- Email: [email protected]
Optical and photoluminescent properties of Al-doped zinc oxide thin films by pulsed laser deposition
Journal of Alloys and Compounds 485 (2009) 529–531
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Optical and photoluminescent properties of Al-doped zinc oxide thin films by pulsed laser deposition Yaodong Liu ∗ , Qiang Li, Huiliang Shao Key Laboratory of Advanced Structrural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 1 February 2009 Received in revised form 2 June 2009 Accepted 2 June 2009 Available online 11 June 2009 Keywords: Doping Photoluminescence Optical properties Al-doped ZnO
a b s t r a c t Al-doped zinc oxide (AZO) thin films have been prepared by pulsed laser deposition. AZO films were observed to grow along the c-axis orientation of (0 0 2). Optical and photoluminescent properties of the AZO films have been investigated. The UV absorption edge was blue shifted with increasing Al doping concentration. AZO films with different Al contents observe only ultraviolet emission without notable DLEs. All films have an average optical transparency over 80% in the visible range. The possible origins responsible for these emission bands have been discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide has recently gained much interest for its potential use in many applications, such as transparent electrodes in solar cells [1], thin film gassensors [2], varistors [3], spintronic devices [4], photodetectors [5], surface acoustic wave devices [6] and light emitting diodes [7,8]. Attributed to its wide and direct band gap, excellent chemical and thermal stability, specific electrical and optoelectronic property, it became a promising semiconductor with a large exciton binding energy. However, doped ZnO thin films are of technological importance because of their great potential applications such as transparent conducting electrodes (doping with III family elements) [1,9], insulating or dielectric layers (doping with Li) [10], and spintronic devices (doping with Mn) [4]. Among them, being noted for the high conductivity and good optical transmittance, Al-doped ZnO (AZO) films have drawn considerable attention for transparent conducting electrodes [9]. It is a wide band gap semiconductor, which shows good optical transmission in the visible wavelength regions (400–700 nm). Furthermore, AZO films show a lower electrical resistivity, which is similar to that of ITO film [11]. There were various methods to produce ZnO film, such as the metal-organic chemical vapor deposition (MOCVD) method [12], the sol–gel method [13], spray hydrolysis [14], sputtering [15] or pulsed laser deposition [16]. In comparison with other techniques, PLD provides several advantages. For example, PLD films crystallize
∗ Corresponding author. Tel.: +86 431 85716421; fax: +86 431 85598512. E-mail address: [email protected] (Y. Liu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.019
at lower substrate temperatures due to the higher energy of the ablated particles in the laser-produced plume and relatively high deposition rates [17]. In some previous reports [18], ceramic targets prepared by sintering the mixture ZnO powder and A12 O 3 powder are used. However, the expensive ceramic targets are usually brittle and would cause the cracking during deposition. In our previous work [19], ZnO film was formed on Si (1 1 1) substrate by pulse laser ablation of Zn target in a controlled oxygen atmosphere, which is a simple method to produce oxide film by PLD. In this paper, Al-doped zinc oxide (AZO) films will be deposited by PLD of Zn–Al alloy in an oxide atmosphere at a relatively low temperature. The effect of aluminum doping on structural, photoluminescence and optical properties of AZO films deposited from metallic targets by PLD on glass substrates are investigated in details. 2. Experiment details The deposition of AZO film was performed in a vacuum chamber. The chamber was evacuated using a turbo-molecular pump to a base pressure of 5 × 10−4 Pa and then filled with oxygen (99.99 wt.% purity) at a fixed pressure of 9 Pa. Metallic Zn–Al target with different Al contents (0–2 wt.%) was ablated by a Nd: YAG pulsed laser with a wavelength of 1064 nm. The pulse duration of 100 ns and frequency of 10 Hz were used for all samples. The target–substrate distance was kept at 2.5 cm and the deposition time of 50 min was maintained. Film thickness was 175–200 nm (determined with a surface roughness detector DEKTAK). The Al doped ZnO thin films were deposited at the substrate temperatures of 150 ◦ C. A low energy fluency of 11 J/cm2 was set for all samples in order to avoid molten droplet. AZO films were grown on 2.5 cm × 2.5 cm quartz glass substrates. X-ray diffraction (XRD, rigaku Dymax) with a Cu target and a mono-chronmator at 50 kV and 300 mA was used to investigate the crystal structure of the films. The elemental compositions of the films were investigated by energy dispersive spectrometer (EDS). The optical properties of the AZO thin films were characterized
530
Y. Liu et al. / Journal of Alloys and Compounds 485 (2009) 529–531
Fig. 1. XRD spectra obtained from Al-doped ZnO films deposited at the substrate temperature of 150 ◦ C, with different Al contents.
