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Optical and electrical properties of URT-IP ZnO thin films for photovoltaic devices
Thin Solid Films 451 – 452 (2004) 212–218
Optical and electrical properties of URT-IP ZnO thin films for photovoltaic devices S. Shirakataa,*, T. Sakemib, K. Awaic, T. Yamamotod a Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan Research and Development Center, Sumitomo Heavy Industries, Ltd., 5-2 Soubiraki-cho, Niihama, Ehime 792-8588, Japan c Sumijyu Technical Center Co., Ltd., 5-2 Soubiraki-cho, Niihama, Ehime 792-8588, Japan d Department of Electronic and Photonic System Engineering, Kochi University of Technology, Tosayamada, Kochi 782-8502, Japan b
Abstract Optical and electrical properties have been studied on thin polycrystalline ZnO films deposited on a glass substrate by Uramoto– Tanaka type ion plating (DC-arc ion-plating) method with relation to the oxygen flow rate (OFR) in the deposition chamber and to the Ga-doping concentration. For undoped films, an increase of OFR led to increase in the resistivity and decrease in the carrier concentration. At OFR of 10 sccm, both the Hall mobility and the near-band-edge photoluminescence (PL) intensity were the maximum, and the good-quality film was obtained at this OFR. Further increase of OFR led to the decrease in the Hall mobility and the increase in the intensities of defect-related PL bands. The Ga-doping reduces the resistivity by one order of magnitude. The Hall mobility and PL intensity were the maximum at OFR of 10 sccm, and the crystal quality is good at this OFR. ZnO:Ga films exhibited very broad near-band-edge PL, and the effect of the Ga-doping on PL is discussed. The optical transmittance was more than 90% invisible. The decrease in the optical transmittance and increase in the reflectance were observed for wavelength longer than 1200 nm for films with high carrier concentration of approximately 1021 cmy3. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Polycrystalline ZnO films; Ion plating; Photoluminescence; Hall effect; Optical transmittance
1. Introduction ZnO is promising as an inexpensive transparent conducting oxide with low resistivity and high optical transmittance. ZnO films with high optical transmittance and low resistivity can be deposited at the low substrate temperature of less than 200 K. Thus, ZnO is expected as an alternative for indium tin oxide. Also, it is noted that ZnO films are stable in hydrogen plasmas, and thus, they can be a window layer for thin-film solar cells based on Si and Cu(InGa)Se2. The main techniques utilized for the deposition of ZnO films for solar cells are the sputtering w1x and the metalorganic chemical vapor deposition w2x. However, in the sputtering, the high-energy particle bombardment creates defects at the interface of solar cells, causing the deterioration of the solar cell performance, especially for Cu(InGa)Se2 solar cells w3x. On the other hand, the DC-arc ion-plating method *Corresponding author. Tel.: q81-89-927-9772; fax: q81-89-9279789. E-mail address: [email protected] (S. Shirakata).
