Journal of Crystal Growth 246 (2002) 230–236

Surface polarity dependence of decomposition and growth of

GaNstudied using in situ gravimetric monitoring

Akinori Koukitu*, Miho Mayumi, Yoshinao Kumagai

Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei,

Tokyo 184-8588, Japan

Abstract

The influence of lattice polarity of wurzite GaN(0 0 0 1) on its decomposition rate was investigated using an in situ

gravimetric monitoring (GM) method with freestanding GaN(0 0 0 1) substrates. At temperatures between 8001C
and 8501C, the decomposition rate of GaN(0 0 0 1) was faster than that of GaN(0 0 0 %1). On the other hand, the
decomposition rate of GaN(0 0 0 %1) was faster than that of GaN(0 0 0 1) at temperatures between 9001C and 9501C.
The relation between the decomposition rate and the H

2

partial pressure (P

H

2

) indicates that the rate-limiting reactions

are N(surface)+

3
2

H

2

(g)

-NH

3

(g) at lower temperatures, but Ga(surface)+

1
2

H

2

(g)

-GaH(g) at higher temperatures. In

addition, we used the GM method to measure the growth rate of GaN(0 0 0 1) on freestanding GaN(0 0 0 1) substrates.
In the low-temperature region, GaN(0 0 0 %1) grew faster than GaN(0 0 0 1), whereas GaN(0 0 0 1) grew faster than
GaN(0 0 0 %1) in the high-temperature region.
r

2002 Elsevier Science B.V. All rights reserved.

Keywords: A1. Etching; A1. Surface processes; A3. Vapor phase epitaxy; B1. Nitrides; B2. Semiconducting gallium compounds

1. Introduction

GaNand related compounds are very attractive

materials

for

blue/green

light-emitting

diodes

(LEDs), violet laser diodes (LDs), and high-
frequency electronic devices. From the view-
point of understanding processes in vapor phase
epitaxy (VPE) such as metalorganic vapor phase
epitaxy (MOVPE) and hydride vapor phase epitaxy
(HVPE), etching/decomposition and growth me-
chanisms of GaNare of great interest. The

VPE growth mechanism of III-nitride semicon-
ductors such as GaNis quite different from that
of the other III–V semiconductors such as GaAs.
This is due to the difference between the growth
reactions. In MOVPE growth, the reaction govern-
ing

GaNdeposition

is

Ga(g)+N

H

3

(g)

-

GaN(s)+

3
2

H

2

(g), whereas that for GaAs deposition

is Ga(g)+

1
4

As

4

(g)

-GaAs(s) [1]. In HVPE growth,

the

reactions

are

instead GaCl(g)+NH

3

(g)

-

GaN(s)+HCl(g)+H

2

(g), where hydrogen is formed

by the GaNdeposition, whereas GaAs deposition
has GaCl(g)+

1
4

As

4

(g)+

1
2

H

2

(g)

-GaAs(s)+HCl(g).

Hydrogen is a reaction product in GaNMOVPE
and HVPE growth. Consequently, hydrogen is
expected to play an important role in the GaN
system.

*Corresponding author. Tel.: [phone]; fax: +81-

423-86-3002.

E-mail address:

[e-mail] (A. Koukitu).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 7 4 6 - 3

On the other hand, because wurtzite GaNis

polar, there are two surface structures along the
c-axis direction: the (0 0 0 1) Ga face, where a Ga
atom combines with three Natoms in bulk GaN,
and the (0 0 0 %1) Nface, where a Natom combines
with three Ga atoms. A few groups have char-
acterized the polarity of GaNgrown on sapphire
(0 0 0 1) substrates, but the reaction between the
GaNsurface and the vapor remains poorly
understood. For example, Kobayashi and Ko-
bayashi studied nitrogen desorption from GaN
surface using an in situ surface photoabsorption
(SPA) method [2]. Groh et al. studied GaN
decomposition by weighing a piece of GaNon
sapphire before and after annealing using an
analytical balance [3], whereas Rebey et al. studied
the same process using laser reflectometry [4].
Although they showed that hydrogen in the carrier
gas had a profound effect on the decomposition,
the decomposition rate is speculative.

A deeper understanding of the decomposition

and growth mechanism in VPE is relevant because
it has implications for our understanding of
the chemistry in GaNVPE. In previous papers,
we have developed an in situ gravimetric monitor-
ing (GM) system [5–9]. The use of this system is
attractive for investigating the decomposition and
growth mechanism of GaNbecause the system
provides direct information on the decomposition
and growth rates at the monolayer (ML) level in
real time. Here, we used the in situ GM system to
determine the polarity dependence of GaNdecom-
position and growth.