by photoluminescence with an Ar ion laser as a light source using an excitation wavelength of 325 nm.
Fig. 3. Optical transmittance spectra of AZO films with various Al doping concentrations. The inset shows the UV absorption edge blue shifted with increasing Al doping concentration.
Al deposited at 150 ◦ C. The elements of O, Zn and Al were found on surface of the AZO film.
3. Results and discussion 3.2. Optical properties 3.1. Structural properties Doping of ZnO films with aluminum significantly affects the structural properties of the films. Fig. 1 shows the X-ray diffraction patterns for Al-doped ZnO films grown at 150 ◦ C under 10 Pa of oxygen pressure. No metallic Zn or Al characteristic peaks are observed within the phase detectability of XRD, Undoped ZnO film was observed to grow along the c-axis orientation of (0 0 2). However, as the increase in the Al content in the targets from 0 to about 2 wt.%, they consist mainly of two peaks that correspond to (0 0 2) and (1 0 1) lattice plane diffraction of hexagonal ZnO, and the FWHM of (0 0 2) peaks is increased with increasing doping concentration, which indicates that the growth of AZO changes from the c-axis orientation to the isotropic growth. The grain growth of ZnO is promoted by fast diffusion of Zn interstitials at higher temperatures [20]. In the case of trivalent cation doping, the concentration of the zinc interstitials is reduced for charge compensation, resulting in suppressed ZnO grain growth and deteriorated crystallinity [21]. Therefore, the deterioration in crystallinity suggests the incorporation of Al into ZnO. The presence of Al in the ZnO film should change the diffusion rate of Zn and O at the surface during deposition. Then it will alter the energetic balance between (0 0 2) and (1 0 1) orientations and lead to the weakening of (0 0 2) orientation and the appearing of preferred (1 0 1) texture in certain conditions. Fig. 2 shows the EDS analysis of the AZO film with 2 wt.% doping of
Fig. 2. The EDS analysis of ZnO film with 2 wt.% doping of Al.
Fig. 3 depicts the optical transmission spectra of AZO films with different Al contents (0–2 wt.%). All films have an average optical transparency over 80% in the visible range. A weak fluctuation in the spectra is mainly due to interference phenomenon between thin film layers. We can also observe in Fig. 3 that in the transmittance curves the threshold of optical absorption shifts to shorter wavelengths (blue shifted) with the increase in Al contents, indicating the broadening of the optical band gap. This broadening in the band gap is known as the Moss–Burstein shift [22]. According to the Moss–Burstein theory, in heavily doped zinc oxide films, the donor electrons occupy states at the bottom of the conduction band. Since the Pauli principle prevents states from being doubly occupied and optical transitions are vertical, the valence electrons require extra energy to be excited to higher energy states in the conduction band. Therefore, the optical band gap of doped zinc oxide is broader than that of undoped zinc oxide films. 3.3. Photoluminescence Fig. 4 shows the PL spectra obtained at room temperature for Al-doped ZnO thin films with different Al contents (0–2 wt.