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(el)=2 MeV, fluence 6 x 10(17) cm(-2)) ZnO samples. The Zn vacancies are identified at concentrations of [V(Zn)] approximately 2 x 10(15) cm(-3) in the as-grown material and [V(Zn)] approximately 2 x 10(16) cm(-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 el = 2 MeV, fluence 6 ϫ 10 17 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 ͓V Zn ͔Ӎc ion Ӎ 2 ϫ 10 16 cm −3 . The O vacancies are introduced at a 10-times-larger concentration ͓V O ͔Ӎ3 ϫ 10 17 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,O m = 1.8± 0.1 eV. The negative ions anneal out between the two annealing stages of the vacancies.

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 x 10 21 -cm -3 Ga donors in ZnO causes the lattice to form 1.7 x 10 20 -cm -3 Zn-vacancy acceptors. The calculated V 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.2

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...

By combining results from positron annihilation and photoluminescence spectroscopy with data from Hall effect measurements, the characteristic deep level emission centered at ∼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.

It is shown that self‐assembled hierarchical porosity in organic polymers can be obtained in a facile manner based on pyrolyzed block‐copolymer–phenolic resin nanocomposites and that a given starting composition can be post‐modified in a wide range from monomodal mesoporous materials to hierarchical micro‐mesoporous materials with a high density of pores and large surface area per volume unit (up to 500–600 m2 g–1). For that purpose, self‐assembled cured composites are used where phenolic resin is templated by a diblock copolymer poly(4‐vinylpyridine)‐block‐polystyrene (P4VP‐b‐PS). Mild pyrolysis conditions lead only to monomodal mesoscale porosity, as essentially only the PS block is removed (length scale of tens of nanometers), whereas during more severe conditions under prolonged isothermal pyrolysis at 420 °C the P4VP chains within the phenolic matrix are also removed, leading to additional microporosity (sub‐nanometer length scale). The porosity is analyzed using transmission electron microscopy (TEM), small‐angle X‐ray scattering, electron microscopy tomography (3D‐TEM), positron annihilation lifetime spectroscopy (PALS), and surface‐area Brunauer–Emmett–Teller (BET) measurements. Furthermore, the relative amount of micro‐ and mesopores can be tuned in situ by post modification. As controlled pyrolysis leaves phenolic hydroxyl groups at the pore walls and the thermoset resin‐based materials can be easily molded into a desired shape, it is expected that such materials could be useful for sensors, separation materials, filters, and templates for catalysis.

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 Oi.