UC Santa Barbara

Efficient power electronic devices are essential for minimizing power losses during power conversion due to the growing worldwide energy consumption and global warming. Ultrawide bandgap semiconductors show high breakdown voltage to achieve efficient power conversion. In particular, β-Ga2O3 has been considered as a promising ultrawide bandgap semiconductor material for next-generation power electronics due to its large bandgap (4.8 eV) and breakdown field (8 MV/cm). The availability of melt-based growth method enables manufacturing of extremely high-quality single-crystal bulk substrates.The optimization of growth orientations is critical toward high-quality β-Ga2O3 epitaxial films. In this study, epitaxial growth of β-Ga2O3 films on (110) substrates have been performed via plasma-assisted molecular beam epitaxy (PAMBE). Atomic force microscopy (AFM) scan shows a very low RMS roughness of 0.08 nm for the surface of the as received (110) substrates. High-resolution X-ray diffraction measurements reveal a 2.5 nm/min growth rate of β-Ga2O3 films on (110) substrates for conventional PAMBE growth conditions (~700 ℃) which is comparable to that of on (010) substrates. The surface morphology of β-Ga2O3 epitaxial films is smooth and has a similar dependence on Ga flux as (010) growth. However, the (110) plane does not have tendency to show a well-defined step-terrace structure in spite of the appearance of (110) facets in growth of (010) β-Ga2O3. Indium catalyzed growth was also demonstrated to improve the growth rate up to 4.5 nm/min and increase the maximum growth temperature up to 900 ℃ of (110) β-Ga2O3. The continuous Si doping in β-Ga2O3 epitaxial films grown by PAMBE through the utilization of a valved effusion cell for the Si elemental source. Secondary ion mass spectroscopy (SIMS) results exhibit that the Si doping profiles in β-Ga2O3 are flat and have sharp turn on/off depth profiles. The Si doping concentration was able to be controlled by either varying the cell temperatures or changing the aperture of the valve of the Si effusion cell. Additionally, the High crystal quality and smooth surface morphologies were confirmed on Si-doped β-Ga2O3 epitaxial films grown on (010) and (001) substrates. The electronic properties of Si-doped (001) β-Ga2O3 epitaxial film showed an electron mobility of 67 cm2/Vs at the Hall concentration of 3.0×1018 cm-3. β-Ga2O3 epitaxial film grown by PAMBE shows outstanding crystal quality. However, the residual nitrogen in the oxygen gas source results in nitrogen incorporation into the β-Ga2O3 epitaxial films. Since nitrogen is a deep acceptor in the β-Ga2O3 materials system, the incorporation of nitrogen will affect the transport properties of β-Ga2O3 films. To identify the nitrogen incorporation level, nitrogen incorporation in β-Ga2O3 films was measured by SIMS with low detection limit of nitrogen. The PAMBE-grown β-Ga2O3 epaxial films showed a nitrogen concentration of 1.0×1017 cm-3 either by conventional MBE growth and MOCATAXY growth. To prevent the nitrogen incorporation, pure ozone source was used as the oxygen source for the growth β-Ga2O3. The ozone concentration was improved to up to 80% by adding a recirculating line between the MBE and the pure ozone generator.