Epitaxial Growth of Compound
Semiconductors Using MOCVD
(Ⅳ)
Sumitomo Chemical Co., Ltd.
IT-related Chemicals Research Laboratory
Tomoyuki TAKADA*1
Noboru FUKUHARA
Hisashi YAMADA*2
Advanced Materials Research Laboratory
Masahiko HATA*3
Yasuyuki KURITA
GaAs based compound semiconductors have been widely used for mobile applications in devices, such as
smartphones, tablet PCs, base stations, and so on, because of their superior RF properties.
One of their major applications is the FEMs (Front-End Modules) of mobile phones, and InGaP-HBT which is
suitable for power amplifiers in FEMs has been developed. In this paper InGaP-HBT epitaxial wafer fabrication
techniques using MOCVD growth method are reviewed.
This paper is translated from R&D Repor t, “SUMITOMO KAGAKU”, vol. 2015.
Introduction
been integrated as shown in Fig. 1. Among the RF frontend modules that have diversified in this manner, power
In recent years, there has been an increase in the
amplifier modules which amplify output signals for mul-
amount of communications data because of cloud com-
tiple bands require high linearity in the amplification
puting and other activities using mobile communications
characteristics for each band. In addition, the power con-
devices typified by mobile phones and the handling of
sumption is the greatest for power amplifier modules
increased data speeds for data communications. With
which process the largest signal in the device, and the
the dissemination of LTE (Long Term Evolution) as a
power consumption tends to increase even more
communication mode, progress has been made with
because of modulation systems becoming more compli-
multiband frequencies, and the number of bands, which
cated, so then even higher power efficiency is required.
was 2 to 3 before 2G, has increased to 10 bands or more.
Currently, electronic devices that are made of Si,
In addition, applications other than mobile phones have
SiGe, GaAs and other semiconductor materials are
recently been frequently equipped with wireless LAN
communication functions, and progress is being made
in increasing the performance of the various electronic
modules that run these communication processes. As a
Antenna Switch Module
Antenna
part of this, progress has been made in integrating vari-
FEMiD
Duplexer
ous semiconductor devices that were discrete devices
Transceiver
up to now to reduce the size and power consumption of
RF front-end modules which receive and transmit RFsignal for communications. The front-end modules for
smart phones, etc. use power amplifiers with integrated
duplexers (PAiD) or front-end modules with integrated
duplexers (FEMiD), etc. into which duplexers have
*1 Currently: Electronic Materials Division
*2 Currently: Advanced Materials Research Laboratory
*3 Currently: Sumika Electronic Materials, Inc.
SUMITOMO KAGAKU 2015
Fig. 1
Schematic diagram of front-end module
in smartphones (example)
1
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
used for the various devices in front-end modules, but
strong requirements for reduction in mounting surface
among them, devices made of GaAs have the charac-
area and volume. Looking at this in terms of control
teristics of high linearity and low power consumption,
systems for output power, high current density can be
making them suitable for the power amplifier applica-
assured from a so-called normally-on type p-HEMT
tions described above. Pseudomorphic high electron
where a settled current flows when the gate voltage is
mobility transistors (p-HEMT), which are used in the
zero, but two power supplies, positive and negative, are
antenna switches and low-noise amplifiers in front-end
required between operating amplitudes during maxi-
modules, are representative of high frequency devices
mum current output (positive gate voltage) and during
that make use of GaAs semiconductors. Technical
shut-off (negative gate voltage). A shut-off state with a
overviews of the development of these p-HEMTs have
gate voltage of zero, where current increases and
times.1)– 3)
On
reductions are only controlled by a positive gate volt-
the other hand, heterojunction bipolar transistors
age, a so-called normally-off type, is also possible, but
(HBT) have especially excellent characteristics for
the effective channel charge density becomes smaller;
power amplifier applications, as will be discussed in the
therefore, current density decreases, and there is the
following, and are widely used today. In addition, the
disadvantage of the device area increasing in order to
markets for these are expanding rapidly, so this report
assure the required output current. On the other hand,
will review the technical development of GaAs-HBT.
with HBT, the current polarity necessary for the con-
been provided in this publication several
trol of the base current signal and the current polarity
Principles and Characteristics of HBT
applied to the collector are the same; therefore, there
is the advantage of drive being possible with a single
Fig. 2 shows a typical HBT structure. The most
power supply, and salient features are the ability to sim-
important difference from a field effect transistor, rep-
plify power supply voltage circuits coupled with higher
resented by p-HEMT used in a switch, is the current
current density per unit surface area than convention-
path, and while p-HEMT has current flowing in parallel
ally, and the ability to form power amplifiers which are
to the surface on the substrate, HBT has current flow-
small and which can control high current outputs.
