Highly mismatched alloys include a class of III-V and II-VI compound semiconductors in which the anion species is partially replaced with an isoelectronic element of much different electronegatively and/or covalent radius. These alloys exhibit large-scale bowing of the bandgap among other interesting properties upon the incorporation of even a few percent of the species being alloyed. Among these HMAs are dilute III-V nitrides, notably GaNxAs1-x, which exhibits a reduction of the band gap by as much as 180 meV per N mole fraction, x (Fig. 1). Comparably large band gap reductions have also been observed in other III-Nx-V1-x alloys such as GaInNAs, GaNP, InNP and AlGaNAs. The strong dependence of the band gap on the N content has made these dilute III-V nitrides important materials for a variety of applications, including long wavelength optoelectronic devices and high efficiency hybrid solar cells.
The unusually strong dependence of the fundamental gap on the N content in the group III-N-V alloys has been explained by a band anticrossing model (BAC). The BAC model takes into account an anticrossing interaction between localized N states and the extended states of the host semiconductor matrix. Such interaction splits the conduction band into two subbands, E- and E+. The downward shift of the lower subband (E-) is responsible for the reduction of the fundamental band gap and the optical transition from the valence band to the upper subband (E+) accounts for the high-energy edge. The model has been successfully used to quantitatively describe the dependencies of the upper and lower subband energies on hydrostatic pressure and on N content of Ga1-yInyNxAs1-x, Ga1-yAlyNxAs1-x, InNxP1-xand GaNxP1-x alloys.
The BAC model not only explains the band gap reduction in III-Nx-V1-x alloys but it also predicts that the N-induced modifications of the conduction band may have profound effects on the transport properties of this material system.
In particular, the downward shift of the conduction band edge and the enhancement of the density of state effective mass in GaInNAs may lead to much enhanced maximum electron concentration nmax. Recent experiments have confirmed such prediction and showed that the modified conduction band in GaNxAs1-x enables a large enhancement in the maximum achievable free electron concentration nmax as compared to GaAs (Fig. 2). While group VI donors (Se, S) led to increased maximum carrier concentration in GaNxAs1-x, group IV donors (Si, Ge) in GaNxAs1-x resulted in a highly resistive layer. This disparity in the behavior of group VI and IV donors can be explained by an entirely new effect in which an electrically active substitutional group IV donor and an isovalent N atom passivate each others' electronic effects. This mutual passivation occurs in Si doped GaNxAs1-x through the formation of nearest neighbor SiGa-NAs pairs. Consequently, Si doping in GaNxAs1-x under equilibrium conditions results in a highly resistive GaNxAs1-x layer with the fundamental band gap governed by a net “active” N, roughly equal to the total N content minus the Si concentration (Fig. 3).