Reactive Ion Beam Sputter Deposition Research Articles

Electrochromic switching of tungsten oxide films grown by reactive ion-beam sputter deposition

Chromogenic thin films are crucial building blocks in smart windows to modulate the flux of visible light and heat radiation into buildings. Electrochromic materials such as tungsten oxide are well established in those devices. Sputter deposition offers a well-suited method for the production of such layers, which can also be used on an industrial scale. Tungsten oxide films were prepared by means of reactive ion-beam sputter deposition. The choice of distinct gas mixtures as well as the growth temperature during the sputtering process allows to tune the properties of the resulting layers. Especially, the variation in the growth temperatures was found to have an impact on the structure of the resulting samples and, as a consequence, on their optical and electrochemical properties. By specific choice of the reactive gas, the deposition of colorless transparent as well as blue films of different composition is possible. The optical transmittance in the visible spectral range was up to 75% for as-deposited oxygen-rich layers. Additionally, hydrogen-doped tungsten oxide samples were grown. Superior electrochromic switching was observed for H^{+}-doped layers, probably by some kind of preconditioning. This resulted in a value for the standardized optical coloration efficiency of 26.5 cm^{2}/C.

X-ray Photoelectron Spectroscopy Analysis of Nitrogen-Doped TiO2 Films Prepared by Reactive-Ion-Beam Sputtering with Various NH3/O2 Gas Mixture Ratios

Nitrogen-doped TiO2 films were prepared by reactive ion-beam sputtering deposition (IBSD) in a mixed atmosphere of NH3 and O2 at a substrate temperature of 400 °C. X-ray photoelectron spectra revealed the presence of six ions, i.e., N3−, N2−, N1−, N+, N2+, and N3+, respectively, in the films. The amorphous films had complex, randomly oriented chemical bonds. The Tauc–Lorentz model was employed to determine the bandgap energy of the amorphous films prepared using different NH3/O2 gas mixing ratios by ellipsometry. In addition, the optical constants of the films were measured. With the increase in the NH3/O2 gas mixture ratio to 3.0, the bandgap of N-doped TiO2 narrowed to ~2.54 eV.

Investigation of effects of assisted ion bombardment on mechanical loss of sputtered tantala thin films for gravitational wave interferometers

Reduction of Brownian thermal noise due to mechanical loss in high-reflectivity mirror coatings is critical for improving the sensitivity of future gravitational wave detectors. In these mirrors, the mechanical loss at room temperature is dominated by the high refractive index component, amorphous tantala (${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$) or tantala doped with titania ($\mathrm{Ti}\ensuremath{\mathbin:}{\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$). Toward the goal of identifying mechanisms that could alter mechanical loss, this work investigates the use of assist ion bombardment in the reactive ion beam sputtering deposition of tantala single layers. Low-energy assist ion bombardment can enhance adatom diffusion. Low-energy assist ${\mathrm{Ar}}^{+}$ and ${\mathrm{Xe}}^{+}$ ion bombardment at different conditions was implemented during deposition to identify trends in the mechanical loss with ion mass, ion energy, and ion dose. It is shown that the atomic structure and bonding states of the tantala thin films are not significantly modified by low-energy assist ion bombardment. The coatings mechanical loss remains unaltered by ion bombardment within errors. Based on an analysis of surface diffusivity, it is shown that the dominant deposition of tantala clusters and limited surface diffusion length of oxygen atoms constrain structural changes in the tantala films. A slower deposition rate coupled with a significant increase in the dose of the low-energy assist ions may provide more favorable conditions to improve adatom diffusivity.

Secondary particle properties for the ion beam sputtering of TiO2 in a reactive oxygen atmosphere

