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

Since the successful demonstration of a blue light-emitting diode (LED)1, potential materials for making short-wavelength LEDs and diode lasers have been attracting increasing interest as the demands for display, illumination and information storage grow2,3,4. Zinc oxide has substantial advantages including large exciton binding energy, as demonstrated by efficient excitonic lasing on optical excitation5,6. Several groups have postulated the use of p-type ZnO doped with nitrogen, arsenic or phosphorus7,8,9,10, and even p–n junctions11,12,13. However, the choice of dopant and growth technique remains controversial and the reliability of p-type ZnO is still under debate14. If ZnO is ever to produce long-lasting and robust devices, the quality of epitaxial layers has to be improved as has been the protocol in other compound semiconductors15. Here we report high-quality undoped films with electron mobility exceeding that in the bulk. We have used a new technique to fabricate p-type ZnO reproducibly. Violet electroluminescence from homostructural p–i–n junctions is demonstrated at room-temperature.

Repeated temperature modulation epitaxy for p-type doping and light-emitting diode based on ZnO

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Optical properties of ZnO nanostructures.

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

(110) oriented YBa2Cu3O7−δ thin films have been epitaxially grown on vicinal (110)SrTiO3 substrates by pulsed-laser deposition. Films with different oxygen deficiencies were obtained by annealing under various oxygen ambient. These films have very smooth surface and with their c axis aligned in one direction in the substrate plane. Electronic transport and polarized far-infrared reflectivity measurements reveal the unique features of the c-axis charge dynamics. A c-axis far-infrared reflectivity edge associated with the Josephson plasma in the superconducting state is observed. Our results demonstrate the potential usage of (110) thin films for the investigation of intrinsic interlayer charge dynamics as well as device application of high Tc superconductors.

Josephson plasma observed in underdoped (110) YBa2Cu3O7−δ films

PROBLEM TO BE SOLVED: To enable light emission with an excitor at a room temperature by providing a zinc oxide thin film into which magnesium is solid-dissolved, in an optical semiconductor device containing groups II oxide. SOLUTION: A thin film of an optical semiconductor device containing group II oxide is manufactured using a laser molecular beam epitaxy method. On a sapphire substrate, a thin film, into which a magnesium composition is solid- dissolved by 4-10 mol%, is formed using a zinc oxide sintered body containing magnesium by, for example, 0-18 mol% as a raw material target. A component of raw material solid is coagulated in a moment on the substrate, and a crystal is formed untill a material becomes steady in a thermodynamic equilibrium condition at a substrate temperature, so that a zinc oxide thin film, into which a magnesium containing a composition and a structure that are not allowed thermodynamically is solid-dissolved, is manufactured. Thus a highly crystalline thin film is manufactured in a laser molecular beam epitaxy method, and light emission in a recombination process of an excitor is enabled at a room temperature. COPYRIGHT: (C)1998,JPO

Optical semiconductor device containing group ii oxide nd manufacture thereof

Time-resolved photoluminescence spectroscopy (TRPL) has been employed to study a set of ZnO/Zn 1-x Mg x O (x = 0.12) multiple quantum wells (MQWs) grown by laser molecular beam epitaxy, with well-width L w , varying from 6.91 to 46.5 A. We have estimated the L w dependence of radiative and nonradiative recombination lifetimes of excitons at low temperature (5 K). Radiative recombination lifetimes were dramatically increased at narrow L w , and the thermal release effect of excitonic localization is discussed. On the other hand, the nonradiative recombination rates were almost constant over the L w range studied, so we conclude that suppression of quantum efficiency due to carrier leakage can be avoided even at narrow L w .

Well-width dependence of radiative and nonradiative lifetimes in ZnO-based multiple quantum wells

Transparent and conductive ZnO:Ga thin films are prepared by laser molecular-beam epitaxy. Their electron properties were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about $n_s \sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs, respectively. In the experimental `mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $\sim 10^{20}$ cm$^{-3}$. Several types of scattering centers including ionized donors and oxygen traps are considered to account for the observed dependence of the Hall mobility on carrier concentration. The scattering mechanism is explained in terms of inter-grain potential barriers and charged impurities. A comparison between theoretical results and experimental data is made.

https://arxiv.org/pdf/cond-mat/0510340.pdf

Majority-Carrier Mobilities in Undoped and \textit{n}-type Doped ZnO Epitaxial Layers

We report on the experimental observation of the radiative recombination of localized biexcitons at low temperature (5 K) in ZnO/ZnMgO multi-quantum wells grown by laser-molecular-beam epitaxy on a lattice-matched ScAlMgO 4 substrate (0001). The emission components due to the recombinations of localized excitons and biexcitons and due to the exciton-exciton scattering were verified by examining their relative energy positions and intensity dependence on excitation power density. The excitation threshold for biexciton emission was significantly lower than that for exciton-exciton scattering. The binding energy of biexcitons in multi-quantum wells is largely enhanced compared with the corresponding value of bulk ZnO.

Y. Segawa

Papers

Josephson plasma observed in underdoped (110) YBa2Cu3O7−δ films

Optical semiconductor device containing group ii oxide nd manufacture thereof

Well-width dependence of radiative and nonradiative lifetimes in ZnO-based multiple quantum wells

Majority-Carrier Mobilities in Undoped and \textit{n}-type Doped ZnO Epitaxial Layers

Observation of Biexciton Emission in ZnO/ZnMgO Multi‐Quantum Wells