INDUSTRY MOCVD

INDUSTRY MOCVD
Left: The UR26K can accommodate ten 6-inch
wafers or six 8-inch wafers

Developers and manufacturers of all of these devices areworking with different types of substrate for chip production.
Sapphire is the most common platform for the LED, but savingsare promised by switching to large area silicon substrates thatenable chip processing in under-utilised, depreciated siliconfabs. Silicon substrates are also popular within the III-N powerelectronics industry, but in both this sector and in that of theLED, if native, low-cost substrate were available, they would
be widely used. In fact, a small proportion of the world’s LEDs
are already being manufactured on GaN, while some groups
pioneering ultraviolet LEDs are using AlN substrates for device
development.
However, regardless of the substrate employed for making theirnitride-based devices, engineers are searching for excellencefrom their MOCVD tools in three areas: control of the gas-phasereaction for high aluminium concentration and high growth rate;
control of carbon doping, from low to high doping densities;
and deposition of high-quality layers at high growth rates.
At Taiyo Nippon Sanso of Tokyo, Japan, we satisfy all theserequirements with a portfolio of MOCVD reactors featuringa ‘horizontal three-layer’ design and growth at atmosphericpressure. Our smaller tools have an enviable reputation with
the R&D community, and our large-scale production machines
share the same design philosophy, making it easy to transfer
recipes from one type of machine to the other.
Although many know of us through our supply of industrial
gases – this activity dates back to the founding of our company
in 1910 – we have a strong track record in MOCVD, with efforts
commencing in 1983. We initially launched systems for thegrowth of materials based on the InP and GaAs families ofmaterials. However, by the late 1980s, we started to develop
our range of GaN MOCVD systems. They now meet theneeds of every customer, from those wanting to carry out
research on a single-wafer 2-inch system, to those requiring alarge-scale machine for volume production that is capable ofaccommodating up to six 8-inch wafers or ten 6-inch wafers(see Table 1).

Faster, better
III-N film growthMOCVD reactors equipped with three-layer nozzles and operating atatmospheric pressure can produce growth rates and doping ranges that
manufacturers of GaN power electronics and ultraviolet LEDs are looking for.
By KOH MATSUMOTO FROM TAIYO NIPPON SANSO
38 www.compoundsemiconductor.net March 2014

THE III-N CHIP generates billions of dollars every year, withrevenues continuing to grow. Sales in this sector are currently
dominated by InGaN-based LEDs, which are backlighting
many screens and driving a revolution in LED lighting. Butother significant markets are emerging: ultraviolet LEDs, which
are attractive replacements for mercury lamps, thanks tosuperior robustness, longer lifetime and portability; and powerelectronics based on GaN that offers a step up in efficiency
compared to silicon incumbents.

The majority of GaN MOCVD reactors employ either a verticalor a horizontal gas flow. Our reactors adopt the latter approach,with precursors supplied from a nozzle upstream of thesubstrate holder, which is a part of the machine that is also
referred to as the susceptor. Materials are consumed along thedirection of gas flow, so rotation of the wafers is required toensure a uniform thickness of film growth. In a single-wafer tool
the wafer is rotated about its centre. Meanwhile, in multi-wafer
reactor, planetary motion is used, with accurate control resulting
from a carefully chosen gear and ball bearing system.
One distinctive feature of our reactors is their three-layered
gas ejection nozzle. Materials are injected into the reactor via
high flow speeds through this nozzle, to enable good control
of organometallics and ammonia, and ultimately the depositionof high-quality GaN, AlN, and AlGaN at high growth rates.
March 2014 www.compoundsemiconductor.net 39

INDUSTRY MOCVD

Table 1: Taiyo Nippon Sanso produces a wide range of MOCVD tools that meet the needs of R&D groups and high-volume manufacturers of compound
semiconductor chips

In our small-scale machines, the nozzle is positioned in theupper-flow section of the susceptor, but in large-scale mass
production systems, it is located in the centre of the susceptor,
so that gas spreads out from there. A resistance heater raises
the temperature of the substrates to that required for growth.
The heating system is zone-controlled to realise the optimum
temperature distribution on the susceptor. The number of zones
varies with the size of the reactor, from just one for the smallest
R&D tool to six for our biggest mass-production machine.

