Highly conductive alumina-added ZnO ceramic target prepared by reduction sintering and its effects o

Highly conductive alumina-added ZnO ceramic target prepared by reduction sintering and its effects on the properties of deposited thin films by direct current magnetron sputtering H.S. Huanga, , , H.C. Tunga, C.H. Chiua, I.T. Honga, R.Z. Chena, J.T. Changa and H.K. Linb a New Materials Research & Development Department, China Steel Corporation, 1 Chung Kang Road, Hsiao Kang, Kaohsiung, 812, Taiwan, R.O.C. b Department of Electrical Engineering, National Central University, 300, Jhongda Road, Jhongli, Taoyuan 320, Taiwan, R.O.C. Received 21 September 2009; revised 9 April 2010; accepted 2 June 2010. Available online 12 June 2010. Abstract To obtain a suitable sputtering target for depositing transparent conducting Al-doped ZnO (AZO) films by using direct current (DC) magnetron sputtering, this study investigates the possibility of using atmosphere controlled sintering of Al2O3 mixed ZnO powders to prepare highly conductive ceramic AZO targets. Experimental results show that a gas mixture of Ar and CO could produce a sintered target with resistivity in the range of 2.23 × 10? 4 Ω cm. The fairly low resistivity was mainly achieved by the formation of both aluminum substitution (AlZn) and oxygen vacancy (VO), thus greatly increasing the carrier concentration. Compared to usual air sintered target, the thin film deposited by the Ar + CO sintered target exhibited lower film resistivity and more uniform spatial distribution of resistivity. A film resistivity as low as 6.8 × 10? 4 Ω cm was obtained under the sputtering conditions of this study. Keywords: Transparent conducting oxide; Aluminum doped zinc oxide (AZO); Direct current magnetron sputtering; Sputtering target; Sintering; Thin film; Conductivity Article Outline 1. Introduction 2. Experimental details 2.1. Preparation of AZO sputtering materials and AZO films 2.2. Property analysis of ceramic AZO materials and AZO films 3. Results and discussion 3.1. Influence of sintering atmosphere on target properties 3.2. Sputtering of sintered AZO targets and their film properties 4. Conclusions References 1. Introduction Aluminum doped zinc oxide (AZO) is known as a transparent conducting oxide (TCO) and has been studied extensively since 1980s [1]. Currently, AZO film has been applied as transparent electrode in thin film photovoltaics such as silicon base [2] and [3] and CIGS (Cu(InGa)Se2) base [4] and [5], because it is much more resistant to hydrogen-plasma reduction and with relatively better optical transmission in the solar spectrum [3]. Moreover, zinc is a rather abundant and cheap metal compared to indium. Therefore, AZO has also been expected to replace the widely used TCO, indium tin oxide (ITO), utilized in liquid crystal display [6]. Various deposition techniques have been utilized for AZO films such as direct current (DC) and radio frequency (RF) magnetron sputtering [7], pulsed laser deposition [8], chemical vapor deposition [9], spray hydrolysis [10], and sol–gel [11]. In order to satisfy the demands of good film quality, high-rate and large-area deposition, and low cost of equipment for commercialization of all TCO films, DC magnetron sputtering has been regarded as one of the most attractive and effective fabrication techniques in the mass production of AZO films. Since high visible light transmission and low electrical resistivity are the basic properties of requirement for all TCO films including AZO, most studies in magnetic sputtering discuss the influences of various sputtering parameters on optical and electrical properties of AZO films. Among these studies substrate heating [12], lower working pressure [13], and hydrogen gas introduction [14] have been proven to improve the performance of AZO films. In addition to the control of sputtering conditions, the properties of TCO sputtering targets are also very important for obtaining good quality of deposited TCO films. A ceramic ITO target, for instance, needs to have high density and low resistivity in order to prevent from abnormal arcing and nodule formation during film depositions [15], [16] and [17]. However, the influence of AZO target with different properties on the performance of AZO film has rarely been studied. In this study, a ceramic AZO sputtering target with a high density of 98.9% and a low resistivity of 2.23 × 10? 4 Ω cm was obtained through sintering of atmosphere controlling. The conducting mechanism of free electrons in the low-resisted AZO target and its effect on the film properties of deposited AZO films by DC magnetron sputtering were investigated. 