Effect of Oxygen Pressure on Magnesium Oxide Dielectrics Grown on Gan by Plasma Assisted Gas Source Molecular Beam Epita
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EFFECT OF OXYGEN PRESSURE ON MAGNESIUM OXIDE DIELECTRICS GROWN ON GaN BY PLASMA ASSISTED GAS SOURCE MOLECULAR BEAM EPITAXY A. H. Onstine1, B. P. Gila1, J. Kim2, D. Stodilka1, K. Allums1, C. R. Abernathy1, F. Ren2 and S. J. Pearton1 1
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 2 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 ABSTRACT The effect of oxygen pressure on MgO grown by RF plasma assisted gas-source molecular beam epitaxy was investigated. Increasing oxygen pressure was found to decrease the growth rate, improve the morphology and reduce the Mg/O ratio to near that obtained from bulk single crystal MgO. By contrast, the electrical characteristics of MgO/GaN diodes showed continual improvement in breakdown field and interface state density as the pressure was decreased. The lowest pressure tested, 1x10-5Torr, produced the lowest Dit, 3x1011 eV-1cm-2, and the highest VBD, 4.4 MV/cm. Cross sectional transmission electron microscopy of the MgO grown at the lowest pressure showed the initial 40 monolayers to be epitaxial, with the remainder of the layer appearing to be fine grained poly-crystal. Comparisons with films grown using an electron cyclotron resonance (ECR) plasma suggest that higher ion energies are desirable for obtaining the best electrical characteristics. INTRODUCTION MgO is a rock salt dielectric which has been explored as an intermediate buffer layer for growth of ferroelectric materials on semiconductors [1,2] or as a potential gate dielectric for GaAs [3,4] or Si.[2] While MgO deposition by MBE has been successfully demonstrated by a number of groups, the crystal quality of the films deposited on GaAs and Si has been poor due to the large lattice mismatch between the MgO and the semiconductor substrate. GaN has a smaller lattice constant than GaAs and is thus a much closer match to MgO. [5,6] An additional advantage of this system is the large bandgap and thus large band offsets that are expected relative to either an n- or p-type semiconductor. Further, the dielectric constant for MgO, 9.8, is substantially higher than for SiO2. We have previously reported on the feasibility of using MgO as a gate dielectric for GaN [7,8] and as a field passivation dielectric for GaN HFET power devices [9]. In this paper we will discuss the optimization of the plasma conditions for growth of MgO dielectrics on GaN. EXPERIMENTAL MgO films were deposited using a RIBER 2300 MBE equipped with a reflection highenergy electron diffraction (RHEED) system and an Oxford radio frequency (RF) oxygen plasma source operating at 13.56 MHz. The RF power was set at 300W. For comparison, some films were also grown with a Wavemat MPDR electron cyclotron resonance (ECR) oxygen plasma source. In all cases, two-micron thick GaN layers grown on sapphire (0001) by MOCVD were
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used as substrates. Elemental Mg from a standard effusion cell at 380°C supplied the Mg. Films were grown using a variety of oxygen pressures, 1x10-5 - 1x10
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