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Effect of the Ti-target arc current on the properties of Ti-doped ZnO thin films prepared by dual-target cathodic arc plasma deposition
Ceramics International 42 (2016) 14438–14442
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of the Ti-target arc current on the properties of Ti-doped ZnO thin films prepared by dual-target cathodic arc plasma deposition Shuo-Fu Hsu a, Min-Hang Weng b, Jyh-Horng Chou a, Chun-Hsiung Fang a, Ru-Yuan Yang c,n a
Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, Taiwan Medical Devices and Opto-electronics Equipment Department, Metal Industries Research & Development Center, Taiwan c Department of Materials Engineering National Ping-Tung University of Science and Technology, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2015 Received in revised form 6 June 2016 Accepted 7 June 2016 Available online 7 June 2016
Ti-doped zinc oxide (Ti:ZnO) films were prepared on glass substrates using the dual-targets cathodic arc plasma deposition process. The effect of the arc current of the Ti target (30, 40, 50 and 60 A) on structural, optical and electrical properties of the deposited films was investigated. All the prepared films exhibited a preferred (002) orientation with the c-axis perpendicular to the substrate, and the intensity of (0 0 2) peak was decreased as increasing the arc current of the Ti target. Zn, Ti and O elements were detected in the deposited film, showing that Ti:ZnO film was successfully deposited. All Ti:ZnO films showed an average transmittance of over 84% in the visible region and the calculated values of the band gap were about 3.22 eV. Under an arc current of Ti target of 30 A, the deposited Ti:ZnO film had the lowest resistivity of 7.9 ×10 3 Ω cm. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Transparent conductive oxide (TCO) Ti:ZnO Transmittance Resistivity
1. Introduction Recently, transparent conductive oxide (TCO) has attracted increased interest for its potential use in light-weight, flat panel applications, including solar cells, cell phones, liquid crystal displays (LCD), and electronic paper due to great tendency [1,2]. Moreover, ZnO has an high exciton binding energy of 60 meV and thus could be expected for ZnO-based light emitting devices (LED). Pure or doped Zinc oxide (ZnO) may be desired to being a good TCO material due to its wide direct band gap of 3.3 eV at room temperature, high transmittance in the visible light range, lowcost fabrication, non-toxicityand suitable transparent and conductive properties at a low substrate temperature (o200 °C) [3,4]. Several technologies have been proposed for the preparation of ZnO films including radio frequency (RF) magnetron sputtering [5], chemical vapor deposition (CVD) [6], electron beam evaporation [7], the sol-gel method [8], electrodeposition [9], and cathodic arc plasma deposition (CAPD) [10–19]. CAPD is a common technique for the preparation of oxide film, since it can provide particles at a high ionization rate and high ion energy (50–150 eV) for condensation on the substrate [19]. The films deposited by CAPD can be processed at a rather low substrate temperature and with a low thermal residual stress in the film compared to other hightemperature processes. n
Corresponding author. E-mail address: [email protected] (R.-Y. Yang).
http://dx.doi.org/10.1016/j.ceramint.2016.06.043 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Many previous studies have examined the doping of Al [20], Ga [21] and Ti [22–25] in ZnO films. In [22–24], Ti:ZnO films were prepared on glass substrates by RF magnetron sputtering. The substrate temperature was 100 °C [22], 200 °C [23] and 300 °C [24]. In [25], Ti doped ZnO films with different concentrations were grown on glass substrates at 400 °C by a spray pyrolysis technique. It was found that the TiO2 was not prone to crystallize at a low temperature ( o400 °C) [25]. High quality ZnO film is not easy to be doped with Ti ion at a low temperature, such as at room temperature, by using sputtering or spray pyrolysis methods. Few papers have discussed the doping effect regarding Tidoped ZnO (Ti: ZnO) thin films especially prepared by using CAPD [26,27]. In [26], the resistivity of the Ti-doped ZnO film was improved when compared to that of un-doped ZnO film, all deposited by CAPD at a low temperature below 40 °C. In [27], the study investigated the post-annealing conditions had a strong affect on microstructure, crystal quality, optical and electrical properties of the Ti-doped ZnO thin films. The best optical quality was obtained due to improved crystallize with a post-annealing atmosphere of N2/H2 mixed gases. In the CAPD process, the control of the arc current is also an important processing parameter. However, no previous studies have investigated the arc current effect on Ti-doped ZnO (Ti: ZnO) thin films using the CAPD. Therefore, in this paper, we deposited Ti:ZnO films on glass substrates using dual-target CAPD to investigate the influence of arc current on the prepared Ti:ZnO films. The structural, optical and electrical properties of the deposited Ti:ZnO films were measured and discussed.