%). The PL spectra are found to be dependent on the Al content. It can be seen that the intensity of UV emission peaks gradually decreases with the increase in the Al content, the UV emission is slightly blue shifted and is deteriorated with increasing Al doping concentration. The blue shift of the UV peak with increasing Al doping concentration is consistent with the results of the transmittance spectra, originating from the band gap broadening known as the Moss–Burstein shift. In addition, the broad visible emission is suppressed on doping with aluminum, it can be seen that Undoped ZnO film shows a broad deep level peak centering around 550 nm. It should correspond to the green emission, different explanations were proposed in recent years. Lin et al. [23] observed the green emission centering at 2.38 eV from the ZnO film deposited on silicon substrate and suggested that green emission should correspond to the electron transition from the bottom of the conduction band to the antisite defect OZn level. It is very interesting to note that the doped ZnO films with different Al contents observe only ultraviolet emission without notable deep level emissions. Inset in Fig. 4 shows
Y. Liu et al. / Journal of Alloys and Compounds 485 (2009) 529–531
531
4. Conclusions AZO films were prepared on quartz glass substrates by PLD in oxygen atmosphere at a relatively lower temperature, by the pulse laser ablation of metallic target with different Al contents. The film crystallinity was deteriorated with increasing Al doping concentration. With increasing Al doping concentration, a broadening in the band gap of the AZO films was observed due to the Moss–Burstein shift. It was observed that Al doped ZnO film obtained optical transparency over 80% in the visible range and the higher UV emission without deep level emission. Acknowledgments This work was supported by Foundation of National Key Basic Research and Development Program (No.2004CB619301) and Project 985-automotive engineering of Jilin University. Fig. 4. Room temperature (298 K) photoluminescence spectra of Al-doped ZnO thin films on quartz glass grown by PLD with different Al contents. The inset gives the full-width at half-maximum of the UV emission peaks.
the full-width at half-maximum (FWHM) of the UV emission peaks is increased with the Aluminum doping, indicating that the film crystallinity is deteriorated, for the intensity and FWHM of the UV emission peaks of ZnO film depend strongly on the microcrystalline structure. The strong UV emission without deep level emission for ZnO films was also observed recently from the ZnO film deposited on NiO buffered sapphire substrate by the laser molecular-beamepitaxy at room temperature in an ultrahigh vacuum [24]. It is understood that [25,26] the deep level emission is probably relative to the variation of the intrinsic defects in ZnO films, different intrinsic defects correspond to various excitated energies in deep level emission. According to the calculation, the energy interval for electronics transition from the bottom of the conduction band to the interstitial oxygen Oi level is 2.28 eV [27]. In our experiments, the deep level emission centered at about 2.23 eV for the ZnO film without doped by Al, which implies that there should be interstitial oxygen defects in the films. As our experiment proceeded at a fixed partial-oxygen pressure of 10 Pa, when the deposited temperature is lower (150 ◦ C), the oxidation reactive velocity maintains a lower level and the continual supply of O atoms is sufficient to combine to ZnO with Zn ions, and there may be still residual O ions which may exist as Oi defects in ZnO films. As a result, deep level emission is expected for the undoped ZnO film (0 wt.%). However a moderate Al doping might act as a surfactant, change the diffusion rate of Zn and O at the surface during deposition, i.e. the reactive velocity assorts with the supply of oxygen under this oxygen pressure, leads to the formation of ZnO film with a lower concentration of intrinsic defects (for example, Oi defects). As a result, highest UV emission with very slight or without deep level emission was observed.
