Materials Science Forum
ISSN: 1662-9752, Vol. 848, pp 477-481
doi:10.4028/www.scientific.net/MSF.848.477
© 2016 Trans Tech Publications, Switzerland
Improve the P-type Conductivity of SnO Films by Na Ion Implantation
Tieying Yang1, 2, 3, a, Jun Zhao1, Xiaolong Li1, 2, Xingyu Gao1, 2, 3, Chaofan Xue1,
Yanqing Wu1 and Renzhong Tai1, 2, 3
1
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
2
Key Laboratory of Interfacial Physics and Technology, Chinese Academy of Sciences, Shanghai,
201204, China
3
Shanghai Institute of Materials Genome, Shanghai, 200444, China
a
yangtieying@sinap.ac.cn
Keywords: P-type transparent conducting oxide, SnO thin film, RF magnetron sputtering,
Ion implantation
Abstract. P-type transparent conducting SnO thin films were directly fabricated using RF
magnetron sputtering. The electronic properties of the SnO thin films were enhanced by Na ion
implantation and annealing at 200°C. The growth and implantation conditions were systemically
investigated. The electronic properties, optical properties, microstructure and surface morphologies
of the films were characterized. It was observed that the structure of the Na-doped SnO films was
crucial to improving their p-type conductivity.
PACS: 81.05.-t; 68.55.-a; 68.55.Ln; 68.55.-Nq;
Introduction
Transparent conducting oxide (TCO) thin films have been widely applied in various fields, such
as solar cells, flat panel displays and light-emitting diodes [1, 2]. Unfortunately, to date, only
features of these films, such as their ability to serve as transparent metals, have been utilized in
industry, whereas their active functions as transparent semiconductors have not been practically
applied due to the lack of suitable p-type TCO thin films [3]. Tin monoxide (SnO) is known to be a
p-type oxide semiconductor with a PbO-type layered structure. The symmetry of SnO is that of the
P4/nmm space group, and the lattice constants are a = b = 3.8029 Å and c = 4.8382 Å [4]. SnO has
been widely studied as an anode material, a catalyst, a coating substance, and a precursor for
producing SnO2 [5, 6] because of its gas sensitivity and metastability in transforming into SnO2.
Compared to those materials with an (n-1)d10ns0 electronic configuration (e.g., Cu+, Ag+, Au+, Sn4+,
and Zn2+), SnO could serve as a better native p-type oxide semiconductor because the energy level
of Sn 5s2 is closer to that of O2p, thus more effectively reducing the hole localization. SnO has been
regarded as the next-generation transparent semiconducting oxide for application in photovoltaics,
sensors, and information storage [7-9].
There exist only a few studies concerning the improvement of p-type SnO films by doping [6],
and the resultant films displayed poor electronic transport properties. Na+ (1.02 Å) was regarded as
a suitable dopant because its ionic radius was very close to that of Sn2+ (1.12 Å). However, because
of the phase instability of SnO and chemical activity of Na, it is very difficult to dope Na in SnO.
Up to now, there has been no related report concerning Na-doped SnO film. Our former effort to
prepare an Na-doped SnO target for RF magnetron sputtering was also unsuccessful. In this study,
we have prepared p-type SnO thin films using RF magnetron sputtering and then improved their
conductivity by Na ion implantation and annealing at 200°C. Na-doped SnO films with higher
p-type conductivity were obtained.
Experiment
SnO thin films were prepared using RF magnetron sputtering. A ceramic SnO target with a
diameter of 76 mm and a thickness of 3 mm was used as the sputtering source. SiO2 (100) substrates
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Functional and Functionally Structured Materials
were carefully cleaned ultrasonically in acetone, deionized water and alcohol, in that order, before
being clamped to a heater. The target-substrate distance was approximately 60 mm, and the
background pressure was 3.0×10−4 Pa. Ar gas was used as the working gas. Pre-sputtering was
conducted for 10 minutes to clean the target surface before each deposition step. The deposition rate
was ~ 4.5 Å/minute, and the film thickness was controlled by adjusting the deposition time. Each
sample was naturally cooled to room temperature after sputtering. Na+ implantation was carried out
on the SnO films with 3.887×1016/cm2 doses. After implantation, the films were annealed in a
vacuum at 200°C for 2 hours.
Na+ implantation was carried out by a 100-keV electromagnetic isotope separator. Electrical
measurements were carried out based on the Hall effect using van der Pauw geometry (Keithley
2400). The microstructure of each sample was characterized by X-ray diffraction (XRD) and X-ray
reflection (XRR) at beamline BL14B1 [10] of the Shanghai Synchrotron Radiation Facility (SSRF).
The thickness of the films was confirmed by XRR and step profiler measurements. The surface
morphologies of the samples were observed by atomic force microscopy (AFM) (Nanoscope IIIa).
Transmittance spectra were recorded via a UV-vis-IR spectrophotometer (Varian Cary 5000).
Results and Discussion
SiO2 (100) was used because of its low lattice mismatch with SnO films, i.e., -1.689% along the
[001] direction and -0.74% along the [110] direction of SnO. The growth conditions of the SnO thin
films were systemically studied in terms of the substrate temperature, working pressure and
sputtering power. When the temperature was between 150°C and 300°C and the Ar pressure was
between 0.5 Pa and 2.0 Pa, optimal p-type conductivity and repeatability were observed. The
implantation and annealing condition were also investigated, and the optimal dose and annealing
temperature were 3.887×1016/cm2 and 200°C, respectively.
