Europe's leading independent research center in the field of
nanoelectronics, and AIXTRON, the world leader in
metal-organic chemical-vapor 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.
deposited, for the first time ever, 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-vapor-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 program.
"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 optimized 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
environment (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, Inc. 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).