Influence of Codoping with Ga on the Electrical and Optical Properties of N-Doped ZnO Films

Article abstract of DOI:10.1149/1.1594732

This study reports on the dependence on Ga concentration of the electrical and optical properties of ZnO films doped at a N concentration of 10 ¹⁸ cm^-3. For Ga concentrations below 0.5 mol %, the N concentration was kept at 10¹⁸ cm^-3. However, as the Ga concentration increased to 5 mol %, the N concentration suddenly increased from 10¹⁸ to 10¹⁹ cm^-3. Furthermore, the donor-acceptor pair emission and yellow luminescence in N single-doped ZnO films were suppressed significantly by doping with Ga at a concentration of 0.1 mol %. However, doping with Ga concentrations above 0.5 mol % resulted in increased red luminescence originating from deep levels generated through the excessive incorporation of Ga atoms. An evaluation of the crystalline quality and electrical properties indicated that the Ga atoms located on the Zn sites changed largely from shallow donor sources to compensating defects trapping electrons at Ga concentrations in the 0.5 to 5 mol % range. These results suggested that the increased N concentration and the enhancement of ultraviolet (UV) emission could be attributed to the role played by the Ga atoms located on the Zn sites in the N single-doped ZnO films.

Full text of DOI:10.1149/1.1594732

G508
Journal of The Electrochemical Society, 150 ͑9͒ G508-G512 ͑2003͒
0
013-4651/2003/150͑9͒/G508/5/$7.00 © The Electrochemical Society, Inc.
Influence of Codoping with Ga on the Electrical and Optical
Properties of N-Doped ZnO Films
z
Hiroaki Matsui, Hiromasa Saeki, Hitoshi Tabata, and Tomoji Kawai
Institute of Scientific and Industrial Research-Sanken, Osaka University, Ibaraki, Osaka 567-0047, Japan
This study reports on the dependence on Ga concentration of the electrical and optical properties of ZnO films doped at a N
1
8
Ϫ3
18
Ϫ3
concentration of 10 cm . For Ga concentrations below 0.5 mol %, the N concentration was kept at 10 cm . However, as the
18
19
Ϫ3
Ga concentration increased to 5 mol %, the N concentration suddenly increased from 10 to 10 cm . Furthermore, the
donor-acceptor pair emission and yellow luminescence in N single-doped ZnO films were suppressed significantly by doping with
Ga at a concentration of 0.1 mol %. However, doping with Ga concentrations above 0.5 mol % resulted in increased red
luminescence originating from deep levels generated through the excessive incorporation of Ga atoms. An evaluation of the
crystalline quality and electrical properties indicated that the Ga atoms located on the Zn sites changed largely from shallow donor
sources to compensating defects trapping electrons at Ga concentrations in the 0.5 to 5 mol % range. These results suggested that
the increased N concentration and the enhancement of ultraviolet ͑UV͒ emission could be attributed to the role played by the Ga
atoms located on the Zn sites in the N single-doped ZnO films.
©
2003 The Electrochemical Society. ͓DOI: 10.1149/1.1594732͔ All rights reserved.
Manuscript submitted August 5, 2002; revised manuscript received February 28, 2003. Available electronically July 10, 2003.
Zinc oxide ͑ZnO͒ is widely known as a wide-gap semiconductor
Experimental
1
2,3
that can display novel ferroelectric and magnetic properties be-
cause it is possible to incorporate a wide variety of monovalent,
divalent, and trivalent ions into the lattice. Additionally, ZnO has a
large exciton binding energy ͑60 meV͒, an attractive feature that can
All ZnO films used in this work were grown by pulse laser depo-
sition ͑PLD͒ at a base pressure of 2 ϫ 10 mbar. An ArF excimer
laser (␭ ϭ 193 nm, 20 ns, and 3 Hz͒ with an energy density of
J/cm was focused on a ZnO single crystal ͑Eagle-Picher: 6 N͒,
Ϫ7
2
1
be used to facilitate the design of novel optical devices utilizing
and Ga -doped ZnO (0 Ϲ x Ϲ 5 mol %) ceramic targets were
excitons.4,5 A variety of dopants and related optical properties for
x
fabricated using a conventional solid-state reaction method. The
target-to-substrate distance was fixed at 4.5 cm. The growth rate and
substrate temperature were kept at 10 Å/min and 400°C, respec-
tively. Glass ͑Corning no. 7059͒ was chosen as the substrate. The
film thickness was approximately 300 nm as measured by a surface
profiler ͑Alpha-step 500͒. Nitrogen and oxygen were supplied si-
multaneously by a radio frequency ͑rf͒ radical source ͑at 13.56
MHz͒ with a rf power of 250 W through the cracking of high purity
N O gas ͑6 N͒. The operating conditions of the rf radical source
were fixed in all experiments.
