Another step towards GaN power devices

IMEC, an independent European research centre in the field of nanoelectronics, and AIXTRON, a metal-organic chemical-vapour deposition equipment supplier, have deposited crack-free AlGaN/GaN structures onto 200 mm Si(111) wafers.

The layers show good crystalline quality as measured by high-resolution x-ray diffraction. Excellent morphology and uniformity were obtained as well.

The high-quality AlGaN and GaN layers were grown in AIXTRON’s application laboratory on the 300 mm CRIUS metal-organic chemical-vapour-phase epitaxy reactor.

“The demonstration of GaN growth on 200 mm Si wafers is an important step towards processing GaN devices on large Si wafers,” said Marianne Germain, program manager of IMEC’s Efficient Power program.

“There is a strong demand for GaN-based solid-state switching devices for power conversion. However, bringing GaN devices to a level acceptable for most applications requires a drastic reduction in the cost of this technology. And that is only possible by processing on large-diameter Si wafers. 150 mm, and then 200 mm, are the minimum wafer sizes we need to fully use today’s silicon processing capabilities.”

The bow of the resulting wafers is still quite large, in the range of 100µm; but IMEC believes that an optimised buffer can reduce this bow drastically, enabling further processing.

“We aim to further develop the growth process and to qualify the wafers to be compatible with Si-CMOS process,” Germain said.

GaN has good capabilities for power, low-noise, high-frequency, high-temperature operations, even in harsh environments (radiation). It also extends the application field of solid-state devices.

Due to the lack of commercially available GaN substrates, GaN heterostructures are grown mainly on sapphire and silicon carbide (SiC).

Si is a cheaper alternative compared to sapphire and SiC. Other benefits include the acceptable thermal conductivity of Si (half of that of SiC) and its availability in large quantities and large wafer sizes.

But, until now, Si wafers with (111) surface orientation were only available with a diameter up to 150 mm.

The 200 mm wafers were custom-made by MEMC Electronic Materials using the Czochralski growth (CZ) method. CZ wafers are suitable for switching applications with large breakdown voltages.

For such devices, the performance is independent of the resistivity of the Si substrate.

Process details

For the AlGaN/GaN heterostructures, a standard layer stack, which had already been demonstrated on 100 and 150 mm Si(111) substrates, was used.

First an AlN layer was deposited onto the Si substrate, followed by an AlGaN buffer which provides compressive stress in the 1 micron thick GaN top layer. The stack was finished with a 20 nm thin AlGaN (26% Al) layer and capped with a 2nm GaN layer.

From in-situ measurements, researchers from IMEC were able to extract the thickness uniformity of the different layers, which show a standard deviation well below 1% over the full 200 mm wafers (5 mm EE).