References [1] B.S. Chua, S. Xu, Y.P. Ren, Q.J. Cheng, K. Ostrikov, J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2009.05.099. [2] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sens. Actuators B Chem. 107 (2005) 379. [3] E. Suvaci, I˙ .Ö. Özer, J. Eur. Ceram. Soc. 25 (2005) 1663. [4] Y. Zheng, J.C. Boulliard, D. Demaille, Y. Bernard, J.F. Pe troff, J. Cryst. Growth 274 (2005) 156. [5] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225 (2001) 110. [6] N.W. Emanetoglu, C. Gorla, Y. Liu, S. Liang, Y. Lu, Mater. Sci. Semicond. Process 2 (1999) 247. [7] W. Gao, Z.W. Li, J. Alloys Compd. 449 (2008) 202–206. [8] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [9] M. Hiramatsu, K. Imaeda, N. Horio, M. Nawata, J. Vacuum Sci. Technol. A 16 (1998) 669. [10] M. Joseph, H. Tabata, T. Kawai, Appl. Phys. Lett. 74 (1999) 2534. [11] K. Tominaga, H. Fukumoto, K. Kondou, Y. Hayashi, K. Murai, T. Moriga, I. Nakabayashi, J. Vacuum 74 (2004) 683. [12] Hui Lin, Shengming Zhou, Hao Tenga, Tingting Jia, Xiao rui Hou, Shulin Gu, Shunming Zhu, Zili Xie, Ping Han, Rong Zhang, J. Alloys Compd. 479 (2009) L8–L10. [13] T. Ratanaa, P. Amornpitoksukb, T. Ratanac, S. Suwanboond, J. Alloys Compd. 470 (2009) 408–412. [14] Y. Natsnme, H. Sakata, Thin Solid Films 372 (2000) 30. [15] Chung Ping Liu, Gwo Rou Jeng, J. Alloys Compd. 468 (2009) 343–349. [16] B.J. Lochande, M.D. Uplane, Appl. Surf. Sci. 167 (2000) 243. [17] X.M. Fan, J.S. Lian, Z.X. Guo, H.J. Lu, J. Cryst. Growth 279 (2005) 447. [18] Chi Shiung His, Boen Houng, Bing Yi Hou, Guo Ju Chen, Shen Li Fu, J. Alloys Compd. 464 (2008) 89–94. [19] X.M. Fan, J.S. Lian, Z.X. Guo, H.J. Lu, Appl. Surf. Sci. 239 (2005) 176. [20] P. Bonasewicz, W. Hirschwald, G. Neumann, J. Electrochem. Soc. 133 (1986) 2270. [21] S. Fujihara, A. Suzuki, T. Kimura, J. Appl. Phys. 94 (2003) 2411. [22] B.E. Sernelius, K.F. Berggren, Z.C. Jim, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244. [23] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. [24] A. Sasaki, W. Hara, A. Matsuda, N. Tateda, K. Saito, M. Yoshimoto, Appl. Phys. Lett. 86 (2005) 231911. [25] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, G. Granqvist, Phys. Rev. B 37 (1988) 10244. [26] B. Lin, Z. Fu, Y. Yia, Appl. Phys. Lett. 79 (2001) 943. [27] J. Hu, R.G. Gordon, J. Appl. Phys. 71 (1992) 880.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Optical and photoluminescent properties of Al-doped zinc oxide thin films by pulsed laser deposition Yaodong Liu ∗ , Qiang Li, Huiliang Shao Key Laboratory of Advanced Structrural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 1 February 2009 Received in revised form 2 June 2009 Accepted 2 June 2009 Available online 11 June 2009 Keywords: Doping Photoluminescence Optical properties Al-doped ZnO
a b s t r a c t Al-doped zinc oxide (AZO) thin films have been prepared by pulsed laser deposition. AZO films were observed to grow along the c-axis orientation of (0 0 2). Optical and photoluminescent properties of the AZO films have been investigated. The UV absorption edge was blue shifted with increasing Al doping concentration. AZO films with different Al contents observe only ultraviolet emission without notable DLEs. All films have an average optical transparency over 80% in the visible range. The possible origins responsible for these emission bands have been discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide has recently gained much interest for its potential use in many applications, such as transparent electrodes in solar cells [1], thin film gassensors [2], varistors [3], spintronic devices [4], photodetectors [5], surface acoustic wave devices [6] and light emitting diodes [7,8]. Attributed to its wide and direct band gap, excellent chemical and thermal stability, specific electrical and optoelectronic property, it became a promising semiconductor with a large exciton binding energy. However, doped ZnO thin films are of technological importance because of their great potential applications such as transparent conducting electrodes (doping with III family elements) [1,9], insulating or dielectric layers (doping with Li) [10], and spintronic devices (doping with Mn) [4]. Among them, being noted for the high conductivity and good optical transmittance, Al-doped ZnO (AZO) films have drawn considerable attention for transparent conducting electrodes [9]. It is a wide band gap semiconductor, which shows good optical transmission in the visible wavelength regions (400–700 nm). Furthermore, AZO films show a lower electrical resistivity, which is similar to that of ITO film [11]. There were various methods to produce ZnO film, such as the metal-organic chemical vapor deposition (MOCVD) method [12], the sol–gel method [13], spray hydrolysis [14], sputtering [15] or pulsed laser deposition [16]. In comparison with other techniques, PLD provides several advantages. For example, PLD films crystallize