Table 1 in Ref. [11] lists the results obtained from the Hall effect measurements of SnO thin
films grown on SiO2 (100) substrates at different deposition temperatures (room temperature, 100°C,
200°C, 300°C, and 400°C). The resistivity of those samples (A, B, C, D, E) were 93.09 Ω⋅cm, 47.52
Ω⋅cm, 8.10×10−1 Ω⋅cm, 8.89×10−4 Ω⋅cm, and 1.20×10−3 Ω⋅cm, respectively. Table 1 below lists the
Hall effect measurements results of SnO thin films after Na ion implantation and annealing. Most
SnO films showed rather stable electronic properties, and no measurable decay could be detected
over 3 months. The conductivity of the optimal film was improved compared with the results before
implantation (ρmin~0.81 Ω⋅cm).
Table 1 Electronic properties of SnO thin films after Na ion implantation and annealing
Resistivity
Carrier dens.
Mobility
Hall coef.
Sample Conditions
Type
−3
2 −1 −1
[Ω⋅cm]
[cm ]
[cm ⋅V ⋅S ]
[cm3⋅C−1]
A2
RT
23.0
2.05
53.21
p
1.32×1017
B2
10.72
2.13
36.77
p
100°C
2.72×1018
−1
19
−3
C2
2.53
p
200°C
6.20×10
1.09×10
3.30×10
−3
22
−4
D2
1.61
n
300°C,
1.85×10
1.41×10
2.53×10
E2
2.15
n
400°C
2.30×10−3
6.91×1021
5.20×10−3
Fig. 1(a) in Ref. [11] shows the XRD spectra of the SnO samples grown at different deposition
temperatures. All of the films grown under the optimal conditions were polycrystalline and showed
no other impurity phase, except for Sn and Sn3O4 in samples D and E and a small amount of Sn in
samples B and C. Fig. 1 below shows the XRD spectra of the SnO samples grown at different
deposition temperatures after implantation and annealing at 200°C.
Better crystallinity means fewer defects (including Sni donor defects), higher hole concentration
and mobility and thus better p-type conductivity. It can be seen that, after implantation and
annealing, the crystallinity of samples A2 and B2 became better compared with that of sample A and
B in Table 1 of Ref. [11], while that of samples C2, D2, and E2 became worse, compared with that of
Materials Science Forum Vol. 848
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sample C, D and E in Table 1 of Ref. [11]. The implantation inevitably damaged the structure of the
films, while the annealing repaired it. For samples A2 and B2 with nearly amorphous structures, the
effect of annealing exceeded that of implantation, while, for samples C2, D2 and E2, the effect was
reversed. For samples D2 and E2, the Sn contents increased while the Sn3O4 contents disappeared.
Due to the increase in the Sn content of the films, samples D2 and E2 showed metallic behavior,
with the resistivity values of 1.85×10−4 Ω⋅cm and 2.30×10−3 Ω⋅cm, respectively.
Fig. 1 XRD profiles of SnO thin films after Na ion implantation and annealing at 200°C.
After implantation and annealing, all the films showed no other impurity phase except for a
small amount of Sn, indicating that the doped Na ions mainly occupied the Sn sites in the lattice and
acted as effective acceptors. As a result, higher hole concentration was obtained, as shown in Table
1.
The lowest p-type resistivity was obtained from sample C2. Compared with sample C in Table 1
of Ref. [11], the hole concentration of sample C2 increased because of Na ion doping, while its
mobility decreased because the crystallinity became poorer after implantation. Therefore, the p-type
resistivity of sample C2 was not ideal as expected, just of the same order of magnitude as that of
sample C. To obtain better p-type conducting Na-doped SnO films, it is crucial to either control the
damage to the film structure during the ion implantation process or explore new Na doping
techniques.
Fig. 2 shows the surface morphologies of samples A, C and C2. The root mean square (RMS)
roughness values of samples A, C and C2 were approximately 1.07 nm, 1.86 nm and 1.1 nm,
whereas the mean grain sizes of the samples were approximately 15 nm, 30 nm, and 20 nm,
respectively, consistent with the XRD results. The optical transmittance was greater than 50% in the
visible range for all of the SnO thin films, as shown in Fig. 3. The optical transmittance was
reduced by approximately 10% after Na ion implantation.
Fig. 2 AFM morphologies of (a) sample A, (b) sample C and (c) sample C2
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Functional and Functionally Structured Materials
Fig. 3 Transmittance spectra of sample A, sample C and sample C2
Conclusions
The p-type transparent and conducting Na-doped SnO thin films were successfully fabricated by
RF magnetron sputtering and Na ion implantation. It was observed that the microstructure of the
films greatly affects the films’ conductivity. Na ion implantation increased the hole concentration
but decreased the mobility of SnO films. It is of great importance to control the damage to the film
structure during the ion implantation process to obtain better p-type conduction for SnO films.
Acknowledgements
The authors thank the staff of beamlines BL14B1 and BL08U1B of the Shanghai Synchrotron
Radiation Facility. This work was financially supported by the National Natural Science Foundation
of China (Grant Nos. 11405253 and U1332205), the Youth Innovation Promotion Association CAS
and the research grant (No.14DZ2261200) from the Science and Technology Commission of
Shanghai Municipality. This work was partly supported by the Open Research Program of Large
Scientific Facility of Chinese Academy of Sciences (Self-assembly Technology and Ultra-high
Density Nano-array Research Project).
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