ing vertically. With this system, an over whelmingly
Next, the band structure of the cr ystals forming
larger current path cross-section can be had than with
HBT will be discussed. With compound semiconduc-
p-HEMT in which electrons flow in a thin film quantum
tors such as GaAs and InP, a variety of band lineups can
well channel width in the tens of nanometers, and these
be designed by forming heterojunctions using alloy
are suitable for power amplifier applications for control-
crystals. For example, a discontinuous band structure
ling large currents.4) In addition, from a different point
can be fabricated by stacking AlGaAs or InGaP, which
of view, there can be a greater amount of current per
have a large band gap, on a GaAs semiconductor while
device surface area; therefore, it can be said that device
matching the lattice constant. In addition, with AlGaAs
sizes can be shrunk, and this is one important merit of
and InGaP, the energy difference differs from the vac-
HBT in applications for mobile terminals, which have
uum level at the lower end of the conduction band and
the upper end of the valence band; therefore, a suitable
Emitter
Base
Collector
n-InGaAs
band lineup can be created according to the purpose
by combining them, and they are frequently used in
electronic and optical devices. As will be described in
n-GaAs
n-InGaP
p-GaAs
the following, an emitter layer formed from AlGaAs or
n-GaAs
high energy barrier is formed between the valence
n-GaAs
InGaP, which have a large band gap, is formed adjacent
to a base layer formed from GaAs with HBT so that a
band of the base layer and the valance band of the emitter layer, and diffusion of minority carriers injected
Semi-insulated GaAs sub.
from the base layer into the emitter layer can be suppressed. Thus, current loss during the driving of the
Fig. 2
Typical HBT (Heterojunction Bipolar
Transistor) structure
SUMITOMO KAGAKU 2015
transistor can be reduced, and a high current gain can
be achieved.
2
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
electron
between the emitter and the base than with a homojunction bipolar transistor; therefore, it is possible to
make IBh smaller, and it is possible to obtain a high β
value.
hole
In addition, the maximum cutoff frequency fT, which
is an index of the highest frequency for current amplification, is inversely proportional to the sum of the elec-
Emitter
Fig. 3
Base
Collector
Band structure of HBT
tron transit time in the base layer and collector layer;
therefore, the appropriate structural design is required
for the HBT operation at high frequencies.4) Specifically, fT can be increased by applying a thinner base layer
Most group III – V compound semiconductors, start-
or by accelerating electrons with a structure that
ing with the GaAs material described here, have high
applies an electric field inside the base layer. In addi-
electron transport velocity; therefore, electrons are
tion, fmax, which is an index of highest frequency for the
used as the majority carrier in most of the electronic
power gain, is known to be dependent on capacitance
devices. For example, while p-HEMT makes for
between the base and the collector and the base resist-
monopolar devices that only control current with elec-
ance. Therefore, it is essential to have good crystallini-
trons, a npn structure where electrons, which have
ty, high doping concentration and low base resistance
high electron transport velocity, are used as the major-
for the base layer to improve fT and fmax .
ity carrier is typical in HBT, but holes must also be controlled along with the electrons as a signal source, and
MOCVD Grown HBT Epitaxial Crystals
not only n type but also p type crystal growth control
is necessary. A typical HBT band structure is shown in
Crystal growth control for p-HEMT using MOCVD
Fig. 3. When no voltage is applied to the base, elec-
has previously been reported in this publication. The
trons are not injected from the emitter into the base
basic MOCVD growth technology is also common to
because of the energy barrier at the pn junction formed
HBT growth, and the technology is used in the same
between the emitter and the base, even when the col-
way, but the crystal materials are different, and the cur-
lector is positively biased to the emitter, but when a
rent control methods in devices are also dif ferent;
positive voltage is applied to the base, then electrons
therefore, growth technology which is different from p-
are injected into the base from the emitter. Some of the
HEMT must be used. In the following we will describe
electrons injected from the emitter recombine with
the relationship between the growth technology and
holes that have been injected into the base layer at the
device characteristics.
emitter/base interface, and a hole recombination current IBr is formed. Electrons that do not recombine flow
into the collector as-is, and form a collector current IC.
1. Growth of p type GaAs crystals and initial drift
phenomena in current drive
On the other hand, some of the holes in the base layer
A p-type GaAs layer is typically used in the base layer
are reverse injected into the emitter layer and form a
for GaAs-HBT. When growing p-type GaAs, group II
reverse injection base current IBh. Therefore the base
elements such as Be, Mg and Zn or group IV elements
current of the transistor is expressed by
such as C are doped as the acceptors.5) Typically group
II dopants are suitable for obtaining high p-type carrier
IB = IBr + IBh
concentrations, and they are used in optical applications such as lasers and LED electrode contact layers;
and the current gain β, which is the ratio of the base
however, there is the problem of the diffusion speed
current and the collector current, is expressed by the
being high within the crystals, and so they are not suit-
following equation.
ed for HBT, which requires high concentrations and a
I
IC
β= C =
IB
IBr + IBh
With HBT there is a greater valence band barrier
SUMITOMO KAGAKU 2015
steep doping profile. If mutual diffusion of dopants in
HBT interfaces arises at a pn junction, they compensate for each other. Since ionized impurity concentration increases within the crystals with the occurrence
3
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
of this compensation of dopants for each other, it
However, there are also problems that arise when
invites a reduction in mobility, in other words, a reduc-
growing p-type GaAs crystals using C as a dopant. Typ-
tion in the electron transport velocity. In addition, in
ically, the maximum carrier concentration obtained is
heterojunction materials, the design of crystal compo-
known to be dependent on the energy band structure
sition interfaces and pn junction interfaces must be
inherent in the semiconductor material in electrical
consistent, but when impurity diffusion arises, it gives
conductivity control using doping in semiconductor
rise to offsetting of pn junction interfaces or degrada-
crystals. Specifically, the lower end of the conduction
tion. Important device parameters such as transistor
band and the energy difference at a charge neutrality
turn-on voltage vary, and since there is a serious effect
level with n-type doping are known from experiments
on their operating characteristics and reliability, a steep
to have a strong correlation with empirical values for
doping profile without mutual diffusion is necessary.