The chemical species generated in the rf plasma cell were char-
acterized using optical emission spectroscopy. A typical optical
ZnO have been anticipated for use in new technological applications
including the use of ͑Zn,Mg͒O and ͑Zn,Cd͒O alloys in the field of
bandgap engineering.6 Accordingly, great importance is placed on
controlling the electrical activity and optical properties of ZnO using
various dopant types and a variety of doping methods such as
chemical doping, ion implantation, and plasma irradiation. In par-
,7
ticular, enhancement of the optical properties of H -plasma irradi-
2
ated ZnO single crystals is a major area of study concerned with the
2
control of deep levels.8
We found recently that the donor-acceptor pair ͑DAP͒ emission
in the UV region, and the yellow luminescence of N single-doped
ZnO ͑ZnO:N͒ films were suppressed significantly by doping with
Ga at a concentration of 0.1 mol %. The suppression of deep-level
emission through the use of codoping has been observed for various
wide-gap semiconductors. Leem et al. reported that the reduction in
yellow luminescence of GaN:Mg, Si films was attributed to a charge
emission spectrum of the N O plasma with an applied rf power of
2
Ϫ3
2
50 W and a pressure of 1.0 ϫ 10 mbar is shown in Fig. 1. The
spectrum shows three types of emissions originating from the
2
ϩ
2
␥
-transition (A ⌺ -X ⌸) of the excited NO molecule (NO*) and
3
3
3
3
ϩ
the second positive (C ⌸ -B ⌸ ) and first positive (B ⌸ -A ⌺ )
u
g
g
u
1
5,16
compensation between Mg2 and Ga vacancies. Watanabe et al.
ϩ
9
transitions of the excited N molecule (N*).
Oxygen was sup-
2
2
plied by the rf plasma cell as O molecules in an unexcited state.
found that a complex of Na and O affected the suppression of deep-
2
10
This was confirmed by quadrupole mass spectroscopy.
level emission in CuInS films. Codoping methods using two dif-
2
The crystalline quality of the films was characterized by X-ray
diffraction ͑XRD͒ using CuK␣ radiation ͑RINT-2000, Rigaku͒. Op-
tical properties were measured by photoluminescence ͑PL͒ spectros-
copy using a He-Cd laser ͑325 nm͒ excitation source with a light
ferent atoms for wide-gap semiconductors have been employed in
an effort to minimize the formation of compensating native defects
and to increase the solubility limits of either dopant, an approach
that has been strongly supported by first-principle computer
2
intensity of 20 mW/cm and 1.5 mm slit width. The incorporation of
stimulations.1
1,12
For example, it is well known that the high hole
N and Ga atoms into the ZnO films was estimated by secondary ions
mass spectroscopy ͑SIMS͒. All SIMS results were calibrated by the
simultaneous measurement of N and Ga ions implanted into a ZnO
single crystal. Electrical properties were measured at room tempera-
ture using the Van der Pauw method.
concentration in ZnSe films codoped with Li and N is due to the
increased N concentration resulting from the simultaneous incorpo-
ration of Li atoms.13 We have also observed an increased N concen-
tration at a Ga concentration of 5 mol % in Ga and N codoped ZnO
͑
ZnO:Ga, N͒ films.14
This paper reports on the interesting finding that the increase in
Results and Discussion
N concentration and the suppressive effect on luminescence from
DAP transitions and deep levels are controlled by the doping con-
centration of Ga in the ZnO:Ga, N films. This study provides new
information toward our understanding of the codoping characteris-
tics of ZnO.
