Growth of ZnO/MgZnO Superlattice on Sapphire
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Growth of ZnO/MgZnO Superlattice on Sapphire J.F. Muth, C.W. Teng, A.K. Sharma', A. Kvit', R.M. Kolbas, J. Narayan' Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695 'Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7916.
ABSTRACT The optical and structural properties of ZnO/ MgZnO superlattices were investigated by transmission electron microscope, transmission measurement and photoluminescence. The uncoupled wells ranged in thickness from -30 A to 75 A. Modulation of the Mg content was observed in Z-contrast TEM indicating the alloy composition was periodic. The density of stacking faults in the superlattice was extremely high, however the photoluminescence in the narrowest well case was blue shifted, and substantially brighter than comparable bulk layers of ZnO and MgZnO indicating that the emission was enhanced. Excitonic features were observed in the optical absorption spectra and also revealed that diffusion of Mg from the barrier layers into the well was occurring. INTRODUCTION A great deal of research has been performed in wide band gap semiconductors resulting in the commercialization of group III nitride blue lasers, blue and green light emitting diodes and ultraviolet photodetectors for use in display optical data storage and solar-blind detection applications.' As an alternative to the GaN material system ZnO and its alloys are of substantial interest. There are many similarities between GaN and
ZnO, they are both wurtzite and have similar band gaps, both exhibit strong excitonic emission. However the exciton binding energy is nearly 3 times as large in ZnO (-60 meV) which permits makes excitonic effects even more pronounced..2 As yet, p-type doping of ZnO not technologically feasible although some reports indicate that nitrogen may act as an acceptor. We have recently been focusing on the growth of MgZnO alloys to investigate the potential of bandgap engineering the ZnO material system. While the equilibrium solubility of Mg in ZnO is -2 percent through pulsed laser deposition we have been able to achieve metastable alloys with Mg concentrations of up to 36 percent.4 The absorption and photoluminesence spectra indicated that the exciton persists despite alloy broadening at room temperature. These alloys have been shown to be thermally stable for temperatures less than 700 TC indicating that formation of stable heterojunction interfaces should be practical. A super lattice structure comprised of ZnO and Mg 0 ,Zn08O has also been demonstrated by Ohtomo et al., indicating that ZnO alloy based quantum structures should be feasible."
J6.7.1
In this work we report on the growth of a MgZnO superlattice by PLD. The superlattice was characterized by high-resolution transmission electron microscopy, transmission measurements and photoluminescence. In optical transmission measurements, the excitonic features of the absorption were enhanced and slightly blue shifted. The photoluminescence from the sample was
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