Comparison Between GaN and SiC From the Viewpoint of Vertical Power Devices

Jun Suda

Department of Electronics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

E-mail: suda@nuee.nagoya-u.ac.jp, Phone: +81-52-789-4642

Keywords: GaN, SiC, vertical power devices, MOSFETs, SBDs, device process

Abstract SiC MOSFETs and SiC SBDs are successfully

Thanks to superior materials properties, GaN has commercialized.
great potential as the next-generation power devices. The During the last 5 years, SiC devices were extensively
author will discuss challenges for GaN vertical devices by implemented in electronic vehicles and railway trains.
referring SiC technologies, which has over 20 years These devices and circuits demonstrate significant
history. improvements in the power utilization efficiency. In Japan,
for example, SiC-based inverter systems are employed in the
INTRODUCTION latest commuter trains of the Yamanote line (the circle line
of Tokyo.)
Wide-bandgap (WBG) semiconductors have attracted Gallium nitride (GaN) is another candidate for power
much attention as materials for the next-generation power devices. AlGaN/GaN HEMTs were originally developed for
devices since they have superior material properties, in high-power high-frequency amplifiers. In the last decade,
particular large breakdown field, compared to silicon (Si). extensive development efforts were carried out on
The most advanced WBG semiconductor for power devices AlGaN/GaN HEMTs grown on Si substrates in hopes to
is silicon carbide (SiC). SiC has polytypism, which makes produce cost-effective and high-efficiency power switching
high-quality crystal growth very difficult. In 1987, a growth devices. Many companies in the U.S., Europe, and Japan
technology called “step-controlled epitaxy” was developed have started into mass production. They have a great impact
that enables a single-phase (single polytype) growth. Among on consumer electronics due to their excellent performance
these polytypes, 4H-SiC has high electron mobility and large of low on-resistance and high switching speed, which can
energy bandgap. Thus, 4H-SiC is generally used for power never be realized simultaneously in Si power devices.
device applications. In 1993-1994, 4H-SiC Schottky-barrier Recently, GaN vertical power devices have attracted
diodes (SBDs) were demonstrated that exceed the power great attention for next-generation power devices with large
performance limit for silicon. In 2001, SiC SBDs were breakdown voltage and high current capability. Required
commercialized by SiCED (later, Infineon). Key process technologies for GaN vertical power devices are
technologies for SiC SBDs were the development of an edge very different from those for lateral AlGaN/GaN HEMTs.
termination by ion implantation to obtain an ideal Many new technologies should be developed to realize high-
breakdown voltage, and a junction barrier Schottky (JBS) performance GaN vertical power devices. In this paper, the
structure to suppress the reverse leakage current. SiC SBDs author would like to discuss what GaN researchers should do
are compatible with Si PiN fast-recovery diodes (FRDs) to catch up and compete with advanced SiC power devices.
which are used in switching power supply units. For
examples, flat panel displays require high-efficiency power SUBSTRATE
supply units and SiC SBDs are suitably used in these units.
The demands of SiC SBDs led to the development of large- 4H-SiC bulk crystals are grown by the seeded
diameter high-quality SiC substrates. The size of SiC sublimation method (modified Lely method). The growth
substrate was 2 inch at the initial stage, and 6 inch SiC temperature is around 2200°C. The source material is high-
substrates are commercially available as of today. purity SiC powder. Typical growth rate is several hundred
The development of SiC power MOSFETs, on the other m/h. The temperature control (shape control) of the seed
hand, took longer than expected due to low channel mobility crystal is very important. It becomes difficult with increasing
at the SiO2/SiC interface and the oxide reliability issues. The crystal diameter. However, manufacturers manage to
channel mobility can be improved by a post-oxidation overcome this challenge and 6-inch-diameter 4H-SiC
nitridation process in NO or N2O ambient. The improved substrates are now commercially available. Some of the
channel mobility is 20-40 cm2/Vs, which is still much less fabricated substrates are reused as seedling crystals. After
than its bulk mobility of 1000 cm2/Vs. By reducing the cell many growth runs by selecting good seedling crystal for next
size and the channel length, low-on-resistance SiC vertical growth, the dislocation density is successfully reduced to the
power MOSFETs as well as power modules consisting of order of 103 cm-2.
 Low resistivity substrates are required for vertical power For GaN, currently available growth method for the drift
devices. N-type doping can be done by introducing nitrogen layer is metal-organic vapor phase epitaxy (MOVPE).
easily. However, heavy doping results in the formation of Because of source material such as metalorganic gallium,
cubic-phase (3C-SiC) inclusion. The minimum resistivity carbon incorporation is inevitable. It is known that carbon
without the inclusion formation is around 15-20 m cm. To substituting nitrogen site in GaN (CN) acts as a deep acceptor.
reduce the substrate resistance, a wafer thinning process is If the donor concentration is less than the concentration of
generally employed in commercial SiC power devices. CN, the layer becomes semi-insulating. Typical carbon
GaN bulk substrates are grown by a hydride vapor phase concentration in MOVPE-grown GaN is 5×1016 cm-3, which