Figure 2 shows the relationship between the Ga and N concen-
trations for the ZnO:Ga, N films. The Ga concentration doped into
the films agreed well with the Ga O content of the ablation targets.
2
3
The initial concentration of N atoms incorporated into the ZnO: N
films under the N O plasma conditions and growth temperature used
2
1
8
Ϫ3
in this work was approximately 10 cm . The N concentration did
not change at Ga concentrations below 0.5 mol %, but suddenly
z
18
19
Ϫ3
E-mail: hiroaki32@sanken.osaka-u.ac.jp
increased from 10 to 2.2 ϫ 10 cm with increasing Ga concen-
Downloaded on 2015-11-26 to IP 130.194.20.173 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 150 ͑9͒ G508-G512 ͑2003͒
G509
Figure 1. Optical emission spectrum from rf excited plasma of N O.
2
trations up to 5 mol %. This result indicates that the incorporation of
N atoms into ZnO is dependent on the Ga doping concentration. The
increase in N concentration through the use of a codoping strategy is
Figure 3. Dependence of Ga concentration on the electrical properties of
ZnO:Ga, N films. Black and white circles indicate resistivity and free elec-
tron density, respectively.
11
also suggested theoretically by first-principle calculations.
Figure 3 details the dependence of electrical properties on Ga
concentration for the ZnO:Ga, N films. Although the ZnO:N films
3
involving vacancies and their complexes. Wang et al. proposed that
displayed high resistivity above 10 ⍀/cm, the free electron density
increased to 1.6 ϫ 1019 cm with increasing Ga concentration due
to the increased number of Ga atoms acting as shallow donor
sources. The origin of the high resistivity in the ZnO:N films may be
attributed to the deep levels related with the yellow luminescence
around 2.2 eV for the PL spectra shown in Fig. 4b. These are dis-
cussed later. The free electron density, on the other hand, decreased
in the ZnO:Ga, N film doped with a Ga concentration of 5 mol %.
Some of the Ga atoms doped into the ZnO created lattice defects
Ϫ3
Zn
interstitial
7
Zn Zn
complexes such as (Ga -O
) and (Ga -V ) must be formed
1
in highly Ga-doped ZnO. In the case of Ga-doped ZnSe, com-
plexes such as (Ga -V ) and (Ga -V ) have been found
Zn
Se
interstitial
Zn
and are known to act as compensating native defects, as determined
from the results of ion channeling measurements and positron beam
1
8,19
analysis.
Therefore, it is considered that the decrease in free
electron density at a Ga concentration of 5 mol % is due to the
presence of vacancies and complexes created by the excessive in-
corporation of Ga atoms into ZnO.
Figure 5 shows the dependence of Ga concentration on the full
width half maximum ͑fwhm͒ value for the ␻-rocking curves of the
͑0002͒ diffraction peak of the ZnO:Ga, N films. A typical XRD
pattern for the ZnO films is shown in the inset of Fig. 5. The XRD
pattern contains only one strong peak at the ͑0002͒ plane of ZnO,
indicating that the films were highly c axis oriented and that second
phases such as ZnGa O and GaN were not observed within the
2
4
measurement limits. The fwhm value underwent large changes with
increasing Ga concentration, while the smallest fwhm value was
obtained in the ZnO:Ga, N film doped with Ga at a concentration of
0.1 mol %. Broadening of the fwhm value for the ␻-rocking curve is
generally observed in samples doped with impurities and may result
from the increased local strain around the impurities or from lattice
2
0
defects generated by the impurities. Thus, doping with Ga at a
concentration of 0.1 mol % should lead to a reduction in the forma-
tion of local strain and lattice defects in the ZnO:N films. On the
other hand, the fwhm value broadened significantly at a Ga concen-
tration of 5 mol %, due to the formation of vacancies and complexes
resulting in a decrease in the free electron density. Additionally, the
N concentration remained almost constant up to a Ga concentration
of 5 mol %, at which point vacancies and complexes were formed
through the excessive incorporation of Ga atoms. Thus, it is sug-
gested that the increased N concentration is closely related to the
presence of vacancies and complexes created by the excessive in-
corporation of Ga atoms. Recently, Nakahara et al. reported that
ZnO:Ga, N films showed a remarkable increase in the N concentra-
tion under excessive supplement conditions of Ga atoms at 8
2
0
Ϫ3 21
Figure 2. Relationship between Ga and N concentration for ZnO:Ga, N
ϫ 10 cm
.
