Filtered vacuum arc deposition of undoped and doped ZnO thin films: Electrical, optical, and structural properties

Filtered vacuum (cathodic) arc deposition (FVAD, FCVD) of metallic and ceramic thin films at low substrate temperature (50-400 °C) is realized by magnetically directing vacuum arc produced, highly ionized, and energetic plasma beam onto substrates, obtaining high quality coatings at high deposition rates. The plasma beam is magnetically filtered to remove macroparticles that are also produced by the arc. The deposited films are usually characterized by their good optical quality and high adhesion to the substrate. Transparent and electrically conducting (TCO) thin films of ZnO, SnO₂, In₂O₃:Sn (ITO), ZnO:Al (AZO), ZnO:Ga, ZnO:Sb, ZnO:Mg and several types of zinc-stannate oxides (ZnSnO₃, Zn₂SnO₄), which could be used in solar cells, optoelectronic devices, and as gas sensors, have been successfully deposited by FVAD using pure or alloyed zinc cathodes. The oxides are obtained by operating the system with oxygen background at low pressure. Post-deposition treatment has also been applied to improve the properties of TCO films.The deposition rate of FVAD ZnO and ZnO:M thin films, where M is a doping or alloying metal, is in the range of 0.2-15 nm/s. The films are generally nonstoichiometric, polycrystalline n-type semiconductors. In most cases, ZnO films have a wurtzite structure. FVAD of p-type ZnO has also been achieved by Sb doping. The electrical conductivity of as-deposited n-type thin ZnO film is in the range 0.2-6 × 10^− 5 Ω m, carrier electron density is 10²³-2 × 10²⁶ m^− 3, and electron mobility is in the range 10-40 cm²/V s, depending on the deposition parameters: arc current, oxygen pressure, substrate bias, and substrate temperature. As the energy band gap of FVAD ZnO films is ∼ 3.3 eV and its extinction coefficient (k) in the visible and near-IR range is smaller than 0.02, the optical transmission of 500 nm thick ZnO film is ∼ 0.90.     

Epitaxial 0.65PbMg1/3Nb2/3O3-0.35PbTiO3 (PMN-PT) thin films grown on LaNiO3/CeO2/YSZ buffered Si substrates

Ferroelectric PMN-PT thin films with a thickness of 600 nm were epitaxially grown on buffered Si (0 0 1) substrates at a substrate temperature that ranged from 550 to 700 °C using pulsed laser deposition (PLD). LaNiO₃ (LNO) electrode thin films with a resistivity of ∼1900 μΩ cm were epitaxially grown on CeO₂/YSZ buffered Si (0 0 1) substrates. The PMN-PT thin films grown at 600 °C on LNO/CeO₂/YSZ/Si substrates had a pure perovskite and epitaxial structure. The PMN-PT films exhibited a high dielectric constant of about 1818 and a low dissipation factor of 0.04 at a frequency of 10 kHz. Polarization-electric-field (P-E) hysteresis characteristics, with a remnant polarization of 11.1 μC/cm² and a coercive field of 43 kV/cm, were obtained in the epitaxial PMN-PT films.     

Influence of growth temperature on the optical and structural properties of ultrathin ZnO films

This study investigates the effect of growth temperature on the optical and structural properties of ultrathin ZnO films on the polished Si substrate. Thickness of the ultrathin ZnO films deposited by atomic layer deposition (ALD) method was about 10 nm. Photoluminescence (PL), X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques were used to measure the properties of ultrathin ZnO films. Experimental results showed that the ultrathin ZnO film deposited at 200 °C had excellent ultraviolet emission intensity, and the average roughness of the film surface was about 0.26 nm.     

Cathodic arc plasma deposition of nano-multilayered ZrN/AlSiN thin films

Thin films of ZrN/AlSiN were deposited on SKD 11 tool steel substrate using Zr and AlSi cathodes in an Ar/N₂ gas mixture in a cathodic arc plasma deposition system. The influence of the AlSi cathode arc current and the substrate bias voltage on the mechanical and structural properties of the films was investigated. X-ray diffraction, electron probe micro-analysis, high resolution transmission electron microscopy, nanoindentation and profilometry were used to characterize the films. The ZrN/AlSiN thin films had a multilayered structure by rotating the substrate in which nano-crystalline ZrN layers alternated with amorphous AlSiN layers. The hardness of the films increased as the AlSi cathode arc current was raised from 35 to 40 A, and then decreased with a further increase of the current. The hardness of the films increased with the increase of the bias voltage from − 50 to − 100 V. Further increase in the bias voltage decreased the hardness. The films exhibited a maximum hardness of 38 GPa. With the increase of bias voltage, the residual stress of the films correlated well with the hardness.     

