APPLIED PHYSICS LETTERS
VOLUME 79, NUMBER 7
13 AUGUST 2001
Green luminescent center in undoped zinc oxide films deposited
on silicon substrates
Bixia Lin and Zhuxi Fua)
Structure Research Laboratory, Academia Sinca and Department of Physics, University of Science and
Technology of China, Hefei, Anhui 230026, China
Yunbo Jia
Structure Research Laboratory, Academia Sinca, University of Science and Technology, Hefei,
Anhui 230026, China
共Received 2 March 2001; accepted for publication 15 June 2001兲
The photoluminescence 共PL兲 spectra of the undoped ZnO films deposited on Si substrates by dc
reactive sputtering have been studied. There are two emission peaks, centered at 3.18 eV 共UV兲 and
2.38 eV 共green兲. The variation of these peak intensities and that of the I – V properties of the ZnO/Si
heterojunctions were investigated at different annealing temperatures and atmospheres. The defect
levels in ZnO films were also calculated using the method of full-potential linear muffin-tin orbital.
It is concluded that the green emission corresponds to the local level composed by oxide antisite
defect OZn rather than oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni , and interstitial
oxygen Oi . © 2001 American Institute of Physics. 关DOI: 10.1063/1.1394173兴
Recently, ZnO film draws much attention because of its
ultraviolet emission.1,2 ZnO is a self-activated crystal of hexagonal wurtzite structure with the lattice constant of
a⫽0.3249 nm, c⫽0.5207 nm. The notable properties of ZnO
are its wide band gap of 3.36 eV at room temperature and
high exciton binding energy 共60 meV兲 which is much higher
than that of ZnSe 共20 meV兲 and GaN 共21 meV兲. Besides,
ZnO can be prepared at a lower temperature than that of
ZnSe and GaN. Owing to these properties, ZnO can be used
as UV or blue emitting materials. Therefore, many researchers have investigated the emitting properties of ZnO films,
including ultraviolet and green emissions. Most authors indicated that the UV emission center could be an exciton
transition.3–5 However, some authors assumed that the green
emission was caused by different intrinsic defects in ZnO
film, such as oxygen vacancy (VO), zinc vacancy (VZn),
interstitial zinc (Zni ), 6 – 8 etc.
In this letter, the influence of annealing conditions on the
photoluminescence 共PL兲 spectra of ZnO films was investigated. The variation of I – V characteristics of the samples
composed by ZnO/Si heterojunction with the annealing conditions was also measured. The levels of the intrinsic defect
in the ZnO film were calculated, too. According to these
results, we suggest that the antisite defect (OZn) plays a major role in the green emission of undoped ZnO film.
The samples were the ZnO films deposited on p-type Si
substrates by dc reactive sputtering that reported in our previous paper.9 The Si substrate and the ZnO film, which are
normally n type, are joined to a p – n heterojunction. After
being sputtered, the sample was cut into several pieces, then
annealed at 850, 950, and 1000 °C for 1 h, in air, pure O2 共1
atm兲 and pure N2 共1 atm兲, respectively. There is only a wide
diffraction peak of ZnO 共002兲 in the x-ray diffraction patterns of all the as-deposited and annealed samples. It is evident that all the films used here are microcrystal ZnO films
with 关001兴 orientation.
a兲
Electronic mail: fuzx@ustc.edu.cn
Figures 1共a兲 and 1共b兲 show the PL spectra of the samples
annealed in air at 850 °C and in oxygen atmosphere at
1000 °C, respectively. In addition to these two spectra, other
samples annealed under various conditions were also measured. All spectra have two emission peaks, centering at 3.18
FIG. 1. The x-ray diffraction patterns of the samples: 共a兲 unannealed and 共b兲
annealed in pure oxygen at 950 °C.
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© 2001 American Institute of Physics
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Appl. Phys. Lett., Vol. 79, No. 7, 13 August 2001
Lin, Fu, and Jia
TABLE I. The relative intensities of green and UV emissions of the samples annealed in different conditions.
Intensity 共arb. unit兲
in N2 共at 1 atm兲
Annealing
temperature 共°C兲
850
950
1000
Green
UV
Green
UV
Green
UV
41
43
17
50
72
38
59
43
32
120
549
35
66
58
eV 共UV兲 and 2.38 eV 共green兲 as in Figs. 1共a兲 and 1共b兲. The
intensity of the green peak depends markedly on annealing
conditions, but that of the UV peak with annealing conditions varies little. Detailed data are shown in Table I. From
Table I, it is clear that the intensity of the green peak increases with the increase of the annealing temperature in the
same atmosphere. On the other hand, with the same annealing temperature, this intensity is enhanced sharply with the
increase of partial oxygen pressure. Therefore, the intensity
of the green peak of the sample annealed in pure oxygen is
much higher than that in air and in pure N2 .
Because the center energy of the green peak, 2.38 eV, is
smaller than the band gap energy of ZnO film, 3.3 eV, the
green emission must be related to a local level in band gap.
Therefore, the intensity variation of the green emission may
be resulted from the variation of the intrinsic defects in ZnO
film, such as zinc vacancy VZn, oxygen vacancy VO, interstitial zinc Zni , interstitial oxygen Oi , and antisite oxygen
OZn.
