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Microstructures, electrical and optical characteristics of ZnO thin films by oxygen plasma-assisted pulsed laser deposition
ARTICLE IN PRESS
Journal of Crystal Growth 305 (2007) 36–39 www.elsevier.com/locate/jcrysgro
Microstructures, electrical and optical characteristics of ZnO thin films by oxygen plasma-assisted pulsed laser deposition Yanfei Gua,b, Xiaomin Lia,, Weidong Yua, Xiangdong Gaoa, Junliang Zhaoa,b, Chang Yanga,b a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 12 March 2007; accepted 30 March 2007 Communicated by D.P. Norton Available online 10 April 2007
Abstract In order to decrease the free-electron concentration and increase the crystalline quality, zinc oxide (ZnO) thin films were deposited on sapphire (0 0 0 1) substrates by oxygen plasma-assisted pulsed laser deposition (PLD). ZnO films showed higher oxygen composition, stronger diffraction intensity of the (0 0 0 2) direction, and larger grain size with regular hexagonal grain shape. The free-electron concentration was decreased greatly from 1019 to 1014 cm3 and the Hall mobility was increased from 6.8 to 37 cm2 V1 s1. Furthermore, the intensity of the resonant Raman scattering and ultraviolet photoluminescence emission was increased. This enhancement of the crystalline, electrical and optical quality would be attributed to the increase of high activity oxygen density introduced by the plasma oxygen source. r 2007 Published by Elsevier B.V. PACS: 81.15.Fg; 81.05.Dz; 68.55.a; 72.20.i; 87.64.Je; 78.55.m Keywords: A1. Crystal structure; A1. Electrical properties; A1. Photoluminescence; A1. Raman scattering; A3. Pulsed laser deposition; B1. Zinc oxide
1. Introduction In view of the demand for fabrication of high-quality optoelectronic devices such as light-emitting diodes [1–3] or ultraviolet detectors [4], zinc oxide (ZnO) has recently attracted widespread research interest because of its exceptional optical and electronic properties [5,6], including direct wide band gap of 3.37 eV and a high exciton bonding energy of 60 meV at room temperature. A variety of methods were employed to grow ZnO thin films, such as RF sputtering [7], chemical-vapor deposition [8], molecular-beam epitaxy [9], and pulsed laser deposition (PLD) [10]. Among these techniques, PLD is widely used for its simplicity and experimental flexibility.
Corresponding author. Tel.: +86 21 52412554; fax: +86 21 52413122.
E-mail address: [email protected] (X. Li). 0022-0248/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2007.03.050
ZnO films grown by these low oxygen pressure thin film deposition methods show low resistivity and large freeelectron concentration. Reducing the background carrier (free electron) concentration in ZnO films is one of the major challenges ahead of realizing high-performance ZnO-based optoelectronic devices [10]. Furthermore, the compensation effect by a large background electron concentration makes the p-type doping of ZnO too difficult [11], which is the bottleneck for the application of ZnO. It is commonly accepted that free electrons in ZnO are generated due to intrinsic defects such as oxygen vacancies (VO) or zinc interstitials (Zni) [12]. Thus, free-electron concentration in ZnO can be deduced by removing the VO or Zni defects at higher oxygen pressure. Unfortunately, in higher oxygen pressure, the ejected species by laser ablation from the target undergo much more collisions with the high density of oxygen molecules. The kinetic energy of the species is decreased greatly, resulting in the
