Microstructures, electrical and magnetic properties of (Ga, Co)-ZnO films by radio frequency magnetron co-sputtering

    Microstructures and electrical and magnetic properties of (Ga, Co)-ZnO films by radio frequency magnetron co-sputtering Sheng-Chi Chen, Chung-Hsien Wang, Hui Sun, Chao-Kuang Wen, Chao-Feng Lu, Chia-Lung Tsai, Yi-Keng Fu, Tung-Han Chuang PII: DOI: Reference:

S0257-8972(16)30188-8 doi: 10.1016/j.surfcoat.2016.03.064 SCT 21037

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Surface & Coatings Technology

Received date: Revised date: Accepted date:

15 November 2015 7 March 2016 22 March 2016

Please cite this article as: Sheng-Chi Chen, Chung-Hsien Wang, Hui Sun, ChaoKuang Wen, Chao-Feng Lu, Chia-Lung Tsai, Yi-Keng Fu, Tung-Han Chuang, Microstructures and electrical and magnetic properties of (Ga, Co)-ZnO films by radio frequency magnetron co-sputtering, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.03.064

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Surface & Coatings Technology

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Microstructures and electrical and magnetic properties of (Ga, Co)-ZnO films by radio frequency magnetron co-sputtering Sheng-Chi Chen a,b,⁎, Chung-Hsien Wang a, Hui Sun a, Chao-Kuang Wen c, Chao-Feng Lu a, Chia-Lung Tsai d, Yi-Keng Fu d, Tung-Han Chuang c

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Article history: Received 15 November 2015 Revised 7 March 2016 Accepted in revised form 22 March 2016 Available online xxxx

Department of Materials Engineering, Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taipei 243, Taiwan Department of Electronic Engineering, Chang Gung University, Taoyuan 333, Taiwan Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan d Electronics and Opto-electronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan b c

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Keywords: rf magnetron co-sputtering Diluted magnetic semiconductors (Ga, Co)-ZnO films Electrical properties Magnetic properties

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In this study, [Co0.05GaxZn(0.95 − x)O] films with different Ga contents were co-sputtered on glass substrates by rf magnetron sputtering. The content of Co in the films was fixed at ~5 at.%. The content of x [Ga/(Ga + Co + Zn)] varied from 0 to 3.2 at.%. As analyzed by Hall effect measurement, the resistivity (ρ) of the film is 42.90 Ω·cm when x content is 0. When the content of x increases to 3.2 at.%, the ρ value drops greatly to 4.93 × 10−3 Ω·cm. It is found that both the surface roughness and grain size of columnar (Ga, Co)-ZnO films decrease significantly after Ga addition into the films, but the phase structure remains almost unchanged. In magnetic properties analysis, two distinct mechanisms of bound magnetic polaron and carrier-mediated exchange lead to the films presenting different ferromagnetic behaviors in various carrier density regions. © 2015 Published by Elsevier B.V.

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behavior of ZnO-based DMSs has been explained by several researchers. Tsai et al. [12] pointed out that the defects in the films were responsible for the magnetic properties of the Co0.2AlxZn0.8 − xO samples; Koshihara et al. stated that the magnetic properties of DMSs are due to the Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect exchange or doubleexchange mechanisms [13]; whereas Coey et al. reported that the ferromagnetic behavior is due to the percolation of bound magnetic polarons (BMPs) [14]. DMSs have been fabricated through various techniques, including sol-gel method [15], pulsed laser deposition (PLD) [16], molecular beam epitaxy (MBE) [17] etc., but there are very few reports about preparation of DMSs using radio frequency (rf) magnetron sputtering. Therefore, in this work we deposited the (Ga, Co)-ZnO films with different Ga contents by rf magnetron co-sputtering, and investigated the influence of Ga content on the microstructures, electrical and magnetic properties of the films.