2. Experimental details 2.1. Preparation of AZO sputtering materials and AZO films Fine ZnO and Al2O3 powders with 99.99% purity and an average particle size of 1 μm and 0.2 μm respectively were used in this study. To prepare the ceramic AZO compact, ZnO powders were mixed with different amounts of Al2O3 through ball milling for 24 h. The mixed slurry was further mixed with polyvinyl alcohol and then spray dried using heated air in order to facilitate the die filling and compaction process. The spray dried powders were pressed into a steel-made disk of 200 mm diameter using a pressure of 50 MPa. The density of the green compacts was about 60%. After debinding at 600 °C for 1 h, the compacts were sintered at 1400 °C for 4 h in two different atmospheric conditions, one in usual air and the other in argon with a trace of carbon monoxide. The sintered AZO compacts from the two different sintering atmospheres were cut, ground, and polished to form circular sputtering targets with a diameter of 152 mm. The AZO targets were sputtered to deposit AZO films by a DC magnetron sputtering system. The sputtering conditions were listed in Table 1. Table 1. Sputtering parameters of deposited AZO films in this study. Sputtering parameters Conditions Substrate (temperature) Cover glass (RT 200 °C) Target to substrate distance 55 mm Power 330– 1000 W Base pressure 4 × 10? 5 Pa Working pressure 0.25–0.4 Pa Ar flow 10–20 sccm Film thickness 800 nm Full-size table View Within Article 2.2. Property analysis of ceramic AZO materials and AZO films The sintered densities of bulk AZO targets were measured by the Archimedes' method, taking 5.67 g/cm3 for the theoretical density of all specimens. To evaluate the effect of target conductivity on the film properties, the resistivity of bulk targets and films were examined by the four-point probe method. In addition, the mobility and carrier concentration of targets and films were further determined at room temperature using the Van der Pauw method by a Hall-effect measurement system (HL5500PC, Nanometrics Inc.). The sample of Hall-effect measurement for AZO targets was prepared by cutting and grinding the bulk materials to form a square sheet with a size of 10 × 10 × 0.25 mm3. Four In electrodes were coated on the four corners of the sample surface in order to obtain a good connection between the sample and the probes. For the transmittance measurement of AZO films, a UV/VIS spectrometer was utilized within the visible light region. The measured location of both electrical and optical properties for AZO films was at the central part of the deposited cover glass with a size of 20 × 20 mm2. Microstructure of sintered AZO targets and surface morphology of sputtered AZO films were observed using a scanning electron microscope (SEM, JSM 6500F, JEOL Ltd.) operated at an accelerating voltage of 10 kV. An X-ray diffractometer (XRD, D8 ADVANCE, Bruker AXS Inc.) with Ni-filtered Cu Kα radiation was employed to compare the second phase, lattice parameter, and crystallinity of AZO targets and films. 3. Results and discussion 3.1. Influence of sintering atmosphere on target properties Fig. 1 shows the relative density of ZnO with respect to different Al2O3 contents after being sintered in air and in Ar + CO respectively at 1400 °C for 4 h. For sintering in usual air, small amounts of Al2O3 slightly decreased the sintered density. However, when the amount of Al2O3 exceeded 2 wt.%, the decrease in density became much more obvious. For sintering in Ar + CO atmosphere, the density curve had a trend similar to that of the air condition, but the final densities of these samples were all higher than those sintered in air. The densities of 2 wt.% Al2O3 addition sintered in air and in Ar + CO were about 98.6% and 98.9% respectively. The grain size shown in Fig. 2 was measured by the quantitative metallography method, which shows that the grain size of AZO gradually decreased with increase in the amounts of Al2O3, but little difference was observed at same level of the additive regardless of sintering atmosphere. Previous literature [18] has reported that the second phase ZnAl2O4 forms during the sintering of Al-doped ZnO in air and thus retards the sintering densification and grain growth. The results from the XRD for phase identif ications shown in Fig. 3 also reveal that a higher level of Al2O3 addition led to a greater formation of the ZnAl2O4 phase. Therefore, the formation of ZnAl2O4 phase is believed to play the major role in