Vacancy defect and defect cluster energetics in ion-implanted ZnO

Abstract

We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Doppler broadening-measured transformation of Zn vacancies to vacancy clusters with annealing shifts defect energies significantly lower in the ZnO band gap. Zn and corresponding O vacancy-related depth distributions provide a consistent explanation of depth-dependent resistivity and carrier-concentration changes induced by ion implantation. DOI: 10.1103/PhysRevB.81.081201 PACS number͑s͒: 72.40.ϩw, 71.55.Gs, 78.60.Hk ZnO is a leading candidate for next generation optoelectronic materials because of its large band gap, high exciton binding energy, thermochemical stability, and environmental compatibility. 1,2 High quality single-crystal bulk ZnO wafers grown by various methods are commercially available 3 and ZnO thin-film growth has attracted intense interest. 4 However, despite nearly sixty years of research, several fundamental issues surrounding ZnO remain unresolved. Chief among these have been the difficulty of p-type doping and the role of compensating native defects. 5,6 Oxygen vacancies ͑V O ͒, V O complexes, Zn interstitial-related complexes, and residual impurities such as hydrogen and aluminum are all believed to be shallow donors in ZnO, while Zn vacancies ͑V Zn ͒ and their complexes are considered to be acceptors. 7,8 Although their impact on carrier compensation is recognized, the physical nature of the donors and acceptors dominating carrier densities in ZnO is unresolved. Thus it remains a challenge to correlate the commonly observed 1.9-2.1 eV "red" and 2.3-2.5 eV "green" luminescence emissions with specific native defects. 9 These and other emissions vary widely in ZnO bulk or thin films grown by various methods. [10][11][12][13][14] Previous optical absorption, photoluminescence, electron paramagnetic resonance, and depth-resolved cathodoluminescence spectroscopy ͑DRCLS͒ ͑Ref. 15͒ studies indicate a correlation between the "green" optical transition and O vacancies ͑V O ͒. 10,16 Still controversial, however, is how such visible emissions correlate with the energetics of Zn/O vacancies, interstitials, and their complexes overall. This work clearly identifies the physical nature of the defects dominating optical features of this widely studied semiconductor and, in turn, these defects provide a consistent explanation for ZnO's effective free-carrier densities on a local scale.Contemporary theoretical approaches are also limited in addressing ZnO defect energetics due to major uncertainties, most notably, the "band-gap problem" within densityfunctional methods. 17 Calculations of such basic ZnO defect properties as formation energy and energy-level relative to band edges vary considerably with different approximations. 5,18-21 Therefore, the determination of energy levels of native point defects and energetics of Zn vacancies versus their clusters provides a method to evaluate methods for calcu...

Changing vacancy balance in ZnO by tuning synthesis between zinc/oxygen lean conditions

“…The relative intensity of the green and red spectral signatures apparently correlates with the synthesis temperatures. Similar DLE components were observed by Dong et al 11 in depth resolved cathodoluminescence ͑DRCL͒ experiments, with the 2 eV emission reportedly in a direct correlation with the vacancy cluster signals measured by PAS. Such association also appears consistent with the present PAS data, specifically considering clear dominance of the 2 eV emission in the films grown at 325°C comparing to the rest of the samples in Fig.…”

“…Interestingly, the ͑S, W͒ parameters of the sample grown at 325°C approach those of the vacancy clusters observed in Li-implanted and flashannealed ZnO, 15 correlated with the characteristic 2 eV optical emission in Ref. 11.…”

“…On the other hand, vacancy-type defects in ZnO thin films have been extensively studied using photoluminescence ͑PL͒, 10 although the exact correlation of deep-level emission ͑DLE͒ peaks with specific intrinsic defects still remains a matter of ongoing discussions. 1,5,11 In this respect, a combination of PAS/PL measurements might be capable to reveal the nature of the dominating intrinsic defects in MOVPE-ZnO films. To make such identification conclusive, it is essential to provide samples exhibiting systematic variations in the intrinsic defect balance.…”

“…In this work, we focus primarily on the spectral region of DLE and employ deconvolution techniques to analyze intensities and positions of the characteristic bands referring to data reported in literature 5,11 as well as to the trends resolved in our PAS measurements. The essence of the vacancy identification with PAS is in positron trapping at neutral and negative vacancy defects, which modifies the annihilation characteristics.…”

Optical property modification of ZnO: Effect of 1.2 MeV Ar irradiation

“…Stable co-existence of Zn I and O V defects have also been identified experimentally in annealed ZnO based ceramics [17]. It has been shown [6,18] The colour of the samples changes from white to dark reddish brown and resistivity is lowered drastically due to 5 × 10 15 ions/cm 2 fluence [6]. Large reduction of resistivity near the subsurface region has been accounted for Zn-O divacancy clusters with abundant O V s in the N implanted ZnO [18].…”