Shifting donor-acceptor photoluminescence in N-doped ZnO

We have grown nitrogen-doped ZnO films grown by two kinds of epitaxial methods on lattice-matched ScAlMgO$_4$ substrates. We measured the photoluminescence (PL) of the two kinds of ZnO:N layers in the donor-acceptor-pair transition region. The analysis of excitation-intensity dependence of the PL peak shift with a fluctuation model has proven that our observed growth-technique dependence was explained in terms of the inhomogeneity of charged impurity distribution. It was found that the inhomogeneity in the sample prepared with the process showing better electrical property was significantly smaller in spite of the similar nitrogen concentration. The activation energy of acceptor has been evaluated to be $\approx 170$ meV, which is independent of the nitrogen concentration.Comment: 4 pages, 3 figures, 1 table, RevTeX4, to appear in the July issue of J. Phys. Soc. Jp

Introduction and recovery of point defects in electron-irradiated ZnO

We have used positron annihilation spectroscopy to study the introduction and recovery of point defects in electron-irradiated n-type ZnO. The irradiation (Eel=2MeV, fluence 6×10 exp 17 cm exp −2) was performed at room temperature, and isochronal annealings were performed from 300 to 600 K. In addition, monochromatic illumination of the samples during low-temperature positron measurements was used in identification of the defects. We distinguish two kinds of vacancy defects: the Zn and O vacancies, which are either isolated or belong to defect complexes. In addition, we observe negative-ion-type defects, which are attributed to O interstitials or O antisites. The Zn vacancies and negative ions act as compensating centers and are introduced at a concentration [VZn]≃cion≃2×10 exp 16 cm exp −3. The O vacancies are introduced at a 10-times-larger concentration [VO]≃3×10 exp 17 cm exp −3 and are suggested to be isolated. The O vacancies are observed as neutral at low temperatures, and an ionization energy of 100 meV could be fitted with the help of temperature-dependent Hall data, thus indicating their deep donor character. The irradiation-induced defects fully recover after the annealing at 600 K, in good agreement with electrical measurements. The Zn vacancies recover in two separate stages, indicating that the Zn vacancies are parts of two different defect complexes. The O vacancies anneal simultaneously with the Zn vacancies at the later stage, with an activation energy of EmV,O = 1.8 ± 0.1 eV. The negative ions anneal out between the two annealing stages of the vacancies.Peer reviewe

Evidence of the Zn Vacancy Acting as the Dominant Acceptor in n-Type ZnO

We have used positron annihilation spectroscopy to determine the nature and the concentrations of the open volume defects in as-grown and electron irradiated (Eel=2   MeV, fluence 6×10 exp 17   cm exp −2) ZnO samples. The Zn vacancies are identified at concentrations of [VZn]≃2×10 exp 15   cm exp −3 in the as-grown material and [VZn]≃2×10 exp 16   cm exp −3 in the irradiated ZnO. These concentrations are in very good agreement with the total acceptor density determined by temperature dependent Hall experiments. Thus, the Zn vacancies are dominant acceptors in both as-grown and irradiated ZnO.Peer reviewe

Self-compensation in semiconductors: The Zn vacancy in Ga-doped ZnO

Self-compensation, the tendency of a crystal to lower its energy by forming point defects to counter the effects of a dopant, is here quantitatively proven. Based on a new theoretical formalism and several different experimental techniques, we demonstrate that the addition of 1.4 × 10 exp 21-cm exp −3 Ga donors in ZnO causes the lattice to form 1.7 × 10 exp 20-cm exp −3 Zn-vacancy acceptors. The calculated VZn formation energy of 0.2 eV is consistent with predictions from density functional theory. Our formalism is of general validity and can be used to investigate self-compensation in any degenerate semiconductor material.Peer reviewe

Deep centers in a free-standing GaN layer

Schottky barrierdiodes, on both Ga and N faces of a ∼300-μm-thick free-standing GaN layer, grown by hydride vapor phase epitaxy(HVPE) on Al2O3 followed by laser separation, were studied by capacitance–voltage and deep level transient spectroscopy(DLTS) measurements. From a 1/C2 vs V analysis, the barrier heights of Ni/Au Schottky contacts were determined to be different for the two polar faces: 1.27 eV for the Ga face, and 0.75 eV for the N face. In addition to the four common DLTS traps observed previously in other epitaxial GaN including HVPE-grown GaN a new trap B′ with activation energyET=0.53 eV was found in the Ga-face sample. Also, trap E1 (ET=0.18 eV), believed to be related to the N vacancy, was found in the N-face sample, and trap C (ET=0.35 eV) was in the Ga-face sample. Trap C may have arisen from reactive-ion-etching damage