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Effect of annealing temperature on the structure and optical properties of In-doped ZnO thin films
Journal of Alloys and Compounds 484 (2009) 575–579
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of annealing temperature on the structure and optical properties of In-doped ZnO thin films L.P. Peng a , L. Fang a,b,∗ , X.F. Yang a , Y.J. Li a , Q.L. Huang a , F. Wu a , C.Y. Kong c a b c
Department of Applied Physics, Chongqing University, Chongqing 400030, PR China Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China, Chongqing University, Chongqing 400030, PR China Department of Applied physics, Chongqing Normal University, Chongqing 400047, PR China
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 28 April 2009 Accepted 29 April 2009 Available online 6 May 2009 Keywords: Thin films Luminescence Optical properties Sputtering
a b s t r a c t High quality In-doped ZnO (ZnO:In) thin films were deposited on the quartz glass substrates at room temperature by using radio frequency magnetron sputtering. The effect of annealing temperature on the structure and optical properties of the ZnO:In films was investigated. It was found that the as-deposited film has a hexagonal wurtzite structure with c-axis perpendicular to the substrate. Accompanying with increasing annealing temperature from 400 ◦ C to 700 ◦ C, the crystal quality of the thin films can be improved, and then it will be deteriorated when the annealing temperature goes over 700 ◦ C. The transmittance of the ZnO:In films was revealed to be 85% in the visible region, and it changes slightly after annealing. The optical band gap increases from 3.17 eV to 3.23 eV with annealing temperature due to the decrease of the tensile strain in the films, and a linear relationship between the band gap and the strain was obtained. PL results show that all the emissions increase while annealing temperature rising from 400 ◦ C to 800 ◦ C and the origins of all the emissions were discussed in detail. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a direct wide band-gap (Eg ∼3.37 eV) semiconductor with a large exciton binding energy (60 meV) [1]. Due to its high conductivity and high transmittance, ZnO thin film has attracted attention as the transparent electrode in applications such as amorphous silicon solar cell, liquid crystal display, heat mirrors, etc. In recent years, great interests are focused on its potential applications in optical devices because of its strong PL spectra and cheap substrates [2]. It was reported that PL spectrum of ZnO consists mainly of two bands. One is in UV region corresponds to near-bandedge (NBE) emission which is attributed to exciton; the other in the visible region because of structural defects and impurities [3]. To obtain high quality ZnO thin film, a variety of techniques including chemical vapor deposition (CVD) [4], magnetron sputtering [5–7], pulsed laser deposition (PLD) [8] and sol–gel technique [9] have been adopted. With the advantages of low deposition temperature, simple processing, high growth rate, low-cost equipment and suitability for large areas deposition, magnetron sputtering is one of the most promising deposition techniques. In order to develop material for special applications, doped ZnO films have been fabricated and investigated by many research
∗ Corresponding author at: Department of Applied Physics, Chongqing University, Chongqing 400030, PR China. Tel.: +86 23 65105870. E-mail address: [email protected] (L. Fang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.139
groups. It was reported that In is one of the most efficient elements utilized to improve the opto-electrical properties [10,11]. Meanwhile, thermal annealing is also a widely used method to improve crystal quality and to decrease structure defect in materials. During the annealing process, dislocations and other structural defects move in the material and adsorption/decomposition may occur at the surface, thus the structure and the stoichiometric ratio of the material will change [12]. Up to now, there is no investigation about the effect of annealing temperature on the PL spectra of In-doped ZnO (ZnO:In) thin film. In this study, ZnO:In films prepared by radio frequency (RF) magnetron sputtering were annealed at the temperature ranging from 400 ◦ C to 800 ◦ C in air, and the effect of the annealing temperature on the crystal properties, transmittance and PL spectra of ZnO:In films have been studied and discussed. 2. Experimental A powder target out of the mixed powder of high purity ZnO (99.99%) and In2 O3 (99.99%) (In/(In + Zn) = 5%) was acted as the sputtering source material. Quartz glass (20 mm × 13 mm × 1 mm) was employed as the substrate, and it was precleaned ultrasonically in acetone, rinsed in alcohol and then dried in hot air. ZnO:In transparent conductive thin films were deposited by using 13.56 MHz RF magnetron sputtering. The distance between the target and substrate was 70 mm. In order to avoid contaminating the film, the chamber was evacuated to a base pressure of 6.6 × 10−4 Pa, and then, argon gas was introduced into the chamber through a mass flow controller. The mass flow of argon gas was fixed at 24 sccm, and the sputtering pressure was 2.0 Pa. The target was pre-sputtered for 15 min to remove contaminants. All the films were deposited at room tem-
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perature (RT) and annealed in air for an hour in the temperature range of 400–800 ◦ C. The structure of the films was characterized by X-ray diffraction (XRD) with a MRD-SingleScan diffractometer using the Cu K␣ line ( = 1.54 Å). The power of XRD was 1200 W and the scan was performed from 15◦ to 85◦ at a speed of 3◦ /min, with a step size of 0.2◦ . The surface morphology and the ratio of atoms of the films were investigated by SEM and EDS. The transmission spectrum of the film was measured by a double-beam ultraviolet/visible spectrophotometer with a wavelength range of 200–800 nm and the optical band gaps were determined from the transmission spectra. The RT photoluminescence spectra (PL) were recorded by a spectraphotometer with excitation wavelength of 325 nm. The film thickness was measured by a Dektak II step-measurer, and it was obtained as around 450 nm.
