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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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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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Akasaki, Appl. Phys. Lett. 67, 2666 1995 .

10 M. Suzuki, M. Onomura, S. Nunoue, L. Sugiura, J. Nishio, and C. Hongo unpublished .

1750 Appl. Phys. Lett., Vol. 72, No. 14, 6 April 1998 Sugiura, Suzuki, and Nishio

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