Synthesis and properties of close packed ZnO:Al submicro-rods deposited by pulsed laser ablation

Al-doped ZnO (AZO) semiconducting thin films consisting of perpendicularly aligned submicro-rods were deposited on silicon substrate by conventional pulsed laser ablation. No catalyst was used in this process. It was found that the rod structure can be grown at relatively high oxygen pressures (1-20 Torr) and relatively high substrate temperatures (550-700 °C). Low resistivity and high carrier concentration can be obtained in these Al-doped ZnO rods with relatively high Al concentrations. Increasing Al doping reduces the electric resistivity and increase carrier concentration. The photoluminescence property measurement indicates an increased UV emission with a small amount Al doping and reduced UV emission with further increase of Al concentration.     

On the effect of acetic acid on physical properties of chemically sprayed fluorine-doped ZnO thin films

Fluorine-doped zinc oxide (ZnO:F) thin films onto sodocalcic glass substrates, starting from a highly concentrated starting solution (0.4 M) containing zinc acetate and hydrofluoric acid diluted in a mixture of deionized water, acetic acid , and methanol, using the chemical spray deposition technique, were deposited and characterized. The effect of the acetic acid content in the starting solution, and the substrate temperature on the electrical resistivity, structure, morphology and optical characteristics was studied. The samples were polycrystalline in nature, but as the acetic acid content in the starting solution increases, the preferential orientation shows a switching from (002) to (101). For a predetermined deposition temperature, as the acetic acid content increases, the film resistivity values show an increase. The minimum resistivity in the order of 6 × 10^− 3 Ω cm was found for the films deposited with the lowest acetic acid content used. The surface morphology varies from agglomerated grains to rod-like shaped grains as a function of the acetic acid content.     

Stability of nano-thick transparent conducting oxide films for use in a moist environment

The stability of nano-thick transparent conducting oxide thin films in a high humidity environment was investigated. The stability of ITO and impurity-doped ZnO thin films prepared with a thickness in the range from approximately 20 to 100 nm on glass substrates at a temperature below 200 °C by a pulsed laser deposition was evaluated in air at a relative humidity of 90% and a temperature of 60 °C. The resistivity of all Al- and Ga-doped ZnO thin films tested was found to increase markedly with test time, whereas that of ITO remained relatively stable; the stability (resistivity increase) of the doped ZnO thin films was considerably affected by film thickness but was relatively independent of the deposition substrate temperature. In particular, doped ZnO thin films with a thickness below approximately 50 nm were very unstable under the test conditions. The resistivity increase of doped ZnO films is mainly attributed to the grain boundary scattering resulting from the adsorption of oxygen on the grain boundary.     

Effect of annealing temperature on ZnO:Al/p-Si heterojunctions

Al-doped, zinc oxide (ZnO:Al) films with a 1.2 at.% Al concentration were deposited on p-type silicon wafers using a sol-gel dip coating technique to produce a ZnO:Al/p-Si heterojunction. Following deposition and subsequent drying processes, the films were annealed in vacuum at five different temperatures between 550 and 900 °C for 1 h. The resistivity of the films decreased with increasing annealing temperature, and an annealing temperature of 700 °C provided controlled current flow through the ZnO:Al/p-Si heterojunction up to 20 V. The ZnO:Al film deposited on a p-type silicon wafer with 1.2 at.% Al concentration was concluded to have the potential for use in electronic devices as a diode after annealing at 700 °C.     

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.     

Optical and electrical properties of ZnO films, codoped with Al and Ga deposited at room temperature by an RF sputtering method

ZnO thin films, codoped with Al and Ga, were prepared on fused quartz (FQ) and cyclo-olefin polymer (COP) substrates using a radial frequency magnetron sputtering technique at room temperature, without the introducing of oxygen. The elemental distributions of Al, Ga, Zn and O throughout the films were found and no compositional variation in working pressure was observed. A resistivity of 0.03-4.07 Ω cm in AGZ/FQ films (Fig. 2b and 0.04-5.73 Ω cm in AGZ/COP films as well as a transmittance of above 85% were obtained by appropriate control of the working pressure. Compared with the band gap energy of single crystal ZnO, the band gap energy of the AGZ/FQ thin film was somewhat higher. The band gap energy of the AGZ/FQ films showed a tendency to increase with the working pressure employed.     