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2. Experimental

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2.1. Sample preparation

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The (Ga, Co)-ZnO films with 100 nm thickness were deposited on Corning Eagle XG glass substrates by radio frequency (rf) cosputtering of ZnO, Co and Ga2O3 targets in pure Ar atmosphere at substrate temperature of 100 °C. A schematic diagram of the apparatus used in our experiment is shown in Fig. 1. It allows the installation of

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1. Introduction

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Zinc oxide (ZnO) with a hexagonal wurtzite structure has a wide direct band gap of 3.37 eV at room temperature, a large exciton binding energy of 60 meV, good electrical properties, and transparency in the visible region. Due to these desirable properties, zinc oxide has also received considerable interest for use in optoelectronic applications such as, solar cells [1], sensors [2], light emitting diodes [3], and stealth technology [4]. For the purpose of improving un-doped ZnO films electrical and magnetic properties, it is necessary to add certain metal atoms into the zinc oxide, for instance trivalent Al [5–7], Ga [8], In [9], and ferromagnetic Co [10]. For at least a decade, diluted magnetic semiconductors (DMSs) have been studied because of their potential applications in magnetic devices, such as Magnetoresistance Random Access Memory (MRAM), Spin Field-Effect Transistors (Spin-FETs), Spin Light-Emitting Diodes (SpinLEDs) and so on. In a previous study [11], ZnO-based DMSs were reported as displaying ferromagnetic behavior at Curie temperature above room temperature, so they can be utilized in next generation spintronic devices. However, the origin of ferromagnetism in ZnO-based DMSs remains a controversial topic. In previous reports, the ferromagnetic

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⁎ Corresponding author at: Department of Electronic Engineering, Chang Gung University, Taoyuan 333, Taiwan. E-mail address: [email protected] (S.-C. Chen).

http://dx.doi.org/10.1016/j.surfcoat.2016.03.064 0257-8972/© 2015 Published by Elsevier B.V.

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Fig. 1. A schematic diagram of the apparatus used in the experiment.

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2.2. Characterization techniques

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The thickness of the (Ga, Co)-ZnO films was measured by a surface profiler (Kosaka Surfcoder). The chemical composition of the

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films was determined using an electron probe microanalyzer (EPMA, JOEL JXA-8900R). The phase structure of the samples was examined by X-ray diffraction (XRD) technique. The X-ray diffractometer (Philips PANalytical – X'Pert PRO MRD) used monochromatic high-intensity Cu Kα1 radiation (λ = 0.15406 nm). The surface particle size and roughness were measured respectively by field emission scanning electron microscopy (FE-SEM) and by atomic force microscopy (AFM). The electrical properties of the films were measured by Hall measurements in the van der Pauw's [18] configuration at room temperature and the magnetic properties were measured by vibrating sample magnetometer (VSM, Lake Shore's Model 7407).

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three targets simultaneously. The chamber was pumped by a scroll pump and a turbo pump to a base pressure below 6.4 × 10−4 Pa. Pure Ar was used as the working gas. The dimensions of sputter targets are 50.8 mm in diameter and 3 mm in thickness. The sputtering powers for the ZnO and Co targets were kept at 80 W and 15 W, respectively. The power of Ga2O3 target was varied in the range of 0 to 25 W. The Ar pressure during deposition of (Ga, Co)-ZnO films was fixed at 1.67 Pa and the distance between the substrate and the target was kept at 10 cm.

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Fig. 2. The variation of Ga content as a function of the sputtering power supplied on Ga2O3 target.

Fig. 3. XRD patterns of pure ZnO and (Ga, Co)-ZnO films with different Ga contents.

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Fig. 4. Variations of resistivity (ρ) in (Ga, Co)-ZnO films with different Ga contents.

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3. Results and discussion

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3.1. Chemical composition analysis

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According to the previous studies, in order to avoid the formation of Co clusters in the (Ga, Co)-ZnO films due to excess Co content, the content of Co [Co / (Ga + Co + Zn)] in the films should be controlled in the range from 5 at.% to 7 at.% [19]. In general, the Co clusters consist of antiferromagnetic materials, which results in the degradation of the magnetic properties of the films. Furthermore, a high Ga content in the (Ga, Co)-ZnO films is also detrimental to the electrical properties due to the formation of Ga clusters [20]. Therefore, the Ga content [Ga / (Ga + Co + Zn)] in the films should be low, in the range of 1 at.% to 3 at.%. In this work, the sputtering power of Co and ZnO targets is fixed at 15 W and 80 W respectively, while the power for Ga2O3 target is varied from 0 to 25 W. Under these conditions, Ga content in the films increases from 0 to 3.2 at.% as the Ga2O3 target sputtering power is raised to 25 W. Meanwhile, Co content decreases slightly from 5.3 at.% to 4.8 at.%, with Zn content dropping greatly from 94.7 at.% to 91.8 at.%. With regard to the changes in both Ga and Zn content, the variation of Co content is relatively insignificant (remaining close to 5 at.%),

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The microstructures of the films were analyzed using a high resolution transmission electron microscope (HR-TEM, JEOL JEM-2100) operated at 200 kV.

