Technology Po w e r el e c t r o n i c s
source gate in situ nitride
drain
1000
double heterostructure
AlGaN
800
GaN channel
600
AlGaN buffer
Al(Ga)N interlayers
buffer breakdown voltage (V)
400
single heterostructure
silicon substrate
200
1.0 1.5 2.0 2.5 3.0 3.5
buffer thickness (µm)
Fig. 1. (left) Diagram of the double-heterostructure (DH) FET. Fig. 2. (right) Voltage breakdown measurements are performed
between two ohmic contacts processed on two different mesas. The novel DHFET structure increases the breakdown voltage by
50% for equivalent buffer thicknesses. Further improvements to 1000 V and beyond can result from thicker buffer layers.
LEDs on silicon
wafer reflectivity and also provides data that can
reveal growth rates and surface roughness. This
IMEC is developing new processes for the fabrication tool has been modified to detect any deviation of
of green LEDs on large-diameter silicon and sapphire the laser beam and reveal any bending or bow-
substrates. Increasing the device efficiency in the ing in the wafer. These tools have played a crucial
green gap is crucial for display applications. By role in the development and implementation of our
incorporating SiN interlayers, the threading dislocation growth processes, which can now produce crack-
density of GaN-on-silicon material has already been free, mirror-like epiwafers with acceptable levels of
lowered by one order of magnitude and the dislocation wafer bow for processing.
density is now comparable to GaN-on-sapphire. The Material for power electronics must also be able
program’s next challenge is to grow InGaN quantum to withstand high operating voltages. We have
wells with sufficient indium for green emission. found that although the standard AlGaN/GaN
HEMT structure is suitable, breakdown voltages
can be increased by more than 50% by adopting a
Our growth is carried out on silicon (111), the double-heterostructure design. This improvement
cheapest form of silicon substrate that is suitable is delivered without any increase in the separation
for III-nitrides. We’ve been developing our growth between the source and drain contacts, and can
process on this material for several years, and in produce devices that reach a breakdown voltage of
2006 we produced the first crack-free AlGaN/GaN 1000 V at a buffer thickness of 3.7 µm. Performance
HEMT epiwafers on 150 mm silicon for RF applica- gains result from greater confinement of electrons
tions. This was followed up by a collabor ation with in the channel and increased aluminum content in
the German MOCVD equipment manufacturer the AlGaN layers (figure 2).
Aixtron that led to the growth of a 1 µm thick crack- We have also discovered that the breakdown volt-
free epitaxial layer of GaN on a 200 mm silicon sub- age is actually limited by the silicon substrate, rather
strate, which we announced this June. than the AlGaN layer, if the contacts are separated
by more than 5 µm. Increasing the buffer thickness
Stresses and strains can deliver further gains in breakdown voltage, as
Controlling the stress within the epilayers is a major long as the stress is controlled.
challenge for III-nitride growth on silicon. Stress After our epitaxial structure has been deposited,
engineering in the heterostructure is the only solu- a Si
3
N
4
layer is added in situ by MOCVD to pro-
tion for deposition on large-diameter substrates, tect the underlying III-nitride layers. This patented
and our approach involves the addition of sufficient depos ition process creates a smooth, passivated
compressive strain during growth. This strain can surface that is stable up to 900 ºC and suitable for
balance the tensile stress that results from the post- device processing.
growth wafer cooling. Our in situ SiN passivation process has several
We employ various tools to assist our strain engin- unique features. It prevents the AlGaN surface from
eering that provide in situ measurements of tem- reacting with air and it allows deposition of high-
perature and wafer bowing. This includes “Argus”, quality AlGaN layers with higher aluminum con-
a temperature measurement tool co-developed tent, by preventing relaxation of this layer. Higher
with Aixtron that features a calibrated photodiode aluminum-content AlGaN layers bring several
array. We use an interferometer that monitors epi- bene fits, including a reduction in sheet resistance
24 compoundsemiconductor.net December 2008 Compound Semiconductor
CSDec08IMEC 23-25.indd 24 18/11/08 13:24:13
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