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.     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Preparation of aluminum doped zinc oxide films and the study of their microstructure, electrical and optical properties

Aluminum doped zinc oxide (AZO) polycrystalline thin films were prepared by sol-gel dip-coating process on optical glass substrates. Zinc acetate solutions of 0.5 M in isopropanol stabilized by diethanolamine and doped with a concentrated solution of aluminum nitrate in ethanol were used. The content of aluminum in the sol was varied from 1 to 3 at.%. Crystalline ZnO thin films were obtained following an annealing process at temperatures between 300 °C and 500 °C for 1 h. The coatings have been characterized by X-ray diffraction, UV-Visible spectrophotometry, scanning electron microscopy, and electrical resistance measurement. The ZnO:Al thin films are transparent (∼ 90%) in near ultraviolet and visible regions. With the annealing temperature increasing from 300 °C to 500 °C, the film was oriented more preferentially along the (0 0 2) direction, the grain size of the film increased, the transmittance also became higher and the electrical resistivity decreased. The X-ray diffraction analysis revealed single-phase ZnO hexagonal wurtzite structure. The best conductors were obtained for the AZO films containing 1 at.% of Al, annealed at 500 °C, 780 nm film thickness.     

ZnO thin films prepared by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by pulsed laser deposition (PLD) in an oxygen-reactive atmosphere. The structural, optical, and electrical properties of the as-prepared thin films were studied in dependence of substrate temperature and oxygen pressure. High quality polycrystalline ZnO films with hexagonal wurtzite structure were deposited at substrate temperatures of 100 and 300 °C. The RMS roughness of the deposited oxide films was found to be in the range 2-9 nm and was only slightly dependent on substrate temperature and oxygen pressure. Electrical measurements indicated a decrease of film resistivity with the increase of substrate temperature and the decrease of oxygen pressure. The ZnO films exhibited high transmittance of 90% and their energy band gap and thickness were in the range 3.26-3.30 eV and 256-627 nm, respectively.     

Influence of the molar concentration and substrate temperature on fluorine-doped zinc oxide thin films chemically sprayed

The effect of both the molar concentration of the starting solution and the substrate temperature on the electrical, morphological, structural and optical properties of chemically sprayed fluorine-doped zinc oxide (ZnO:F) thin films deposited on glass substrates is analyzed in this work. All the starting solutions employed were aged for 10 days before the deposition. The results show that as the molar concentration increases, a decrease in the electrical resistivity values is obtained, reaching the minimum resistivity in films ZnO:F deposited from a 0.4 M solution at 500 °C. A further increase in the molar concentration leads to a very slight increase in the resistivity. On the other hand, as the substrate temperature is increased, the resistivity decreases and a tendency towards to minimum value is evidenced; taking the molar concentration as parameter, minimum values are reached at 500 °C. The obtaining of ZnO:F thin films, with a resistivity as low as 7.8 × 10^− 3 Ω cm (sheet resistance of 130 Ω/□ and film thickness of 600 nm) measured in as-deposited films is reported here for the first time. The concurrent effect of the high molar concentration of the starting solution, the substrate temperature values used, and the ageing of the starting solution, which might cause polymerization of the zinc ions with the fluorine species, enhance the electrical properties. The structure of the films is polycrystalline, with a (002) preferential growth. Molar concentration rules the surface morphology as at low concentration an hexagonal and porous structure is developed changing to a uniform compact and small grain size surface in the films deposited with the high molar concentrations.     

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.     