S.-F. Hsu et al. / Ceramics International 42 (2016) 14438–14442
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2. Experiment Ti-doped ZnO films were deposited onto glass substrates in adual-targetCAPD system. Fig. 1 shows the schema of the CAPD system with the dual targets arrangement [25]. The cathode targets were metallic Zn and Ti, both with a diameter of 100 mm and a purity of 99.99% held in an alumina ceramic tube. Metallic Zn and Ti were separately held in two alumina ceramic tubes as cathode targets. High purity (99.99%) O2 gas was used as the reactant gas. Before deposition, the glass substrates with a thickness of 2 mm were washed with alcohol and then ultrasonically cleaned in alcohol for 10 min, then and then blown dry with nitrogen gas. When depositing Ti:ZnO films, arc current was used to generate a cathode plasma. Flow rates of Ar and O2 were directly controlled by the gas flow controllers. During the deposition process, parameters for the Ti:ZnO films were set as follows: substrate rotation of 2 rpm, base pressure was kept at 3 ×10 4 Torr Pa, and the substrate-anode distance was approximately 21 cm. The substrate temperature still was increased slightly during deposition even without external heating due to the energetic ion flux to the substrate [11]. It was reported that the processing temperature for depositing ZnO films was less than a maximum temperature of 70 °C without any further substrate heating [16,17]. To investigate the effect of arc currentof the Ti target on prepared Ti:ZnO films, O2 and Ar gas flow rates were respectively fixed at 160 and 20 sccm, and the arc current of Zn target was maintained at 50 A, while the arc current of the Ti target was set to 30, 40, 50 and 60 A. However, to control and obtain the doping effect of the Ti element, a shutter was set to partly cover the Ti target, as shown in Fig. 1. The structural properties of the films were specified by X-ray diffraction (XRD, BRUKER D8 ADVANCE) equipped with CuKα radiation with an average wavelength of 1.5406 Ǻ. X-ray diffractograms was taken at 2θ between 20° and 80° at a scan speed of 4.5°/min. The constituent elements of the films were detected by the energy dispersive spectrometer (EDS, JSM-6400F. JEOL, Japan) analysis. The optical properties of the films were measured using a UV–vis spectrometer (Thermo Evolution-300) with a wavelength range of 300–800 nm. Moreover, the absorption coefficient α could be determined from the absorption spectra. The thickness of the deposited films was measured by an Alpha-step (α-step, Kosaka Laboratory Ltd. ET-4000), and the all Ti:ZnO films were kept at a constant thickness of 250 nm. The room-temperature sheet resistance of the films was determined using the standard fourpointprobe method (Jiehan 5000, SRS 4000). Carrier concentration and Hall mobility were obtained from Hall-effect measurements (ECOPIA HMS-3000).
Fig. 1. Schematic of the cathodic arc plasma deposition system.
Fig. 2. XRD patterns of the deposited Ti:ZnO films on glass substrates using CAPD with different arc currents for the Ti target.