Another feature of our systems is continuously controllablepressure, which can be varied from 10 KPa to 100 KPa. Thanks
to well-controlled gas phase reactions, adjustments in pressure
can tune the material properties, which depend on growthpressure.
Quartz is used for the flow channel components. One merit ofquartz is that it is easy to clean – and that is a big advantage
for growth of GaN on silicon, because component cleanness
is critical for reproducible growth. Note that mass production
machines are equipped with an automated component
exchange robot, making it relatively easy to carry out
maintenance on the tool.

Higher growth rates
The capability of our reactors is highlighted by the high qualityof various epiwafers produced by them. This includes electronic
devices based on AlGaN and GaN that can be formed by highspeed growth and feature a very wide range of carbon doping
levels; high-voltage GaN-on-GaN diodes with well-controlled,shallow silicon doping levels; and AlGaN films with a highaluminium content, which can be used for producing ultraviolet
LEDs.
Growth of high-quality, wide-bandgap electronic structures onsilicon demands a multi-layer buffer structure incorporating
GaN, AlN and AlGaN. And if a HEMT is to offer high-voltage
power-switching, this buffer must be thick enough to enable ahigh breakdown voltage.
The Taiyo Nippon Sanso UR26K is the company’s biggest
MOCVD reactor

40 www.compoundsemiconductor.net March 2014

One downside of any thick layer is that it adds to growthtimes and thus reduces the number of epiwafers that can be

INDUSTRY MOCVD

INDUSTRY MOCVD
low doping concentrations is not easy, requiring control ofthe very diluted SiH4 gas supply and a low level of carbon
impurities, so that it is possible to produce low compensationn-type GaN.
Realising GaN films with these attributes requires a suitable
reactor design and a good growth process. As previously
stated, the growth rate of GaN in a conventional reactor leads todevice deposition times that can be 10 hours or more. But withour system, because it is possible to cut carbon concentration
via atmospheric pressure growth, n-type GaN growth rates canbe very high, leading to runs of just several hours for epiwafers
production.

Figure 1: Process gases enter the horizontal ﬂow reactor through a three-layered gas-injection nozzle

produced per day from an MOCVD system. With a conventional
reactor, the growth rate for the buffer is limited to 1 - 3 µm/h due
to parasitic reactions.
However, with our systems, far faster growth rates are possible,
thanks to the combination of a laminar high flow velocity anda specially designed three-layer-flow, gas-injection nozzle.
Equipped with these attributes, engineers using our tools havea better control over vapour phase reactions and can realise
shorter process cycle times.

Superior doping control
For a power switching device, the usual approach is to heavily
dope the GaN buffer layer with carbon so that it is highly
resistive, while employing a pure (undoped) GaN layer for thechannel. Research groups have shown that increasing carbon
concentration boosts breakdown voltage, while increasing
growth pressure cuts yellow luminescence in GaN and suggests
suppression of current collapse.

Growth rates for III-Ns depend on the constituents, withmaximum values of 27 μm/h, 3.8 μm/h, and 11.4 μm/h for
GaN, AlN and AlGaN, respectively (see figure 2). Employing avery high growth rate, we have deposited a 3 μm-thick HEMT
test structure on 6-inch silicon, using a growth rate of 8.5 μm/h
for the AlGaN/AlN strained layer superlattice and 7.5 μm/h for
GaN. The net time for film growth, which excludes temperature
ramping and reactor purging, is only 41 minutes, compared to88 minutes for our standard growth time.