During annealing, the variation of these defects with the
oxygen pressure (p O2 ) can be expressed as the following:
x
x
1/2O2⫹VO
⫽OO
,
x
⫺1/2
,
关 VO
兴⬀ pO
2
共1兲
x
x
1/2O2⫽VZn
⫽OO
,
x
1/2
,
关 VZn
兴⬀ pO
2
共2兲
Zni ⫹1/2O2 共 g 兲 ⫽ZnZn⫹OO ,
1/2O2⫽Oi ,
in O2 共at 1 atm兲
in air
⫺1/2
关Zni ]⬀ p O
,
2
1/2
,
关 Oi 兴 ⬀ p O
2
ing on ZnO forms and preparation conditions. Because the
ZnO films in our experiment grew in higher oxygen partial
pressure, the VO and Zni would be lower, so that the VZn,
Oi , and OZn can be easily formed when annealing in oxygen
atmosphere at high temperature. Therefore, the earlier results
can be observed. In order to test this result, we deposited
ZnO films on quartz and n-type Si substrates and measured
their PL variation with annealing atmosphere and temperature. The results are almost similar.
The variation of these defects can be also demonstrated
by the variation of the I – V properties of the heterojunction
formed by n-ZnO/p-Si. Generally, VO and Zni are donors,
and VZn, Oi , and OZn are acceptors in ZnO films. Their
concentration variations must change the Fermi level of the
ZnO film, and result in the variation of the I – V properties.
Figures 2共a兲 and 2共b兲 show the I – V curves of the samples
共3兲
共4兲
1/2
1/2O2 共 g 兲 ⫹VZn⫽OZn 关OZn]⬀ p O
关VZn].
2
共5兲
x
x
Here, 关 VO
兴 and 关 VZn
兴 are the concentrations of the nonionized vacancies of oxygen and zinc, respectively. 关 Zni 兴 ,
关 Oi 兴 , and 关 OZn兴 are the concentrations of the interstitial zinc
Zni , interstitial oxygen Oi , and antisite oxygen OZn, respectively. Equations 共1兲 and 共3兲 indicate that concentrations of
the oxygen vacancy and the interstitial zinc ought to decrease
with the increase of the oxygen pressure p O . These variations are all contrary to the intensity variation of the green
emission we measured in the experiments. Therefore, the
green emission must be independent on Zni and VO. It must
correspond to VZn, Oi , or OZn, whose concentration variations conform to the intensity variation of the green emission.
We have taken notice of that our results, PL variation
with annealing atmosphere and temperature, are different
from the results reported by Vanheusden et al.10 and Ogsata
et al.11 There is also difference between Vanheusden’s and
Ogsata’s results. We suggest that all these differences are
FIG.to
2. the
Theterms
I – V curves
of the samples: 共a兲 annealed at 850 °C inDownloaded
air and 共b兲 to IP:
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at: http://scitation.aip.org/termsconditions.
owing to the kinds and numbers of intrinsic defects dependannealed at 1000 °C in oxygen.
129.97.58.73 On: Sat, 09 Nov 2013 00:50:16
Appl. Phys. Lett., Vol. 79, No. 7, 13 August 2001
Lin, Fu, and Jia
FIG. 3. The draft of the ZnO/Si heterojunction band diagram: 共a兲 sample A
annealed at 850 °C in air and 共b兲 sample B annealed at 1000 °C in pure
oxygen.
annealed for 1 h in air at 850 °C 共sample A兲 and in pure
oxygen at 1000 °C 共sample B兲, respectively. The forward
bias is that the cathode is ZnO film and anode is Si substrate.
The reverse bias is polarity reversed. These two curves of
Figs. 2共a兲 and 2共b兲 are quite different. We suppose it is due to
different heterojunction band structure. The band diagram of
samples A and B are shown in Figs. 3共a兲 and 3共b兲, respectively. It is known that the band gap energy of Si 共1.09 eV兲 is
smaller than half of the ZnO’s band gap energy 共3.36 eV兲.
After the n-type ZnO and p-type Si are joined, the bending
of the valence band is much higher than the bending of the
conduction band. The higher barrier in the valence band prevents the hole’s movement. Therefore, the conductive property of this heterojunction is mainly determined by the electrons in the conduction band. For sample A, we suppose its
band diagram 关Fig. 3共a兲兴 is almost similar to a homogenous
p – n junction, because they have similar I – V curves, as
shown in Fig. 2共a兲. However, the ZnO film of sample B,
which is annealed at 1000 °C in pure oxygen, has more holes
because of more acceptors VZn or OZn. Its Fermi level must
move away from the bottom of the conduction band. It
makes the conduction-band bottom of p-type Si lower than
that of n-type ZnO, in order to keep the Fermi level to be
constant in the two materials. So that, the conduction band
energy spike and the potential well appear in the interface
between ZnO and Si, as shown in Fig. 3共b兲. Under forward
bias, these energy spike and potential well impede electron
moving, and the forward current increases slowly with the
increase of the applied voltage. As the applied reverse voltage increases, the bottom of the conduction band of the Si
approaches the bottom of ZnO’s conduction band and the
energy spike and potential well in the conduction band become narrower so that the electrons can tunnel them easily.
Therefore, the reverse current increases rapidly, as shown in
Fig. 2共b兲.
Using full-potential linear muffin-tin orbital method,
Sun12 calculated the energy levels of the intrinsic defects in
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FIG. 4. The draft of the calculated defect’s levels in ZnO film.
ZnO films. The result shows in Fig. 4. It is seen from Fig. 4
that the energy interval from the bottom of the conduction
band to the OZn level 共2.38 eV兲 is exactly consistent with the
energy of the green emission observed in our experiment.
The energy interval between the bottom of the conduction
band and the Oi level 共2.28 eV兲 also approximately conforms
to the green emission, but the probability of forming Oi is
little due to large diameter of oxygen atom. The VZn could
not be related to the green emission, because the energy interval, 3.06 eV, is too large.
According to the analysis of the experimental phenomena and the calculation of the defect levels in the ZnO films,
we suggest that green emission corresponds to the electron
transition from the bottom of the conduction band to the
antisite defect OZn level.
Supported by Natural Science Foundation of China under Grant No. 5987203, and Natural Science Foundation of
Anhui Province No. 98641550.
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