3. Results and discussion Fig. 2 displays the XRD patterns of the deposited films on glass substrates with different arc currents for the Ti target. All the deposited films exhibit a diffraction peak for ZnO (0 0 2) at about θ ¼ 34.2°, indicating that the films are highly oriented to the c-axis normal to the substrate. It is known that the c-axis preferred orientation of the deposited thin films are typically formed from a self-ordering effect due to the minimization of the crystal surface free energy [13]. Moreover, since the used substrate is glass, the crystallographic structure of the substrate does not therefore affect the film crystal orientation. This means that ZnO based films were successfully deposited at a low temperature without extra substrate heating. It is known that Ti ion could be incorporated in ZnO as an interstitial atom, since Zn ion has a radius of 7.4 nm and Ti ion has a radius of 6.8 nm. In this study, the intensity of (0 0 2) peak is decreased significantly as the arc current of the Ti target is increased. The result is similar to those in other reports [22,25]. In [22], the Ti:ZnO film showed a poor crystallinity as compared with pure ZnO film. Besides, such a crystallinity became poorer as Ti content was increased. In [25], the crystallinity also became lower when the doping concentration was further increased above 0.7%. Namely, the doping Ti into the ZnO film suppresses the crystallite quality since the TiO2 is not prone to crystallize at a low temperature (o 400 °C) [25]. As the arc current of the Ti target is increased, more Ti elements would substitute the Zn site and distorted the crystallite of Ti:ZnO films. In other words, too much arc current of the Ti target would reduce, rather than improve, the crystalline quality of the films. Fig. 3 shows the EDS data of prepared films on glass substrates by dual-target CAPD with various arc currents of the Ti target. The results confirm that Zn, Ti and O elements exist in the prepared films, indicating that the Ti:ZnO films can be successfully deposited by using dual-target CAPD. The atomic percentage of Ti element in the deposited film is increased with the increase of arc current for the Ti target. It is reasonable that the particles of generation by arc plasma and the probability of the particles arriving at the glass substrate are both increased. Thus the doping amount of the Ti element in the ZnO film is also increased even without substrate heating. Ti is expected to provide more electrons, thus the doping amount of Ti shall be carefully controlled to avoid being the scattering centers [24]. Typically, the doping amount of Ti in the ZnO film is as less as 1.5%, as reported in many reports [22,23]. In this study, the atomic percentages of the elements in the prepared
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Fig. 3. EDS data of the deposited Ti:ZnO films on glass substrates using CAPD with different arc currents for the Ti target..
films are 0.64%, 0.72%, 0.76% and 0.83% with arc currents for the Ti target of 30, 40, 50 and 60 A, respectively. Although the doping amounts of Ti element in ZnO film are actually rare, the values are still in a reasonable range. The doping amounts of Ti element in ZnO film using CAPD without substrate heating are only slightly less than that (1.1%) of reports using sputtering with substrate heating [22,23], but are much less than that (3%) of report using spray pyrolysis technology with substrate temperature as high as 400 °C [25]. When depositing such less Ti doping amount using CAPD, too small arc current for Ti target (less than 1 A) can not from the Ti based plasma. Therefore, arc currents for Ti target (30, 40, 50 and 60 A) are required to form the plasma. In order to control the doping effect of the Ti element, a shutter was set to partly cover the Ti target, as shown in Fig. 1. In this experiment, the substrate temperature is less than 70 °C since TiO2 is not prone to crystallize at a low temperature (o400 °C) [25]. From XRD result and EDS result, it is suggested that the deposited films by using dual-target CAPD in this study are the Ti:ZnO films instead of the Zn:TiO films. Fig. 4 shows the full width at half maximum (FWHM) and crystalline size of the Ti:ZnO films for various arc currents of the Ti target. The average crystal size of the Ti:ZnO films can be determined by applying the Scherrer equation to the full width at
half maximum (FWHM) of the (002) peaks, as given by [11]:
Fig. 4. FWHM and crystalline size of the deposited Ti:ZnO films on glass substrates using CAPD as a function of different arc currents for the Ti target.
Fig. 5. Optical transmittance spectra of the deposited Ti:ZnO films on glass substrates using CAPD with various arc currents for the Ti target.
D=
0.9λ ω cos θ
(1)
where ω is the angular FWHM of the selected diffraction line in radians, θ is the Bragg angle and λ is the X-ray wavelength (0.15406 nm). The crystal size is decreased from 13.02 to 8.87 nm as the arc current of the Ti target is increased from 30 to 60 A. The crystal size is found to decrease linearly with the arc current of the Ti target. In general, a larger arc current is associated with a higher ionization rate, indicating increased Ti doping. Therefore, the results imply that the crystal size of the Ti:ZnO films is closely related to the titanium dopant content, and a similar behavior has been reported elsewhere in the literature [22,23]. Fig. 5 illustrates the optical transmittance spectra of the Ti:ZnO films on glass substrates with different arc currents for the Ti target. All films are found to be highly transparent in the visible wavelength region with an average transmittance exceeding 84%. The transmittance is increased with the arc current of the Ti target, with respective average transmittances in the visible region of 84.32%, 86.61%, 87.05% and 87.05% for Ti:ZnO films deposited on glass substrates with an arc current of 30, 40, 50 and 60 A,
S.-F. Hsu et al. / Ceramics International 42 (2016) 14438–14442
Fig. 6. (αhν)2 as a function of the photon energy (hν) for the deposited Ti:ZnO films on glass substrates using CAPD with different arc currents for the Ti target.
