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High-quality ZnO films are receiving increased interest for use in low-loss high-frequency surface acoustic wave (SAW) devices, acousto-optic and optical modulators, as buffer layers for III-nitride growth, and as the active material in ultraviolet solid state lasers. In this work, high quality epitaxial ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The structural, piezoelectric, and optical properties of the ZnO films on R sapphire have been investigated. The epitaxial relationship between ZnO and R-Al2O3 was found to be (112̄0) ZnO∥(011̄2) Al2O3, and [0001] ZnO∥[01̄11] Al2O3. The interface between as-grown ZnO and R sapphire was atomically sharp and semicoherent, as evaluated by transmission electron microscopy. On annealing the films at temperatures above 850 °C, a solid state reaction occurred between ZnO and Al2O3, resulting in the formation of ZnAl2O4 (spinel) at the interface. A 15–20 nm spinel layer formed when the ZnO film was annealed at 850 °C for 30 min, whereas a 150 nm layer formed when the film was annealed at 1000 °C for 150 min. To prevent this reaction from occurring, the maximum process temperature should be below 750 °C. The surface acoustic wave properties of the piezoelectric ZnO were evaluated by fabricating SAW devices on (112̄0) ZnO/(011̄2) Al2O3. An effective electromechanical coupling coefficient, keff2, of 6% was achieved for a 1.5 μm thick ZnO film, which is close to the value for bulk single-crystal ZnO. The photoluminescence spectra were obtained both at room temperature and at 11 K. The full width at half maximum of the 3.363 eV band edge emission photoluminescence peak measured at 11 K was 6 meV, which is close to that for single-crystal ZnO. We also evaluated the anisotropic absorption characteristics of the (112̄0) ZnO film, which can be used for a high contrast ultraviolet light modulator.

We report on the realization of wide band gap (5–6 eV), single-phase, metastable, and epitaxial MgxZn1−xO thin-film alloys grown on sapphire by pulsed laser deposition. We found that the composition, structure, and band gaps of the MgxZn1−xO thin-film alloys depend critically on the growth temperature. The structural transition from hexagonal to cubic phase has been observed for (Mg content greater than 50 at. %) (1⩾x⩾0.5) which can be achieved by growing the film alloys in the temperature range of 750 °C to room temperature. Interestingly, the increase of Mg content in the film has been found to be beneficial for the epitaxial growth at relatively low growth temperature in spite of a large lattice mismatch between sapphire and cubic MgZnO alloys.

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In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s-i-n+/s-i-p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The case of uniform field, obtained with this specialized structure as well as that of the nonuniform field, are addressed. The various experimental techniques are also briefly reviewed. We then summarize the various experimental results obtained in the last few years using these special structures and FKOs and find that, even in this short period, good use has been made of the technique and the structure. This is followed by a brief review of the work on nonuniform fields. In this case, the work on actual device structures has significant technological implications. Important issues such as metallization and processing, the effects of surface treatment and thermal annealing, Schottky barrier heights of different metals, piezoelectric fields in (111) grown strained InGaAs/GaAs quantum wells, and Fermi level in low-temperature grown GaAs have been studied using this structure. This structure has also been used to study the dynamics of photomodulation, revealing the nature of the cw photoreflectance.

We report on the epitaxial growth of wide-band-gap cubic-phase MgxZn1−xO thin films on Si(100) by pulsed-laser deposition and fabrication of oxide-semiconductor-based ultraviolet photodetectors. The challenges of large lattice and thermal expansion mismatch between Si and MgxZn1−xO have been overcome by using a thin SrTiO3 buffer layer. The heteroepitaxy of cubic-phase MgxZn1−xO on Si was established with epitaxial relationship of MgxZn1−xO(100)//SrTiO3(100)//Si(100) and MgxZn1−xO[100]//SrTiO3[100]//Si[110]. The minimum yield of the Rutherford backscattering ion channeling in MgxZn1−xO layer was only 4%, indicating good crystalline quality of the film. Smooth surface morphology with rms roughness of 0.6 nm was observed using atomic force microscopy. Photodetectors fabricated on Mg0.68Zn0.32O/SrTiO3/Si show peak photoresponse at 225 nm, which is in the deep UV region.

We report on the fabrication and characterization of visible blind ultraviolet photodetectors based on MgxZn1−xO. Using pulsed laser deposition technique, Mg0.34Zn0.66O thin films with a bandgap of 4.05 eV were epitaxially grown on c-plane sapphire substrates. The structural, electrical, and optical properties of epilayers were characterized using various techniques. Based on the Mg0.34Zn0.66O films, planar geometry photconductive type metal–semiconductor–metal photodetectors were fabricated. At a 5 V bias, a high responsivity of 1200 A/W was achieved at 308 nm, and the visible rejection (R308 nm/R400 nm) was more than four orders of magnitude. The 10%–90% rise and fall time were 8 ns and 1.4 μs, respectively.

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