Highly mismatched alloys include a class of IIIV and IIVI 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 largescale 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 IIIV nitrides, notably GaN_{x}As_{1x}, 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 IIIN_{x}V_{1x} alloys such as GaInNAs, GaNP, InNP and AlGaNAs. The strong dependence of the band gap on the N content has made these dilute IIIV 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 IIINV 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 highenergy 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 Ga_{1y}In_{y}N_{x}As_{1x}, Ga_{1y}Al_{y}N_{x}As_{1x}, InN_{x}P_{1x}and GaN_{x}P_{1x} alloys. The BAC model not only explains the band gap reduction in IIIN_{x}V_{1x} alloys but it also predicts that the Ninduced 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 GaN_{x}As_{1x} enables a large enhancement in the maximum achievable free electron concentration n_{max} as compared to GaAs (Fig. 2). While group VI donors (Se, S) led to increased maximum carrier concentration in GaN_{x}As_{1x}, group IV donors (Si, Ge) in GaN_{x}As_{1x} 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 GaN_{x}As_{1x} through the formation of nearest neighbor SiGaNAs pairs. Consequently, Si doping in GaN_{x}As_{1x} under equilibrium conditions results in a highly resistive GaN_{x}As_{1x} 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). Selected Publications:

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