DC-sputtered Films Research Articles

Pressure effects in RF and DC sputtered Sb2Te3 thin films and its applications into solar cells

In this work we developed the synthesis and characterization of Sb2Te3 thin films, which were grown by RF as well as DC sputtering as a function of the deposition pressure (Pd), 5–15 mTorr. Sb2Te3 films were characterized through X-Ray Diffraction, Energy Dispersive Spectrometry (EDS), Scanning Electron Microscopy (SEM) and electrical resistivity measurements. As a part, contact layer, of the CdTe-based Solar Cell, Current-Voltage (I–V) as well as External Quantum Efficiency (EQE) measurements were carried out. Our results indicate that the radio-frequency sputtered Sb2Te3 are polycrystalline with predominant rhombohedral crystalline structure, whereas the DC-sputtered films crystallized mainly in the monoclinical structure. Both set of samples showed a resistivity of the order of 10−4 Ω-cm. Concerning the CdTe-based solar cell, the incorporation of the Sb2Te3 as a back surface film in the back contact improves the solar cell efficiency up to 8.01%, 10 mTorr pressures into the growth chamber, as compared to the CdTe-based solar cell, 4.82% efficiency, without the Sb2Te3 layer.

Evolution of morphology and crystalline quality of DC-sputtered AlN films with high-temperature annealing

High-temperature annealing (HTA) is a promising technique to improve the crystalline quality of AlN films. Combining HTA with RF sputtering, Miyake et al. have reported the fabrication of high-quality AlN templates on sapphire [J. Cryst. Growth 456, 155 (2016)], which paves the way to realizing low-cost AlGaN UV optical devices. In terms of AlN growth, DC sputtering is interesting because of its low cost and high throughput. We studied the effect of HTA on the morphology and crystalline quality of DC-sputtered AlN films. As with RF sputtering, HTA substantially improved the crystalline quality of DC-sputtered AlN films. In contrast, hillocks were formed on the surface after HTA, which plausibly stems from the rough surface with spiked structures of the DC-sputtered film. Additional growth of AlN on annealed samples improved the surface morphology and crystalline quality except for the sample containing large hillocks.

Fabrication and characterization of He-charged ODS-FeCrNi films deposited by a radio-frequency plasma magnetron sputtering technique

He-charged oxide dispersion strengthened (ODS) FeCrNi films were prepared by a radio-frequency (RF) plasma magnetron sputtering method in a He and Ar mixed atmosphere at 150 °C. As a comparison, He-charged FeCrNi films were also fabricated at the same conditions through direct current (DC) plasma magnetron sputtering. The doping of He atoms and Y2O3 in the FeCrNi films was realized by the high backscattered rate of He ions and Y2O3/FeCrNi composite target sputtering method, respectively. Inductive coupled plasma (ICP) and x-ray photoelectron spectroscopy (XPS) analysis confirmed the existence of Y2O3 in FeCrNi films, and Y2O3 content hardly changed with sputtering He/Ar ratio. Cross-sectional scanning electron microscopy (SEM) shows that the FeCrNi films were composed of dense columnar nanocrystallines and the thickness of the films was obviously dependent on He/Ar ratio. Nanoindentation measurements revealed that the FeCrNi films fabricated through DC/RF plasma magnetron sputtering methods exhibited similar hardness values at each He/Ar ratio, while the dispersion of Y2O3 apparently increased the hardness of the films. Elastic recoil detection (ERD) showed that DC/RF magnetron sputtered FeCrNi films contained similar He amounts (∼17 at.%). Compared with the minimal change of He level with depth in DC-sputtered films, the He amount decreases gradually in depth in the RF-sputtered films. The Y2O3-doped FeCrNi films were shown to exhibit much smaller amounts of He owing to the lower backscattering possibility of Y2O3 and the inhibition effect of nano-sized Y2O3 particles on the He element.

Magnetism of Amorphous and Nano-Crystallized Dc-Sputter-Deposited MgO Thin Films.

