Layer uniformity of glancing angle deposition

Some results of an investigation on the layer thickness uniformity of glancing angle deposition are presented. A zirconia monolayer has been deposited by glancing angle deposition to analyze the layer thickness uniformity. The experimental results indicate that the thickness variation over the substrate is less than 0.1%, which is considered as good uniformity. It is found that the non-uniformity of experimental results is larger than that of the theoretical results.     

The structure of Ta nanopillars grown by glancing angle deposition

Regular arrays of Ta nanopillars, 200 nm wide and 500 nm tall, were grown on SiO₂ nanosphere patterns by glancing angle sputter deposition (GLAD). Plan-view and cross-sectional scanning electron microscopy analyses show dramatic changes in the structure and morphology of individual nanopillars as a function of growth temperature T_s ranging from 200 to 700 °C. At low temperatures, T_s ≤ 300 °C, single nanopillars develop on each sphere and branch into subpillars near the pillar top. In contrast, T_s ≥ 500 °C leads to branching during the nucleation stage at the pillar bottom. The top branching at low T_s is associated with surface mounds on a growing pillar that, due to atomic shadowing, develop into separated subpillars. At high T_s, the branching occurs during the nucleation stage where multiple nuclei on a single SiO₂ sphere develop into subpillars during a competitive growth mode which, in turn, leads to intercolumnar competition and the extinction of some nanopillars.     

Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition

Nanostructured tungsten (W) and tungsten trioxide (WO₃) films were prepared by glancing angle deposition using pulsed direct current magnetron sputtering at room temperature with continuous substrate rotation. The chemical compositions of the nanostructured films were characterized by X-ray photoelectron spectroscopy, and the film structures and morphologies were investigated using X-ray diffraction and high resolution scanning electron microscopy. Both as-deposited and air annealed tungsten trioxide films exhibit nanostructured morphologies with an extremely high surface area, which may potentially increase the sensitivity of chemiresistive WO₃ gas sensors. Metallic W nanorods formed by sputtering in a pure Ar plasma at room temperature crystallized into a predominantly simple cubic β-phase with <100> texture although evidence was found for other random grain orientations near the film/substrate interface. Subsequent annealing at 500 °C in air transformed the nanorods into polycrystalline triclinic/monoclinic WO₃ structure and the nanorod morphology was retained. Substoichiometric WO₃ films grown in an Ar/O₂ plasma at room temperature had an amorphous structure and also exhibited nanorod morphology. Post-deposition annealing at 500 °C in air induced crystallization to a polycrystalline triclinic/monoclinic WO₃ phase and also caused a morphological change from nanorods into a nanoporous network.     

Growth and properties of the CuInS2 thin films produced by glancing angle deposition

We use the glancing angle deposition technique (GLAD) to grow CuInS₂ thin films by a vacuum thermal method onto glass substrates. During deposition, the substrate temperature was maintained at 200 °C. Due to shadowing effect the oblique angle deposition technique can produce nanorods tilted toward the incident deposition flux. The evaporated atoms arrive at the growing interface at a fixed angle θ measured from the substrate normal. The substrate is rotated with rotational speed ω fixed at 0.033 rev s^− 1. We show that the use of this growth technique leads to an improvement in the optical properties of the films. Indeed high absorption coefficients (10⁵–3.10⁵ cm^− 1) in the visible range and near-IR spectral range are reached. In the case of the absence of the substrate rotation, scanning electron microscopy pictures show that the structure of the resulting film consists of nanocolumns that are progressively inclined towards the evaporation source as the incident angle was increased. If a rapid azimuthal rotation accompanies the substrate tilt, the resulting nanostructure is composed of an array of pillars normal to the substrate. The surface morphology show an improvement without presence of secondary phases for higher incident angles (θ > 60°).     