using a pressure-gradient plasma-gun w4x, named Uramoto–Tanaka type ion plating (URT-IP), is promising for the deposition of ZnO films for solar cells because of the high deposition rate, very small plasma damage and the large area deposition. So far, polycrystalline films of Si w5x and ITO w6,7x and ZnO:Ga w8,9x were deposited by the URT-IP method. However, almost nothing is known about properties of ZnO films deposited by this method. The authors reported the preliminary result on electrical and optical properties of URT-IP deposited ZnO films w10x. In this paper, optical and electrical properties of the ZnO polycrystalline films deposited on a glass substrate were studied in detail with relation to the film deposition condition (the oxygen flow rate: OFR) and the Ga-doping condition. Photoluminescence (PL), the optical transmittance and the optical reflectance were discussed with relation to electrical properties. 2. Experimental Polycrystalline ZnO films have been deposited on an alkali-free glass substrate at 200 8C using sintered ZnO
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.093
S. Shirakata et al. / Thin Solid Films 451 – 452 (2004) 212–218
source precursors by the URT-IP method, similar to the deposition of ITO w6x. Details of the deposition method will be published w8x. For the Ga-doping, source ZnO pellets were prepared by sintering of ZnO powder containing small amount of Ga2O3 (2–4 wt.%). The Ga-doping level is only nominal value. The oxygen gas was introduced into the deposition chamber in order to compensate the oxygen deficiencies. The OFR was varied as a deposition parameter. Films with thickness of 200 nm were studied. Hall effect measurements were carried out using the van der Pauw method. The In dot electrodes approximately 0.5 mm in diameter were formed at four corners of square-shaped samples. The magnetic field was 1.7 T. For PL measurements, the sample was photo-excited by a 325-nm line of a He–Cd laser. PL was dispersed by a double monochromator (SPEX, model 1680B) and detected by a photomultiplier (Hamamatsu, model R955). PL was filtered by the long-pass filter (Melles Gliot WG345) in order to reduce the stray light due to the excitation source, and thus, PL spectra are reliable for photon energy less than 3.6 eV. For optical transmittance and reflectance measurements, a double-beam spectrometer (Hitachi, model U4000) was used. The value of the transmittance used in this paper is that measured with a reference glass substrate. 3. Results and discussion 3.1. Undoped ZnO films The resistivity, the carrier concentration and the Hall mobility have been measured as a function of OFR. When OFR increases from 0 to 40 sccm, the resistivity increases from 4.2=10y3 to 9.6=10y1 V cm, and correspondingly, the carrier concentration changes from 1.0=1020 to 1.2=1018 cmy3 w10x. This may be due to the decrease of the oxygen vacancy acting as a donor. The Zn-vacancy, which is an acceptor, may compensate residual donors with increasing OFR. Fig. 1 shows the Hall mobility plotted as a function of OFR. The Hall mobility tends to have the maximum value of 28 cm2 Vy1 sy1 at OFR of 10 sccm. Further increase of OFR led to the decrease of the Hall mobility. Fig. 2 shows PL spectra at 298 and 77 K of undoped ZnO films for various OFR. In Fig. 3a and b, integrated PL intensity and the full-width at the half-maximum (FWHM) value of the near-band-edge PL at 298 K are plotted as a function of OFR, respectively. Integrated PL intensity tends to be the maximum at OFR of 10 sccm. It is noted that integrated PL intensity is clearly correlated to the Hall mobility in terms of OFR. The PL FWHM values are large (220–250 meV) for OFR of 0–5 sccm. For OFR more than 15 sccm, the FWHM
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Fig. 1. Hall mobility of undoped ZnO films plotted as a function of OFR.
value is small (150–160 meV). The FWHM value tends to be the minimum at OFR of 10 sccm. It can be seen in Fig. 2b that PL spectra at 77 K are strongly dependent on OFR. In addition to the nearband-edge PL, two PL peaks (A and B in Fig. 2) can be seen at OFR more than 20 sccm. At OFR of 10 sccm, the PL spectrum is dominated by the near-bandedge PL, and no other PL has been observed. Although the PL spectrum for the film deposited without oxygen flow (0 sccm) exhibited only the near-band-edge PL, PL intensity is weaker than that at OFR of 10 sccm by one order of magnitude. In addition, the near-band-edge PL peak is very broad. At high OFR more than 20 sccm, two PL peaks, A (3.2 eV) and B (2.2 eV), are observed. Their relative intensities increase as OFR increases. PLs of these films can be seen as an orange color in the human eye. The origins of these two peaks are unknown. The absence of the green emission suggests that films are not oxygen-deficient because the green emission is sometimes reported for ZnO deficient in oxygen. These PL peaks (A and B) may be related to defects created under excess oxygen partial pressure. In view of them, the crystal quality of ZnO films is strongly dependent on OFR, and films with low defect concentration are considered to have been grown at OFR of 10 sccm. Fig. 4 shows optical transmittance spectra of undoped ZnO films. The transmittance is more than 80% in the visible region for films deposited with OFR more than 10 sccm. For OFR of 0 and 5 sccm, the transmittance is poor and the absorption edge is clearly shifted to the short wavelength due to the Burstein–Moss shift because of the high carrier concentration of approximately 1020 cmy3. These results show that the undoped ZnO film deposited at OFR of 10 sccm is of good
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Fig. 2. PL spectra of undoped ZnO films at 298 and 77 K for various OFRs. Near-band-edge emission is denoted as NBE in the figure.