maximum carrier concentration obtained in various
Because of this, C, which has an extremely small diffu-
semiconductor materials. The maximum carrier con-
sion coefficient, is suitable as an acceptor element for
centration for a given semiconductor material can be
the base layer. Besides halomethane gases such as
thought of being determined largely by the relative
CBr 4 and CBrCl 3 , organic arsenic gases such as
position of the neutrality level in the band structure of
trimethyl As and tertiary butyl As, which can also be
that semiconductor material and the conduction band
used as As sources, can be used as the dopant sources
and valence band positions.7) When the dopant is sup-
for doping with C. In addition, when trimethylgallium
plied in a greater amount than the maximum carrier
is used as a Ga source, it can also be used as a source
concentration determined by this band structure, the
of C, making use of the phenomenon of C being natu-
dopant atoms are taken into the crystal, but by intro-
rally incorporated into the cr ystal from its methyl
ducing defects, etc., that compensate for this into the
groups. Typically, from the standpoint of assuring crys-
crystal, it can move to a stable energy state. In terms
tallinity and film thickness controllability, cr ystal
of these defects, ones caused by the formation of vacan-
growth is most often carried out while strictly control-
cies and interstitial atoms as well as complexes of those
ling the Ga source flow rate under Ga source material
defects can be cited, but when doping with C is carried
supply rate limiting conditions while supplying an
out in GaAs crystals by MOCVD, H is taken up in the
excess of As, which has a large decomposition and dis-
cr ystal and bonds with C, compensating for the C
sociation pressure, when carr ying out GaAs growth
acceptors. Thus, if HBT with a C-doped GaAs crystal
using MOCVD. Therefore, cases using halomethane
as the base layer is fabricated, the device operation is
doping where the amount of C doping can be con-
affected by H compensating for the C acceptors. This
trolled independently from the As and Ga source mate-
is what is called the burn-in effect,8) and when the HBT
rials, which also require strict control from the stand-
turns on, changes in the C-H bond state arise because
point of these crystal controls, are
common.6)
of the electrical and thermal stress, and the C acceptor
compensation by H is partly eliminated. As a result, the
ef fective carrier concentration of the base layer
changes, and as a result of that the collector current
0.10
7×1017 cm–3
Collector current IC (A)
0.08
and current gain change (Fig. 4). The concentration of
H atoms in the p-GaAs cr ystal must be kept low in
2×1018 cm–3
advance to control these changes in characteristics
0.06
0.04
0.02
0.00
0.01
with initial turning on of the device, and typically, an insitu post annealing process is used after C-doped GaAs
7×1018 cm–3
layer growth. Even when high concentration p-type
2×1019 cm–3
GaAs with a p-type carrier concentration of around
Hydrogen
concentration
4 × 10 19cm –3 is used for the base layer, the H concentra-
0.1
tion of greater than 7 × 1018cm–3 right after base layer
1
10
100
Time (s)
Fig. 4
Initial collector current drift (Burn-in
effect)
SUMITOMO KAGAKU 2015
1000
growth can be reduced to less than or equal to
7× 1017cm – 3 by applying the post annealing process,
and the collector current variations in initial operation
of the device after turning on can be suppressed. By
4
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
using p-type GaAs base layers that make use of these
contact resistance must be lowered similarly for the
C acceptors, practically acceptable performance for
collector layer formed from the n-type GaAs layer and
current mobile communications for the fT and fmax val-
the emitter layer formed from the n-type InGaAs layer.
ues described above have been achieved. This method
In particular, formation of an electrode surface area
of using post annealing carries out C doping at a high
that is very much smaller than the collector electrode
concentration while maintaining the energy difference
and base electrode, which have comparatively large
between the charge neutrality level and the upper end
surface areas, is necessary for the emitter; therefore,
of the valence band by inactivating with H during the
the reduction of the contact resistance of the electrode,
crystal growth, and is an extremely effective method
which increases in inverse proportion to the electrode
because it is able to obtain a high carrier concentration
surface area and reduction of the resistance in the ver-
not normally achieved by MOCVD growth conditions
tical direction of the emitter layer to the base are
close to thermal equilibrium by activating the C in the
important problems to be solved. A method for forming
post annealing process after growth. In this connection,
ohmic contact on a GaAs compound semiconductor is
there is the example of success in p-type carrier activa-
by forming a film by vapor deposition, etc., of an elec-
tion and increased concentration that exceed by far the
trode material that includes dopant elements and the
maximum doping carrier concentration predicted from
dopant elements being diffused in the crystal by carry-
the band structure previously described by separating
ing out heat treatment afterward, forming a high carri-
hydrogen by post growth annealing of Mg acceptors
er concentration region and reducing the contact resist-
that have been deactivated by hydrogen during growth
ance. This diffusion technology is commonly used in
using a similar method with an Mg-doped GaN layer
source and drain electrodes for horizontal-type devices
used as a p-type layer in blue light-emitting devices,
such as field effect transistors (FETs), but in HBT,
which received the Nobel prize in 2014.