respectively. In particular, the average overall transmittances of the deposited Ti:ZnO films are higher than that of the pure ZnO film under similar processing conditions [18]. The average transmittances of prepared Ti:ZnO films are increased with increasing Ti concentration. The result is consistent with that reported in Ref. [25]. The variation of the absorption coefficient α with the photon energy hν can be given as [11]:
α=
B(hν − Eg )1/2 hν
(2)
where α is estimated from the transmittance data, and B is a constant depending on the material properties. Fig. 6 shows the plots of (αhν)2 as a function of hν for the prepared Ti:ZnO films with different arc currents of the Ti target. As shown in Fig. 6, the energy gap of all Ti:ZnO films fell within a range of 3.21–3.22 eV. The typical energy gap of un-doped ZnO film deposited on glass substrates is 3.2 eV [24]. Thus the optical band gap of the deposited Ti:ZnO film shifts slightly to a higher energy level as a result of the doping of the Ti element. Since the increased arc current for the Ti target increases the atomic percentage of Ti element in the Ti:ZnO film. This phenomenon is probably related to the increased carrier concentration according to the BursteinMoss effect [11]. Fig. 7 shows the resistivity, mobility, and carrier concentration of the Ti:ZnO thin films as a function of arc current of the Ti target. Hall measurement results shows the Ti:ZnO films deposited by CAPD with various arc currents of the Ti target generally exhibit n-type conductivity. The carrier concentration is increased from 5.75 1019 to 3.12 1020 cm 3 as the arc current of Ti is increased
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from 30 to 60 A. This is attributed to a greater number of Ti atoms being doped into the deposited films. Instead, the carrier mobility is decreased with the arc current of the Ti target. Hall mobility is known as the reflection of the scattering processes of carriers in the films. In a polycrystalline film, carriers would be scattered by intrinsic defects and grain boundaries [28]. As shown in Fig. 2, when the arc current of the Ti target is increased from 30 to 60 A, the excess Ti dopant would reduce the crystalline quality and form scattering centers, thus reducing the carrier mobility. The resistivity is known to be related to the carrier concentration and mobility. Although the carrier concentration of the Ti: ZnO films is increased due to the extension of the arc current of the Ti target, the mobility is decreased accordingly due to carrier scattering. The Ti:ZnO film has the lowest resistivity of 7.9 ×10 3 Ω cm when the arc current of the Ti target is 30 A, after which the resistivity is increased linearly with the arc current of the Ti target. Thus, the correlation between resistivity and arc current of the Ti target is mainly caused by the decreased carrier mobility with increasing the arc current of the Ti target. The resistivity of the Ti-doped ZnO film is improved when compared to that of un-doped ZnO film [25]. Moreover, the lowest resistivity of the deposited Ti:ZnO film is competitive with previous reports [18] under similar film thicknesses, indicating that doping the Ti element into pure ZnO films will improve their electrical properties.
4. Conclusions In this paper, we deposited successfully Ti-doped ZnO films onto glass substrates using the dual-target cathodic arc plasma deposition. The properties of the Ti:ZnO films deposited with various arc currents were investigated. Through X-ray diffraction measurements, almost all deposited films revealed a preferred orientation with the c-axis perpendicular to the substrates. The intensity of (0 0 2) peak was decreased significantly as the arc current of the Ti target was increased. The transmittance of almost all deposited Ti:ZnO films exceeded 84% in the range of 380– 780 nm. With the extension of the arc current of the Ti target, the carrier concentration of the Ti:ZnO films was increased and the mobility was decreased. The deposited Ti:ZnO film had the lowest resistivity of 7.9 ×10 3 Ω cm when the arc current of the Ti target was 30 A. This was mainly caused by the increased carrier concentration although the carrier scattering was increased and the mobility was decreased with the arc current of the Ti target. From the above, the arc current played an important role in the optical and electrical properties of Ti:ZnO films.
Acknowledgments This work was partially funded by the Ministry of Science and Technology, Taiwan, and Southern Taiwan Science Park Administration (STSPA), Taiwan under Contract NSC 101-2628-E-020-002MY3, and 102GE06. The authors would also like to thank the National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for the supporting the experimental equipment.