Their report and our work indicate that the increase
films.
in N concentration is an essential phenomenon that is dependent on
Downloaded on 2015-11-26 to IP 130.194.20.173 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G510
Journal of The Electrochemical Society, 150 ͑9͒ G508-G512 ͑2003͒
Figure 4. PL spectra for ZnO:Ga, N
films at ͑a͒ room temperature and ͑b͒ 77
K.
neither the substrate type nor the thin film fabrication technique
used.
broad peak at 3.245 eV ͑Fig. 6͒. The INBE shifted to a lower energy
from 3.356 to 3.246 eV with increasing temperature. This shift oc-
curred as a result of the decrease in bandgap with increasing tem-
perature, indicating that the INBE is produced by the recombination
of excitons. On the other hand, the broad peak at 3.245 eV with the
longitudinal optical ͑LO͒ phonon replicas of 3.171 eV at intervals of
Figure 4a and b show the PL spectra at room temperature and 77
K, respectively, for the ZnO:Ga, N films doped with various Ga
concentrations. All PL spectra were normalized against the maxi-
mum emission peak in order to visually compare the relative inten-
sity ratio of the UV and deep-level emissions. In the case of the
ZnO:N films, the UV emission at 77 K can be divided further into
the near band-edge emission (INBE) at 3.350 eV and the broad peak
at 3.245 eV. The temperature dependence of each emission peak in
the temperature range from 10 K to room temperature was investi-
gated in an effort to clarify the origin of the INBE at 3.350 eV and the
74 meV moved to slightly higher energies, followed by the complete
Figure 6. Temperature dependence of the near band-edge emission (INBE
)
Figure 5. Dependence of Ga concentration on the fwhm value for X-ray
and DAP emission in the temperature range from 10 K to room temperature.
The insert details the PL spectra at 10 K in the ultraviolet region for ZnO:N
films grown on a ZnO ͑a͒ single crystal and ͑b͒ on a glass substrates.
␻-rocking curves of the ͑0002͒ peak of ZnO:Ga, N films. The insert figure is
a typical XRD pattern for ZnO:Ga, N films.
Downloaded on 2015-11-26 to IP 130.194.20.173 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 150 ͑9͒ G508-G512 ͑2003͒
disappearance of the emission peak above 190 K. The inset in Fig. 6
G511
shows that these emission peaks at 10 K were similar to the DAP
emission with clear LO phonon series of ZnO homoepitaxial film on
a
ZnO substrate doped with a N concentration of 1.0
1
9
Ϫ3
ϫ 10 cm . Thonke et al. reported recently that the zero phonon
peak of the DAP emission for a ZnO single crystal containing few N
2
2
atoms ͑2 ppm͒ was located at 3.220 eV at 5 K. These results
indicate that acceptor levels are formed in the ZnO:N films grown
on the glass, although the detailed mechanism of the DAP transition
requires further investigation.
It should be noted that the phenomenon of deep-level emission
for ZnO is quite complex and our understanding remains controver-
sial. It has been suggested that the blue and green emissions located
2
3
at around 2.8 and 2.4 eV are related to Zn vacancies and Cu
impurities,24 respectively. The PL intensity and the fwhm value of
the emission peak of the yellow luminescence in the ZnO:N film,
shown in Fig. 4b gradually increased and narrowed, respectively,
with decreasing temperature. This behavior is similar to that of the
2
5,26
localized luminescence centers observed in ZnS and ZnSe films.