which is a vertical device, the diffusion of dopant ele-
As described above, there are two methods for C
ments may affect the transistor operation. Specifically,
doping in GaAs, and one is the method of introducing
the carrier concentration may vary between the base
a CBr4 or CBrCl3 from the outside, and the other is
layer and the emitter layer for which carrier concentra-
adjusting the supply ratio of trimethylgallium (TMG,
tion control should be carried out if diffusion process-
Ga(CH 3 ) 3 ) and arsine (AsH 3 ), which are the con-
ing is carried out on the emitter layer formed on the
stituent raw materials for Ga and As, that is the V/III
outermost epitaxial layer, and by extension, the
ratio. We have discovered technology for controlling
response characteristics when the bias voltage between
the current amplification factor in HBT by adjusting the
the base and emitter is changed in comparison to the
V/III ratio and, by reducing the hydrogen concentra-
original design values. Therefore, it is preferable to be
tion taken into the crystal, have achieved device char-
able to achieve low contact resistance in emitter elec-
acteristics that sufficiently reduce the burn-in effect.
trodes without diffusion by heat treatment, and use of
materials with small energy barriers and high carrier
2. HBT characteristics and relationship with high
concentration n-type doping in crystals
concentration doping that makes barrier tunneling possible is effective. InGaAs with a high In composition is
During operation, HBT devices can be thought of as
an emitter contact layer material with a small energy
a type of resistance element. From the standpoint of
barrier. As the In composition in InGaAs increases, the
reducing power loss, it is important to lower the resist-
electron affinity becomes larger, and the energy barrier
ance of the various HBT layers and also to reduce con-
for electron injection is reduced. In addition, an impor-
tact resistance in the forming of electrodes, which are
tant merit is the ability to obtain a much higher doping
the external output terminals for the main layers con-
concentration than GaAs because of the band structure
stituting HBT. To achieve this, selection of suitable
characteristics described previously. However, since
electrode materials and epitaxial design are required.
the lattice constants for InGaAs and GaAs dif fer,
The main point is lowering the energy barrier between
defects are easily introduced into the crystal by lattice
the electrodes and the semiconductor as much as pos-
mismatch. The emitter contact layer is positioned as
sible in the epitaxial design. The contact resistance is
the topmost par t in the HBT structure, and even
reduced by the high p-type carrier concentration dop-
though there is no problem with propagation of crystal
ing technology described above for the base layer, but
defects above that, layer structure design and growth
SUMITOMO KAGAKU 2015
5
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
conditions must be optimized to be able to form a layer
the crystal during crystal growth from the sub-collec-
with few cr ystal defects, because the effects of the
tor layer onward, penetrating the collector layer and
defect level when the layer is formed lead to current
reaching the base layer and emitter layer, which large-
loss when there are too many of these crystal defects.
ly control the transistor characteristics. When the sub-
Si, Ge, S, Sn, Se and Te are used as donor elements in
collector layer is excessively doped, it causes a phe-
n-type high concentration doping of InGaAs emitter
nomenon where the transistor current gain is reduced.
contact layers, and doping with high concentrations
Therefore, it is important to use a doping concentra-
greater than
2 × 1019cm –3
is possible. A low ionization
tion of an extent that does not introduce cr ystal
energy for a donor element is desirable from the stand-
defects (an example of the data is shown in Fig. 5).9)
point of donor activation, but on the other hand, use of
However, the introduction of crystal defects is largely
dopant sources having the characteristic of releasing
dependent on the crystal growth conditions, in other
when adsorbed on the walls and other parts of the reac-
words, the maximum carrier concentration value can
tor becomes a problem from the standpoint of mass
be increased by stoichiometric control. Moreover,
production stability. It can be said that Si, which has lit-
technology for crystal defect control in this sub-collec-
tle adsorption and desorption effect and can form a
tor layer and increasing carrier concentration can be
steep doping profile, is suitable for mass production.
applied in a similar manner for the sub-emitter layer
On the other hand, use of a GaAs layer for the con-
formed from an n-GaAs layer that is positioned below
tact layer on the collector side is typical. One reason
the emitter contact layer and performs the role of link-
for using the GaAs contact layer is that there is no cur-
ing electron transport between the InGaP emitter layer
rent pathway for transistor operation below the collec-
and the emitter contact layer.
tor contact layer and diffusion treatment of the electrode is possible. Another reason is that the growth of
200
a high quality GaAs epitaxial layer on the InGaAs con-
180
tact layer is difficult because the difference in lattice
160
locations are introduced in the GaAs layer. This is different from the emitter contact layer which is positioned at the uppermost epitaxial part. However, even
Current Gain
constant between InGaAs and GaAs is large, and dis-
140
120
100
in the collector contact layer, where electrode diffu-
80
sion can be used, high carrier concentration doping is
60
required to a certain extent. This is not only because
40
of the contact resistance with the electrode, but also
0
2
3
4
5
6
Carrier concentration (×1018 cm–3)
for reducing the series resistance component in the
path of electron flow horizontally from the electrode.
