Piezoelectric ZnO films by r.f. sputtering

Piezoelectric zinc oxide films are used in microelectromechanical systems (MEMS) applications, where they can be used in sensors to detect, e.g., pressure or acceleration. Beside sensors, ZnO films are applied in activation devices, where force is needed. Conductive-doped zinc oxide (most often with aluminum) is also used in optoelectronics. Piezoelectric films including AlN and ZnO are more difficult to produce than the corresponding conductive materials. In order to achieve good piezoelectricity in ZnO films, they have to possess high purity, a (0 0 1) orientation (ZnO has hexagonal crystal structure), high resistivity, and fine columnar microstructure perpendicular to the substrate. We have used r.f. magnetron (13.56 MHz) sputtering from a ZnO target in an oxygen atmosphere to achieve the piezoelectric ZnO. The aim of this work has been to develop an r.f. sputtering process for ZnO to achieve highly piezoelectric thin films. As a test vehicle to measure the piezoelectricity of the ZnO films we have fabricated resonators and passband filters in the 1–2 GHz range using standard microelectronics photolithography, deposition, and etching techniques on 100-mm diameter Corning glass or silicon wafers. The influence of the sputtering-process parameters on the film properties has been studied by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and electrical measurements. In this study, the effects of the process parameters on the final material properties of the ZnO film are discussed in detail.     

D.c. properties of ZnO thin films prepared by r.f. magnetron sputtering

In the development of ZnO-based varistors the electrical properties of ZnO/Bi₂O₃ junctions and of the two individual oxides are being investigated. Following our recent work on a.c. conductivity in Al---ZnO---Al sandwich structures we currently report d.c. measurements. The structures were prepared by r.f. magnetron sputtering in an argon/oxygen mixture in the ratio 4:1. Capacitance-voltage data confirm that the Al/ZnO interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 9.7. With increasing voltage the current density changed from an ohmic to a power-law dependence with exponent n≈3. Furthermore measurements of current density as a function of reciprocal temperature showed a linear dependence above about 240 K, with a very low activation energy below this temperature consistent with a hopping process. The higher temperature results may be explained assuming a room-temperature electron concentration n₀ and space-charge-limited conductivity, dominated by traps exponentially distributed with energy E below the conduction band edge according to N = N₀exp(−E/kT_t), where k is Boltzmann's constant. Typical derived values of these parameters are: n₀ = 7.2 × 10¹⁶ m⁻³, N₀ = 1.31 × 10⁴⁵ J⁻¹ m⁻³ and T_t = 623 K. The total trap concentration and the electron mobility were estimated to be 1.13 × 10²⁵ m⁻³ and (5.7−13.1) ×10⁻³m²V⁻¹s⁻¹ respectively.     

Humidity sensitivity of optical structures prepared by r.f.-biased r.f. sputtering

In an attempt to produce optical structures with a lower humidity sensitivity than that of structures deposited by conventional evaporation, we used r.f.-biased r.f. sputtering for the preparation of coatings. The sensitivity was effectively reduced owing to the high density of bias-sputtered films which are subjected to controlled ion bombardment during their growth. Our experiments suggest that neither rate monitoring nor optical monitoring is necessary, at least not in the preparation of simple structures with few layers, if glow discharge sputtering processes comprising r.f. substrate biasing in diode, triode or magnetron set-ups are used to deposit the coatings.     

ZnO:Ga透明导电膜的低温制备及特性研究

采用射频磁控溅射法在玻璃衬底上低温制备出高质量的镓掺杂氧化锌(ZnOGa)透明导电膜,对薄膜的结构和光电特性以及制备参数对薄膜性能的影响进行了研究.制备的ZnOGa是具有六角纤锌矿结构的多晶薄膜,最佳择优取向为(002)方向.薄膜的最低电阻率达到了3.9×10-4Ωcm,方块电阻～4.6Ω/□,薄膜具有良好的附着性,在可见光区的平均透过率达到90%以上.     

Crystallization behaviour of amorphous SiC films prepared by r.f. sputtering

The crystallization behaviour of amorphous SiC (a-SiC) films prepared by r.f. diode sputtering was studied using IR measurements and transmission electron microscopy. The absorption band at around 800 cm^-1 in the IR spectrum became sharper and more intense when a sample was annealed above 1000°C, corresponding to the phase transition in the film from amorphous to polycrystalline β-SiC. From cross-sectional transmission electron microscopy observations, crystallization occured homogeneously in the film. The crystallization behaviour was independent of the film thickness and the substrate temperature during preparation. The measured overall activation energy of crystallization of a-SiC films is about 5.0 eV.     

