Impurity–defect emission from undoped Cd$_{1-x}$Zn$_{x}$Te single crystals near the fundamental absorption edge

Shallow impurity–defect states in undoped Cd$_{1-x}$Zn$_{x}$Te ($x\sim$ 3–6%) single crystals have been studied using low-temperature photoluminescence measurements. It has been found that the effect exerted by zinc is mainly reduced to a rigid shift of all the specific features associated with the exciton radiation, which made it possible, with a high ($\sim$0.3 meV) accuracy, to measure the band gap and the zinc concentration in solid solutions. Hydrogen-like donors with the ground-state energy of $\sim$14 meV and four types of acceptors with average activation energies of 59.3 $\pm$ 0.6 meV, 69.6 $\pm$ 1.5 meV, 155.8 $\pm$ 2.0 meV, and 52.3 $\pm$ 0.6 meV have been identified in all the crystals studied. Based on a comparison with the results of the analysis of the impurity background and the data available in the literature on impurity–defect emission in undoped CdTe, the first three acceptors can be assigned to the substitutional impurities Na$_{\mathrm{Cd}}$, P$_{\mathrm{Te}}$, and Cu$_{\mathrm{Cd}}$, respectively. The most shallow acceptor (52.3 $\pm$ 0.6 meV) is a complex defect in which there is a nonstandard excited level separated by only 7 meV from the ground level. This level is formed apparently due to the removal of degeneracy, which is characteristic of $T_D$ acceptors, by the low-symmetry potential of the complex defect.