【World Express】High purity β-type Gallium Oxide crystals were grown at high speed by Metal-Organic Vapor Deposition

      Professor Yoshinao Kumagai and Assistant Professor Ken Goto of the Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, National University of Japan (President: Kazuhiro Chiba), and Special Assistant Professor Shogo Ssasaki of the Future Value Creation Research and Education Special Zone of the University, together with Mr. Yoshinaga Junya, Mr. Guanxi Piao, and Dr. Kazutada Ikenaga from TAIYO NIPPON SANSO CORPORATION. (President: Mr. Kenji Nagata) CSE Business Department of the Innovation unit and Dr. Yuzaburo Ban from TNSC CSE Corporation (President: Mr. Aida Takashi).  In order to achieve high-purity β-Gallium Oxide (β-Ga₂O₃) crystals of semiconductor crystals as next-generation power devices, the metal-organic vapor phase epitaxy (MOVPE)^注1 process^注2, which has been difficult to grow at high speed, has been successfully used to achieve high-speed growth, which is crucial for improving power control and conversion efficiency. Based on the results of reaction analysis in a unique crystal growth furnace, it has been demonstrated that high purity β-Ga₂O₃ crystals can be grown at high speed without impurities and carbon. This achievement means that we can look forward to the practical application of mass production technology for β-Ga₂O₃ power devices to realize an energy-efficient society in the future.

      The research results were published in the English journal Applied Physics Express (APEX) on September 28.

      Title of the paper: High-speed growth of thick high-purity β-Ga₂O₃ layers by low-pressure hot-wall metalorganic vapor phase epitaxy

URL: https://doi.org/10.35848/1882-0786/acf8ae

Background

      In order to suppress loss and promote energy saving during energy conversion, the use of wide band gap semiconductor crystals instead of silicon (Si) crystals whose material properties have reached the limit to realize high voltage and low loss power devices (diodes, transistors, etc.) has been widely concerned. β-Ga₂O₃ crystals have larger bandgap widths compared to Silicon Carbide (SiC) and Gallium Nitride (GaN) crystals (both of which have been studied), and further reductions in power loss are expected when implementing devices (about 1/3000 compared to silicon devices). In addition, the ability to mass produce single crystal silicon wafers from melt has led to a significant reduction in device manufacturing costs, and industry, government, and academia are undertaking research and development efforts worldwide in anticipation of industrial applications.

Structure of Research

      Kumagaya Laboratory at Tokyo A&M University has been working on the practical application of various semiconductor crystal growth techniques using chemical reactions in the vapor phase. In order to mass-produce β-Ga₂O₃ power devices, it is necessary to obtain the homoepitaxial wafers^注3 of the electrically controlled homoepitaxial β-Ga₂O₃ films grown on single crystal β-Ga₂O₃ wafer. Kumagaya Research has established a high speed growth technology for high purity β-Ga₂O₃ using the halide vapor phase growth (HVPE) method^注4, and supplies homogeneous epitaxial wafers to the market based on this intellectual property. However, the HVPE method is unable to cope with the formation of complex equipment structures. Therefore, we hope to apply the metal-organic vapor phase epitaxy (MOVPE) method, which is good at manufacturing complex device structures and has been widely used in the mass production of Gallium Arsenide (GaAs) and Gallium Nitride (GaN) based devices, to β-Ga₂O₃ growth and establish relevant technologies to promote its industrial development.

      In the Kumagaya Research Lab, in a cooperative study with the largest MOVPE equipment manufacturers in the country, TNSC and TNSC CSE, we explored the reaction mechanism for organometallic compounds of Gallium and Oxygen (O₂) and the crystallization growth conditions of β-Ga₂O₃. The results showed that high purity β-Ga₂O₃ without carbon and hydrogen pollution can be grown by the MOVPE method under conditions of complete combustion of organometre-derived carbon and hydrogen into carbon dioxide and water. Based on these results, we developed a decompression hot-wall MOVPE device (FR2000-OX) with a maximum 2-inch wafer placed side down and installed it at Tokyo A&M University (Figure 1). We selected trimethyl gallium (TMGa), which is considered to be a high vapor pressure and suitable for high speed growth, as an organometallic compound of gallium, and investigated the high speed growth of high purity β-Ga₂O₃ crystals.

Figure 1: (a) Appearance of FR2000-OX reduced pressure Hot Wall MOVPE growth system (Kumagaya Laboratory, Tokyo A&M University),(b) Schematic cross-section of the growth furnace. Use high purity Argon (Ar) as the carrier gas.