We report a systematic study of room-temperature ferromagnetism (RTFM) in pristine MgO thin films in their amorphous and nano-crystalline states. The as deposited dc-sputtered films of pristine MgO on Si substrates using a metallic Mg target in an O2 containing working gas atmosphere of (N2 + O2) are found to be X-ray amorphous. All these films obtained with oxygen partial pressure (PO2) ~10% to 80% while maintaining the same total pressure of the working gas are found to be ferromagnetic at room temperature. The room temperature saturation magnetization (MS) value of 2.68 emu/cm3 obtained for the MgO film deposited in PO2 of 10% increases to 9.62 emu/cm3 for film deposited at PO2 of 40%. However, the MS values decrease steadily for further increase of oxygen partial pressure during deposition. On thermal annealing at temperatures in the range 600 to 800 °C, the films become nanocrystalline and as the crystallite size grows with longer annealing times and higher temperature, MS decreases. Our study clearly points out that it is possible to tailor the magnetic properties of thin films of MgO. The room temperature ferromagnetism in MgO films is attributed to the presence of Mg cation vacancies.

Characterization of iridium oxide thin films deposited by pulsed-direct-current reactive sputtering

Thin films of iridium oxide were deposited on silicon and borosilicate glass substrates by pulsed-direct-current (pulsed-DC) reactive sputtering of iridium metal in an oxygen-containing atmosphere. Optimum deposition conditions were identified in terms of plasma pulsing conditions, oxygen partial pressures, and substrate temperature. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and Rutherford backscattering spectroscopy. According to the results, it was possible to obtain films that are near-stoichiometric, smooth and uniform in texture. The films deposited without substrate heating were amorphous, and those deposited at substrate temperatures above 300 °C were found to have a homogeneous polycrystalline structure. The results also showed that pulsed-DC sputtered iridium oxide films were smoother and had lower micro-inclusions density than DC-sputtered films obtained under otherwise similar deposition conditions. This improvement in the film quality is at the expense of a tolerable decrease in the deposition rate.

Comparison of radio-frequency and direct-current magnetron sputtered thin In 2O 3 : Sn films

Polycrystalline In 2O 3 : Sn films have been prepared by direct-current (dc) and radio-frequency (rf) magnetron sputtering at 300 °C substrate temperature and analyzed by optical transmittance spectra, X-ray diffraction spectra and atomic force micrography. The optical and electronic properties of the samples were derived from a simulation of the transmittance spectra based on dielectric modeling. Generally, the rf-sputtered films have larger lattice distortion, microstrain and a lower carrier density than the dc-sputtered films. The predominant orientation of rf-sputtered films is (222), instead of the (400) orientation found for dc-sputtered films. Contrary to dc-sputtering, there was no clear influence of the oxygen partial pressure on the texture of rf-sputtered films. A grain–subgrain structure was sometimes observed and correlated with the texture of both rf- and dc-sputtered films. Correlations between surface roughness, crystal lattice expansion and packing density are observed. The experimental results are explained by oxygen interstitials forming acceptor centers and different plasma parameters for rf- and dc-sputtering. The electron mobility in all samples is quantitatively explained by scattering at ionized donors and donor–acceptor complexes.

Development of Thin Film Multilayer Structures with Smooth Surfaces for HTS SFQ Circuits

We have fabricated a multilayer structure for single flux quantum (SFQ) circuit application using a high-temperature superconductor (HTS). La 0.2 -Y 0.9 Ba 1.9 Cu 3 O x (La-YBCO) base electrode layers were prepared by a dc or rf magnetron sputtering method. The reproducibility of film quality for dc-sputtered La-YBCO films was better than that for rf-sputtered films, and the dc sputtered films exhibited the average surface roughness R a less than 1.0 nm and a T c zero value of 88 K. By using the dc-sputtered La-YBCO films, a multilayer structure of SrSnO 3 /La-YBCO/ SrSnO 3 /La-YBCO on MgO substrate with R a below 2.0 nm was obtained. Interface-modified ramp-edge junctions with La 0.2 -Yb 0.9 Ba 1.9 Cu 3 O x (La-YbBCO) counter electrodes have been fabricated by using this multilayer structure with dc-sputtered films. The fabricated junctions exhibited RSJ-type I - V characteristics with I c R n products of about 3 mV at 4.2 K. We also obtained a 1-σ I c spread of 8% for a 1000-junction series-array. The sheet inductance values at 4.2 K for the base and counter electrodes on La-YBCO ground planes were 0.8 pH and 0.7 pH per square, respectively. Operation of several types of elementary SFQ circuits has been successfully demonstrated hv using this multilayer structure.

