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p-type conduction in as-grown Mg-doped GaN grown by metalorganicchemical vapor deposition
Lisa Sugiura,a) Mariko Suzuki, and Johji NishioAdvanced Semiconductor Devices Research Laboratories, R&D Center, Toshiba Corporation,1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan
Received 24 November 1997; accepted for publication 9 February 1998
We have clarified the effect of H2 and NH3 on the passivation of Mg acceptor in p-type GaN filmsgrown by metalorganic chemical vapor deposition. It has been found that the small amount of H 2carrier gas strongly influences the electrical property of the Mg-doped GaN films. Low-resistivity
p-type GaN has been obtained by H2-free growth without any post-treatments. Its acceptor
concentration is as high as that obtained by conventional H2-rich growth with subsequent thermal
annealing. It has also been clarified that hydrogen produced by NH 3 dissociation does not prevent
Mg from electrically activating in H2-free growth. 1998 American Institute of Physics.
S0003-6951 98 02714-4
GaN and other group III nitrides have recently attracted
extensive attention because of their potential application to
optoelectronic devices operating in blue and ultraviolet spec-tral regions. The pioneering efforts have realized p-type GaN
films,1,2 which are one of the keys to the success of GaN-
based light-emitting devices such as light-emitting diodes
LEDs 3,4 and laser diodes LDs .5 7 At present, the domi-
nant growth technique for GaN-based devices is metalor-
ganic chemical vapor deposition MOCVD . p-type doping
in MOCVD grown GaN is typically achieved using magne-
sium Mg as an acceptor dopant. The as-grown Mg-doped
GaN films grown by MOCVD show high resistivity and the
p-type conduction cannot be obtained. Hence, the devices
grown by MOCVD require an additional processing step to
electrically activate Mg, such as low-energy electron beam
irradiation LEEBI 1 treatment or thermal annealing.2 Hydro-
gen passivation of Mg acceptor is considered to be a main
reason for the hole compensation.8,9 Nakamura et al. re-
ported that low-resistivity p-type GaN films obtained by
N2-ambient thermal annealing or LEEBI treatment were
changed to high-resistivity films by NH3-ambient thermal an-
nealing at temperatures above 600 C, and that atomic hy-
drogen produced by NH3 dissociation at temperatures above
400 C is related to the hole compensation mechanism.8
In the present research, we have tried N2-ambient
MOCVD growth using N2 as a main carrier gas and investi-
gated the effect of H2 carrier gas and NH3 on the passivation
of Mg acceptor. For this purpose, the doping characteristic ofMg-doped GaN grown by N2-ambient MOCVD has been
investigated by varying the amount of H2 carrier gas. We
demonstrate that the small amount of H2 in a reactor strongly
influences the electrical property of the Mg-doped GaN
films. Low-resistivity p-type GaN films have been obtained
by the H2-free MOCVD growth without any posttreatment.
The acceptor concentration of these films was as high as that
of the p-type GaN films obtained by the conventional
method: H2-rich ambient growth and post-thermal annealing.
The samples investigated here were grown on 0001
sapphire substrates by atmospheric-pressure MOCVD using
trimethylgallium TMG , NH3, and bis-cyclopentadienyl-magnesium (Cp2Mg) as Ga, N, and Mg sources, respec-
tively. The Mg-doped GaN films were grown under nitrogen
(N2)-rich ambient with a small amount H2/ N2H22.4% of hydrogen ( H2), and also under hydrogen-free
ambient. Their characteristics were compared with those of
the p-type GaN films obtained under H2 rich-ambient growth
H2/ N2H2 75% with post-thermal annealing. After the
growth, the sample was cooled down naturally in the atmo-
sphere of NH3 and N2. The Mg-doped GaN films were
grown at the temperature of 1040 C under which a specular
surface morphology and a fine structural property with nar-
row X-ray full width at half-maximum were obtained.