Ordered arrays of nanocolumns grown by the oblique angle deposition technique on a self-assembled layer of polystyrene spheres

Ordered arrays of upright nanocolumns of chalcogenide glass were fabricated by combining the oblique angle deposition technique with substrate rotation. Self-assembled close-packed arrays of polystyrene spheres of 200 nm and 500 nm in diameter were used as templates. Our method allows the fabrication of GeSbSe nanocolumns of controlled height, cross-sectional diameter and two-dimensional distribution. Finally, preliminary optical characterization suggests the possible use of the ordered arrays of GeSbSe nanocolumns in optical devices, in particular photonic crystals.     

纳米结构的倾斜角度沉积及性能优化

纳米材料发展的关键是纳米结构的制备、形貌调控和性能优化.倾斜角度沉积是以较大的角度(大于75°)倾斜入射沉积薄膜,通过控制沉积参数,得到具有特殊形貌纳米结构的方法,具有适用范围广,操作便捷,制备的薄膜面积大、纯度高、结构规整等特点,是一种理想的制备纳米材料的方法.本文介绍了采用倾斜角度沉积技术制备氧化铪抗反射薄膜和银基表面增强拉曼基底,详细分析了该方法的参数调控对纳米结构的形貌和性能的影响,并指出将倾斜角度沉积与其他先进技术相结合(以原子层沉积为例),可进一步优化纳米结构的性能,提高倾斜角度沉积的使用范围.     

Vapor deposited sculptured nano-porous titania films by glancing angle deposition for efficiency enhancement in dye-sensitized solar cells

Sculptured porous titania films as photoanode in dye-sensitized solar cell (DSSC) were prepared using an electron-beam evaporation system with glancing angle deposition (GLAD) method. By controlling the substrate rotation rate and the incident angle of evaporant, titania films of various thicknesses were prepared on ITO glasses. The as-deposited nano-porous films are comprised of helical nano-columns and assembled in an orderly manner with gaps or pores in between, which offer large internal surface area for dye adsorption and direct electron transfer path. There is a positive correlation between the film thickness, film effective surface area, amount of absorbed dye and cell efficiency. The nano-porous films provide a synergistic effect of high surface area, effective route for electron transfer, tight interfaces, and enhanced light trapping, which are all beneficial for higher cell efficiency. The DSSC consisting of a 6 μm titania film, deposited at substrate rotating rate 0.17 rpm and incident angle 73°, gave a cell efficiency of 6.1%.     

Nanospring pressure sensors grown by glancing angle deposition

Arrays of Cr zigzag nanosprings and slanted nanorods, 15-55 nm and 40-80-nm-wide, respectively, were grown on SiO2/Si substrates by glancing angle deposition. The arrays exhibit a reversible change in resistivity upon loading and unloading, by 50% for nanosprings and 5% for nanorods, indicating their potential as pressure sensors. The resistivity drop is due to a compression of nanosprings (by a measured 19% for an applied external force of 10(-10) N per spring), which causes them to physically touch their neighbors, providing a path for electric current to flow between nanosprings. Repeated loading and unloading at large loads (> or =1 MPa) results in irreversible plastic deformation and a degradation of the pressure sensitivity.     

Film morphology modification in ion-assisted glancing angle deposition

Porous structured films grown with the glancing angle deposition technique have been widely studied for thin film optical device applications. We report the use of ion assistance to modify the structural and optical properties of porous silicon dioxide and titanium dioxide columnar thin films grown at deposition angles of 70° and 85°. Optical characterization studies show that tilted columnar structures will undergo an increase in tilt angle and film density with increasing ion dose. These two trends contrast with unassisted films where film density and column tilt angle are primarily controlled by the deposition angle. Thus, a regime of film structures simultaneously exhibiting high film density and large column tilt angle is enabled by incorporating an ion-assisted process. The phisweep substrate motion algorithm for minimizing columnar anisotropy used in conjunction with ion-assisted deposition provides additional control over film morphology and expands the utility of this modified fabrication process.     

Structural and photoluminescence properties of aligned Sb-doped ZnO nanocolumns synthesized by the hydrothermal method

Aligned Sb-doped ZnO nanocolumns were synthesized by a simple hydrothermal method. Based on the analyses of the X-ray diffraction and photoluminescence result, it could be confirmed that the Sb has successfully doped in the ZnO crystal lattices to form an accepter energy level. At 85 K, the recombination of the acceptor-bound exciton was predominant in PL spectrum, which was attributed to the transition of the (Sb_Zn-2V_Zn) complex bound exciton. The acceptor binding energy had been calculated to be 123 meV.     