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poor crystalline quality, low electron mobility and poor optical properties [10]. In this article, we deposited ZnO thin films by PLD employing an oxygen plasma source to decrease the background free-electron concentration greatly and enhance the crystalline quality. The composition, structural, electrical and optical properties of ZnO thin films were also described. 2. Experiments ZnO thin films were deposited on sapphire (0 0 0 1) substrates by a PLD system employing a plasma oxygen source (ZnOplasma) and gaseous oxygen source (ZnOgaseous), respectively. The O plasma source was introduced by a plasma generator (KYKY Technology Development Ltd., Shenyang, China), with the working voltage of 400 V, and the current of 35 mV. A ceramic ZnO (99.99% purity) was used as target and an KrF excimer laser (Lambda Physik COMpex, wavelength of 248 nm, energy of 200 mJ/pulse, repetition rate of 5 Hz) was applied to ablate the target. The substrate temperature was fixed to be 750 1C. The vacuum chamber was initially evacuated to a pressure of 1 104 Pa. Then a pure O2 gas was introduced into the chamber (the flow rate was 20 sccm) and the pressure during deposition was maintained at 1 Pa. The compositions of obtained ZnO films were characterized by X-ray photoelectron spectroscopy (XPS) (Microlab 310F, Al Ka source). The crystallinity was investigated by X-ray diffraction (XRD) (D/MAX-2550 V Cu Ka). The microstructure and surface morphology were analyzed by field emission scanning electron microscopy (FESEM, JSM-6700F). Electrical properties were measured at room temperature by Hall measurements with the Van der Pauw configuration. The optical properties were characterized by room-temperature photoluminescence (PL) and Raman scattering spectra excited by the He–Cd laser with the wavelength of 325 nm, and measured by a Jobin Yvon LabRAM HR 800UV system. 3. Results and discussion 3.1. Microstructure Atomic concentrations of zinc and oxygen were analyzed by XPS. For the ZnOgaseous film, atomic concentration of Zn was found to be 55.501% and that of O was 44.499% and in ZnOplasma film atomic concentration of Zn was found to be 44.472% and that of O was 55.528%. It is well known that the oxygen is insufficient in the ZnO deposited by conventional PLD method. With the assistant of O plasma, the density of active O ions in plume was increased evidently which have higher binding energy with Zn element, resulting in the increasing of the atomic concentration of O. The thicknesses of the deposited films were all about 250 nm. Fig. 1 shows XRD patterns of ZnOplasma and
Fig. 1. XRD spectra of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
ZnOgaseous films. Only (0 0 0 2) diffraction peaks of ZnO were detected, showing that both films are of hexagonal wurtzite structure and with their c-axis perpendicular to the substrate surface. The diffraction intensity of the (0 0 0 2) direction in ZnOplasma film was 30 times greater than ZnOgaseous film. The full-width at half-maximum (FWHM) of the (0 0 0 2) diffraction peak for ZnOplasma and ZnOgaseous film was 0.201 and 0.281, respectively. The stronger diffraction intensity and smaller FWHM for ZnOplasma film indicates the higher crystal quality. The microstructure and surface morphology of the ZnO films were investigated by FESEM. Fig. 2 shows the SEM images of ZnOplasma and ZnOgaseous film. For the ZnOgaseous film (Fig. 2a), the crystal grains were irregular in shape, and the grains size was around 100 nm. In contrast, the ZnOplasma film was dominated by a typical ‘‘honeycomb’’ like structure with well-faced regular hexagonal grains (Fig. 2b). The grain size was about 200–300 nm. Regular hexagonal grain shape and larger grain size for ZnOplasma film indicate that the O plasma significantly improves the microstructure quality of the PLD grown ZnO film, which is consistent with the XRD analysis. In the traditional PLD process, the ejected species from the target collide with the O2 molecules greatly at higher oxygen pressure. The kinetic energy of the species is decreased strongly, resulting in the poor crystalline quality. The high-energy O ions generated by the O plasma increase the species energy and enhance the mobility on the substrate surface, and benefit the grain growth of ZnO thin film. 3.2. Electrical characteristics Table 1 shows the electrical properties such as surface resistivity rs, carrier concentration N, and the Hall mobility m of ZnOgaseous and ZnOplasma films, respectively. Both films