The capacitancevoltage (C V) measurements wereperformed in order to evaluate the net acceptor concentration
(NA ND). We employed the electrochemical technique for
C V measurements.10 Secondary ion mass spectrometry
SIMS was used to determine the Mg concentration (NMg)
and hydrogen concentration. The Hall measurement 300 K
was also conducted for the samples obtained under the opti-
mized growth conditions. Indium solder was used approxi-
mately at 300 C to fabricate small area electrical contacts
for Hall measurement.
Figure 1 demonstrates the Mg concentration (NMg) and
the net acceptor concentration (NA ND) in Mg-doped GaN
films grown under N2-rich ambient with 2.4%-H2 and
H2-free ambient as a function of the mole ratio of Cp2Mg to
the group III source ( Cp2Mg / III ). The increment of
Cp2Mg / III ratio corresponds to the increase in Cp2Mg
flow rate, since the flow rate of TMG was constant. There
was no difference in the Mg concentration between the
samples grown under N2-rich ambient with 2.4%- H2 and
H2-free ambient. This result indicates that the small amount
of H2 in the atmosphere does not affect the incorporation
ratio of Mg into GaN. All the as-grown Mg-doped GaN films
obtained in this experiment showed p-type conduction with-
out any subsequent treatment, although the as-grown films
grown under H2-rich ambient do not show p-type conduc-a Electronic mail: [email protected]
APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 14 6 APRIL 1998
17480003-6951/98/72(14)/1748/3/$15.00 1998 American Institute of PhysicsDownloaded 03 Jul 2008 to 132.234.251.211. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
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tion. A distinctive difference can be seen in the net acceptor
concentration of the sample grown under N2-rich ambient
with 2.4%-H2 and H2-free ambient. The maximum value of
the net acceptor concentration of the sample grown under
H2-free ambient is 81018 cm3 which is twice that of the
one grown under N2-rich ambient with 2.4%-H2. It should
be noted that acceptor concentration of the sample grown
under N2-rich ambient with 3.7%-H2 was the lowest of all
the samples in Fig. 1 despite high Cp2Mg / III ratio. The
post-thermal annealing was also performed under 770 C for
30 min in a N2-ambient furnace for the films grown under
N2-rich ambient. The net acceptor concentration of the post-annealed films is also shown in Fig. 1. A remarkable increase
in the net acceptor concentration was seen in the post-
annealed samples. The increase of the acceptor concentration
of the sample grown under N2-rich ambient with 3.7%-H2indicates that the low acceptor concentration of this as-
grown sample is due not to the low Mg incorporation but to
the hydrogen passivation.
Figure 2 shows the relationship between the Mg concen-
tration and hydrogen concentration, which was determined
by SIMS, in Mg-doped GaN. The hydrogen concentration in
Mg-doped GaN films grown under N2-rich ambient is about
twice that of the ones grown under the H2-free ambient. The
hydrogen existing in the films grown under the H 2-free am-bient might originate from the sources. The difference in the
hydrogen concentration is due to the existence of hydrogen
in a carrier gas. The hydrogen concentration increases in
proportion to the Mg concentration. These results clearly
show that the incorporation of hydrogen is accompanied by
Mg incorporation.
Table I shows the resistivity , the Mg concentration
(NMg), the acceptor concentration (NA ND), the carrier
concentration (p) and the Hall mobility of p-type Mg-
doped GaN obtained by the conventional H2-rich growth
with subsequent thermal annealing, N2-rich growth
(2.4%-H2) and H2-free growth. Comparing the results of
N2-rich and H2-free growth, it has been demonstrated that thesmall amount of H2 in a carrier gas prevents Mg acceptor
from activating, and strongly influences the electrical prop-
erties. By the growth under H2-free ambient, we have ob-
tained as-grown p-type Mg-doped GaN with the resistivity
of 0.81 cm, the acceptor concentration of 6 8
1018 cm3 the carrier concentration of 0.8 2 1018 cm3
and the Hall mobility of 5 10 cm2/V s. These values are
approximately the same as that of the films obtained under
the conventional H2-rich growth with subsequent thermal an-
nealing see Table I . The details of the characteristics of
Mg-doped GaN grown under H2-rich ambient with various
conditions are reported elsewhere.10 The carrier concentra-
FIG. 1. Mg concentration (NMg) and net acceptor concentration (NA ND)
of Mg-doped GaN films as a function of Cp2Mg / III . : NMg for
N2-rich growth unannealed , : NMg for H2-free growth unannealed ,
: NA ND for N2- rich growth unannealed , : NA ND for N2-rich
growth
annealed
,
: NA
ND
for H2-free growth
unannealed
.
