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Production of Nitrogen Acceptors in ZnO by Thermal Annealing Production of Nitrogen Acceptors in ZnO by Thermal Annealing
N. Y. Garces
N. C. Giles
L. E. Halliburton
G. Cantwell
D. B. Eason
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Garces, N. Y., Giles, N. C., Halliburton, L. E., Cantwell, G., Eason, D. B., Reynolds, D. C., & Look, D. C. (2002).
Production of Nitrogen Acceptors in ZnO by Thermal Annealing.
Applied Physics Letters, 80
(8),
1334-1336.
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Production of nitrogen acceptors in ZnO by thermal annealing
N. Y. Garces, N. C. Giles, and L. E. Halliburton
a)
Department of Physics, West Virginia University, Morgantown, West Virginia 26506
G. Cantwell and D. B. Eason
Eagle-Picher Technologies, LLC, Miami, Oklahoma 74354
D. C. Reynolds and D. C. Look
Semiconductor Research Center, Wright State University, Dayton, Ohio 45435
共Received 19 October 2001; accepted for publication 11 December 2001兲
Nitrogen acceptors are formed when undoped single crystals of zinc oxide 共ZnO兲 grown by the
chemical-vapor transport method are annealed in air or nitrogen atmosphere at temperatures
between 600 and 900 °C. After an anneal, an induced near-edge absorption band causes the crystals
to appear yellow. Also, the concentration of neutral shallow donors, as monitored by electron
paramagnetic resonance 共EPR兲, is significantly reduced. A photoinduced EPR signal due to neutral
nitrogen acceptors is observed when the annealed crystals are exposed to laser light 共e.g., 364, 442,
458, or 514 nm兲 at low temperature. The nitrogens are initially in the nonparamagnetic singly
ionized state (N
⫺
) in an annealed crystal, because of the large number of shallow donors, and the
light converts a portion of them to the paramagnetic neutral acceptor state (N
0
). © 2002 American
Institute of Physics. 关DOI: 10.1063/1.1450041兴
At the present time, zinc oxide is receiving considerable
attention because of its potential application as an ultraviolet
light emitter.
1–3
One of the major obstacles in the develop-
ment of this material, however, is the difficulty encountered
in finding an efficient p-type dopant. Most of today’s ZnO
crystals contain significant concentrations of shallow donors
and, thus, are n type. Known acceptors in ZnO include
lithium,
4,5
copper,
6,7
and zinc vacancies,
8,9
but all of these are
deep acceptors and do not contribute significantly to hole
conduction. Recently, thin-film growers have focused on ni-
trogen as a shallower acceptor in ZnO.
10–13
They have dem-
onstrated that nitrogen will enter the films if N
2
OorNH
3
,
depending on the growth technique, is used as a source. With
these successes, it is important to better understand the prop-
erties and behavior of nitrogen in ZnO.
In the present letter, we describe the production of nitro-
gen acceptors in undoped single crystals of ZnO by anneal-
ing in air, or nitrogen, to temperatures between 600 and
900 °C. These treatments cause the following effects: 共1兲 an
optical absorption band appears in the near-edge region, ex-
tending out to 550 nm; 共2兲 the concentration of neutral shal-
low donors is greatly decreased; and 共3兲 the electron para-
magnetic resonance 共EPR兲 signal of the neutral nitrogen
acceptor can be photoinduced. This nitrogen EPR signal has
been recently reported by Carlos, Glaser, and Look
14
and is
unambiguously assigned to the neutral nitrogen acceptor be-
cause of its uniquely characteristic three-line hyperfine pat-
tern, arising from a nearly 100% abundant I⫽ 1 nucleus 共in
this case, the
14
N isotope兲. We have found that this neutral
nitrogen EPR signal can be photoinduced at low temperature
with a variety of laser wavelengths 共e.g., 364, 442, 458, and
514 nm兲. Our results suggest that, following a thermal an-
neal, the nitrogen acceptors compensate a portion of the do-
nors 共i.e., the nitrogens are in the nonparamagnetic singly
ionized charge state (N
⫺
) and the concentration of neutral
shallow donors has decreased兲. If an annealed sample is ex-
posed to light at low temperature, some of these singly ion-
ized nitrogen acceptors are converted to the paramagnetic
neutral acceptor charge state (N
0
).