With conventional reactors, the range of carbon concentrations
at a given growth rate is held back by the limited range of
V/III ratios and growth pressure. It is possible to deposit GaN
layers with very low carbon concentrations, but this requires
low growth rates, and that leads to long growth times. In stark
contrast, with our tools the range of V/III ratios and growthpressures that can be used is far wider, and this allows
engineers to obtain the carbon concentration they wish at ahigh GaN growth rate. For example, it is possible to produce
GaN films at carbon concentrations from 1016 cm-3 to 1020 cm-3
with growth rates in excess of 3 μm/h (see Figure 3).

This trimming of the growth time by just over 50 percent did
not lead to a significant deterioration in electrical properties.
According to Van der Pauw measurements, in the high-growthrate epiwafer the typical electron mobility was 1530 cm2 V-1 s-1
at a sheet carrier density of 8.9×1012 cm-2. It is possible thatfurther optimisation of the growth process could enable evenshorter growth times, and thus deliver an additional hike in the
productivity of III-N wafers for electronic applications.

An alternative, attractive architecture for GaN-based electronics
is the vertical diode. This device, which offers easy wiringand packaging and high area efficiency, consists of n-type
and p-type GaN layers that are grown on a conductive GaN
substrate. Challenges for manufacturing this device include: areduction in growth time, because the n-type GaN needs to betens of microns thick; and uniform silicon doping of GaN fromconcentrations of 1015 cm-3 to 1019 cm-3. Producing films withFigure 2. Growth rates for III-N binary and ternary ﬁlms deposited in a Taiyo Nippon Sanso reactor

42 www.compoundsemiconductor.net March 2014

The carrier concentration in n-type GaN grown on 6-inch
sapphire depends on the ratio of SiH4 to tri-methyl-gallium (seeFigure 4). Uniform doping at 5×1015 cm-3 is possible under
growth rates as high as 3.6 μm/h, indicating that our reactor isvery suitable for growth of nitride-based vertical diodes.

Figure 4. The carrier concentration in n-type GaN depends on the ratioof SiH4 and tri-methyl-gallium and the V/III ratios. The inset reveals theuniformity of doping across a 6-inch epiwafer

Ultraviolet LED structures
Essential ingredients for the ultraviolet LED – which can be used
for curing, money checking, and air purification – are the growthof AlN and high aluminium composition AlGaN. These layers
are normally deposited on sapphire, and when conventional
equipment is used, within-wafer uniformity and crystal quality
are hampered by an excessive gas phase reaction. This impacts
productivity and yield.

Figure 5. A high level of uniformity of peak emission wavelength indicatesthe potential of the Taiyo Nippon Sanso reactor for ultraviolet LEDproduction.

Figure 3. Increasing the growth pressure leads to faster growth rates
with a low carbon concentration

However, if engineers use our tool, the parasitic reaction iscontrolled and high-quality, uniform AlN and AlGaN films canbe grown at high growth rates. For example, it is possible todeposit AlGaN with an aluminium content of about
60 percent on 4-inch sapphire at 6.4 μm/h. Photoluminescencemeasurements of an InAlGaN multi-quantum well structure
produce intense emission at 330 nm, and a high level of
uniformity in the peak emission wavelength across the wafer(see Figure 5). These results demonstrate that our reactors
have the capability to increase productivity for manufacturers
of ultraviolet LEDs, just like they can do for the makers ofelectronic devices based on nitride materials.
© 2014 Angel Business Communications.
Permission required.

Further reading
K. Matsumoto et. al. Proc. SPIE. 8262 826202 (2012)
H. Tokunaga et. al. Phys. Stat. Sol. 5 3017-3019 (2008)
Y. Yano et. al. Jpn. J. Appl. Phys. 52 08JB06 (2013)
Y. Yano et. al. Taiyo Nippon Sanso Technical Report
32 27-28 (2013) (in Japanese).
S. Kato et. al. J. Cryst. Growth 298 831 (2007)
J. Selvaraj et. al. Jpn. J. Appl. Phys. 48 121002 (2009)
A. Fujioka et. al. Appl. Phys. Express 3 041001 (2010)

March 2014 www.compoundsemiconductor.net 43