epitaxy (HVPE) on foreign substrates such as GaAs or
limits the controllable range of n-type doping. Recently,
sapphire (Al2O3) with high growth rate of several hundred
carbon concentration was reduced by controlling growth
m/h. At the present time, 2, 3, and 4 inch substrates are
commercially available. 6-inch-diameter substrates were condition [1]. The smallest value is 1-3 × 1015 cm-3. The
demonstrated in the R&D level, which showcase the value is small enough for 1kV-class devices but not enough
scalability of the HVPE method. Due to the heteroepitaxial for >3kV-class devices. The growth rate for such low carbon
growth, the GaN crystal contains threading dislocations of 3 layers is around 1-3 m/h, and is too low for manufacturing.
×106 cm-2. Since current GaN substrates are produced for High-speed, low-background-impurity growth methods
for GaN drift layers are needed. One of the promising
LDs or LEDs, the resistivity is set at 20 – 50 mꞏcm to keep methods is the HVPE. Generally HVPE-grown GaN has
good optical transparency. The threading dislocation density high residual Si and O concentrations. However, a recently
seems too large for vertical power devices however. The report showed that a well-designed reactor and optimized
resistivity should also be reduced. If the optical transparency growth condition could enable high-purity GaN growth with
is not of major concern, GaN substrates with low resistivity
high growth rate (60 m/h) in HVPE reactors [2].
(<7 mꞏcm) can be grown by the HVPE method.
To compete with SiC vertical power devices, low-cost, ION IMPLANTATION
large-size, low-dislocation-density, and low-resistivity GaN
substrates are needed. The epitaxial growth on a foreign Selective doping by ion implantation is essential for the
substrate is also a limiting factor. One of the promising making of vertical power devices. To fully utilize superior
approaches is a combination of an ammonothermal method material properties in WBG materials, electric field
and the HVPE. Ammonothermal method (a GaN growth crowding should be avoided by utilizing a junction
condition under supercritical ammonia) has low growth rate. termination extension (JTE) structure, which can be formed
However, it can achieve strain-free GaN bulk crystal with by ion implantation. Both n-type and p-type doping in SiC
large substrate size and low dislocation density. By using can be made possible by nitrogen and aluminum ion
high-quality crystal as the seed, long period of HVPE implantation, followed by high temperature annealing at
growth can be possible, and multiple GaN substrates can be 1650°C. Graphite is a nice capping material for high
obtained from one ingot. temperature annealing.
For GaN, there are many reports on n-type doping by Si
DRIFT LAYER ion implantation. The p-type doping was difficult. However,
Mg/H co-implantation methods [3] and AlN epi capping
The drift layer is the most important part of vertical layer method [4] are recently proposed. Although further
power devices. The drift layer support applied voltage in the R&D are needed, especially for proper annealing methods
off-state. For kV-class devices, low doping (5×1015 cm-3) for GaN materials, the proof-of-concept work for both n-
and thick growth (10-30 m) are required. An reduction of type and p-type doping in GaN materials by ion implantation
the background impurity and point defects that form donor is encouraging.
or acceptor states in the bandgap is important. In addition,
high-speed growth is required to reduce the manufacturing MOS CHANNEL
cost.
For SiC, the drift layer is grown by VPE using silane and As mentioned above, SiC suffers from low channel
propane with hydrogen carrier gas. The growth reactor is mobility. The origin has not yet revealed. However, many
made by high-purity graphite (some parts are coated by researchers believe carbon play an important role.
high-purity poly-SiC). Thanks to the simple chemistry, high- Commercial SiC power MOSFETs seem to use the channel
mobility of 20-30 cm2/V-s. The channel resistance is
purity SiC with background donor concentration of <1×1014
reduced by a proper design in the device geometry.
cm-3, can be grown by VPE. The growth rate can be 100 GaN have great potential from the perspective of the
m/h, which is good enough. The deep-level trap channel mobility. There are some reports on the inversion-
concentration is much less than the background donor channel in GaN MOSFETs. A channel mobility of > 120
concentration. cm2/Vs with normally-off characteristics was demonstrated
in lateral GaN MOSFETs [5]. It is clear that SiC devices
cannot replace Si power MOSFETs with a blocking voltage
600-900 V due to their low channel mobility. GaN
MOSFETs, however have a chance to become a contender in
this application space as long as the channel mobility of
greater than 300 cm2/Vs can be achieved.

CONCLUSIONS

As mentioned above, SiC is the most advanced WBG

semiconductor for power device applications. There are
many things to be developed for GaN vertical power
devices. However, GaN have a great potential to provide
low-cost substrate and drift layer in the future. Realization of
high-performance low-cost WBG power devices have a
strong impact to the society.

REFERENCES

[1] G. Piao, et al., J. Crys. Growth 456, 137 (2016).

[2] H. Fujikura, et al., Jpn. J. Appl. Phys. 56, 085503 (2017).

[3] T. Narita, et al., Appl. Phys. Exp. 10, 016501 (2017).

[4] T. Niwa, et al., Appl. Phys. Exp. 10, 091002 (2017).

[5] S. Takashima, et al., Appl. Phys. Exp. 10, 121004

(2017).

ACRONYMS

FRD: Fast Recovery Diode

HVPE: Hydride Vapor Phase Epitaxy
JBS: Junction Barrier Schottky
JTE: Junction Termination Extension
MOVPE: Metal-Organic Vapor Phase Epitaxy
SBD: Schottky Barrier Diode
WBG: Wide BandGap