Volume 22 Issue 3 April / May 2016

@compoundsemi

www.compoundsemiconductor.net

The precarious
promise of 5G

Evaluating theIII-V MOSFET

Taiyo Nippon Sanso

Smart options forthe infrared LED

Advancing UV LEDs and power devices

Going orange
with modified wells

Lighting up silicon
photonic chips

News Review, News Analysis, Features, Research Review and much more.

Free Weekly E News round up , go to www.compoundsemiconductor.net

VENDOR VIEW MOCVD

Driving
diversificationin GaN device
production
MOCVD reactors that deliver fast
production of aluminium-rich epiwafers
with carefully controlled doping profilescan drive a growth in shipments ofUV LEDs and vertical power devices
BY KOH MATSUMOTO FROMTAIYO NIPPON SANSOBY FAR the biggest selling GaN chip of today is theLED. It is backlighting countless screens and fuelling a
revolution in efficient, solid-state lighting.
Revenues for GaN LEDs will continue to growthroughout this decade, but could plateau soon after.
So, if GaN chip sales are to continue increasing in
the long term, the LED will have to be joined by newbreeds of device delivering significant commercialsuccess.
Two of the most promising GaN-based candidates
are the UV LED and the vertical electron device. The
former is an attractive replacement for mercury lamps,
and could be used for various applications, including

22 www.compoundsemiconductor.net April / May 2016sterilizing viruses, purifying water, optical processing,curing resins, degrading environmental pollutants
and preventing hospital infection. There are many
good reasons for replacing mercury lamps with UV
LEDs, including: increasing the lifetime of the lamp;
enabling the introduction of a broader, wavelengthtuneable, more powerful source; boosting wall-plug
efficiency; trimming the size of the unit; and reducing
environmental impact.
Meanwhile, commercial introduction of competitive,
GaN-based power devices could take market sharefrom the incumbent silicon devices, which areheld back by a smaller bandgap, a slower electron
velocity and a lower breakdown electric field.

Copyright Compound Semiconductor

VENDOR VIEW MOCVD

Many of the researchers that are working with widebandgap power electronics are developing vertical
GaN electron devices that are grown on bulk GaN
substrates, because this simplifies wiring and enablesthe device to be housed in a small package.

innovation is the introduction of a high temperatureelement to our GaN MOCVD tools, which have arich history − the development of the first-generationof these reactors can be traced back to thelate 1980s.

To enable UV LEDs and power devices to generatesignificant and increasing revenues, it is essentialfor MOCVD tools to provide: control of gas-phasereactions at high aluminium concentrations; control ofcarbon incorporation; and deposition of high-quality
layers at high growth rates. At Taiyo Nippon Sanso ofTokyo, Japan, we satisfy all these requirements with aportfolio of reactors featuring a ‘horizontal three-layer’design and growth at atmospheric pressure. Our latest

One of the biggest challenges with UV LEDs andvertical GaN power devices is the growth of highquality layers of AlN and high-aluminium-content
AlGaN. Particles tend to form, due to a gas phasereaction between trimethylaluminium (TMA),trimethylgallium (TMG), and ammonia. To overcome
this issue, a few years ago we introduced a high-flowrate MOCVD tool, the SR4000, which we described inthe March 2014 edition of Compound Semiconductor.

Copyright Compound Semiconductor

THE SR4000HT
can reach wafer
temperaturesof 1300°C ormore, thanks
to an effectiveheat shield,a high-power
input and animproved innerreactor design.

April / May 2016 www.compoundsemiconductor.net 23

VENDOR VIEW MOCVD

Table I
Specification ofSR4000HT.

The high-temperature element is present in ourrecently released SR4000HT, which is capable ofreaching wafer temperatures of 1300 °C or more,thanks to an effective heat shield, a high-power input
and an improved inner reactor design (see Figure1 for temperature uniformities on the susceptor
surface). Higher temperatures are ideal for forming
thick, aluminium-rich layers, which are needed
when fabricating UV LEDs and GaN power devices.
The latter device can have an epitaxial stack witha thickness of tens of microns, while the hightemperatures are beneficial for the baking of thereactor in between growth runs.
Our SR4000HT shares many of the hallmarks of
our SR4000, which has proved our most popularmachine for R&D and a small-scale production ofInGaN-based lasers and GaN-based electron devices(the specification of the SR4000HT is summarisedin Table I). The SR4000HT retains the original

Figure 1. The SR4000HT produces an excellent degree of temperature
uniformity across the susceptor surface in the 2-inch wafer region.

