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) ps and that for positrons trapped in defects (257 +/- 2) ps. 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 3.7x10(17) cm(-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.