2. Experimental procedure

Fig. 1 shows the in situ GM system used for

monitoring the decomposition rate of GaN. The
GM system, consisting of a vertical quartz reactor
and a recording microbalance, provides direct
information on the substrate weight change caused
by the decomposition of GaNfrom the surface
under atmospheric pressure. This system has a
sensitivity of 0.004 mg under dynamical conditions.
We used a freestanding GaN(0 0 0 1) substrate
(1.0
1.0
0.03 cm

3

) that was prepared by meta-

lorganic hydrogen chloride vapor phase epitaxy

(MOHVPE) on GaAs(1 1 1)A substrate [10]. One
side of the GaNsubstrate was covered by a
protective SiO

2

mask. The substrate was sus-

pended from the microbalance with a fused quartz
fiber. The carrier gases used were H

2

and He as an

inert gas. NH

3

was introduced over the GaN

substrate while heating the furnace to prevent
GaNdecomposition before measurements. The
GaNdecomposition rate under each fixed set of
conditions was measured after switching off the
NH

3

flow. By monitoring the weight change of

the GaNsubstrate, the decomposition rates of
both GaN(0 0 0 1) and GaN(0 0 0 %1) faces were
measured in the temperature range from 7301C to
9501C with P

H

2

¼ 1 atm. In addition, we mon-

itored the dependence of decomposition rate on
hydrogen partial pressure P

H

2

in the carrier gas by

varying the ratio of H

2

to H

2

+He.

Fig. 2 shows the in situ monitoring system for

measuring the growth rate of GaNby HVPE.
GaCl, which was obtained by reacting metallic Ga
with HCl at 7801C, flowed onto the substrate
surface using a suction pump controlled by a
computer. NH

3

was used as a nitrogen source and

Fig. 1. Schematic of the GM system for GaNdecomposition.

A. Koukitu et al. / Journal of Crystal Growth 246 (2002) 230–236

231

was introduced from a separate tube using air-
operated valves. Also, the substrates used were
freestanding GaN(0 0 0 1) substrates prepared by
MOHVPE on GaAs(1 1 1)A surfaces. One side of
the GaNsubstrate was covered by a SiO

2

film.

3. Decomposition of GaN(0 0 0 1) surfaces

The decomposition rates of GaN(0 0 0 1) and

GaN(0 0 0 %1) faces as a function of temperature
are shown in Fig. 3. The partial pressure of H

2

in

the carrier gas was kept constant at 1 atm and the
decomposition

temperature

was

varied

from

7301C to 9501C. Regardless of the polarity, the
decomposition rates of both GaN(0 0 0 1) surfaces
increase markedly with increasing temperature.
This result agrees with our previous study using an
epitaxial GaNfilm grown on sapphire (0 0 0 1) [8].
The data show that the dependence on tempera-
ture is different at low and high temperatures
(Fig. 3): from 8501C to 9501C, the activation
energies for GaN(0 0 0 1) and GaN(0 0 0 %1) are
242 and 259 kJ/mol, respectively; whereas from
7301C to 8501C, the activation energies for

Fig. 2. Schematic of the GM system for GaNgrowth.

10

−

1

10

0

10

1

1000

900

800

700

Decomposition rate (

µ

m/h)

1000/T(K

−

1

)

Temperature (

°

C)

0.8

0.9

1.0

GaN (0001) Ga

GaN (0001) N

Fig. 3. Decomposition rates of GaN(0 0 0 1) and GaN(0 0 0 %1)
as a function of temperature at P

H

2

¼ 1 atm.

A. Koukitu et al. / Journal of Crystal Growth 246 (2002) 230–236

232

GaN(0 0 0 1) and GaN(0 0 0 %1) are 143 and 191 kJ/
mol, respectively. These results indicate that the
rate-limiting reactions for decomposition on both
surfaces change with temperature.

Furthermore, the decomposition rate depends

on the polarity of the GaNsurface. Below about
8201C, the decomposition rate of GaN(0 0 0 1) is
faster than that of GaN(0 0 0 %1). Conversely, the
decomposition rate of GaN(0 0 0 %1) is faster than
that of GaN(0 0 0 1) at temperatures ranging from
about 8501C to 9501C. We argue below that these
differences in decomposition rate for the two
different lattice polarities is due to the different
bonding configuration of the GaNsurfaces.

