The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

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Fundamentals of zinc oxide as a semiconductor

We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++‐Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susc...

Diluted magnetic semiconductors

We have performed a comprehensive first-principles investigation of native point defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem. Oxygen deficiency, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional $n$-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native point defects are very unlikely to be the cause of unintentional $n$-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in $n$-type ZnO (and are therefore unlikely to form). Zinc interstitials are shallow donors, but they also have high formation energies in $n$-type ZnO and are fast diffusers with migration barriers as low as $0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; they are therefore unlikely to be stable. Zinc antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equilibrium conditions. We have, however, identified a different low-energy atomic configuration for zinc antisites that may play a role under nonequilibrium conditions such as irradiation. Zinc vacancies are deep acceptors and probably related to the frequently observed green luminescence; they act as compensating centers in $n$-type ZnO. Oxygen interstitials have high formation energies; they can occur as electrically neutral split interstitials in semi-insulating and $p$-type materials or as deep acceptors at octahedral interstitial sites in $n$-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Based on our results for migration energy barriers, we calculate activation energies for self-diffusion and estimate defect-annealing temperatures. Our results provide a guide to more refined experimental studies of point defects in ZnO and their influence on the control of $p$-type doping.

Native point defects in ZnO

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Beryllium incorporation was studied for both Ga-polarity and N-polarity GaN using a series of Be step-doped epitaxial layers. Dopant concentration profiles indicated that surface polarity-related incorporation differences are not pronounced for Be. Significant surface accumulation of Be occurs during growth with surface accumulations approaching a monolayer for heavier doping levels. Transmission electron microscopy studies indicate the surface layer of Be has a significant effect on the microstructure, particularly for near monolayer coverage.

Incorporation-related structural issues for beryllium doping during growth of GaN by rf-plasma molecular-beam epitaxy

The introduction of phosphorus and arsenic dopants into bulk Cd1−xMnxTe crystals grown by the Bridgman–Stockbarger technique has been studied with respect to the resulting electrical and optical properties. Uncompensated acceptor concentrations as high as 1015–1016 cm−3 are obtained. Samples with a Mn composition in the range 0.10<x<0.30, both as‐grown and annealed, are studied. Hall‐effect and resistivity measurements are used to determine carrier concentrations, mobilities, and acceptor activation energies. A combination of room‐temperature transmittance and reflectance measurements over the spectral range from the ultraviolet to the far infrared has been used to gain information concerning structural quality and point defects. Low‐temperature photoluminescence measurements (1.6–5 K) are used to determine optical quality and excitonic energies. The effect of alloy broadening on luminescence linewidth is calculated and compared with measured values.

Electrical and optical properties of P-and As-doped Cd1−xMnxTe

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Triplet ground state of the neutral oxygen-vacancy donor in rutile TiO 2

Electron paramagnetic resonance (EPR) is used to experimentally determine the (0/−) level of the Mg acceptor in an Mg-doped β-Ga2O3 crystal. Our results place this level 0.65 eV (±0.05 eV) above the valence band, a position closer to the valence band than the predictions of several recent computational studies. The crystal used in this investigation was grown by the Czochralski method and contains large concentrations of Mg acceptors and Ir donors, as well as a small concentration of Fe ions and an even smaller concentration of Cr ions. Below room temperature, illumination with 325 nm laser light produces the characteristic EPR spectrum from neutral Mg acceptors ( M g G a 0). A portion of the singly ionized Ir4+ donors are converted to their neutral Ir3+ state at the same time. For temperatures near 250 K, the photoinduced EPR spectrum from the neutral M g G a 0 acceptors begins to decay immediately after the laser light is removed, as electrons are thermally excited from the valence band to the Mg acceptor. Holes left in the valence band recombine with electrons at the deeper Ir3+ ions and restore the Ir4+ ions. An activation energy for the thermal decay of the M g G a 0 acceptors, and thus a value for the (0/−) level, is obtained by using a general-order kinetics model to analyze a set of five isothermal decay curves taken at temperatures between 240 and 260 K.

https://scholar.afit.edu/cgi/viewcontent.cgi?article=1676&context=facpub

Experimental determination of the (0/−) level for Mg acceptors in β-Ga2O3 crystals

Electron paramagnetic resonance (EPR) is used to identify and characterize neutral zinc acceptors in Zn-doped β-Ga2O3 crystals. Two EPR spectra are observed at low temperatures, one from Zn ions at tetrahedral Ga(1) sites (the Z n Ga 1 0 acceptor) and one from Zn ions at octahedral Ga(2) sites (the Z n Ga 2 0 acceptor). These Zn acceptors are small polarons, with the unpaired spin localized in each case on a threefold coordinated oxygen O(I) ion adjacent to the Zn ion. Resolved hyperfine interactions with neighboring 69Ga and 71Ga nuclei allow the EPR spectra from the two acceptors to be easily distinguished: Z n Ga 1 0 acceptors interact equally with two Ga(2) ions and Z n Ga 2 0 acceptors interact unequally with a Ga(1) ion and a Ga(2) ion. The as-grown crystals are compensated, with the Zn ions initially present as singly ionized acceptors ( Z n Ga 1 − and Z n Ga 2 −). Exposing a crystal to 325 nm laser light, while being held at 140 K, primarily produces neutral Z n Ga 2 0 acceptors when photoinduced holes are trapped at Z n Ga 2 − acceptors. This suggests that there may be significantly more Zn ions at Ga(2) sites than at Ga(1) sites. Warming the crystal briefly to room temperature, after removing the light, destroys the EPR spectrum from the shallower Z n Ga 2 0 acceptors and produces the EPR spectrum from the more stable Z n Ga 1 0 acceptors. The Z n Ga 2 0 acceptors decay in the 240–260 K region with a thermal activation energy near 0.65 eV, similar to Mg Ga 2 0 acceptors, whereas the slightly deeper Z n Ga 1 0 acceptors decay close to room temperature with an approximate thermal activation energy of 0.78 eV.

Nancy C. Giles

Papers

Incorporation-related structural issues for beryllium doping during growth of GaN by rf-plasma molecular-beam epitaxy

Electrical and optical properties of P-and As-doped Cd1−xMnxTe

Triplet ground state of the neutral oxygen-vacancy donor in rutile TiO 2

Experimental determination of the (0/−) level for Mg acceptors in β-Ga2O3 crystals

Zn Acceptors in β-Ga2O3 Crystals