Investigations on Properties of (ZnO:Al) Films Prepared by RF/DC Co-sputtering

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1 TELKOMNIKA, Vol.10, No.5, September 2012, pp. 947~952 e-issn: X accredited by DGHE (DIKTI), Decree No: 51/Dikti/Kep/ Investigations on Properties of (ZnO:Al) Films Prepared by RF/DC Co-sputtering Meng Chao, Diao Xungang*, Ju Wenbo, Wang Tao, Feng Lan *Beijing Institute of Aeronautical Materials, Beijing, China Abstract Aluminum doped zinc oxide (ZnO:Al) thin films with high conductivity, resistance to ion irradiation, high transparency in the visible range and moderate costs will be one of the most promising candidates for the present predominant transparent conducting materials. However, there are still some problems such as long preparation cycle and high surface resistance. In this paper, adding a certain amount of radio frequency (RF) power to the applied DC power, this technique known as RF-DC co-sputtering ionization is mainly driven by oscillating electrons in the bulk plasma, there is an enhanced substrate bombardment by plasma ions (mainly Ar+) of moderate energy to improve the optical and electrical properties of the film. Such a moderate energy ion bombardment can assist the film growth and lead to better and denser films. By variation of the RF/DC power ratio, we prepared at a relatively higher sputter rates and higher discharge voltage. The samples of crystal structure, surface morphology, electrical conductivity were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), four-probe meter, as well as visible spectrophotometer. The results show that the average transmittance in the visible was 89% and the resistivity was reduced by 80% compared to other single technique. When the radio frequency power makes up around 50% of the total, the resistivity reach a minimum of Ω cm, improve the homogeneity of the film Effectively. Keywords: RF-DC co-sputtering; ZAO thin films; transparent conductive films; optical and electrical properties Copyright 2012 Universitas Ahmad Dahlan. All rights reserved. 1. Introduction Aluminum-doped zinc oxide (ZnO: Al, referred to as ZAO) thin film is typical of heavily doped, high-degenerate semiconductor, the band gap of about 3eV, and its optical band gap can be controlled by adjusting the aluminum doping ratio. It has a high transmittance in the visible range of the spectrum (380~780nm) (typically greater than 86%), and good electrical conductivity (resistivity close to 10-4 Ω cm)[1]. Al-doped ZnO thin films is very broad studied, its outstanding advantages of cheap and abundant, cost-effective and is easy to fabricate, nontoxic, easy to implement doping and good stability in plasma, which may be an alternative of ITO, especially in solar cells, transparent electrode field. Conventional magnetron sputtering methods which prepared ZAO films have their special characteristics. DC magnetron sputtering process requires relatively high voltage, and secondary electron emission by the target surface can maintain a continuous and stable working current in the procedure. Due to the load of the higher discharge voltage, the sputtering rate and sputtering efficiency of DC magnetron sputtering is considered to be relatively cheap and efficient preparation method. In RF magnetron sputtering process, the gas ionization and plasma generation in the course of their work is dependent on the charged ions in a shock state [2], the shock of movement of charged ions also increases the probability of collision with nonionized atom, thus significantly increase the plasma density, and significantly reduces the discharge voltage, at the same time the substrate was bombarded by a large number of medium - energy Ar + ion, contribute to the film growth and densification, but this process has a low sputtering rate, because of the low sputtering voltage. In order to prepare dense, high transparent conductive thin film,and have a high deposition rate in the preparation of this experiment. This article introduces the combination with the respective advantages of the DC sputtering and RF sputtering, and discusses a method which was mentioned in some papers to make the DC/RF sputtering power by reforming power supply. By adjusting process parameters to control thin film growth in the limited dynamic Received June 7, 2012; Revised September 2, 2012; Accepted September 11, 2012