Iwata et al. also reported that ZnO:N films showed yellow lumines-
cence at around 2.2 eV at low temperatures.27 The localized lumi-
nescence is produced by an internal transition of the localized cen-
ters created by the trapping of electrons at anion vacancies. The
luminescence efficiency always decreases at room temperature since
the probability of nonradiative transitions is higher than that of ra-
diative transitions with increasing temperature. We consider that the
intense yellow luminescence at 77 K in the ZnO:N film may be
associated with an increase of intrinsic defects in ZnO due to the
incorporation of N atoms.
The optical properties of ZnO:N films were enhanced greatly by
doping with Ga. In Fig. 4a and b, the DAP emission and yellow
luminescence were suppressed significantly at both room tempera-
ture and 77 K as the Ga concentration increased up to 0.1 mol %.
However, as the Ga concentration exceeded 0.5 mol %, the INBE
declined and a red luminescence at around 1.9 eV was observed at
room temperature. With a further increase of Ga concentration, the
PL spectrum could not be measured due to the very weak emission
intensity. The increase of red luminescence along with the reduction
of INBE is thought to be the result of luminescence produced by deep
levels related to excessive incorporation of Ga atoms. Moreover, we
confirmed that the PL properties depended on the doping concentra-
tion of Ga given the increase in the fwhm value for the emission
peak of INBE with increasing Ga concentration. Broadening of the
emission peak by the incorporation of donor impurities is generally
attributed to a screening effect of the excitons caused by the random
distribution of donor impurities.28 Accordingly, we conclude that Ga
doping at 0.1 mol % assisted in reducing the formation of acceptor
levels and localized centers in the ZnO:N films.
Figure 7. Dependence of Ga concentration on the PL intensity ratio of INBE
to IDE at room temperature and 77 K. The insert shows the PL spectra for
ZnO:Ga, N film doped with a Ga concentration of 0.1 mol % on glass ͑filled
line͒ and the undoped ZnO single crystal ͑empty circles͒.
C. G. van de Walle suggested that H atoms could, theoretically, form
shallow donor levels in ZnO and thus introduce large distortions of
the tetrahedral structure centered around the Zn ions due to the
3
2
formation of strong H-O bonds. Ohashi et al. proposed that the
suppression of deep-level emission by H -plasma irradiation arises
2
from a charge compensation between H ions and lattice defects or
3
3
residual impurities. We measured the d-value of the ͑0002͒-plane
for the ZnO:Ga, N films using a standard Si substrate. In the ZnO:N
films the d-value was 2.607 Å, which resembles the d-value of
nondoped ZnO films grown in an O /O atmosphere. On the other
2
3
hand, ZnO:Ga, N films doped with Ga concentrations of 0.1 and 5
mol % displayed d-values of 2.612 and 2.616 Å, respectively, indi-
cating that the length of the c axis increased despite that the ionic
Figure 7 shows the dependence of the relative PL intensity ratio
of INBE to IDL (INBE /IDE) on the Ga concentration for the ZnO:Ga,
N films. As the Ga concentration increased, the INBE /IDE ratio
gradually increased at both room temperature and 77 K. The maxi-
mum INBE /IDE ratio was obtained for the ZnO:Ga, N film with a Ga
concentration of 0.1 mol %. As shown in the inset of Fig. 7, a
INBE /IDE ratio as high as 95 was achieved even at room temperature,
which is equivalent to the INBE /IDE ratio of a high quality ZnO
single crystal ͑Eagle-Picher, 6 N͒. We then investigated the PL prop-
erties of ZnO films codoped with N and other dopants aside from Ga
to further clarify the effect of the suppression of the yellow lumi-
nescence by codoping. As shown in Fig. 8, suppression effects were
also observed in ZnO films doped with Al and N, but they were not
seen in ZnO films doped with Li and N. This result confirmed that
3ϩ
2ϩ
radius of Ga ions ͑0.61 Å͒ is smaller than that of Zn ions ͑0.72
Å͒. Furthermore, lattice distortion resulting from the doping of Ga
into ZnO has been verified by nuclear magnetic resonance measure-
34
ments for ZnO:Ga and ZnO:Al powder samples. Therefore, the
increase in length of the c axis must be related to the local lattice
distortion due to the replacement of Zn ions by Ga ions. We propose
that suppression of the DAP emission and yellow luminescence in
ZnO:Ga, N films are closely related to the mechanism involved in
the enhancement of UV emission of H -plasma irradiated ZnO.