Boron doping of amorphous hydrogenated silicon films prepared by r.f. sputtering

Amorphous hydrogenated silicon (a-Si:H) films 1 μm thick were prepared at 190 and 250 °C by r.f. sputtering in an Ar-H₂-B₂H₆ gas mixture whose composition Y_g = [B₂H₆]/([Ar] + [H₂]) was varied between zero and 10^-2. Measurements of the optical gap E₀ and the dark conductivity σ_d give evidence for the existence of three regions as a function of Y_g. Up to Y_g = 10^-6, the conductivity activation energy E_a increases towards the intrinsic value E₀/2. For Y_g between 10^-6 and 10^-3, E₀ and E_a decrease to 1.5 eV and 0.4 eV respectively, indicating that efficient p-type doping is achieved. At higher diborane concentrations a boron-rich a-Si:B:H compound is formed which is not conductive. The photoconductivity ratio σ_ph/σ_d reaches its maximum value (of 10³) at Y_g ≈ 10^-6 and drops to unity for Y_g ? 10^-4. The loss in σ_ph is related to an increase in the density of states near the Fermi level as revealed by the onset of an important hopping conductivity on the σ_d curves.     

Properties of cadmium stannate films prepared by r.f. sputtering from powder targets

The composition, electrical conductivity and optical transmission of sputtered thin films of Cd₂SnO₄ were studied in relation to r.f. sputtering power and deposition rate and also in relation to the effects of post-deposition heat treatment. Sputtering conditions can be varied to give a stoichiometric cadmium:tin ratio in the film at which the conductivity is shown to have a maximum value. Design data are given relating the transmission of coated soda glass to film resistance in ohms per square.     

Morphological and compositional properties of MoSe2 films prepared by r.f. magnetron sputtering

Thin MoSe₂ films were prepared by r.f. planar magnetron sputtering. Their morphological and growth characteristics were examined by scanning electron microscopy and transmission electron microscopy. The specimens show a lamellar-type microstructure with the basal plane of the crystallites perpendicular to the substrate. The film morphology is strongly influenced by the substrate temperature. The chemical composition of sputtered MoSe₂ films, as a function of process parameters, was determined by X-ray microanalysis. It was found that under well-selected conditions samples with the correct stoichiometry can be obtained whereas substrate biasing decreases drastically the selenium concentration in the films.     

Electrical and optical properties of MoSe2 films prepared by r.f. magnetron sputtering

The electrical resistivity of MoSe₂ films prepared by r.f. magnetron sputtering was measured between 300 and 10 K. The main sputtering parameter governing the physical properties of the films was found to be the substrate temperature T_sub. The room temperature resistivity of the as-sputtered films increased from $\text{1.7 × 10}{}^{-1}\text{Ω}\text{cm}$(T_sub = -70 °C) to $\text{1.4 × 10}{}^1\text{Ω}\text{cm}\text{(T}{}_{\mathrm{<mtext>sub</mtext>}}\text{= 150 °}\text{C}\text{)}$. A check of the thermo-electrical response showed that the majority charge carriers are holes except for films deposited at T_sub = 150 °C which are n type. Hall effect measurements indicated very low Hall mobilities (3–5 cm² V^-1 s^-1). Thermal annealing increased the room temperature resistivities by more than one order of magnitude for the specimens sputtered at a low substrate temperature. The optical properties were weakly influenced by the process conditions. The optical gap was determined to be 1.06 eV.     

射频磁控溅射制备TiO2-xNx薄膜及其光催化特性研究

利用射频磁控溅射在玻璃衬底上制备了透明TiO2 和 TiO2-x Nx 薄膜样品,通过 X 射线衍射(XRD)、原子力显微镜(AFM)及 UV Vis分光光度计等测试手段表征了样品的结构、形貌和光催化性能。结果表明制备的薄膜为锐钛矿相结构。随着 N2/Ar气流比的增大薄膜样品出现新的物相,吸收光谱向可见光方向展宽,在N2/Ar流量比为 3∶100 时,制备的薄膜在可见光区具有很好的光催化性能。     

Highly conductive and transparent ZnO thin films prepared by r.f. magnetron sputtering in an applied external d.c. magnetic field

Highly conductive films of undoped and aluminum-doped ZnO were prepared by r.f. magnetron sputtering in an applied external magnetic field in pure argon gas. Films with a resistivity as low as $\text{2 × 10}{}^{-4}\text{Ω}\text{cm}$ and a transmittance above 80% at wavelengths between 400 and 800 nm can be produced on a non-intentionally heated substrate by the sputtering of a ZnO target to which 2 wt.% Al₂O₃ had been added. The characteristic features of aluminum-doped ZnO films are their high carrier concentration, low mobility and blue shift in the absorption edge in comparison with the properties of undoped ZnO films.     

