ZnO nanostructures prepared by RF sputtering

Advances in Electrical and Electronic Engineering 330 ZnO NANOSTRUCTURES PREPARATED BY RF SPUTTERING S. Flickyngerová a) , V. Tvarožek a) , I. Novotný a) , A. Šatka a) , P. Šutta b) a) Slovak University of Technology, Dep. of Microelectronics, Ilkovicova 3, SK-812 19, Bratislava, Slovakia b) West Bohemia University, New technologies - Research Center, Univerzitni 8, 306 14 Plzen, Czech Republic e-mail: sona.fli[email protected] Summary ZnO is shortly reviewed as significant material for nanotechnology. Sputtered ZnO thin films showed colummar polycrystalline structure with preffered orientation in <002> direction, resistivity ~ 1 Ωcm and optical bandgap Eg = 3,33 eV. Nanoclusters of Au and ZnO were formed by use of RF sputtering. 1. INTRODUCTION Zinc oxide (ZnO) is a unique material that exhibits semiconducting, piezoelectric, and pyroelectric multiple properties which are a key phenomenon in building electro-/mechanical, optical, acoustic, thermal coupled sensors and actuators. Pure ZnO is colorless and crystallizes in the hexagonal (wurtzite) system with lattice parameters a = 0,3296 nm and c = 0,52065 nm. The structure ZnO can be simply described by alternating planes composed of tetrahedraly coordinated O + and Zn 2+ ions, stacked alternately along the c-axis (Fig 1) [1]. Fig. 1. Wurtzite structure model of ZnO with tetrahedral configuration Zinc oxide is a transparent semiconductor with a direct bandgap of 3.37 eV and large excitation binding energy (60 meV), and exhibits near-UV emission and absorption, as well as high transparency and the natural n-type conductivity. The semiconducting nanostructures such as nano-/fibers, wires, rods, tubes, belts, saws, springs, rings, bows and propellers (Fig. 2) have caught considerable attention due to zinc oxide great application potential in nanoactuators and nanosensors [2]. ZnO is bio-safe and biocompatible, and can be used also for biomedical applications. The highly one-dimensional ZnO nanostructures (wire, tubes, etc.) were realized using various methods such as vapor transport and deposition, thermal decomposition [3] and evaporation [4] (e.g. vapor liquid solid (VLS) process [6], metal-organic chemical vapor deposition (MOCVD) [7], metalorganic vapor phase epitaxy (MOVPE) [8], growth by site-selective molecular beam epitaxy (MBE) [9]. Fig. 2. Typical morphologies of one-dimensional nanostructures of ZnO a) nanowires fibers , b) nanorod, c) nanotubes, d) nanobelts, e) nanoring, f) nanospiral, g) nanohelixes [1, 5] RF sputtering is capable: a low -temperature ion – assisted deposition of metals, semiconductors, insulators, the before/post deposition modification of substrate/thin - film surface by ions on the micro- /nanolevel; change of deposition rate in wide range (0,1 to 10 nm/s); to control further parameters which are important for thin film growth (substrate temperature, plasma density, composition of working gas, ion bombardment of film during deposition). Fig. 3. Schematic illustration of nanostructure growth a) initial nucleation of Au atoms; b) formation of Au clusters; c) initial ZnO growth on the Au seeds; d) growth of ZnO nanofibres In this work we report nanostructured surface on ZnO thin layer on silicon substrates using RF ZnO nanostructures prepared by RF sputtering 331 sputtering technique with the use of gold as a catalyst. Therefore, it is key to be able to accurately control the position and size of the seeds Au particles during growth. For growth of nanostructures are dominant initial growth phases: seed formation, nucleation growth of clusters and islands, their coalescency. Schematic simplified growth of ZnO structures (nanodots, nanofibers) on Au particles is shown Fig 3. 2. EXPERIMENTAL PROCEDURE The Au/ZnO structures were RF diode sputter deposited on Si and Corning glass substrates using a ZnO target (99.99 % purity, in 20 cm diameter) and metal Au target (99.95 % purity, in 20 cm diameter). The substrates were cleaned by standard chemical method. The sputtering chamber was pumped down to 10 -5 Pa before admission of the sputtering gas Ar (99,9995 %) and total gas pressure 1,3 Pa was kept constant. The thickness of the ZnO thin film was 200 nm and the deposited Au thickness was approximately 10 nm ranges. The morphology of Au and ZnO structures were characterized using scanning electron microscope SEM LEO 1550. The crystal orientation and microstrains of the films were investigated by an automatic powder X-ray diffractometer AXS Bruker D8 with Eulerian cradle and 2D detector (CoK,  = 0.179 nm). The resistivity and carrier concentration in ZnO films were obtained from Van der Pauw measurements. 