To better understand the surface reaction of H

2

on GaN, we measured the decomposition rates of
GaNfor P

H

2

values ranging from 0 to 1. Fig. 4

shows the P

H

2

dependence of decomposition rates

for GaN(0 0 0 1) and GaN(0 0 0 %1) in the tempera-
ture range from 8001C to 9501C. The decomposi-
tion rates of both GaN(0 0 0 1) and GaN(0 0 0 %1)
increased with increasing P

H

2

at a given tempera-

ture. Therefore, it is clear that H

2

plays an

important role in the decomposition of GaN.
To make this more specific, we fit the data to the
rate equation for GaNdecomposition r ¼ k P

n
H

2

;

where r; k; and n are the decomposition rates, rate
constants, and order of reaction, respectively. In
Fig. 4, the values of n are drawn at each
temperature. The values of n for GaN(0 0 0 1)
and GaN(0 0 0 %1) are

1
2

from 8751C to 9501C, and

from 9251C to 9501C (hereafter the high-tempera-

ture region), respectively. On the other hand, the
values of n for GaN(0 0 0 1) and GaN(0 0 0 %1) are

3
2

from 8001C to 8501C, and from 8001C to 8751C
(hereafter the low-temperature region), respec-
tively.

Thus,

the

decomposition

rates

of

GaN(0 0 0 1) and GaN(0 0 0 %1) are proportional
to P

1=2
H

2

in the high-temperature region, whereas the

decomposition rates of both surfaces are propor-
tional to P

3=2
H

2

in the low-temperature region. These

results indicate that the rate-limiting reactions for
decomposition of both surfaces are

GaðsurfaceÞ þ

1
2

H

2

ðgÞ

-GaHðgÞ ðhigh-temperature regionÞ;

ð1Þ

and

NðsurfaceÞ þ

3
2

H

2

ðgÞ

-NH

3

ðgÞ ðlow-temperature regionÞ:

ð2Þ

Consequently,

the

rate-limiting

reactions

for

the decomposition of both GaN(0 0 0 1) and
GaN(0 0 0 %1) likely shift from reaction (1) to
reaction (2) with increasing temperature.

As described above, decomposition of GaNis

limited by the Ga surface on the GaNsubstrate in
the high-temperature region. The topmost Ga
atoms on the GaN(0 0 0 1) surface each combine
with three Natoms (three back-bonds) in the bulk,
whereas Ga atoms on the GaN(0 0 0 %1) surface
each form one back-bond with only one Natom in
the bulk. This is the likely reason why the
decomposition rate of GaN(0 0 0 %1) is faster than
that of GaN(0 0 0 1) in the high-temperature

0.0

0.2

0.4

0.6

0.8

1.0

800

850

900

950

1

2

3

4

5

0.0

0.2

0.4

0.6

0.8

1.0

800

850

900

950

1

2

3

4

5

n=3/2

n=3/2

n=3/2

n=3/2

n=0.7

n=3/2

n=1.0

n=1/2

n=1/2

n=1/2

n=1/2

n=1/2

Decompositio

n

Rate

(

µ

m

/h)

Decompositio

n

Rate

(

µ

m/h)

Hydr

ogen

Partial

Pressure

(atm)

Hydrogen

Partial

Pressure

(atm)

Tempe

rature(

°

C)

Tempe

rature(

°

C)

GaN (0001)

GaN (0001)

(a)

(b)

Fig. 4. P

H

2

influence on decomposition rate for: (a) GaN(0 0 0 1) and (b) GaN(0 0 0 %1) in the temperature range from 8001C to 9501C.

A. Koukitu et al. / Journal of Crystal Growth 246 (2002) 230–236

233

region. On the other hand, the limiting surface is
the Nsurface on the GaNsubstrate in the low-
temperature region. The decomposition rate of
GaN(0 0 0 1) is faster than that of GaN(0 0 0 %1)
because each Natom on the GaN(0 0 0 1) surface
combines with only one Ga atom in the bulk,
whereas each Natom on GaN(0 0 0 %1) forms back-
bonds to three Ga atoms in the bulk. Thus, the
different bonding configuration of the GaN
surfaces can explain the difference of the decom-
position

rates

between

GaN(0 0 0 1)

and

GaN(0 0 0 %1).

H

2

is generally used as a carrier gas in MOVPE

and HVPE growth. Additionally, H

2

is formed as

a by-product of the reaction that forms GaN.
Therefore, the reaction between H

2

gas and the

GaNsurface should cause some GaNdecom-
position even during growth. For example, in the
high-temperature region, the net growth rate of
GaN(0 0 0 1) should be faster than that

of

GaN(0 0 0 %1) because the net growth rate in VPE
is determined by the growth rate minus the
decomposition rate. This growth rate difference
agrees with the data on MOVPE growth of
GaNreported by Sumiya et al. [11] and Rouviere
et al. [12].

4. Growth on GaN(0 0 0 1) surfaces

The kinetics of HVPE is considerably more

complex. In the low-temperature region, an in-
crease in GaCl partial pressure actually results in a
decrease in the growth rate because GaCl hinders
desorption of HCl from the surface. On the other
hand, the growth rate has a maximum at an
intermediate temperature due to competing tem-
perature trends from thermodynamics and surface
kinetics. In particular, because growth is generally
exothermic, the growth rate should decrease with
increasing temperature. In fact, this decrease in the
high-temperature region is approximately equal to
that predicted from thermodynamics. But in the
low-temperature region, growth is limited by sur-
face kinetics such that the growth rate increases
with increasing temperature. Consequently, the
temperature of maximum growth rate depends on
the amount of GaCl or NH

3

in the vapor.

