Helicon Magnetron Sputtering Research Articles

STRUCTURE AND PHOTO- FUNCTIONAL PROPERTIES OF TiO2 THIN FILM PREPARED BY RF HELICON MAGNETRON SPUTTERING

Fabrication of titanium dioxide (TiO2) thin film (200nm) has been done by using RF helicon magnetron sputtering from metallic Ti target under changing of O2 flow rate and substrate temperature. Film structure was measured by X-ray diffraction (XRD). The films were examined by an X-ray photoelectron spectroscopy (XPS) to study the structure; atomic force microscopy (AFM) and field-emission scanning electron microscope (FE-SEM) were employed to investigate the surface morphology and the cross section of films, respectively. Optical properties and optical band gap were calculated by using UV-VIS spectrophotometer. Photocatalytic activity was evaluated by light induced degradation of methylene blue (MB) solution using UV and Vis light. TiO2 thin film with a rutile phase at substrate temperature 100oC and O2 flow rate to 0.5 - 1.0sccm can be obtained, also an anatase phase can be fabricated when set substrate temperature to 300 oC and O2 flow rate to 2.5sccm. Obtained films showed a high dense with smooth surface and high crystalline. Maximum degradation rate (about 85%) of MB has been indicated at anatase phase under UV light irradiation, while under Vis light irradiation rutile and anatase mixture phase showed a maximum degradation rate (about 15%). In this work, the morphological, structural, optical, and photocatalytic properties of TiO2 thin films that were fabricated by RF helicon magnetron sputtering under change of O2 flow rate and substrate temperature have been investigated. It is expected that the change of substrate temperature and/or O2 flow rate will make ability to control the film structure.

Electron spin resonance (ESR) analysis of Cu-doped ZnO and AlN films

Cu-doped ZnO and AlN films both exhibiting room-temperature (RT) ferromagnetism (FM) were deposited by helicon magnetron sputtering. Magnetic anisotropy was observed for all the films; saturated magnetization was larger under the applied field perpendicular to the sample surface than the in-plane field, which indicates intrinsic ferromagnetism. The easy axis of magnetization was along the [0 0 0 2] direction. Electron spin resonance (ESR) analysis showed that spins tended to align in a perpendicular direction to the surface, which might account for the observed magnetic anisotropy.

Ferromagnetism in Cu-doped AlN films

AlN films doped with 3.2–8.2 at. % Cu were deposited by helicon magnetron sputtering. The films exhibited ferromagnetism with a Curie temperature above 360 K. The observed magnetic anisotropy and exclusion of ferromagnetic contamination indicated that the ferromagnetism was the intrinsic property of Cu-doped AlN films. Room-temperature (RT) saturation magnetization (Ms) tended to decrease with increasing Cu concentration and the maximum RT Ms obtained was about 8.5 emu/cm3 (0.6 μB/Cu). Vacuum annealing increased the RT Ms values and meanwhile reduced ratio of N/(Al+Cu), which suggested that Al interstitial and N vacancy might contribute to the observed ferromagnetic behavior.

Effect of substrate temperature on the room-temperature ferromagnetism of Cu-doped ZnO films

Wurtzite structure ZnO films doped with ~2 at% Cu were deposited at substrate temperatures ( T s) from 350 to 600 °C by helicon magnetron sputtering. All the films exhibited room-temperature (RT) ferromagnetism (FM) and the maximum saturation magnetization ( M s) was 1.2 emu/cm 3 (~0.15 μ B/Cu). Cu ions were mainly in a divalent state as identified by X-ray photoelectron spectroscopy. FM tended to increase with decreasing T s, and vacuum annealing enhanced the M s. These results suggested that oxygen vacancies and/or zinc interstitials might contribute to the ferromagnetic performance. Thus, the observed FM was explained in terms of the defect related mechanism.