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Fig. 7. Resistivity (ρ), mobility (μ) and carrier concentration (n) of the deposited Ti: ZnO films on glass substrates using CAPD with various arc currents for the Ti target.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of the Ti-target arc current on the properties of Ti-doped ZnO thin films prepared by dual-target cathodic arc plasma deposition Shuo-Fu Hsu a, Min-Hang Weng b, Jyh-Horng Chou a, Chun-Hsiung Fang a, Ru-Yuan Yang c,n a
Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, Taiwan Medical Devices and Opto-electronics Equipment Department, Metal Industries Research & Development Center, Taiwan c Department of Materials Engineering National Ping-Tung University of Science and Technology, Taiwan b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2015 Received in revised form 6 June 2016 Accepted 7 June 2016 Available online 7 June 2016
Ti-doped zinc oxide (Ti:ZnO) films were prepared on glass substrates using the dual-targets cathodic arc plasma deposition process. The effect of the arc current of the Ti target (30, 40, 50 and 60 A) on structural, optical and electrical properties of the deposited films was investigated. All the prepared films exhibited a preferred (002) orientation with the c-axis perpendicular to the substrate, and the intensity of (0 0 2) peak was decreased as increasing the arc current of the Ti target. Zn, Ti and O elements were detected in the deposited film, showing that Ti:ZnO film was successfully deposited. All Ti:ZnO films showed an average transmittance of over 84% in the visible region and the calculated values of the band gap were about 3.22 eV. Under an arc current of Ti target of 30 A, the deposited Ti:ZnO film had the lowest resistivity of 7.9 ×10 3 Ω cm. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Transparent conductive oxide (TCO) Ti:ZnO Transmittance Resistivity
1. Introduction Recently, transparent conductive oxide (TCO) has attracted increased interest for its potential use in light-weight, flat panel applications, including solar cells, cell phones, liquid crystal displays (LCD), and electronic paper due to great tendency [1,2]. Moreover, ZnO has an high exciton binding energy of 60 meV and thus could be expected for ZnO-based light emitting devices (LED). Pure or doped Zinc oxide (ZnO) may be desired to being a good TCO material due to its wide direct band gap of 3.3 eV at room temperature, high transmittance in the visible light range, lowcost fabrication, non-toxicityand suitable transparent and conductive properties at a low substrate temperature (o200 °C) [3,4]. Several technologies have been proposed for the preparation of ZnO films including radio frequency (RF) magnetron sputtering [5], chemical vapor deposition (CVD) [6], electron beam evaporation [7], the sol-gel method [8], electrodeposition [9], and cathodic arc plasma deposition (CAPD) [10–19]. CAPD is a common technique for the preparation of oxide film, since it can provide particles at a high ionization rate and high ion energy (50–150 eV) for condensation on the substrate [19]. The films deposited by CAPD can be processed at a rather low substrate temperature and with a low thermal residual stress in the film compared to other hightemperature processes. n
Corresponding author. E-mail address: [email protected] (R.-Y. Yang).
http://dx.doi.org/10.1016/j.ceramint.2016.06.043 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Many previous studies have examined the doping of Al [20], Ga [21] and Ti [22–25] in ZnO films. In [22–24], Ti:ZnO films were prepared on glass substrates by RF magnetron sputtering. The substrate temperature was 100 °C [22], 200 °C [23] and 300 °C [24]. In [25], Ti doped ZnO films with different concentrations were grown on glass substrates at 400 °C by a spray pyrolysis technique. It was found that the TiO2 was not prone to crystallize at a low temperature ( o400 °C) [25]. High quality ZnO film is not easy to be doped with Ti ion at a low temperature, such as at room temperature, by using sputtering or spray pyrolysis methods. Few papers have discussed the doping effect regarding Tidoped ZnO (Ti: ZnO) thin films especially prepared by using CAPD [26,27]. In [26], the resistivity of the Ti-doped ZnO film was improved when compared to that of un-doped ZnO film, all deposited by CAPD at a low temperature below 40 °C. In [27], the study investigated the post-annealing conditions had a strong affect on microstructure, crystal quality, optical and electrical properties of the Ti-doped ZnO thin films. The best optical quality was obtained due to improved crystallize with a post-annealing atmosphere of N2/H2 mixed gases. In the CAPD process, the control of the arc current is also an important processing parameter. However, no previous studies have investigated the arc current effect on Ti-doped ZnO (Ti: ZnO) thin films using the CAPD. Therefore, in this paper, we deposited Ti:ZnO films on glass substrates using dual-target CAPD to investigate the influence of arc current on the prepared Ti:ZnO films. The structural, optical and electrical properties of the deposited Ti:ZnO films were measured and discussed.
