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1-1-2008
Migration and luminescence enhancement effects of deuterium in Migration and luminescence enhancement effects of deuterium in
ZnO/ZnCdO quantum wells ZnO/ZnCdO quantum wells
W. Lim
D. P. Norton
S. J. Pearton
X. J. Wang
W. M. Chen
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Recommended Citation Recommended Citation
Lim, W.; Norton, D. P.; Pearton, S. J.; Wang, X. J.; Chen, W. M.; Buyanova, I. A.; Osinsky, A.; Dong, J. W.;
Hertog, B.; Thompson, A. V.; Schoenfeld, W. V.; Wang, Y. L.; and Ren, F., "Migration and luminescence
enhancement effects of deuterium in ZnO/ZnCdO quantum wells" (2008).
Faculty Bibliography 2000s
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618.
https://stars.library.ucf.edu/facultybib2000/618
Appl. Phys. Lett. 92, 032103 (2008); https://doi.org/10.1063/1.2836946 92, 032103
© 2008 American Institute of Physics.
Migration and luminescence enhancement
effects of deuterium in quantum
wells
Cite as: Appl. Phys. Lett. 92, 032103 (2008); https://doi.org/10.1063/1.2836946
Submitted: 21 December 2007 . Accepted: 31 December 2007 . Published Online: 22 January 2008
W. Lim, D. P. Norton, S. J. Pearton, X. J. Wang, W. M. Chen, I. A. Buyanova, A. Osinsky, J. W. Dong, B.
Hertog, A. V. Thompson, W. V. Schoenfeld, Y. L. Wang, and F. Ren
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Migration and luminescence enhancement effects of deuterium
in ZnO/ZnCdO quantum wells
W. Lim, D. P. Norton, and S. J. Pearton
a兲
Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA
X. J. Wang, W. M. Chen, and I. A. Buyanova
Department of Physics, Chemistry and Biology, Linköping University, S-581 83 Linköping, Sweden
A. Osinsky, J. W. Dong, and B. Hertog
SVT Associates, Eden Prairie, Minnesota 55344, USA
A. V. Thompson and W. V. Schoenfeld
CREOL, University of Central Florida, Orlando, Florida 32816, USA
Y. L. Wang and F. Ren
Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA
共Received 21 December 2007; accepted 31 December 2007; published online 22 January 2008兲
ZnO/ ZnCdO/ ZnO multiple quantum well samples grown on sapphire substrates by molecular beam
epitaxy and annealed in situ were exposed to D
2
plasmas at 150 ° C. The deuterium showed
migration depths of ⬃0.8
m for 30 min plasma exposures, with accumulation of
2
H in the ZnCdO
wells. The photoluminescence 共PL兲 intensity from the samples was increased by factors of 5 at 5 K
and ⬃20 at 300 K as a result of the deuteration, most likely due to passivation of competing
nonradiative centers. Annealing up to 300 °C led to increased migration of
2
H toward the substrate
but no loss of deuterium from the sample and little change in the PL intensity. The initial PL
intensities were restored by annealing at 艌400 ° C as
2
H was evolved from the sample 共⬃90% loss
by 500 °C兲. By contrast, samples without in situ annealing showed a decrease in PL intensity with
deuteration. This suggests that even moderate annealing temperatures lead to degradation of ZnCdO
quantum wells. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2836946兴
There is an extensive current interest in developing the
ZnMgCdO system for blue and ultraviolet optoelectronic
devices.
1,2
ZnO is attractive for optoelectronic applications
due to its large exciton binding energy 共60 meV兲,
3
which
allows excitonic recombination above room temperature. In
addition, ZnO substrates are commercially available and
ZnO films of excellent optical quality can be grown at rela-
tively low temperatures. ZnO-based alloys, such as
Zn
1−x
Mg
x
O and Zn
1−x
Cd
x
O, add tunability in the alloy band-
gap for bandgap engineering. The larger gap ZnMgO can be
used as cladding layers in double heterostructures and with
active layers of ZnCdO, the bandgap energy can be tuned as
desired within the blue-yellow spectral range for light-
emitting diodes.
4–9
There is great interest in the fundamental
optical properties and associated recombination processes in
ZnCdO. In ZnCdO / MgZnO quantum wells with very low
Cd content of 0.4%, the low temperature emission is attrib-
uted to recombination of excitons localized by potential fluc-
tuations due to alloy disorder and well width variations.
10
The role of hydrogen on ZnCdO quantum wells is unclear.
Hydrogen is a common impurity in ZnO and related alloys
and produces a donor state in pure ZnO.
11–25
It has a high
diffusivity in the binary
14
and is thought to be responsible for
part of the commonly observed n-type background in most
ZnO.
15–18
Hydrogen may passivate nitrogen acceptors in va-
por phase grown ZnO 共Ref. 16兲 and also form a variety of
complexes with O.
12,13,17–25
In this letter, we report on the incorporation of deuterium
into ZnCdO/ ZnO quantum wells by exposure to a
2
H
plasma, its migration during annealing, and its effect on en-
hancing the luminescence intensity from the quantum wells.
The samples investigated consisted of wurtzite 2 nm
ZnCdO multiple quantum wells with 6 nm ZnO barriers on a
1.0
m ZnO:Ga buffer layer grown on 共0001兲 sapphire sub-
strates by molecular-beam epitaxy at 500 °C and annealed in
situ at 550 ° C under O
2
to improve crystalline quality.
26,27
This type of annealing has been shown to enhance the optical
properties of ZnCdO multiple quantum wells.
