Effect of thickness on the structural,optical and electrical properties of RF magnetron sputtered GZO thin films

Gallium-doped zinc oxide (GZO) thin films with very high conductivity and transparency were successfully deposited by RF magnetron sputtering at a substrate temperature of 400 °C. The dependence of the film properties over the thickness was investigated. X-ray diffraction (XRD) results revealed the polycrystalline nature of the films with hexagonal wurtzite structure having preferential orientation along [001] direction normal to the substrate. The lowest resistivity obtained from electrical studies was 5.4×10⁻⁴ Ω cm. The optical properties were studied using a UV–vis spectrophotometer and the average transmittance in the visible region (400–700 nm) was found to be 92%, relative to the transmittance of a soda–lime glass reference for a GZO film of thickness 495 nm and also the transparency of the films decreases in the near IR region of the spectra. The mobility of the films showed a linear dependence with crystallite size. GZO film of thickness 495 nm with the highest figure of merit indicates that the GZO film is suitable as an ideal transparent conducting oxide (TCO) material for solar cell applications.     

Influence of thermal annealing on electrical and optical properties of indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) thin films with the thickness of 300 nm were deposited on quartz substrates via electron beam evaporation, and five of them post-annealed in air atmosphere for 10 min at five selected temperature points from 200 °C to 600 °C, respectively. An UV–vis spectrophotometer and Hall measurement system were adopted to characterize the ITO thin films. Influence of thermal annealing in air atmosphere on electrical and optical properties was investigated in detail. The sheet resistance reached the minimum of 6.67 Ω/sq after annealed at 300 °C. It increased dramatically at even higher annealing temperature. The mean transmittance over the range from 400 nm to 800 nm reached the maximum of 89.03% after annealed at 400 °C, and the figure of merit reached the maximum of 17.79 (Unit: 10⁻³ Ω⁻¹) under the same annealing condition. With the annealing temperature increased from 400 °C to 600 °C, the variations of transmittance were negligible, but the figure of merit decreased significantly due to the deterioration of electrical conductivity. With increasing the annealing temperature, the absorption edge shifted towards longer wavelength. It could be explained on the basis of Burstein–Moss shift. The values of optical band gap varied in the range of 3.866–4.392 eV.     

Fabrication of Ag nanoparticle/ZnO thin films using dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with photoreduction method and its application

We report a novel method to grow silver nanoparticle/zinc oxide (Ag NP/ZnO) thin films using a dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with a photoreduction method. The crystalline quality, optical properties, and electrical characteristics of Ag NP/ZnO thin films depend on the AgNO₃ concentration or Ag content and annealing temperature. Optimal Ag NP/ZnO thin films have been grown with a AgNO₃ concentration of 0.12 M or 2.54 at%- Ag content and 500 °C- rapid thermal annealing (RTA); these films show orientation peaks of hexagonal-wurtzite-structured ZnO (002) and face-center-cubic-crystalline Ag (111), respectively. The transmittance and resistivity for optimal Ag NP/ZnO thin films are 85% and 6.9×10⁻⁴ Ω cm. Some Ag NP/ZnO transparent conducting oxide (TCO) films were applied to InGaN/GaN LEDs as transparent conductive layers. The InGaN/GaN LEDs with optimal Ag NP/ZnO TCO films showed electric and optical performance levels similar to those of devices fabricated with indium tin oxide.     

Ultraviolet-assisted annealing for low-temperature solution-processed p-type gallium tin oxide (GTO) transparent semiconductor thin films

Solution-processed p-type gallium tin oxide (GTO) transparent semiconductor thin films were prepared at a low temperature of 300 °C using ultraviolet (UV)-assisted annealing instead of conventional high-temperature annealing (> 500 °C). We report the effects of UV irradiation time on the structural, optical, and electrical properties of sol-gel derived GTO thin films and a comparison study of the physical properties of UV-assisted annealed (UVA) and conventional thermally annealed (CTA) GTO thin films. The Ga doping content was fixed at 15 at% in the precursor solution ([Ga]/[Sn]+[Ga] = 15%). After a spin-coating and preheating procedure was performed two times, the dried sol-gel films were heated on a hotplate at 300 °C under UV light irradiation for 1–4 h. Each UVA GTO thin film had a dense microstructure and flat free surface and exhibited an average optical transmittance approaching 85.0%. The level of crystallinity, crystallite size, and hole concentration density of the GTO thin films increased with increasing UV irradiation time. In this study, the UVA 4 h thin film samples exhibited the highest hole concentration (9.87 × 10¹⁷ cm⁻³) and the lowest resistivity (1.8 Ω cm) and had a hole mobility of 5.1 cm²/Vs.     

