- Email: [email protected]
Influence of different post-treatments on the structure and optical properties of zinc oxide thin films
Applied Surface Science 242 (2005) 346–352 www.elsevier.com/locate/apsusc
Influence of different post-treatments on the structure and optical properties of zinc oxide thin films Ruijin Hong*, Jianbing Huang, Hongbo He, Zhengxiu Fan, Jianda Shao Research and Development Center for Optical Thin Film Coatings, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China Received in revised form 25 August 2004; accepted 31 August 2004 Available online 12 October 2004
Abstract Zinc oxide (ZnO) films with c-oriented were grown on fused quartz glass substrates at room temperature using dc reactive magnetron sputtering. The as-grown films were annealed at 700 8C in air and bombarded by ion beam, respectively. The effects of post-treatments on the structural and optical properties of the ZnO films were investigated by X-ray diffraction (XRD), photoluminescence (PL), optical transmittance and absorption measurements. The XRD spectra indicate that the crystal quality of ZnO films has been improved by both the post-treatments. Compared with the as-grown sample, both annealed and bombarded samples exhibited blueshift in the UV emission peaks, and a strong green emission was found in the annealed ZnO film. In both optical transmittance and absorption spectra, a blueshift of the band-gap edge was observed in the bombarded film, while a redshift was observed in the annealed film. # 2004 Elsevier B.V. All rights reserved. PACS: 78.66.Db; 71.20.Mq; 78.55.Ap Keywords: Zinc oxide; Thin films; Annealing; Ion beam bombardment
1. Introduction In recent years, wide-band-gap semiconductor compounds have attracted a great deal of attention because of the intense commercial interest in developing practical short-wavelength semiconductor * Corresponding author. Tel.: +86 21 699 18492; fax: +86 21 699 18028. E-mail address: [email protected] (R. Hong).
diode lasers for the huge market needs. Wurtzite ZnO is a wide-band-gap (3.37 eV at room temperature) semiconductor material and has a large excitation binding energy of 60 meV. In this regard, ZnO is a promising material [1–4]. ZnO thin films have been prepared by many methods such as sputtering [5], reactive thermal and electron-beam evaporation [6], pulse laser deposition [7], chemical vapor deposition [8], oxidation of metallic Zn [9] and molecular beam epitaxy [10]. Among all, magnetron sputtering is
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.08.037
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
characterized by several advantages: (1) low substrate temperature (down to room temperature); (2) good adhesion of films on substrates; (3) very good thickness uniformity and high density of the films; and (4) directive deposition from elemental (metallic) targets by reactive sputtering in rare/reactive gas mixtures [11]. The growth conditions such as substrates, deposition temperature, and background gas for depositing ZnO films have been extensively studied. The influence of growth conditions on electrical characteristics and crystallographic relationships between epitaxial ZnO thin films and substrate have also been reported [12]. Usually, as-grown films require a special treatment of the surface to reinforce the stability of the films and to reduce the possible undesirable influence of the surface. The method of conventional thermal annealing of zinc oxide films has been performed on a number of experimental conditions, showing an improvement in film quality as regards optical and acoustical propagation losses. As a consequence of furnace thermal annealing, an improvement in crystallite orientation has been observed, as evidenced by increased alignment of (0 0 2) crystal planes parallel to the substrate, possible crystallite growth, and considerable stress relief. Bombarding the specimen with ions of reactive elements is a promising way of modifying the surface properties of mechanical or electronic components. The low-energy ion beam bombardment is often effective in material properties’ enhancement. It usually increases the film adhesion to the substrate and the surface coverage. Moreover, it is possible to control the density, the hardness and preferred crystal orientation of the film [13]. In this work, we report post-treatments performed by both thermal annealing and ion beam bombardment on the as-grown ZnO thin films, and discuss the influences of annealing and ion beam bombardment on the structure and optical properties of ZnO thin films.
347
oxygen were used as the sputtering and reactive gases, respectively. The target-to-substrate distance was 15 cm. The cathode was mounted on a water-cooled copper plate. The chamber was pumped to a base pressure of 1 103 Pa before deposition. Film growth was carried out in the growth ambient with a mixture of argon (40%) and oxygen (60%) and at a constant working pressure of 0.15 Pa. Both annealing and ion beam bombardment were introduced as posttreatment in the experiment. For annealing, the asgrown films were heated to 700 8C in air at a rate of 100 8C/h. The temperature was then maintained at 700 8C for 8 h after which cooling was also carried out at a rate of 100 8C/h. A 12-cm RF ion source (Ion Tech, Inc.) was employed during the course of ion beam bombardment. The energy of the ion beam was fixed at 550 V, while its current was fixed at 150 mA. In the case of ion beam treatment, a gas mixture of argon (40%) and oxygen (60%) was introduced into the discharge chamber as the bombarding species to keep the same ambient as in the forming one. The bombardment process lasted for 20 min. The crystal structure of the films was characterized by X-ray diffraction (XRD) using a Rigaku D/Max-B system, with Cu Ka radiation (l = 0.15408 nm). Biaxial film stress (s) and the average crystallite size D were evaluated from XRD u/2u scans. PL spectra were acquired in a JASCO fluorespectrometer from 300 nm to 750 nm at room temperature; the excitation light was the 284 nm line of a xenon lamp, the filter wavelength was 290 nm. The optical transmittance and absorption of the films were measured with UV–visible spectroscopy (Lambda900 UV–vis–NIR double beam spectrophotometer). The thickness of ZnO thin films was measured to be in the range of 200–300 nm. All the measurements were carried out at room temperature.