This work deals with the properties of secondary particles, i.e., sputtered target and scattered primary particles, for the ion beam sputtering of TiO2 in a reactive oxygen atmosphere. The angular and energy distributions in dependence on ion beam parameters (ion energy, ion species, or ion mass) and geometrical parameters (ion incidence angle, polar emission angle, or scattering angle) were investigated. The angular distributions of sputtered Ti particles were determined by the collection method, i.e., by growing TiO2 thin films and measuring the thickness, composition, and mass density. The distributions can be described by the superposition of an isotropic and an anisotropic part. The anisotropic part relative to the isotropic part gets more pronounced with decreasing ion energy, increasing ion incidence angle or changing the process gas from Ar to Xe. An energy-selective mass spectrometer was used to measure mass and energy distributions of secondary ions. The most prevalent species were identified to be O+, O2+, Ti+, TiO+, Ar+, and Xe+ ions. The energy distributions of Ti+ and TiO+ ions show a low-energy maximum between 10 eV and 30 eV, followed by an exponential decay. When decreasing the scattering angle, the high-energy tail extends to higher energies. O+, O2+, Ar+, and Xe+ ions show a low-energy maximum between 5 eV and 20 eV, which is followed by a sudden signal drop for scattering angles larger than 90°. For scattering angles smaller than 90°, additional peak(s) at higher energy appear, which are assigned to direct scattering and/or direct sputtering events. The experimental results are compared to calculations based on elastic collision theory and to simulations done with the Monte Carlo code SDTrimSP. The results are also discussed in relation to secondary particle properties for the reactive ion beam sputtering of Ti and to properties of TiO2 thin films grown by reactive ion beam sputter deposition with the same setup and the same parameters. Among others, it was found that sputtering of the TiO2 target exhibits less anisotropy contributions than sputtering of the Ti target.

Controlling the p-type conductivity of SnO by doping with nitrogen and hydrogen

Two series of SnO thin films, one doped with N and one doped with H, were deposited on c-plane sapphire by reactive ion beam sputter deposition starting from growth parameters optimized for stoichiometric SnO. The amounts of dopants incorporated into the SnO:H and SnO:N samples were quantified by secondary ion mass spectroscopy. The influence on the structural and electrical properties of SnO thin films was studied as a function of dopant concentration. In the case of N doping, all N incorporated, probably as NO, are active as the acceptor and exhibit long-term stability. We assign an acceptor activation energy of 100 to 150 meV to NO. However, we observe a change in the film morphology at a critical N concentration of about 7⋅1017cm−3, which deteriorates the structural properties of the films. In the case of SnO:H, the situation is different. We observe an outdiffusion of H after growth, i.e., the samples are not stable in the long term. Nevertheless, all H incorporated up to a H-content of 1019cm−3 seem to be electrically active and exhibit an activation energy between 150 and 250 meV, likely corresponding to Hi. Furthermore, at H contents above 1019cm−3, we observe molecular H2 inside the SnO:H thin films. We conclude that N doping of SnO is better suited for tuning the p-type conductivity of SnO. However, it will be essential to overcome the morphology change observed at the critical N concentration to fully explore the tunability of the p-type conductivity in device applications.

Ion beam sputter deposition of TiO2 films using oxygen ions

TiO2 thin films were grown by ion beam sputter deposition (IBSD) using oxygen ions, with the ion energy and geometrical parameters (ion incidence angle, polar emission angle, and scattering angle) being varied systematically. Metallic Ti and ceramic TiO2 served as target materials. The thin films were characterized concerning thickness, growth rate, surface topography, structural properties, mass density, and optical properties. It was found that the scattering geometry has the main impact on the film properties. Target material, ion energy, and ion incidence angle have only a marginal influence. Former studies on reactive IBSD of TiO2 using Ar and Xe ions reported equivalent patterns. Nevertheless, the respective ion species distinctively affects the film properties. For instance, mass density and the refractive index of the TiO2 thin films are remarkably lower for sputtering with oxygen ions than for sputtering with Ar or Xe ions. The variations in the thin film properties are tentatively attributed to the angular and the energy distribution of the film-forming particles, especially, to those of the backscattered primary particles.

Percolated Si:SiO₂ Nanocomposites: Oven- vs. Millisecond Laser-Induced Crystallization of SiOx Thin Films.

Three-dimensional nanocomposite networks consisting of percolated Si nanowires in a SiO matrix, Si:SiO, were studied. The structures were obtained by reactive ion beam sputter deposition of (x ≈ 0.6) thin films at 450 C and subsequent crystallization using conventional oven, as well as millisecond line focus laser treatment. Rutherford backscattering spectrometry, Raman spectroscopy, X-ray diffraction, cross-sectional and energy-filtered transmission electron microscopy were applied for sample characterization. While oven treatment resulted in a mean Si wire diameter of 10 nm and a crystallinity of 72% within the Si volume, almost single-domain Si structures of 30 nm in diameter and almost free of amorphous Si were obtained by millisecond laser application. The structural differences are attributed to the different crystallization processes: conventional oven tempering proceeds via solid state and millisecond laser application via liquid phase crystallization of Si. The five orders of magnitude larger diffusion constant in the liquid phase is responsible for the three-times larger Si nanostructure diameter. In conclusion, laser treatment offers not only significantly shorter process times, but moreover, a superior structural order of nano-Si compared to conventional heating.