1
Fig. 5
The maximum carrier concentration that can be
Impact of sub-collector carrier concentration on current gain
obtained with n-type high carrier concentration doping
of GaAs is lower than that for the p-type at less than
1× 1019cm –3. This is because the lower end of the GaAs
conduction band is separated in energy from the
3. Application in InGaP crystal growth and emitter
layer
Fermi level stabilization position more than the upper
As described above, an emitter layer formed from a
end of the valence band. In addition, as with the p-type
material with a larger band gap than that of the base
doping described above, crystal defects that compen-
layer is used to prevent the base current from leaking
sate for the doping are gradually introduced as the
to the emitter in HBT. Either AlGaAs or InGaP can be
amount of doping increases. With n-type doping, there
used as the emitter material in GaAs lattice-matched
is no passivation because of H atoms, and carrier com-
HBT, and it is possible to have better performance by
pensation because of self-forming lattice defects such
using InGaP because it can prevent hole current loss
as Ga vacancies and atoms in interstitial positions is
with a large energy barrier at the valence band, while
typical. The diffusion rate for these lattice defects in
reducing electron injection resistance at the conduc-
the crystal is extremely high and they diffuse within
tion band edge between the emitter and the base. In
SUMITOMO KAGAKU 2015
6
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
addition, since AlGaAs cr ystals include chemically
junction interface is affected. Specifically, for example,
active Al, the density of the defects at the surface of
when InGaP crystal is formed on GaAs crystal, switch-
the crystal exposed to the environment during device
ing from the As material to the P material is carried
processing easily becomes high, and these defects at
out, but after the switching to the P material has been
the surface form an electron or hole trap by which the
carried out, substitutions between the As atoms and
device characteristics and reliability are af fected.
the P atoms arise at the crystal surface because of the
Because of this, it can be said to be more beneficial to
time elapsed before supply starts of the group III mate-
use InGaP than AlGaAs, but there are also distinctive
rial, in other words Ga material and In material, and
problems in terms of crystal growth in InGaP crystals.
cr ystals such as GaAsP may be formed. This is the
One of them is interface control. In MOCVD growth
degradation of the steepness of the heterojunction
of crystal systems using As and P as group V atoms,
interface in a so-called interface transition layer and is
the dissociation pressure for the group V atoms is high
a cause of a deterioration in device characteristics. To
at the crystal growth surface, and group V material is
suppress this phenomenon, optimization of designs for
typically supplied at a rate of several tens to several
MOCVD gas supply systems and gas switching condi-
hundred times that of the group III material. Crystal
tions suitable for those systems is necessary.
growth is carried out at supply rate limiting conditions
The second problem in using InGaP crystals is the
for the group III material. Therefore, group III element
formation of a natural superlattice. In InGaP crystals
switching is comparatively easy, and a steep hetero
formed by MOCVD growth, it is known that the lattice
interface can be formed in heterojunctions with differ-
energy can be lowered with a lattice structure by reg-
ent group III atoms. On the other hand, control of het-
ularly altering the In-rich group-III sub-lattice plane and
erojunctions with different group V atoms is difficult.
the Ga-rich sub-lattice and this phenomenon is the so-
Since the group V atom dissociation pressure is high
called natural superlattice formation.10)
as described previously, the group V material supply
The formation of a regular arrangement of such
must also be interrupted to suppress desorption of
altered sub-lattices in the <110> direction with InGaP
group V atoms from the crystal surface while interrupt-
crystal growth on a (100) crystal surface has been con-
ing growth at the MOCVD crystal growth surface for
firmed by x-ray diffraction (Fig. 6). It is known that if
group III – V compound semiconductors. During the for-
a natural superlattice is formed, distor tion arises
mation of hetero interfaces with different group V ele-
because of the difference in the bonding state of Ga
ments, there is switching of this group V material, but
atoms and In atoms with P atoms and in the lattice con-
cases occur in which substitution of the group V atoms
stant, and then a local polarization arises due to the
on the crystal surface arises because of the magnitude
change of charge balance induced by the distortion.
of the dissociation pressure for the group V atoms at
This charging annihilates itself between adjacent sub-
the crystal surface and the gas phase group V gas pres-
lattice planes inside the InGaP cr ystal, but a charge
sure. Therefore, the steepness of the hetero interface
remains at the upper and lower inter faces for the
formed by the gas switching sequence at the hetero-
InGaP layer as a whole. The amount of charge varies
Growth direction
In(111)
Ga(111)
[111]
ordered
Fig. 6
disordered
Ga
In
P
[001]
[111]
[110]
Natural superlattice formation
(electron beam diffraction pattern difference - order/disorder)
SUMITOMO KAGAKU 2015
7
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
with the degree of formation of the natural superlattice,
n-InGaP
and that degree of formation depends on the growth
conditions. In addition, it is known that the InGaP layer
Band structure change caused
by interfacial charge
p-GaAs
n-GaAs
band gap value varies according to the degree of formation of the natural superlattice. Specifically, the
hole
hole
greater the degree of formation of the natural superlattice is, the smaller the band gap becomes, and the
reduction of the band gap at this time is thought to
mainly arise on the conduction band side. This means
a) Without interfacial charge
b) With interfacial charge
Band structure change caused by interfacial charge between InGaP and GaAs
Fig. 7
that, while the energy barrier against hole injection
from the base layer to the emitter layer is maintained
on the valence band side, the energy barrier against
electron injection from the emitter to the base gets
100
smaller, so it is an advantageous direction for HBT
operation. However, if HBT is fabricated using an
ral superlattice present in the upper and lower interfaces of this layer, the device characteristics are affected by the interface charging. In InGaP crystal growth
on GaAs (100) crystals, a positive charge and free electrons are generated at the lower interface of the InGaP
crystal because of the polarity induced, and a negative
10–4
IB
10–6
10–8
IC : Collector Current
IB : Base Current
10–10
0.5
charge and free holes are generated at the upper inter-
0.7
0.9
1.1
1.3
VBE (V)
face. With the HBT structure shown in Fig. 1, a p-type
base layer doped with high carrier concentration is
IC
10–2
Current IC , IB (A)
InGaP layer with interface charging induced by a natu-
without n-doping
with
n-doping
Fig. 8
present on the lower side of the InGaP emitter; there-
Effect of InGaP interfacial charge on HBT
Gummel plot
fore, the positive interface charging generated by the
natural superlattice is canceled out by negative charging because of ionization acceptors in the base layer.
side interface of the InGaP crystal, and the increase in
When an n-type doped layer with high carrier concen-
the base current in the low-voltage range as shown in
tration is present at the upper interface of the InGaP
Fig. 8 can be suppressed by using this method.11)
crystal, a similar interface charging cancellation is possible, but there are also cases of n-type layers doped
Problems for Making HBT Practical
with low carrier concentration because of HBT design.