3. Results and discussion 2. Experiment Zinc oxide films (ZnO) were grown by dc planar magnetron sputtering using an 8.6 cm-in-diameter Zn target (99.99%) on fused silica at room temperature. The quartz glass substrates were rinsed in acetone, ethanol and distilled water sequentially. Argon and
X-ray diffraction (XRD) u/2u patterns of the ZnO films are shown in Fig. 1, inset gives the XRD v-rocking curves of the (0 0 2) diffraction peak of the ZnO films. The XRD spectra reveal the influence of post-treatment on the structure of the ZnO thin film. It was apparent that all the ZnO films consisted of a single ZnO phase oriented with its c-axis normal to the
348
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
Fig. 1. XRD patterns of the as-grown, bombarded and annealed ZnO films.
substrate surface, no diffraction from randomly oriented grains or impurity phases can be observed from the X-ray patterns. According to Fig. 1, both the thermal annealing and ion bombardment have the effects of narrowing the diffraction peak, indicating that grain growth has occurred, and shifting the (0 0 2) peaks to higher 2u angles, a result of the partial relief of residual stresses within as-treated films. Compared with the zinc oxide powders, all the films formed in this experiment exhibit discrepancy in d-value (d is interplanar spacing), which is due to the variation of residual stress in the films [14]. The full width at half maximum (FWHM) of as-grown, ion bombarded and annealed films have the value of 1.3768, 1.3338and 0.3318, respectively. The diffraction peaks are 33.758, 33.778and 34.508 (2u), respectively. From the integral width and peak position of the (0 0 2) peak, the grain size and the residual stress in the films are calculated (see Table 1). The average grain size of the film can be estimated by Scherrer formula, using FWHM value of the XRD diffraction
peaks as follows [15]: 0:9l (1) B cos u where D, l, u and B are the mean grain size, the X-ray wavelength of 0.154 nm, Bragg diffraction angle and the FWHM of the diffraction peak of the (0 0 2) direction at around 348 (2u) for ZnO films, respectively. The calculation of the film stress is based on ðc c Þ the biaxial strain model. The strain e ¼ filmcbulkbulk in c-axis, i.e., perpendicular to the substrate surface, was calculated by XRD data, where cfilm and cbulk are the lattice constants (c) of ZnO film and bulk (or powder), respectively. To derive the film stress sfilm parallel to the film surface, the following formula is used, which is valid for a hexagonal lattice:
D¼
s XRD film ¼
2c213 c33 ðc11 þ c12 Þ cfilm cbulk 2c13 cbulk
(2)
For the elastic constants cij, the following values for single crystal ZnO are used: c11 = 208.8 GPa,
Table 1 The data evaluated from XRD u/2u scans for the samples before and after treatments Sample
Interplanar spacing d (nm)
FWHM (8)
Average grain size (nm)
sfilm (GPa)
As-grown Bombarded Annealed
0.2652 0.2651 0.2596
1.376 1.333 0.331
6.0 6.2 25.1
4.4 4.3 0.6
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
c33 = 213.8 GPa, c12 = 119.7 GPa, c13 = 104.2 GPa [16]. This yields the following numerical relation for the stress derived from XRD: sfilm = 233 e (GPa). Biaxial stresses, calculated from (0 0 2) peak positions, are shown in Table 1. Large residual stresses are commonly found in sputtered thin films. Since the temperature rise during deposition is less than 40 8C, the main residual stresses in the as-grown films are the intrinsic stresses, which arise from the film density and structure [17]. From Table 1, the residual stresses in the films exhibit compressive, which were observed to reduce from 4.4 GPa, 4.3 GPa, to 0.6 GPa. The average gain size of asgrown, bombarded and annealed films are 6.0 nm, 6.2 nm and 25.1 nm, respectively. Fig. 2 shows the PL spectra of as-grown, annealed and bombarded films, respectively. In general, PL spectrum of ZnO consists of two bands, near band edge (NBE) excitonic UV emission and defect related deep level emission (DLE) in the visible range [18]. The UV emission band centering at 3.11 eV can be observed from as-grown film. Compared with the asgrown films, the energy of UV luminescence of ion beam bombarded films shifted from 3.11 eV to 3.22 eV. The shift of band-gap energy is related to the ion bombardment. Additional energy has been added to the film when it is bombarded with lowenergy ions, which leads to part loss of oxygen and
349
