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

Epitaxial growth of ZnO films

ZnO is attracting considerable attention for its possible application to light-emitting sources due to its advantages over GaN. We review the recent progress in the growth of ZnO epitaxial films, doping control, device fabrication processes including etching and ohmic contact formation, and finally the prospects for fabrication and characteristics of ZnO light-emitting diodes.

https://iopscience.iop.org/article/10.1088/0022-3727/40/22/R01/pdf

ZnO thin films and light-emitting diodes

Sol-gel preparation of transparent zinc oxide films with highly preferential crystal orientation

Metalorganic chemical vapor phase deposition of ZnO with different O-precursors

C- and O-incorporation in (AlGa)As epitaxial layers grown by MOVPE using TBAs

High-quality ZnO was grown by metal organic vapor phase epitaxy on 1.3 µm thick GaN layers on Si(111) using dimethylzinc and tertiary-butanol as precursors. The variation of the growth temperature shows a strong correlation with the microstructure as observed by atomic force and scanning electron microscopy. With increasing growth temperature we find an increasing size of ZnO crystallites and a transition from 3D to 2D growth. Moreover, increasing the growth temperature leads to a reduction of tensile stress in the ZnO as observed by X-ray diffraction. In highly spatially resolved cathodoluminescence measurements we observe narrow (A0,X) luminescence from the ZnO surface and a strong donator correlated luminescence at macroscopic defects.

http://iopscience.iop.org/article/10.1143/JJAP.42.7474/pdf

Metal Organic Vapor Phase Epitaxy of ZnO on GaN/Si(111) Using Tertiary-Butanol as O-Precursor

Heteroepitaxy and nitrogen doping of high-quality ZnO

Metal organic chemical vapor phase epitaxy-grown ZnO layers using iso-propanol and DMZn as oxygen and zinc precursors, respectively, are presented. The layers were grown on GaN templates in the temperature range 400-475°C at a reactor pressure of 300-500 mbar with VI/II ratios around 45. A columnar structure with an average column size of 6.5 μm in diameter and a roughness of 70 nm (r.m.s.) is observed by atomic force microscopy measurements. In X-ray diffractometry reciprocal space maps the ZnO peak splits up indicating differently strained layers. An approximately 100 nm wide distorted transition region between the GaN and ZnO layers is found in these measurements. The low-temperature cathodoluminescence spectra are dominated by narrow near-band-edge emission and almost no deep defect-related luminescence is found. A pronounced spectral blue shift is found in the growth direction, which is attributed to the relaxation of the tensile stress and associated with inner fields. A strong microscopic correlation between the surface morphology and the emission of the acceptor bound excitons is observed.

A. Greiling

Papers

Metalorganic chemical vapor phase deposition of ZnO with different O-precursors

C- and O-incorporation in (AlGa)As epitaxial layers grown by MOVPE using TBAs

Metal Organic Vapor Phase Epitaxy of ZnO on GaN/Si(111) Using Tertiary-Butanol as O-Precursor

Heteroepitaxy and nitrogen doping of high-quality ZnO

Growth of ZnO layers by metal organic chemical vapor phase epitaxy