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News Article

Here today, GaN tomorrow

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IMEC and AIXTRON set important step towards low cost GaN power devices. Results were presented at the 14th international conference on metal organic vapour phase epitaxy in Metz, France.
IMEC, Europe's independent research centre in the field of nanoelectronics, and AIXTRON, a provider in metal organic chemical vapour deposition (MOCVD) equipment, have demonstrated the growth of high quality and uniform AlGaN/GaN heterostructures on 200mm silicon wafers. This demonstration is a milestone towards fabricating low cost GaN power devices for high efficiency/high power systems beyond the silicon limits.

IMEC and AIXTRON deposited, crack free AlGaN/GaN structures onto 200mm Si (111) wafers. The layers show good crystalline quality as measured by high resolution x-ray diffraction (HR-XRD). Excellent morphology and uniformity were obtained as well. The high quality AlGaN and GaN layers were grown in AIXTRON’s application laboratory on the 300mm CRIUS metal organic chemical vapour phase epitaxy (MOVPE) reactor.

“The demonstration of GaN growth on 200mm Si wafers is an important
step towards processing GaN devices on large Si wafers”, said Marianne Germain,
Program Manager of IMEC’s Efficient Power programme. “There is a strong demand for
GaN based solid state switching devices in the field of 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. 150mm, and then 200mm are the minimum wafer sizes
we need to fully leverage 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.
Marianne Germain: “We aim to further develop the growth process and to qualify
the wafers to be compatible with Si-CMOS process.”

Gallium nitride (GaN) has outstanding capabilities for power, low noise, high frequency,
high temperature operations, even in harsh environments (i. e. radiation); it
considerably extends the application field of solid state devices. Due to the lack of
commercially available GaN substrates, GaN heterostructures are nowadays grown
mainly on sapphire and silicon carbide (SiC). Si is a very attractive alternative,
being much cheaper than 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 150mm. The 200mm wafers were custom made
by MEMC Electronic Materials, using the Czochralski growth (CZ) method. CZ wafers are ideally suited for switching applications with large breakdown voltages. For such devices, the performance is independent of the resistivity of the Si substrate.

For the AlGaN/GaN heterostructures, a standard layer stack, that had already been
successfully demonstrated on 100 and 150mm 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 20nm 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 200mm wafers (5mm EE).
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