2 948 e-issn: X conditions, the high-quality films, good transmittance conductive properties has been obtained, With a continuous tuning of the discharge characteristics from a pure DC-discharge to a RFdischarge by changing the RF-portion of the total sputtering power, the film properties could be improved. The influence of the surface morphology, mechanical, electrical and optical properties was investigated. 2. Experimental In this experimental set-up the sputtering cathode was excited simultaneously with SKY-type DC (a maximum 800W power) and SY-type RF(13.6MHz)power. The two power supply were connected to form a co-sputtering power, shown in Figure 1. In order to solve the mutual interference between the power supply, before the DC output connected to the RF wire adding an inductor coil as a low-pass filter, blocking the RF electric field to interfere with DC source. Add a high-frequency capacitance in the circuit, one end of the capacitance connected with DC output and the other end connected with ground, thereby maximizing remove the interference of the RF electric field on the DC power [3]. SKY-DC sputtering power SY-RF sputteri ng low-pass filter Auto match Target cathod Figure 1. DC / RF co-sputtering schematic diagram Table1. Different preparation conditions of power on sample Sample No. RF power / total power The flms examined in this work were prepared with an JGP-350C inline sputtering system for production Purposes, Floatglas and Quartz were used as substrate materials, and deposited by sputtering 60 cm diameter ZnO:Al targets in RF-DC co-sputtering system with Pa. The following parameters were found to be suitable and were used in film preparation: the content of Al 2 O 3 in the used target was 3%, the input total power was 120W, the separation between targets and substrates was 8cm and the Ar pressure was 0.8Pa. The cathode was excited simultaneously by a DC-power supply and a RF-generator. The DC-power supply was protected from the RF-waves by a RF-filter. The FR-power was coupled capacitively via a matching network, which was tuned for minimum reflected power. An arc suppression unit provided stable process conditions by controlling both the DC-power supply and the FRgenerator. The experimental arrangement and the properties of the FR-superimposed DC gas discharge are described more detailed in table Sample Characterization and Analysis The thin film s thickness was measured by elliptical polarization Instrument. The optical transmittances of the studied films were determined at room temperature for photon wavelengths ranging from 300 to 900nm, using a HITACHI U-3010UV-Vis spectrophotometer. The sheet resistivities were measured with an SD-510four probe instrument. Crystallographic and phase structures were determined by an X-ray diffractometer (RIGAKU D/MAX2200PC) with Cu Ka radiation. The growth morphologies and thickness of the deposited films were observed by using HITACHI scanning electron microscopy. TELKOMNIKA Vol. 10, No. 5, September 2012 :

3 TELKOMNIKA e-issn: X The Film Sheet Resistance Values and Deposition Rate Figure 2 shares for the RF power changes in proportion with the film sheet resistance and deposition rate corresponds to the diagram can be seen from the figure, the ZAO thin film samples were prepared with pure RF sputtering resistance is far greater than the total sputtering thin film resistors. In the same time, The proportion of the introduction with the RF power gradually increases, the sheet resistance of the sample gradually decreased. when RF power accounted for less than 46.3% of the total power in all deposited samples. With the RF components of total power further increasing, the resistance value gradually increase, the final resistance value get close to value at the pure RF sputtering. The results showed that the RF power ratio near 46% electrical properties have the optimal performance, you can get the sheet resistance, which is 6.725Ω/. Co-sputtering can influence the film quality of the film, the Co-sputtering of the filmforming rate is greater than the DC sputtering, rather than RF sputtering, with the speed characteristics of the DC sputtered film; and co-sputtering the film conductivity than pure DC sputtering film, this is the role of RF ingredients to improve the film quality [4] Sheet Resistance ( Ω / ) Prf (W) Figure 2. The