The ZnO crystals used in the present study were grown
at Eagle-Picher 共Miami, OK兲 using the chemical-vapor trans-
port method. Although material from six separate growth
runs was investigated 共with similar results from all six兲, most
of the data reported here were taken from one sample cut
from a 1-mm-thick c plate. The dimensions of this sample
were 8⫻ 2.5⫻ 1mm
3
. During a thermal anneal, the sample
was placed in a quartz tube extending through a small hori-
zontal furnace. The ends of the tube were either left open to
the air, giving a semistatic atmosphere, or one end was con-
nected to a source of flowing nitrogen or helium gas. At the
start of a thermal anneal, the furnace was stabilized at the
desired temperature, and then the sample was inserted. At the
end of an anneal, the sample was removed from the hot
furnace and cooled to room temperature in less than 1 min.
The EPR data were obtained using a Bruker EMX spec-
trometer operating near 9.477 GHz. An Oxford Instruments
model ESR-900 helium-gas flow system provided tempera-
ture control. The excitation sources for the photoinduced
EPR were a cw argon-ion laser 共364, 458, and 514 nm兲 and
a cw helium–cadmium laser 共442 nm兲. Optical data were
taken on a Cary 14 spectrophotometer.
As shown in Fig. 1共a兲, an EPR signal due to neutral
shallow donors was present in our as-grown, unannealed
ZnO samples. The data in Fig. 1 were taken near 6.5 K with
the magnetic field perpendicular to the c axis of the crystal.
This large EPR signal, with g
储
⫽ 1.957 and g
⬜
⫽ 1.956, has
been widely reported in the ZnO literature. Its position is
independent of the shallow donor identity 共i.e., earlier inves-
tigators have shown that Al, Ga, and In give identical
signals兲.
15–18
At the shallow-donor concentrations present in
a兲
Electronic mail: lhallibu@wvu.edu
APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 8 25 FEBRUARY 2002
13340003-6951/2002/80(8)/1334/3/$19.00 © 2002 American Institute of Physics
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our as-grown samples, there is no resolved hyperfine pattern
associated with this signal. It is important to note that this
EPR signal at ‘‘g⫽ 1.96’’ is not due to singly ionized oxygen
vacancies. A different EPR signal, with g
储
⫽ 1.9945 and g
⬜
⫽ 1.9960, is normally seen only after particle irradiation and
has been unambiguously assigned to the paramagnetic state
of the oxygen vacancy.
19
In Fig. 1共b兲, we show the EPR spectrum taken after the
ZnO crystal was annealed at 700 °C in air for 30 min. Prior
to this 700 °C anneal, the crystal had been annealed for 30
min each at a series of lower temperatures, starting at 300 °C
and increasing by 50 °C increments until 700 °C was
reached. There is a significant reduction in the size of the
neutral shallow donor EPR signal and its shape also has
changed dramatically. Comparing the spectra in Figs. 1共a兲
and 1共b兲 shows that the annealing treatments decreased the
donor EPR signal by a factor of 50. The spectrum in the
annealed sample appears to have its ‘‘phase’’ inverted, and
this suggests that there is microwave power saturation of the
signal 共i.e., a longer spin relaxation time arises when the
interaction between adjacent neutral donors is reduced be-
cause of larger separation distances兲. The decrease in the
concentration of neutral shallow donors, caused by the ther-
mal anneal and observed in the EPR experiments, has been
verified by recent Hall-effect measurements on similar ZnO
samples, and the results will be reported in a later paper. In
that study, we found that annealing a ZnO crystal in air at
750 °C for 30 min decreased the room-temperature electron
concentration from 1.0⫻ 10
17
to 5.1⫻ 10
16
cm
⫺ 3
.