∗ Corresponding author. Tel.: +86 431 85716421; fax: +86 431 85598512. E-mail address: [email protected] (Y. Liu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.019
at lower substrate temperatures due to the higher energy of the ablated particles in the laser-produced plume and relatively high deposition rates [17]. In some previous reports [18], ceramic targets prepared by sintering the mixture ZnO powder and A12 O 3 powder are used. However, the expensive ceramic targets are usually brittle and would cause the cracking during deposition. In our previous work [19], ZnO film was formed on Si (1 1 1) substrate by pulse laser ablation of Zn target in a controlled oxygen atmosphere, which is a simple method to produce oxide film by PLD. In this paper, Al-doped zinc oxide (AZO) films will be deposited by PLD of Zn–Al alloy in an oxide atmosphere at a relatively low temperature. The effect of aluminum doping on structural, photoluminescence and optical properties of AZO films deposited from metallic targets by PLD on glass substrates are investigated in details. 2. Experiment details The deposition of AZO film was performed in a vacuum chamber. The chamber was evacuated using a turbo-molecular pump to a base pressure of 5 × 10−4 Pa and then filled with oxygen (99.99 wt.% purity) at a fixed pressure of 9 Pa. Metallic Zn–Al target with different Al contents (0–2 wt.%) was ablated by a Nd: YAG pulsed laser with a wavelength of 1064 nm. The pulse duration of 100 ns and frequency of 10 Hz were used for all samples. The target–substrate distance was kept at 2.5 cm and the deposition time of 50 min was maintained. Film thickness was 175–200 nm (determined with a surface roughness detector DEKTAK). The Al doped ZnO thin films were deposited at the substrate temperatures of 150 ◦ C. A low energy fluency of 11 J/cm2 was set for all samples in order to avoid molten droplet. AZO films were grown on 2.5 cm × 2.5 cm quartz glass substrates. X-ray diffraction (XRD, rigaku Dymax) with a Cu target and a mono-chronmator at 50 kV and 300 mA was used to investigate the crystal structure of the films. The elemental compositions of the films were investigated by energy dispersive spectrometer (EDS). The optical properties of the AZO thin films were characterized
530
Y. Liu et al. / Journal of Alloys and Compounds 485 (2009) 529–531
Fig. 1. XRD spectra obtained from Al-doped ZnO films deposited at the substrate temperature of 150 ◦ C, with different Al contents.
by photoluminescence with an Ar ion laser as a light source using an excitation wavelength of 325 nm.
Fig. 3. Optical transmittance spectra of AZO films with various Al doping concentrations. The inset shows the UV absorption edge blue shifted with increasing Al doping concentration.
Al deposited at 150 ◦ C. The elements of O, Zn and Al were found on surface of the AZO film.
3. Results and discussion 3.2. Optical properties 3.1. Structural properties Doping of ZnO films with aluminum significantly affects the structural properties of the films. Fig. 1 shows the X-ray diffraction patterns for Al-doped ZnO films grown at 150 ◦ C under 10 Pa of oxygen pressure. No metallic Zn or Al characteristic peaks are observed within the phase detectability of XRD, Undoped ZnO film was observed to grow along the c-axis orientation of (0 0 2). However, as the increase in the Al content in the targets from 0 to about 2 wt.%, they consist mainly of two peaks that correspond to (0 0 2) and (1 0 1) lattice plane diffraction of hexagonal ZnO, and the FWHM of (0 0 2) peaks is increased with increasing doping concentration, which indicates that the growth of AZO changes from the c-axis orientation to the isotropic growth. The grain growth of ZnO is promoted by fast diffusion of Zn interstitials at higher temperatures [20]. In the case of trivalent cation doping, the concentration of the zinc interstitials is reduced for charge compensation, resulting in suppressed ZnO grain growth and deteriorated crystallinity [21]. Therefore, the deterioration in crystallinity suggests the incorporation of Al into ZnO. The presence of Al in the ZnO film should change the diffusion rate of Zn and O at the surface during deposition. Then it will alter the energetic balance between (0 0 2) and (1 0 1) orientations and lead to the weakening of (0 0 2) orientation and the appearing of preferred (1 0 1) texture in certain conditions. Fig. 2 shows the EDS analysis of the AZO film with 2 wt.% doping of
Fig. 2. The EDS analysis of ZnO film with 2 wt.% doping of Al.