3. RESULT AND DISCUSSION ZnO thin films were deposited by RF sputtering from the ZnO target in pure Ar gas. The resistivities of the of the sputter films was slightly dependent on the RF power, ρ = 0,4 ÷ 1,0 Ωcm. The ZnO resistivity was determined by the carrier concentration which varied from n = 1,4 x10 19 cm -3 to n = 7 x 10 18 cm -3 . Transparency in VIS region was 85 %, with absorption edge in near UV region 3,33 eV which is close to the bandgap of bulk ZnO E g = 3.37 eV. The XRD diffraction lines of ZnO <100>, <002> and <101>, were observed (Fig 4.). The set of two-dimensional diffraction patterns of ZnO films deposited at Ar in sputtering gas displays three Debye rings of ZnO Bragg reflections (Fig. 5). The white circle is a Laue spot diffracted by substrate material (c-Si) due to the continuous X-ray radiation. The morphologies of the deposited Au/ZnO structures are shown in Fig. 6 – Fig. 11. RF diode sputtered Au nanoparticles of mean diameter of 10 th nm (Fig. 6) were as seeds for growth of nanostructured surface of ZnO thin films in thickness of 200 nm (Fig. 10). Fig 4. Two dimensional diffraction pattern of ZnO film Fig 5. Sputtered ZnO thin films showed colummar polycrysralline structure with preffered orientation in <002> direction All substrates were bombarded by Ar ions before deposition Au to increase the cleanness and micro roughness. Fig. 6 shows Au surface after to the high rate deposition at the temperature of 300°C. This caused an increase of roughness and decrease of reflexion. The clusters have an average linear dimension of 1 µm. In comparison with that, the sample of the same Au thickness prepared at room temperature exhibits sparkle a surface with smaller seeds (Fig. 7). Fig. 8 shows Au-coated Si substrate by lower deposition rate and after deposition in annealed vacuum at temperature of 300°C. The Au formed separate seeds with diameters of 10 - 40nm. The size of seeds became more homogenous when we used two - step deposition. In the first step we deposited Au film with the rate of 0,8 nm/s and thickness of 250 nm. In the second step we used low deposition rate (0,3 nm/s) and amount of Au corresponding to thickness of 50 nm. Substrate temperatures were 300°C during deposition and after deposition substrate was annealed in vacuum (150°C) (Fig 9). Au-seeds on Si Corning glass substrates were used as base for forming of ZnO nano - crystalline structures by RF sputtering (Fig. 10 and Fig 11). 30 32 34 36 38 40 42 44 46 48 50 0 5 10 15 20 25 30 35 40 Intensity (cps) 2 ϑ (degrees) (100) (002) (101) Advances in Electrical and Electronic Engineering 332 Linear size of these nanostructures were lower or equal to 100 nm. Fig. 6. Au on the Si substrates, the Au rate deposition 0,8 nm/s at temperature t = 300°C. Fig. 7. Au on the Si substrates, the Au rate deposition 0,8 nm/s at temperature t = 23°C. Fig. 8. Au on the Si substrates , the Au rate deposition 0,3 nm/s at temperature t = 300°C, annealing in vacuum at t = 300°C. Fig. 9. Au on the Si substrates, the Au rate deposition 0,8 nm/s and 0,3 nm/s at temperature t = 23°C, annealing in vacuum at t = 150°C. Fig. 10. Nanostructured surface of ZnO thin film on the Si substrate. Fig. 11. Nanostructured surface of ZnO thin film on the Corning glass substrate 4. CONCLUSIONS ZnO offers the broad spectrum of unique physical and chemical properties, what is getting it into group of the most important nanomaterials for integration with microsystems and biotechnology. Our preliminary results have confirmed that ZnO nanostructures prepared by RF sputtering 333 sputtering is one of available deposition method for forming of chosen ZnO nanostructures. Future target is to use of nanostructured surface of ZnO thin films for modern DNA-based biosensors. Acknowledgement This work was supported by SK Grant VEGA 1/3098/06 and CZ project MŠMT 1M06031. REFERENCES [1] Wang, Z. L., Kong, X. Y.: Adv. Funct. 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