proportion of RF power thinfilm sheet resistance and deposition rate of the corresponding diagram sputer rate ( nm/min) θ /( ) Intensity / a.u. 0 sample 10 sample 9 sample 7 sample 5 sample 3 sample 1 Figure 3. XRD spectra for ZAO films with different Proportion of RF power The Crystal Structure of the Film It can be seen from the figure, sample 1 prepared by pure RF magnetron sputtering has no obvious diffraction peak, which is typical of amorphous structure, whereas the film has not fully crystallized, the thickness of the film is very thin. Samples of pure DC sputtering and RF power accounted for 50% have a relatively sharp diffraction peaks, the other samples into a single diffraction peak, diffraction peaks are basically near 2θ 34.6, that the films are polycrystalline film with hexagonal structure and a single preferred direction in the (002) direction. On the sample 5, (002) preferential growth is more obvious, Due to the decrease of the residual stress of interface, grain growth, a grain boundary carrier scattering are almost not affected and weakened,so it improve the electrical conductivity of the film. Through the increase of RF power can effectively change the degree of crystallinity of the film [5] The Surface Morphology of the Film Figure 4 shows the fracture surface and surface morphology of SEM of samples 1 and 2. Observed from cross-section of the sample 1 prepared by DC sputtering, the crystal growth has obvious directionality in the direction perpendicular to the substrate surface, and forms columnar grains. The observation from the surface of the film shows that the surface particles is relatively uniform granular diameter of 100nm. Between the grains has Clearer boundaries, and exist the gap. Crystal orientation of Sample 2 has slightly irregular crystal shape, but the boundaries are relatively clear, the surface roughness less than sample 1, surface bump, and the gap is very obvious. Investigations on Properties of (ZnO:Al) Films Prepared by RF/DC Co-sputtering (Meng Chao)

4 950 e-issn: X Sample 1 Sample 1 Sample 2 Sample 2 Figure 4. Samples 1 and 2 cross-section and surface morphology of SEM micrographs Figure 5. (a) Samples 3(29.4%)surface morphology of SEM micrographs (b) Samples 5(50%) surface morphology of SEM micrographs (c) Samples 9 (83.9%)surface morphology of SEM micrographs (d) Samples 10(100%)surface morphology of SEM micrographs Figure 5 shows the surface morphology of the four samples. Samples 10 prepared by pure RF sputtering, the surface grain has relatively small particle size of about 100nm, there are clearly boundaries and no gap, and compact appearance is completely flat. The grain of the sample 3 obviously grew up, and the particle size is about 300nm. Samples 5,9 grains obviously grew up bigger, sample 3 grain shows the ridge-like particles, and sample 9 shows the clusters particles, the two surfaces are very smooth, there are obvious gaps and bulges, the film forming process is very similar with heat-treated the film s surface morphology. Comprehensive comparison of the surface morphology of the film formed by pure DC sputtering, pure RF sputtering and DC/RF co-sputtering, Under co-sputtering condition, the film was obtained a particle size larger grain. because of existence of the RF electric field, argon are more easily ionized in the vacuum chamber, the formation of a large number of Ar+ ion, part of the ion bombarded the target surface, involved in the sputtering. Another part of the ions in a shock state, they may impact the substrate, the films acquired by the conversion of kinetic energy can lead to heat the substrate, the samples were heat treatment. The heat treatment will reduce the defects in the film to improve the film properties. The smoothness is poor largely TELKOMNIKA Vol. 10, No. 5, September 2012 :

