Effects of sputtering power and pressure on properties of ZnO:Ga thin films prepared by oblique-angle deposition

The effects of power and pressure on radiofrequency (RF) diode sputtering in oblique-angle (80°) deposition arrangement are presented. Oblique-angle sputtering of ZnO:Ga (GZO) thin films resulted in a tilted columnar crystalline structure and inclination of the c-axis by an angle of approximately 9° with respect to the substrate. This improved their structural, electrical and optical properties in comparison with films deposited perpendicularly to the substrate. GZO films sputtered by an RF power of 600 W at room temperature of the substrate in Ar pressure 1.3 Pa showed strong crystalline (002) texture, lowest electrical resistivity 3.4 × 10^− 3 Ωcm, highest electron mobility 10 cm² V^− 1 s^− 1, high electron concentration 1.8 × 10²⁰ cm^− 3 and good optical transparency up to 88%. The small inclination angle of the film structure is caused by the high kinetic energy of sputtered species and additional energetic particle bombardment causes random surface diffusion, which is suppressing the shadow effect produced by oblique-angle sputtering.     

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.     

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.     

Thermally stable transparent conducting and highly infrared reflective Ga-doped ZnO thin films by metal organic chemical vapor deposition

Highly transparent conductive Ga-doped ZnO (GZO) thin films have been prepared on glass substrates by metal organic chemical vapor deposition. The effect of Ga doping on the structural, electrical and optical properties of GZO films has been systematically investigated. Under the optimum Ga doping concentration (∼4.9 at.%), c-axis textured GZO film with the lowest resistivity of 3.6 × 10⁻⁴ Ω cm and high visible transmittance of 90% has been achieved. The film also exhibits low transmittance (<1% at 2500 nm) and high reflectance (>70% at 2500 nm) to the infrared radiation. Furthermore, our developed GZO thin film can well retain the highly transparent conductive performance in oxidation ambient at elevated temperature (up to 500 °C).     

Half wave rectification of inorganic/organic heterojunction diode at the frequency of 1 kHz

An inorganic/organic vertical heterojunction diode has been demonstrated with p-type Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) deposited by spin coating on n-type Ga-doped ZnO (GZO) thin films. Transparent conducting GZO thin films are deposited on glass substrate by rf-magnetron sputtering. Electrical properties of GZO thin films are investigated depending on the processing temperatures. The resistivity, mobility and carrier concentration of the GZO thin films deposited at processing temperatures of 500 °C are measured to be about 3.6 × 10⁻⁴ Ω cm, 23.8 cm²/Vs and 7.1 × 10²⁰ cm³, respectively. The root mean square surface roughness of the GZO thin films is calculated to be ~ 0.9 nm using atomic force microscopy. Current-voltage characteristics of the n-GZO/p-PEDOT:PSS heterojunction diode present rectifying operation. Half wave rectification is observed with the maximum output voltage of 1.85 V at 1 kHz. Low turn-on voltage of about 1.3 V is obtained and the ideality factor of the n-GZO/p-PEDOT:PSS diode is derived to be about 1.8.     

Tungsten-doped ZnO transparent conducting films deposited by direct current magnetron sputtering

Highly conducting and transparent thin films of tungsten-doped ZnO (ZnO:W) were prepared on glass substrates by direct current (DC) magnetron sputtering at low temperature. The effect of film thickness on the structural, electrical and optical properties of ZnO:W films was investigated. All the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity first decreases with film thickness, and then increases with further increase in film thickness. The lowest resistivity achieved was 6.97 × 10⁻⁴ Ω cm for a thickness of 332 nm with a Hall mobility of 6.7 cm² V⁻¹ s⁻¹ and a carrier concentration of 1.35 × 10²¹ cm⁻³. However, the average transmittance of the films does not change much with an increase in film thickness, and all the deposited films show a high transmittance of approximately 90% in the visible range.     

Conductive Ga doped ZnO/Cu/Ga doped ZnO thin films prepared by magnetron sputtering at room temperature for flexible electronics

Ga doped ZnO(GZO)/Cu/GZO multilayers were deposited by magnetron sputtering on polycarbonate substrates at room temperature. We investigated the structural, electrical, and optical properties of multilayers at various thicknesses of Cu and GZO layers. The lowest resistivity value of 3.3 × 10^− 5 Ω cm with a carrier concentration of 2.9 × 10²² cm^− 3 was obtained at the optimum Cu (10 nm) and GZO (10 nm) layer thickness. The highest value of figure of merit φ_TC is 2.68 × 10^− 3 Ω^− 1 for the GZO (10 nm)/Cu(10 nm)/GZO(10 nm) multilayer. The highest average near infrared reflectivity in the wavelength range 1000-2500 nm is as high as 70% for the GZO(10 nm)/Cu(10 nm)/GZO(10 nm) multilayer.     

