First Observation of Atomic Long Range Ordering in Metal-Oxide Based ZnMgO Wide Bandgap Heterostructures
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HETEROSTRUCTURES R. D. VISPUTE"', S. CHOOPUN', Y. H. LI2 , D. M. CHALK', S. B. OGALE', R. P. 3 3 SHARMA', T. VENKATESAN" , AND A. ILIADIS 1. CSR, Departmentof Physics, University of Maryland, College Park, Maryland 20742. 2. also in Department of Materialsand Nuclear Engineering,University of Maryland, College Park, MD 20742. 3. Departmentof ElectricalEngineering,University of Maryland, College Park, Maryland 20742.
ABSTRACT We report the first observation of atomic long range ordering in the metal-oxide based wide bandgap (3.68 eV) ZnMgO alloy semiconductor thin films grown on sapphire (0001) by pulsed laser deposition. The ZnMgO system has the hexagonal wurtzite structure. The atomic ordering was deduced from the observations of forbidden x-ray diffraction peaks in the wurtzite structure. We show that under optimum thin film growth conditions, Zn and Mg preferentially occupy hcp sublattice. The relative intensity of the superlattice peak was largest for the Mg content in the range of 10%, which is about 50% of the solubility of Mg in ZnO. The observation of long range order in ZnMgO alloy semiconductor is quite important in the context of understanding issues related to the growth kinetics, surface reconstruction, and phase stability of metal-oxide based heterostructures and devices.
INTRODUCTION Wide bandgap semiconductors have been under intense investigation due to their potential for short-wavelength light emitting devices [1]. There has been a significant progress in III-nitrides that has resulted in demonstration of blue LEDs, laser diodes, and high temperature/high power electronic devices. Another material analogous to GaN is ZnO, which is a direct bandgap semiconductor with energy gap of 3.3 eV at room temperature and is being considered as a futuristic material for UV and blue light emitting devices [2,3]. In this system, band gap engineering can be achieved by alloying ZnO with Mg and Cd for enlarging and narrowing the bandgap, respectively. Thin films of ZnMgO alloys with wider band gaps than the ZnO are being considered for the fabrication of ZnMgO/ZnO/ZnMgO quantum well heterostructures and optically pumped lasers. Using virtual crystal approximation, alloying of MgO with ZnO is possible. The structural and optical properties of MgO and ZnO are summarized in Table I. Recently, it is shown that MgO solubility is more than 25% by atomic mole percent in ZnO [4,5]. The alloyed films have wider band gaps up to 4eV. Moreover, the ionic radius of Mg2+ (0.66 A) is close to that of Zn 2, (0.74 A). Therefore, the lattice constant of ZnMgO alloy is expected to be close to that of ZnO. Thus, the ZnMgO alloyed films may be ideal for the fabrication of metal-oxide based wide band gap heterostructures. Although, the fabrication of ZnMgO films has been reported recently [4,5], there is no effort towards understanding of atomic long range ordering phenomenon in the ZnMgO system.
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Mat. Res. Soc. Symp. Proc. Vol. 583 © 2000 Materials Research Society
Table I: Summary of atomic radius, crystal structure, latti
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