Assessing the growth window of stannous oxide by ion beam sputter deposition (IBSD)

SnOx films were fabricated on soda-lime glass and sapphire substrates by reactive ion beam sputter deposition (IBSD). A detailed growth series on c-plane sapphire was prepared using heater temperatures from room temperature to 600°C with different oxygen-to-argon ratios to investigate the impact of those parameters on thin film characteristics. By applying X-ray diffraction analysis and Raman spectroscopy an operating window was defined, in which SnO was grown without inclusions of parasitic Sn-related phases. The chemical bonding of the films was examined by X-ray photoelectron spectroscopy (XPS). Only a very narrow regime of oxygen flux was found with a composition close to stoichiometry. Furthermore, variation of the composition in SnO1±δ as well as the growth temperature were found to influence the morphology of the grown layer. The surface morphology was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). An average grain size of ≈10–50 nm was revealed for films grown at 200°C, which increases for annealed samples and samples grown at elevated temperatures. In addition to the expected influence on grain growth and surface roughness, disintegration of the SnO crystallites was found for inappropriate oxygen flux and heater temperature, giving way to incorporation of Sn or SnO2 species.

Systematic investigation of the reactive ion beam sputter deposition process of SiO2

Ion beam sputter deposition (IBSD) is an established physical vapour deposition technique that offers the opportunity to tailor the properties of film-forming particles and, consequently, film properties. This is because of two reasons: (i) ion generation and acceleration (ion source), sputtering (target) and film deposition (substrate) are locally separated. (ii) The angular and energy distribution of sputtered target atoms and scattered primary particles depend on ion incidence angle, ion energy, and ion species. Ion beam sputtering of a Si target in a reactive oxygen atmosphere was used to grow SiO2 films on silicon substrates. The sputtering geometry, ion energy and ion species were varied systematically and their influence on film properties was investigated. The SiO2 films are amorphous. The growth rate increases with increasing ion energy and ion incidence angle. Thickness, index of refraction, stoichiometry, mass density and surface roughness show a strong correlation with the sputtering geometry. A considerable amount of primary inert gas particles is found in the deposited films. The primary ion species also has an impact on the film properties, whereas the influence of the ion energy is rather small.

Stable Cu2O Photoelectrodes by Reactive Ion Beam Sputter Deposition

Cu2O has been deposited on quartz substrates by reactive ion beam sputter deposition. Experimental results show that by controlling argon/oxygen flow rates, both n‐type and p‐type Cu2O samples can be achieved. The bandgap of n‐type and p‐type Cu2O were found to be 2.3 and 2.5 eV, respectively. The variable temperature photoluminescence study shows that the n‐type conductivity is due to the presence of oxygen vacancy defects. Both samples show stable photocurrent response that photocurrent change of both samples after 1,000 seconds of operation is less than 5%. Carrier densities were found to be 1.90 × 1018 and 2.24 × 1016 cm−3 for n‐type and p‐type Cu2O, respectively. Fermi energies have been calculated, and simplified band structures are constructed. Our results show that Cu2O is a plausible candidate for both photoanodic and photocathodic electrode materials in photoelectrochemical application.

Characterization of n-type Cu2O deposited by reactive ion beam sputter deposition

Cuprous oxide (Cu2O) has been successfully deposited by reactive ion beam sputter deposition at 450 °C with various oxygen flow rates. At high oxygen flow rates, single phase polycrystalline Cu2O thin film was attained while low oxygen flow rates results in the formation of Cu2O nanorods. X-ray diffraction, Raman, and x-ray photoelectron spectroscopy analyses indicate that both samples are composed of Cu2O phase only without the presence of CuO while samples deposited with low oxygen flow rates exhibit improved crystalline quality. Photocurrent measurement result indicates that Cu2O samples prepared under low oxygen flow rate are of n-type. Photoluminescence study suggests that this n-type conductivity is due to the presence of intrinsic oxygen vacancy defects.

