Effect of thermal annealing on electrical properties of transparent conductive Ga-doped ZnO films prepared by ion-plating using direct-current arc discharge

The effect of thermal annealing on the electrical properties of highly transparent conductive Ga-doped ZnO (GZO) films deposited on glass substrates at 200 °C by an ion-plating deposition was investigated. GZO films were annealed in the temperature range from 200 to 600 °C for 30 min under the atmospheric pressure of high-purity N₂ gas. Up to 300 °C, GZO films were electrically very stable, and there was little change in resistivity. When the annealing temperature exceeded 400 °C, resistivity increased rapidly, originating from an abrupt decrease in carrier concentration. It was suggested to be due to both desorption of Zn from GZO films and grain boundary segregation of Ga dopants.     

Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications

The present status and prospects for further development of reduced or indium-free transparent conducting oxide (TCO) materials for use in practical thin-film transparent electrode applications such as liquid crystal displays are presented in this paper: reduced-indium TCO materials such as ZnO-In₂O₃, In₂O₃-SnO₂ and Zn-In-Sn-O multicomponent oxides and indium-free materials such as Al- and Ga-doped ZnO (AZO and GZO). In particular, AZO thin films, with source materials that are inexpensive and non-toxic, are the best candidates. The current problems associated with substituting AZO or GZO for ITO, besides their stability in oxidizing environments as well as the non-uniform distribution of resistivity resulting from dc magnetron sputtering deposition, can be resolved. Current developments associated with overcoming the remaining problems are also presented: newly developed AZO thin-film deposition techniques that reduce resistivity as well as improve the resistivity distribution uniformity using high-rate dc magnetron sputtering depositions incorporating radio frequency power. In addition, stability tests of resistivity in TCO thin films evaluated in air at 90% relative humidity and 60 °C have demonstrated that sufficiently moisture-resistant AZO thin films can be produced at a substrate temperature below 200 °C when the film thickness was approximately 200 nm. However, improving the stability of AZO and GZO films with a thickness below 100 nm remains a problem.     

Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering

In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO.     

ZnO thin films prepared by a single step sol-gel process

ZnO thin films were prepared on fused silica from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C₂H₃O₂)₂], monoethanolamine [H₂NC₂H₄OH ] and isopropanol. Crystallization annealing was performed over the range 500 to 650 °C. X-ray analysis showed that thin films were preferentially orientated along the [002] c-axis direction of the crystal. The films had a transparency of greater than 85% in the visible region for sol-gels with a zinc content of up to 0.7 M and exhibited absorption edges at ∼ 378 nm. The optical band-gap energy was evaluated to be 3.298-3.306 eV. Photoluminescence showed a strong emission centered at ca. 380 nm along with a broad yellow-orange emission centered at ca. 610 nm. Single step sol-gel thin film deposition in the film thickness range from 80 nm to 350 nm was demonstrated. The effect of sol-gel zinc concentration, film thickness and crystallization temperature on film microstructure, morphology and optical transparency is detailed. A process window for single spin coating deposition of c-axis oriented ZnO discussed.     

Effect of spatial relationship between arc plasma and substrate on the properties of transparent conducting Ga-doped ZnO thin films prepared by vacuum arc plasma evaporation

The effect of the spatial relationship between the arc plasma flow and the substrate surface on the resulting film thickness and electrical properties is investigated in transparent conducting Ga-doped ZnO (GZO) thin films deposited by a vacuum arc plasma evaporation (VAPE) method. It was found that the resulting electrical properties of GZO thin films produced by a VAPE deposition on a fixed substrate were considerably dependent on both the film thickness and the location on the substrate surface, extending from the area nearest the arc plasma source to that at the opposite end of the substrate in a direction parallel to the arc plasma flow; with GZO thin films deposited with various thicknesses in the range from 20 to 200 nm, the films exhibited a thickness dependence of resistivity that was considerably affected by the location on the substrate surface. The variation of resistivity relative to the location on the substrate surface was related to that of carrier concentration, which is mainly attributed to the distribution of the amount of oxygen reaching the substrate surface. In GZO thin films deposited with a thickness of 30-40 nm at a substrate temperature of 250 °C, a resistivity as low as 4 × 10^− 4 Ω cm was obtained in the area of the substrate nearest the arc plasma source.     