Photocatalytic activity studies of TiO2 thin films prepared by r.f. magnetron reactive sputtering

A series of transparent titanium dioxide thin films have been obtained on microscope glass slides by means of r.f. magnetron reactive sputtering using Ar and O₂ mixed gases. The photocatalytic activity of the TiO₂ thin films was evaluated by the degradation of rhodamine B dye wastewater. The influences of substrate temperatures and total sputtering pressures on the photocatalytic activity of the TiO₂ films were investigated. It was observed that substrate temperature had little influences on the photocatalytic activity, but the photocatalytic activity of the TiO₂ thin films was improved by decreasing the total sputtering pressure.     

薄膜厚度对ZnO:Ga透明导电膜性能的影响

采用射频磁控溅射法在玻璃衬底上低温制备出镓掺杂氧化锌(ZnOGa)透明导电膜,研究了薄膜的结构、电学和光学性质随薄膜厚度的变化关系.制备的ZnOGa是具有六角纤锌矿结构的多晶薄膜,最佳择优取向为(002)方向.随着薄膜厚度的增加,衍射峰明显增强,晶粒增大.薄膜的最低电阻率为3.9×10-4Ω·cm,在可见光范围内平均透过率达到了85%以上.     

Transparent conducting ZnO : Al films on different organic substrates deposited by r.f. sputtering

1. IntroductionThansparellt conducting zinc oxide films have beenextensively studied in recede years, because of theirlow material cost, relatively low deposition temperature and stability in hydrogen plasma compared withITO and SnOZ films[1]. These adVatages are of considerable interest for electrrvoptical conversion device.Compared with undoped ZnO, Al-doped ZnO filmshave lobed resistivity and better stability. nansparellt conducting films deposited on organic substrateshave many lments…     

Structural properties of TiO2–WO3 thin films prepared by r.f. sputtering

TiO₂–WO₃ thin films were prepared by radio frequency (r.f.) reactive sputtering from metallic target. Structural and morphological properties of the thin films have been studied through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The influence of the annealing on the phase composition TiO₂–WO₃ system was studied. The binding energies of titanium and tungsten are characteristic for Ti⁴⁺ and W⁶⁺. The influence of tungsten on anatase–rutile phase transition in TiO₂ was observed. The structural modeling has been performed to account the preferred orientation in tungsten doped titanium oxide.     

Preparation of Fe-N films by r.f. sputtering

Fe-N films over a wide compositional range have been prepared by the reactive sputtering method. Fe-N sputtered films were composed of a single or two phases such as-Fe₂N,-Fe_3.02N,-Fe_3.82N,-Fe₄N and-Fe; however, an unknown phase was observed at a higher nitrogen pressure. A remarkable preferred orientation of the-Fe_3.02N (110) plane parallel to the film surface was observed. The Curie temperature of the sputtered -Fe_3.82N sample was 490° C, which was almost the same as that of-Fe₄N prepared by metal nitriding. The saturation magnetization, _s, of the sputtered Fe-N samples decreased from 151.8 to 42.4 e.m.u.g^–1 with increasing nitrogen content from 7.94 to 24.87 at%, and its coercive force,₁ H _c was found to lie in the range 150 to 600 Oe in the powder form at room temperature.     

Effect of r.f. sputtering parameters on ZnO films deposited onto GaAs substrates

In order to compensate the piezoelectrical property of GaAs-based material for monolithic integrated acousto-optic circuit applications, a ZnO film with a highly preferred orientation should be deposited. R.f. sputtering was employed to deposit ZnO films with various technological parameters. The optimal conditions to deposit high quality ZnO film onto GaAs-based substrates were deduced and investigated. Low loss surface acoustic wave devices fabricated on ZnO/GaAs wafers were obtained. The total insertion loss measured at a centre frequency of 25.37 MHz was 20 dB.     

Effects of CdCl2 treatment on the properties of CdS films prepared by r.f. magnetron sputtering

Effects of the thickness of CdCl₂ layer and the annealing on structural and optical properties of sputter-deposited CdS films were investigated. The annealing process of evaporated CdCl₂ was carried out by heating the sample in air at 350-500 °C for 20 min. As the thickness of the CdCl₂ increases, the (002) peak of CdS becomes weak and the other peaks of CdS increases. Especially, for 200 nm CdCl₂, the preferential orientation of the (002) plane disappears and the c-axis of the CdS film tends to orient parallel to the substrate. As the CdCl₂ layer is thicker, the grains are enlarged significantly. The improvement of optical properties of CdS films with thicker CdCl₂ layer might be successfully employed in achieving better conversion efficiencies in solar cells.