Correlation of process parameters and properties of TiO2 films grown by ion beam sputter deposition from a ceramic target

The correlation between process parameters and properties of TiO2 films grown by ion beam sputter deposition from a ceramic target was investigated. TiO2 films were grown under systematic variation of ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle, polar emission angle) and characterized with respect to film thickness, growth rate, structural properties, surface topography, composition, optical properties, and mass density. Systematic variations of film properties with the scattering geometry, namely the scattering angle, have been revealed. There are also considerable differences in film properties when changing the process gas from Ar to Xe. Similar systematics were reported for TiO2 films grown by reactive ion beam sputter deposition from a metal target [C. Bundesmann et al., Appl. Surf. Sci. 421, 331 (2017)]. However, there are some deviations from the previously reported data, for instance, in growth rate, mass density and optical properties.

Systematic investigation of the properties of TiO2 films grown by reactive ion beam sputter deposition

TiO2 films were grown by ion beam sputter deposition under systematic variation of ion beam parameters (ion species, ion energy) and geometrical parameters (ion incidence angle, polar emission angle) and characterized with respect to film thickness, growth rate, structural properties, surface topography, composition, optical properties, and mass density. The angular distributions of film thickness and growth rate show an over-cosine shape, which is tilted in forward direction. All films are amorphous and the surface roughness is below 0.22nm. The investigation of the composition revealed stoichiometric TiO2 with implanted backscattered primary particles. The optical properties were analysed using the Tauc Lorentz (TL) model. The amplitude parameter of the TL model was found to vary systematically with the scattering angle, whereas the impact on the other TL parameters is negligible. Mass density follows the same trends as the optical properties, i.e. optical properties and mass density are correlated. Surface roughness, atomic fraction of implanted primary particles, optical properties and mass density show similar systematic variations with process parameters, especially, with the scattering geometry (i.e. scattering angle). Ion species, ion energy and ion incidence angle have no or only a small impact. The variations in the film properties are tentatively assigned to changes in the angular and energy distribution of the sputtered target particles and backscattered primary particles.

Cupric and cuprous oxide by reactive ion beam sputter deposition and the photosensing properties of cupric oxide metal–semiconductor–metal Schottky photodiodes

Cupric (CuO) and cuprous (Cu2O) oxide thin films have been deposited by reactive ion beam sputter deposition at 400°C with an Ar:O2 ratio from 2:1 to 12:1. With an Ar:O2 ratio of 2:1, single phase polycrystalline CuO thin films were obtained. Decreasing oxygen flow rate results in CuO+Cu2O and Cu2O+Cu mixed thin films. As Ar:O2 ratio reaches 12:1, Cu2O nanorods with diameter of 250nm and length longer than 1μm were found across the sample. Single phase CuO thin film exhibits an indirect band gap of 1.3eV with a smooth surface morphology. CuO metal–semiconductor–metal (MSM) Schottky photodiodes (PD) were fabricated by depositing Cu interdigitated electrodes on CuO thin films. Photosensing properties of the CuO PD were characterized from 350 to 1300nm and a maximum responsivity of 43mA/W was found at λ=700nm. The MSM PD is RC limited with a decay time constant less than 1μs.

Influence of Oxygen to Argon Ratio on the Structural and Morphological Properties of Nb-Doped SrTiO<sub>3</sub> Epitaxial Films Grown by Reactive Ion Beam Sputter Deposition

Nb-doped SrTiO3 (STNO) films were grown on (001)-oriented LaAlO3 substrates by a reactive ion beam sputter deposition at various mixing ratios (OMRs) with a substrate temperature of 800oC. The STNO films exhibited good crystallinity with an epitaxial orientation as characterized by high-resolution X-ray diffraction, grazing-incidence X-ray diffraction, and in-plane pole figure analysis. A decrease of out-of-plane and in-plane lattice constants was observed with an increase of OMR. The surface morphology of the STNO films showed a very dense fine-grain structure. The root-mean-square roughness was found to be increased as the OMR increased. Moreover, the elemental compositions of the STNO films were examined by X-ray photoelectron spectroscopy.

Vanadium Oxide Thin Films Synthesized by Reactive Ion Beam Sputter Deposition: Influence of Processing Parameters

Vanadium oxide thin films were deposited by reactive ion beam sputtering deposition onto glass substrates. The films were prepared by sputtering from a metallic vanadium target with an argon+oxygen ion beam in vacuum. Different processing conditions were evaluated with focus in obtaining monoclinic VO2(M) phase, which is known to exhibit a semiconducting-metal phase transition near room temperature. X-ray diffractometry (XRD) analyses revealed amorphous films for temperatures below 500°C. In crystalline films, the co-existence of VO2(M) with other phases was suppressed by pre-depositing a very thin metallic vanadium seeding layer which showed to promote the formation of single phase VO2(M) films. The VO2(M) films showed clearly the distinctive optical modulation behavior at the near-infrared range when going through the phase transition. The temperature dependence of sheet resistance supports the optical analyses revealing an evident semiconducting-metal behavior change up to over 2 orders of magnitude.