stoichiometric disturbances in the ZnO lattice. As well known, ZnO is naturally n-type material, and the Fermi level will be inside of the conduction band by the quantity jn when there is loss of oxygen. Since the states below jn of the conduction band are filled, the absorption edge should shift to the higher energy by jn. The NBE emission shifts to the higher energy correspondingly [13,19–21]. Therefore, the energy of UV emission known as near band edge emission increased from 3.11 eV to 3.22 eV. After annealing treatment at 700 8C in air for 8 h, two emission bands can be observed from the sample. One is UV emission centering at 3.15 eV, the other is the green emission at 2.41 eV. The energy of UV luminescence shifted from 3.11 eV to 3.15 eV. The blueshift of band-gap energy is believed to originate from the residual stress along the c-axis due to the lattice distortion. The as-grown ZnO thin film exhibits a compressive stress in nature. According to XRD analysis, the decrease of the length of c-axis was found in the annealed film and the stress was transited from the compressive to a tensile one. If the tensile is accumulated, the band-gap energy is increased. Therefore, the energy of UV emission was observed to shift from 3.11 eV to 3.15 eV. From Fig. 2, we also find that the intensity of light emission increased after post-treatment. It has been reported that the PL emission characteristics of ZnO films are strongly dependent on both the crystal
Fig. 2. Room temperature photoluminescence spectra of the as-grown, bombarded and annealed ZnO films.
350
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
quality of the film and the film stoichiometry [22,23]. The luminescence efficiency of the light emission can be described by the following formula [24]: h¼
IR ðIR þ IIR Þ
(3)
where, h is luminescence efficiency, IR and INR are radiative and non-radiative transition probabilities, respectively. For the as-grown ZnO films prepared by dc reactive sputtering method, the chemical composition was nonstoichiometric, and usually consisted of excess Zn atoms [25]. Therefore, many lattice defects and surface defects were contained in the as-grown ZnO films. These defects produced various non-radiative centers and reduced light emission from the ZnO. Thus, the non-radiative transition probability INR is high for the as-grown film, and the light emission intensity was very low in the as-grown films spectrum. After annealing in air at high temperature, these non-radiative-related defects could be reduced and re-structured. However, the high temperature anneal would also reduce the traps for non-radiative transition. Consequently, the visible light emission associated with structural defect was enhanced. Such reducing and re-structuring made the crystal structure more perfect and lead to a decrease in the non-radiative transition probability INR. Accordingly, the light
emission was enhanced markedly by annealing in air. We have taken notice of our results, the green peak is sensitive to annealing temperature, the intensity of the green peak is much higher than that of UV emission peak. In the case of ion beam bombardment, the ion bombardment has the effects of narrowing the diffraction peak, indicating that grain growth has occurred. The crystal quality of the films has improved. The nonradiative transition probability INR was suppressed in a certain extent by ion beam bombardment. Thus, the light emission enhanced slightly. The above analysis was supported by the results of the XRD characterization shown in Fig. 1. The full width at half maximum (FWHM) values of the (0 0 2) diffraction peaks of ZnO thin films annealed in air, bombarded by ion beam were 0.3318 and 1.3338, respectively which were smaller than that of the as-deposited film (FWHM = 1.3768). These confirmed that the annealing in air and bombardment with ion beam would improve the crystal quality of the films. Fig. 3 shows the transmittance spectra for the asgrown ZnO film, annealed film in air and bombarded film, respectively. The transmittance spectra have shown that all films exhibit high transmittance in the 400–1200 nm range. Transmission, however, falls very sharply in the UV region due to the onset of fundamental absorption. Post-deposition annealing in
Fig. 3. Optical transmittance spectra of the as-grown, bombarded and annealed ZnO films.
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
351
Fig. 4. a2 vs. hn curves for the optical band-gap determination in the as-grown, bombarded and annealed ZnO films.