quality in view of the Hall mobility, PL properties and the optical transmittance. 3.2. Ga-doped ZnO films As a result of Ga doping (2–3 wt.%), the resistivity decreases and the carrier concentration increases by one order of magnitude compared with those of undoped ZnO films. These results indicate that the Ga impurity doped in the film acts as a donor occupying the substitutional Zn site. The detail of the OFR dependence of electrical properties will be published w10x. Thus, in this paper, we only summarize the results of the 3 and 4 wt.% Ga-doped ZnO films as follows. Electrical properties in 3 wt.% Ga-doped ZnO films are very similar to those in 4% Ga-doped samples. For Ga-doping concentrations of 3 and 4 wt.%, the resistivity is low (2.6=10y4 to 3.0=10y4 V cm) for OFR of 0– 15 sccm. The lowest resistivity is 2.6=10y4 V cm for 4 wt.% Ga-doped films at OFR of 10 sccm. The increase
in OFR from 15 to 30 sccm led to the rapid increase in the resistivity from approximately 3.0=10y4 V cm to approximately 1=10y2V cm. The carrier concentration decreases slowly from approximately 1=1021 cmy3 to approximately 7=1020 cmy3 with increase in OFR from 0 to 15 sccm. The further increase of OFR from 15 to 30 sccm led to the drastic decrease in the carrier concentration from approximately 7=1020 cmy3 to approximately 1=1019 cmy3. The decrease of the carrier concentration with OFR is considered to be due to the compensation of donors (substitutional Ga atoms at Zn site) with acceptors such as the Zn vacancy. In Fig. 5a and b, the Hall mobility and PL intensity of the near-band-edge emission at 298 K are plotted as a function of OFR, respectively. The Hall mobility tends to be the maximum at OFR of 10 sccm. Further increase of OFR led to the remarkable decrease of the Hall mobility, the result being similar to the case of undoped ZnO films. Similar to the Hall mobility, PL intensity depends on OFR and it tends to be the maximum at
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Fig. 3. Integrated PL intensity and FWHM value of the PL at 298 K plotted as a function of the OFR for undoped ZnO films.
OFR of 5–10 sccm, and PL intensity decreases suddenly with increasing OFR more than 15 sccm, as can be seen in Fig. 5b. Also, the increase in the Ga-doping level causes the decrease in PL intensity. Such decreases in
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PL intensity may be due to the increase in the concentration of non-radiative centers, which is introduced into films as a result of both Ga-doping and excess oxygen partial pressure. These results clearly show that there is the optimum OFR (10 sccm) in view of the crystal quality. Fig. 6 shows PL spectra at Ga-doped ZnO films (3 wt.%) for various OFR. PL spectra of ZnO:Ga exhibited only one near-band-edge emission. No other PL was found. This may be due to the screening of defects with high concentration of electrons ()1020 cmy3). As a result of the Ga-doping, PL spectra become very broad and the peak energy shifts to the high energy side compared with undoped films, as can be seen in Fig. 6a. The FWHM value of the near-band-edge emission is very large ()500 meV). The PL peak has the characteristic low energy tail. The emission higher than the band-gap energy may be due to the optical transition from the degenerated conduction band to the valence band. In this case, electrons in the conduction band with energy up to the Fermi level can recombine with holes in the valence band because of the breakdown of the k selection rule, which is caused by the random distribution of the ionized impurities w11x. The characteristic low energy tail of the PL peak may be due to the transition related to the tail state of the band that is perturbed by the strain field in the presence of the large amount of Ga impurities. It is noted that low temperature (77 K) PL spectra are very similar to those at room temperature (298 K), as can be seen in Fig. 6b. For films with large carrier concentration (close to 1=1021 cmy3) grown at OFR of 0 and 10 sccm, PL peak energy at 77 K is slightly lower than that at 298 K. This energy shift does not stand for temperature dependence of the band-gap, but it may reflect the thermal distribution of
Fig. 4. Transmittance spectra of undoped ZnO films for various OFRs.