3. Result and discussions 3.1. Structure properties Fig. 1 shows the SEM image of the as-deposited ZnO:In thin films. It depicts that the surface of the films is smooth and dense, and the grain size is uniformly distributed with an average size of about 80 nm. EDS analysis indicates that the films contained approximately 5.08 at.% In, which is higher than that in the target (5 at.%). Cai et al.’s results indicate that the content of Al in the ZnO thin films was less than that in the target [13]. Ma et al. reported that the Ga atomic concentration for Zn1 − x Mgx O:Ga thin films is larger than that in the targets, and it was affected by the concentration of Mg [14]. The reason why the concentration of the dopant in the thin films prepared from sputtering is different from the target may be that the sputtering rate and re-evaporation rate from the substrate before being oxidized vary from element to element. Such problems need to be further investigated. XRD patterns of the ZnO:In films before and after annealing at various temperatures are shown in Fig. 2. It illustrates that the films are single phased with a wurtzite structure characteristic of pure ZnO. All the films exhibit preferential orientation with c-axis perpendicular to the substrate surface. No phase corresponding to In oxide or Zn–In compounds was observed, which implies that a low level of In incorporates into the ZnO lattice [15], and In atom replaces Zn atom in the hexagonal wurtzite lattice and/or In segregates to the no-crystalline region in the boundary [7]. Fig. 3 shows the intensity and the full width at maximum (FWHM) for (0 0 2) peak as a function of the annealing temperature. It was found that the intensity of the XRD diffraction (0 0 2) peak decreases with annealing temperature below 400 ◦ C, whereas it increases with increasing annealing temperature from 400 ◦ C up to 700 ◦ C. Higher annealing temperature provided more energy that would cause a
Fig. 1. SEM image of the as-deposited ZnO:In thin films.
Fig. 2. X-ray diffraction patterns of the ZnO:In thin films before and after annealing at different temperatures.
decrease of defects in the films and thus improve the crystal quality. The (0 0 2) peak intensity becomes weak when the annealing temperature exceeds 700 ◦ C, which can be explained by the presence of porosity. Fang et al. demonstrated a similar result as they studied the influence of post-annealing treatment on the structural properties of ZnO films [16]. Meanwhile, the FWHM decrease significantly with the annealing temperature up to 600 ◦ C, and then it decreases slightly above 600 ◦ C. According to Scherrer formula, the decrease of the FWHM indicates the increase of grain size of the films [17]. According to the XRD patterns, one can infer that a proper annealing temperature can improve the crystal quality, and 700 ◦ C is the best annealing temperature in our study. For the hexagonal system, using Bragg function, the d-spacing is related to the lattice parameters by the following equations: 2d sin = n, 1 4 = 3 d2
h2
+ hk a2
+ k2
(1)
+
l2 c2
,
(2)
where and are the diffraction angle and the wavelength; h, k and l are the miller indices; a and c are the lattice parameters, respectively. The calculated values of lattice constants c of the films are shown in Table 1. It is well known that the films with value of lattice parameter c greater than the theoretical value possess a positive or extensive strain in them whereas those with lower values
Fig. 3. The intensity of the (0 0 2) XRD peaks and the full width at half maximum (FWHM) for the ZnO:In thin films before and after annealing at different temperatures.