S.-F. Hsu et al. / Ceramics International 42 (2016) 14438–14442
14439
2. Experiment Ti-doped ZnO films were deposited onto glass substrates in adual-targetCAPD system. Fig. 1 shows the schema of the CAPD system with the dual targets arrangement [25]. The cathode targets were metallic Zn and Ti, both with a diameter of 100 mm and a purity of 99.99% held in an alumina ceramic tube. Metallic Zn and Ti were separately held in two alumina ceramic tubes as cathode targets. High purity (99.99%) O2 gas was used as the reactant gas. Before deposition, the glass substrates with a thickness of 2 mm were washed with alcohol and then ultrasonically cleaned in alcohol for 10 min, then and then blown dry with nitrogen gas. When depositing Ti:ZnO films, arc current was used to generate a cathode plasma. Flow rates of Ar and O2 were directly controlled by the gas flow controllers. During the deposition process, parameters for the Ti:ZnO films were set as follows: substrate rotation of 2 rpm, base pressure was kept at 3 ×10 4 Torr Pa, and the substrate-anode distance was approximately 21 cm. The substrate temperature still was increased slightly during deposition even without external heating due to the energetic ion flux to the substrate [11]. It was reported that the processing temperature for depositing ZnO films was less than a maximum temperature of 70 °C without any further substrate heating [16,17]. To investigate the effect of arc currentof the Ti target on prepared Ti:ZnO films, O2 and Ar gas flow rates were respectively fixed at 160 and 20 sccm, and the arc current of Zn target was maintained at 50 A, while the arc current of the Ti target was set to 30, 40, 50 and 60 A. However, to control and obtain the doping effect of the Ti element, a shutter was set to partly cover the Ti target, as shown in Fig. 1. The structural properties of the films were specified by X-ray diffraction (XRD, BRUKER D8 ADVANCE) equipped with CuKα radiation with an average wavelength of 1.5406 Ǻ. X-ray diffractograms was taken at 2θ between 20° and 80° at a scan speed of 4.5°/min. The constituent elements of the films were detected by the energy dispersive spectrometer (EDS, JSM-6400F. JEOL, Japan) analysis. The optical properties of the films were measured using a UV–vis spectrometer (Thermo Evolution-300) with a wavelength range of 300–800 nm. Moreover, the absorption coefficient α could be determined from the absorption spectra. The thickness of the deposited films was measured by an Alpha-step (α-step, Kosaka Laboratory Ltd. ET-4000), and the all Ti:ZnO films were kept at a constant thickness of 250 nm. The room-temperature sheet resistance of the films was determined using the standard fourpointprobe method (Jiehan 5000, SRS 4000). Carrier concentration and Hall mobility were obtained from Hall-effect measurements (ECOPIA HMS-3000).
Fig. 1. Schematic of the cathodic arc plasma deposition system.
Fig. 2. XRD patterns of the deposited Ti:ZnO films on glass substrates using CAPD with different arc currents for the Ti target.
