Growth of ZnO/MgZnO Superlattice on Sapphire
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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 makes excitonic effects even more pronounced.2 As yet, p-type doping of ZnO is not technologically feasible although some reports indicate that nitrogen may act as an acceptor.3 We have recently been focusing on the growth of MgZnO alloys to investigate the potential of bandgap engineering for 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 'C, indicating that formation of stable heterojunction interfaces should be practical. 5 A superlattice structure comprised of ZnO and Mg02Zn080 has also been demonstrated by Ohtomo et al., indicating that ZnO alloy based quantum structures should be feasible6 . In this work we report on the growth of a MgZnO superlattice by PLD. The superlattice was characterized by high-resolution transmission electron microscopy,
353 Mat. Res. Soc. Symp. Proc. Vol. 623 © 2000 Materials Research Society
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 very bright and blue shifted from the corresponding ZnO band edge value. While several samples were examined optically, only one sample has been analyzed by TEM. In transmission electron microscopy, the zcontrast technique indicated that the Mg content was modulated according to the expected period of the superlattice. High-resolution transmission microscopy revealed numerous horizontal stacking faults. The interface between the MgZnO barriers and ZnO wells was poorly defined. The well thickness was also wider than expected, lessening the confinement which complicates the analysis. However, we found the study illuminating since it provides insight into the broadening mechanisms and growth issues that are expected to be important in the growth of quantum wells and heterostructures in this material system.
EXPERIMENTAL DETAILS The MgZnO superlattice and bulk films in this study were deposited by pulsed laser deposition on c-plane double-side polished sapphire. Before deposition the sapphire was cleaned in an ultrasonic bath using acetone and methanol. The vacuum system was evacuated to -5x 10' Torr and the substrate temperature was maintained at 650 tC during deposition. The low temperature was intended to minimize Mg diffusion. During growth a pulsed KrF excimer laser (X=248 nm, pulse width=25 ns, and repetition rate=10 Hz) with laser energy densities in the range of 3-4 J/cm 2 was used to ablate MgZnO and ZnO sintered targets. The composition of t
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