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Fig. 3. Raman spectra of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
Fig. 2. FESEM morphologies of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
Table 1 Electrical properties of ZnO films deposited by PLD with plasma oxygen and gaseous oxygen Sample
Gaseous oxygen Plasma oxygen
N (cm3)
rs (O/&)
1.759 10
4
2.549 108
1.731 10
m (cm2 V1 s1) 19
3.353 1014
6.8 37
Conductivity type n n
were n-type conductivity. The ZnOgaseous film showed low resistivity with the surface resistivity rs of 1.759 104 O/&, and the electron concentration was as high as 1.731 1019 cm3. With the assistance of O plasma, the ZnOplasma film showed high resistivity with the surface resistivity rs of 2.549 108 O/&, and the electron concentration was decreased drastically to 3.353 1014 cm3 (which is the lowest value for undoped ZnO film to our knowledge). The free electron in unintentionally doped ZnO is related to the intrinsic defects such as VO or Zni caused by oxygen deficiency. From the composition analysis, the oxygen deficiency in ZnO film is inhibited by the high density of active oxygen ions in the plume due to the O plasma. As a result, the concentration of VO or Zni decreases greatly, leading to the lower electron concentration. The Hall mobility was 6.8 cm2 V1 s1 for ZnOgaseous film, and 37 cm2 V1 s1 for ZnOplasma film. The electron
transport properties of the semiconductor are influenced by the impurities scattering, electronic correlation mechanisms, and scattering from defects. In these undoped ZnO thin film, the impurities scattering can be ignored. The increase of the mobility for ZnOplasma film is contributed by the decrease of the electron concentration, and the decrease of defects density such as intrinsic point defects and grain boundaries due to the enhancement of the crystal quality. 3.3. Optical characteristics To investigate the optical characteristics of the ZnO thin films, Raman scattering measurements were performed using a 325 nm He–Cd laser as the excitation source. The room-temperature resonant Raman spectra are shown in Fig. 3. In our measurement range, from 200 to 1400 cm1, multiphonon (2LO) signals were observed. The first-order LO phonon peaks are centered around 570 and 574 cm1 for ZnOgaseous film and ZnOplasma film, respectively. These peaks were assigned to the A1(LO) phonon mode of ZnO thin film [13]. Mode A1(LO) of ZnOgaseous film had a redshift to low-frequency direction. Kumar et al. [14] proposed several reasons to explain this redshift in nanocrystal ZnO: (1) the optical phonon confinement, (2) the local heating of nanocrystallites, and (3) the presence of impurity defects. The average sizes of the ZnO films were 100–300 nm, which is markedly greater than and is not comparable to the exciton Bohr radius of ZnO (2.34 nm) [15]. So, the former two reasons may not play the key role on the redshift phenomenon. In addition, there are no impurities present in our undoped ZnO film, confirmed by the XPS measurement. Furthermore, the oxygen in ZnOgaseous film is deficient markedly. Thus, we believe that the oxygen vacancies defects due to the oxygen deficiency may play the role to induce the mode A1(LO) redshift of ZnOgaseous film, similarly to the impurity defects.