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Fig. 5. Hall mobility and PL intensity plotted as a function of the OFR for ZnO:Ga films.
Fig. 6. PL spectra of ZnO:Ga films for various OFRs, (a) PL spectra of ZnO:Ga films at 298 K are compared with those for the undoped films and (b) PL spectra of ZnO:Ga films at 77 and 298 K.
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Fig. 7. Transmittance and reflectance spectra for ZnO:Ga films for various OFRs.
the carrier and the enhancement of the defect-related luminescence at the low energy tail of the PL peak at low temperature. Fig. 7a and b show optical transmittance and reflectance spectra of ZnO:Ga films, respectively. It is noted that the transmittance is almost more than 90% for 400– 1000 nm for all films. For the wavelength longer than 1200 nm, the transmittance of ZnO:Ga films decreases due to the plasma absorption especially for films with high carrier concentration close to approximately 1=1021 cmy3 (OFR: 0–15 sccm). The infrared reflectivity (wavelength longer than 1200 nm) is high for the corresponding films as can be seen in Fig. 7b. For the use of ZnO:Ga films as a window layer of the CIGS solar cells, the carrier concentration should be decreased in order to improve the infrared transmittance without increasing the resistivity. Thus, the electron mobility should be improved by the further optimization of film deposition conditions. 4. Summary PL, optical transmittance, optical reflectance and Hall effect measurements were carried out on thin polycrystalline ZnO films deposited on a glass substrate by URT-
IP method. For undoped films, an increase of OFR led to increase in the resistivity and decrease in the carrier concentration due to the decrease of oxygen vacancies. At OFR of 10 sccm, both the Hall mobility and PL intensity were maximum. The increase of OFR more than 20 sccm led to the deterioration of the crystal quality of films as can be seen as the decrease of the Hall mobility and the enhancement of defect-related PL. Thus, the good quality film was obtained at OFR of approximately 10 sccm. The Ga-doping reduced the resistivity by one order of magnitude due to the activation of Ga donors. For ZnO:Ga films, the crystal quality was good at OFR of 10 sccm. ZnO:Ga films with high Ga-doping level exhibited very broad near-band-edge PL. The optical transmittance was more than 90% for visible. The decrease of the optical transmittance was observed for near the infrared region for films with high carrier concentration. This study showed the importance of the control of oxygen partial pressure for the deposition of ZnO films, and basic film properties in terms of the OFR are clarified. Good quality undoped and Ga-doped ZnO films were found to be grown. Application of the URTIP deposited ZnO film to the Cu(InGa)Se2 solar cell is
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in progress, and properties of ZnO films studied in this work are expected to be reflected as solar cell parameters. Acknowledgments The authors would like to thank A. Miyata and S. Hamamoto for technical assistance, Dr T. Terasako, Dr K. Iwata, Dr S. Niki, Prof. K. Yoshino and Prof. T. Ikari for discussion. This research was sponsored the Shikoku consortium grant from the Ministry of Economy, Trade and Industry of Japan. References w1x T. Minami, H. Nanto, S. Tanaka, Jap. J. Appl. Phys. 24 (1984) L605.