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Fig. 6. SEM observations of the top surface of (a) ZnO, (b) Co-ZnO and (c) (Ga, Co)-ZnO (Ga: 2.0 at.%) films (insets are AFM images).

Fig. 5. Variations of carrier concentration (n) and mobility (μ) in (Ga, Co)-ZnO films with different Ga contents.

which we expect to have little influence on the films properties. The 126 variation of each metal's content in the films with increasing in Ga2O3 127 target sputtering power is shown in Fig. 2. 128

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Fig. 7. Cross-sectional TEM images of (a) pure ZnO film, (b) Co-ZnO film, and (c) (Ga, Co)-ZnO film (Ga:2.0 at.%).

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3.3. Electrical properties

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Fig. 3 shows the XRD patterns of the pure ZnO and (Ga, Co)-ZnO films with various Ga contents. As can be seen on the JCPD card (Card No. 89-1397), all the films display only ZnO peaks of (002), (102) and (103), appearing at around 34.3°, 47.3° and 62.7° respectively, with no other phases. Furthermore, the intensity of these peaks remains almost constant when the Ga content in the films is increased continuously from 0 to 3.2 at.%, indicating that the phase structures of the films cannot be changed by the addition of small amounts of Ga.

The electrical properties of (Ga, Co)-ZnO films with various Ga contents in this work were measured by Hall effect using van der Pauw's method [18]. As can be seen in Fig. 4, when the Ga content in the films is 0 at.% (i.e. Co-ZnO film), the resistivity (ρ) is 42.90 Ω·cm. This value drops to 7.01 Ω·cm as 0.3 at.% Ga is added into the films and it continues to decrease as the content of Ga further increases. The ρ value decreases markedly to 4.93 × 10−3 Ω·cm when the films have a high Ga content of 3.2 at.%.

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SEM observation of top surfaces of ZnO, Co-ZnO, and (Ga, Co)-ZnO (Ga: 2.0 at.%) films are shown in Fig. 6. The films' roughness, illustrated by atomic force microscopy, is given in the inset images. Pure ZnO film (Fig. 6(a)) has the roughest surface with the Ra surface roughness of 1.86 nm. The ZnO particles with size in the range of 20–60 nm can be clearly observed. In the Co-ZnO film (Fig. 6(b)), the growth of ZnO particles can be suppressed by Co doping, which causes a particle refinement in the ZnO films. We believe that the aggregated or precipitated Co clusters could separate the growing ZnO particles, and thus prevent the ZnO particles from coarsening. In the (Ga, Co)-ZnO (Ga: 2.0 at.%) film, this phenomenon becomes more evident due to the aggregation or precipitation of Ga atoms. Consequently, the film roughness decreases to 1.10 nm in the Ga doped Co-ZnO film. A similar result can be seen by TEM observation. The cross-sectional images with different resolutions corresponding to pure ZnO, Co-ZnO, and (Ga, Co)-ZnO films (Ga: 2.0 at.%) are shown in Fig. 7. The columnar

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Fig. 5 shows the carrier concentration (n) and carrier mobility (μ) of the (Ga, Co)-ZnO films with different Ga contents. It is found that the carrier concentration of the films rises with increasing Ga content. It increases significantly from 9.05 × 1015 to 3.92 × 1020 cm−3 after 3.2 at.% of Ga is added into the Co-ZnO films, and all the films show n-type conduction. We believe that Zn2+ ions in the ZnO lattice are replaced by Ga3+ ions, leading to the formation of n-type conduction, which also results in an increase in carrier concentration with a subsequent decrease in the ρ value in the films. Although the carrier concentration rises with increasing Ga content, the carrier mobility drops greatly from 16.1 to 3.25 cm2 V−1 s−1 as 3.2 at.% of Ga content is added into Co-ZnO films. Lu et al. prepared (Ga, Co)-ZnO thin films using molecular beam epitaxy (MBE) method [20]. They found that the highest carrier concentration of about 5 × 1020 cm− 3 was achieved at Ga content of 2 at.%; this value then decreases due to the aggregation or precipitation of excess Ga. In our findings, a relatively lower carrier concentration (~2.23 × 1020 cm−3) is detected when Ga content is about 2 at.%. This is because compared to the MBE method, the film's higher growth rate using rf magnetron sputtering could result in insufficient substitution of Ga atoms into the ZnO lattice. On further raising the Ga content to 3.2 at.%, the carrier concentration continuously increases due to more Ga substitution occurring in the films. Since the magnetron sputtering method possesses high deposition rate, excellent uniformity over large area deposition, and good controllability over chemical composition, this method is the favored technique used in depositing films for industrial applications.