2
Conclusions
The codoping characteristics of ZnO were demonstrated using
various ratios of Ga and N. The increase in N concentration ob-
served in the highly doped region at Ga concentration of 5 mol %
was attributed to the formation of vacancies and complexes associ-
ated with compensating centers of electrons. On the other hand, the
N concentration remained almost unchanged in the doped regions
consisting of Ga concentrations below 0.5 mol %, where Ga can act
as a shallow donor source at Zn sites. Furthermore, a significant
trivalent ions such as Ga3 and Al , which act as donor sources in
ZnO, and have an opposite charge to anions, play an important role
in suppressing the DAP emission and the yellow luminescence.
In recent years, many researchers have reported that the suppres-
sion of deep-level emission and the increase in free electron density
ϩ
3ϩ
8
,29-31
were observed at the same time in H -plasma irradiated ZnO.
2
Downloaded on 2015-11-26 to IP 130.194.20.173 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G512
Journal of The Electrochemical Society, 150 ͑9͒ G508-G512 ͑2003͒
2
3
. K. Ueda, H. Tabata, and T. Kawai, Appl. Phys. Lett., 79, 988 ͑2001͒.
. T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, and M. Kawasaki, Appl. Phys. Lett.,
5, 3366 ͑1999͒.
7
4
5
6
7
8
9
. Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y.
Segawa, Appl. Phys. Lett., 72, 3270 ͑1998͒.
. A. Ohtomo, K. Tamura, M. Kawasaki, T. Makino, Y. Segawa, Z. K. Tang, G. K. L.
Wong, and H. Koinuma, Appl. Phys. Lett., 77, 2204 ͑2000͒.
. K. Sakurai, T. Kubo, D. Kajita, T. Tanabe, H. Takasu, S. Fujita, and S. Fujita, Jpn.
J. Appl. Phys., Part 2, 39, L1146 ͑2000͒.
. A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y.
Yoshida, and Y. Segawa, Appl. Phys. Lett., 72, 2466 ͑1998͒.
. T. Sekiguchi, N. Ohashi, and Y. Terada, Jpn. J. Appl. Phys., Part 2, 36, L289
͑
1997͒.
. J. Y. Leem, C. R. Lee, S. K. Noh, Y. S. Kwon, Y. H. Ryu, and S. J. Son, J. Cryst.
Growth, 193, 491 ͑1998͒.
1
0. T. Watanabe and T. Yamamoto, Jpn. J. Appl. Phys., Part 2, 39, L1280 ͑2000͒.
11. T. Yamamoto and H. Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2, 38, L166
͑
1999͒.
1
1
2. Y. Yan, S. B. Zhang, and S. T. Pantelides, Phys. Rev. Lett., 86, 5723 ͑2001͒.
3. S. W. Lim, T. Honda, F. Koyama, K. Iga, K. Inoue, Y. Yanashina, H. Munekata, and
H. Kukimoto, Appl. Phys. Lett., 65, 2437 ͑1994͒; M. Yoneta, H. Uechi, K. Nanami,
M. Ohishi, H. Saito, K. Yoshino, and K. Ohmori, Physica B, 302-303, 166 ͑2001͒.
4. M. Joseph, H. Tabata, and T. Kawai, Jpn. J. Appl. Phys., 38, L2505 ͑1999͒.
5. L. G. Piper and W. T. Rawlins, J. Phys. Chem., 90, 320 ͑1986͒.
6. Y. C. Kim, J. S. Chun, and W. J. Lee, Thin Solid Films, 288, 67 ͑1995͒.
7. R. Wang, A. W. Sleight, and D. Cleary, Chem. Mater., 8, 433 ͑1996͒.
8. T. Miyajima, H. Okuyama, K. Akimoto, Y. Mori, L. Wei, and S. Tanigawa, Appl.
Phys. Lett., 59, 1482 ͑1991͒.