w2x W.W. Wenas, A. Yamada, M. Konagai, K. Takahashi, Jap. J. Appl. Phys. 30 (1991) L441. w3x B. Sang, K. Kushiya, D. O’Kumura, O. Yanase, Sol. Energy Mater. Sol. Cells 67 (2001) 237. w4x J. Uramoto, J. Vacuum Soc. Jpn. 25 (1982) 781, in Japanese. w5x M. Yoshira, T. Saida, S. Okada, M. Akamatsu, K. Kondo, Thin Solid Films 335 (1998) 127. w6x M. Tanaka, H. Makino, R. Chikugo, T. Sakemi, K. Awai, J. Vacuum Soc. Jpn. 44 (2001) 435, in Japanese. w7x Y. Suzuki, F. Niino, K. Katoh, J. Non-Cryst. Solids 218 (1997) 30. w8x K. Iwata, H. Sakemi, A. Yamada, P. Fons, K. Awai, T. Yamamoto, K. Matsubara, S. Niki, Thin Solid Films, in press. w9x H. Hasegawa, M. Yoshida, S. Nakamura, Y. Suzuki, S. Okada, K. Kondo, Sol. Energ. Mat. Sol. Cells 67 (2001) 231. w10x S. Shirakata, H. Sakemi, K. Awai, T. Yamamoto, Thin Solid Films, in press. w11x D. Olego, M. Cardona, Phys. Rev. B 22 (1980) 886.
Optical and electrical properties of URT-IP ZnO thin films for photovoltaic devices S. Shirakataa,*, T. Sakemib, K. Awaic, T. Yamamotod a Faculty of Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan Research and Development Center, Sumitomo Heavy Industries, Ltd., 5-2 Soubiraki-cho, Niihama, Ehime 792-8588, Japan c Sumijyu Technical Center Co., Ltd., 5-2 Soubiraki-cho, Niihama, Ehime 792-8588, Japan d Department of Electronic and Photonic System Engineering, Kochi University of Technology, Tosayamada, Kochi 782-8502, Japan b
Abstract Optical and electrical properties have been studied on thin polycrystalline ZnO films deposited on a glass substrate by Uramoto– Tanaka type ion plating (DC-arc ion-plating) method with relation to the oxygen flow rate (OFR) in the deposition chamber and to the Ga-doping concentration. For undoped films, an increase of OFR led to increase in the resistivity and decrease in the carrier concentration. At OFR of 10 sccm, both the Hall mobility and the near-band-edge photoluminescence (PL) intensity were the maximum, and the good-quality film was obtained at this OFR. Further increase of OFR led to the decrease in the Hall mobility and the increase in the intensities of defect-related PL bands. The Ga-doping reduces the resistivity by one order of magnitude. The Hall mobility and PL intensity were the maximum at OFR of 10 sccm, and the crystal quality is good at this OFR. ZnO:Ga films exhibited very broad near-band-edge PL, and the effect of the Ga-doping on PL is discussed. The optical transmittance was more than 90% invisible. The decrease in the optical transmittance and increase in the reflectance were observed for wavelength longer than 1200 nm for films with high carrier concentration of approximately 1021 cmy3. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Polycrystalline ZnO films; Ion plating; Photoluminescence; Hall effect; Optical transmittance
1. Introduction ZnO is promising as an inexpensive transparent conducting oxide with low resistivity and high optical transmittance. ZnO films with high optical transmittance and low resistivity can be deposited at the low substrate temperature of less than 200 K. Thus, ZnO is expected as an alternative for indium tin oxide. Also, it is noted that ZnO films are stable in hydrogen plasmas, and thus, they can be a window layer for thin-film solar cells based on Si and Cu(InGa)Se2. The main techniques utilized for the deposition of ZnO films for solar cells are the sputtering w1x and the metalorganic chemical vapor deposition w2x. However, in the sputtering, the high-energy particle bombardment creates defects at the interface of solar cells, causing the deterioration of the solar cell performance, especially for Cu(InGa)Se2 solar cells w3x. On the other hand, the DC-arc ion-plating method *Corresponding author. Tel.: q81-89-927-9772; fax: q81-89-9279789. E-mail address: [email protected] (S. Shirakata).