Fig. 3 depicts the optical transmission spectra of AZO films with different Al contents (0–2 wt.%). All films have an average optical transparency over 80% in the visible range. A weak fluctuation in the spectra is mainly due to interference phenomenon between thin film layers. We can also observe in Fig. 3 that in the transmittance curves the threshold of optical absorption shifts to shorter wavelengths (blue shifted) with the increase in Al contents, indicating the broadening of the optical band gap. This broadening in the band gap is known as the Moss–Burstein shift [22]. According to the Moss–Burstein theory, in heavily doped zinc oxide films, the donor electrons occupy states at the bottom of the conduction band. Since the Pauli principle prevents states from being doubly occupied and optical transitions are vertical, the valence electrons require extra energy to be excited to higher energy states in the conduction band. Therefore, the optical band gap of doped zinc oxide is broader than that of undoped zinc oxide films. 3.3. Photoluminescence Fig. 4 shows the PL spectra obtained at room temperature for Al-doped ZnO thin films with different Al contents (0–2 wt.%). The PL spectra are found to be dependent on the Al content. It can be seen that the intensity of UV emission peaks gradually decreases with the increase in the Al content, the UV emission is slightly blue shifted and is deteriorated with increasing Al doping concentration. The blue shift of the UV peak with increasing Al doping concentration is consistent with the results of the transmittance spectra, originating from the band gap broadening known as the Moss–Burstein shift. In addition, the broad visible emission is suppressed on doping with aluminum, it can be seen that Undoped ZnO film shows a broad deep level peak centering around 550 nm. It should correspond to the green emission, different explanations were proposed in recent years. Lin et al. [23] observed the green emission centering at 2.38 eV from the ZnO film deposited on silicon substrate and suggested that green emission should correspond to the electron transition from the bottom of the conduction band to the antisite defect OZn level. It is very interesting to note that the doped ZnO films with different Al contents observe only ultraviolet emission without notable deep level emissions. Inset in Fig. 4 shows
Y. Liu et al. / Journal of Alloys and Compounds 485 (2009) 529–531
531
4. Conclusions AZO films were prepared on quartz glass substrates by PLD in oxygen atmosphere at a relatively lower temperature, by the pulse laser ablation of metallic target with different Al contents. The film crystallinity was deteriorated with increasing Al doping concentration. With increasing Al doping concentration, a broadening in the band gap of the AZO films was observed due to the Moss–Burstein shift. It was observed that Al doped ZnO film obtained optical transparency over 80% in the visible range and the higher UV emission without deep level emission. Acknowledgments This work was supported by Foundation of National Key Basic Research and Development Program (No.2004CB619301) and Project 985-automotive engineering of Jilin University. Fig. 4. Room temperature (298 K) photoluminescence spectra of Al-doped ZnO thin films on quartz glass grown by PLD with different Al contents. The inset gives the full-width at half-maximum of the UV emission peaks.
the full-width at half-maximum (FWHM) of the UV emission peaks is increased with the Aluminum doping, indicating that the film crystallinity is deteriorated, for the intensity and FWHM of the UV emission peaks of ZnO film depend strongly on the microcrystalline structure. The strong UV emission without deep level emission for ZnO films was also observed recently from the ZnO film deposited on NiO buffered sapphire substrate by the laser molecular-beamepitaxy at room temperature in an ultrahigh vacuum [24]. It is understood that [25,26] the deep level emission is probably relative to the variation of the intrinsic defects in ZnO films, different intrinsic defects correspond to various excitated energies in deep level emission. According to the calculation, the energy interval for electronics transition from the bottom of the conduction band to the interstitial oxygen Oi level is 2.28 eV [27]. In our experiments, the deep level emission centered at about 2.23 eV for the ZnO film without doped by Al, which implies that there should be interstitial oxygen defects in the films. As our experiment proceeded at a fixed partial-oxygen pressure of 10 Pa, when the deposited temperature is lower (150 ◦ C), the oxidation reactive velocity maintains a lower level and the continual supply of O atoms is sufficient to combine to ZnO with Zn ions, and there may be still residual O ions which may exist as Oi defects in ZnO films. As a result, deep level emission is expected for the undoped ZnO film (0 wt.%). However a moderate Al doping might act as a surfactant, change the diffusion rate of Zn and O at the surface during deposition, i.e. the reactive velocity assorts with the supply of oxygen under this oxygen pressure, leads to the formation of ZnO film with a lower concentration of intrinsic defects (for example, Oi defects). As a result, highest UV emission with very slight or without deep level emission was observed.