Room‐temperature ferromagnetism of Cu‐doped ZnO films deposited by helicon magnetron sputtering

AbstractWurtzite structure ZnO films doped with 0.5 and 1.7 at% Cu were deposited by helicon magnetron sputtering. The prepared films exhibited room‐temperature (RT) ferromagnetism (FM). Maximum RT saturation magnetization of 2 emu/cm3 (∼0.3μB/Cu) was observed for ZnO film with 1.7 at% Cu. Cu ions were in a bivalent state as identified by X‐ray photoelectron spectroscopy (XPS). In photoluminescence spectra, the green emission peak increased and redshifted due to the incorporation of Cu or defects induced by Cu incorporation. Since Cu and Cu‐related oxides are not RT ferromagnetic, and no trace of ferromagnetic contamination was detected in XPS results, the observed FM is considered to be an intrinsic property of Cu‐doped ZnO films. The FM was thought to originate from defect‐related mechanisms. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of In2O3 thin films prepared by rf helicon magnetron sputtering

AbstractPolycrystalline In2O3 films with cubic crystal structure and film thicknesses of 239, 472, 579 and 745 nm respectively were grown on quartz substrates at substrate temperature 450 °C by rf helicon magnetron sputtering. The preferred crystal growth orientation is varied from <111> to <211> with increasing film thickness as confirmed by X‐ray diffractometry. The photon energy dependent optical properties such as complex dielectric functions and absorption coefficient, optical band gap, and Urbach energy width of the samples have been obtained from spectroscopic ellipsometry analysis in the energy range from 1.5 eV to 5.0 eV. The relation between those optical properties and crystal structure and/or crystallinity of the films was discussed. The band gap value, which is averaged over two typical samples with good crystallinity, is 3.765 eV and agrees satisfactorily with reported value on that of single crystal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunable Ag surface-plasmon-resonance wavelength and its application on the photochromic behavior of TiO 2–Ag films

Surface and bulk plasmon resonance of noble metal particles play an essential role in the multicolor photochromism of semiconductor systems containing noble metal particles, Here we examined several key parameters affecting surface plasmon resonance wavelength (SPRW) of Ag particles and investigated the relation between surface plasmon and photochromic reaction wavelength. From the transmission spectra of sandwiched (TiO 2/Ag/TiO 2) and overcoated (Ag/TiO 2) films deposited on quartz substrates at room temperature by rf helicon magnetron sputtering, we demonstrated that the SPRW can be made tunable by changing the surrounding media and thickness of the metal layer. The coloration and bleaching in visible light region due to photochromism were clearly observed for the films inserted with a 0.55 nm Ag layer.

Spectroscopic ellipsometry analysis of multilayered TiO2‐Ag thin films for photochromic application

AbstractBy altering the surrounding media of metal particles, the wavelength of surface plasma resonance can be tailored, which offers the potential for utilizing of plasma resonance related phenomenon, such as photochromism. Both sandwiched (TiO2/Ag/TiO2) and overcoated (TiO2/Ag) films with different thicknesses were deposited on quartz substrate by rf helicon magnetron sputtering method. The structure of multilayered TiO2‐Ag films and the concentration of loaded metal Ag in each layer were determined by spectroscopic ellipsometry in high accuracy. The spectral dependent dielectric functions of each layer were obtained, which exhibited typical semiconductor TiO2, metal Ag and mixture TiO2‐Ag behaviors, respectively. The surface plasma resonance peak was observed for several extremely thin mixture layers. The apparent redshift of SPR peak from 700 nm to 780 nm was happened with increasing of Ag amount from 35% to 50%. The broadening of SPR peak was found for layer 3 in sample 1 with low concentration Ag embedded in TiO2 media. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopic ellipsometry study of In2O3 thin films

In2O3 films grown by helicon magnetron sputtering with different thicknesses were characterized by spectroscopic ellipsometry in the energy range from 1.5 to 5.0 eV. Aside from one amorphous sample prepared at room substrate temperature, polycrystalline In2O3 films with cubic crystal structure were confirmed for other four samples prepared at the substrate temperature of 450 °C. Excellent SE fittings were realized by applying 1 and/or 2 terms F&B amorphous formulations, building double layered film configuration models, and further taking account of void into the surface layer based on Bruggeman effective medium approximation for thinner films. Spectral dependent refractive indices and extinction coefficients were obtained for five samples. The curve shapes were well interpreted according to the applied dispersion formulas. Almost similar optical band gap values from 3.76 to 3.84 eV were obtained for five samples by Tauc plot calculation using extinction coefficients under the assumption of direct allowed optical transition mode.