Characterization of nitrogen doped p-type ZnO thin films prepared by reactive ion beam sputter deposition

Nitrogen doped ZnO (ZnO:N) thin films have been successfully prepared by reactive ion beam sputter deposition. The ZnO:N thin films show a preferred growth orientation along the (002) direction regardless of nitrogen flow rates. Raman spectroscopy analysis shows nitrogen related local vibration modes at 275 and 576cm−1 in addition to the ZnO E2 (high) mode at 436cm−1, indicating a successful incorporation of nitrogen into the ZnO. Both of the peak intensities of 275 and 576cm−1 reach a maximum after post-growth annealing at 500°C. ZnO:N deposited with a 0.5sccm nitrogen flow rate exhibits p-type conductivity with a hole concentration of 2.1×1017/cm3 and a mobility of 3cm2V−1s−1 after annealing at 500°C. Conversion to p-type conductivity was not observed on ZnO:N deposited with higher nitrogen flow rates. The p-type conductivity remains stable after it was stored at ambient conditions for more than two months. The p-type ZnO:N thin film is transparent in the visible range with a transmittance larger than 83%. SIMS analysis indicates that nitrogen concentration of less than 1.2at.% results in the successful preparation of p-type ZnO:N. ZnO:N deposited with higher nitrogen concentration results in n-type conductivity which is likely due to the formation of molecular nitrogen replacing oxygen sites that act as double donors (N2)O.

Effect of nanoscale ripples on the formation of ZnO quantum dots

Nano-scale ripples on Si (100) substrates were prepared by argon ion beam sputtering at 6, 8, and 10 keV with an ion fluence of 5 × 1017 ions/cm2. The spatial wavelength of the nano-scale ripple increases as the ion beam energy increases, regardless of ion beam incident angles, indicating that ion beam induced diffusion is the dominant diffusion mechanism. ZnO quantum dots (QDs) with diameters less than 20 nm and heights less than 4 nm can be prepared over the temperature range from 200 ∼ 300 °C by reactive ion beam sputter deposition. Compared with ZnO QDs deposited on Si substrates without nano-scale ripple, the ion-beam textured substrate provides a wider processing window, improved dot size, and increased QD density.

Growth of ZnO quantum dots on Si nano ripples

Substrates with nano-scale ripples are excellent templates for the deposition of semiconductor nanostructures. We have prepared quasi periodical nano-scale ripples on Si (100) substrates with spatial wavelength λ from ~ 70 to ~ 150 nm by ion beam sputtering. ZnO QDs with diameters from 17 to ~ 30 nm and heights from 2 to ~ 4 nm have been successfully deposited by reactive ion beam sputter deposition. The QD size and distribution were found to be dependent both on growth conditions and spatial wavelength of the nano-scale ripple. On substrates with λ ~ 150 nm, ZnO QDs were distributed evenly across the wafer, while on substrates with λ ~ 70 nm, ZnO QDs were preferentially located along the crest of the nano-scale ripples. As the QD height decreases from ~ 4 to 2 nm, room temperature photoluminescence UV emission energy blue shifts by 80 meV. Possible sources of the blue shift are presented.

Cooperative upconversion and optical gain in ion-beam sputter-deposited Er_xY_2-xSiO_5 waveguides

Single-phase, polycrystalline Er(x)Y(2-x)SiO(5) thin films were deposited by reactive ion-beam sputter deposition and rapid thermal annealing. Due to the crystalline nature, the silicate thin films provide peak Er(3+) emission cross-section of 0.9 +/- 0.02 x 10(-20) cm(2) that is higher than that in silica. Optical gain, with near 60% inversion, is achieved via optical pumping of a single-mode, ridge-type waveguide with the silicate core with an Er concentration of 1.7 x 10(20) cm(-3). Analysis of pump-power dependence of the optical gain and spontaneous emission intensity of Er(3+) indicate that the gain is limited by cooperative upconversion process, whose coefficient is determined to be (8 +/- 3) x 10(-17) cm(3)/sec.