In such cases, the band of the InGaP layer is raised by
While the basic performance of HBT devices using
the negative charge of the interface on the InGaP upper
the technology described above satisfying practical
side interface, and this effect extends to the base/emit-
standards has been achieved, problems arise in manu-
ter interface (Fig. 7). Fig. 8 shows a Gummel plot in
facturing the epitaxial substrates and in operating con-
that instance. When an interfacial charge is present, it
ditions. In the following, examples of these will be
is shown that the base current (IB) increases in the low-
introduced.
voltage range, and as a result, the current gain decreases in the low current range. This is thought to be
1. Crystal defects in bulk GaAs substrates
caused by the hole barrier at the base/emitter inter-
As has been discussed up to this point, current gain
face being lowered by the raising of the band at the
and some other characteristics of HBT deteriorate by
upper-side InGaP crystal interface and hole injection to
increase of recombination centers originated from
the emitter layer from the base layer increasing. Com-
crystal defects, impurities, etc. We have described the
pensating for the charge on the upper side of the InGaP
defects introduced by growth conditions, doping and
cr ystal can eliminate this effect. Specifically, n-type
other processes during epitaxial growth, but defects
doping can be carried out in the vicinity of the upper
propagated from bulk GaAs substrates are also a cause
SUMITOMO KAGAKU 2015
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Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
of effects. Vertical gradient freeze (VGF), vertical boat
doped at a high concentration, and base resistance that
(VB) and liquid encapsulated Czochralski (LEC) are
limits high-speed operation can be reduced. However,
manufacturing methods for GaAs substrates. Basically
during high density current drive, the current density
these methods are a kind of melt growth method, and
in the peripheral parts of the emitter near the base elec-
crystals are grown near the melting point of GaAs, but
trode increases because of the ef fect of horizontal
at such high temperatures, the concentration of various
resistance in the base, and a current density difference
types of point defects such as vacancies and interstitial
with the center part where the resistance becomes
atoms increases due to thermodynamic reasons. The
large arises. To avoid the effects of this lack of unifor-
point defect density or dislocation density can be also
mity, a design for forming an arrangement of multiple
seriously affected by the manufacturing technique and
small electrodes is normally employed for the emitter.
the growth conditions through the fluctuation of stoi-
Therefore, the reduction of contact resistance with the
chiometry and/or residual stress during crystal forma-
electrode in emitters or of resistance in the emitter
tion. Dislocation in the substrate is usually evaluated
crystal layer is an important problem as was indicated
by the etch pit technique, and the substrates with dis-
earlier.
1 × 103cm –2
can be typical-
However the troublesome problem, so-called ther-
ly grown with VGF and VB, and ones with around
mal runaway, is typically present in bipolar transistors,
1× 104cm–2 with LEC. Since these dislocations are in
including HBT.12) The thermal runaway mechanism is
themselves active as recombination centers, they can
extremely simple. When a for ward bias is applied
be thought of as having large effects on HBT charac-
across the emitter and the base, the current flowing
teristics and reliability when they are propagated to the
from the emitter to the collector increases exponential-
active area of HBT. There is also variation in the dislo-
ly, and generates heat due to resistance along the cur-
cation density itself, and that variation is a cause of vari-
rent pathway. In parts where the amount of heat gener-
ations in HBT characteristics, but the dislocations in
ated is large, the band gap of the semiconductor crystal
themselves are also used as sources of absorption or
is made smaller by the increase in temperature; there-
release of various types of point defects as described
fore, the energy barrier for injection current from the
above, and the behavior of dislocations and the various
emitter is reduced, and the current density increases
types of point defects are typically complex. In addition,
further. Typically current density dif ferences arise
some of these defects, as with the defects during high
between emitters or even within an emitter forming
concentration doping of sub-collectors described previ-
HBT because of some variation in size of each emitter
ously, can be thought of as propagating to the active
due to limited processing capability among the multiple
area of HBT and affecting important characteristics
emitters or differences in distance from the base elec-
such as the current gain, etc. One measure for prevent-
trode even within the same emitter (resistance differ-
ing these effects is introducing a layer that can block
ences), and further, because of natural fluctuations
dislocation into the epitaxial layer. A typical dislocation
when multiple emitters are present in parallel. Once
propagation blocking method carried out with epitaxy
current density differences arise, a positive feedback
in optical applications such as lasers is the method of
action that concentrates current further in parts with
multiple layer stacking of a pair of layers with different
high current density because of temperature differ-
lattice constants (strained superlattice), bending dislo-
ences that occur there arises, and finally, crystal break-
cations in the interface direction using that strain ener-
down arises because of the electric field caused by
gy and elimination to the outside of the crystal. In addi-
abnormal current density or thermal effects.