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microstructures are observed in all the films. Compared with pure ZnO film (Fig. 7(a)), the grain size of columnar crystals decreases in the CoZnO film (Fig. 7(b)) and further in the (Ga, Co)-ZnO (Ga: 2.0 at.%) film. This confirms the suppressive effect on ZnO grain growth by Co and Ga doping.

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Fig. 8. The relationship between the saturation magnetization (Ms) and carrier concentration (n) of (Ga, Co)-ZnO films.

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The magnetic properties of (Ga, Co)-ZnO films were measured by vibrating sample magnetometer (VSM). Fig. 8 shows the relationship between saturation magnetization (Ms) and carrier concentrations. Noticeable ferromagnetic behavior is detected at both low carrier concentration region (n b 1017 cm−3) and high carrier concentration region (n N 1019 cm−3). However, in the intermediate carrier concentration region (1017 cm−3 b n b 1019 cm−3), the ferromagnetic behavior is weak. Behan et al. [21] reported that the ferromagnetic origin in these three regions is very different. They have defined these three regions as being insulating regime, intermediate regime and metallic regime, respectively, according to the carrier concentration. In the insulating regime, the film with low carrier concentration possesses poor electrical properties, and the carriers are localized by the defects in the films. According to the simulation results reported by Wu et al. [22], under Orich conditions, the formation energy of intrinsic defects of Zn vacancies (VZn) and interstitial O (Oi) is low in Ga doped ZnO, which indicates that VZn and Oi are easier to form than other defects in this work due to the O content in our films always being higher than 60% (O-rich condition). The exchange interactions between localized carriers by VZn/Oi and the surrounding Co2+ ions align the Co spins around the carrier localization center, and form bound magnetic polarons (BMPs). The increase in the number of BMPs with increasing Ga content in the films, as well as the overlapping between BMPs, results in the rise of the film's ferromagnetism. However, with the carrier concentration increasing (1017 cm−3–1019 cm−3), there are too many free carriers created in the film, so that they cannot all be localized by the defects. As a result, the BMPs become unstable and the film's magnetism decreases (intermediate regime). In metallic regime, when the carrier concentration exceeds 1019 cm−3, the film's ferromagnetism rises from the carriermediated mechanisms. In this region, numerous mobile carriers can induce carrier-mediated exchange interactions, resulting in the film's ferromagnetism. A similar phenomenon is also found in (Al, Co)-ZnO films [23].

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4. Conclusions

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The (Ga, Co)-ZnO films with different Ga contents were co-sputtered on glass substrates by rf magnetron sputtering. In chemical composition analysis, the contents of Co and Ga are relatively consistent at around 5 at.% and 0 to 3.2 at.%, respectively. Hall effect analysis shows the electrical resistivity of the films decreases and the carrier concentration increases as Ga content in the films is increased; meanwhile, the carrier mobility drops. When the Ga content is 3.2 at.%, the resistivity, carrier concentration, and mobility are 4.93 × 10−3 Ω·cm, 3.92 × 1020 cm−3, and 3.25 cm2 V− 1 s−1, respectively. In magnetic properties analysis, three distinct regions can be defined according to the carrier concentration. BMP and carrier-mediated exchange mechanisms can be given as reasons to explain the variation in the film's ferromagnetic behavior in different carrier density regions.

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Acknowledgements

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This work was supported by the Ministry of Science and Technology of Taiwan through Grant No. 103-2221-E-131-004. Thanks also to Prof. H.C. Lin and Mr. C.Y. Kao of Instrumentation Center, National Taiwan University for EPMA experiments.

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