air influences the optical properties of ZnO films exhibiting band-gap shrinkage. After ion beam bombardment, the absorption edge of the film was observed to shift towards lower wavelength. We can further explain the phenomena of absorption edge shifts from the plots of a2 versus photon energy. Fig. 4 shows the a2 versus photon energy curves of as-grown film, annealed film in air and bombarded film, respectively. Where, a represents the absorption coefficient and hn photon energy. The as-grown ZnO, annealed ZnO and bombarded ZnO samples have optical band-gap energy of 3.31 eV, 3.26 eV and 3.33 eV, respectively. In the case of ion beam bombardment, the optical band-gap was shifted from 3.31 eV to 3.33 eV. This shift to a high-energy value can be explained in terms of the Burstein–Moss bandfilling [19]. After annealing at 700 8C in air for 8 h, the optical band-gap was found to shift from 3.31 eV to 3.26 eV for the increase of ZnO grain size and a change in the nature and strength of the interaction potentials between defects and host materials, which increases the tailing of the absorption edge and hence reduces the band-gap. The UV emission was shifted from 3.11 eV to 3.15 eV after annealing at 700 8C, while the shift of optical band-gap from 3.31 eV to 3.26 eV. The shift is different from the result of PL in the range of UV. In the case of ion beam bombard-
ment, the shift is consistent with the result of PL in the range of UV. The UV emission was shifted from 3.11 eV to 3.22 eV by the shift of optical band-gap from 3.31 eV to 3.33 eV.
4. Conclusion We have shown both annealing and ion bombardment influences the structure and the emission characteristics of ZnO films. XRD results reveal that both the thermal annealing and ion bombardment have the effects of narrowing the diffraction peak, indicating that grain growth has occurred, and shifting the (0 0 2) peaks to higher 2u angles, a result of the partial relief of residual stresses within as-treated films. The UV emission peaks of both the annealed and bombarded films were observed to shift towards lower wavelength. In addition, an emission peak centering at 2.41 eV was found in the annealed film PL spectrum. The intensity of UV emission was improved. The band-gap edge shifts were observed in both optical transmittance and absorption spectra after different post-treatments. Post-deposition annealing in air influences the optical properties of ZnO films exhibiting a band-gap shrinkage for a change in the nature and strength of the interaction
352
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
potentials between defects and host materials. The absorption edge of the bombarded film was observed to shift towards lower wavelength due to Burstein– Moss band-filling.
References [1] D.C. Look, Mater. Sci. Eng. B 80 (2001) 383. [2] D. Wiersma, Nature (Lond.) 406 (2000) 132. [3] P.H. Holloway, T.A. Trottier, B. Abrams, C. Kondoleon, S.L. Jones, J.S. Sebastian, W.J. Thomas, H. Swart, J. Vac. Sci. Technol. B 17 (1999) 758. [4] H. Cao, J.Y. Xu, D.Z. Zhang, S.H. Chang, S.T. Ho, E.W. Seelig, X. Liu, R.P.H. Xu, Phys. Rev. Lett. 84 (2000) 5584. [5] K.K. Kim, J.H. Song, H.J. Jung, S.J. Park, J. Appl. Phys. 87 (2000) 3573. [6] H.Z. Wu, K.M. He, D.J. Qui, D.M. Huang, J. Cryst. Growth 217 (2000) 131. [7] S.H. Bae, S.Y. Lee, B.J. Jin, S. Im, Appl. Surf. Sci. 154 (2000) 458. [8] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [9] Y.G. Wang, S.P. Lau, H.W. Lee, S.F. Yu, B.K. Tay, X.H. Zhang, H.H. Hng, J. Appl. Phys. 94 (2003) 354. [10] Y.F. Chen, D. Bagnall, T.F. Yao, Mater. Sci. Eng. B 75 (2000) 190.
[11] K. Ellmer, J. Phys. D Appl. Phys. 33 (2000) 17. [12] V. Srikant, V. Sergo, D.R. Clarke, J. Am. Ceram. Soc. 78 (1995) 1931. [13] G. Gautherin, Chr. Weissmantel, Thin Solid Films 50 (1978) 135. [14] T.J. Vink, W. Walrave, J.L.C. Daams, P.C. Baarslag, J.E.A.M. van den Meerakker, Thin Solid Films 266 (1995) 145. [15] R. Cebulla, R. Wendt, K. Ellmer, J. Appl. Phys. 83 (1998) 1087. [16] B.D. Cullity, Elements of X-Ray Diffractions, AdditionWesley, Reading, MA, 1978. [17] B. Window, K.H. Muller, Thin Solid Films 171 (1989) 183. [18] D.C. Reynolds, D.C. Look, B. Jobai, C.W. Litton, T.C. Collins, W. Harsch, G. Cantwell, Phys. Rev. B 57 (1998) 12151. [19] E. Burstein, Phys. Rev. 93 (1954) 632. [20] F.K. Shan, B.I. Kim, G.X. Liu, Z.F. Liu, J.Y. Sohn, W.J. Lee, B.C. Shin, Y.S. Yu, J. Appl. Phys. 95 (2004) 4772. [21] S.O. Kucheyev, J.S. Williams, C. Jagadish, J. Zou, Phys. Rev. B 67 (2003) 094115. [22] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. [23] Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 (1998) 3270. [24] S. Shionoya, W.M. Yen, Phosphor Handbook, Chemical Rubber, Cleveland, 1999. [25] H.Q. Ni, Y.F. Lu, Z.M. Ren, Jpn. J. Appl. Phys. Part 1 40 (2001) 4103.