L.P. Peng et al. / Journal of Alloys and Compounds 484 (2009) 575–579
577
Table 1 Lattice parameter, strain along c-axis and bandgap of the ZnO:In thin films before and after annealing at different temperatures. Annealing temperature (◦ C)
2 (◦ )
Lattice constant c (nm)
Strain, εzz (%)
Bandgap (eV)
As-deposited 400 500 600 700 800
33.47 33.67 33.70 33.76 33.92 34.09
5.354 5.323 5.320 5.307 5.287 5.260
2.81 2.24 2.17 1.93 1.54 1.02
3.28 3.17 3.18 3.19 3.21 3.23
have a negative or compressive strain. The larger value of lattice constant c of the films compared to the unstressed powder value shows that the unit cell is enlarged along the c-axis, and tensile force in the plane of the films. The tensile force becomes weaker as the annealing temperature increases, resulting in a decrease in “c” value. The larger value of lattice parameter (c) observed in the films reveals that the added element is in the interstitial site of the host matrix, and the dopants are activated after annealing, thus the lattice parameter decreases [18,19]. There are two aspects that attribute to the strain in the films, i.e., intrinsic strain introduced by impurities and defects in the lattice, and thermal expansion coefficient mismatch between the film and the substrate [20]. The strain value along c-axis, εzz , can be calculated by the following formula [21]: εzz
c − c0 = × 100%, c0
(3)
where c is the lattice parameter of strained ZnO:In thin films calculated from XRD result and c0 (0.521 nm) is the unstrained parameter of bulk ZnO. The obtained results of the strain along c-axis and the position of (0 0 2) peak of ZnO:In thin films are plotted in Fig. 4 as a function of annealing temperature. The films are highly strained with the εzz value up to 2.8% in the as-deposited film. With increasing annealing temperature, the strain is partially relieved. The maximal εzz value in our films is much larger than that (εzz ∼0.4%) of ZnO films deposited on (0 0 1) Al2 O3 substrate via PLD reported by Fouchet at al. [22], but is comparable to the result (εzz ∼2.15%) of ZnO films on glass substrate via DC magnetron sputtering by He et al. [20]. Ghosh et al. have ever obtained different strain values for ZnO films on different substrates by sol–gel technique [23]. The reported strain values of ZnO thin films are quite different from each other, and may be because the deposited method and the substrate type play important roles.
Fig. 4. The strain along c-axis and 2 value of the (0 0 2) peaks for the ZnO:In thin films before and after annealing at different temperatures.
3.2. Optical properties Fig. 5 shows the transmittance spectra of ZnO:In films in the range of 300–800 nm before and after annealing at different temperatures. All the films exhibit an average transmittance of around 85% in the visible region and a sharp fundamental absorption edge. It indicates in the insert of Fig. 5 that the absorption edge shows red shift after annealing, and with increasing annealing temperature, the value of the red shift decreases. The absorption coefficient (˛) is calculated by using the following equation [24]: ˛=
Ln(1/T ) , d
(4)
where T is the transmittance index and d is the film thickness. In the direct transition semiconductor, the optical band-gap dependence on the absorption coefficient is given by the following equation [25]: 2
(˛hv) = A(hv − Eg ),
(5)
where A, Eg and h are constant, optical band gap, and photo energy, respectively. The Eg can be determined by plotting the curve of (˛h)2 versus photo energy h and extrapolating the linear portion of the curve to the h-axis. Band-gap energy estimated from the absorption edge of the film annealed at 400 ◦ C is shown in Fig. 6 and a band gap of 3.17 eV was obtained. The band gaps obtained from transmittance are listed in Table 1. The band gaps of all the films obtained are smaller than those of the bulk ZnO. The as-deposited film has a band gap of 3.28 eV, and it decreases to 3.17 eV after annealing at 400 ◦ C. The band gap increases from 3.17 eV to 3.28 eV with annealing temperature increasing from 400 ◦ C to 800 ◦ C. Li et al. obtained a similar band-gap value (3.11–3.26 eV) for ZnO:In thin films deposited by DC reactive magnetron sputtering [26].
Fig. 5. Transmission spectra of the ZnO:In thin films before and after annealing at different temperatures: (a) as-deposited; (b) 400 ◦ C; (c) 500 ◦ C; (d) 600 ◦ C; (e) 700 ◦ C; and (f) 800 ◦ C.
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Fig. 6. Plot of (˛h)2 as a function of photo energy for the ZnO:In thin film annealed at 400 ◦ C.
It is well known that the band gap of a semiconductor is affected by the residual strain in the films, and a tensile strain will result in a decrease in the band gap while compressive strain will case an increase. The optical band gap of ZnO films as a function of corresponding strain is shown in Fig. 7. Note that the band gap for the as-deposited thin film was not plotted for it has a high carrier concentration, which will enlarge the band gap of the film due to Burstein-Moss effect. It is evident that the band gap decreases linearly with increasing tensile strain. By least-square regression, the band gap Eg of ZnO:In film after annealing can be expressed in terms of stress εzz : Eg = 3.28 − 0.045 εzz .
(6)
The relationship indeed shows that the band gap of the ZnO:In thin films will increase for negative strain and decrease for positive strain. Therefore, one can find out the band gap knowing the strain value along c-axis from Eq. (6). 3.3. PL properties Photoluminescence is a sensitive technique for examining the film quality, especially its optical properties. Fig. 8 shows the RT PL spectra of ZnO:In thin films as-deposited and annealed at 500 ◦ C and 800 ◦ C, respectively. It indicates that the PL spectra of the films
Fig. 7. The optical band gap of the ZnO:In thin films as a function of the corresponding strain along c-axis.