A closer examination, at higher temperature, of the shal-
low donor EPR signal in Fig. 1共b兲 shows that there are two
contributing centers, and that one of the centers has a par-
tially resolved hyperfine pattern. Figure 2 shows these donor
signals, taken at 22 K with the magnetic field perpendicular
to the c axis. As the temperature is raised from 6 to 22 K, the
donor signal evolves from a distorted, highly saturated shape
to a normal unsaturated derivative shape 关i.e., compare the
spectrum in Fig. 1共b兲 to the spectrum in Fig. 2兴. Although the
structure in Fig. 2 is not well resolved, we suggest that a
four-line pattern is present, indicated by the stick diagram,
and that a single larger line is present just to the low-field
side of the center of the four lines. We attribute the four-line
donor signal in Fig. 2 to neutral shallow gallium donors, with
the four lines arising from the hyperfine interactions with the
two isotopes. The hyperfine splitting for the gallium spec-
trum in Fig. 2 is 6.7 G, and this compares favorably to the
estimate of 4.2 G made by Gonzalez et al.
18
from an unre-
solved EPR spectrum in a gallium-doped sample. It is pos-
sible that chlorine donors might be responsible for the four-
line pattern in Fig. 2, but we consider this to be less likely
because of the agreement with the earlier Ga study. We did
not observe, in our samples, the ten-line EPR spectrum pre-
viously assigned to indium shallow donors.
18
In Fig. 1共c兲, we show the effect of laser light on our
crystal that was annealed in air at 700 °C. The sample was
held in the microwave cavity at 6.5 K with the magnetic field
perpendicular to the c axis, and the laser beam entered the
cavity through slots. Before exposing the sample to the laser,
no acceptor signals are observed 关see Fig. 1共b兲兴. With the
light on, we see a large three-line EPR spectrum appear 关see
Fig. 共1c兲兴. This spectrum has been assigned to the neutral
nitrogen acceptor by Carlos, Glaser, and Look.
14
Its three-
line hyperfine pattern arises from an interaction with one
14
N
nucleus. These three primary lines shift, but do not split,
when the magnetic field is rotated relative to the crystal axes.
From the angular dependence, the acceptor is shown to have
axial symmetry about the c axis 共g
储
⫽ 1.9948, g
⬜
⫽ 1.9632,
A
储
⫽ 81.3 MHz, and A
⬜
⫽ 9.5 MHz兲. When the magnetic field
is nearly perpendicular to the c axis, ‘‘forbidden’’ transitions
FIG. 1. EPR spectra taken at 6.5 K from a ZnO crystal 共a兲 before a thermal
anneal; 共b兲 in the dark after a thermal anneal at 700 °C in air for 30 min; and
共c兲 with 458 nm illumination after the 700 °C anneal. The magnetic field
was perpendicular to the c axis. Note that trace 共a兲 is actually ten times
larger than shown. The signal at higher field is due to neutral shallow donors
and the photoinduced signal at lower field is due to neutral nitrogen accep-
tors.
FIG. 2. EPR spectrum of the neutral shallow donors in a ZnO crystal an-
nealed in air at 700 °C. The temperature was 22 K and the magnetic field
was perpendicular to the c axis.
1335Appl. Phys. Lett., Vol. 80, No. 8, 25 February 2002 Garces
et al.
Downloaded 25 Sep 2012 to 130.108.121.217. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
acquire significant intensity and the EPR spectrum appears
more complicated than a simple three-line pattern. Also, hy-
perfine lines from the four neighboring
67
Zn nuclei are ob-
served, with splittings of 13.2 G for the axial neighbor and
6.5 G for the three basal plane neighbors when the field is
parallel to the c axis. The neutral nitrogen EPR spectrum is
best observed between 5 and 10 K. After being produced
with light at 6.5 K, the EPR signal decays in the dark at this
same temperature over a period of several hours.