inhibiting densification and grain growth of sintered AZO. -------------------------------------------------------------------------------- Full-size image (26K) High-quality image (148K) Fig. 1. Sintered density of ZnO with different amounts of Al2O3. View Within Article -------------------------------------------------------------------------------- Full-size image (20K) High-quality image (93K) Fig. 2. Grain size of sintered ZnO with different amounts of Al2O3. View Within Article -------------------------------------------------------------------------------- Full-size image (39K) High-quality image (239K) Fig. 3. X-ray diffraction pattern of sintered AZO compacts with different amounts of Al2O3. View Within Article To understand the effects of amount of Al2O3 addition and sintering atmosphere on conductivity of sintered AZO, the resistivity of specimens of all sintering conditions was measured by the four-point probe method. Fig. 4 shows that the optimum addition of Al2O3 for achieving the lowest resistivity of bulk AZO was about 1.5–2 wt.%, regardless of the sintering atmospheres. However, sintering in Ar + CO obtained far lower resistivity than sintering in usual air. With 2 wt.% addition of Al2O3, the resistivity of ZnO sintered in air and Ar + CO were 2.14 × 10? 3 Ω cm and 2.23 × 10? 4 Ω cm, respectively. Such a low resistivity value for a ceramic AZO target has not been widely reported. In addition, the difference of resistivity between the two atmospheres almost reached an order of magnitude, indicating that the impact of sintering atmosphere on conductivity of AZO target was much more prominent than that of alumina addition. -------------------------------------------------------------------------------- Full-size image (23K) High-quality image (117K) Fig. 4. Bulk resistivity as a function of Al2O3 content for sintered AZO in air and Ar + CO. View Within Article To further illustrate the difference between air and Ar + CO atmospheres on electrical conductivity of AZO target, samples with 2 wt.% Al2O3 sintered in both atmospheres were examined by the Hall-effect measurements and shown in Table 2. The result of electrical resistivity measured by the Hall-effect method again confirmed that sintering in Ar + CO atmosphere produced bulk AZO with much lower resistivity. Furthermore, the Hall mobility of AZO sintered in Ar + CO was only about 1.1 times higher than that sintered in air, but the carrier concentration was about 4 times higher. Therefore, the lower resistivity of AZO sintered in Ar + CO is mainly attributed to the enhancement of the carrier concentration. Since the source of transport carriers of n-type ZnO was electrons from extrinsic doped impurities and native point defects, the enhanced conductivity of bulk AZO within the same amount of Al2O3 for sintering in the Ar + CO atmosphere should be associated with the increment of native point defects. The increased concentration of native point defects occurred due to CO being a reducing atmosphere, which reacted with ZnO to form non-stoichiometric ZnO with oxygen deficiency. To prove that, a slightly lower oxygen content of bulk AZO sintering in Ar + CO had been confirmed by chemically quantitative analysis by an electron probe microanalyzer. -------------------------------------------------------------------------------- Table 2. Comparison of electrical properties for AZO (2 wt.% Al2O3) targets sintered in air and Ar + CO. AZO (2 wt.% Al2O3) target Sintered in air Sintered in Ar + CO Hall mobility (cm2/V s) 76.6 87.0 Carrier concentration (cm? 3) 9.24 × 1019 3.72 × 1020 Resistivity (Ω cm)a 8.85 × 10? 4 1.93 × 10? 4 Full-size table a The values were measured by the Hall-effect apparatus. View Within Article Previous literature [19] has shown that there are two kinds of native point defects in oxygen-deficient ZnO that may form and contribute significantly to free electrons and thus its conductivity. One is the formation of oxygen vacancy VO, the other is the formation of zinc interstitial Zni. It is however, yet to resolve which of the two is more dominant to form and thus creates free carriers to lead to the conductivity of oxygen-deficient ZnO [20], [21], [22] and [23]. In our case, it is believed that if the point defect of oxygen-deficient ZnO is VO dominant, formation of vacancy should result in lattice shrinkage. In the contrary, formation of interstitials should result in lattice expansion. Thus, pure ZnO before and after Ar + CO sintering were further examined with XRD. Fig. 5 shows that the diffraction peak of (002) of ZnO after