Fig. 8. Plot of photoluminescence (PL) spectra of the as-deposited and the annealed (at 500 ◦ C and 800 ◦ C) ZnO:In thin films.
depend on annealing temperature significantly. The as-deposited film exhibits a weak emission peak and the PL intensity of emission peaks increases with increase of the annealing temperature. There are four PL emissions: 396 nm (3.13 eV, violet), 446 nm (2.78 eV, blue), 482 nm (2.57 eV, green) and 527 nm (2.35 eV, green) for all the films. It is believed that the near-band-edge emission comes from the ZnO films with less crystal defect or from (0 0 2) oriented ZnO films, and the deep level emission of ZnO relates to the intrinsic defect ZnO films [27]. There are five main intrinsic defects in ZnO films, such as zinc vacancy, oxygen vacancy, interstitial zinc, interstitial oxygen, and antisite oxygen. The violet emission is a NBE emission, but the energy of the NBE emission is smaller than the band gap calculated from transmittance spectra. The luminescence peak shows red shift from the absorption threshold, which is known as Stokes shift. Makino has ever obtained a similar result for ZnO:Ga thin films deposited on ScMgO4 substrate by laser molecular epitaxial growth [28]. In n-type ZnO, an absorptive optical transition occurs from the valence band to the Fermi level or conduction band, while an emissive transition occurs from an impurity-donor band to the valence band. The NBE emission peak in PL has a small intensity in comparison with the Ref. [29]. The reason may be that they had better crystal quality films. The 446 nm blue emission and the 482 nm green emission were observed and the physical mechanism was also discussed by Zhang et al. [30]. They believed that the 446 nm blue emission originated from the electron transition from the shallow donor level of oxygen vacancies to the valence band and electron transition from the shallow donor level of zinc interstitials to the valence band. They indicated that the green emission of 488 nm wavelength originated from the electron transition from the deep donor level of the ionized oxygen vacancies to the valance band. Vanheusden et al. proposed that there are three different charge states of oxygen vacancies: the neutral vacancy (VO ), the single ionized oxygen vacancy (VO + ), and the doubly ionized oxygen vacancy (VO ++ ), and only VO + can act as luminescent centers [31]. During the annealing process, the concentration of single ionized oxygen vacancies increased, so that the corresponding defect luminescence appears stronger [12]. The 527 nm green emission could be ascribed to the electron transition from the bottom of the conduction band to the antisite oxygen level, which was calculated theoretically by Sun [32] and studied experimentally by Lin and Fu [33]. The intensity of the 527 nm green emission increases with increasing annealing temperature, indicating more antisite oxygen or zinc vacancy was produced when the films was annealed at higher temperature in air. Some researchers believe that the increase this emission after O2 annealing dues to the increase of
L.P. Peng et al. / Journal of Alloys and Compounds 484 (2009) 575–579
zinc vacancy [34]. Similar increase of luminescence intensity for the films annealed at higher temperature was reported by Vinodkumar et al. for undoped ZnO films prepared by pulsed laser ablation [2]. The intensity ratio between the blue peak (446 nm) and green peak (527 nm) increases from 3.1 to 5.7 after annealing at 800 ◦ C, indicating that the crystal quality of the films can be improved by annealing [35]. The intense PL emission of the films annealed at 800 ◦ C suggests the possibility of the films for application as blue light emitter. 4. Conclusion In summary, we have discussed the effect of post-annealing temperature on the structure, optical properties and PL spectra of ZnO:In thin films. The crystal quality of the films is highly dependent on the annealing temperature: it can be improved along with the films annealed from 400 ◦ C to 700 ◦ C, and then deteriorates above 700 ◦ C. The average transmittance of the films is about 85% in the visible range, and the band gap increases from 3.17 eV to 3.23 eV with annealing temperature going up from 400 ◦ C to 800 ◦ C. The increase of the band gap is attributed to the decrease of the tensile strain in the films and a linear increment of the optical band gap with the strain was obtained. Due to Stokes shift, the NBE emission energy (3.13 eV) of the films is smaller than the band gap (3.23 eV). The films annealed at 800 ◦ C shows intense PL spectra in the blue region, which means ZnO:In thin films may have application as blue light emitter. Acknowledgements This work was supported by the Program for the New Century Excellent Talents in University of China under grant No. NCET05-0764 and the Graduate Innovation Foundation of Chongqing University under grant No. 200801A1B0060265. The authors also thank Prof. C.Z. Cai in Department of Applied Physics, Chongqing University for the English revision. References [1] S.J. Pearson, D.P.N. Orton, K. Ip, Y.W. Hoe, T. Steiner, Prog. Mater. Sci. 50 (2005) 293–340.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of annealing temperature on the structure and optical properties of In-doped ZnO thin films L.P. Peng a , L. Fang a,b,∗ , X.F. Yang a , Y.J. Li a , Q.L. Huang a , F. Wu a , C.Y. Kong c a b c
Department of Applied Physics, Chongqing University, Chongqing 400030, PR China Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China, Chongqing University, Chongqing 400030, PR China Department of Applied physics, Chongqing Normal University, Chongqing 400047, PR China