24 www.compoundsemiconductor.net April / May 2016

features of the three-layered gas ejection nozzle,
wide controllability from low pressure to atmosphericpressure, and a high-flow-rate of carrier gases.
Production capability per run is either three 2-inch
wafers, or a single 4-inch wafer.

Making high-quality UV LED epiwafers
We have demonstrated the capability of ourSR4000HT by using it to grow AlN and AlGaN films.
Scrutinising the epilayers indicates a very high qualityof material produced by our reactor. UV LEDs have an
underlying buffer layer of AlN, plus high-aluminiumcontent AlGaN, so the ideal substrate is bulk AlN.
However, the lack of a mature production technique
for the growth of this material has led to widespreaduse of sapphire as a foundation for the UV LED.
Growth of high-quality AlN and AlGaN layers on
sapphire demands higher growth temperatures, which
enhances the migration of aluminium atoms on thegrowing surface, but also a parasitic vapor phasereaction. Our SR4000HT meets this need and canrealise growth rates of 10.6 μm/h for AlN and7.2 μm/h for Al0.6Ga0.4N (see Figure 2 for growth rates
at various TMA and TMG supply concentrations).
The growth rate for AlGaN is almost constant for
growth temperatures spanning 1000°C to 1160°C(see Figure 3), indicating that the gas-phase reaction
is controlled well.
It is possible to use our SR4000HT to produce AlN
template wafers, and to also carry out AlN regrowth onthese engineered substrates. We have re-grown 3 μmthick AlN films on AlN-on-sapphire templates, whichwere annealed at high temperatures under a mixtureof nitrogen and carbon monoxide, and prepared byreserachers at Tohoku University and Mie University.
X-ray rocking curve measurements on the templateproduce a full-width at half-maximum (FWHM) of 64
arcsec for AlN(002) and 191 arcsec for AlN(102).
Similar values are obtained for a 3 μm-thick film of AlN
deposited on the AlN template wafer – in this case, theFWHM is 49 arcsec for AlN(002), and 214 arcsec forAlN(102). These results indicate that high-quality AlN

Copyright Compound Semiconductor

VENDOR VIEW MOCVD

Figure 2. The
high growth
rates for AlN
and Al0.6Ga0.4N
enable therelatively quick
growth ofthick epilayer
structures.

with a low dislocation density can be obtained by thecombination of these techniques. In addition to thenarrow X-ray diffraction FWHM, good quality AlN withan atomic step on the surface was grown at 1300 °C.
Scanning electron microscopy and atomic forcemicroscopy reveal a flat, high-quality surface for the
3 μm-thick AlN layer (see Figure 4). The surface hasa root-mean-square roughness of 0.103 nm andfeatures atomic steps, according to atomic force
microscopy.
High-performance UV LEDs require n-type AlGaN withgood electrical characteristics. To grow such a layer, it
is essential to reduce impurities that can compensate
for donors. Cutting out carbon is crucial, as it can beincorporated on nitrogen sites within GaN, where it acts
as a deep acceptor. We have found that the carbonconcentration can be reduced from 5.4×1018 cm-3
to 2×1017 cm-3, even for growth rates as high as 7.2
μm/h. This result underlines the capability of theSR4000HT for producing UV LEDs (see Figure 5).

Figure 3. MOCVD process engineers will benefit from an
AlGaN growth rate that produces very little variation withgrowth temperature.

Further proof of the capability of our tool has comeFigure 4. (a) SEM image of AlN template surface annealed at a high temperature after MOCVD growth. SEM (b) and AFM (c) images of an AlNsurface re-grown on the template shown in Figure 4 (a).

