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. Lett. 16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett. 18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors 2, 624 (1968)], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)], ZnO∕ZnSe n-p junctions (1975) by Tsurkan et al. [Semiconductors 6, 1183 (1975)], and Al∕Au Ohmic contacts by Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B 241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.

The optical properties of a high quality bulk $\mathrm{ZnO}$, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at $3.3605\mathrm{eV}$ having a linewidth of $0.7\mathrm{meV}$ and dominating the PL spectrum at $10\mathrm{K}$. This bound exciton transition was visible only below $150\mathrm{K}$, whereas the A-free exciton transition at $3.3771\mathrm{eV}$ persisted up to room temperature. A-free exciton binding energy of $60\mathrm{meV}$ is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of $\sim 170$ and $\sim 864\mathrm{ps}$ for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.

High quality $n\text{-}\mathrm{Zn}\mathrm{O}$ films on commercial $p$ -type 6H–SiC substrates have been grown by plasma-assisted molecular-beam epitaxy, and $n\text{-}\mathrm{Zn}\mathrm{O}∕p\text{-}\mathrm{Si}\mathrm{C}$ heterojunction mesa structures have been fabricated. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than $2\times {10}^{-4}\mathrm{A}∕{\mathrm{cm}}^2$ at $-10\mathrm{V}$ , a breakdown voltage greater than $20\mathrm{V}$ , a forward turn on voltage of $\sim 5\mathrm{V}$ , and a forward current of $\sim 2\mathrm{A}∕{\mathrm{cm}}^2$ at $8\mathrm{V}$ . Photosensitivity of the diodes was studied at room temperature and a photoresponsivity of as high as $0.045\mathrm{A}∕\mathrm{W}$ at $-7.5\mathrm{V}$ reverse bias was observed for photon energies higher than $3.0\mathrm{eV}$ .