Defects in virgin and N+ -implanted ZnO single crystals studied by positron annihilation, Hall effect, and deep-level transient spectroscopy

High-quality single crystals of ZnO in the as-grown and N⁺ ion-implanted states have been investigated using a combination of three experimental techniques---namely, positron lifetime/slow positron implantation spectroscopy accompanied by theoretical calculations of the positron lifetime for selected defects, temperature-dependent Hall (TDH) measurements, and deep level transient spectroscopy (DLTS). The positron lifetime in bulk ZnO is measured to be (151±2)0.3em0exps and that for positrons trapped in defects (257±2)0.3em0exps. On the basis of theoretical calculations the latter is attributed to Zn+O divacancies, existing in the sample in neutral charge state, and not to the Zn vacancy proposed in previous experimental work. Their concentration is estimated to be 3.7×10¹⁷0.3em0excm^-3. From TDH measurements the existence of negatively charged intrinsic defects acting as compensating acceptors is concluded which are invisible to positrons---maybe interstitial oxygen. This view is supported from TDH results in combination with DLTS which revealed the creation of the defect $E1$, and an increase in concentration of the defect $E3$ after N⁺ ion implantation, and peculiarities in the observation of the defect $E4$.