Structural, electrical, and optical properties of p-type ZnO thin films ...

Structural, electrical, and optical properties of p-type ZnO thin films
with Ag dopant
Hong Seong Kang, Byung Du Ahn, Jong Hoon Kim, Gun Hee Kim, Sung Hoon Lim,
Hyun Woo Chang, and Sang Yeol Lee a�
Department of Electrical and Electronic Engineering, Yonsei University, 134 Shinchon-dong,
Seodaemoon-ku, Seoul 120-749, Korea
�Received 11 January 2006; accepted 29 March 2006; published online 17 May 2006�
p-type ZnO films have been fabricated on a �0001� Al2O3 substrate, using Ag2O as a silver dopant
by pulsed laser deposition. The structural property of those films is systematically characterized by
observing the shift of �0002� peak to investigate the substitution of Ag + for Zn + . Narrow deposition
temperature for Ag-doped p-type ZnO films has been obtained in the range of 200–250 °C with the
hole concentration of 4.9�1016 –6.0�1017 cm−3 . A neutral acceptor bound exciton has been clearly
observed by photoluminescence emitted at 3.317 eV in Ag-doped p-type ZnO thin films. © 2006
American Institute of Physics. �DOI: 10.1063/1.2203952�
ZnO is a promising material for the use in ultraviolet or
visible optoelectronic devices, because of its direct wide
band gap �3.37 eV� and large exciton binding energy
�60 meV�. 1–5 Although it has several advantages compared
to GaN, the difficulty of p-type doping inhibits applications
to optoelectronic devices. 4,5 It is difficult to fabricate p-type
ZnO because ZnO is naturally only n-type conduction due to
a large number of native defects, such as oxygen vacancies
and zinc interstitials. Recently, research on ZnO has been
focused on the synthesis of p-type ZnO using various techniques
and dopants. 6–17 p-type doping in ZnO may be possible
by substituting either group I elements �Li, Na, and K�
for Zn sites or group V elements �N, P, and As� for O sites. 18
Group I elements tend to occupy the interstitial sites rather
than substitutional sites, in part mitigated by their smaller
ionic radii than Zn ionic radius; therefore they act mainly as
donors. 19 Ag, as a group Ib element, could also act as an
acceptor in ZnO, if incorporated on substitutional Zn sites. 20
Fan and Freer suggested that Ag acted as an amphoteric dopant,
existing both on substitutional Zn sites and in the interstitial
sites. 21 Kanai reported that Ag behaves as an acceptor
with a deep level �0.23 eV below the conduction band. 22
However, there is no report for the fabrication of p-type ZnO
by doping Ag yet. In this study, p-type ZnO films with Ag
dopant were fabricated by pulsed laser deposition and characterized
depending on deposition temperatures. The p-type
conduction of Ag-doped ZnO was confirmed by structural,
electrical, and optical characterizations.
Ag, doped ZnO �ZnO:Ag� thin films were grown on
�0001� sapphire substrates by pulsed laser deposition �PLD�.
Ag-doped ZnO targets were prepared by a conventional ceramic
powder process. A 2 wt. % Ag 2O was mixed with ZnO
powder for the Ag-doped ZnO target. ZnO:Ag thin films
were deposited at various temperatures from 100 °C to
400 °C under the oxygen pressure of 3.5�10 −1 Torr. The
film thickness was measured to be about 1000 nm confirmed
by scanning electron microscopy �SEM�. The structural
property of the samples has been investigated by using x-ray
diffraction �XRD� where a Ni-filtered Cu K� ��
=1.54056 � source was used. The electrical property of Ag-
a� Electronic mail: sylee@yonsei.ac.kr
APPLIED PHYSICS LETTERS 88, 202108 �2006�
doped ZnO films has been investigated by Hall measurements
in the van der Pauw configuration at a room temperature.
The optical properties were characterized by
photoluminescence �PL� with a HeCd laser as a light source
using the excitation wavelength of 325 nm and the power of
20 mW.