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 28 April 2009 Accepted 29 April 2009 Available online 6 May 2009 Keywords: Thin films Luminescence Optical properties Sputtering
a b s t r a c t High quality In-doped ZnO (ZnO:In) thin films were deposited on the quartz glass substrates at room temperature by using radio frequency magnetron sputtering. The effect of annealing temperature on the structure and optical properties of the ZnO:In films was investigated. It was found that the as-deposited film has a hexagonal wurtzite structure with c-axis perpendicular to the substrate. Accompanying with increasing annealing temperature from 400 ◦ C to 700 ◦ C, the crystal quality of the thin films can be improved, and then it will be deteriorated when the annealing temperature goes over 700 ◦ C. The transmittance of the ZnO:In films was revealed to be 85% in the visible region, and it changes slightly after annealing. The optical band gap increases from 3.17 eV to 3.23 eV with annealing temperature due to the decrease of the tensile strain in the films, and a linear relationship between the band gap and the strain was obtained. PL results show that all the emissions increase while annealing temperature rising from 400 ◦ C to 800 ◦ C and the origins of all the emissions were discussed in detail. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a direct wide band-gap (Eg ∼3.37 eV) semiconductor with a large exciton binding energy (60 meV) [1]. Due to its high conductivity and high transmittance, ZnO thin film has attracted attention as the transparent electrode in applications such as amorphous silicon solar cell, liquid crystal display, heat mirrors, etc. In recent years, great interests are focused on its potential applications in optical devices because of its strong PL spectra and cheap substrates [2]. It was reported that PL spectrum of ZnO consists mainly of two bands. One is in UV region corresponds to near-bandedge (NBE) emission which is attributed to exciton; the other in the visible region because of structural defects and impurities [3]. To obtain high quality ZnO thin film, a variety of techniques including chemical vapor deposition (CVD) [4], magnetron sputtering [5–7], pulsed laser deposition (PLD) [8] and sol–gel technique [9] have been adopted. With the advantages of low deposition temperature, simple processing, high growth rate, low-cost equipment and suitability for large areas deposition, magnetron sputtering is one of the most promising deposition techniques. In order to develop material for special applications, doped ZnO films have been fabricated and investigated by many research
∗ Corresponding author at: Department of Applied Physics, Chongqing University, Chongqing 400030, PR China. Tel.: +86 23 65105870. E-mail address: [email protected] (L. Fang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.139
groups. It was reported that In is one of the most efficient elements utilized to improve the opto-electrical properties [10,11]. Meanwhile, thermal annealing is also a widely used method to improve crystal quality and to decrease structure defect in materials. During the annealing process, dislocations and other structural defects move in the material and adsorption/decomposition may occur at the surface, thus the structure and the stoichiometric ratio of the material will change [12]. Up to now, there is no investigation about the effect of annealing temperature on the PL spectra of In-doped ZnO (ZnO:In) thin film. In this study, ZnO:In films prepared by radio frequency (RF) magnetron sputtering were annealed at the temperature ranging from 400 ◦ C to 800 ◦ C in air, and the effect of the annealing temperature on the crystal properties, transmittance and PL spectra of ZnO:In films have been studied and discussed. 2. Experimental A powder target out of the mixed powder of high purity ZnO (99.99%) and In2 O3 (99.99%) (In/(In + Zn) = 5%) was acted as the sputtering source material. Quartz glass (20 mm × 13 mm × 1 mm) was employed as the substrate, and it was precleaned ultrasonically in acetone, rinsed in alcohol and then dried in hot air. ZnO:In transparent conductive thin films were deposited by using 13.56 MHz RF magnetron sputtering. The distance between the target and substrate was 70 mm. In order to avoid contaminating the film, the chamber was evacuated to a base pressure of 6.6 × 10−4 Pa, and then, argon gas was introduced into the chamber through a mass flow controller. The mass flow of argon gas was fixed at 24 sccm, and the sputtering pressure was 2.0 Pa. The target was pre-sputtered for 15 min to remove contaminants. All the films were deposited at room tem-
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perature (RT) and annealed in air for an hour in the temperature range of 400–800 ◦ C. The structure of the films was characterized by X-ray diffraction (XRD) with a MRD-SingleScan diffractometer using the Cu K␣ line ( = 1.54 Å). The power of XRD was 1200 W and the scan was performed from 15◦ to 85◦ at a speed of 3◦ /min, with a step size of 0.2◦ . The surface morphology and the ratio of atoms of the films were investigated by SEM and EDS. The transmission spectrum of the film was measured by a double-beam ultraviolet/visible spectrophotometer with a wavelength range of 200–800 nm and the optical band gaps were determined from the transmission spectra. The RT photoluminescence spectra (PL) were recorded by a spectraphotometer with excitation wavelength of 325 nm. The film thickness was measured by a Dektak II step-measurer, and it was obtained as around 450 nm.