using a pressure-gradient plasma-gun w4x, named Uramoto–Tanaka type ion plating (URT-IP), is promising for the deposition of ZnO films for solar cells because of the high deposition rate, very small plasma damage and the large area deposition. So far, polycrystalline films of Si w5x and ITO w6,7x and ZnO:Ga w8,9x were deposited by the URT-IP method. However, almost nothing is known about properties of ZnO films deposited by this method. The authors reported the preliminary result on electrical and optical properties of URT-IP deposited ZnO films w10x. In this paper, optical and electrical properties of the ZnO polycrystalline films deposited on a glass substrate were studied in detail with relation to the film deposition condition (the oxygen flow rate: OFR) and the Ga-doping condition. Photoluminescence (PL), the optical transmittance and the optical reflectance were discussed with relation to electrical properties. 2. Experimental Polycrystalline ZnO films have been deposited on an alkali-free glass substrate at 200 8C using sintered ZnO
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.093
S. Shirakata et al. / Thin Solid Films 451 – 452 (2004) 212–218
source precursors by the URT-IP method, similar to the deposition of ITO w6x. Details of the deposition method will be published w8x. For the Ga-doping, source ZnO pellets were prepared by sintering of ZnO powder containing small amount of Ga2O3 (2–4 wt.%). The Ga-doping level is only nominal value. The oxygen gas was introduced into the deposition chamber in order to compensate the oxygen deficiencies. The OFR was varied as a deposition parameter. Films with thickness of 200 nm were studied. Hall effect measurements were carried out using the van der Pauw method. The In dot electrodes approximately 0.5 mm in diameter were formed at four corners of square-shaped samples. The magnetic field was 1.7 T. For PL measurements, the sample was photo-excited by a 325-nm line of a He–Cd laser. PL was dispersed by a double monochromator (SPEX, model 1680B) and detected by a photomultiplier (Hamamatsu, model R955). PL was filtered by the long-pass filter (Melles Gliot WG345) in order to reduce the stray light due to the excitation source, and thus, PL spectra are reliable for photon energy less than 3.6 eV. For optical transmittance and reflectance measurements, a double-beam spectrometer (Hitachi, model U4000) was used. The value of the transmittance used in this paper is that measured with a reference glass substrate. 3. Results and discussion 3.1. Undoped ZnO films The resistivity, the carrier concentration and the Hall mobility have been measured as a function of OFR. When OFR increases from 0 to 40 sccm, the resistivity increases from 4.2=10y3 to 9.6=10y1 V cm, and correspondingly, the carrier concentration changes from 1.0=1020 to 1.2=1018 cmy3 w10x. This may be due to the decrease of the oxygen vacancy acting as a donor. The Zn-vacancy, which is an acceptor, may compensate residual donors with increasing OFR. Fig. 1 shows the Hall mobility plotted as a function of OFR. The Hall mobility tends to have the maximum value of 28 cm2 Vy1 sy1 at OFR of 10 sccm. Further increase of OFR led to the decrease of the Hall mobility. Fig. 2 shows PL spectra at 298 and 77 K of undoped ZnO films for various OFR. In Fig. 3a and b, integrated PL intensity and the full-width at the half-maximum (FWHM) value of the near-band-edge PL at 298 K are plotted as a function of OFR, respectively. Integrated PL intensity tends to be the maximum at OFR of 10 sccm. It is noted that integrated PL intensity is clearly correlated to the Hall mobility in terms of OFR. The PL FWHM values are large (220–250 meV) for OFR of 0–5 sccm. For OFR more than 15 sccm, the FWHM
213
Fig. 1. Hall mobility of undoped ZnO films plotted as a function of OFR.