Figure 1 shows the �-2� x-ray diffraction patterns of
ZnO:Ag films deposited at various deposition temperatures
in the range of 100–400 °C. The �21 ¯1 ¯0�, �21 ¯1 ¯3�, and
�21 ¯1 ¯6� peaks and a preferential strong �0002� peak have
been observed in the films grown at the deposition temperature
below 300 °C. Ag-doped ZnO thin films exhibit the
sharp �0002� peak at deposition temperature of above
300 °C, indicating highly c-axis orientated, as shown in
Fig. 1�d�.
FIG. 1. �-2� x-ray diffraction patterns of ZnO:Ag films grown at various
deposition temperatures such as �a� 100 °C, �b� 200 °C, �c� 300 °C, and �d�
400 °C.
0003-6951/2006/88�20�/202108/3/$23.00 © 2006 American Institute of Physics
88, 202108-1
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202108-2 Kang et al. Appl. Phys. Lett. 88, 202108 �2006�
FIG. 2. �a� The change of �0002� peak positions of ZnO:Ag films, and �b�
that of c-axis lattice constant depending on deposition temperature.
The �0002� peak positions of Ag-doped ZnO thin films
are shifted as changing the deposition temperature, as shown
in Fig. 2�a�. The structural properties of Ag-doped ZnO films
exhibit three different regions according to their deposition
temperatures, as shown in Fig. 2. Region I is from
100 to 175 °C, region II from 175 to 275 °C, and region III
from 275 to 400 °C. The existence of three regions is
closely related with the lattice sites of Ag in ZnO, as reported
by Rita et al. 23 Ag in ZnO can act as an amphoteric dopant,
existing both on substitutional Zn sites and in the interstitial
sites. 21 The quasichemical reaction for Ag in ZnO thin films
may be described as follows:
*
Ag2O → AgZn� +Agi + ZnO. �1�
In region I, the Ag + ions cannot substitute Zn2+ ions but
form defects or metallic clusters in the ZnO thin films. This
could be explained mainly because the diffusion behavior of
Ag in ZnO thin film is restricted at low temperature of below
175 °C. According to Bragg rule, as the peaks of �0002�
Ag-doped ZnO films shift to a lower angle as increasing
deposition temperature in region II, the c-axis lattice constant
increases, as shown in Fig. 2�b�. While as the peaks of
�0002� Ag-doped ZnO shift back to a higher angle as increasing
deposition temperature in region III, the c-axis lattice
constant decreases, as shown in Fig. 2�b�. Because the radius
of Ag + ion �1.22 � is a greater than that of Zn + ion
�0.72 �, the increase of the number of Ag + ion in the Zn +
lattice sites expands the lattice parameter.
Figure 3 shows the carrier concentration �ne or nh� and
the resistivity ��� of Ag-doped ZnO films at various deposi-
FIG. 3. Carrier concentration and resistivity of Ag-doped ZnO films depending
on deposition temperatures ��: hole concentration, �: electron concentration,
and �: resistivity�.
tion temperatures. This result is well agreed with three deposition
temperature regions divided by the XRD results, as
shown in Fig. 2. As in the case of Ag-doped ZnO films
deposited at region I, it shows semi-insulator behavior, as
shown in Fig. 3�b�. It is noticeable that Ag-doped ZnO films
deposited at region II exhibit p-type conductivity. The hole
concentration, hole mobility, and resistivity of p-type
Ag-doped ZnO films are 4.9�1016 –6.0�1017 cm−3 ,
0.29–2.32 cm2 /V s, and 34 to 54 � cm, respectively. The
large increase of carrier concentration was observed between
regions I and II. From the repeated experiments, the existence
of critical temperature related to huge increase of carrier
concentration has been deserved between 150 and
200 °C. The Ag-doped ZnO films deposited at region III
show a conversion of n-type from p-type conductivity with
the electron concentration being increased to 1020 cm−3 .