Influence of different post-treatments on the structure and optical properties of zinc oxide thin films Ruijin Hong*, Jianbing Huang, Hongbo He, Zhengxiu Fan, Jianda Shao Research and Development Center for Optical Thin Film Coatings, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China Received in revised form 25 August 2004; accepted 31 August 2004 Available online 12 October 2004
Abstract Zinc oxide (ZnO) films with c-oriented were grown on fused quartz glass substrates at room temperature using dc reactive magnetron sputtering. The as-grown films were annealed at 700 8C in air and bombarded by ion beam, respectively. The effects of post-treatments on the structural and optical properties of the ZnO films were investigated by X-ray diffraction (XRD), photoluminescence (PL), optical transmittance and absorption measurements. The XRD spectra indicate that the crystal quality of ZnO films has been improved by both the post-treatments. Compared with the as-grown sample, both annealed and bombarded samples exhibited blueshift in the UV emission peaks, and a strong green emission was found in the annealed ZnO film. In both optical transmittance and absorption spectra, a blueshift of the band-gap edge was observed in the bombarded film, while a redshift was observed in the annealed film. # 2004 Elsevier B.V. All rights reserved. PACS: 78.66.Db; 71.20.Mq; 78.55.Ap Keywords: Zinc oxide; Thin films; Annealing; Ion beam bombardment
1. Introduction In recent years, wide-band-gap semiconductor compounds have attracted a great deal of attention because of the intense commercial interest in developing practical short-wavelength semiconductor * Corresponding author. Tel.: +86 21 699 18492; fax: +86 21 699 18028. E-mail address: [email protected] (R. Hong).
diode lasers for the huge market needs. Wurtzite ZnO is a wide-band-gap (3.37 eV at room temperature) semiconductor material and has a large excitation binding energy of 60 meV. In this regard, ZnO is a promising material [1–4]. ZnO thin films have been prepared by many methods such as sputtering [5], reactive thermal and electron-beam evaporation [6], pulse laser deposition [7], chemical vapor deposition [8], oxidation of metallic Zn [9] and molecular beam epitaxy [10]. Among all, magnetron sputtering is
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.08.037
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
characterized by several advantages: (1) low substrate temperature (down to room temperature); (2) good adhesion of films on substrates; (3) very good thickness uniformity and high density of the films; and (4) directive deposition from elemental (metallic) targets by reactive sputtering in rare/reactive gas mixtures [11]. The growth conditions such as substrates, deposition temperature, and background gas for depositing ZnO films have been extensively studied. The influence of growth conditions on electrical characteristics and crystallographic relationships between epitaxial ZnO thin films and substrate have also been reported [12]. Usually, as-grown films require a special treatment of the surface to reinforce the stability of the films and to reduce the possible undesirable influence of the surface. The method of conventional thermal annealing of zinc oxide films has been performed on a number of experimental conditions, showing an improvement in film quality as regards optical and acoustical propagation losses. As a consequence of furnace thermal annealing, an improvement in crystallite orientation has been observed, as evidenced by increased alignment of (0 0 2) crystal planes parallel to the substrate, possible crystallite growth, and considerable stress relief. Bombarding the specimen with ions of reactive elements is a promising way of modifying the surface properties of mechanical or electronic components. The low-energy ion beam bombardment is often effective in material properties’ enhancement. It usually increases the film adhesion to the substrate and the surface coverage. Moreover, it is possible to control the density, the hardness and preferred crystal orientation of the film [13]. In this work, we report post-treatments performed by both thermal annealing and ion beam bombardment on the as-grown ZnO thin films, and discuss the influences of annealing and ion beam bombardment on the structure and optical properties of ZnO thin films.
347
oxygen were used as the sputtering and reactive gases, respectively. The target-to-substrate distance was 15 cm. The cathode was mounted on a water-cooled copper plate. The chamber was pumped to a base pressure of 1 103 Pa before deposition. Film growth was carried out in the growth ambient with a mixture of argon (40%) and oxygen (60%) and at a constant working pressure of 0.15 Pa. Both annealing and ion beam bombardment were introduced as posttreatment in the experiment. For annealing, the asgrown films were heated to 700 8C in air at a rate of 100 8C/h. The temperature was then maintained at 700 8C for 8 h after which cooling was also carried out at a rate of 100 8C/h. A 12-cm RF ion source (Ion Tech, Inc.) was employed during the course of ion beam bombardment. The energy of the ion beam was fixed at 550 V, while its current was fixed at 150 mA. In the case of ion beam treatment, a gas mixture of argon (40%) and oxygen (60%) was introduced into the discharge chamber as the bombarding species to keep the same ambient as in the forming one. The bombardment process lasted for 20 min. The crystal structure of the films was characterized by X-ray diffraction (XRD) using a Rigaku D/Max-B system, with Cu Ka radiation (l = 0.15408 nm). Biaxial film stress (s) and the average crystallite size D were evaluated from XRD u/2u scans. PL spectra were acquired in a JASCO fluorespectrometer from 300 nm to 750 nm at room temperature; the excitation light was the 284 nm line of a xenon lamp, the filter wavelength was 290 nm. The optical transmittance and absorption of the films were measured with UV–visible spectroscopy (Lambda900 UV–vis–NIR double beam spectrophotometer). The thickness of ZnO thin films was measured to be in the range of 200–300 nm. All the measurements were carried out at room temperature.