location densities around the
tion, another possible method, an impurity doping
This is the so-called thermal runaway phenomenon.
technique, has also been proposed to eliminate the dis-
To suppress this phenomenon, countermeasures such
location to reduce the effects of dislocations on HBT
as positioning of emitter electrodes or making temper-
characteristics.
ature uniform by heat exchange through wiring connecting those emitters and improvements in heat dis-
2. Thermal runaway problem in HBT
sipation are employed. When the temperature
In HBT, reverse injection current to the emitter from
increases in homo bipolar transistors or AlGaAs/GaAs
the base can be suppressed by a heterobarrier formed
HBT where the valence band barrier is comparatively
at the base-emitter interface; therefore, the base can be
small, the base current flows out to the emitter side to
SUMITOMO KAGAKU 2015
9
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
a certain extent; the base potential rises, and there is
number of combinations is 2 3×3 = 512, and conducting
a negative feedback action that reduces the current
an experiment for all of them is not realistic if we con-
injection from the emitter, while InGaP/GaAs HBT,
sider the limited development period and costs. Fur-
etc., that have large valence band barriers, are espe-
thermore, the three layers above are divided more fine-
cially vulnerable to thermal runaway even though they
ly into the emitter contact layer, sub-emitter layer,
have superior temperature characteristics. Thus,
ballast layer, sub-collector layer, interface layer etc.,
mechanisms are used for suppressing current concen-
and, further, the parameters to be optimized and the
tration by forming a so-called ballast resistance, which
levels to be studied for them make for an astronomical
has thermistor characteristics of resistance increasing
number. Sumitomo Chemical had already developed a
during increases in temperatures in emitter electrode
device simulator for epitaxial wafers for p-HEMT, and
parts, and automatically increasing resistance in parts
to address this problem, we made use of that develop-
where temperature increases have arisen. This ballast
ment and created an HBT device simulator for han-
resistance is commonly formed on top of emitter elec-
dling HBT development. An overview of that technolo-
trode par ts, but it is also frequently formed within
gy is described in the following.
semiconductor cr ystal layers. Most semiconductors
The HBT simulator we created is based on the drift
normally have negative temperature characteristics,
diffusion method.13) The drift diffusion method is a
and it is possible to have a function as a ballast layer
method that expresses the current by the sum of the
by achieving comparatively large negative temperature
drift current dependent on electric field strength and
characteristics. However, these ballast layers cause
the diffusion current dependent on the gradient of the
loss just as resistance in the typical operating condi-
carrier (electrons or holes) concentration, but it is a
tions for transistors does; therefore, the development
rough approximation of the Boltzmann transport equa-
of an HBT crystal structure having lower resistance
tion (Wigner transpor t equation when considering
and an effective thermal runaway suppressing function
quantum mechanics). Therefore, the calculation speed
is one challenge for the future.
of the drift diffusion method is more rapid than that of
the hydrodynamic method or the Monte-Carlo method,
HBT Device Simulation
with less approximation, but the difference between
the calculated and measured electrical characteristics
To design HBT having the targeted electrical char-
is large. To make this difference smaller, we referred
acteristics, three major parameters; the layer thickness,
to multiple HBT current-voltage characteristics meas-
composition (Al ratio in AlGaAs, etc.) and impurity con-
ured by Sumitomo Chemical, and we adjusted the phys-
centrations (Si, C and other impurity concentrations)
ical parameters such as electron and hole mobility, as
must be optimized for each layer. HBT requires many
well as band offset energy at different types of com-
processes, much time and effort for device formation,
pound semiconductor interfaces (hetero interfaces).
including microprocessing at a practical level, but on
(Naturally, when a technique with less approximation
the other hand, HBT which has a large emitter area of
is used sacrificing calculation speed, these physical
around 100 µm requires no special microprocessing,
parameters take values closer to those measured.)
and devices can be formed with a time of approximately
Another area in which ingenuity was used was the cal-
three hours at the shortest using simple processing
culation method for current in the vicinity of hetero
technology. This large emitter device is naturally differ-
interfaces. In a typical semiconductor device simulator,
ent from a commercial product device, but several
the thermionic-field emission boundar y condition
important device parameters, including current gain in
(specifically, boundary conditions calculating the inter-
low current density regions, can be extracted in a short
face current with the idea of “using the thermal energy
cycle time, and the technique is extremely useful from
and tunnel effect when the carrier overcomes the ener-
the standpoint of epitaxial crystal development for HBT
gy barrier at the hetero interface”) is set only at the
and quality assurance for products, so it is widely used.
