Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the main experimental and theoretical analysis techniques are described. The usage of these methods is illustrated through examples in technologically important elemental and compound semiconductors. Future challenges include the analysis of noncrystalline materials and of transient defect-related phenomena.

We have used positron annihilation spectroscopy to determine the nature and the concentrations of the open volume defects in as-grown and electron irradiated ($E_{\mathrm{e}\mathrm{l}}=2\mathrm{M}\mathrm{e}\mathrm{V}$, fluence $6\times {10}^{17}{\mathrm{c}\mathrm{m}}^{-2}$) ZnO samples. The Zn vacancies are identified at concentrations of $[V_{\mathrm{Z}\mathrm{n}}]\simeq 2\times {10}^{15}{\mathrm{c}\mathrm{m}}^{-3}$ in the as-grown material and $[V_{\mathrm{Z}\mathrm{n}}]\simeq 2\times {10}^{16}{\mathrm{c}\mathrm{m}}^{-3}$ 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.

We have used positron annihilation spectroscopy to study the introduction and recovery of point defects in electron-irradiated $n$-type ZnO. The irradiation ($E_{\mathrm{el}}=2\mathrm{MeV}$, fluence $6\times {10}^{17}{\mathrm{cm}}^{-2}$) 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 $\left[V_{\mathrm{Zn}}\right]\simeq c_{\mathrm{ion}}\simeq 2\times {10}^{16}{\mathrm{cm}}^{-3}$. The O vacancies are introduced at a 10-times-larger concentration $\left[V_{\mathrm{O}}\right]\simeq 3\times {10}^{17}{\mathrm{cm}}^{-3}$ 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 $E_{V,\mathrm{O}}^m=1.8\pm 0.1\mathrm{eV}$. The negative ions anneal out between the two annealing stages of the vacancies.

The authors have applied positron annihilation spectroscopy to study the vacancy defects in undoped and Si-doped Ga₂O₃ thin films. The results show that Ga vacancies are formed efficiently during metal-organic vapor phase epitaxy growth of Ga₂O₃ thin films. Their concentrations are high enough to fully account for the electrical compensation of Si doping. This is in clear contrast to another n-type transparent semiconducting oxide In₂O₃, where recent results show that n-type conductivity is not limited by cation vacancies but by other intrinsic defects such as O_i.

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${}^{21}$-cm${}^{-3}$ Ga donors in ZnO causes the lattice to form 1.7 × 10${}^{20}$-cm${}^{-3}$ Zn-vacancy acceptors. The calculated $V_{\mathrm{Zn}}$ 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.

By combining results from positron annihilation and photoluminescence spectroscopy with data from Hall effect measurements, the characteristic deep level emission centered at $\sim${}1.75 eV and exhibiting an activation energy of thermal quenching of 11.5 meV is associated with the zinc vacancy. Further, a strong indication that oxygen interstitials act as a dominating acceptor is derived from the analysis of charge carrier losses induced by electron irradiation with variable energy below and above the threshold for Zn-atom displacement. We also demonstrate that the commonly observed green emission is related to an extrinsic acceptorlike impurity, which may be readily passivated by oxygen vacancies.