Gallium oxide (Ga2O3) is positively researched as one of the ultra-wide-bandgap semiconductor materials which are expected to realize cost-effective power devices. To demonstrate device performances, many efforts have been paid on the investigation of crystal growth methods to prepare high-quality drift layers. Among them, halide vapor phase epitaxy (HVPE) has advanced as a capable growth method for n-type conductivity-controlled β-Ga2O3 homoepitaxial layers with a wide range by Si doping. Recently, the fabrication of SBDs and FETs using the β-Ga2O3 homoepitaxial wafers have been reported by many research groups.
In our group, the HVPE growth of Ga2O3 and In2O3 was investigated in an atmospheric pressure system based on thermodynamic analyses, using group-III monochlorides (GaCl and InCl) and oxygen (O2) as precursors and nitrogen (N2) carrier gas. It was found that high-purity single-crystal layers can be grown at around 1000°C. The growth rate was found to be controlled by the input partial pressure of group-III monochloride and reach above 10 μm/h.
In the homoepitaxy on β-Ga2O3(001) substrates, the n-type carrier density in the range 1E15 - 1E18 cm-3 was achieved. For the layer with the carrier density of 3E15 cm-3, the highest room-temperature mobility of 149 cm2/Vs was confirmed. In the heteroepitaxy of c-In2O3(111) on sapphire (0001) substrates, the lowest n-type carrier density of 2.2E16 cm-3 with relatively high mobility of 235 cm2/Vs was achieved. These results indicate that HVPE-grown single-crystal sesquioxides can be applicable to the fabrication of power devices.
Gallium oxide (Ga2O3) is an emerging material for power electronics. The final penetration in the market is limited by several issues, including a stable and effective isolation between different devices and between different regions of the same device. In this work, we analyze lateral and vertical isolation structures, obtained by Mg implantation and annealing at 1000°C in Halide Vapor Phase Epitaxy β-Ga2O3. By means of repeated current-voltage characterization, it is possible to detect a severe current collapse, which can be completely recovered by white light illumination. When a constant bias is applied, the current collapse increases in magnitude at higher bias, showing a stronger filling of the deep levels. The transients closely follow the stretched-exponential model, an indication that the charge trapping is originated by extended defects, mini-bands or surface states. From the recovery transients carried out at various temperatures, it is possible to extrapolate a dominant thermal activation energy of 0.34 eV. The results of the recovery transients under monochromatic illumination show gradual variation in a broad energy range, consistent with the presence of extended defects. Temperature-dependent current-voltage characterization highlights the good performance of the bulk isolation and the presence of a significant surface leakage. Long-term stability tests show that the lateral structure is able to withstand a higher voltage level before catastrophic failure, but is less stable and is affected by a time-dependent degradation process. Charge trapping at the surface may act as a field-limiting element and partially explain the experimental findings.
Gallium oxide (β-Ga2O3) is a suitable material for next generation high power devices because of its huge critical electric field strength. However, most current device structures are not enough to take advantage of the full potential of Ga2O3 because these structures are optimized for material properties of silicon. To bring out the potential of Ga2O3, we propose a trench structure. First, we made Ga2O3 metal-oxide-semiconductor Schottky barrier diodes (MOSSBDs). The HfO2 film was deposited on the trench bottom and sidewall. Ga2O3 MOSSBD had a small leakage current level, and had about a 40% lower forward voltage than that of the commercially available SiC SBDs. We thus successfully demonstrated that the performance of Ga2O3 devices can exceed that of SiC devices. Next, we made Ga2O3 junction barrier Schottky (JBS) diodes. p-type region was made by p-type NiO. The Ga2O3 JBS diode had several orders of magnitude smaller leakage current than that of the normal SBD. This result indicates that the electric field at the Schottky junction decreased as a result of using the JBS structure. Finally, we fabricated Ga2O3 trench MOS field effect transistors. We used a static induction transistor-type structure that can be made only with n-type semiconductors. Si-doped Ga2O3 n+ contact and n-drift layers were grown on Sn-doped (001) Ga2O3 substrate with HVPE. The gate dielectric was HfO2. The device showed clear current modulation characteristics and a maximum current density of 1.36 kA/cm2. The device had a high on-off ratio of over 107.
Gallium oxide (Ga2O3) has emerged as a new competitor to SiC and GaN in the race toward next-generation power switching and harsh environment electronics by virtue of the excellent material properties and the relative ease of mass wafer production. In this proceedings paper, an overview of our recent development progress of Ga2O3 metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes will be reported.
This paper describes the bulk crystal growth of β-Ga2O3 using edge-defined film-fed growth (EFG) process. We first describe the method of the crystal growth and show that large-size crystal with width of up to 6 inch can be grown. Then, we discuss the way to control electrical properties. In the discussion, we give some experimental results of residual impurity measurement, intentional doping using Si and Sn for n-type doping and Fe for insulating doping.