Oxygen diffusion into SiO2-capped GaN during annealing

Article abstract of DOI:10.1063/1.125194

SiO₂ layers were deposited on p-GaN (hole concentration 9 × 10¹⁷ cm^-3) by inductively coupled plasma chemical vapor deposition using an ¹⁷O-enriched O₂ precursor. The samples were then annealed at 500

Full text of DOI:10.1063/1.125194

Oxygen diffusion into SiO 2 -capped GaN during annealing
S. J. Pearton, H. Cho, J. R. LaRoche, F. Ren, R. G. Wilson, and J. W. Lee
Citation: Applied Physics Letters 75, 2939 (1999); doi: 10.1063/1.125194
View online: http://dx.doi.org/10.1063/1.125194
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APPLIED PHYSICS LETTERS
VOLUME 75, NUMBER 19
8 NOVEMBER 1999
Oxygen diffusion into SiO2-capped GaN during annealing
S. J. Peartona) and H. Cho
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611
J. R. LaRoche and F. Ren
Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611
R. G. Wilson
Consultant, Stevenson Ranch, California 91381
J. W. Lee
Plasma-Therm, Inc., St. Petersburg, Florida 33716
͑Received 19 July 1999; accepted for publication 7 September 1999͒
SiO2 layers were deposited on p-GaN ͑hole concentration 9ϫ1017 cmϪ3) by inductively coupled
plasma chemical vapor deposition using an 17O-enriched O2 precursor. The samples were then
annealed at 500–900 °C and the SiO2 was removed. Secondary ion mass spectrometry profiling
showed significant indiffusion of 17O into the GaN under these conditions, with an incorporation
depth of ϳ0.18 ␮m after the 900 °C anneal. The 17O diffusion profiles indicate that the high
dislocation density in the GaN strongly affects the effective penetration depth. The GaN remained
p type upon incorporation of the oxygen. © 1999 American Institute of Physics.
͓S0003-6951͑99͒00445-3͔
Oxygen is one of the most important residual impurities
in GaN, and is generally believed to be responsible for the
n-type conductivity of unintentionally doped material.1–16
The source of the residual oxygen is generally as an impurity
in NH3 used as a precursor for metal organic chemical vapor
deposition ͑MOCVD͒ or the remnant water vapor in molecu-
lar beam epitaxy ͑MBE͒ chambers. In the latter case, oxygen
may also be leached from the SiO2 or Al2O3 plasma contain-
ment sections used in some plasma sources for creating
atomic nitrogen from N2 feedstock. It has also been reported
that bulk GaN grown at high temperatures (1700 °C) and
high N2 pressures ͑20 k bar͒ from Ga solutions typically con-
tains у1018–1019 cmϪ3 of oxygen.9 Implantation of Oϩ ions
creates shallow donor levels (ϳ30 meV) in GaN after acti-
vation annealing and under these conditions the oxygen dif-
fusivity is very small (Ͻ10Ϫ14 cm2 sϪ1 at 1400 °C).6 Inten-
tional oxygen doping during MOCVD growth produced a
donor level at EcϪ78 meV, with banding occurring at high
concentrations.1 Theoretically, ON in GaN was found to cre-
ate a shallow donor level with low formation energy.2,3 Oxy-
gen may also be important in co-doping with acceptors to
increase the effective concentration in GaN.17–20
There has been little work on understanding the role of
oxygen during thermal processing of GaN. It is know that the
temperature at which surface dissociation becomes signifi-
cant during annealing of GaN is several hundred degrees
lower in O2-containing ambients relative to pure N2
ambients.21 The strong affinity of O for GaN may lead to
autodoping in situations such as epitaxial regrowth over SiO2
windows, as in the epitaxial lateral overgrowth technique, or
during annealing with SiO2 encapsulant layers. However, if
the acceptor concentration in the GaN were low
(Ͻ1017 cmϪ3), it is clear that indiffused oxygen could even
cause type conversion if a significant fraction of it was elec-
trically active. In this letter we report on the observation of
17O diffusion into p-type GaN during SiO2-capped annealing.
We employed isotopically enriched O2 as a precursor for
SiO2 deposition, and were able to follow the 17O diffusion
with high sensitivity using secondary ion mass spectrometry
͑SIMS͒. The activation energy and prefactor for 17O diffu-
sion were determined over the temperature range
500–900 °C.
Epitaxial layers of GaN consisting of 1 ␮m undoped
(nϳ1016 cmϪ3) followed by 0.3 ␮m of Mg-doped ͑hole con-
centration, pϳ9ϫ1017 cmϪ3) were grown on c-plane Al2O3
substrates by radio frequency plasma activated MBE.22
These layers were capped with 1800 Å of SiO2 grown by
inductively coupled plasma chemical vapor deposition ͑ICP–
CVD͒ at 50 °C using SiH4 and 17O-enriched ͑50%͒ O2 in a
Plasma-Therm 790 reactor. The ICP source power was 300
W ͑2 MHz͒, while the sample position was biased at Ϫ5 V
using 5 W of 13.56 MHz power. Under these conditions
there is minimal change in the electrical or optical properties
of the GaN as measured by Hall and cathodoluminescence
measurements, due to the low energy of the incident ions
(Ͻ30 eV). Sections of these samples were annealed for 5
min at 500, 700, or 900 °C under N2 in a Heatpulse 610
furnace. SIMS measurements were performed on the as-
deposited and annealed samples both before and after re-
moval of the SiO2 in buffered HF solution at 25 °C.
Figure 1 shows SIMS depth profiles of the as-deposited
structure. There is a high concentration of 17O in the SiO2
film and the interface with the GaN is abrupt. Samples an-
nealed up to 900 °C showed no change in the profiles of the
lattice elements, including the Mg in the GaN. The SiO2 was
selectively removed in buffered HF and there was little dif-
ference in its etch rate before and after annealing, indicating
a͒
Electronic mail: spear@mse.ufl.edu
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0003-6951/99/75(19)/2939/3/$15.00 2939 © 1999 American Institute of Physics
129.12.235.198 On: Tue, 09 Dec 2014 17:22:38

