Defects in virgin and N+ ion-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 both neutral and single negative charge states, and not to the Zn vacancy proposed in previous experimental work. From TDH measurements the concentrations of negatively-charged and neutral Zn+O divacancies are estimated to lie between 4 × 1015 and 6.3 × 1016 cm-3. DLTS revealed the creation of the defect E1 and an increase in concentration of the defect E3 after N+ ion implantation. Furthermore, a p-conducting layer is formed at the surface after the implantation/annealing procedure that can be used to inject holes during a DLTS measurement. In this way, the acceptor traps A2 and A3 with thermal activation energies of about 150 and 280 meV, respectively, have been detected for the first time by DLTS.