Novel properties of AZO film sputtered in Ar+H2 ambient at high temperature

AZO (ZnO:Al) transparent conductive thin film was prepared by RF magnetron sputtering with a AZO (98 wt% ZnO 2 wt% Al₂O₃) ceramic target in the same Ar+H₂ ambient at different substrate temperatures ranging from 100 to 300 °C. The minimum resistivity of AZO films was 7.9×10⁻⁴ Ω cm at the substrate temperature of 200 °C. The average transmission in the visible rang was more than 90%. Scanning electron microscopy and XRD analyses showed that the surface morphology of the AZO samples altered with the increasing of the substrate temperature. AZO film prepared at 200 °C in the pure Ar ambient was also made as comparison about the resistivity, carrier concentration and the average crystallite size. The resistivity became about 3 times higher. The carrier concentration became lower and the average crystallite size was smaller.     

Influence of working pressure on ZnO:Al films from tube targets for silicon thin film solar cells

Mid-frequency magnetron sputtering of aluminum doped zinc oxide films (ZnO:Al) from tube ceramic targets has been investigated for silicon based thin film solar cell applications. The influence of working pressure on structural, electrical, and optical properties of sputtered ZnO:Al films was studied. ZnO:Al thin films with a minimum resistivity of 3.4 × 10⁻⁴ Ω cm, high mobility of 50 cm²/Vs, and high optical transmission close to 90% in visible spectrum region were achieved. The surface texture of ZnO:Al films after a chemical etching step was investigated. A gradual increase in feature sizes (diameter and depth) was observed with increasing sputter pressure. Silicon based thin film solar cells were prepared using the etched ZnO:Al films as front contacts. Energy conversion efficiencies of up to 10.2% were obtained for amorphous/microcrystalline silicon tandem solar cells.     

Structures and properties of the Al-doped ZnO thin films prepared by radio frequency magnetron sputtering

Al-doped ZnO thin films were deposited by radio frequency magnetron sputtering using a ZnO target with 2 wt.% Al₂O₃. The structures and properties of the films were characterized by the thin film X-ray diffraction, high resolution transmission electron microscopy, Hall system and ultraviolet/visible/near-infrared spectrophotometer. The Al-doped ZnO film with high crystalline quality and good properties was obtained at the sputtering power of 100 W, working pressure of 0.3 Pa and substrate temperature of 250 °C. The results of further structure analysis show that the interplanar spacings d are enlarged in other directions besides the direction perpendicular to the substrate. Apart from the film stress, the doping concentration and the doping site of Al play an important role in the variation of lattice parameters. When the doping concentration of Al is more than 1.5 wt.%, part of Al atoms are incorporated in the interstitial site, which leads to the increase of lattice parameters. This viewpoint is also proved by the first principle calculations.     

Room temperature deposition and properties of ZnO:Al thin films by nonreactive DC magnetron sputtering

Transparent and conductive ZnO:Al (AZO) thin films were prepared at room temperature by nonreactive DC magnetron sputtering from ceramic ZnO:Al targets. The effects of Al doping level and argon gas pressure on microstructure, growth behavior, resistivity and transmittance of AZO thin films were investigated. The experimental results show that AZO thin films change from polycrystalline to preferred c-axis-orientation just at high argon gas pressure. The resistivity of AZO films first decreases with the increase of Al content under 3 wt%, then increases when the Al content is over 3 wt%. The argon gas pressure also effect on the resistivity of AZO thin films due to the change of dispersion related the grain and crystal boundary. When the argon gas pressure from 0.6 to 3.0 Pa, the resistivity of AZO films decreases to a lowest resistivity of 1.4 × 10⁻³ Ω cm when the argon gas pressure is 1.5 Pa, then increases gently. The Al content and argon gas pressure had a little influence on transmittance, and the average optical transmittances of AZO thin films were from 86% to 90%, but the absorption edge has a blue shift with the increase of doping level and argon gas pressure.     