value is small (150–160 meV). The FWHM value tends to be the minimum at OFR of 10 sccm. It can be seen in Fig. 2b that PL spectra at 77 K are strongly dependent on OFR. In addition to the nearband-edge PL, two PL peaks (A and B in Fig. 2) can be seen at OFR more than 20 sccm. At OFR of 10 sccm, the PL spectrum is dominated by the near-bandedge PL, and no other PL has been observed. Although the PL spectrum for the film deposited without oxygen flow (0 sccm) exhibited only the near-band-edge PL, PL intensity is weaker than that at OFR of 10 sccm by one order of magnitude. In addition, the near-band-edge PL peak is very broad. At high OFR more than 20 sccm, two PL peaks, A (3.2 eV) and B (2.2 eV), are observed. Their relative intensities increase as OFR increases. PLs of these films can be seen as an orange color in the human eye. The origins of these two peaks are unknown. The absence of the green emission suggests that films are not oxygen-deficient because the green emission is sometimes reported for ZnO deficient in oxygen. These PL peaks (A and B) may be related to defects created under excess oxygen partial pressure. In view of them, the crystal quality of ZnO films is strongly dependent on OFR, and films with low defect concentration are considered to have been grown at OFR of 10 sccm. Fig. 4 shows optical transmittance spectra of undoped ZnO films. The transmittance is more than 80% in the visible region for films deposited with OFR more than 10 sccm. For OFR of 0 and 5 sccm, the transmittance is poor and the absorption edge is clearly shifted to the short wavelength due to the Burstein–Moss shift because of the high carrier concentration of approximately 1020 cmy3. These results show that the undoped ZnO film deposited at OFR of 10 sccm is of good
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214
Fig. 2. PL spectra of undoped ZnO films at 298 and 77 K for various OFRs. Near-band-edge emission is denoted as NBE in the figure.
quality in view of the Hall mobility, PL properties and the optical transmittance. 3.2. Ga-doped ZnO films As a result of Ga doping (2–3 wt.%), the resistivity decreases and the carrier concentration increases by one order of magnitude compared with those of undoped ZnO films. These results indicate that the Ga impurity doped in the film acts as a donor occupying the substitutional Zn site. The detail of the OFR dependence of electrical properties will be published w10x. Thus, in this paper, we only summarize the results of the 3 and 4 wt.% Ga-doped ZnO films as follows. Electrical properties in 3 wt.% Ga-doped ZnO films are very similar to those in 4% Ga-doped samples. For Ga-doping concentrations of 3 and 4 wt.%, the resistivity is low (2.6=10y4 to 3.0=10y4 V cm) for OFR of 0– 15 sccm. The lowest resistivity is 2.6=10y4 V cm for 4 wt.% Ga-doped films at OFR of 10 sccm. The increase
in OFR from 15 to 30 sccm led to the rapid increase in the resistivity from approximately 3.0=10y4 V cm to approximately 1=10y2V cm. The carrier concentration decreases slowly from approximately 1=1021 cmy3 to approximately 7=1020 cmy3 with increase in OFR from 0 to 15 sccm. The further increase of OFR from 15 to 30 sccm led to the drastic decrease in the carrier concentration from approximately 7=1020 cmy3 to approximately 1=1019 cmy3. The decrease of the carrier concentration with OFR is considered to be due to the compensation of donors (substitutional Ga atoms at Zn site) with acceptors such as the Zn vacancy. In Fig. 5a and b, the Hall mobility and PL intensity of the near-band-edge emission at 298 K are plotted as a function of OFR, respectively. The Hall mobility tends to be the maximum at OFR of 10 sccm. Further increase of OFR led to the remarkable decrease of the Hall mobility, the result being similar to the case of undoped ZnO films. Similar to the Hall mobility, PL intensity depends on OFR and it tends to be the maximum at
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Fig. 3. Integrated PL intensity and FWHM value of the PL at 298 K plotted as a function of the OFR for undoped ZnO films.
