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

We investigated the optical properties of epitaxial n-type ZnO films grown on lattice-matched ScAlMgO4 substrates. As the Ga doping concentration increased up to 6×1020cm−3, the absorption edge showed a systematic blueshift, consistent with the Burstein–Moss effect. A bright near-band-edge photoluminescence (PL) could be observed even at room temperature, the intensity of which increased monotonically as the doping concentration was increased except for the highest doping level. It indicates that nonradiative transitions dominate at a low doping density. Both a Stokes shift and broadening in the PL band are monotonically increasing functions of donor concentration, which was explained in terms of potential fluctuations caused by the random distribution of donor impurities.

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Gallium concentration dependence of room-temperature near-band-edge luminescence in n-type ZnO:Ga

We investigated the optical properties of epitaxial \textit{n}-type ZnO films grown on lattice-matched ScAlMgO$_4$ substrates. As the Ga doping concentration increased up to $6 \times 10^{20}$ cm$^{-3}$, the absorption edge showed a systematic blueshift, consistent with the Burstein-Moss effect. A bright near-bandedge photoluminescence (PL) could be observed even at room temperature, the intensity of which increased monotonically as the doping concentration was increased except for the highest doping level. It indicates that nonradiative transitions dominate at a low doping density. Both a Stokes shift and broadening in the PL band are monotonically increasing functions of donor concentration, which was explained in terms of potential fluctuations caused by the random distribution of donor impurities.

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Gallium concentration dependence of room-temperature near-bandedge luminescence in n-type ZnO:Ga

We report on temperature dependence of excitonic photoluminescence (PL) from ZnO/(Mg, Zn)O multiple quantum wells (MQWs). Two kinds of MQWs having different barrier heights grown by laser molecular-beam epitaxy showed significantly different temperature dependences of PL spectra; in ZnO/Mg0.27Zn0.73O MQWs, the PL peak energy at 50–200 K was a monotonically increasing function of temperature, which was opposite to that ascribed by band gap shrinkage. Moreover, spectra taken at 95–200 K encompassed two peaks, both of which originated from recombination of localized excitons. The temperature-induced shift (redshift-blueshift-peak duplication-redshift) at 5–300 K is caused by a change in the exciton dynamics with increasing temperature due to inhomogeneity and the exciton localization effect. On the other hand, the corresponding dependence in ZnO/Mg0.12Zn0.88O MQWs (lower barrier height) was similar to that in bulk II–VI semiconductors.

https://dr.ntu.edu.sg/bitstream/10356/104086/1/GetPDFServlet3.pdf

Temperature dependence of near ultraviolet photoluminescence in ZnO/(Mg, Zn)O multiple quantum wells

Self-assembled ZnO nano-crystals and exciton lasing at room temperature

We studied photoluminescence (PL) properties of eighteen samples of wurtzite ZnO/MgxZn1−xO multiple quantum wells (x=0.12 and 0.27) with various well widths (Lw) of 0.7–4.65 nm. Radiative recombination of the electron–hole pairs that are spatially separated due to the quantum-confined Stark (QCS) and Franz–Keldish (QCFK) effects was observed in two thicker samples at 5 K. This PL band is located ≈ 40 meV in energy below the emission band of the localized excitons and ≈ 60 meV below the absorption energy of the free exciton transition. One can not observe such kind of luminescence unless both of the following conditions are accomplished: (1) higher Mg concentration (x=0.27) and (2) Lw⩾4.23 nm. These experimental findings do not contradict the following two characteristic features for the QCS and QCFK effects; the magnitude of the electric field due to spontaneous and piezoelectric polarizations and the depth of the triangle-shaped potential wells are the monotonically increasing functions of Mg concentration and the Lw, respectively. The coupling strength with longitudinal-optical phonons, which is determined from the relative luminescence intensities of the phonon replicas, is significantly larger than that between the localized excitons and phonons. It is considered that the strong electric field increases the distance between electron and hole charge distributions from that determined by the Coulomb force and leads to the enhancement in the phonon interaction.

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Radiative recombination of electron–hole pairs spatially separated due to quantum-confined Stark and Franz–Keldish effects in ZnO/Mg0.27Zn0.73O quantum wells

Y. Segawa

Papers

Gallium concentration dependence of room-temperature near-band-edge luminescence in n-type ZnO:Ga

Gallium concentration dependence of room-temperature near-bandedge luminescence in n-type ZnO:Ga

Temperature dependence of near ultraviolet photoluminescence in ZnO/(Mg, Zn)O multiple quantum wells

Self-assembled ZnO nano-crystals and exciton lasing at room temperature

Radiative recombination of electron–hole pairs spatially separated due to quantum-confined Stark and Franz–Keldish effects in ZnO/Mg0.27Zn0.73O quantum wells