Optical thin-film linear polarization selector fabricated by oblique angle deposition

A linear polarization selector was made from a three-section sculptured thin film deposited using an oblique angle deposition technique. In this device, the circular Bragg reflector with a left-handed helical structure was sandwiched in between two quarter-wave plates with opposite in-plane birefringence. Within the Bragg regime, the normal incident S-polarized light on this device was reflected, whereas the incident P-polarized light was transmitted through it. The microstructure of the linear polarization selector was also examined by scanning electron microscopy.     

Silicon nanorods prepared by glancing angle catalytic chemical vapor deposition

Vertically oriented amorphous and microcrystalline Si nanorods grown on different substrates were successfully obtained by Cat CVD with the glancing angle incident silane flux at low temperatures. The influences of the substrate type, substrate temperature, post treatment and hydrogen dilution on the microstructure of Si nanorods were investigated. The density and diameter of nanorods are varying with the substrates. The hydrogen dilution of silane dominates the crystallization of Si nanorods rather than high substrate temperature at 550 °C and annealing at 900 °C in nitrogen for 6 h. The crystallized Si nanorods with crystalline volume fraction, Xc, of 0.55 were achieved under a low substrate temperature of 140 °C.     

Low temperature synthesis of single crystalline ZnO nanorods by oblique angle deposition

The authors report the growth of single crystalline ZnO nanorods by direct current magnetron sputtering in the oblique angle deposition configuration near room temperature. These isolated nanorods have a diameter of  40 nm, an inter-rod spacing of  20 nm, and a height of  100 nm. The nanorods show a (002) orientation along the rod-axis which is normal to the substrate. The low temperature fabrication of single crystal ZnO nanorods may find potential applications in optoelectronics and energy conversion devices.     

Study on TiO2 thin films grown by advanced pulsed laser deposition on ITO

TiO₂ films were grown by an advanced pulsed laser deposition method (PLD) on ITO substrates to be used as functional electrodes in the manufacturing of solar cells. A pure titanium target (99.99%) was irradiated by a Nd:YAG laser (355 and 532 nm, 5 ns, 35 mJ, 3 J/cm²) in an oxygen atmosphere at different pressures (20-160 mTorr) and at room temperature. After deposition, the films were subjected to an annealing process at 350 °C. The film structure, surface morphology, thickness, roughness, and optical transmission were investigated. Regardless of the wavelength used, the films deposited at room temperature presented only Ti₂O and TiO peaks. After thermal treatment, the TiO₂ films became strongly crystalline, with a tetragonal structure and in the anatase phase; the threshold temperature value was 250 °C. The deposition rate was in the range of 0.035-0.250 nm/pulse, and the roughness was 135-305 nm. Optical transmission of the films in the visible range was between 40% and 60%.     

Thermal properties of TiO2 films grown by atomic layer deposition

We study the thermal properties of amorphous TiO₂ thin films of various thicknesses t, grown by atomic layer deposition. The thermo-optic coefficient dn/dT and the temperature coefficient dρ/dT of film density ρ are determined from ellipsometric data in wavelength range 380 < λ < 1800 nm with the Cauchy model and the Lorentz-Lorenz relation. It is found that dn/dT exhibits negative values for films with t < 150 nm and positive values for thicker films, while no significant changes in the two coefficients take place if t < 200 nm. A qualitative physical explanation based on porosity of the thin films is suggested. Films with t = 60 nm are illustrated in detail at λ = 640 nm: the room-temperature values of the coefficients are found to be dn/dT = − 3.1 × 10^− 5 °C^− 1 and dρ/dT = − 4.8 × 10^− 5g cm^− 3° C^− 1.     