Research Results

      Through thermodynamic analysis of the reactions of TMGa and O₂, and analysis of the types of molecules present in the growth furnace using a Time of Flight Mass Spectrometer, we determined the conditions under which the hydrocarbon from TMGa is completely burned and the carbon and hydrogen are not absorbed by the β-Ga₂O₃ growth film as impurities. By increasing the supply of O₂, TMGa can be completely burned, resulting in uniform growth of β-Ga₂O₃ on a 2-inch diameter wafer. A constant growth rate can be obtained only in the furnace pressure range of 2.4 to 3.4 kPa (atmospheric pressure of 101 kPa), and the pollution of carbon is inhibited (Figure 2). Other impurities, such as Hydrogen (H), Nitrogen (N), and Silicon (Si), were also not detected, setting the stage for future control of electrical conductivity by intentional doping of impurities. Based on the above results, on the β-Ga₂O₃ (010) wafer, the growth temperature is 1000℃, the pressure in the furnace is 2.4 kPa, the oxygen supply partial pressure is 570 Pa, and the TMGa supply varies from 34 to 550 μmol/min (micromol /min). The growth rate is increased from 0.9 to 16.2 μm/h (micron/hour), achieving a high growth rate comparable to the HVPE method (FIG. 3). The homogeneous epitaxial thick film grown for 1 hour at 16.2 μm/h had the same structural quality as the wafer used and was confirmed to have excellent surface flatness (Figure 4).

Figure 2: Growth rate of β-Ga₂O₃ and concentration of carbon impurities in the grown film as a function of furnace pressure with a growth temperature of 1000 °C, TMGa feed rate of 180 µmol/min, and O/Ga feed ratio of 970. Under furnace pressure conditions of 2.4 to 3.4 kPa, the growth rate is about 5 microns per hour and the carbon impurity concentration is low.

Figure 3: Relationship between β-Ga₂O₃ homoepitaxial growth rate and TMGa feed rate; The growth rate increases linearly with the increase of TMGa feed rate; When the TMGa feed rate is 550 µmol/min, the growth rate is up to 16.2 microns per hour.

Figure 4: Electron microscope image of a homogeneous epitaxial film grown on a β-Ga₂O₃(010) wafer at a growth rate of 16.2 microns per hour for 1 hour. As can be seen from the figure, a flat and uniform homogeneous epitaxial film has been grown.

Future Development

      Due to concerns about the violent reactivity of gallium organomethallic compounds contaminated with Oxygen, Carbon and Hydrogen impurities, the MOVPE method was rarely considered before to achieve the high-speed growth of high purity β-Ga₂O₃ thick films. The MOVPE device can handle wafers up to 2 inches in diameter, enabling high throughput homoepitaxy wafers for device development. This is expected to boost research in the field. Further research on intentional doping and β-(Al_xGa_1-x)₂O₃ mixed crystal growth techniques is expected to promote the practical application of β-Ga₂O₃ power devices. The CSE Division of TNSC’s Innovation Unit (Business Manager: Takayuki Arai) plans to expand FR2000-OX into a device for small batch production of homoepitaxy wafers, and then develop a large scale production device for 2-inch epitaxy wafers based on FR2000-OX.

Phraseology

Note 1) β-type Gallium Oxide (β-Ga₂O₃) crystallization:

An oxide semiconductor crystal formed by combining gallium (Ga) atoms and oxygen (O) atoms at a stoichiometric ratio of 2:3. It has a energy gap of about 4.5 eV (electron volt), which is larger than Si(1.1 eV), 4H-SiC(3.3 eV), and GaN(3.4 eV), and has a high insulating breakdown electric field strength (8 MV/cm).

Note 2) Metal Organic Vapor Phase Epitaxy (MOVPE) :

A crystal growth technique using organometallic compound gases of metallic elements as raw materials. The film thickness can be precisely controlled to one atomic layer, and it is widely used in the manufacture of Nitride semiconductor light-emitting devices and Arsenide and Nitride high-speed transistors that need to be designed in nanoscale structure. However, in oxide crystal growth, this method has not been widely studied due to concerns about carbon and hydrogen contamination caused by organometallic compounds.

* A nanometer is a billionth of a meter.

Note 3) Homogeneous epitaxial wafers:

On a single crystal substrate, homogeneous crystals that grow coaxially with the substrate and have different electrical conductivity. According to the needs of equipment design, the thickness and conductivity of homologous epitaxial film are required to be controlled.

Note 4: Halide Vapor Phase Epitaxy (HVPE) :

A crystallization growth technique using halide gas of metallic elements as raw materials. Although high-speed growth of high purity crystals can be achieved, it is difficult to control the film thickness at the nanoscale. Currently, this method is used to fabricate homologous epitaxial wafer (film thickness about 10 microns) for vertical power devices consisting of a single layer structure.