Copyright Compound Semiconductor

April / May 2016 www.compoundsemiconductor.net 25

VENDOR VIEW MOCVD

ratios. This flexibility equips our tool with impurity
controllability at a high growth rate. In the remainderof this feature, we detail the reactor’s capability tocontrol carbon and silicon impurities, and dopant
concentration, in the growth of a GaN p-n diode on anative substrate.
We have investigated the growth of the bipolar p-ndiode, rather than the mono-polar Schottky barrier
diode, because it has a smaller on-resistance. Thecarrier concentration in the silicon-doped, n-typelayer of this bipolar device must be controlled over
the range 1015 cm-3 to 1019 cm-3, while the carbon
concentration is maintained to no more than
1×1016 cm-3. For the lower end of n-type carrier
concentrations, silane must be supplied at a very low
flow rate and carbon impurities must be very low.
Meeting these requirements depends on the reactor
design and growth techniques.
With theSR4000HT,
diode structures
can be grownin just a few
hours, thanksto low carbon
concentrations
at high growth
rates.

from engineers at the Industrial Technology ResearchInstitute in Taiwan. They have used our SR4000
to produce a UV LED. The electroluminescence
spectrum for this device has a peak intensity at
284 nm, a full-width at half maximum of 15 nm, and avery small deep-level-luminescence (see Figure 6).

Improving diode doping
The other leading candidate for driving up GaN chipsales, the vertical electron device, has a superior
efficiency over the silicon incumbent, due to a lower
on-resistance. Minimising this on-resistance requires
careful control of dopant concentration and minimising
impurities. Another challenge for MOCVD processengineers is to produce epiwafers using high growth
rates, to reduce the time taken to grow the thick layersthat are needed for a high breakdown voltage.

Conventional reactors are often unsuitable for thegrowth of diodes, because they have a low growthrate for n-type GaN that leads to total growth timesof several tens of hours or more. With our reactors,
however, epiwafer growth can be completed in just afew hours, thanks to the low carbon concentrations athigh growth rates (see Figure 7).
At a growth rate of 2.3 μm/h under atmosphericpressure, the carbon concentration can be as low
as 3.7×1015 cm-3. Lower carbon concentration athigh growth rates can be obtained at atmosphericpressure. The far higher carbon concentrations athigh growth rates can be also be beneficial in somestructures, such as GaN-on-silicon devices, because
they introduce high resistivity.

Our reactors are ideal for growing power devices,because they accommodate a tremendously wide
range of growth pressures, growth rates and V/III

We have used our reactor to produce vertical GaN
electron devices with an architecture that has been
pioneered by researchers at Cornell University andHosei University (see Figure 8). Growth rates ofFigure 5. n-type Al0.6Ga0.4N with low carbon
concentrations are possible, even at growth
rates of 7.2 μm/h.

Figure 6. The sharp electro-luminescence spectrum
highlights the capability of the SR4000HT for the growthof UV LEDs.

26 www.compoundsemiconductor.net April / May 2016

Copyright Compound Semiconductor

VENDOR VIEW MOCVD

Figure 7.
Even at highgrowth rates,
the carbon
concentration
can be adjusted
in the range
1015 cm-3 to1020 cm-3 byappropriate
selection of theV/III ratio andgrowth pressure.

2.3 μm/h for the undoped GaN and 5.4 μm/h for then-type GaN have been used to produce p-n diodes on
a GaN substrate. These devices have been scrutinisedby secondary ion mass spectrometry (see Figure 9).
This technique reveals low values for carbon andsilicon concentrations in the undoped GaN, andaccurate doping of these elements in the p-typelayers. Note that it took less than four hours to growthe 20 μm-thick n-type GaN drift layer.
Our diodes are currently undergoing electricalcharacterisation. We anticipate that these results will
support our case for the suitability of our reactors for
growing GaN diodes. And as we have shown earlier,their high-temperature capability makes them a great
choice for producing UV LEDs.

Figure 8. The p-n diode structure grown by engineers atTaiyo Nippon Sanso.

Further reading
H. Miyake et. al. Appl. Phys.
Express 9 025501 (2016)
K. Ikenaga et. al. to be published in JJAP (2016)
K. Nomoto et. al. IEDM 2015 237 (2015)

Figure 9. SIMS profiles highlight the capability of the SR4000 tool to realise material with low levels of impurities and carefully controlled
doping. (a) SIMS profile to a depth of 9 μm (b) SIMS profile to a depth of 0.8 μm. The detection limits are as follows: 6 x 1014 cm-3 for carbon,
2 x 1015 cm-3 for silicon, and 1 x 1016 cm-3 for magnesium.

28 www.compoundsemiconductor.net April / May 2016

Copyright Compound Semiconductor