3. Results and discussion 2. Experiment Zinc oxide films (ZnO) were grown by dc planar magnetron sputtering using an 8.6 cm-in-diameter Zn target (99.99%) on fused silica at room temperature. The quartz glass substrates were rinsed in acetone, ethanol and distilled water sequentially. Argon and
X-ray diffraction (XRD) u/2u patterns of the ZnO films are shown in Fig. 1, inset gives the XRD v-rocking curves of the (0 0 2) diffraction peak of the ZnO films. The XRD spectra reveal the influence of post-treatment on the structure of the ZnO thin film. It was apparent that all the ZnO films consisted of a single ZnO phase oriented with its c-axis normal to the
348
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
Fig. 1. XRD patterns of the as-grown, bombarded and annealed ZnO films.
substrate surface, no diffraction from randomly oriented grains or impurity phases can be observed from the X-ray patterns. According to Fig. 1, both the thermal annealing and ion bombardment have the effects of narrowing the diffraction peak, indicating that grain growth has occurred, and shifting the (0 0 2) peaks to higher 2u angles, a result of the partial relief of residual stresses within as-treated films. Compared with the zinc oxide powders, all the films formed in this experiment exhibit discrepancy in d-value (d is interplanar spacing), which is due to the variation of residual stress in the films [14]. The full width at half maximum (FWHM) of as-grown, ion bombarded and annealed films have the value of 1.3768, 1.3338and 0.3318, respectively. The diffraction peaks are 33.758, 33.778and 34.508 (2u), respectively. From the integral width and peak position of the (0 0 2) peak, the grain size and the residual stress in the films are calculated (see Table 1). The average grain size of the film can be estimated by Scherrer formula, using FWHM value of the XRD diffraction
peaks as follows [15]: 0:9l (1) B cos u where D, l, u and B are the mean grain size, the X-ray wavelength of 0.154 nm, Bragg diffraction angle and the FWHM of the diffraction peak of the (0 0 2) direction at around 348 (2u) for ZnO films, respectively. The calculation of the film stress is based on ðc c Þ the biaxial strain model. The strain e ¼ filmcbulkbulk in c-axis, i.e., perpendicular to the substrate surface, was calculated by XRD data, where cfilm and cbulk are the lattice constants (c) of ZnO film and bulk (or powder), respectively. To derive the film stress sfilm parallel to the film surface, the following formula is used, which is valid for a hexagonal lattice:
D¼
s XRD film ¼
2c213 c33 ðc11 þ c12 Þ cfilm cbulk 2c13 cbulk
(2)
For the elastic constants cij, the following values for single crystal ZnO are used: c11 = 208.8 GPa,
Table 1 The data evaluated from XRD u/2u scans for the samples before and after treatments Sample
Interplanar spacing d (nm)
FWHM (8)
Average grain size (nm)
sfilm (GPa)
As-grown Bombarded Annealed
0.2652 0.2651 0.2596
1.376 1.333 0.331
6.0 6.2 25.1
4.4 4.3 0.6
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
c33 = 213.8 GPa, c12 = 119.7 GPa, c13 = 104.2 GPa [16]. This yields the following numerical relation for the stress derived from XRD: sfilm = 233 e (GPa). Biaxial stresses, calculated from (0 0 2) peak positions, are shown in Table 1. Large residual stresses are commonly found in sputtered thin films. Since the temperature rise during deposition is less than 40 8C, the main residual stresses in the as-grown films are the intrinsic stresses, which arise from the film density and structure [17]. From Table 1, the residual stresses in the films exhibit compressive, which were observed to reduce from 4.4 GPa, 4.3 GPa, to 0.6 GPa. The average gain size of asgrown, bombarded and annealed films are 6.0 nm, 6.2 nm and 25.1 nm, respectively. Fig. 2 shows the PL spectra of as-grown, annealed and bombarded films, respectively. In general, PL spectrum of ZnO consists of two bands, near band edge (NBE) excitonic UV emission and defect related deep level emission (DLE) in the visible range [18]. The UV emission band centering at 3.11 eV can be observed from as-grown film. Compared with the asgrown films, the energy of UV luminescence of ion beam bombarded films shifted from 3.11 eV to 3.22 eV. The shift of band-gap energy is related to the ion bombardment. Additional energy has been added to the film when it is bombarded with lowenergy ions, which leads to part loss of oxygen and
349
stoichiometric disturbances in the ZnO lattice. As well known, ZnO is naturally n-type material, and the Fermi level will be inside of the conduction band by the quantity jn when there is loss of oxygen. Since the states below jn of the conduction band are filled, the absorption edge should shift to the higher energy by jn. The NBE emission shifts to the higher energy correspondingly [13,19–21]. Therefore, the energy of UV emission known as near band edge emission increased from 3.11 eV to 3.22 eV. After annealing treatment at 700 8C in air for 8 h, two emission bands can be observed from the sample. One is UV emission centering at 3.15 eV, the other is the green emission at 2.41 eV. The energy of UV luminescence shifted from 3.11 eV to 3.15 eV. The blueshift of band-gap energy is believed to originate from the residual stress along the c-axis due to the lattice distortion. The as-grown ZnO thin film exhibits a compressive stress in nature. According to XRD analysis, the decrease of the length of c-axis was found in the annealed film and the stress was transited from the compressive to a tensile one. If the tensile is accumulated, the band-gap energy is increased. Therefore, the energy of UV emission was observed to shift from 3.11 eV to 3.15 eV. From Fig. 2, we also find that the intensity of light emission increased after post-treatment. It has been reported that the PL emission characteristics of ZnO films are strongly dependent on both the crystal