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composition in ZnO film was increased, resulted in drastic decrease of the free-electron concentration from 1019 to 1014 cm3. With the assistance of oxygen plasma, the ZnO film showed regular hexagonal shape grain with larger size, higher Hall mobility, stronger resonant Raman scattering and UV NBE emission, indicated the higher crystal quality. This enhancement of the crystalline, electrical and optical quality would be attributed to the increase of high activity oxygen density introduced by the plasma oxygen source. Acknowledgments This work was supported by National Nature Science Foundation of China (90401010), and Project of ShanghaiAM Materials Research and Development Fund (0516). Fig. 4. Photoluminescence spectra of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
Additionally, the intensity of mode A1(LO) for ZnOplasma film is increased, which may be related to the higher crystal quality. Room-temperature PL spectra were shown in Fig. 4. The ZnOgaseous film shows two luminescent bands: one is the sharp near-band-edge (NBE) emission in the ultraviolet (UV) region centered at 375 nm (FWHM ¼ 12.74 nm), which is attributed to the free exciton recombination [16], and the other was the broad deep-level (DL) emission band centered at 511 nm. For ZnOplasma film, there is no DL emission can be detected in the PL spectrum, and only strong NBE emission (FWHM ¼ 9.44 nm) appeared. The origin of the DL emission band is ascribed to VO or Zni defects [17]. The disappearance of the DL emission for ZnOplasma film indicates that the VO or Zni may be inhibited effectively by the O plasma. Compare to the ZnOgaseous film, the ZnOplasma film has greater NBE intensity with small FWHM value. Structure defects, such as dislocations and grain boundaries can trap photogenerated carriers into a nonradiative recombination process before the NBE recombination occurs. This nonradiative relaxation process decreases the emission intensity [18]. The remarkably increase of the intensity of NEB emission shows the ZnOplasma film has higher optical quality due to its higher crystal quality. 4. Conclusion ZnO thin films were deposited by oxygen plasma-assisted PLD method on sapphire (0 0 0 1) substrates. The oxygen
References [1] R.F. Service, Science 276 (1997) 895. [2] J.-H. Lim, C.-K. Kang, K.-K. Kim, I.-K. Park, D.-K. Hwang, S.-J. Park, Adv. Mater. 18 (2006) 2720. [3] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2005) 42. [4] C.-Y. Lu, S.-J. Chang, S.-P. Chang, C.-T. Lee, C.-F. Kuo, H.-M. Chang, Y.-Z. Chiou, C.-L. Hsu, I.-C. Chen, Appl. Phys. Lett. 89 (2006) 153101. [5] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293. [6] D.C. Look, B. Claflin, Phys. Stat. Sol. 241 (2004) 624. [7] M.-L. Tu, Y.-K. Su, C.-Y. Ma, J. Appl. Phys. 100 (2006) 053705. [8] B. Theys, V. Sallet, F. Jomard, A. Lusson, J.-F. Rommelue`re, Z. Teukam, J. Appl. Phys. 91 (2002) 3922. [9] M. Sano, K. Miyamoto, H. Kato, T. Yao, J. Appl. Phys. 95 (2004) 5527. [10] S. Choopun, R.D. Vispute, W. Noch, A. Balsamo, R.P. Sharma, T. Venkatesan, A. Iliadis, D.C. Look, Appl. Phys. Lett. 75 (1999) 3947. [11] K. Minegishi, Y. Koiwai, Y. Kikuchi, K. Yano, M. Kasuga, A. Shimizu, Jpn. J. Appl. Phys. 36 (1997) L1453. [12] D.C. Look, J.W. Hemsky, J.R. Sizelove, Phys. Rev. Lett. 82 (1999) 2552. [13] N. Ashkenov, B.N. Mbenkum, C. Bundesmann, V. Riede, M. Lorenz, D. Spemann, E.M. Kaidashev, A. Kasic, M. Schubert, M. Grundmann, G. Wagner, H. Neumann, V. Darakchieva, H. Arwin, B. Monemar, J. Appl. Phys. 93 (2003) 126. [14] B. Kumar, H. Gong, S.Y. Chow, S. Tripathy, Y. Hua, Appl. Phys. Lett. 89 (2006) 071922. [15] R.T. Senger, K.K. Bajaj, Phys. Rev. B 68 (2003) 045313. [16] D.C. Look, G.C. Farlow, P. Reunchan, S. Limpijumnong, S.B. Zhang, K. Nordlund, Phys. Rev. Lett. 95 (2005) 225502. [17] Z.Q. Chen, S. Yamamoto, M. Maekawa, A. Kawasuso, X.L. Yuan, T. Sekiguch, J. Appl. Phys. 94 (2003) 4807. [18] T. Matsumoto, H. Kato, K. Miyamoto, M. Sano, E.A. Zhukov, Appl. Phys. Lett. 81 (2002) 1231.