FIG. 2. Relationship between Mg concentration and hydrogen concentration
in Mg-doped GaN. : N2-rich growth unannealed , : H2-free growth
unannealed .
TABLE I. Resistivity , Mg concentration (NMg), acceptor concentration (NA ND), carrier concentration (p)
and Hall mobility of p-type Mg-doped GaN obtained by the conventional H2-rich growth with subsequent
thermal annealing, N2-rich growth (2.4%-H2) and H2-free growth. The symbol indicates that the value is
unable to be measured.
p( cm) NMg(cm3) NA ND(cm
3) p(cm3) (cm2/V s)
H2-rich growth
as-grown
high resistivity
H2-rich growth
annealed12 61019 6101018 3 51017 510
N2-rich growth
as-grown
34 361019 3 41018 2 31017 514
H2-free growth
as-grown
0.81 61019 6 81018 0.821018 510
1749Appl. Phys. Lett., Vol. 72, No. 14, 6 April 1998 Sugiura, Suzuki, and Nishio
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7/28/2019 APL1998ptypedoiping
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tion of p-type GaN grown under H2-free ambient was a little
higher and the resistivity was a little lower than that of
H2-rich growth. Further study is required to explain the rea-
son for this result. The reason that NA ND is one order
lower than NMg is presumably that Mg is not involved in the
Ga site, or the complexes such as MgMg are formed, and
also the reason that p is one order lower than NA ND might
be that the deep levels are formed at Mg acceptors.
It should be noted that p-type activation of Mg acceptor
is not caused by cooling process in N2 ambient after the
growth. This fact is verified by the following result. When
the Mg-doped GaN films grown in H2-rich ambient were
cooled down naturally in N2 ambient, the p-type conduction
could not be obtained.
In order to investigate the influence of NH3 on the hy-
drogen passivation, the growth of Mg-doped GaN was per-
formed under the lower NH3 flow rate, which is 2/3 of that
used in the above-mentioned experiment. There was little
difference in the Mg incorporation and the net acceptor con-
centration compared with those in Fig. 1. It seems that hy-
drogen produced by NH3
dissociation does not prevent Mg
from electrically activating in N2-ambient growth. If we con-
sider our experimental results and the previous report con-
cerning the formation of MgH complexes accompanied by
NH3 dissociation causes hole compensation,8 these facts
might be explained if the hydrogenation process of Mg ac-
ceptor and the dissociation process of hydrogen atom from
MgH complexes occur simultaneously at the surface in
N2-ambient growth.
In summary, low-resistivity p-type Mg-doped GaN has
been obtained by N2-ambient MOCVD growth without any
post-treatments. The small amount of H2 carrier gas in a
reactor strongly influenced the electrical property of the Mg-
doped GaN films. It has been demonstrated that hole com-pensation caused by hydrogen passivation is not an obstacle
in obtaining low-resistivity p-type GaN in the N2-ambient
MOCVD growth. Hydrogen produced by NH3 dissociation
does not prevent Mg from electrically activating in
N2-ambient growth. Hydrogenation process of Mg acceptor
and dissociation process of hydrogen atom from MgH com-
plexes might occur simultaneously at the surface in
N2-ambient growth. H2-free growth realized the as-grown
p-type GaN with the acceptor concentration as high as that
ofp-type GaN obtained by conventional H2-rich growth with
subsequent thermal annealing. This result indicates that the
device fabrication process can be simplified by employing
H2-free growth.
The authors would like to thank C. Hongo for the SIMS
measurement. They are also grateful to Y. Kokubun, M. Ish-
ikawa, and K. Itaya for their encouragement.
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1750 Appl. Phys. Lett., Vol. 72, No. 14, 6 April 1998 Sugiura, Suzuki, and Nishio
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