hetero interfaces.13) We proposed a method for setting
However, when we simply think of the design of three
values for the carrier current density in the vertical
layers for the emitter, base and collector constituting
direction to hetero interfaces by taking the weighted
HBT, even if there are two levels for each of the three
average of the carrier current density calculated based
major parameters above for the three layers, their total
on the drift diffusion equation and that calculated based
SUMITOMO KAGAKU 2015
10
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
on the thermionic-field emission boundary condition in
communications. Recently, a BiHEMT str ucture
the vicinity of hetero interfaces.14)
where HBT power amplifiers, low noise amplifiers and
In this method, the drift diffusion current is the main
switches formed by p-HEMT can be integrated has
one at a point a certain distance away from the hetero
also come into use; increased complexity and greater
interface, but as the hetero interface is approached, the
functionality in the structures of epitaxial substrates
ratio of the thermionic-field emission boundary condi-
have been progressing, and quality, which is required
tion current to the drift diffusion current increases, and
from the standpoint of manufacturing, has also been
at the hetero interface, only the thermionic-field emis-
increasing. To achieve these things, we must continue
sion boundary condition current is used. If current-volt-
to make progress in strengthening development func-
age characteristics are calculated by this method, the
tions and improving manufacturing technology. Com-
difference between the calculated and measured cur-
pound semiconductor materials are more replete with
rent-voltage characteristics is smaller than when the
variety in terms of material design and control of char-
thermionic-field emission boundary condition is only
acteristics than Si, and there are expectations for fur-
interface.14)
An example of the cur-
ther expansion of devices that can make use of com-
rent-voltage characteristics (Gummel plot) calculated
pound semiconductors. The factors that determine the
using the HBT simulator described above is shown in
performance limits of equipment related to electronic
Fig. 9. The calculated results agree well with experi-
information, which is becoming more complex and
mental results. Currently, at Sumitomo Chemical, the
more various, can be said to be in a large part the
HBT simulator described above is fully utilized for
characteristics of the semiconductor materials them-
designing and optimizing the epitaxial layer structure,
selves. At Sumitomo Chemical, we are making the
with numerical predictions of HBT Gummel plots, cur-
utmost use of crystal growth and analysis evaluation
rent gain, collector-base voltage resistance, emitter-
technology, and we would like to continue to hold up
base voltage resistance, elucidation and suppression of
our end of the expansion of the compound semicon-
the thermal runaway phenomenon, etc.
ductor market.
set at the hetero
Collector Current
1.0E-01
1) M. Hata, N. Fukuhara, Y. Matsuda and T. Maeda,
1.0E-02
SUMITOMO KAGAKU, 1994-!, 34 (1994).
1.0E-03
Current (A)
References
2) M. Hata, N. Fukuhara, Y. Sasajima and Y. Zempo,
SUMITOMO KAGAKU, 2000-!, 10 (2000).
1.0E-04
Base Current
1.0E-05
3) M. Hata, T. Inoue, N. Fukuhara, T. Nakano, T.
Osada, J. Hada and Y. Kurita, SUMITOMO KAGAKU,
2008-!, 4 (2008).
1.0E-06
4) K. Honjou, “Microwave Semiconductor Circuit;
1.0E-07
Basic and Applications” supervised by Y. Konishi,
Experiment
Simulation
1.0E-08
1.0E-09
0.8
0.9
1.0
1.1
1.2
1.3
1.4
Base-Emitter Voltage (V)
Fig. 9
Gummel plot comparison between simulation and experimental results
Nikkan Kogyo Shinbun (1993).
5) G. B. Stringfellow, “Organometallic Vapor-Phase
Epitaxy: Theor y and Practice”, Second Edition,
Academic Press (1999).
6) Sumitomo Chemical Co., Ltd., JP 2000-49094 A, US
6191014 B1 (2001).
7) E. Tokumitsu, Jpn. J. Appl. Phys., 29 (5), L698
(1990).
Conclusion
Based on the technology described up to this point,
Sumitomo Chemical has manufactured and sold HBT
epitaxial wafers for power amplifiers targeting mobile
SUMITOMO KAGAKU 2015
8) M. Borgarino, R. Plana, S. Sylvain, L. Delage, F.
Fantini and J. Graf feuil, IEEE Trans. Electron.
Devices, 46, 10 (1999).
9) Sumitomo Chemical Co., Ltd., JP 5543302 B2
(2014).
11
Epitaxial Growth of Compound Semiconductors Using MOCVD (IV)
10) Y. S. Chun, Y. Hsu, I. H. Ho, T. C. Hsu, H. Murata,
G. B. Stringfellow, J. H. Kim and T. –Y. Seong, J.
of Electron. Materials, 26 (10), 1250 (1997).
11) Sumitomo Chemical Co., Ltd., JP 5108694 B2
(2012).
12) W. Liu, A. Khatibzadeh, J. Sweder and H-F. Chau,
IEEE Trans. Electron Devices, 43 (2), 245 (1996).
13) V. Palankovski and R. Quay, “Analysis and Simulation of Heterostructure Devices”, Springer (2004).
14) Sumitomo Chemical Co., Ltd., JP 2006-302964 A.
PROFILE
Tomoyuki TAKADA
Masahiko HATA
Sumitomo Chemical Co., Ltd.
IT-related Chemicals Research Laboratory
Senior Research Specialist, Group Manager
Sumitomo Chemical Co., Ltd.
Advanced Materials Research Laboratory
Senior Research Specialist
(Currently: General Manager, Compound Semiconductor Materials Dept., Electronic Materials Division)
(Currently: Sumika Electronic Materials, Inc.)
Noboru FUKUHARA
Yasuyuki KURITA
Sumitomo Chemical Co., Ltd.
IT-related Chemicals Research Laboratory
Senior Research Associate
Sumitomo Chemical Co., Ltd.
Advanced Materials Research Laboratory
Senior Research Specialist,
Doctor of Agriculture.
Hisashi YAMADA
Sumitomo Chemical Co., Ltd.
IT-related Chemicals Research Laboratory
Senior Research Associate,
Doctor of Engineering
(Currently: Advanced Materials Research Laboratory)
SUMITOMO KAGAKU 2015
12