sintering had an obvious shift to the right, indicating the occurrence of lattice shrinkage. The calculated lattice parameters of c axis of ZnO before and after Ar + CO sintering were 5.2069 ? (very close to the 5.2066 ? in the JCPDS data of ZnO (36–1451)) and 5.1998 ?, respectively. Therefore, the formation of oxygen vacancies is believed to be responsible for the increase in carrier concentration and conductivity of AZO when sintering in Ar + CO atmosphere in this study. -------------------------------------------------------------------------------- Full-size image (25K) High-quality image (138K) Fig. 5. Comparison of the XRD patterns for pure ZnO before and after Ar + CO sintering at 1400 °C. The (002) peak of ZnO shifted to the right upon sintering. View Within Article Based on the previous discussions, the chemical reactions during the sintering process and the mechanism of electrical conduction for Ar + CO sintered AZO can be described as follows. Fig. 6 illustrates that as the Al2O3 mix ed ZnO compact was heated in Ar + CO atmosphere, Al atoms from Al2O3 started to diffuse into the nearby areas of ZnO and to substitute into the lattice site of Zn to form the point defects of AlZn. Moreover, ZnO reacted with small amount of CO to form non-stoichiometric ZnO1 ? x with oxygen deficiency at the same time. Since both point defects of VO and AlZn were produced after sintering from chemical reduction and intentional Al doping respectively, which provided large amount of free electrons, the carrier concentration was improved, leading to enhanced conductivity. -------------------------------------------------------------------------------- Full-size image (71K) High-quality image (422K) Fig. 6. Schematics of the chemical reactions and conduction mechanism of the Ar + CO sintered AZO. View Within Article 3.2. Sputtering of sintered AZO targets and their film properties To evaluate the performance of sintered AZO with the large difference in conductivity obtained from the two different sintering atmospheres, 2 wt.% Al2O3 added AZO targets sintered in both air and Ar + CO respectively were performed to deposit AZO films on cover glasses by the DC magnetron sputtering method. The optical and electrical properties of the AZO films were compared. Fig. 7a shows the transmittance spectra in the optical wavelength range of 200–1000 nm for AZO films with a thickness of 800 nm. Fig. 7b presents the comparison of average transmittance calculated from the visible optical region. All films exhibited an average transmittance of more than 80%, regardless of the different sputtering conditions; however, proper substrate heating during the deposition indeed improved the transmittance, which is consistent with previous literature [12]. The transmittance of the AZO film deposited on a heated glass from the Ar + CO sintered target was 87%, slightly lower than that of the air sintered target with a transmittance of 88.2%. -------------------------------------------------------------------------------- Full-size image (49K) High-quality image (505K) Fig. 7. Comparison of (a) Transmittance spectra and (b) average transmittance in visible region for AZO thin films deposited from air sintered and Ar + CO sintered targets. View Within Article In comparing electrical properties, Fig. 8 shows the resistivity of AZO films in various sputtering power. Without substrate heating, the resistivity exhibited about 1.2–1.5 × 10? 3 Ω cm, showing little difference between the air and Ar + CO sintered targets. However, when the substrate was heated at 200 °C, the resistivity of the AZO film deposited from the Ar + CO sintered target was decreased significantly to 6.8 × 10? 4 Ω cm at 1000 W, much lower than the resistivity value of 9.8 × 10? 4 Ω cm obtained from the air sintered target. In order to understand the difference in resistivity, carrier concentration and mobility, the AZO films were measured with a Hall-effect apparatus. Table 3 shows that although the AZO film prepared from the Ar + CO sintered target without substrate heating exhibited higher carrier concentration than that of the air sintered AZO, the Hall mobility was lower, thus making almost no difference in resistivity. When the substrate temperature was heated to 200 °C, the carrier concentration shows a very slight decrease for both AZO films. However, the Hall mobility of the thin film deposited from the Ar + CO target was significantly increased from 4.91 cm2/V s to 8.63 cm2/V s, much higher than that deposited from the air sintered target, which only showed a slight increase