Journal of Crystal Growth 305 (2007) 36–39 www.elsevier.com/locate/jcrysgro
Microstructures, electrical and optical characteristics of ZnO thin films by oxygen plasma-assisted pulsed laser deposition Yanfei Gua,b, Xiaomin Lia,, Weidong Yua, Xiangdong Gaoa, Junliang Zhaoa,b, Chang Yanga,b a
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 12 March 2007; accepted 30 March 2007 Communicated by D.P. Norton Available online 10 April 2007
Abstract In order to decrease the free-electron concentration and increase the crystalline quality, zinc oxide (ZnO) thin films were deposited on sapphire (0 0 0 1) substrates by oxygen plasma-assisted pulsed laser deposition (PLD). ZnO films showed higher oxygen composition, stronger diffraction intensity of the (0 0 0 2) direction, and larger grain size with regular hexagonal grain shape. The free-electron concentration was decreased greatly from 1019 to 1014 cm3 and the Hall mobility was increased from 6.8 to 37 cm2 V1 s1. Furthermore, the intensity of the resonant Raman scattering and ultraviolet photoluminescence emission was increased. This enhancement of the crystalline, electrical and optical quality would be attributed to the increase of high activity oxygen density introduced by the plasma oxygen source. r 2007 Published by Elsevier B.V. PACS: 81.15.Fg; 81.05.Dz; 68.55.a; 72.20.i; 87.64.Je; 78.55.m Keywords: A1. Crystal structure; A1. Electrical properties; A1. Photoluminescence; A1. Raman scattering; A3. Pulsed laser deposition; B1. Zinc oxide
1. Introduction In view of the demand for fabrication of high-quality optoelectronic devices such as light-emitting diodes [1–3] or ultraviolet detectors [4], zinc oxide (ZnO) has recently attracted widespread research interest because of its exceptional optical and electronic properties [5,6], including direct wide band gap of 3.37 eV and a high exciton bonding energy of 60 meV at room temperature. A variety of methods were employed to grow ZnO thin films, such as RF sputtering [7], chemical-vapor deposition [8], molecular-beam epitaxy [9], and pulsed laser deposition (PLD) [10]. Among these techniques, PLD is widely used for its simplicity and experimental flexibility.
Corresponding author. Tel.: +86 21 52412554; fax: +86 21 52413122.
E-mail address: [email protected] (X. Li). 0022-0248/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2007.03.050
ZnO films grown by these low oxygen pressure thin film deposition methods show low resistivity and large freeelectron concentration. Reducing the background carrier (free electron) concentration in ZnO films is one of the major challenges ahead of realizing high-performance ZnO-based optoelectronic devices [10]. Furthermore, the compensation effect by a large background electron concentration makes the p-type doping of ZnO too difficult [11], which is the bottleneck for the application of ZnO. It is commonly accepted that free electrons in ZnO are generated due to intrinsic defects such as oxygen vacancies (VO) or zinc interstitials (Zni) [12]. Thus, free-electron concentration in ZnO can be deduced by removing the VO or Zni defects at higher oxygen pressure. Unfortunately, in higher oxygen pressure, the ejected species by laser ablation from the target undergo much more collisions with the high density of oxygen molecules. The kinetic energy of the species is decreased greatly, resulting in the
ARTICLE IN PRESS Y. Gu et al. / Journal of Crystal Growth 305 (2007) 36–39
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poor crystalline quality, low electron mobility and poor optical properties [10]. In this article, we deposited ZnO thin films by PLD employing an oxygen plasma source to decrease the background free-electron concentration greatly and enhance the crystalline quality. The composition, structural, electrical and optical properties of ZnO thin films were also described. 