Ex situ and in situ spectroscopic ellipsometry of MF and DC-sputtered TiO2 and SiO2 films for process control

Titanium oxide and silicon oxide films were deposited on floatglass substrates by magnetron sputtering employing both the DC and the MF (mid-frequency) technique. The films were grown at different working points between transition and oxide mode, target power densities up to 7.5 W/cm2. Ex-situ ellipsometry at different angles of incidence was applied to study the optical properties and the morphology of the films. For modeling the spectra, the Lorentz model with one single oscillator was used within the spectral range between 380 and 850 nm. For SiO2, the ellipsometric data could be fitted using a model with one single homogeneous film. The ellipsometric investigation of TiO2 films with thicknesses below 150 nm show that, in general inhomogeneities of the refractive index in the growth direction have to be taken into account. It is concluded that both plasma heating as well as ion bombardment are responsible for these inhomogeneities. Refractive indices of 2.43 for DC sputtered films up to 2.58 for MF sputtered films are observed. A four-layer SiO2–TiO2 antireflective coating on glass was fabricated using both plasma and ellipsometric control. The reflectivity, calculated from the in situ ellipsometric analysis, is in a good agreement with the measured reflectivity.

Comparison of high-pressure dc-sputtering and pulsed laser deposition of superconducting YBa2Cu3O x thin films

Superconducting YBa2Cu3Ox thin films were deposited on NdGaO3 (110) substrates using two different techniques: dc sputtering at high oxygen pressure and pulsed laser deposition. The structure, electrical properties, and surface morphology of the obtained films were compared. The superior crystal quality of dc-sputtered films fabricated at the same temperature and at oxygen pressure of the same range as for laser-deposited films can be explained by a lower deposition rate providing time for recrystallization processes. The re-evaporation becomes significant for dc sputtering at high deposition temperatures and results in Badeficient films. The high mobility of atoms on the surface of the growing film during laser deposition helps in the formation of smoothc-oriented areas of the film.

定量的解析手法によるＩＴＯスパッタ時の酸素添加メカニズムの研究

The dependence of resistivity on the oxygen flow rate in sputtered ITO films prepared by RF is quite different from that of ones prepared by DC. The minimum resistivity in RF-sputtered films is about half that in DC-sputtered films. The oxygen flow rate for minimum resistivity in RF-sputtered films is much lower than that in DC-sputtered films. However, these oxygen-doping mechanisms are still unclear.In order to better understand these mechanisms, oxygen partial pressures under ITO sputtering were measured using a quadrupole mass analyzer with a differential pumping system. The data were interpreted using a quantitative analysis method.The results reveal that 1) the ratio of gas-phase oxygen to all phases of oxygen coming from the ITO target is the same for DC and RF, 2) the total amounts of all phases of oxygen doped into films are the same in the optimum condition for minimum resistivity, 3) the absorption efficiency of oxygen gas is larger for RF-sputtered films and 4) the oxygen concentration in ITO films is one of the basic factors for determining the resistivity, however there should be other factors which determine the difference between the DC- and RF-sputtered films.

High resolution transmission electron microscopy study of interface structures and growth defects in epitaxial Bi2Sr2Can−1CunO4+2n + δ films on SrTiO3 and LaAlO3

The defect structure of epitaxial, c-oriented Bi2Sr2Can−1CunO4+2n+δ (BSCCO) thin films grown by dc-sputtering and layer-by-layer MBE on SrTiO3 and LaAlO3 single crystal substrates was investigated by high-resolution transmission electron microscopy (HRTEM). Particular emphasis was put on the structure of the film/substrate interface. The films grown by dc-sputtering show a rather perfect structure involving a regular stacking of the unit cells. In spite of this regularity, there are many defects, such as twins, chemical stacking faults, and precipitates, as well as interfacial dislocations accommodating the film/substrate lattice misfit. The MBE-grown films contain twins and interfacial dislocations, but most prominent are precipitates of various size and rather high number density. Composition and structure of the precipitates were analyzed. Interfacial dislocations were found to be located in the films at a distance of up to 3 nm from the film/substrate interface. The experiments showed that the quality of the film/substrate interface in MBE-grown films is considerably higher with respect to smoothness, sharpness, and regularity, if the layer-by-layer MBE process starts with a Sr–O layer instead of a Bi–O layer. This observation is in correspondence to the observed interface structure of the dc-sputtered films, where the first film layer was a Sr–O layer, not a Bi–O layer, in spite of the films being sputtered from a composite target. A structure model of the Bi2Sr2Can−1CunO4+2n+δ/(100)SrTiO3 interface is proposed. The prolonged MBE process was shown to imply a chemical interaction between the SrTiO3 substrate and the growing film, resulting in the formation of Sr-rich phases in the near-interface substrate regions.

