Structural and Optical Property Investigations on Mg-Alloying in Epitaxial Zinc Oxide Films on Sapphire
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Abstract We have synthesized single-crystal epitaxial MgZnO films by pulsed-laser deposition. High-resolution transmission electron microscopy, X-ray diffraction and Rutherford backscattering spectroscopy/ion channeling were used to characterize the microstructure, defect content, composition and epitaxial single-crystal quality of the films. In these films with up to ~ 34 atomic percent Mg incorporation, an intense ultraviolet band edge photoluminescence at room temperature and 77 K was observed. The highly efficient photoluminescence is indicative of the excitonic nature of the material. Transmission spectroscopy revealed that the excitonic structure of the alloys was clearly visible at room temperature. Post-deposition annealing in oxygen reduced the number of defects and improved the optical properties of the films. The potential applications of MgZnO alloys in a variety of optoelectronic devices are discussed.
Introduction The intense interest in blue and ultraviolet light emitters and detectors has promoted enormous research efforts into the wide band gap semiconductors. In this context, the major efforts have been put into the development of high quality films of IIInitrides and their alloys. Recently, the successful syntheses of high quality GaN and its alloys have resulted in the commercialization of blue lasers, light-emitting-diodes (LEDs) and ultraviolet photodetectors [1]. With these successes, the LEDs in the visible range have been realized and the major concern now is to make improvements in these devices. Now, the immense interest has been generated in the development of ultraviolet light emitters, and truly solar blind photodetectors based on GaN material system [2-4], which are transparent to the visible and near UV portion of the spectrum. Compact ultraviolet sources and detectors could be useful to monitor or catalyze specific chemical reactions or to excite fluorescence in various proteins. As an alternative to the GaN material system, ZnO and its alloys [5,6] are of substantial interest. Alloying ZnO films with MgO or CdO may potentially permit the band gap to be controlled between 2.8 to 4 eV and higher. Zinc oxide or zincite (ZnO) is hexagonal, whereas magnesium oxide or periclase (MgO) is cubic. However, the similarity in ionic radii between Mg++ (1.36Å) and Zn++ (1.25 Å) allows some replacement in either structure. In the ZnO lattice, the solid solubility of Mg is limited to only 2% maximum [7]. In the present work, using pulsed laser deposition (PLD), we have achieved nonequilibrium phase space corresponding to 34 at. % Mg in ZnO, while maintaining the ZnO hexagonal structure with a lattice constant close to that of ZnO. These films also maintained favorable optical characteristics of wide band gap materials, including transparency in the visible and high excitonic binding energy (~60
F99W3.87
meV) [8]. Direct observation of the excitonic nature of the films at room temperature was also observed, despite alloy broadening.
Experiment The MgxZn1-xO films were synthesized by PLD using
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