Epitaxial growth of Ge thin film on Si (001) by DC magnetron sputtering

We have demonstrated that sub-10 nm-thick heteroepitaxial Ge films on Si (001) having smooth surfaces can be obtained by DC magnetron sputtering. Ge films grown at 350 °C preserve the smooth surfaces with a roughness root mean square (RMS) of 0.39 nm, whereas, the Ge films grown at 500 °C show significant roughness with an island-like morphology. In samples grown at 350 °C, it is confirmed that the Ge films are grown epitaxially by cross-section transmission electron microscopy (TEM) and X-ray diffraction (XRD) rocking curve measurements. Rapid thermal annealing (RTA) at 720 °C is effective in improving the crystalline quality and the degradation in the roughness is negligible. Raman spectra and an XRD reciprocal space map reveal that the epitaxial Ge grown at 350 °C show an in-plane compressive strain and that the strain continues to remain after a 720 °C RTA.     

Effect of co-doping concentration on structural,morphological, optical and electrical properties of aluminium and indium co-doped ZnO thin films deposited by ultrasonic spray pyrolysis

In this work, we reported a chemical approach to prepare aluminium and indium co-doped ZnO thin films (AIZO) by ultrasonic spray pyrolysis. Film depositions were carried out on soda lime glass substrates at 425 °C by using a spray solution containing zinc acetate as zinc precursor, aluminium acetylacetonate as Al dopant source and indium (III) acetate as In dopant source. Physical properties such as structural, morphological, optical and electrical properties were studied with respect to the equal variations in co-dopants concentration (0.5–3 at%). X-ray diffraction patterns proved that films are poly crystalline with (002) preferential orientation. Scanning electron microscopy analysis showed that AIZO films grown like hexagonal nanopyramids, elongated grains and irregular trigonals. Optical transmittance ~85% and a minimum resistivity of 1.3×10⁻³ Ω cm, are achieved for films when co-doped with 1.5 at% of Al and 1.5 at% of In, confirm that AIZO films are suitable for transparent conductive oxide (TCO) applications.     

Study on the adhesive mechanism between the Ga doped ZnO thin film and the polycarbonate substrate

The Ga doped ZnO (GZO) film was deposited on the polymer substrate at room temperature by magnetron sputtering. The resistivity is 8.9×10⁻⁴ Ω cm. The average transmittance in the visible region is over 85%. According to the resistivity and transmittance in the visible light, it is obtained that the film exhibits excellent electrical and optical properties, which satisfies the application for optoelectronic devices. However, the adhesion between the film and the polymer substrate is very weak. In order to figure out the reason of the weak adhesion, we study the adhesive mechanism between the GZO film and the polymer substrate through using depth profiling XPS method, residual stress test, and SIMS method for the first time. The residual stress of the film is a compressive stress. According to the SIMS results, an element diffusion exists at the interface. However, according to the depth profiling XPS results, there is no chemical bonding between the GZO film and the polymer substrate.     

Effects of oxygen concentration on the properties of Al-doped ZnO transparent conductive films deposited by pulsed DC magnetron sputtering

The effect of oxygen concentration on the properties of Al-doped ZnO (AZO) transparent conductive films has been investigated on the films deposited by pulsed DC magnetron sputtering using a cylindrical ZnO target containing 2 wt% Al. AZO films were deposited at 230 °C to the thickness of about 1000 nm and the oxygen concentration was controlled by varying the O₂/Ar supply ratio from 0 to 0.167. With the increasing O₂/Ar ratio, crystallinity of the AZO films deteriorated while the film surface became smooth. Accompanying this, electrical properties also deteriorated significantly. When the O₂/Ar ratios were 0 and 0.033, the AZO films showed metallic conduction behavior with the electrical resistivity in the mid 10^?4 Ω cm range. However, when the ratios were 0.100 and 0.167, the films showed poor electrical conduction behavior similar to semiconductors as deduced from the transmittance behavior. Spectroscopic analysis showed that such deteriorating properties are due to the formation of condensed oxide group through the reaction between excess oxygen and dopant aluminum.     

