The firm, a spin-out from the Kyoto University, is aiming to commercialise crystalline α-Ga2O3 (corundum/sapphire/ruby structure) in power transistors and Schottky diodes – with the intention of manufacturing 600V TO-220 diodes in 2018 to compete against silicon carbide power devices.
Its second intellectual property is ‘mist CVD’, developed by Professor Shizuo Fujita et al and branded ‘Mistepitaxy’, a novel form of non-vacuum chemical vapour deposition with which it deposits Ga2O3, as well as many other oxides – see later.
“By using Mistepitaxy, we succeeded in forming a very good single crystal corundum structure gallium oxide semiconductor layer with a diameter of 4inch on a sapphire substrate,” said Flosfia. “Carbon impurities, which had been a problem until now, have also been reduced.”
Gallium oxide has a higher bandgap (5.3eV) than gallium nitride (GaN), which is starting to be used in power devices, but has poorer thermal conductivity.
However, if mist CVD works out for Ga2O3, it could under-cut GaN on production cost because GaN needs expensive vacuum processing.
To beat the thermal conductivity limitation, after it has grown gallium oxide on a sapphire substrate, Flosfia separates the 20μm thick α-Ga2O3 film using an un-disclosed lift-off process. “This film can reduce the resistance to about 1/100 compared with commercially available SiC substrates. It also solves the problem of low thermal conductivity, which is a disadvantage of gallium oxide,” said Flosfia.
The nature of any carrier substrate used to support the gallium oxide film prior to packaging – or other support technique used – is not yet been disclosed, Flosfia told Electronics Weekly.
Predicted breakdown field for the resulting film is >8MV/cm and the firm has made a vertical-conduction gallium oxide Schottky diode with an on-resistance of 0.1mΩcm2 – claiming this as a world record and comparing it to 0.7mΩcm2 for silicon carbide (SiC) Schottky diodes. From a diagram released by the firm the diode appear to use alternating layers of 20nm Ga2O3 and 1nm (AlGa)2O3.
Flosfia is claiming fast reverse recovery time, capacitance down to 130pF, and 13.9°C/W thermal resistance in a moulded TO-220 package – which the firm compares with 12.5°C/W for a commercial SiC Schottky although the gallium chip is less that half the SiC chip size.
For the creation of normally-off mosfets, Flosfia has grown iridium oxide (α-Ir2O3), a p-type material with 2.3cm2/Vs hole mobility (1×1021/cm3 concentration) and 0.3% lattice mismatch compared with α-Ga2O3.
Mist CVD can also be used to prepare oxide films including: ZnO, SnO2, Al2O3 (sapphire), CuO, SiO2, TiO2 and Cr2O3 – in single-crystal, polycrystalline and amorphous forms. Solar cells, fuel cells and organic devices are potential applications.
- (1) Make reaction solution mist
- Dissolve metal compound in solvent
- Ultrasonic atomisation
- (2) heating cause chemical reaction
- mist heated
- Chemical reaction of metal compound
- nm precision growth of oxide film
“The mist CVD method can produce an oxide semiconductor thin film using a raw material solution and a heater to make a mist. A chemically stable metal compound solute can be used as raw material in water or an organic solvent” – Flosfia.
if you are a Japanese speaker and could kindly contribute to translating the diagrams, please comment below.