OFR of 5–10 sccm, and PL intensity decreases suddenly with increasing OFR more than 15 sccm, as can be seen in Fig. 5b. Also, the increase in the Ga-doping level causes the decrease in PL intensity. Such decreases in
215
PL intensity may be due to the increase in the concentration of non-radiative centers, which is introduced into films as a result of both Ga-doping and excess oxygen partial pressure. These results clearly show that there is the optimum OFR (10 sccm) in view of the crystal quality. Fig. 6 shows PL spectra at Ga-doped ZnO films (3 wt.%) for various OFR. PL spectra of ZnO:Ga exhibited only one near-band-edge emission. No other PL was found. This may be due to the screening of defects with high concentration of electrons ()1020 cmy3). As a result of the Ga-doping, PL spectra become very broad and the peak energy shifts to the high energy side compared with undoped films, as can be seen in Fig. 6a. The FWHM value of the near-band-edge emission is very large ()500 meV). The PL peak has the characteristic low energy tail. The emission higher than the band-gap energy may be due to the optical transition from the degenerated conduction band to the valence band. In this case, electrons in the conduction band with energy up to the Fermi level can recombine with holes in the valence band because of the breakdown of the k selection rule, which is caused by the random distribution of the ionized impurities w11x. The characteristic low energy tail of the PL peak may be due to the transition related to the tail state of the band that is perturbed by the strain field in the presence of the large amount of Ga impurities. It is noted that low temperature (77 K) PL spectra are very similar to those at room temperature (298 K), as can be seen in Fig. 6b. For films with large carrier concentration (close to 1=1021 cmy3) grown at OFR of 0 and 10 sccm, PL peak energy at 77 K is slightly lower than that at 298 K. This energy shift does not stand for temperature dependence of the band-gap, but it may reflect the thermal distribution of
Fig. 4. Transmittance spectra of undoped ZnO films for various OFRs.
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Fig. 5. Hall mobility and PL intensity plotted as a function of the OFR for ZnO:Ga films.
Fig. 6. PL spectra of ZnO:Ga films for various OFRs, (a) PL spectra of ZnO:Ga films at 298 K are compared with those for the undoped films and (b) PL spectra of ZnO:Ga films at 77 and 298 K.
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Fig. 7. Transmittance and reflectance spectra for ZnO:Ga films for various OFRs.
the carrier and the enhancement of the defect-related luminescence at the low energy tail of the PL peak at low temperature. Fig. 7a and b show optical transmittance and reflectance spectra of ZnO:Ga films, respectively. It is noted that the transmittance is almost more than 90% for 400– 1000 nm for all films. For the wavelength longer than 1200 nm, the transmittance of ZnO:Ga films decreases due to the plasma absorption especially for films with high carrier concentration close to approximately 1=1021 cmy3 (OFR: 0–15 sccm). The infrared reflectivity (wavelength longer than 1200 nm) is high for the corresponding films as can be seen in Fig. 7b. For the use of ZnO:Ga films as a window layer of the CIGS solar cells, the carrier concentration should be decreased in order to improve the infrared transmittance without increasing the resistivity. Thus, the electron mobility should be improved by the further optimization of film deposition conditions. 4. Summary PL, optical transmittance, optical reflectance and Hall effect measurements were carried out on thin polycrystalline ZnO films deposited on a glass substrate by URT-
IP method. For undoped films, an increase of OFR led to increase in the resistivity and decrease in the carrier concentration due to the decrease of oxygen vacancies. At OFR of 10 sccm, both the Hall mobility and PL intensity were maximum. The increase of OFR more than 20 sccm led to the deterioration of the crystal quality of films as can be seen as the decrease of the Hall mobility and the enhancement of defect-related PL. Thus, the good quality film was obtained at OFR of approximately 10 sccm. The Ga-doping reduced the resistivity by one order of magnitude due to the activation of Ga donors. For ZnO:Ga films, the crystal quality was good at OFR of 10 sccm. ZnO:Ga films with high Ga-doping level exhibited very broad near-band-edge PL. The optical transmittance was more than 90% for visible. The decrease of the optical transmittance was observed for near the infrared region for films with high carrier concentration. This study showed the importance of the control of oxygen partial pressure for the deposition of ZnO films, and basic film properties in terms of the OFR are clarified. Good quality undoped and Ga-doped ZnO films were found to be grown. Application of the URTIP deposited ZnO film to the Cu(InGa)Se2 solar cell is
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