Effect of oxygen pressure of SiOx buffer layer on the electrical properties of GZO film deposited on PET substrate

The present work was made to investigate the effect of oxygen pressure of SiO_x layer on the electrical properties of Ga-doped ZnO (GZO) films deposited on poly-ethylene telephthalate (PET) substrate by utilizing the pulsed-laser deposition at ambient temperature. For this purpose, the SiO_x buffer layers were deposited at various oxygen pressures ranging from 13.3 to 46.7 Pa. With increasing oxygen pressure during the deposition of SiO_x layer as a buffer, the electrical resistivity of GZO/SiO_x/PET films gradually decreased from 7.6 × 10^− 3 to 6.8 × 10^− 4 Ω·cm, due to the enhanced mobility of GZO films. It was mainly due to the grain size of GZO films related to the roughened surface of the SiO_x buffer layers. In addition, the average optical transmittance of GZO/SiO_x/PET films in a visible regime was estimated to be ~ 90% comparable to that of GZO deposited onto a glass substrate.     

SiO2 thin film deposition on the inner surface of a poly(tetra-fluoroethylene) narrow tube by atmospheric-pressure glow microplasma

SiO₂ thin films were deposited on the inner surfaces of a commercial poly(tetrafluoroethylene) narrow tube with an inner diameter of 0.5 mm using tetraethoxysilane/O₂ feedstock gases and He carrier gas by atmospheric-pressure microplasma-enhanced chemical vapor deposition. A glow microplasma was generated inside the tube by radio frequency (RF) capacitively coupled discharge. X-ray photoelectron spectroscopy spectra showed that the tube inner surface was covered by a SiO₂ thin film. Transparent SiO₂ thin films were obtained with a deposition rate of 230 nm/min at an RF power of 6 W and substrate temperature of 100 °C. The wettability of the SiO₂-coated tube was about 3 times as large as that of an untreated sample tube.     

Structural evolution of magnetron sputtered shape memory alloy Ni-Ti films

Near equiatomic and Ti-rich Ni-Ti polycrystalline films have been deposited by magnetron co-sputtering using a chamber installed at a synchrotron radiation beamline. The in situ X-ray diffraction studies enabled the identification of different steps of the structural evolution during film processing.The depositions on a 140 nm amorphous SiO₂ buffer layer heated at 520 °C (without applying bias voltage, V_b, to the substrate) led to a preferential growth of <100> oriented grains of the Ni-Ti B2 phase from the beginning of film growth until the end of the deposition. Films exhibiting a preferential growth of <110> oriented grains of the Ni-Ti B2 phase from the beginning of the deposition were obtained (without and with a V_b of −45 V) by using a TiN coating with a topmost layer formed by <111> oriented grains. Those trends have been observed for the growth of near equiatomic (≈50.0 at.% Ti-Ni) and Ti-rich (≈50.8 at.% Ti-Ni) Ni-Ti films.Additionally, an ion gun had been commissioned, which allows ion bombardment during sputter deposition or post-deposition ion irradiation. In this first series of experiments, a Ni-Ti film was irradiated with He ions after deposition (without exposing the film to the atmosphere, i.e., avoiding surface oxide formation), thus modifying deliberately the microstructure of the film locally.     

Effect of growth temperature on the structural and transport properties of magnetite thin films prepared by pulse laser deposition on single crystal Si substrate

Magnetite (Fe₃O₄) thin films are prepared by pulsed laser deposition using an α-Fe₂O₃ target on silicon (111) substrate in the substrate temperature range of 350 °C to 550 °C. X-ray diffraction (XRD) measurement shows that the film deposited at 450 °C is a single phase Fe₃O₄ film oriented along [111] direction. However, the film grown at 350 °C reveals mixed oxide phases (FeO and Fe₃O₄), while the film deposited at 550 °C is a polycrystalline Fe₃O₄. X-ray photoelectron spectroscopy study confirms the XRD findings. Raman measurements reveal identical spectra for all the films deposited at different substrate temperatures. We observe abrupt increase in the resistivity behavior of all the films around Verwey transition temperature (T_V) (125 K-120 K) though the transition is broader in the film deposited at 350 °C. We observe that the optimized temperature for the growth of Fe₃O₄ film on Si is 450 °C. The electrical transport behavior follows Shklovskii and Efros variable range hopping type conduction mechanism below T_V for the film deposited at 450 °C possibly due to the granular growth of the film.     