5 TELKOMNIKA e-issn: X 951 attributed to the co-sputtering stability than the other two sputtering methods, resulting in the local film growth rate differences, so that the formation of the more obvious bulge and the gap Transmittance of the Films Figure 6 shows DC sputtering, RF sputtering, DC/RF co-sputtering of the sample transmittance in the visible range; including sample 1 prepared in the pure DC sputtering conditions, the average transmittance in the visible range is 88.5 %, and has very good uniformity. Sample 10 was prepared in the pure RF sputtering conditions, the average transmittance in the visible range of is 93.9%, of the samples, however the thinnest film thickness is almost 1/3 of the other samples, but the electrical conductivity is low, so it is not an ideal sample. Sample 3 was prepared under sputtering conditions, has the best conductivity, the average visible light transmittance reach 88.7% in pure DC sputtering, but also better uniformity. The sample 5 prepared in DC-RF co-sputtering sputtering conditions, the conductivity is far better than sample 1, and the visible light transmittance is 89.1%. So in the appropriate proportion of RF power, the samples prepared by co-sputtering with good visible light transmittance and lower the resistance Transmittance(%) W avelength (nm ) Figure 6. Transmittance spectra of ZAO films with different proportion of RF power Moreover, the co-sputtering of the samples undergo "red shift" in the near ultraviolet cut-off band, that is, move the cutoff wavelength to long wavelength phenomenon. According to the band theory, atoms combine to form crystals, the outervalence electrons by the shackles of the weakest, it is also subject to the original belongs to the atom and other atoms together, in fact, by all the atoms in the crystal, called electron sharing. When the Interatomic distance decreases, each level of the isolated atoms will evolve into quasi-continuous band of intensive level. The higher degree of sharing electron, the bigger corresponding energy band width is. The higher the degree of crystallinity and the bigger energy band of the crystal width is, and the h more narrow the fobidden band gap becomes, E g = we can see the band gap bacomes smaller, when the corresponding cutoff wavelength will be larger phenomenon[6]. The cutoff wavelength of visible light transmittance of the "red shift " phenomenon shows that the cosputtering film performance improvement is largely attributed to the promotion of grain growth [7]. λ Investigations on Properties of (ZnO:Al) Films Prepared by RF/DC Co-sputtering (Meng Chao)

6 952 e-issn: X 4. Conclusion DC/RF sputtering technique can promote the deposition rate has been markedly improved, the average deposition rate is twice the rate of sputtering deposition, and four times rate of RF sputtering deposition, mainly due to the high-frequency oscillatory electric field excited state of shock charged ions, significantly increase the plasma density, high-energy ion bombardment further substrate surface to achieve the effect of heat treatment. When the total sputtering RF power to total power ratio of 50% for the optimal value in the vicinity of the optimal value can be an excellent optical performance, its sheet resistance 6.725Ω/, and average transmittance of visible light 89.1 %, and effectively improve the uniformity of the film surface. References [1] R. Cebulla, R. Wendt, K. Ellmer. Al-doped Zinc Oxide Films Deposited by Simultaneous RF And DC Excitation of a Magnetron Plasma: Relationships between Plasma Parameters and Structural and Electrical Film Properties. J. Appl. Phys. 1998; 83: [2] M. Bender, J. Trube and J. Stollenwerk. Characterization of a RF/dc-Magnetron Discharge for the Sputter Deposition of Transparent and Highly Conductive ITO Films. Applied Physics A: Materials Science & Processing. 1999; 69(4): [3] Zahra Malekjamshidi, Mohammad Jafari, Kourosh Mahmoodi. Operation of a Fuzzy Controlled Half- Bridge DC-Converter as a Welding Current-Source. TELKOMNIKA Indonesian Journal of Electrical Engineering. 2012; 10(1): [4] P.K. Petrov, E.K. Hollmann, V.A. Volpyas, A.V. Volpyas, T. Tanaka, K. Kawabata. Preparation of Argon-free Refractory Thin Films using RF-DC Coupled Magnetron Sputtering. 1999; 53(1-2): [5] Michael Stowell, Joachim Müller, Manfred Ruske, Mark Lutz and Thomas Linz. RF-superimposed DC and Pulsed DC Sputtering for Deposition of Transparent Conductive Oxides. Thin Solid Films. 2007; 515(19): [6] S.S. Lin,J.L. Huang,P. Sajgalik. Effects of Substrate Temperature on the Properties of Heavily Al- Doped ZnO Films by Simultaneous R.F. and D.C. Magnetron Sputtering. Surface and Coatings Technology. 2005; 190: [7] D. Ghodsi Nahri, H. Arabshahi.Static Characterization of InAs/AlGaAs Broadband Self-Assembled Quantum Dot Lasers. TELKOMNIKA Indonesian Journal of Electrical Engineering. 2012; 10(1): TELKOMNIKA Vol. 10, No. 5, September 2012 :