Low temperature pulsed laser deposition of textured γ′-Fe4N films on Si (1 0 0)

Low-temperature reactive pulsed laser deposition (PLD) was used to prepare iron nitride films. The textured γ′-Fe₄N films with (0 0 1)-orientation were deposited on Si (1 0 0) substrate with Fe buffer layer at a substrate temperature as low as 150 °C. The (0 0 1)-oriented γ′-Fe₄N film grew on the Fe buffer layer with a 3.5-nm thick amorphous interlayer, which eliminated the lattice mismatch stress between them. The films showed a columnar granular morphology with an average lateral grain size of approximately 110 nm. The films exhibited good soft magnetic properties with a high in-plane M_r/M_s value of 0.84. The magneto-optic Kerr effect results indicated an in-plane magnetic isotropy and confirmed the high remnant ratio of the γ′-Fe₄N films.     

Cathodic arc plasma deposition of nano-multilayered Zr-O/Al-O thin films

Thin films of Zr-O/Al-O were deposited on SKD 11 tool steel substrate using Zr and Al cathodes in a cathodic arc plasma deposition system. The substrates were mounted on a rotating holder which alternatively exposed them to plasma from the two cathodes. The influence of the Zr and Al cathode arc currents and the substrate bias on the mechanical and the structural properties of the films were investigated. Films with a nano-layered structure of alternating Al-rich and Zr-rich layers were obtained. The Zr layers contained nano-crystallites of (101) oriented t-ZrO structure. Crystallites with α-Al₂O₃ structure were observed only when the substrate was negatively biased in the 100-150 V range. The hardness of the film decreased with the increase of Zr cathode current from 60 to 80 A, increased when the Al cathode current increased from 25 to 30 A, and decreased when the Al cathode current increased from 30 to 35 A. The hardness of the film increased with the increase of bias voltage up to − 150 V and then decreased with further increase of the negative bias. The film structure was elucidated by HRTEM microscopy. Good correlation between the residual stress and the hardness enhancement of the films was observed.     

Microstructure and thickness dependent magnetic properties of nanogranular Co-Zn-O thin films for microwave applications

Co-based granular thin films with in-plane anisotropy were deposited on Si substrate by magnetron sputtering. The films have a phase structure of Co nanocrystallites and amorphous Zn-O inter-granular phases. The Co nanograins with uniform size of 8-10 nm are evenly distributed in the amorphous matrix. This structure gives the films relatively high resistivities. The as-deposited films with thickness larger than 100 nm have low coercivity (<10 Oe) along both easy and hard directions. The dynamic properties in the frequency range up 5 GHz for the films with various thicknesses have been investigated. High values of permeability (μ′ up to 560 and μ″ up to 1000) and ferromagnetic resonance frequency (FMR) up to 4.1 GHz have been obtained in these films. The FMR frequency decreases with increasing thickness, because of the increases in real and imaginary permeabilities. The high frequency characteristics have complicated dependences on the resistivity, anisotropy field, and magnetization. The microwave properties of Co-Zn-O films can be adjusted in a relatively wide range by changing film thickness, which makes these films promising for absorber applications.     

Preparation of mesoporous silica thin films on polystyrene substrate by electrochemically induced sol-gel technique

The mesoporous silica thin films with an oriented hexagonal mesostructure were prepared on polystyrene (PS) substrate by electrochemically induced sol-gel technique using tetraethoxyorthosilicate (TEOS) as silica source and cetyltrimethyl-ammonium bromide (CTAB) as structure-directing agent. Prior to coating deposition, the PS substrate was made hydrophilic by sulfonation with concentrated sulfuric acid for 72 h to provide better adhesion of silica films to the substrate. The effects of synthesis parameters required to obtain well-ordered crack-free layers, such as deposition voltage and deposition time, were evaluated in detail. The samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared-Attenuated total reflectance (FT-IR-ATR), small-angle X-ray diffraction (SAXRD) and transmission electron microscopy (TEM). According to the experimental results, the deposition voltage of 3.6 V and the deposition time of 10 s were determined as the optimum conditions. The silica films with the thickness of ca. 1.5 μm obtained under this condition was crack-free smooth and had a hexagonally ordered pore array pattern nanostructure. The pore diameter was about 3 nm and the distance between the neighboring pore centers was ca. 4.6 nm.     