As described above, the temperature of max-

imum growth rate depends on the partial pressures
of the source materials. In the in situ monitoring
system for GaNgrowth, the input GaCl pressure
used was low compared with typical values used
for GaNHVPE because this allows more precise
measurements in the low-growth rate region. The
growth rates of GaN(0 0 0 1) and GaN(0 0 0 %1)
surfaces increase with increasing temperature in
the low-temperature region up to 7001C, then they
decrease with an increase of temperature (Fig. 5).
The activation energies for GaNgrowth obtained
from the slopes in the low-temperature region are
68 kJ/mol for GaN(0 0 0 1) and 64 kJ/mol for
GaN(0 0 0 %1).

The growth rate of the GaN(0 0 0 %1) surface is

faster than that of the GaN(0 0 0 1) surface in the
low-temperature region, which is the reverse of the
relation in the high-temperature region. Although
the growth rate dependence on the temperature
was shifted to low temperature due to the low
input partial pressure of GaCl used in this work,

1

10

800 750

700

650

600

550

P

: 1x10

atm

P

: 8x10

atm

GaCl

−

4

NH

−

2

3

Growth Rate (

µ

m/h)

GaN(0001)Ga

GaN(0001)N

_

1000/T(K

−

1

)

1.25

1.20

1.15

1.10

1.05

1.00

0.95

Temperature (

°

C)

Fig. 5. Growth rates of GaNon GaN(0 0 0 1) and (0 0 0 %1) as a
function of temperature.

A. Koukitu et al. / Journal of Crystal Growth 246 (2002) 230–236

234

because of the precise in situ monitoring, the
relation of growth rates with the GaNpolarities is
very close to the nature indicated by the results of
the GaNdecomposition.

To help clarify the difference of growth rates

between the two polarities, we show the growth
rates as a function of the input V/III ratio in
Fig. 6. For these measurements, the input GaCl
pressure was kept constant at 1
10

4

atm and the

input NH

3

pressure was varied from 4
10

2

to

8
10

2

atm. All the growth rates increase with an

increase of the input V/III ratio at 7001C (i.e., in
the low-temperature region in this work) and
7501C (high-temperature region in this work). The
dependence of the growth rate on the lattice
polarity is significantly clarified here. In the low-
temperature region, the growth rate on the
GaN(0 0 0 %1) substrate is faster than that on the
GaN(0 0 0 1) substrate, whereas the growth rate on
the GaN(0 0 0 1) is faster than that on the
GaN(0 0 0 %1) in the high-temperature region.

5. Conclusions

The dependence of GaNdecomposition on the

lattice polarity has been investigated by an in situ
GM method using a freestanding GaN(0 0 0 1)
substrate. In the low-temperature region, the
decomposition rate of GaN(0 0 0 1) was faster

than that of GaN(0 0 0 %1). On the other hand, in
the high-temperature region, the decomposition
rate of GaN(0 0 0 %1) was faster than that of
GaN(0 0 0 1). The decomposition rates of both
surfaces increased rapidly with increasing tem-
perature. Dependence of the GaNdecomposition
rate on P

H

2

showed that the decomposition rates

for both polarities were proportional to P

3=2
H

2

and

P

1=2
H

2

in the low- and the high-temperature regions,

respectively. Based on these results, the relations
between the lattice polarity and the decomposition
of GaNcan be explained clearly by considering the
rate-limiting reactions and the bonding configura-
tions on GaNsurfaces.

From the preliminary in situ monitoring of the

growth rates, it is found that the growth rate on
the GaN(0 0 0 %1) surface is faster than that on
GaN(0 0 0 1) in the low-temperature region, but
the reverse relation holds in the high-temperature
region. The relation of growth rates with the GaN
polarities is very close to that predicted from
considering the GaNdecomposition.

Acknowledgements

The authors would like to express their sincere

thanks to Y. Matsuo for preparation of our GaN
samples. This work was partly supported by the
Foundation for Promotion Material Science and
Technology of Japan (MST) and by the Grant-in-
Aid for Scientific Research from the Ministry
of

Education,

Culture,

Sports,

Science

and

Technology.

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600

800

2

3

4

5

GaN(0001) N

GaN(0001) Ga

750

°

C

700

°

C

P

GaCl

: 1x10

−

4

atm

Growth

Rate

(

µ

m/h)

Input V/III Ratio

Fig. 6. Growth rates of GaNas a function of input V/III ratio.

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