Effect of NaOH solution on surface textured ZnO: Al films prepared by pulsed direct current magnetron sputtering

Transparent conducting Al-doped ZnO (ZnO:Al, AZO) thin films were prepared at substrate temperature of 270 °C by pulsed direct current magnetron sputtering. NaOH solution (5 wt%) was employed to etch the AZO films at room temperature, and the surface textured AZO films were obtained successfully. The relationship between the surface textured structures and the etching process controlled by etching time was discussed. The textured morphology of the etched AZO films became clear as increasing the etching time, and the AZO film etched for 30 min exhibited uniformly and distinctly crater-like surface textured structure. Correspondingly, the haze and the resistivity increased with the increasing etching time. And the resistivity of the AZO film etched for 30 min was 3.2×10⁻³ Ω cm.     

Properties of CuAlO2 thin films deposited by polyacrylamide gel route

CuAlO₂ films were deposited on clean glass substrates by the acrylamide sol–gel dip coating technique. The coated films were dried in air oven for 30 min followed by heat treatment in air at different temperatures in the range of 350–500 °C. The films annealed at low temperatures exhibited weak x-ray diffraction (XRD) peaks. As the post anneal temperature increased beyond 375 °C, the XRD pattern exhibited the diffraction peaks of rhombohedral CuAlO₂. Surface morphology of the films indicated that the films annealed at low temperatures exhibit small grains. As the annealing temperature increases larger grains are observed. The root mean square (rms) value of the surface roughness increases with annealing temperature. The films exhibited optical transmission above 75%. The films post annealed at low temperature exhibited lower transmission. Optical band gap in the range of 3.43–3.75 eV was obtained for the films annealed at different temperature. Hall measurements indicated p-type conductivity. Resistivity of the films decreased from 25.0 to 2.0 Ω cm as the anneal temperature increased. Mobility and carrier density increased with annealing temperature.     

Structural and electrical studies of Cu-doped CdO prepared by solid state reaction

This paper reports synthesis, crystal structure and electrical properties of Cu-doped CdO (CdO:Cu) powders. X-ray diffraction shows that majority of the samples are monophase and has the cubic structure. The limit solubility of Cu ions in CdO lattice is found to be 2 mol% (after heating at 900 °C), whereby the impurity phase was determined to be the monoclinic-CuO. For monophase CdO:Cu samples synthesized at 900 °C, the lattice parameter decreased with increasing Cu concentration. Electrical conductivity of undoped CdO and 2 mol% Cu-doped CdO (after heating at 900 °C) were found to be 79 and 191 Ω^?1 cm^?1, respectively, at 100 °C and 912 and 1549 Ω^?1 cm^?1, respectively, at 900 °C. Thus, it appears that electrical conductivity slightly increases with Cu doping. Finally, the activation energy of monophase CdO:Cu (after heating at 900 °C) is shown to decrease with Cu concentration.     

Top illuminated organic photodetectors with dielectric/metal/dielectric transparent anode

The top illuminated organic photodetectors (OPDs) with a Dielectric/Metal/Dielectric (DMD) transparent anode are fabricated. The transparent electrode is composed of molybdenum trioxide (MoO₃)/silver (Ag)/MoO₃ layers and zinc oxide (ZnO)/aluminum (Al) is used for bottom cathode. The optimized DMD electrode has an optical transmittance of 85.7% at the wavelength of 546 nm and sheet resistance of ∼6 Ω/sq. The fabricated OPDs exhibit a high detectivity and wide range linearity.     