28
The Cd com-
position in the wells was ⬃12 at. %. The photoluminescence
共PL兲 peak from the ZnCdO quantum wells 共QWs兲 was cen-
tered at ⬃2.9 eV at 300 K. Based on transmission electron
microscopy and selected area diffraction pattern measure-
ments, the alloy has excellent crystalline quality with no
evidence of second phase formation. The samples were ex-
posed to a
2
H plasma for 30 min at 150 ° C with a chamber
pressure of 800 mtorr and a rf power 共13.56 MHz兲 of
30 mW cm
−2
. Some samples were annealed at temperatures
up to 500 ° C for 5 min under O
2
ambient following the deu-
teration treatment. The
2
H depth profile was obtained by sec-
ondary ion mass spectrometry 共SIMS兲 using a 14.5 keV Cs
+
ion beam and detecting negative secondary ions. The con-
centrations were quantified using ion implanted standards. A
Verdi/MBD-266 laser system 共 = 266 nm兲 was used as an
excitation source during PL measurements. Optical signals
during PL measurements were dispersed by a grating mono-
chromator and detected by a charge-coupled detector. An os-
cillating pattern seen in the PL spectra is likely a result of
interference within the multilayer structure.
a兲
Electronic mail: spear@mse.ufl.edu.
APPLIED PHYSICS LETTERS 92, 032103 共2008兲
0003-6951/2008/92共3兲/032103/3/$23.00 © 2008 American Institute of Physics92, 032103-1
Figure 1 shows the
2
H profile in the structure after deu-
teration and subsequent annealing at different temperatures.
Deuterium incorporates to depths of ⬃0.8
m and shows
little additional migration until anneals of 300 ° C. By
500 ° C, the concentration near the surface is lowered as deu-
terium migrates out of the crystal and its profile extends all
the way through the ZnO buffer. Note the higher concentra-
tion of deuterium in the region of the ZnCdO quantum wells
both after deuteration and all subsequent anneals.
A closeup of deuterium profile in the quantum well re-
gion is shown in Fig. 2. The deuterium concentration peaks
in the ZnCdO wells, which are regions of strain. Similar
accumulation of hydrogen is observed in many similar inter-
faces or strained regions in multilayered semiconductor
structures.
29
As the annealing temperature is increased, the
deuterium concentration decreases in the first well, and ini-
tially increases in the second well as migration into the
sample occurs, before decreasing in both wells at the higher
temperatures.
Figure 3 shows the areal density of deuterium remaining
in the samples as a function of annealing temperature, ob-
tained by integrating the area under the SIMS profiles for
each condition. Significant loss of deuterium is only ob-
served above 300 °C, somewhat higher than the temperature
of ⬃200 ° C reported for bulk ZnO.
29
The broad nature of
the curve in Fig. 3 is indicative that more than one form of
deuterium exists in the samples, since if all the deuterium
were present as one configuration, most of it should evolve
from the crystal over a narrow temperature range.
30
We ex-
pect the presence of the various types of oxygen-deuterium
centers, deuterium molecules, and deuterium trapped at lat-
tice defects lead to this broad temperature range for evolu-
tion of deuterium from the samples.
17
As shown in Fig. 4, at 5 K, after deuteration we ob-
served a substantial 共more than five times兲 increase of inten-
sities of all recorded emissions and a strong blueshift
共⬃70 meV兲 of the broad emission band peaking at around
3.07 eV. In addition, there was the appearance of a new PL
peak at 3.342 eV. At 300 K, there was a strong 共⬃20 times兲
increase of intensities of all recorded emissions. The changes
observed in the 5 K PL spectra remain unaffected by anneal-
ing performed within the temperature range of 150–300 °C,
but were reversed by annealing at 艌400 ° C.
The observed changes in the PL spectra correlate with
the SIMS measurements which imply that they are due to the
presence of deuterium. The enhancement of the PL efficiency
likely reflects passivation of competing nonradiative recom-
bination centers, as commonly observed in other semicon-
ductors. Assuming that the emission occurs via the tail states,
as commonly seen in ZnCdO,
31
the blueshift of the 3.07 eV
line after hydrogenation may reflect passivation of nonradi-
ative recombination centers. This would enhance radiative
efficiency of optical transitions via the tail states shallower in
energy. The effect of deuteration on samples grown at
500 ° C but not given an in situ anneal was quite different. In
that case, we typically saw a small decrease 共of the order of
10%兲 in the overall PL intensity, probably due to an increase
in surface recombination from ion impingement damage dur-
ing the exposure. This suggests that even an in situ annealing
at 550 ° C of the ZnCdO was sufficient to create nonradiative
centers that were passivated by hydrogen and establishes the
limit for the onset of thermal degradation. This is consistent
FIG. 1. 共Color online兲 SIMS depth profile of deuterium in a ZnCdO/ ZnO
multiple QW 共MQW兲 sample grown on a thick ZnO buffer and exposed to
aD
2
plasma for 30 min at 150 ° C and subsequently annealed at different
temperatures.
FIG. 2. 共Color online兲 Closeup of SIMS depth profile of deuterium in the
MQW region.
FIG. 3. 共Color online兲 Areal concentration of deuterium remaining after
deuteration and subsequent annealing at different temperatures.
FIG. 4. 5 K PL spectra from ZnCdO/ ZnO MQW sample before and after
hydrogenation and after subsequent annealing at different temperatures.
032103-2 Lim et al. Appl. Phys. Lett. 92, 032103 共2008兲
