NEDO project achieves crucible-free crystal growth, reducing gallium oxide substrate costs by 90%

The New Energy and Industrial Technology Development Organization (NEDO) is implementing the "Development of Material Technologies for High-Power and High-Efficiency Power Devices/High-Frequency Devices" project under the Key and Advanced Technology R&D through Cross Community Collaboration Program, commonly known as the K Program. Through this project, Novel Crystal Technology, Inc. (Sayama City, Saitama Prefecture), which is engaged in the development of gallium oxide (β-Ga₂O₃) wafers, power devices, and power modules, has successfully developed a crystal growth method that does not use platinum group metal crucibles. This new method has been named the "drop-fed growth" (DG) method, reflecting its characteristic of supplying raw material melt in droplet form.

Compared with conventional edge-defined film-fed growth (EFG) methods, the DG method significantly reduces the amount of iridium, which is an extremely expensive precious metal required in the growth process. This enables the manufacturing cost of gallium oxide substrates—which are one type of wide-bandgap semiconductor composed of gallium and oxygen—to be reduced to one-tenth of the conventional cost.

Gallium oxide's superior material properties enable the fabrication of low-loss power devices. Its use is expected in power electronics for medium-voltage (several hundred volts) application markets such as home appliances, industrial equipment, and electric vehicles, as well as high-voltage (several kilovolts) markets including railway vehicles and power grids.

Novel Crystal Technology, Inc. has been manufacturing gallium oxide substrates using the EFG method since 2015. However, because the melting point of the raw material is approximately 1,800℃, the EFG method requires large amounts of iridium to ensure the crucible serving as the raw material container can withstand high temperatures, making cost reduction difficult.

To address this, the company undertook the development of a proprietary gallium oxide crystal growth method that does not rely on iridium crucibles, advancing this effort through the NEDO project from fiscal year 2024. As a result, they succeeded in growing gallium oxide crystals with a diameter of 95 mm without using iridium crucibles by devising a heating method and raw material supply method.

The DG method employs "induction heating." In crystal growth, this method generates a magnetic field by passing a high-frequency current through a coil, which in turn induces eddy currents in metallic crucibles placed inside the coil, producing heat. A metal cylinder is heated in this way to raise the temperature inside the heating chamber, causing the crystal (seed crystal) surface to be heated and melted by radiation from the chamber opening.

The shape of this opening facilitates precise control of the temperature distribution on the crystal surface. This makes it easier to achieve optimal temperature distribution in accordance with the crystal diameter, thereby enabling larger diameters. Additionally, by lowering the crystal while continuously supplying molten raw material to its surface in droplet form, crystal growth can be achieved without using the iridium crucibles previously required.

Patents for this DG method have been filed, and intellectual property protection is being pursued in Japan, the United States, China, and Europe. Novel Crystal Technology, Inc. is continuing efforts to increase crystal diameter and improving the crystal quality of the DG method, with plans to ship 150 mm (6-inch) diameter substrates from 2029 and 200 mm (8-inch) diameter substrates from 2035.