Properties of fluorine-doped SnO2 thin films by a green sol–gel method

Fluorine doped tin oxide (FTO) films were fabricated on a glass substrate by a green sol–gel dip-coating process. Non-toxic SnF₂ was used as fluorine source to replace toxic HF or NH₄F. Effect of SnF₂ content, 0–10 mol%, on structure, electrical resistivity, and optical transmittance of the films were investigated using X-ray diffraction, Hall effect measurements, and UV–vis spectra. Structural analysis revealed that the films are polycrystalline with a tetragonal crystal structure. Grain size varies from 43 to 21 nm with increasing fluorine concentration, which in fact critically impacts resultant electrical and optical properties. The 500 °C-annealed FTO film containing 6 mol% SnF₂ shows the lowest electrical resistivity 7.0×10⁻⁴ Ω cm, carrier concentration 1.1×10²¹ cm⁻³, Hall mobility 8.1 cm²V⁻¹ s⁻¹, optical transmittance 90.1% and optical band-gap 3.91 eV. The 6 mol% SnF₂ added film has the highest figure of merit 2.43×10⁻² Ω⁻¹ which is four times higher than that of un-doped FTO films. Because of the promising electrical and optical properties, F-doped thin films prepared by this green process are well-suited for use in all aspects of transparent conducting oxide.     

Ultrafast near-infrared curing of PEDOT:PSS

In the printed electronics industry, in order to produce conducting layers of suitably low resistance (<0.015 Ω cm) PEDOT:PSS solutions are attractive as transparent conductors. The wet film is currently heated using conventional convection ovens at temperatures of 120–140 °C for several minutes. Near infrared (NIR) radiation curing is shown to reduce the minimum drying time from 240 s in a conventional oven (giving 0.014 Ω cm) to 2 s (giving 0.011 Ω cm). Here we show it is the NIR absorbance of the PEDOT:PSS itself that gives rise to the rapid curing and this limits the energy density of NIR used.     

Evaporated iron disulfide thin films with sulfurated annealing treatments

FeS₂ thin films were grown on a glass substrate using a physical vapor deposition technique at room temperature. Subsequently, the thin films were annealed in two different atmospheres: vacuum and vacuum-sulfur. In the vacuum-sulfur atmosphere a graphite box was used as sulfur container and the films were sulfurated successfully at 200–350 ºC. It was found that annealing in a vacuum-sulfur atmosphere was indispensable in order to obtain polycrystalline FeS₂ thin films. The polycrystalline nature and pure phase were determined by XRD and Raman techniques and the electrical properties by the Hall effect. Using the sulfurating technique, the n-type semiconductor was prepared at 200–350 °C and a p-type at 500 °C. The carrier concentrations were between 1.19×10²⁰ and 2.1×10²⁰ cm⁻³. The mobility was 9.96–5.25 cm² V⁻¹ s⁻¹ and the resistivity was 6.31×10⁻² to 1.089×10⁻² Ω cm. The results obtained from EDS showed that the films prepared in the vacuum-sulfur atmosphere were close to stoichiometric and that the indirect band gap varied between 1.03 and 0.945 eV.     

Effect of substrate temperature on the properties of ZnO thin films prepared by spray pyrolysis

Sprayed ZnO films were grown on glass at different substrate temperatures from 200 °C to 500 °C and their structural, optical and electrical properties were investigated. All films are polycrystalline with hexagonal wurtzite structure. ZnO films at substrate temperatures above 400 °C appear to be better crystalized with (002) plane as preferential orientation. Optical transmission spectrum shows that ZnO films have high transmission (above 80%) in visible region for substrate temperatures above 400 °C. Photoluminescence spectra at room temperature show an ultraviolet emission and two visible emissions at 2.82 eV and 2.37 eV. The resistivity of ZnO films increases with increasing substrate temperatures (above 400 °C). The ZnO film deposited at 400 °C shows highest figure of merit.     

Characteristics of Ga-doped ZnO films deposited by pulsed DC magnetron sputtering at low temperature

Characteristics of Ga-doped ZnO (GZO) transparent conductive oxide films have been investigated based on the absorption behavior and chemical states of dopant Ga in the film. GZO samples were prepared by pulsed DC magnetron sputtering at 423 K by varying the sputtering power from 0.6 to 2.4 kW and the Ga₂O₃ concentration in the targets from 0.6 to 5.7 wt%. Absorption spectra of the GZO films in the visible to ultraviolet range were characterized by long absorption tails and shoulders near the absorption edges indicating the presence of impurity states or bands that overlap with the conduction band. X-ray photoelectron spectroscopy and X-ray diffraction revealed that substantial portion of the dopant exists as finely dispersed or amorphous metallic Ga and oxide of Ga, which would be related to the formation of the impurity bands or states, especially in the samples with lower Ga content. Presence of these species is correlated to the limited doping efficiency observed in the GZO films.     