Deposition of nanolayered CrN/AlBN thin films by cathodic arc deposition: Influence of cathode arc current and bias voltage on the mechanical properties

Thin films of CrAlBN were deposited on SKD 11 tool steel substrate using Cr and AlB cathodes in a cathodic arc plasma deposition system. The influence of AlB cathode arc current and substrate bias voltage on the mechanical and the structural properties of the films was investigated. The CrAlBN thin films had a multilayered structure in which the nano-crystalline CrN layer alternated with the amorphous AlBN layer. The hardness of the films increased as the AlB cathode arc current was raised from 35 to 45 A, and then decreased with further increase of the current. The hardness of the films increased rapidly with the increase of the bias voltage from − 50 to − 150 V. Further increase in the bias voltage decreased the hardness. The maximum hardness of 48 GPa was obtained at the bias voltage of − 150 V. With the increase of bias voltage, a good correlation between the residual stress and the hardness of the films was observed.     

Study on the Si penetration into Fe sheets using PVD method and its application in the fabrication of Fe-6.5 wt.% Si alloys

FeSi (12 wt.% Si) and Si were alternatively deposited on pure iron (Fe) substrates by direct current magnetron sputtering. Subsequent annealing in vacuum at 1150-1190 °C results in penetration of Si into the substrate. Cross-sectional microstructure and Si concentration were investigated by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The penetration mechanism is found to depend greatly on Si amount in the as-deposited films. When FeSi/Si/FeSi/Si/FeSi was deposited on the Fe substrate, the Si penetration is controlled by phase-boundary migration, while a diffusion-controlling penetration is observed in FeSi/Si/FeSi deposited samples. Fe-6.5 wt.% Si sheet with thickness of 0.35 mm is obtained through the deposition of FeSi/Si multilayer on a Fe-3 wt.% Si sheet together with subsequent annealing at 1180 °C for 2 h.     

Al and Zn film deposition using a vacuum arc plasma source with a refractory anode

The radially expanding plasma plume generated in a Hot Refractory Anode Vacuum Arc was used to deposit thin Al and Zn films on glass substrates. The electrode separation was 10 mm, arc time varied up to 165 s, and current (I) was 100-225 A. The cathode was a water-cooled Al or Zn cylinder. A graphite anode with 9 or 30 mm height was used with the Al cathode, and 10 or 30 mm height Mo anode was used with the Zn cathode. A mechanical shutter controlled the substrate exposure onset and duration (15 s) to the anodic plasma. The distance from the arc axis to the substrate (L) was varied between 80 and 165 mm. The film thickness was measured with a profilometer, and macroparticle (MP) presence on the coating surface was examined by optical microscopy.It was found that the deposition rate increased as a function of time to a peak, and then decreased to a steady-state value. The peak occurred sooner using the 9 mm anode than with the 30 mm anode. The peak deposition rate increased and the peak time decreased with I. The steady-state deposition rate was larger for Zn (~ 2 μm/min) than for Al cathodes (~ 1 μm/min) at I = 225 A and L = 110 mm. The arc voltage for Al was ~ 20-22 V and for Zn it was 11 V. The deposition rate peak appeared due to MPs evaporating from the hot anode, where they had initially condensed during the conventional arc stage when the anode was still cold. This effect was significant with low melting temperature Al and Zn cathodes, and negligible with Cu and Ti cathodes studied previously.     

Influence of substrate bias, deposition temperature and post-deposition annealing on the structure and properties of multi-principal-component (AlCrMoSiTi)N coatings

Nitride films are deposited from a single equiatomic AlCrMoSiTi target by reactive DC magnetron sputtering. The influence of the substrate bias and deposition temperature on the coating structure and properties are investigated. The bias is varied from 0 to − 200 V while maintaining a substrate temperature of 573 K. And the temperature is changed from 300 to 773 K whilst maintaining a substrate bias of − 100 V. From X-ray diffraction analysis, it is found that all the as-deposited coatings are of a single phase with NaCl-type FCC structure. This is attributed to the high mixing entropy of AlN, CrN, MoN, SiN, and TiN, and the limited diffusion kinetics during coating growth. Specific aspects of the coating, namely the grain size, lattice constant and compressive stress, are seen to be influenced more by substrate bias than deposition temperature. In fact, it is possible to classify the deposited films as large grained (~ 15 nm) with a reduced lattice constant (~ 4.15 Å) and low compressive residual stresses for lower applied substrate biases, and as small grained (~ 4 nm) with an increased lattice constant (~ 4.25 Å) and high compressive residual stresses for applied biases of − 100 V or more. A good correlation between the residual stress and lattice constant under various deposition conditions is found. For the coatings deposited at − 100 V, and at temperatures above 573 K, the hardness could attain to the range of 32 to 35 GPa.Even after annealing in vacuum at 1173 K for 5 h, there is no notable change in the as-deposited phase, grain size or lattice constant of the coatings but an increase in hardness. The thermal stability of microstructure is considered to be a result of the high mixing entropy and sluggish diffusion of these multi-component coatings. For the anneal hardening it is proposed that the overall bonding between target elements and nitrogen is enhanced by thermal energy during annealing.     