1
1
1
1
1
1
9. T. Hino, T. Haga, Y. Abe, T. Miyajima, H. Okuyama, and K. Akimoto, Appl. Phys.
Lett., 82, 1196 ͑1997͒.
2
2
0. K. Ohkawa, T. Mitsuyu, and O. Yamazaki, J. Appl. Phys., 62, 3216 ͑1987͒.
1. K. Nakahara, H. Takasu, P. Fons, A. Yamada, K. Iwata, K. Matsubara, R. Hunger,
and S. Niki, Appl. Phys. Lett., 79, 4139 ͑2001͒.
2
2. K. Thonke, Th. Gruber, N. Teofilov, R. Sch o¨ nfelder, A. Waag, and R. Sauer,
Physica B, 308-310, 945 ͑2001͒.
2
2
2
3. B. J. Jin, S. Im, and Y. Shin, Thin Solid Films, 366, 107 ͑2000͒.
4. R. Dingle, Phys. Rev. Lett., 23, 579 ͑1969͒.
5. T. K. Tran, W. Park, W. Tong, M. M. Kyi, B. K. Wagner, and C. J. Summeres, J.
Appl. Phys., 81, 2803 ͑1997͒.
Figure 8. PL spectra at 77 K for ZnO: N film and ZnO films codoped with
N and M (M ϭ Ga, Al, and Li͒.
e
e
2
2
2
2
3
6. T. Yoda, T. Koyama, H. Ueda, and K. Yamashita, J. Appl. Phys., 65, 2726
͑
1989͒.
7. K. Iwata, P. Fons, A. Yamada, K. Matsubara, and S. Niki, J. Cryst. Growth, 209,
26 ͑2000͒.
8. E. F. Schubert, I. D. Goepfert, W. Grieshaber, and J. M. Redwing, Appl. Phys. Lett.,
1, 921 ͑1997͒.
9. J. M. Lee, K. K. Kim, S. J. Park, and W. K. Choi, Appl. Phys. Lett., 78, 3842
2001͒.
5
suppressive effect in the ZnO:N film was found when using a Ga
concentration of 0.1 mol %. A suppressive effect was also observed
in ZnO films codoped with Al and N, indicating that trivalent ions
play an important role in suppressing the formation of acceptor lev-
els and localized centers in the ZnO:N films. Finally, the codoping
of Ga into N single-doped ZnO that led to an enhancement in UV
emission and the formation of shallow donor levels was similar to
the electrical and optical behavior observed with H-doped ZnO.
7
͑
0. D. H. Hofmann, A. Hofstaetter, F. Leiter, H. Zhou, F. Henecker, B. K. Meyer, S. B.
Orlinskii, J. Schmidt, and P. G. Baranov, Phys. Rev. Lett., 88, 045504 ͑2002͒.
31. S. F. J. Cox, E. A. Davis, S. P. Cottrell, P. J. C. King, J. S. Lord, J. M. Gil, H. V.
Alberto, R. C. Vilao, J. P. Duarte, N. Ayres de Campos, A. Weidinger, R. L. Lichti,
and S. J. C. Irvine, Phys. Rev. Lett., 86, 2601 ͑2001͒.
Osaka University assisted in meeting the publication costs of this article.
3
3
2. C. G. van de Walle, Phys. Rev. Lett., 85, 1012 ͑2000͒.
3. N. Ohashi, T. Ishigaki, N. Okada, T. Sekiguchi, I. Sakaguchi, and H. Haneda, Appl.
Phys. Lett., 80, 2869 ͑2002͒.
References
1
. A. Onodera, N. Tamaki, K. Jin, and H. Yamashita, Jpn. J. Appl. Phys., Part 1, 36,
008 ͑1997͒.
34. N. Roberts, R. P. Wang, A. W. Sleight, and W. W. Warren, Jr., Phys. Rev. B, 57,
5734 ͑1998͒.
6
Downloaded on 2015-11-26 to IP 130.194.20.173 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).