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

Recent progress in processing and properties of ZnO

ZnO nanostructures for optoelectronics: Material properties and device applications

This Letter focuses on the discovery of a transparent conducting oxide (TCO), anatase Ti1−xNbxO2 films with x=0.002–0.2. The resistivity of films with x⩾0.03 is 2–3×10−4Ωcm at room temperature. The carrier density of Ti1−xNbxO2 can be controlled in a range of 1×1019to2×1021cm−3. The internal transmittance for films with x⩽0.03 (40nm thickness) is about 97% in the visible light region. The transport and optical parameters are comparable to those of typical TCOs, such as In2−xSnxO3 and ZnO.

A transparent metal: Nb-doped anatase TiO2

Zinc oxide (ZnO) films were grown on sapphire (1120) substrates by molecular beam epitaxy under oxygen radical irradiation. The effect of the growth conditions, including the Zn/O ratio supplied to the film surface, on the electrical properties of ZnO films was studied in relation to the film morphology. We found that the growth rate strongly depended on the Zn flux from the Knudsen cell and the optimum condition for high growth rate was very narrow. The grain size in the lateral direction increased with increasing growth rate in the thickness direction. The increase in growth rate, especially in the lateral direction, resulted in the carrier mobility increasing up to 42 cm2 V−1 s−1. The carrier concentration N was sensitive to the substrate temperature, while the value of N was not sensitive to the source supplying ratio Zn/O. We discuss the decrease of the carrier concentration with increasing substrate temperature in regard to the formation of nonequilibrium defects.

Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxy

Undoped and aluminum-doped ZnO epitaxial films were grown on (001) sapphire substrates by an ion-beam sputtering method with or without the irradiation of oxygen radicals. The effect of oxygen-radical irradiation was notable in the undoped ZnO films when the growth temperature was relatively low. The irradiation improved the crystallinity and decreased the oxygen-vacancy concentration, while it induced internal stress into the films. The carrier concentration of the undoped ZnO films was decreased by the oxygen-radical irradiation, which was attributable to a decrease in the oxygen-vacancy concentration. The Hall mobility of the undoped ZnO films was as low as 1–3 cm2 V-1 s-1. The low mobility was explained by carrier scattering due to the potential barriers at the grain boundaries. The height of the potential barriers at the grain boundaries decreased with increasing carrier concentration. This behavior was well explained by a simple model assuming a single defect state at grain boundaries.

https://iopscience.iop.org/article/10.1143/JJAP.38.3682/pdf

Electric properties of zinc oxide epitaxial films grown by ion-beam sputtering with oxygen-radical irradiation

Deep donor levels in ZnO single crystals doped with transition metal (TM; Co or Mn) were characterized by isothermal capacitance transient spectroscopy (ICTS) applied to ZnO-based Schottky junctions, Au/ZnO (0001) or Ag/ZnO (0001). The barrier height at the junction and donor concentration was not influenced by TM. A deep donor level at 0.28 eV was detected by ICTS; however, its energy dispersion and concentration was composition independent. The effect of doping with TM was found in the magnitude of leakage current; in other words, the leakage current at the Au/ZnO:Mn junction was lower than the other junctions on undoped or Co-doped crystals.

Isothermal capacitance transient spectroscopy for deep levels in co- and Mn-doped ZnO single crystals

Band-edge emission of ZnO at around room temperature was investigated by measuring the temperature dependence of cathodoluminescence spectra at 20–300 K. Undoped crystals grown by a vapor transport method and Al-doped crystals by flux method were employed to elucidate the effect of doping on luminescence properties. For the Al-doped crystals, the free-exciton emission was weak through out the temperature range T<300 K. The most intense emission peak of the Al-doped crystal was energetically close to bound exciton annihilation emission. On the other hand, for undoped crystals, it was found that the most intense emission peak at room temperature was at E≈Eg−60 meV and this peak was not assignable to free-exciton annihilation emission. It was also found that this peak is not a reason for the reduction in emission efficiency.

Band-edge emission of undoped and doped ZnO single crystals at room temperature

Zinc oxide films doped with Mn (Mn:ZnO) were prepared by implanting Mn+ ions into ZnO films deposited by pulsed laser deposition, and their structure and magnetic properties were studied. The Raman spectra of the films indicated that Mn ions occupied the Zn site of ZnO after annealing, while the as-implanted films were amorphous like the ones with very low crystallinity. Magnetic measurements revealed that neither as-implanted nor annealed Mn:ZnO films showed ferromagnetism. The Mn:ZnO films demonstrated paramagnetism that was likely due to Mn2+ ions at the substitutional Zn site.

Takeshi Ohgaki

Papers

Growth condition dependence of morphology and electric properties of ZnO films on sapphire substrates prepared by molecular beam epitaxy

Electric properties of zinc oxide epitaxial films grown by ion-beam sputtering with oxygen-radical irradiation

Isothermal capacitance transient spectroscopy for deep levels in co- and Mn-doped ZnO single crystals

Band-edge emission of undoped and doped ZnO single crystals at room temperature

Structural and magnetic properties of Mn-ion implanted ZnO films