Epitaxial growth of Ba 8Ga 16Ge 30 clathrate film on Si substrate by RF helicon magnetron sputtering with evaluation on thermoelectric properties

Epitaxial Ba 8Ga 16Ge 30 clathrate thin films were successfully grown on Si substrate by using helicon magnetron sputtering. The (1 0 0) lattice of Ba 8Ga 16Ge 30 was identified grown on four Si(2 0 0) lattices in small mismatch (0.1%). Both the color of samples and XRD results suggest 600 °C is the optimal substrate temperature for the growth of high quality Ba–Ga–Ge clathrate film on Si substrates. High Seebeck coefficients and electrical resistivities for the deposited clathrate thin films in comparison with those of bulk are obtained. The high crystal quality and thermionic effects in heterostructures may contribute to the larger Seebeck coefficients, while the increasing of interface scattering for electrons probably is the reason for large electrical resistivities. Although the thermoelectric (TE) results are not ideal as designed, our results are significant due to the first successful work on epitaxial growth of Ba 8Ga 16Ge 30 clathrate thin films on Si substrate with large Seebeck coefficient.

Preparation of five-layered Si/Si xGe 1− x nano-films by RF helicon magnetron sputtering

Five-layered Si/Si x Ge 1− x films on Si(1 0 0) substrate with single-layer thickness of 30 nm, 10 nm and 5 nm, respectively were prepared by RF helicon magnetron sputtering with dual targets of Si and Ge to investigate the feasibility of an industrial fabrication method on multi-stacked superlattice structure for thin-film thermoelectric applications. The fine periodic structure is confirmed in the samples except for the case of 5 nm in single-layer thickness. Fine crystalline Si x Ge 1− x layer is obtained from 700 °C in substrate temperature, while higher than 700 °C is required for Si good layer. The composition ratio ( x) in Si x Ge 1− x is varied depending on the applied power to Si and Ge targets. Typical power ratio to obtain x = 0.83 was 7:3, Hall coefficient, p-type carrier concentration, sheet carrier concentration and mobility measured for the sample composed of five layers of Si (10 nm)/Si 0.82Ge 0.18 (10 nm) are 2.55 × 10 6 /°C, 2.56 × 10 12 cm −3, 1.28 × 10 7 cm −2, and 15.8 cm −2/(V s), respectively.

Optical Properties of Indium-Doped ZnO Films

Indium-doped ZnO (IZO) films with low In content (<6 at. %) were fabricated by rf helicon magnetron sputtering. The uniformity of the composites was confirmed by elemental analysis. The formation of an In–Zn–O solid solution was verified using X-ray diffraction (XRD) patterns. A wide, high-transmittance region (400–2000 nm) and >80% transmittance in the window of fiber optics telecommunication (1.30–1.55 µm) were observed. The incorporation of indium enhances the optical transmission in the designated visible and infrared wavelengths. The optical band gap shows a slight blue-shift with increasing In doping which can be explained by the Burstein–Moss effect. The Urbach tail parameter E0 increases with increasing indium content, which coincides with the increase in the full width half maximum (FWHM) of (0002) planes in XRD patterns. A decline in crystal quality with In incorporation in IZO films is also confirmed from photoluminescence (PL) spectra.