2. Experiments ZnO thin films were deposited on sapphire (0 0 0 1) substrates by a PLD system employing a plasma oxygen source (ZnOplasma) and gaseous oxygen source (ZnOgaseous), respectively. The O plasma source was introduced by a plasma generator (KYKY Technology Development Ltd., Shenyang, China), with the working voltage of 400 V, and the current of 35 mV. A ceramic ZnO (99.99% purity) was used as target and an KrF excimer laser (Lambda Physik COMpex, wavelength of 248 nm, energy of 200 mJ/pulse, repetition rate of 5 Hz) was applied to ablate the target. The substrate temperature was fixed to be 750 1C. The vacuum chamber was initially evacuated to a pressure of 1 104 Pa. Then a pure O2 gas was introduced into the chamber (the flow rate was 20 sccm) and the pressure during deposition was maintained at 1 Pa. The compositions of obtained ZnO films were characterized by X-ray photoelectron spectroscopy (XPS) (Microlab 310F, Al Ka source). The crystallinity was investigated by X-ray diffraction (XRD) (D/MAX-2550 V Cu Ka). The microstructure and surface morphology were analyzed by field emission scanning electron microscopy (FESEM, JSM-6700F). Electrical properties were measured at room temperature by Hall measurements with the Van der Pauw configuration. The optical properties were characterized by room-temperature photoluminescence (PL) and Raman scattering spectra excited by the He–Cd laser with the wavelength of 325 nm, and measured by a Jobin Yvon LabRAM HR 800UV system. 3. Results and discussion 3.1. Microstructure Atomic concentrations of zinc and oxygen were analyzed by XPS. For the ZnOgaseous film, atomic concentration of Zn was found to be 55.501% and that of O was 44.499% and in ZnOplasma film atomic concentration of Zn was found to be 44.472% and that of O was 55.528%. It is well known that the oxygen is insufficient in the ZnO deposited by conventional PLD method. With the assistant of O plasma, the density of active O ions in plume was increased evidently which have higher binding energy with Zn element, resulting in the increasing of the atomic concentration of O. The thicknesses of the deposited films were all about 250 nm. Fig. 1 shows XRD patterns of ZnOplasma and
Fig. 1. XRD spectra of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
ZnOgaseous films. Only (0 0 0 2) diffraction peaks of ZnO were detected, showing that both films are of hexagonal wurtzite structure and with their c-axis perpendicular to the substrate surface. The diffraction intensity of the (0 0 0 2) direction in ZnOplasma film was 30 times greater than ZnOgaseous film. The full-width at half-maximum (FWHM) of the (0 0 0 2) diffraction peak for ZnOplasma and ZnOgaseous film was 0.201 and 0.281, respectively. The stronger diffraction intensity and smaller FWHM for ZnOplasma film indicates the higher crystal quality. The microstructure and surface morphology of the ZnO films were investigated by FESEM. Fig. 2 shows the SEM images of ZnOplasma and ZnOgaseous film. For the ZnOgaseous film (Fig. 2a), the crystal grains were irregular in shape, and the grains size was around 100 nm. In contrast, the ZnOplasma film was dominated by a typical ‘‘honeycomb’’ like structure with well-faced regular hexagonal grains (Fig. 2b). The grain size was about 200–300 nm. Regular hexagonal grain shape and larger grain size for ZnOplasma film indicate that the O plasma significantly improves the microstructure quality of the PLD grown ZnO film, which is consistent with the XRD analysis. In the traditional PLD process, the ejected species from the target collide with the O2 molecules greatly at higher oxygen pressure. The kinetic energy of the species is decreased strongly, resulting in the poor crystalline quality. The high-energy O ions generated by the O plasma increase the species energy and enhance the mobility on the substrate surface, and benefit the grain growth of ZnO thin film. 3.2. Electrical characteristics Table 1 shows the electrical properties such as surface resistivity rs, carrier concentration N, and the Hall mobility m of ZnOgaseous and ZnOplasma films, respectively. Both films
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Fig. 3. Raman spectra of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
Fig. 2. FESEM morphologies of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
Table 1 Electrical properties of ZnO films deposited by PLD with plasma oxygen and gaseous oxygen Sample
Gaseous oxygen Plasma oxygen
N (cm3)
rs (O/&)
1.759 10
4
2.549 108
1.731 10
m (cm2 V1 s1) 19
3.353 1014
6.8 37
Conductivity type n n
were n-type conductivity. The ZnOgaseous film showed low resistivity with the surface resistivity rs of 1.759 104 O/&, and the electron concentration was as high as 1.731 1019 cm3. With the assistance of O plasma, the ZnOplasma film showed high resistivity with the surface resistivity rs of 2.549 108 O/&, and the electron concentration was decreased drastically to 3.353 1014 cm3 (which is the lowest value for undoped ZnO film to our knowledge). The free electron in unintentionally doped ZnO is related to the intrinsic defects such as VO or Zni caused by oxygen deficiency. From the composition analysis, the oxygen deficiency in ZnO film is inhibited by the high density of active oxygen ions in the plume due to the O plasma. As a result, the concentration of VO or Zni decreases greatly, leading to the lower electron concentration. The Hall mobility was 6.8 cm2 V1 s1 for ZnOgaseous film, and 37 cm2 V1 s1 for ZnOplasma film. The electron