The properties of radio frequency sputtered transparent and conducting ZnO:F films on polyethylene naphthalate substrate

We report on the properties of ZnO:F films deposited by RF sputtering on polyethylene naphthalate (PEN) substrates and compared them with films deposited on glass. Detailed and systematic investigations of various properties of films were deposited on PEN substrates were carried out as functions of thickness and annealing ambient. The films were deposited at room temperature and annealed at 150 °C in either Ar or 7% H₂/Ar ambients. These films exhibited carrier concentrations between 2 × 10¹⁸/cm³ and 9.5 × 10¹⁹/cm³, mobility between 3 and 11 cm²/V-s, and resistivity between 10^− 1 and 10^− 2 Ω-cm. Hall mobility variation with concentration has been explained assuming ionized impurity and lattice scattering to be the dominant mechanisms. The transmission of the films varied from 68 to 80% with increasing thickness and the absorption edge was limited by the absorption of the PEN substrate. The mechanical flexibility of the films was measured in terms of its critical radius of bending which was determined from the onset of a sharp increase in electrical resistance. The critical radius varied between 6.5 and 17 mm for film thicknesses varying from 20 to 200 nm. The thickness dependence of critical strain and critical radius can be explained by Griffith defect theory.     

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.     

Effect of process parameters on the growth and properties of impurity-doped zinc oxide transparent conducting thin films by RF magnetron sputtering

Zinc oxide transparent conducting thin films co-doped with aluminum and ruthenium were grown on polyethylene terephthalate substrates at room temperature using RF magnetron sputtering. The crystal growth and physical properties of the films were investigated with respect to the variation of discharge power density from 1.5 to 6.1 W/cm² and sputtering pressure from 0.13 to 2.0 Pa. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) showed that the films grown with 3.6 W/cm² power density and sputtering pressure of 0.4 Pa had the best crystallinity and larger pyramid-like grains. The optimized electrical resistivity had a lowest measured value of about 9 × 10⁻⁴ Ω cm. The low carrier mobilities of the films (3-8.9 cm² V⁻¹ s⁻¹) have been discussed in terms of what is believed to be the dominant effect of ionized impurity scattering, but in addition chemisorption of oxygen on the film surface and effect of grain boundaries are also thought to be significant. The transmittances of the films in the visible range are greater than 80%, while the optical band gaps are in the order of 3.337-3.382 eV.     

Effects of substrate on the structural, electrical and optical properties of Al-doped ZnO films prepared by radio frequency magnetron sputtering

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited on substrates including alkali-free glass, quartz glass, Si, and SiO₂ buffer layer on alkali-free glass by using radio frequency magnetron sputtering. The effects of different substrates on the structural, electrical and optical properties of the AZO films were investigated. It was found that the crystal structures were remarkably influenced by the type of the substrates due to their different thermal expansion coefficients, lattice mismatch and flatness. The AZO film (100 nm in thickness) deposited on the quartz glass exhibited the best crystallinity, followed sequentially by those deposited on the Si, the SiO₂ buffer layer, and the alkali-free glass. The film deposited on the quartz glass showed the lowest resistivity of 5.14 × 10^− 4 Ω cm among all the films, a carrier concentration of 1.97 × 10²¹ cm^− 3 and a Hall mobility of 6.14 cm²/v·s. The average transmittance of this film was above 90% in the visible light spectrum range. Investigation into the thickness-dependence of the AZO films revealed that the crystallinity was improved with increasing thickness and decreasing surface roughness, accompanied with a decrease in the film resistivity.     

Study on In2O3-SnO2 transparent and conductive films prepared by d.c. sputtering using high density ceramic targets

The influence of SnO₂ concentration in the target on the film characteristics was studied in order to make the useful database for the device design when low discharge voltage sputtering method and a high density In₂O₃-SnO₂ ceramic targets were used. In the case of the films deposited on unheated substrate, X-ray diffraction profile showed amorphous structure. Minimum resistivity of 358 μΩ cm was obtained by In₂O₃ film with mobility of 40.1 cm² (V s)⁻¹ and carrier density of 4.35E+20 cm⁻³. With the increase of SnO₂ contents, resistivity of the films increased because of the decrease in both carrier density and mobility. Whereas, the films deposited on heated substrates showed polycrystalline structure. Resistivity was reduced, ranging from 5 to 20 wt.% SnO₂, and minimum resistivity of 136 μΩ cm was obtained at 15 wt.% with mobility of 40.5 cm² (V s)⁻¹ and carrier density of 1.13E+21 cm⁻³. Transmittance and reflectance of these films strongly depend on carrier density.     

Fabrication of low resistivity tin-doped indium oxide films with high electron carrier densities by a plasma sputtering method

Tin-doped indium oxide (ITO) films fabricated on glass substrates using a hot-cathode plasma sputtering method exhibited low resistivity of 9.7 × 10⁻⁵ Ω cm, which is due to a high carrier density of 2.1 × 10²¹ cm⁻³. The change in the number of carriers, N, as a function of film thickness d, strongly suggests that oxygen extraction in the initial stages of ITO film growth on the glass substrate surface, creates oxygen vacancies as an electron carrier source for improvement in the resistivity of the films.     

