shallower acceptor/donor levels than monodoping and triatomic codoping. (v) There is a strong attractive interaction between positively charged donors and negatively charged acceptors. Therefore, a high concentration of the charge-neutral dopant pairs exists in the alloy. This finding explains why free carriers in a nonisovalent alloy have a high mobility. (vi) Our results also explain the asymmetric dependence of the band gap on the alloy composition. Specifically, adding a small amount of Ga+As into ZnSe leads to a sharp drop in the band gap of the host crystal, whereas adding Zn+Se into GaAs does not change the band gap very much.