3. Results and discussion Fig. 2 displays the XRD patterns of the deposited films on glass substrates with different arc currents for the Ti target. All the deposited films exhibit a diffraction peak for ZnO (0 0 2) at about θ ¼ 34.2°, indicating that the films are highly oriented to the c-axis normal to the substrate. It is known that the c-axis preferred orientation of the deposited thin films are typically formed from a self-ordering effect due to the minimization of the crystal surface free energy [13]. Moreover, since the used substrate is glass, the crystallographic structure of the substrate does not therefore affect the film crystal orientation. This means that ZnO based films were successfully deposited at a low temperature without extra substrate heating. It is known that Ti ion could be incorporated in ZnO as an interstitial atom, since Zn ion has a radius of 7.4 nm and Ti ion has a radius of 6.8 nm. In this study, the intensity of (0 0 2) peak is decreased significantly as the arc current of the Ti target is increased. The result is similar to those in other reports [22,25]. In [22], the Ti:ZnO film showed a poor crystallinity as compared with pure ZnO film. Besides, such a crystallinity became poorer as Ti content was increased. In [25], the crystallinity also became lower when the doping concentration was further increased above 0.7%. Namely, the doping Ti into the ZnO film suppresses the crystallite quality since the TiO2 is not prone to crystallize at a low temperature (o 400 °C) [25]. As the arc current of the Ti target is increased, more Ti elements would substitute the Zn site and distorted the crystallite of Ti:ZnO films. In other words, too much arc current of the Ti target would reduce, rather than improve, the crystalline quality of the films. Fig. 3 shows the EDS data of prepared films on glass substrates by dual-target CAPD with various arc currents of the Ti target. The results confirm that Zn, Ti and O elements exist in the prepared films, indicating that the Ti:ZnO films can be successfully deposited by using dual-target CAPD. The atomic percentage of Ti element in the deposited film is increased with the increase of arc current for the Ti target. It is reasonable that the particles of generation by arc plasma and the probability of the particles arriving at the glass substrate are both increased. Thus the doping amount of the Ti element in the ZnO film is also increased even without substrate heating. Ti is expected to provide more electrons, thus the doping amount of Ti shall be carefully controlled to avoid being the scattering centers [24]. Typically, the doping amount of Ti in the ZnO film is as less as 1.5%, as reported in many reports [22,23]. In this study, the atomic percentages of the elements in the prepared
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Fig. 3. EDS data of the deposited Ti:ZnO films on glass substrates using CAPD with different arc currents for the Ti target..
films are 0.64%, 0.72%, 0.76% and 0.83% with arc currents for the Ti target of 30, 40, 50 and 60 A, respectively. Although the doping amounts of Ti element in ZnO film are actually rare, the values are still in a reasonable range. The doping amounts of Ti element in ZnO film using CAPD without substrate heating are only slightly less than that (1.1%) of reports using sputtering with substrate heating [22,23], but are much less than that (3%) of report using spray pyrolysis technology with substrate temperature as high as 400 °C [25]. When depositing such less Ti doping amount using CAPD, too small arc current for Ti target (less than 1 A) can not from the Ti based plasma. Therefore, arc currents for Ti target (30, 40, 50 and 60 A) are required to form the plasma. In order to control the doping effect of the Ti element, a shutter was set to partly cover the Ti target, as shown in Fig. 1. In this experiment, the substrate temperature is less than 70 °C since TiO2 is not prone to crystallize at a low temperature (o400 °C) [25]. From XRD result and EDS result, it is suggested that the deposited films by using dual-target CAPD in this study are the Ti:ZnO films instead of the Zn:TiO films. Fig. 4 shows the full width at half maximum (FWHM) and crystalline size of the Ti:ZnO films for various arc currents of the Ti target. The average crystal size of the Ti:ZnO films can be determined by applying the Scherrer equation to the full width at
half maximum (FWHM) of the (002) peaks, as given by [11]:
Fig. 4. FWHM and crystalline size of the deposited Ti:ZnO films on glass substrates using CAPD as a function of different arc currents for the Ti target.
Fig. 5. Optical transmittance spectra of the deposited Ti:ZnO films on glass substrates using CAPD with various arc currents for the Ti target.
D=
0.9λ ω cos θ
(1)
where ω is the angular FWHM of the selected diffraction line in radians, θ is the Bragg angle and λ is the X-ray wavelength (0.15406 nm). The crystal size is decreased from 13.02 to 8.87 nm as the arc current of the Ti target is increased from 30 to 60 A. The crystal size is found to decrease linearly with the arc current of the Ti target. In general, a larger arc current is associated with a higher ionization rate, indicating increased Ti doping. Therefore, the results imply that the crystal size of the Ti:ZnO films is closely related to the titanium dopant content, and a similar behavior has been reported elsewhere in the literature [22,23]. Fig. 5 illustrates the optical transmittance spectra of the Ti:ZnO films on glass substrates with different arc currents for the Ti target. All films are found to be highly transparent in the visible wavelength region with an average transmittance exceeding 84%. The transmittance is increased with the arc current of the Ti target, with respective average transmittances in the visible region of 84.32%, 86.61%, 87.05% and 87.05% for Ti:ZnO films deposited on glass substrates with an arc current of 30, 40, 50 and 60 A,
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Fig. 6. (αhν)2 as a function of the photon energy (hν) for the deposited Ti:ZnO films on glass substrates using CAPD with different arc currents for the Ti target.