2940
Appl. Phys. Lett., Vol. 75, No. 19, 8 November 1999
Pearton et al.
FIG. 3. Arrhenius plot of 17O diffusivity in p-GaN.
incorporation. Second, SiO2 is not a suitable choice as an
encapsulant or annealing of implanted GaN, for the same
reason.
FIG. 1. SIMS profiles of as-deposited SiO2 layer on GaN.
The 17O diffusion profiles in Fig. 2 clearly do not follow
an error function ͑erfc͒ distribution and the fact the logarithm
of the concentration is almost linear in penetration depth is a
clear signature of pipe diffusion.23–26 The expanded regions
around threading dislocations provide an easy path for diffu-
sion of impurities, with an activation energy typically half
that for diffusion in the bulk of the material.27 In a simple
picture one can consider the bulk and pipe diffusion to be
independent, with the square of the diffusion distance given
by23
that there was little densification resulting from the high tem-
perature process.
Figure 2 shows SIMS profiles of 17O in the GaN after
different annealing temperatures. Note that there is the usual
high concentration, near-surface (р200 Å) signal character-
istic of SIMS profiling through a surface or interface and
which is basically an artifact of the measurement. In the
as-deposited sample the 17O signal is returned to the back-
ground level of ϳ1017 cmϪ3 at a distance of 400–500 Å. By
sharp contrast, in the samples annealed prior to removal of
the SiO2, the 17O concentration and depth of incorporation
show clear increases for higher annealing temperatures. The
17O clearly diffuses from the SiO2 into the GaN during an-
nealing. There is no indication of pairing of oxygen with Mg
to form Mg–O complexes, which should be evident as a
plateau in the O profile. There are some obvious implications
of these results: First, the use of SiO2 masks for ELO of GaN
could produce n-type autodoping of the material by oxygen
X2ϭ2Ddtϩ2Dbt,
where Dd,b are the diffusivities in the dislocations or bulk,
respectively, and t is the diffusion time. If DdӷDb and the
oxygen-dislocation binding energy forward partitioning to
the dislocation then the overall mass transport of the oxygen
would be dominated by the pipe diffusion. Since this appears
to be the case from Fig. 2, we can obtain a rough estimate of
Dd from X2/2t, where X is the depth at which the oxygen
concentration falls to 1017 cmϪ3 and tϭ300 s. Figure 3
shows the resulting values of D in Arrhenius form. At least
squares fit to the data yielded the relationship
Dϭ4.5Ϯ2.2ϫ10Ϫ12 exp Ϫ0.23Ϯ0.12eV/kT͒,
͑
where k is Boltzmann’s constant. The activation is approxi-
mately half that expected for diffusion of interstitials in bulk
material. We do not know the final lattice position of the
oxygen, but can estimate that Ͻ30% is substitutional based
on the fact that we could not measure any change in the
carrier concentration, obtained by Hall effect using a two
band model28 in the 900 °C diffused sample.
The diffusion behavior of oxygen originating from SiO2
encapsulant layers is obviously quite different from im-
planted oxygen.6 In the latter situation the high vacancy con-
centration created by the implanted Oϩ ions may enhance the
occupation probability of substitutional sites, i.e., there is a
higher efficiency for creation of ON . There are still many
unanswered questions concerning the effect of strain induced
in the near-surface region by the SiO2 cap ͑which is known
FIG. 2. SIMS profiles of 17O in GaN annealed at different temperatures for
5 min prior to removal of the SiO2 layer.
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to enhance point defect diffusion in other compound semi-
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Appl. Phys. Lett., Vol. 75, No. 19, 8 November 1999
Pearton et al.
2941
29
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