References [1] B.S. Chua, S. Xu, Y.P. Ren, Q.J. Cheng, K. Ostrikov, J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2009.05.099. [2] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sens. Actuators B Chem. 107 (2005) 379. [3] E. Suvaci, I˙ .Ö. Özer, J. Eur. Ceram. Soc. 25 (2005) 1663. [4] Y. Zheng, J.C. Boulliard, D. Demaille, Y. Bernard, J.F. Pe troff, J. Cryst. Growth 274 (2005) 156. [5] S. Liang, H. Sheng, Y. Liu, Z. Huo, Y. Lu, H. Shen, J. Cryst. Growth 225 (2001) 110. [6] N.W. Emanetoglu, C. Gorla, Y. Liu, S. Liang, Y. Lu, Mater. Sci. Semicond. Process 2 (1999) 247. [7] W. Gao, Z.W. Li, J. Alloys Compd. 449 (2008) 202–206. [8] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [9] M. Hiramatsu, K. Imaeda, N. Horio, M. Nawata, J. Vacuum Sci. Technol. A 16 (1998) 669. [10] M. Joseph, H. Tabata, T. Kawai, Appl. Phys. Lett. 74 (1999) 2534. [11] K. Tominaga, H. Fukumoto, K. Kondou, Y. Hayashi, K. Murai, T. Moriga, I. Nakabayashi, J. Vacuum 74 (2004) 683. [12] Hui Lin, Shengming Zhou, Hao Tenga, Tingting Jia, Xiao rui Hou, Shulin Gu, Shunming Zhu, Zili Xie, Ping Han, Rong Zhang, J. Alloys Compd. 479 (2009) L8–L10. [13] T. Ratanaa, P. Amornpitoksukb, T. Ratanac, S. Suwanboond, J. Alloys Compd. 470 (2009) 408–412. [14] Y. Natsnme, H. Sakata, Thin Solid Films 372 (2000) 30. [15] Chung Ping Liu, Gwo Rou Jeng, J. Alloys Compd. 468 (2009) 343–349. [16] B.J. Lochande, M.D. Uplane, Appl. Surf. Sci. 167 (2000) 243. [17] X.M. Fan, J.S. Lian, Z.X. Guo, H.J. Lu, J. Cryst. Growth 279 (2005) 447. [18] Chi Shiung His, Boen Houng, Bing Yi Hou, Guo Ju Chen, Shen Li Fu, J. Alloys Compd. 464 (2008) 89–94. [19] X.M. Fan, J.S. Lian, Z.X. Guo, H.J. Lu, Appl. Surf. Sci. 239 (2005) 176. [20] P. Bonasewicz, W. Hirschwald, G. Neumann, J. Electrochem. Soc. 133 (1986) 2270. [21] S. Fujihara, A. Suzuki, T. Kimura, J. Appl. Phys. 94 (2003) 2411. [22] B.E. Sernelius, K.F. Berggren, Z.C. Jim, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244. [23] B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. [24] A. Sasaki, W. Hara, A. Matsuda, N. Tateda, K. Saito, M. Yoshimoto, Appl. Phys. Lett. 86 (2005) 231911. [25] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, G. Granqvist, Phys. Rev. B 37 (1988) 10244. [26] B. Lin, Z. Fu, Y. Yia, Appl. Phys. Lett. 79 (2001) 943. [27] J. Hu, R.G. Gordon, J. Appl. Phys. 71 (1992) 880.