Constitution, microstructure, and battery performance of magnetron sputtered Li-Co-O thin film cathodes for lithium-ion batteries as a function of the working gas pressure

Li-Co-O thin film cathodes have been deposited onto Si and stainless steel substrates by RF magnetron sputtering from a ceramic LiCoO₂ target at various working gas pressures from 0.15 to 25 Pa. Composition, crystal structure and thin film morphology were examined and properties such as intrinsic stress, conductivity and film density were determined. As-deposited films at 0.15 Pa as well as in the range between 5 Pa and 10 Pa working gas pressure showed a nanocrystalline metastable rocksalt structure with disordered cation arrangement and were nearly stoichiometric. To induce a cation ordering the films were annealed in a furnace at temperatures between 100 and 600 °C for 3 h in argon/oxygen atmosphere (Ar:O₂ = 4.5:5) of 10 Pa. This cation ordering process was observed by XRD and Raman spectroscopy. For the films deposited at 10 Pa gas pressure an annealing temperature of 600 °C leads to the formation of the high temperature phase HT-LiCoO₂ with a layered structure. The Raman spectrum of the films deposited at 0.15 Pa and annealed at 400 °C indicates the formation of the low temperature phase LT-LiCoO₂ with a cubic spinel-related structure, which is assumed to be stabilized due to high compressive stress in the film. The electrochemical characterisation of annealed thin film cathodes revealed that the discharge capacity strongly depends on the crystal structure. Thin Li-Co-O films with a perfect layered HT-LiCoO₂ structure showed the highest discharge capacities.     

Structural and optical properties of electrodeposited ZnO thin films on conductive RuO2 oxides

ZnO thin films were grown on the 150 nm-thick RuO₂-coated SiO₂/Si substrates by electrochemical deposition in zinc nitrate aqueous solution with various electrolyte concentrations and deposition currents. Crystal orientation and surface structure of the electrodeposited ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy, respectively. The XRD results show the as-electrodeposited ZnO thin films on the RuO₂/SiO₂/Si substrates have mixed crystallographic orientations. The higher electrolyte concentration results in the ZnO thin films with a higher degree of c-axis orientation. Moreover, the use of an ultra-thin 5 nm-thick ZnO buffer layer on the RuO₂/SiO₂/Si substrate markedly improves the degree of preferential c-axis orientation of the electrodeposited ZnO crystalline. The subsequent annealing in vacuum at a low temperature of 300 °C reduces the possible hydrate species in the electrodeposited films. The electrodeposited ZnO thin films on the 5 nm-thick ZnO buffered RuO₂/SiO₂/Si substrates grown in 0.02 M electrolyte at −1.5 mA with a subsequent annealing in vacuum at 300 °C had the best structural and optical properties. The UV to visible emission intensity ratio of the film can reach 7.62.     

X-ray reflectivity studies on DLC films deposited by microwave ECR CVD: Effect of substrate bias

Diamond like carbon (DLC) films were deposited at room temperature on Si (111) substrates by microwave electron cyclotron resonance (ECR) plasma chemical vapor deposition (CVD) process using plasma of methane diluted with argon gas. During deposition, dc self bias (− 25 V to − 200 V) on substrate was varied by application of RF power to the substrate. The influence of substrate bias on density of the deposited films was studied by X-ray reflectivity (XRR). The results from these measurements are further correlated with the results from UV and visible Raman spectroscopy. DLC film is modeled as a structure having three different layers such as low density surface, bulk and interface with the substrate. This three-layer model is used to fit the measured XRR data to evaluate the surface, interface and interlayer roughness, thickness and density of these films. The surface roughness obtained from XRR is correlated with the results from Atomic Force Microscopy (AFM) measurements. The observed results are explained based on the subplantation model for DLC film growth.     

