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Direct observation of the crystal structure changes in the MgxZn1 − xO alloy system

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Abstract

We directly observed the crystal structure changes of the MgxZn1 − xO alloy thin film deposited on Si (111) substrates. Through the in situ heating transmission electron microscopy study, it was determined that the crystal structure changes did not occur up to at 400 °C, whereas the disappearance of the hexagonal structure was observed at 500 °C in the layer of nanosized grains. Additionally, the decreased intensity of the Zn L-edge was analyzed in the comparison of the core loss electron energy loss spectroscopy spectra of the Zn L-edge and the Mg K-edge obtained at room temperature and 500 °C. Based on these experimental results, the process of crystal structure changes could be explained by the evaporation of Zn atoms in the MgxZn1 − xO alloy system. This phenomenon is prominent in the improvement of the microstructure of the MgxZn1 − xO alloy thin film by controlling the thermal annealing temperature.

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Zn O / Mg Zn O single quantum wells (QWs) in which the well width changes continuously were grown on sapphire (11 2 0) substrates by metalorganic chemical vapor deposition. Photoluminescence (PL) measurement revealed two emission peaks: one is position dependent and the other is not. Polarized PL spectra obtained from cleaved facets demonstrated perfect two-dimensional features of the position-dependent emission peak. The position-dependent peak was attributed to emissions due to excitons confined in the ZnO well layer, and the position-independent peak was attributed to emissions due to excitons in MgZnO barrier layers. The width dependence of the emission energy from the ZnO QW was interpreted by a simple theoretical model. Typical PL decay time of the QW emission was 360 ps at 77 K . It was shorter than that of the MgZnO barrier, 470 ps , due to the enhanced confinement effect in the QW.
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We have investigated the properties of ZnMgO epilayers and ZnO – ZnMgO quantum well structures grown by metalorganic vapor-phase epitaxy. A well-controlled incorporation of magnesium, x≤0.10 , could be confirmed resulting in a blueshift of the photoluminescence emission wavelength of the Zn <sub>1-x</sub> Mg <sub>x</sub> O layers up to 200 meV . Using ZnMgO as barrier material, ZnO – ZnMgO quantum well structures with different well widths have then been fabricated. The confinement effect in the ZnO quantum wells leads to the expected increase of the corresponding quantum well emission energy with decreasing well width. A comparison to calculations also suggests a further enhancement of the exciton binding energy in the quantum wells of up to 90 meV .
Article
Single-phase wurtzite Zn1−xMgxO alloy films with 0⩽x⩽0.45 were successfully grown on (1 1 1)-oriented Si substrates by molecular beam epitaxy. Although the Zn1−xMgxO alloy films with x>0.3 exceeded the solid solubility limit at 600 °C, the growth at lower temperatures was found to be effective to raise the limit up to x=0.45. Both energies of the cathodoluminescence peak and optical absorption edge showed continuous blueshifts with increasing the x value till the solubility limit, although the shift of the former energy became shorter than that of the latter one presumably due to the spatial inhomogeneity in alloy films. Photoresponse measurement for Zn1−xMgxO/Si photoconduction cells revealed visible-blind characteristics with specific cutoff wavelengths in accordance with relevant bandgap energies to their x values.
Article
Un-doped ZnO and MgZnO thin films were deposited on c-plane sapphire substrates by molecular-beam epitaxy (MBE) and subsequently annealed in hydrogen ambient at 200–500 °C with a step of 100 °C. Hall-effect measurements show that annealing temperature has great effect on the electrical property of both ZnO and MgZnO films. The electron concentration of both ZnO and MgZnO films increases with annealing temperature ranging from 200 °C to 400 °C, and then decreases, which is attributed to incorporation of H into ZnO as a shallower donor during the annealing process and change of solid solubility of hydrogen in ZnO and MgZnO films with annealing temperature. The D0X emission is related to the hydrogen in MgZnO film and the donor level of the H is estimated to be 33.5 meV. It is also found that the controversial luminescence band at 3.310 eV can be formed in un-doped ZnO film upon annealing and its intensity increases with increasing annealing temperature, implying that this band may be not related to p-type doping.
Article
This is a report on the effect of a ZnO buffer layer on the microstructures and optical properties of MgZnO thin films grown on Si (100) substrates by radio frequency magnetron sputtering. For the sample without the ZnO buffer layer, the microstructural analyses carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the formation of Mg2Si in the interface between the Si substrate and the MgZnO thin film. Mg2Si induced the random oriented polycrystalline MgZnO thin film. For the sample with the ZnO buffer layer, a few Mg2Si were observed. An epitaxial relationship between the Si substrate and the MgZnO thin film was formed. In both samples, the photoluminescence (PL) investigation showed a small blue shift of the emission peak, which was owing to the incorporation of Mg atoms in ZnO by co-sputtering the MgO and ZnO targets. In addition, the sample with the ZnO buffer layer showed the enhanced PL intensity, when compared with the sample without the buffer layer.
Article
Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.
Article
Amorphous Zn1-xMgxO (alpha-Zn1-xMgxO) ternary alloy thin films across the full compositional range were synthesized by a low-cost sol-gel method on quartz substrates. The amorphous property of the alpha-Zn1-xMgxO films was verified by x-ray diffraction, and atomic force microscopy revealed a smooth surface with sub-nanometer root-mean square roughness. The current phase segregation issue limiting application of crystalline Zn1-xMgxO with 38% < x < 75% was completely eliminated by growing amorphous films. Optical transmission measurements showed high transmissivity of more than 90% in the visible and near infrared regions, with optical bandgap tunability from 3.3 eV to more than 6.5 eV by varying the Mg content. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3604782]