3. Result and discussions 3.1. Structure properties Fig. 1 shows the SEM image of the as-deposited ZnO:In thin films. It depicts that the surface of the films is smooth and dense, and the grain size is uniformly distributed with an average size of about 80 nm. EDS analysis indicates that the films contained approximately 5.08 at.% In, which is higher than that in the target (5 at.%). Cai et al.’s results indicate that the content of Al in the ZnO thin films was less than that in the target [13]. Ma et al. reported that the Ga atomic concentration for Zn1 − x Mgx O:Ga thin films is larger than that in the targets, and it was affected by the concentration of Mg [14]. The reason why the concentration of the dopant in the thin films prepared from sputtering is different from the target may be that the sputtering rate and re-evaporation rate from the substrate before being oxidized vary from element to element. Such problems need to be further investigated. XRD patterns of the ZnO:In films before and after annealing at various temperatures are shown in Fig. 2. It illustrates that the films are single phased with a wurtzite structure characteristic of pure ZnO. All the films exhibit preferential orientation with c-axis perpendicular to the substrate surface. No phase corresponding to In oxide or Zn–In compounds was observed, which implies that a low level of In incorporates into the ZnO lattice [15], and In atom replaces Zn atom in the hexagonal wurtzite lattice and/or In segregates to the no-crystalline region in the boundary [7]. Fig. 3 shows the intensity and the full width at maximum (FWHM) for (0 0 2) peak as a function of the annealing temperature. It was found that the intensity of the XRD diffraction (0 0 2) peak decreases with annealing temperature below 400 ◦ C, whereas it increases with increasing annealing temperature from 400 ◦ C up to 700 ◦ C. Higher annealing temperature provided more energy that would cause a
Fig. 1. SEM image of the as-deposited ZnO:In thin films.
Fig. 2. X-ray diffraction patterns of the ZnO:In thin films before and after annealing at different temperatures.
decrease of defects in the films and thus improve the crystal quality. The (0 0 2) peak intensity becomes weak when the annealing temperature exceeds 700 ◦ C, which can be explained by the presence of porosity. Fang et al. demonstrated a similar result as they studied the influence of post-annealing treatment on the structural properties of ZnO films [16]. Meanwhile, the FWHM decrease significantly with the annealing temperature up to 600 ◦ C, and then it decreases slightly above 600 ◦ C. According to Scherrer formula, the decrease of the FWHM indicates the increase of grain size of the films [17]. According to the XRD patterns, one can infer that a proper annealing temperature can improve the crystal quality, and 700 ◦ C is the best annealing temperature in our study. For the hexagonal system, using Bragg function, the d-spacing is related to the lattice parameters by the following equations: 2d sin = n, 1 4 = 3 d2
h2
+ hk a2
+ k2
(1)
+
l2 c2
,
(2)
where and are the diffraction angle and the wavelength; h, k and l are the miller indices; a and c are the lattice parameters, respectively. The calculated values of lattice constants c of the films are shown in Table 1. It is well known that the films with value of lattice parameter c greater than the theoretical value possess a positive or extensive strain in them whereas those with lower values
Fig. 3. The intensity of the (0 0 2) XRD peaks and the full width at half maximum (FWHM) for the ZnO:In thin films before and after annealing at different temperatures.