Deposition of low-resistivity gallium-doped zinc oxide films by low-temperature radio-frequency magnetron sputtering

The transparent and conductive gallium-doped zinc oxide (GZO) film was deposited on 1737F Corning glass using the radio-frequency (RF) magnetron sputtering system with a GZO ceramic target. (The Ga₂O₃ contents are approximately 5 wt. %). In this study, the effect of the sputtering pressure on the structural, optical and electrical properties of GZO films upon the glass or polyester film (PET) substrate was investigated and discussed in detail. The GZO film was grown under a steady RF power of 400 W and a lower substrate temperature from room temperature up to 200 °C. The crystal structure and orientation of GZO thin films were examined by X-ray diffraction. All of the GZO films under various sputtering pressures had strong c-axis (002)-preferred orientation. Optical transparency was high (> 80%) over a wide spectral range from 380 nm to 900 nm. According to the experimental data, the resistivity of a single-layered GZO film was optimized at  8.3 × 10^− 4 Ω cm and significantly influenced by the sputtering pressure. In further research, the sandwich structure of the GZO film/Au metal/GZO film was demonstrated to improve the electrical properties of the single-layered GZO film. The resistivity of the sandwich-structured GZO film was around 2.8 × 10^− 4 Ω cm.     

Anode material properties of Ga-doped ZnO thin films by pulsed DC magnetron sputtering method for organic light emitting diodes

This study examined the anode material properties of Ga-doped zinc oxide (GZO) thin films deposited by pulsed DC magnetron sputtering along with the device performance of organic light emitting diodes (OLEDs) using GZO as the anode. The structure and electrical properties of the deposited films were examined as a function of the substrate temperature. The electrical properties of the GZO film deposited at 200 °C showed the best properties, such as a low resistivity, high mobility and high work function of 5.3 × 10^− 4Ω cm, 9.9 cm²/Vs and 4.37 eV, respectively. The OLED characteristics with the GZO film deposited under the optimum conditions showed good brightness > 10,000 cd/m². These results suggest that GZO films can be used as the anode in OLEDs, and a lower deposition temperature of 200 °C is suitable for flexible devices.     

Structural, electrical and optical properties of Ga-doped ZnO films on cyclo-olefin polymer substrates

Ga-doped ZnO (GZO) films with a thickness of 100 nm were prepared on cyclo-olefin polymer (COP) and glass substrates at various temperatures below 100 °C by ion plating with direct-current arc discharge. The dependences of the characteristics of GZO films on the substrate temperature Ts were investigated. All the polycrystalline GZO films, which exhibited a high average visible transmittance of greater than 86%, were crystallized with a wurtzite structure oriented along the c-axis. The lowest resistivities of the GZO films were 5.3 × 10^− 4 Ωcm on the glass substrate and 5.9 × 10^− 4 Ωcm on the COP substrate.     

Deposition of WNxCy thin films for diffusion barrier application using the dimethylhydrazido (2) tungsten complex (CH3CN)Cl4W(NNMe2)

Tungsten nitride carbide (WN_xC_y) thin films were deposited by chemical vapor deposition using the dimethylhydrazido (2⁻) tungsten complex (CH₃CN)Cl₄W(NNMe₂) (1) in benzonitrile with H₂ as a co-reactant in the temperature range 300 to 700 °C. Films were characterized using X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy and four-point probe to determine film crystallinity, composition, atomic bonding, and electrical resistivity, respectively. The lowest temperature at which growth was observed from 1 was 300 °C. For deposition between 300 and 650 °C, AES measurements indicated the presence of W, C, N, and O in the deposited film. The films deposited below 550 °C were amorphous, while those deposited at and above 550 °C were nano-crystalline (average grain size < 70 Å). The films exhibited their lowest resistivity of 840 µΩ-cm for deposition at 300 °C. WN_xC_y films were tested for diffusion barrier quality by sputter coating the film with Cu, annealing the Cu/WN_xC_y/Si stack in vacuum, and performing AES depth profile and XRD measurement to detect evidence of copper diffusion. Films deposited at 350 and 400 °C (50 and 60 nm thickness, respectively) were able to prevent bulk Cu transport after vacuum annealing at 500 °C for 30 min.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

High density plasma treatment of polyimide substrate to improve structural and electrical properties of Ga-doped ZnO films

We investigated the effects of a high density O₂ plasma treatment on the structural and electrical properties of sputter-deposited GZO films. The GZO films were deposited on polyimide substrate without substrate heating by RF magnetron sputtering from a ZnO target mixed with 5 wt.% Ga₂O₃. Prior to the GZO film growth, we treated a polyimide substrate with highly dense inductively coupled oxygen plasma. The optical transmittance of the GZO film, about 80%, was maintained regardless of the plasma pre-treatment. However, the resistivity of the film was strongly influenced by the plasma pre-treatment. The resistivity of the GZO film decreased from 1.02 × 10^− 2 Ω cm without an O₂ plasma pre-treatment to 1.89 × 10^− 3 Ω cm with an O₂ plasma pre-treatment.     

Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10^− 4 and 4 × 10^− 4Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.     

Ion sputter etching of ZnO:Ga thin film surfaces

In this article the modification of surface morphology of ZnO:Ga (GZO) thin films by ion sputter etching is presented. GZO thin films were prepared at room temperature on Corning glass substrates by both normal and oblique angle RF diode sputtering from ZnO:2%Ga ceramic target in Ar gas. Ion sputter etching was performed by RF re-sputtering of GZO thin films on substrates. During RF sputter etching, Ar pressure of 1.3 Pa and RF power of 250 W were kept constant, only the time of sputter etching was changed. Ion sputter etching had remarkable influence on surface morphology of GZO thin films: increase of roughness R_q and the “homogenization” of film surfaces, i.e. skewness (R_sk) and spikiness (R_ku) parameters (R_sk ≈ 0/R_ku ≈ 3).Surface root-mean-square roughness (R_q) increased from 15.3 nm (after sputter deposition) to 29.1 nm (after ion sputter etching). For obliquely thin films increased from 16.5 nm (after sputter deposition) to 38.2 nm. Changes of these parameters R_q, R_sk, R_ku influenced optical properties of GZO films, increased Haze parameter up to values 7.7% and width of optical band gap 3.44 eV, respectively.     

The effects of impurity and temperature for transparent conducting oxide properties of Al:ZnO deposited by dc magnetron sputtering

Aluminum-doped zinc oxide films (ZnO:Al) were deposited on Si wafers and glass substrates by dc magnetron sputtering from a ZnO target mixed with 2 wt% Al₂O₃ for photovoltaic films. The effect of base pressure, additional oxygen, and substrate temperature were studied in detail. By dc magnetron sputtering at room temperature, the resistivity and the average transmittance in visible range was 2.3 × 10⁻³ Ω cm and 77.3%, respectively. And these were improved up to 3.3 × 10⁻⁴ Ω cm and 86% at the substrate temperature of 400 °C by high deposition rate and low impurity ambient. The mobility and the carrier concentration were improved by the increased preferred orientation of (002) plane and grain size of film with increasing deposition temperature. This advanced AZO film with good resistivity and transmittance can be expected as the front TCO of thin film solar cells.     

Low refractive index silicon oxide coatings at room temperature using atmospheric-pressure very high-frequency plasma

Low refractive index silicon oxide films were deposited using atmospheric-pressure He/SiH₄/CO₂ plasma excited by a 150-MHz very high-frequency power. Significant increase in deposition rate at room temperature could prevent the formation of dense SiO₂ network, decreasing refractive index of the resulting film effectively. As a result, a silicon oxide film with the lowest refractive index, n = 1.24 at 632.8 nm, was obtained with a very high deposition rate of 235 nm/s. The reflectance and transmittance spectra showed that the low refractive index film functioned as a quarter-wave anti-reflection coating of a glass substrate.     

The growth of BaTiO3 films on (001) MgAl2O4 substrates by pulsed laser deposition technique

The microstructural evolution of the BaTiO₃ films grown on (001) MgAl₂O₄ spinel substrates at different temperatures by means of pulsed laser deposition technique is studied via transmission electron microscopy (TEM). The BaTiO₃ film grown at 850 °C consists of columnar grains of random orientations. Once the substrate temperature is over 900 °C, the BaTiO₃ films grow on (001) MgAl₂O₄ substrates epitaxially. The cross-sectional TEM study reveals that the boundaries and interfaces act as the sources to emit stacking faults and twins which are detrimental to the film quality. The quality of epitaxial films increases with the growth temperature, and is optimized at the growth temperature of 1050 °C. The evolution of film microstructures with the growth temperature is discussed in view of the growth temperature, the surface structure of MgAl₂O₄ substrates, and the phase transition of BaTiO₃.