from 6.61 cm2/V s to 6.91 cm cm2/V s. This indicates that the scattering probability of transition carriers for AZO thin films obtained from the Ar + CO sintered target is much easier to be reduced through the substrate heat treatment, resulting in the improvement of carrier mobility and thus conductivity. -------------------------------------------------------------------------------- Full-size image (27K) High-quality image (161K) Fig. 8. Film resistivity as a function of sputtering power for AZO films deposited from air sintered and Ar + CO sintered targets with and without substrate heating. View Within Article -------------------------------------------------------------------------------- Table 3. Comparison of electrical properties for AZO films deposited from air sintered and Ar + CO sintered AZO (2 wt.% Al2O3) targets. AZO film Target sintered in air Target sintered in Ar + CO RT 200 °C RT 200°C Hall mobility (cm2/V s) 6.61 6.91 4.91 8.63 Carrier concentration (cm? 3) 9.49 × 1020 9.11 × 1020 1.29 × 1021 1.23 × 1021 Resistivity (Ω cm)a 9.95 × 10? 4 9.92 × 10? 4 9.84 × 10? 4 Full-size table a The values were measured by the Hall-effect apparatus. View Within Article Although the above results indicated that the conductivity of AZO films was enhanced by sputtering an AZO target with higher conductivity, [Table 2] and [Table 3] show that the dominant factor of conductivity between the targets and films was totally different. AZO targets, prepared by either air sintering or Ar + CO sintering, exhibited higher Hall mobility but lower carrier concentration, while AZO films exhibited reverse characteristics, irrespective of the kind of target used. Because the microstructural analysis obtained from the SEM and XRD showed that sintered AZO targets exhibited much larger grains and better crystallinity compared to AZO films, it is logical to believe that the higher electron mobility of AZO targets was ascribed to the decrease of electron scattering from crystal and boundary defects. Furthermore, although the AZO films were formed from sputtering of AZO targets prepared by sintering of mixed Al2O3 and ZnO powders, the atomic rearrangement among Zn, Al, and O atoms during the deposition generally helped to obtain an AZO film with higher doping concentration and better doping uniformity. Therefore, the carrier concentration in the AZO films was systematically higher than that in the AZO targets. In addition to minimum resistivity, the uniformity in spatial distribution of resistivity is even more important for the criteria of practical use. Minami et al. [6], [24], [25], [26] and [27] have reported that when AZO films are prepared by DC magnetron sputtering, non-uniform distribution of resistivity occurs, especially with a marked increase in resistivity at substrate locations that correspond to the erosion areas of the targets. It is generally believed that the phenomenon is mainly associated with the activity and amount of oxygen reaching the substrate surface [24], and thus suggesting some controllable deposition conditions such as RF + DC sputtering, a less oxidizing atmosphere, or the use of AZO targets with lower oxygen content [25], [26] and [27]. Fig. 9 compares the resistivity distribution of AZO films prepared from the air sintered and Ar + CO sintered targets, respectively. Higher resistivity was indeed observed at substrate surfaces that correspond to the erosion areas for both thin films. However, the effect was considerably inhibited when the Ar + CO sintered AZO target was used. The results prove that using an AZO target with lower oxygen content and higher conductivity for DC magnetron sputtering not only enhanced the conductivity, but also improved the uniformity of resistivity distribution of AZO thin films. Therefore, the ceramic AZO target sintered in Ar + CO atmosphere is more suitable for practical sputtering use than those sintered with usual air. -------------------------------------------------------------------------------- Full-size image (26K) High-quality image (146K) Fig. 9. Spatial distributions of resistivity for AZO films deposited from air sintered and Ar + CO sintered targets. View Within Article 4. Conclusions The effect of sintering atmosphere on properties of ceramic AZO target was investigated. It has been shown that the conductivity of AZO target strongly depended on the sintering atmosphere. An AZO target with high density (> 98.5%) and low resistivity (< 3 × 10? 4 Ω cm) was achieved in a controlled Ar + CO atmosphere. 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