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Table 1 Lattice parameter, strain along c-axis and bandgap of the ZnO:In thin films before and after annealing at different temperatures. Annealing temperature (◦ C)
2 (◦ )
Lattice constant c (nm)
Strain, εzz (%)
Bandgap (eV)
As-deposited 400 500 600 700 800
33.47 33.67 33.70 33.76 33.92 34.09
5.354 5.323 5.320 5.307 5.287 5.260
2.81 2.24 2.17 1.93 1.54 1.02
3.28 3.17 3.18 3.19 3.21 3.23
have a negative or compressive strain. The larger value of lattice constant c of the films compared to the unstressed powder value shows that the unit cell is enlarged along the c-axis, and tensile force in the plane of the films. The tensile force becomes weaker as the annealing temperature increases, resulting in a decrease in “c” value. The larger value of lattice parameter (c) observed in the films reveals that the added element is in the interstitial site of the host matrix, and the dopants are activated after annealing, thus the lattice parameter decreases [18,19]. There are two aspects that attribute to the strain in the films, i.e., intrinsic strain introduced by impurities and defects in the lattice, and thermal expansion coefficient mismatch between the film and the substrate [20]. The strain value along c-axis, εzz , can be calculated by the following formula [21]: εzz
c − c0 = × 100%, c0
(3)
where c is the lattice parameter of strained ZnO:In thin films calculated from XRD result and c0 (0.521 nm) is the unstrained parameter of bulk ZnO. The obtained results of the strain along c-axis and the position of (0 0 2) peak of ZnO:In thin films are plotted in Fig. 4 as a function of annealing temperature. The films are highly strained with the εzz value up to 2.8% in the as-deposited film. With increasing annealing temperature, the strain is partially relieved. The maximal εzz value in our films is much larger than that (εzz ∼0.4%) of ZnO films deposited on (0 0 1) Al2 O3 substrate via PLD reported by Fouchet at al. [22], but is comparable to the result (εzz ∼2.15%) of ZnO films on glass substrate via DC magnetron sputtering by He et al. [20]. Ghosh et al. have ever obtained different strain values for ZnO films on different substrates by sol–gel technique [23]. The reported strain values of ZnO thin films are quite different from each other, and may be because the deposited method and the substrate type play important roles.
Fig. 4. The strain along c-axis and 2 value of the (0 0 2) peaks for the ZnO:In thin films before and after annealing at different temperatures.
3.2. Optical properties Fig. 5 shows the transmittance spectra of ZnO:In films in the range of 300–800 nm before and after annealing at different temperatures. All the films exhibit an average transmittance of around 85% in the visible region and a sharp fundamental absorption edge. It indicates in the insert of Fig. 5 that the absorption edge shows red shift after annealing, and with increasing annealing temperature, the value of the red shift decreases. The absorption coefficient (˛) is calculated by using the following equation [24]: ˛=
Ln(1/T ) , d
(4)
where T is the transmittance index and d is the film thickness. In the direct transition semiconductor, the optical band-gap dependence on the absorption coefficient is given by the following equation [25]: 2
(˛hv) = A(hv − Eg ),
(5)
where A, Eg and h are constant, optical band gap, and photo energy, respectively. The Eg can be determined by plotting the curve of (˛h)2 versus photo energy h and extrapolating the linear portion of the curve to the h-axis. Band-gap energy estimated from the absorption edge of the film annealed at 400 ◦ C is shown in Fig. 6 and a band gap of 3.17 eV was obtained. The band gaps obtained from transmittance are listed in Table 1. The band gaps of all the films obtained are smaller than those of the bulk ZnO. The as-deposited film has a band gap of 3.28 eV, and it decreases to 3.17 eV after annealing at 400 ◦ C. The band gap increases from 3.17 eV to 3.28 eV with annealing temperature increasing from 400 ◦ C to 800 ◦ C. Li et al. obtained a similar band-gap value (3.11–3.26 eV) for ZnO:In thin films deposited by DC reactive magnetron sputtering [26].
Fig. 5. Transmission spectra of the ZnO:In thin films before and after annealing at different temperatures: (a) as-deposited; (b) 400 ◦ C; (c) 500 ◦ C; (d) 600 ◦ C; (e) 700 ◦ C; and (f) 800 ◦ C.
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Fig. 6. Plot of (˛h)2 as a function of photo energy for the ZnO:In thin film annealed at 400 ◦ C.
It is well known that the band gap of a semiconductor is affected by the residual strain in the films, and a tensile strain will result in a decrease in the band gap while compressive strain will case an increase. The optical band gap of ZnO films as a function of corresponding strain is shown in Fig. 7. Note that the band gap for the as-deposited thin film was not plotted for it has a high carrier concentration, which will enlarge the band gap of the film due to Burstein-Moss effect. It is evident that the band gap decreases linearly with increasing tensile strain. By least-square regression, the band gap Eg of ZnO:In film after annealing can be expressed in terms of stress εzz : Eg = 3.28 − 0.045 εzz .
(6)
The relationship indeed shows that the band gap of the ZnO:In thin films will increase for negative strain and decrease for positive strain. Therefore, one can find out the band gap knowing the strain value along c-axis from Eq. (6). 3.3. PL properties Photoluminescence is a sensitive technique for examining the film quality, especially its optical properties. Fig. 8 shows the RT PL spectra of ZnO:In thin films as-deposited and annealed at 500 ◦ C and 800 ◦ C, respectively. It indicates that the PL spectra of the films
Fig. 7. The optical band gap of the ZnO:In thin films as a function of the corresponding strain along c-axis.