Properties of In2O3 films obtained by thermal oxidation of sprayed In2S3

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using indium chloride and thiourea as precursors at a molar ratio of S:In=2.5. The deposition was carried out at 350 °C on quartz substrates. The film thickness is about 1 µm. The films were then annealed for 2 h at 550, 600, 650 and 700 °C in oxygen flow. This process allows the transformation of nanocrystal In₂O₃ from In₂S₃ and the reaction is complete at 600 °C. X-ray diffraction spectra show that In₂O₃ films are polycrystalline with a cubic phase and preferentially oriented towards (222). The film grain size increases from 19 to 25 nm and RMS values increase from 9 to 30 nm. In₂O₃ films exhibit transparency over 70–85% in the visible and infrared regions due to the thickness and crystalline properties of the films. The optical band gap is found to vary in the range 3.87–3.95 eV for direct transitions. Hall effect measurements at room temperature show that resistivity is decreased from 117 to 27 Ω cm. A carrier concentration of 1×10¹⁶ cm^?3 and mobility of about 117 cm² V^?1 s^?1 are obtained at 700 °C.     

Transparent conductors with an ultrathin nickel layer for high-performance photoelectric device applications

A thin nickel (Ni) layer of thickness 5 nm was inserted in between the indium tin oxide (ITO) layers of thickness 50 nm each so as to increase the conductivity of ITO without affecting much of its transmittance nature. ITO layers with and without Ni film were prepared by reactive DC sputtering on both Si and glass substrates. The influence of Ni layer on the optical and electrical properties of prepared devices was investigated. Due to the insertion of thin Ni layer, the resistivity of ITO/Ni/ITO sample (3.2×10⁻⁴ Ω cm) was reduced 10 times lesser than that of ordinary ITO layer (38.6×10⁻⁴ Ω cm); consequently it increased the mobility of ITO/Ni/ITO device. The external and internal quantum efficiencies (EQE and IQE) of ITO/Ni/ITO device exhibited better performance when compared to ITO layer that has no Ni film. At wavelengths of 350 and 600 nm, the photoresponses of ITO/Ni/ITO device were predominant than that of reference ITO device. This highly conductive and photoresponsive Ni inserting ITO layers would be a promising device for various photoelectric applications.     

Effect of substrate temperature on structural,morphological, optical and electrical properties of MnIn2S4 thin films prepared by nebulizer spray pyrolysis technique

Manganese indium sulphide (MnIn₂S₄) thin films were deposited using an aqueous solution of MnCl₂, InCl₃ and (NH₂)₂CS in the molar ratio 1:2:4 by simple chemical spray pyrolysis technique. The thin film substrates were annealed in the temperature range between 250 and 350 °C to study their various physical properties. The structural properties as studied by X-ray diffraction showed that MnIn₂S₄ thin films have cubic spinel structure. The formation of cube and needle shaped grains was clearly observed from FE-SEM analysis. The energy dispersive spectrum (EDS) predicts the presence of Mn, In and S in the synthesized thin film. From the optical studies, it is analyzed that the maximum absorption co-efficient is in the order between 10⁴ and 10⁵ cm⁻¹ and the maximum transmittance (75%) was noted in the visible and infrared regions. It is noted that, the band gap energy decreases (from 3.20 to 2.77 eV) with an increase of substrate temperature (from 250 to 350 °C). The observations from photoluminescence studies confirm the emission of blue, green, yellow and red bands which corresponds to the wavelength range 370–680 nm. Moreover, from the electrical studies, it is observed that, as the substrate temperature increases the conductivity also increases in the range 0.29–0.41×10⁻⁴ Ω⁻¹ m⁻¹. This confirms the highly semiconducting nature of the film. The thickness of the films was also measured and the values ranged between 537 nm (250 °C) to 483 nm (350 °C). This indicates that, as the substrate temperature increases, the thickness of the film decreases. From the present study, it is reported that the MnIn₂S₄ thin films are polycrystalline in nature and can be used as a suitable ternary semiconductor material for photovoltaic applications.