respectively. In particular, the average overall transmittances of the deposited Ti:ZnO films are higher than that of the pure ZnO film under similar processing conditions [18]. The average transmittances of prepared Ti:ZnO films are increased with increasing Ti concentration. The result is consistent with that reported in Ref. [25]. The variation of the absorption coefficient α with the photon energy hν can be given as [11]:
α=
B(hν − Eg )1/2 hν
(2)
where α is estimated from the transmittance data, and B is a constant depending on the material properties. Fig. 6 shows the plots of (αhν)2 as a function of hν for the prepared Ti:ZnO films with different arc currents of the Ti target. As shown in Fig. 6, the energy gap of all Ti:ZnO films fell within a range of 3.21–3.22 eV. The typical energy gap of un-doped ZnO film deposited on glass substrates is 3.2 eV [24]. Thus the optical band gap of the deposited Ti:ZnO film shifts slightly to a higher energy level as a result of the doping of the Ti element. Since the increased arc current for the Ti target increases the atomic percentage of Ti element in the Ti:ZnO film. This phenomenon is probably related to the increased carrier concentration according to the BursteinMoss effect [11]. Fig. 7 shows the resistivity, mobility, and carrier concentration of the Ti:ZnO thin films as a function of arc current of the Ti target. Hall measurement results shows the Ti:ZnO films deposited by CAPD with various arc currents of the Ti target generally exhibit n-type conductivity. The carrier concentration is increased from 5.75 1019 to 3.12 1020 cm 3 as the arc current of Ti is increased
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from 30 to 60 A. This is attributed to a greater number of Ti atoms being doped into the deposited films. Instead, the carrier mobility is decreased with the arc current of the Ti target. Hall mobility is known as the reflection of the scattering processes of carriers in the films. In a polycrystalline film, carriers would be scattered by intrinsic defects and grain boundaries [28]. As shown in Fig. 2, when the arc current of the Ti target is increased from 30 to 60 A, the excess Ti dopant would reduce the crystalline quality and form scattering centers, thus reducing the carrier mobility. The resistivity is known to be related to the carrier concentration and mobility. Although the carrier concentration of the Ti: ZnO films is increased due to the extension of the arc current of the Ti target, the mobility is decreased accordingly due to carrier scattering. The Ti:ZnO film has the lowest resistivity of 7.9 ×10 3 Ω cm when the arc current of the Ti target is 30 A, after which the resistivity is increased linearly with the arc current of the Ti target. Thus, the correlation between resistivity and arc current of the Ti target is mainly caused by the decreased carrier mobility with increasing the arc current of the Ti target. The resistivity of the Ti-doped ZnO film is improved when compared to that of un-doped ZnO film [25]. Moreover, the lowest resistivity of the deposited Ti:ZnO film is competitive with previous reports [18] under similar film thicknesses, indicating that doping the Ti element into pure ZnO films will improve their electrical properties.
4. Conclusions In this paper, we deposited successfully Ti-doped ZnO films onto glass substrates using the dual-target cathodic arc plasma deposition. The properties of the Ti:ZnO films deposited with various arc currents were investigated. Through X-ray diffraction measurements, almost all deposited films revealed a preferred orientation with the c-axis perpendicular to the substrates. The intensity of (0 0 2) peak was decreased significantly as the arc current of the Ti target was increased. The transmittance of almost all deposited Ti:ZnO films exceeded 84% in the range of 380– 780 nm. With the extension of the arc current of the Ti target, the carrier concentration of the Ti:ZnO films was increased and the mobility was decreased. The deposited Ti:ZnO film had the lowest resistivity of 7.9 ×10 3 Ω cm when the arc current of the Ti target was 30 A. This was mainly caused by the increased carrier concentration although the carrier scattering was increased and the mobility was decreased with the arc current of the Ti target. From the above, the arc current played an important role in the optical and electrical properties of Ti:ZnO films.
Acknowledgments This work was partially funded by the Ministry of Science and Technology, Taiwan, and Southern Taiwan Science Park Administration (STSPA), Taiwan under Contract NSC 101-2628-E-020-002MY3, and 102GE06. The authors would also like to thank the National Nano Device Laboratories, and the Precision Instrument Center of National Pingtung University of Science and Technology for the supporting the experimental equipment.
References
Fig. 7. Resistivity (ρ), mobility (μ) and carrier concentration (n) of the deposited Ti: ZnO films on glass substrates using CAPD with various arc currents for the Ti target.
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