Unusual uniaxial stress results on the stretch mode of OH related defects in ZnO

Some uniaxial stress studies of the frequency dependence of O-H-related defects in ZnO have produced surprising results$^{1,2}$. For example, the Li:OH defect in ZnO (H-I*) is oriented along the c-axis, yet the OH stretch mode decreases in frequency when stress is applied along the c direction and increases when stress is perpendicular to the c-direction. Another example is the Cu:OH defect, in which the OH is aligned along one of the three non-c tetrahedral directions. Stress along the c-direction produces a strongly non-linear increase in frequency. These examples and others indicate something unusual is happening in these systems. One possibility is that the piezoelectric effect in ZnO is responsible for the ``backward'' behavior of the frequency shift of these defects. The piezoelectric effect in ZnO is caused by the lack of cancellation between the ``clamped-ion'' term (i.e., electronic contribution) and the term related to the change in the u parameter (``internal strain''), with the latter dominating$^{3}$. When c stress is applied, the value of u increases, thus the two interpenetrating hexagonal lattices (one for Zn, the other for O) increase their overlap. We will attempt to explain the experimental results within this framework. $^{1}$Lavrov and Weber, Phys. Rev. B, \textbf{73}, 035208 (2006), and $^{2}$ Phys.Stat. Sol. (b) \textbf{243}, 2657 (2006) $^{3}$Corso, et al Phys. Rev B. \textbf{50}, 10715 (1994)