Laser deposition of copper oxide thin films: contrast with sputtering

This work reports a systematic study of the growth of copper oxide thin films by laser evaporation from a Cu foil or a Cu sintered powder target in an oxygen atmosphere. The experimental conditions to deposit Cu 2O and CuO thin films are determined. Particle-free single-phase films are obtained from the foil target. The laser-deposited films have good optical properties. For comparison, DC-sputtered films deposited from the same Cu foil were studied. To obtain the same phase the former method requires higher oxygen pressure during deposition. The microstructure formed in the laser-deposited samples is always polycrystalline with a homogeneous grain size whereas sputtering can form columnar growth.

Neutron diffraction and the structure of amorphous Ni0.95Tb0.05.

Neutron diffraction measurements have been performed on amorphous Ni/sub 0.95/Tb/sub 0.05/ and crystalline Ni. Thick dc-sputtered films of the amorphous alloy were prepared by high-rate triode magnetron sputtering into liquid-nitrogen-cooled substrates. The contributions of structural and thermal disorder to the first-neighbor radial distribution function were obtained using the crystalline Ni results along with corrections for finite Q/sub max/. The results indicate a Ni-Ni first-neighbor structural disorder whose width, sigma/sub 1//r-bar/sub 1/approx. =3%, is substantially lower than values suggested for amorphous metals. Comparisons with theoretical models of the pair distribution function of a-Ni and a-Fe based on relaxed dense random-packed structures indicate good agreement with the results of Heimendahl. Density measurements have yielded the packing fraction for a-Ni/sub 0.95/Tb/sub 0.05/. Extrapolation of this value has allowed an estimate of the packing fraction of pure a-Ni to be obtained. The results indicate a density greater than the maximum for a hard-sphere, dense random-packing model, but significantly below that of crystalline values found in some relaxed models.

Structural analysis of rf and dc sputtered Nb3Ge thin films containing oxygen

Using electron microscopy, diffraction, microprobe, scanning microscopy, and nuclear back-scattering techniques to analyze rf and dc sputtered N3Ge thin films, it was found that the film structure was strongly dependent on film preparation methods. Amorphous Nb3Ge films were prepared by both sputtering techniques, on substrates held at liquid nitrogen temperature. Identical crystallization heat treatments were then carried out in a He furnace at 750°C for 65 h. Analyses by nuclear back-scattering indicated that crystallized rf-prepared films contained about three times as much oxygen as crystallized dc films. Structure studies showed that although both types of crystallized films exhibited the A-15 structure, the data on the rf films indicate the presence of the crystalline oxide, Nb2O5. In addition, the lattice parameter for the rf film was slightly smaller * dc-prepared films while the grain size of the former is about 10 times larger. These data appear to be correlated to differences in the superconducting transition temperature of both types of films. A high Tc (∠16 K) was observed for rf-prepared samples. The annealed dc-sputtered films did not become superconducting for temperatures above 1.5 K. In summary, the results of this work indicate that large grain size is conducive to high Tc superconductivity in Nb–Ge system and provide some evidence for oxygen stabilization of the high TcA-15 phase.

Optical waveguide characteristics of reactive dc-sputtered niobium pentoxide films

Reactively dc-sputtered films of Nb2O5 have very good optical characteristics when prepared under the proper conditions. Substrate preparation, sputtering pressure, gas composition, and deposition power have beem determined which yield films having refractive index from 2.1 to 2.27 and waveguide loss less than 2 dB/cm at 6328 Å.

The Structure of Evaporated- and dc-Sputtered Films of Gold and Silver Deposited on Glass

Evaporated- and dc-sputtered films of Au and Ag deposited on glass substrates were characterized in terms of topography and crystal size. The evaporated films exhibited less surface irregularity than the sputtered films, but contained a much wider range of crystal sizes. Little correlation was found between crystal size and surface topography.