The photoinduced neutral nitrogen acceptor signal in
Fig. 1共c兲 can be formed with wavelengths below the band
gap. We found that lasers operating at 364, 442, 458, and 514
nm are all effective in producing the nitrogen EPR signal,
while excitation at 633 nm did not produce the signal. These
observations are consistent with a yellow coloration that
forms in the crystals when annealed above 600°C in air or
nitrogen. We show this absorption in Fig. 3 for the crystal
heated to 700 °C for 30 min 共this is the same sample used in
Figs. 1 and 2兲. These data were taken with unpolarized light
propagating along the c axis. A near-edge absorption is
present in the annealed sample. At 10 K, the optical absorp-
tion band narrows slightly and the fundamental absorption
edge moves to higher energy, but the peak is still not re-
solved. The laser wavelengths that are effective in producing
the nitrogen EPR signal all fall within the new band. We
suggest that this near-edge absorption may represent transi-
tions from singly ionized nitrogen acceptors to shallow do-
nors and the conduction band or from singly ionized nitrogen
acceptors to ‘‘deep’’ levels, such as transition-metal impuri-
ties or intrinsic defects.
The following observations relate to the process by
which nitrogen acceptors are formed in ZnO. We found that
measurable concentrations of the neutral nitrogen acceptor
EPR signal could not be photoinduced in ZnO crystals an-
nealed in air at temperatures below 600 °C or above 900 °C.
Also, there was only a small reduction 共a factor of 2兲 in the
neutral donor concentration and no photoinduced nitrogen
EPR signal in a sample annealed at 750 °C in flowing helium
gas, even though similar anneals in air and nitrogen pro-
duced large effects. A final result refers to the distribution of
photoactive nitrogen acceptors in a crystal annealed in air at
700 °C for 1 h 共the sample was a c plate initially 1.0 mm
thick兲. A ‘‘thinning’’ experiment was performed, where the
neutral nitrogen EPR signal was repeatedly measured as ma-
terial was removed from the two sides by grinding 共i.e., after
each removal step, the sample was cooled to 6.5 K and illu-
minated with 442 nm light to induce the EPR signal兲.We
found that the photoinduced nitrogen acceptors were not dis-
tributed uniformly, even though they appeared through most
of the crystal. Specifically, the number of photoinduced ac-
ceptors dropped to 58% of the 1-mm-thick value when the
crystal was thinned to 0.95 mm, they dropped to 23% of the
1 mm value when the thickness was 0.80 mm, and they
dropped to 12% of the 1 mm value when the thickness was
0.50 mm. These results suggest that our production of nitro-
gen acceptors in ZnO does not occur simply as a result of the
thermal activation of nitrogen that may have been uniformly
incorporated in the crystal during growth. It is also clear that
the production of nitrogen acceptors is not restricted to a
small region near the surface of the crystal.
In summary, we have described the production of nitro-
gen acceptors in ZnO by annealing in air or nitrogen at tem-
peratures in the 600–900 °C range. The active nitrogen ac-
ceptors introduced during these treatments provide
compensation for the shallow impurity donors. We have
shown that, after an anneal, the EPR signal of the neutral
nitrogen acceptor can be photoinduced with below-band-gap
light. This is consistent with the induced optical absorption
that accompanies the formation of nitrogen acceptors and
extends from the band edge to 550 nm. We did not find EPR
signals associated with zinc vacancies, zinc interstitials, or
oxygen vacancies in our as-grown or annealed crystals.
This work was supported at West Virginia University by
the Air Force Office of Scientific Research 共Grant No.
F49620-00-1-0301兲.
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FIG. 3. Optical absorption spectrum of a ZnO crystal annealed in air at
700 °C. The data were taken at room temperature. Trace 共a兲 was taken
before the anneal and trace 共b兲 was taken after the anneal.
1336 Appl. Phys. Lett., Vol. 80, No. 8, 25 February 2002 Garces
et al.
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