Effects of post-annealing on Schottky contacts of Pt/ZnO films toward UV photodetector

The Schottky contact of Pt/ZnO was formed by depositing ZnO films oriented along c-axis by pulsed-laser deposition on Pt/Ti buffer layer supported by SiO₂/Si substrate. Effects of the post-annealing on the crystallinity, uniformity and native defects of ZnO film as well as Schottky contacts of Pt/ZnO films were investigated. Results show that the annealing can improve the crystallinity of ZnO film, suppress the native defects, and enhance the performance of Pt/ZnO Schottky contacts dramatically. The best Schottky diode shows the largest barrier height of 0.8 eV with reverse leakage current of 1.5 × 10⁻⁵ A/cm². The zero-biased photodetector based on the best Schottky diode possesses responsivity of 0.265 A/W at 378 nm, fast photo-response component with rise time of 10 ns and fall time of 17 ns. This report demonstrates possibility of ZnO films/Pt hetero-junction with large area Schottky contact, and establishes the potential of this material for use in UV photodetector devices.     

Preparation and characterization of CoFe2O4 powders and films via the sol-gel method

The CoFe₂O₄ (CFO) starting precursor solutions were prepared by two sol-gel methods. The XRD results show that the second sol-gel method is a better method to obtain CFO materials with high purity. The CFO precursor solutions prepared by the second sol-gel method were spin-coated onto the Pt/Ti/SiO₂/Si substrate to obtain CFO films. With the increase of annealing temperature, the relative amounts of secondary phases in CFO films are decreased. When annealed at 700 °C, CFO films are almost composed of the main phase and the substrate phase without secondary phases. The CFO film is crack-free and has compact structure without any pore. The thickness of CFO film is about 49 nm. The starting precursor solution with the concentration of 0.15 mol L⁻¹ is better for preparing CFO films. The CFO films with nano-scaled film thicknesses have better magnetic properties than the CFO powders.     

In situ accelerated micro-wear — A new technique to fill the measurement gap

A novel accelerated microtribological capability was implemented on a commercial ultra-low drift nanomechanical test system (NanoTest) by modification of the instrument's hardware. 10 and 25 μm spheroconical and Berkovich diamond probes were used in this study. To compare the accelerated micro-wear capability with existing nano-scratch tests, a range of thin film samples previously characterised were evaluated, including 80 nm ta-C film deposited on Si, 150 nm a-C:H thin film deposited on Si, metal-containing molybdenum disulphide (MoST) 70-150 nm, 70 nm a-C:H and 1 μm a-C films deposited on Si, multilayered 20 nm Si3N4/20 nm NiCr/80 nm Si3N4 multilayer coating deposited on float glass and additionally bulk Cu sample. Operational principles of the experimental setup are explained and reliability of the method is validated with a number of experiments. Results are presented and discussed following four experimental sections of this paper: (i) constant load micro-wear of various films on Si, (ii) constant load micro-wear kinetics of bulk Cu, (iii) ramped load micro-wear of thin films and (iv) tangential force calibration.     

Hexagonal diamond formation on steel substrates by negative bias microwave plasma enhanced chemical vapor deposition

This paper reports for the first time the synthesis of hexagonal diamond thin films on high-speed steel substrates by multi-mode microwave plasma enhanced chemical vapor deposition. Before deposition of the films, the substrate surface was treated by scratching with diamond powder. The deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy. The XRD patterns of (100) and (101) planes and the Raman peaks at ~ 1317-1322 cm^− 1 were observed, confirming the formation of hexagonal diamond phase in the prepared films. The effects of voltage bias on the phase formation, microstructure and hardness of the films were also studied by setting the voltage to 0, − 70, − 150 and − 190 V. The highest hardness of 23.8 GPa was found in the film having clusters of size about 550 nm deposited under a bias voltage of − 150 V. These clusters were built up of grains of size about 14 nm.     

The effect of film composition on the structure and mechanical properties of NiTi shape memory thin films

Shape memory NiTi-based thin films approximately 2 μm thick were deposited onto Si (100) substrates at room temperature by simultaneous DC magnetron sputter deposition from separate elemental Ni and Ti targets. The effect of composition on film structure, surface morphology, transformation temperature and mechanical behavior was studied using variable temperature X-ray diffraction, atomic force microscopy, electrical resistivity, and nanoindentation. The films showed the expected shape memory and superelasticity behavior corresponding to the different film compositions, comparable with bulk properties. The transformation from the low temperature martensitic phase to the high temperature parent phase takes place above room temperature in Ti-rich and near-equiatomic films, and below room temperature in Ni-rich films. Mechanical properties of films investigated at room temperature by a series of nanoindentations at mN loads (indentation depth < 200 nm) with a spherical indenter demonstrate superelasticity in Ni-rich material and martensitic deformation for Ti-rich and near-equiatomic compositions.