Other groups also reported same transition behavior related
with group V elements doping. 14,24 These results on the dependence
of the conductivity of Ag-doped ZnO films on the
deposition temperature suggest that a narrow temperature
window exists to be p-type semiconductor.
Low-temperature �11 K� PL spectra of an undoped ZnO
film and p-type Ag-doped ZnO thin film are compared in
Fig. 4. The PL spectra of an undoped ZnO show various
peaks at 3.359, 3.318, 3.246, and 3.174 eV, as shown in Fig.
4�a�. The peaks at 3.359 and at 3.318 eV can be assigned to
a neutral donor bound exciton �D 0 X� and a donor acceptor
pair �DAP� emission. The peaks at 3.246 and 3.174 eV exhibit
phonon replicas of DAP separated by approximately
�E=72 meV, which correlates with the longitudinal optical
phonon energy observed in ZnO. 25 The origin of emission
line at 3.317 eV of Ag doped p-type ZnO, as shown in Fig.
4�b�, suggested that it was related to neutral acceptor bound
exciton �A 0 X�. The origin of emission line at 3.317 eV in
Fig. 4�b� could be explained as two electron satellites �TESs�
or DAP emission compared to other references and our result,
as shown in Fig. 4�a�. 20,25,26 However, it is difficult to
observe such a large intensity of TES compared to the D 0 X
emission intensity because TES is a kind of replica of D 0 X
and characteristics having specific energy difference �about
29.9 or 55 meV�. 28–30 In the case of DAP emission, at low
temperature like 10 K, the intensity of DAP emission mainly
shows a low intensity compared with bound exciton
emission, 26,28–30 and DAP emission is observed in company
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202108-3 Kang et al. Appl. Phys. Lett. 88, 202108 �2006�
FIG. 4. PL spectra of �a� undoped ZnO, and �b� Ag-doped p-type ZnO films
measured at 11 K.
with D 0 X or A 0 X in higher energy region. Therefore, the
origin of emission line at 3.317 eV, as shown in Fig. 4�b�,
could be suggested as a neutral bound acceptor exciton of
Ag-doped p-type ZnO. This deep acceptor bound exciton
was observed elsewhere. The neutral acceptor bound exciton
emission lines of As- and N-doped ZnO were reported at
3.32 eV measured at 20 K and 3.315 eV at 2 K, respectively,
by Ryu et al. 27 and Look et al. 28 Also, in the case of N-doped
p-type ZnO, small D 0 X emission and large A 0 X emission at
3.315 eV were simultaneously observed at 2 K. These p-type
ZnO thin films having deep acceptor exciton emission show
the DAP located at about below 3.25 eV. 27,28 These tendencies
are consistent with the results in Fig. 4�b�. The D 0 X
emission at 3.359 eV was observed very weakly because of
the large and broad emission peak of A 0 X, as shown in Fig.
4�b�. Peaks at 3.218, 3.147, and 2.988 eV were not observed
on PL spectra of undoped ZnO, as shown in Fig. 4�a�. These
peaks shown in Fig. 4�b� were observed only from Ag-doped
p-type ZnO and suggest that they are closely related to Agrelated
impurity or defect level transition. However, further
investigation is necessary to verify exact origin of these
peaks.
In summary, Ag-doped p-type ZnO thin films have been
fabricated and characterized. A systematic investigation on
the effect of deposition temperature has been carried out for
Ag-doped ZnO films. A narrow window of deposition temperature
existed for films doped with Ag, in which p-type
ZnO films could be obtained. The fabricated Ag-doped
p-type ZnO film shows the hole concentration in the
range of 4.9�10 16 –6.0�10 17 cm −3 , the hole mobility of
0.29–2.32 cm 2 /V s, and the resistivity of 34–54 � cm. A
neutral acceptor bound exciton peak of 3.317 eV was also
observed in Ag-doped ZnO.
This work was supported by Grant No. R01-2004-000-
10195-0 �2005� from the Basic Research Program of the
Korea Science and Engineering Foundation.
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