Fig. 8. Plot of photoluminescence (PL) spectra of the as-deposited and the annealed (at 500 ◦ C and 800 ◦ C) ZnO:In thin films.
depend on annealing temperature significantly. The as-deposited film exhibits a weak emission peak and the PL intensity of emission peaks increases with increase of the annealing temperature. There are four PL emissions: 396 nm (3.13 eV, violet), 446 nm (2.78 eV, blue), 482 nm (2.57 eV, green) and 527 nm (2.35 eV, green) for all the films. It is believed that the near-band-edge emission comes from the ZnO films with less crystal defect or from (0 0 2) oriented ZnO films, and the deep level emission of ZnO relates to the intrinsic defect ZnO films [27]. There are five main intrinsic defects in ZnO films, such as zinc vacancy, oxygen vacancy, interstitial zinc, interstitial oxygen, and antisite oxygen. The violet emission is a NBE emission, but the energy of the NBE emission is smaller than the band gap calculated from transmittance spectra. The luminescence peak shows red shift from the absorption threshold, which is known as Stokes shift. Makino has ever obtained a similar result for ZnO:Ga thin films deposited on ScMgO4 substrate by laser molecular epitaxial growth [28]. In n-type ZnO, an absorptive optical transition occurs from the valence band to the Fermi level or conduction band, while an emissive transition occurs from an impurity-donor band to the valence band. The NBE emission peak in PL has a small intensity in comparison with the Ref. [29]. The reason may be that they had better crystal quality films. The 446 nm blue emission and the 482 nm green emission were observed and the physical mechanism was also discussed by Zhang et al. [30]. They believed that the 446 nm blue emission originated from the electron transition from the shallow donor level of oxygen vacancies to the valence band and electron transition from the shallow donor level of zinc interstitials to the valence band. They indicated that the green emission of 488 nm wavelength originated from the electron transition from the deep donor level of the ionized oxygen vacancies to the valance band. Vanheusden et al. proposed that there are three different charge states of oxygen vacancies: the neutral vacancy (VO ), the single ionized oxygen vacancy (VO + ), and the doubly ionized oxygen vacancy (VO ++ ), and only VO + can act as luminescent centers [31]. During the annealing process, the concentration of single ionized oxygen vacancies increased, so that the corresponding defect luminescence appears stronger [12]. The 527 nm green emission could be ascribed to the electron transition from the bottom of the conduction band to the antisite oxygen level, which was calculated theoretically by Sun [32] and studied experimentally by Lin and Fu [33]. The intensity of the 527 nm green emission increases with increasing annealing temperature, indicating more antisite oxygen or zinc vacancy was produced when the films was annealed at higher temperature in air. Some researchers believe that the increase this emission after O2 annealing dues to the increase of
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zinc vacancy [34]. Similar increase of luminescence intensity for the films annealed at higher temperature was reported by Vinodkumar et al. for undoped ZnO films prepared by pulsed laser ablation [2]. The intensity ratio between the blue peak (446 nm) and green peak (527 nm) increases from 3.1 to 5.7 after annealing at 800 ◦ C, indicating that the crystal quality of the films can be improved by annealing [35]. The intense PL emission of the films annealed at 800 ◦ C suggests the possibility of the films for application as blue light emitter. 4. Conclusion In summary, we have discussed the effect of post-annealing temperature on the structure, optical properties and PL spectra of ZnO:In thin films. The crystal quality of the films is highly dependent on the annealing temperature: it can be improved along with the films annealed from 400 ◦ C to 700 ◦ C, and then deteriorates above 700 ◦ C. The average transmittance of the films is about 85% in the visible range, and the band gap increases from 3.17 eV to 3.23 eV with annealing temperature going up from 400 ◦ C to 800 ◦ C. The increase of the band gap is attributed to the decrease of the tensile strain in the films and a linear increment of the optical band gap with the strain was obtained. Due to Stokes shift, the NBE emission energy (3.13 eV) of the films is smaller than the band gap (3.23 eV). The films annealed at 800 ◦ C shows intense PL spectra in the blue region, which means ZnO:In thin films may have application as blue light emitter. Acknowledgements This work was supported by the Program for the New Century Excellent Talents in University of China under grant No. NCET05-0764 and the Graduate Innovation Foundation of Chongqing University under grant No. 200801A1B0060265. The authors also thank Prof. C.Z. Cai in Department of Applied Physics, Chongqing University for the English revision. References [1] S.J. Pearson, D.P.N. Orton, K. Ip, Y.W. Hoe, T. Steiner, Prog. Mater. Sci. 50 (2005) 293–340.
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