transport properties of the semiconductor are influenced by the impurities scattering, electronic correlation mechanisms, and scattering from defects. In these undoped ZnO thin film, the impurities scattering can be ignored. The increase of the mobility for ZnOplasma film is contributed by the decrease of the electron concentration, and the decrease of defects density such as intrinsic point defects and grain boundaries due to the enhancement of the crystal quality. 3.3. Optical characteristics To investigate the optical characteristics of the ZnO thin films, Raman scattering measurements were performed using a 325 nm He–Cd laser as the excitation source. The room-temperature resonant Raman spectra are shown in Fig. 3. In our measurement range, from 200 to 1400 cm1, multiphonon (2LO) signals were observed. The first-order LO phonon peaks are centered around 570 and 574 cm1 for ZnOgaseous film and ZnOplasma film, respectively. These peaks were assigned to the A1(LO) phonon mode of ZnO thin film [13]. Mode A1(LO) of ZnOgaseous film had a redshift to low-frequency direction. Kumar et al. [14] proposed several reasons to explain this redshift in nanocrystal ZnO: (1) the optical phonon confinement, (2) the local heating of nanocrystallites, and (3) the presence of impurity defects. The average sizes of the ZnO films were 100–300 nm, which is markedly greater than and is not comparable to the exciton Bohr radius of ZnO (2.34 nm) [15]. So, the former two reasons may not play the key role on the redshift phenomenon. In addition, there are no impurities present in our undoped ZnO film, confirmed by the XPS measurement. Furthermore, the oxygen in ZnOgaseous film is deficient markedly. Thus, we believe that the oxygen vacancies defects due to the oxygen deficiency may play the role to induce the mode A1(LO) redshift of ZnOgaseous film, similarly to the impurity defects.
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composition in ZnO film was increased, resulted in drastic decrease of the free-electron concentration from 1019 to 1014 cm3. With the assistance of oxygen plasma, the ZnO film showed regular hexagonal shape grain with larger size, higher Hall mobility, stronger resonant Raman scattering and UV NBE emission, indicated the higher crystal quality. This enhancement of the crystalline, electrical and optical quality would be attributed to the increase of high activity oxygen density introduced by the plasma oxygen source. Acknowledgments This work was supported by National Nature Science Foundation of China (90401010), and Project of ShanghaiAM Materials Research and Development Fund (0516). Fig. 4. Photoluminescence spectra of ZnO thin film deposited by PLD with (a) gaseous oxygen and (b) plasma oxygen.
Additionally, the intensity of mode A1(LO) for ZnOplasma film is increased, which may be related to the higher crystal quality. Room-temperature PL spectra were shown in Fig. 4. The ZnOgaseous film shows two luminescent bands: one is the sharp near-band-edge (NBE) emission in the ultraviolet (UV) region centered at 375 nm (FWHM ¼ 12.74 nm), which is attributed to the free exciton recombination [16], and the other was the broad deep-level (DL) emission band centered at 511 nm. For ZnOplasma film, there is no DL emission can be detected in the PL spectrum, and only strong NBE emission (FWHM ¼ 9.44 nm) appeared. The origin of the DL emission band is ascribed to VO or Zni defects [17]. The disappearance of the DL emission for ZnOplasma film indicates that the VO or Zni may be inhibited effectively by the O plasma. Compare to the ZnOgaseous film, the ZnOplasma film has greater NBE intensity with small FWHM value. Structure defects, such as dislocations and grain boundaries can trap photogenerated carriers into a nonradiative recombination process before the NBE recombination occurs. This nonradiative relaxation process decreases the emission intensity [18]. The remarkably increase of the intensity of NEB emission shows the ZnOplasma film has higher optical quality due to its higher crystal quality. 4. Conclusion ZnO thin films were deposited by oxygen plasma-assisted PLD method on sapphire (0 0 0 1) substrates. The oxygen
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