Fig. 2. Room temperature photoluminescence spectra of the as-grown, bombarded and annealed ZnO films.
350
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
quality of the film and the film stoichiometry [22,23]. The luminescence efficiency of the light emission can be described by the following formula [24]: h¼
IR ðIR þ IIR Þ
(3)
where, h is luminescence efficiency, IR and INR are radiative and non-radiative transition probabilities, respectively. For the as-grown ZnO films prepared by dc reactive sputtering method, the chemical composition was nonstoichiometric, and usually consisted of excess Zn atoms [25]. Therefore, many lattice defects and surface defects were contained in the as-grown ZnO films. These defects produced various non-radiative centers and reduced light emission from the ZnO. Thus, the non-radiative transition probability INR is high for the as-grown film, and the light emission intensity was very low in the as-grown films spectrum. After annealing in air at high temperature, these non-radiative-related defects could be reduced and re-structured. However, the high temperature anneal would also reduce the traps for non-radiative transition. Consequently, the visible light emission associated with structural defect was enhanced. Such reducing and re-structuring made the crystal structure more perfect and lead to a decrease in the non-radiative transition probability INR. Accordingly, the light
emission was enhanced markedly by annealing in air. We have taken notice of our results, the green peak is sensitive to annealing temperature, the intensity of the green peak is much higher than that of UV emission peak. In the case of ion beam bombardment, the ion bombardment has the effects of narrowing the diffraction peak, indicating that grain growth has occurred. The crystal quality of the films has improved. The nonradiative transition probability INR was suppressed in a certain extent by ion beam bombardment. Thus, the light emission enhanced slightly. The above analysis was supported by the results of the XRD characterization shown in Fig. 1. The full width at half maximum (FWHM) values of the (0 0 2) diffraction peaks of ZnO thin films annealed in air, bombarded by ion beam were 0.3318 and 1.3338, respectively which were smaller than that of the as-deposited film (FWHM = 1.3768). These confirmed that the annealing in air and bombardment with ion beam would improve the crystal quality of the films. Fig. 3 shows the transmittance spectra for the asgrown ZnO film, annealed film in air and bombarded film, respectively. The transmittance spectra have shown that all films exhibit high transmittance in the 400–1200 nm range. Transmission, however, falls very sharply in the UV region due to the onset of fundamental absorption. Post-deposition annealing in
Fig. 3. Optical transmittance spectra of the as-grown, bombarded and annealed ZnO films.
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
351
Fig. 4. a2 vs. hn curves for the optical band-gap determination in the as-grown, bombarded and annealed ZnO films.
air influences the optical properties of ZnO films exhibiting band-gap shrinkage. After ion beam bombardment, the absorption edge of the film was observed to shift towards lower wavelength. We can further explain the phenomena of absorption edge shifts from the plots of a2 versus photon energy. Fig. 4 shows the a2 versus photon energy curves of as-grown film, annealed film in air and bombarded film, respectively. Where, a represents the absorption coefficient and hn photon energy. The as-grown ZnO, annealed ZnO and bombarded ZnO samples have optical band-gap energy of 3.31 eV, 3.26 eV and 3.33 eV, respectively. In the case of ion beam bombardment, the optical band-gap was shifted from 3.31 eV to 3.33 eV. This shift to a high-energy value can be explained in terms of the Burstein–Moss bandfilling [19]. After annealing at 700 8C in air for 8 h, the optical band-gap was found to shift from 3.31 eV to 3.26 eV for the increase of ZnO grain size and a change in the nature and strength of the interaction potentials between defects and host materials, which increases the tailing of the absorption edge and hence reduces the band-gap. The UV emission was shifted from 3.11 eV to 3.15 eV after annealing at 700 8C, while the shift of optical band-gap from 3.31 eV to 3.26 eV. The shift is different from the result of PL in the range of UV. In the case of ion beam bombard-
ment, the shift is consistent with the result of PL in the range of UV. The UV emission was shifted from 3.11 eV to 3.22 eV by the shift of optical band-gap from 3.31 eV to 3.33 eV.