Silicon metal-semiconductor-metal photodetector with zinc oxide transparent conducting electrodes

Transparent conducting oxides thin layers, due to their optical and electrical properties, can be used as transparent electrodes in various optoelectronic devices. We present a metal-semiconductor-metal photodiode (MSM-PD) on silicon as optically active layer with zinc oxide (ZnO) thin layer as interdigitated Schottky transparent electrodes. The advantage of using a ZnO thin layer as Schottky electrodes consists in the improvement of the photoresponse by eliminating the shadowing of the active area by opaque metallic electrodes. ZnO thin layers were deposited on 10 Ω cm resistivity silicon epitaxial wafers by the vacuum thermal evaporation method. High purity metallic powders were mixed with an (Al + Sn)/Zn ratio of 0.03. In order to obtain transparent layers the metallic depositions were thermally treated at 450 °C for 2 h. The Al, Sn co-doped ZnO layers of 0.5-0.8 μm were investigated regarding structural, optical and electrical properties and surface morphology. The obtained thin layers have a high transparency (T > 85%) over a large spectral range and the resistivity is quite low, ρ ~ 10^− 4 Ω cm. The interdigitated Schottky contacts of ZnO were configurated onto the optically active Si layer providing an MSM-PD structure of 0.143 mm² active area and finger spacing and finger width of 6 μm. The optoelectronic characteristics were measured and the Schottky barrier height of 0.62 eV was determined from the current-voltage characteristic. A responsivity of 0.2 A/W at 475 nm and a capacitance of 1.4 pF at 10 V bias were obtained for the MSM-PD structure with transparent conducting ZnO Schottky electrodes.     

Structural, electrical and optical properties of ZnO:Al films deposited on flexible organic substrates for solar cell applications

Aluminum doped ZnO thin films (ZnO:Al) were deposited on glass and poly carbonate (PC) substrate by r.f. magnetron sputtering. In addition, the electrical, optical properties of the films prepared at various sputtering powers were investigated. The XRD measurements revealed that all of the obtained films were polycrystalline with the hexagonal structure and had a preferred orientation with the c-axis perpendicular to the substrate. The ZnO:Al films were increasingly dark gray colored as the sputter power increased, resulting in the loss of transmittance. High quality films with the resistivity as low as 9.7 × 10^− 4 Ω-cm and transmittance over 90% have been obtained by suitably controlling the r.f. power.     

Effects of sputtering pressure and Al buffer layer thickness on properties of AZO films grown by rf magnetron sputtering

Transparent and conductive Al-doped (2 wt.%) zinc oxide (AZO) films were deposited on inexpensive soda-lime glass substrates by using rf magnetron sputtering at room temperature. This study analyzed the effects of argon sputtering pressure, which varied in the range from 0.46 to 2.0 Pa, on the morphological, electrical and optical properties of AZO films. The only (0 0 2) diffraction peak of the film were observed at 2θ～34.45°, exhibiting that the AZO films had hexagonal ZnO wurtzite structure, and a preferred orientation with the c-axis perpendicular to the substrate. By applying a very thin aluminum buffer layer with the thickness of 2 nm, findings show that the electrical resistivity was 9.46 × 10⁻⁴ Ω-cm, and the average optical transmittance in the visible part of the spectra was approximately 81%. Furthermore, as for 10 nm thick buffer layer, the electrical resistivity was lower, but the transmittance was decreased.     

Structural,electrical and photoluminescence properties of ZnO:Al films grown on MgO(0 0 1) by direct current magnetron sputtering with the oblique target

ZnO:Al films were deposited on MgO(0 0 1) substrates at 300 K and 673 K by direct current magnetron sputtering with the oblique target. The Ar pressure was adjusted to 0.4 Pa and 1.2 Pa, respectively. All the films have a wurtzite structure and a c-axis orientation in the film growth direction. The films deposited at 300 K initially grow with thin columnar grains and subsequently grow with large granular grains on the thin columnar grains. However, the films grown at 673 K consist mainly of dense columnar grains perpendicular to the substrate surface. The ZnO:Al film deposited at 673 K and 0.4 Pa has the lowest resistivity, the highest free electron concentration and Hall's mobility. A temperature dependence of the resistivity within 5–300 K reveals that the films grown at 300 K exhibit a semiconducting behavior and those grown at 673 K show a metal–semiconductor transition. The carrier transport mechanism is Mott's variable range hopping in the temperature range below 90 K for all the films and thermally activated band conduction above 215 K for the films grown at 300 K. Room temperature photoluminescence spectra for wavelengths between 300 nm and 800 nm reveal mainly blue-green emissions centered at 452 nm, 475 nm and 515 nm.