Conformity and structure of titanium oxide films grown by atomic layer deposition on silicon substrates

Conformity and phase structure of atomic layer deposited TiO₂ thin films grown on silicon substrates were studied. The films were grown using TiCl₄ and Ti(OC₂H₅)₄ as titanium precursors in the temperature range from 125 to 500 °C. In all cases perfect conformal growth was achieved on patterned substrates with elliptical holes of 7.5 μm depth and aspect ratio of about 1:40. Conformal growth was achieved with process parameters similar to those optimized for the growth on planar wafers. The dominant crystalline phase in the as-grown films was anatase, with some contribution from rutile at relatively higher temperatures. Annealing in the oxygen ambient resulted in (re)crystallization whereas the effect of annealing depended markedly on the precursors used in the deposition process. Compared to films grown from TiCl₄, the films grown from Ti(OC₂H₅)₄ were transformed into rutile in somewhat greater extent, whereas in terms of step coverage the films grown from Ti(OC₂H₅)₄ remained somewhat inferior compared to the films grown from TiCl₄.     

Carrier dynamics in coalescence overgrowth of GaN nanocolumns

Coalescence overgrowth of pattern-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of metal organic chemical vapor deposition layers, overgrown on the columnar structure with varying diameters of columns. Nano-imprint lithography was applied to open circular holes of 250, 300, 450, and 600 nm diameter on the SiO₂ layer, deposited on the GaN layer on the c-plane sapphire template. After the growth of ~ 1 μm high GaN nanocolumns, the further coalescence conditions led to an overgrown layer ~2 μm thickness. Photoelectrical and optical properties of the overgrown layers and a reference sample were investigated by time-resolved picosecond transient grating and time-integrated photoluminescence. We note a 3-4 fold increase in carrier lifetime in the overgrown epilayers when the diameter of columns increased from 250 to 450 nm. This feature is a clear indication of an ~4-fold reduced defect density.     

Effect of deposition angle on the structure and properties of pulsed-DC magnetron sputtered TiAlN thin films

This article reports the comparison of structure and properties of titanium aluminum nitride (TiAlN) films deposited onto Si(100) substrates under normal and oblique angle depositions using pulsed-DC magnetron sputtering. The substrate temperature was set at room temperature, 400 °C and 650 °C, and the bias was kept at 0, − 25, − 50, and − 80 V for both deposition angles. The surface and cross-section of the films were observed by scanning electron microscopy. It was found that as the deposition temperature increases, films deposited under normal incidence exhibit distinct faceted crystallites, whereas oblique angle deposited (OAD) films develop a kind of “tiles of a roof” or “stepwise structure”, with no facetted crystallites. The OAD films showed an inclined columnar structure, with columns tilting in the direction of the incident flux. As the substrate temperature was increased, the tilting of columns nearly approached the substrate normal. Both hardness and Young's modulus decreases when the flux angle was changed from α = 0° to 45° as measured by nanoindentation. This was attributed to the voids formed due to the shadowing effect. The crystallographic properties of these coatings were studied by θ-2θ scan and pole figure X-ray diffraction. Films deposited at α = 0° showed a mixed (111) and (200) out-of-plane orientation with random in-plane alignment. On the other hand, films deposited at α = 45° revealed an inclined texture with (111) orientation moving towards the incident flux direction and the (200) orientation approaching the substrate normal, showing substantial in-plane alignment.     

Structure and properties of nitrogen-doped titanium dioxide thin films grown by atmospheric pressure chemical vapor deposition

Using TiCl₄, O₂, and N₂O as precursors, N-doped titanium dioxide thin films with large area and continuous surface were obtained by atmospheric pressure chemical vapor deposition. Measurements of X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscope, transmission electron microscope and ultravoilet-Visible transmission spectra were performed. Using N₂O as N-doped source, anatase-rutile transformation is accelerated through oxygen vacancies formation, and the mean grain size of rutile crystallites decreases with the increase of N₂O flow rate. Compared to the pure TiO₂, N-doped TiO₂ films give a relative narrow optical band-gap, and their visible-light induced photocatalysis is much enhanced. Visible-light-induced hydrophilicity of the TiO₂ thin films enhances with the increase of N₂O flow rate, which might be due to the dentritic islands structure on the surface of the N-doped TiO₂ thin films.