4. Conclusion We have shown both annealing and ion bombardment influences the structure and the emission characteristics of ZnO films. XRD results reveal that both the thermal annealing and ion bombardment have the effects of narrowing the diffraction peak, indicating that grain growth has occurred, and shifting the (0 0 2) peaks to higher 2u angles, a result of the partial relief of residual stresses within as-treated films. The UV emission peaks of both the annealed and bombarded films were observed to shift towards lower wavelength. In addition, an emission peak centering at 2.41 eV was found in the annealed film PL spectrum. The intensity of UV emission was improved. The band-gap edge shifts were observed in both optical transmittance and absorption spectra after different post-treatments. Post-deposition annealing in air influences the optical properties of ZnO films exhibiting a band-gap shrinkage for a change in the nature and strength of the interaction
352
R. Hong et al. / Applied Surface Science 242 (2005) 346–352
potentials between defects and host materials. The absorption edge of the bombarded film was observed to shift towards lower wavelength due to Burstein– Moss band-filling.
References [1] D.C. Look, Mater. Sci. Eng. B 80 (2001) 383. [2] D. Wiersma, Nature (Lond.) 406 (2000) 132. [3] P.H. Holloway, T.A. Trottier, B. Abrams, C. Kondoleon, S.L. Jones, J.S. Sebastian, W.J. Thomas, H. Swart, J. Vac. Sci. Technol. B 17 (1999) 758. [4] H. Cao, J.Y. Xu, D.Z. Zhang, S.H. Chang, S.T. Ho, E.W. Seelig, X. Liu, R.P.H. Xu, Phys. Rev. Lett. 84 (2000) 5584. [5] K.K. Kim, J.H. Song, H.J. Jung, S.J. Park, J. Appl. Phys. 87 (2000) 3573. [6] H.Z. Wu, K.M. He, D.J. Qui, D.M. Huang, J. Cryst. Growth 217 (2000) 131. [7] S.H. Bae, S.Y. Lee, B.J. Jin, S. Im, Appl. Surf. Sci. 154 (2000) 458. [8] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [9] Y.G. Wang, S.P. Lau, H.W. Lee, S.F. Yu, B.K. Tay, X.H. Zhang, H.H. Hng, J. Appl. Phys. 94 (2003) 354. [10] Y.F. Chen, D. Bagnall, T.F. Yao, Mater. Sci. Eng. B 75 (2000) 190.
[11] K. Ellmer, J. Phys. D Appl. Phys. 33 (2000) 17. [12] V. Srikant, V. Sergo, D.R. Clarke, J. Am. Ceram. Soc. 78 (1995) 1931. [13] G. Gautherin, Chr. Weissmantel, Thin Solid Films 50 (1978) 135. [14] T.J. Vink, W. Walrave, J.L.C. Daams, P.C. Baarslag, J.E.A.M. van den Meerakker, Thin Solid Films 266 (1995) 145. [15] R. Cebulla, R. Wendt, K. Ellmer, J. Appl. Phys. 83 (1998) 1087. [16] B.D. Cullity, Elements of X-Ray Diffractions, AdditionWesley, Reading, MA, 1978. [17] B. Window, K.H. Muller, Thin Solid Films 171 (1989) 183. [18] D.C. Reynolds, D.C. Look, B. Jobai, C.W. Litton, T.C. Collins, W. Harsch, G. Cantwell, Phys. Rev. B 57 (1998) 12151. [19] E. Burstein, Phys. Rev. 93 (1954) 632. [20] F.K. Shan, B.I. Kim, G.X. Liu, Z.F. Liu, J.Y. Sohn, W.J. Lee, B.C. Shin, Y.S. Yu, J. Appl. Phys. 95 (2004) 4772. [21] S.O. Kucheyev, J.S. Williams, C. Jagadish, J. Zou, Phys. Rev. B 67 (2003) 094115. [22] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. [23] Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 (1998) 3270. [24] S. Shionoya, W.M. Yen, Phosphor Handbook, Chemical Rubber, Cleveland, 1999. [25] H.Q. Ni, Y.F. Lu, Z.M. Ren, Jpn. J. Appl. Phys. Part 1 40 (2001) 4103.