Effects of Indium Incorporation on the Optical Properties of ZnO Films

Transparent indium-doped ZnO (IZO) films with low In content (&lt;6at%) were fabricated through radio-frequency (rf) helicon magnetron sputtering. Formation of In-Zn-O solid solution was confirmed by X-ray diffraction (XRD) patterns. Incorporation of indium into ZnO films enhances the optical transmission in the visible wavelength. The optical band-gaps slightly increase from 3.25eV (ZnO) to 3.28eV (In0.04Zn0.96O) and to 3.30eV (In0.06Zn0.94O) due to Burstain-Moss effect. The Urbach tail parameter E0, which is believed to be a function of structural disorder, increases from 79meV (ZnO), to 146meV (In0.04Zn0.96O), and to 173meV (In0.06Zn0.94O), which is consistent with increase of Full-Width Half-Maximum (FWHM) in corresponding XRD patterns. Decreasing in crystal quality with increasing indium concentration is also confirmed by photoluminescence spectra.

AB-INITIO CALCULATIONS OF THE OPTICAL BAND GAP OF TiO2 THIN FILMS

Titanium dioxide has been extensively studied in recent decades for its important photocatalytic application in environmental purification. The search for a method to narrow the optical band gap of TiO 2 plays a key role for enhancing its photocatalytic application. The optical band gap of epitaxial rutile and anatase TiO 2 thin films deposited by helicon magnetron sputtering on sapphire and on SrTiO 3 substrates was correlated to the lattice constants. The optical band gap of 3.03 eV for bulk-rutile increased for the thin films to 3.37 on sapphire. The band gap of 3.20 eV for bulk-anatase increases to 3.51 on SrTiO 3. In order to interpret this expansion, ab-initio calculations were performed using the Vienna ab-initio software. The calculations for rutile as well anatase show an almost linear increase of the band gap width with decreasing volume or increasing lattice constant a. The calculated band gap fits well with the experimental values. The conclusion from these calculations is, in order to achieve a smaller band gap for both, rutile or anatase, the lattice constant c has to be compressed, and a has to be expanded.

Microstructure and bactericidal ability of photocatalytic TiO2 thin films prepared by rf helicon magnetron sputtering

The present study investigates the microstructure and bactericidal ability of polycrystalline and epitaxial TiO2 thin films with anatase and rutile structure. Phase formation and the distortion of the lattice of the film compared with bulk are proved by TEM observation. HRTEM images assure the good crystal quality and compact structure of the films prepared by rf helicon magnetron sputtering. This obtained larger band gap due to the lattice deformation could contribute to the better photocatalytic effect of the anatase film.Bactericidal abilities evaluated by the photokilling E. Coli using film stick method under UV irradiation show the following result: Both single and polycrystal anatase films exhibit obvious bactericidal abilities while no distinguishing difference for them. On the other hand, no bactericidal ability is observed for rutile films in this work. The band gap value of rutile film is lower than the energy potential required to O2/O2− reductions according to the principle proposed by Fujishima et al. This resulted in the observed poor photocatalyst effect of rutile film.

Simultaneous epitaxial growth of anatase and rutile TiO 2 thin films by RF helicon magnetron sputtering

Epitaxial films of TiO 2 with rutile structure on sapphire and anatase structure on SrTiO 3 were simultaneously grown by RF helicon magnetron sputtering of a TiO 2 target in Ar atmosphere. X-ray diffraction using θ−2 θ scan and pole figure plots confirmed the epitaxial relationship, which were rutile (1 0 1)∥sapphire (1 1 0), (0 1 0) f∥(0 0 1) s, and anatase (0 0 1)∥SrTiO 3(0 0 1), (1 0 0) f∥(1 0 0) s, where suffix f and s stand for the film and substrate, respectively. Moreover, observation by transmission electron microscopy identified the epitaxial film growth of single crystalline anatase and rutile structure with slight lattice distortion compared with bulk. The lattice constants of a and b, which were calculated from electron diffraction spots and lattice image in TEM of the films were contracted while that of c being expanded. According to the results of spectroscopic ellipsometry, the films show very high refractive indices ( n) at the designated wavelength range in comparison with the past reports on TiO 2 thin films. Although there are no bulk references in the anatase case, the values n of the rutile film are comparable to the bulk in the data-book. Such high refractive indices of the films indicate the compact texture of the epitaxial films fabricated by helicon sputtering.