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.     

Effect of post-annealing on the optoelectronic properties of ZnO:Ga films prepared by pulsed direct current magnetron sputtering

In this study, highly conductive films of ZnO:Ga (GZO) were deposited by pulsed direct current magnetron sputtering to explore the effect of post-annealing on the structural, electrical and optical properties of the films. XRD patterns showed that after annealing, the intensity of c-axis preferentially oriented GZO (002) peak was apparently improved. GZO film annealing at 300 °C for 0.5 h exhibits lowest resistivity of 1.36 × 10^− 4 Ω cm. In addition, the film shows good optical transmittance of 88% with optical band gap, 3.82 eV. Carrier concentration and optical band gap both decreases with the annealing temperature. Besides, the near-infrared transmittance at 1400 nm is below 5%, while the reflectivity at 2400 nm is as high as 70%.     

Structural and optoelectronic properties of Al-doped zinc oxide films deposited on flexible substrates by radio frequency magnetron sputtering

Al-doped zinc oxide (AZO) thin films were deposited onto flexible polyethylene terephthalate substrates, using the radio frequency (RF) magnetron sputtering process, with an AZO ceramic target (The Al₂O₃ content was about 2 wt.%). The effects of the argon sputtering pressure (in the range from 0.66 to 2.0 Pa), thickness of the Al buffer layer (thickness of 2, 5, and 10 nm) and annealing in a vacuum (6.6 × 10^− 4 Pa), for 30 min at 120 °C, on the morphology and optoelectronic performances of AZO films were investigated. The resistivity was 9.22 × 10^− 3 Ω cm, carrier concentration was 4.64 × 10²¹ cm^− 3, Hall mobility was 2.68 cm²/V s and visible range transmittance was about 80%, at an argon sputtering pressure of 2.0 Pa and an RF power of 100 W. Using an Al buffer decreases the resistivity and optical transmittance of the AZO films. The crystalline and microstructure characteristics of the AZO films are improved by annealing.     

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.     

Properties of indium tin oxide films deposited on unheated polymer substrates by ion beam assisted deposition

The optical, electrical and mechanical properties of indium tin oxide (ITO) films prepared on polyethylene terephthalate (PET) substrates by ion beam assisted deposition at room temperature were investigated. The properties of ITO films can be improved by introducing a buffer layer of silicon dioxide (SiO₂) between the ITO film and the PET substrate. ITO films deposited on SiO₂-coated PET have better crystallinity, lower electrical resistivity, and improved resistance stability under bending than those deposited on bare PET. The average transmittance and the resistivity of ITO films deposited on SiO₂-coated PET are 85% and 0.90 × 10^− 3 Ω cm, respectively, and when the films are bent, the resistance remains almost constant until a bending radius of 1 cm and it increases slowly under a given bending radius with an increase of the bending cycles. The improved resistance stability of ITO films deposited on SiO₂-coated PET is mainly attributed to the perfect adhesion of ITO films induced by the SiO₂ buffer layer.     

Transparent conducting Al and Y codoped ZnO thin film deposited by DC sputtering

Transparent conducting Al and Y codoped zinc oxide (AZOY) thin films with high transparency and low resistivity were deposited by DC magnetron sputtering. The effects of substrate temperature on the structural, electrical and optical properties of AZOY thin films deposited on glass substrates have been investigated. X-ray diffraction spectra indicate that no diffraction peak of Al₂O₃ or Y₂O₃ except that of ZnO (0 0 2) is observed. The AZOY thin film prepared at substrate temperature of 250 °C has the optimal crystal quality inferring from FWHM of ZnO (0 0 2) diffraction peak, but the AZOY thin film deposited at 300 °C has the lowest resistivity of 3.6 × 10⁻⁴ Ω-cm, the highest mobility of 30.7 cm² V⁻¹ s⁻¹ and the highest carrier concentration of 5.6 × 10²⁰ cm⁻³. The films obtained have disorderly polyhedral surface morphology indicating possible application in thin film solar cell with good quality and high haze factor without the need of post-deposition etching.     

Study on Raman line at 1080.2 cm in ZnO thin films prepared under high RF power

ZnO thin films were prepared on glass or on homo-buffer/glass by a RF magnetron sputtering method at RF power of 100-550 W. The structural and Raman characteristics of the films were analyzed by X-ray diffraction and Raman scattering. There appeared a sharp peak of 1080.2 cm⁻¹ near the A₁(2LO) mode (1156 cm⁻¹) of ZnO in the Raman spectra when the RF power was higher than 300 W. In this case, the (100) peak of ZnO film appeared obviously. It was speculated that the Raman mode at 1080.2 cm⁻¹ was induced by the ordered distribution of Zn_i defects in ZnO lattice.