ZAO Films Research Articles

Tuning the optical, electrical resistivity and structural properties of DC magnetron sputtered aluminum zinc oxide films by changing the oxygen flow rate

Thin films of aluminum zinc oxide (AZO) were fabricated by DC reactive magnetron sputtering from 50 wt% Al/Zn metallic target. The structure and optical constants of AZO films were controlled by changing the oxygen (O2) flow rate. X-Ray diffraction revealed that the addition of oxygen transforms the crystalline nature of the metallic Al/Zn to completely amorphous film at 4.5 sccm. The film density, surface roughness and deposition rate were evaluated from X-ray reflectometry measurements and they were found to decrease strongly upon transformation from metallic to oxidic sputtering mode. The optical constants were extracted from the ellipsometry measurements. As O2 flow increased from 4.5 to 30 sccm, the optical band gap increased from 4.74 to 5.33 eV whereas the refractive index decreased simultaneously. Low resistivity films were obtained as O2 flow increased from 0 to 4 sccm whereas insulating films were obtained for O2 flows above 4 sccm. The results indicated that by regulating the oxygen flow diverse ZAO films with diverse chemical stoichiometries and properties can be tuned.

Stability enhancement of low temperature thin-film transistors with atomic-layer-deposited ZnO:Al channels

The electrical characteristics of TFTs with atomic-layer-deposited ZnO:Al (ZAO) channels have been studied in this work. By increasing Al doping concentration, the ZAO film changes from polycrystalline to amorphous, and its bandgap widens as well. With post-annealing at 200°C, the superior electrical stabilities under illumination and gate bias stress were achieved in ZAO TFTs compared with ZnO TFTs. For the strong immunity to illumination in ZAO TFTs, it is attributed to the widening bandgap of channel material for the reduction of the carrier concentration. While for the improved electrical stability under positive bias stress, it is mainly due to the suppression of interactions between the amorphous channel and the surrounding ambient, which is verified by the observations in N2 ambient.

Improved visible transparency of SIO2/ZNO:AL /CEO2-TIO2/SIO2 multilayer films with high UV absorption and infrared reflection rate

New visible transparent, UV absorption, and high infrared reflection properties have been realized by depositing multilayer SiO2/ZnO: Al/CeO2-TiO2/SiO2 films onto glass substrates at low temperature by radio frequency magnetron sputtering. Optimum thickness of SiO2, ZnO: Al (ZAO) and CeO2-TiO2 (CTO) films were designed with the aid of thin film design software. The degree of antireflection can be controlled by adjusting the thickness and refractive index. The outer SiO2 film can diminish the interference coloring and increase the transparency; the inner SiO2 film improves the adhesion of the coating on the glass substrate and prevents Ca2+, Na+ in the glass substrate from entering the ZAO film. The average transmittance in the visible light range increases by nearly 18%-20%, as compared to double layer ZAO/CTO films. And the films display high infrared reflection rate of above 75% in the wavelength range of 10-25 μm and good UV absorption (> 98%) properties. These systems are easy to produce on a large scale at low cost and exhibit high mechanical and chemical durability. The triple functional films with high UV absorption, antireflective and high infrared reflection rate will adapt to application in flat panel display and architectural coating glass, automotive glass, with diminishing light pollution as well as decreasing eye fatigue and increasing comfort.

Metamorphosis of strain/stress on optical band gap energy of ZAO thin films via manipulation of thermal annealing process

We report on the growth of Al-doped ZnO (ZAO) thin films prepared by the sol–gel technique associated with dip-coating onto Corning 7740 glass substrates. The influence of varying thermal annealing (Ta) temperature on crystallisation behaviour, optical and electrical properties of ZAO films has been systematically investigated. All films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction 〈002〉. The metamorphosis of strain/stress effects in ZAO thin films has been investigated using X-ray diffraction. The as growth films have a large compressive stress of 0.55GPa, which relaxed to 0.25GPa as the Ta was increased to 500°C. Optical parameters such as optical transmittance, absorption coefficient, refractive index and optical band gap energy have been studied and discussed with respect to Ta. All films exhibit a transmittance above 80–90% along the visible–NIR range up to 1500nm and a sharp absorption onset below 400nm corresponding to the fundamental absorption edge of ZnO. Experimental results show that the tensile stress in the films reveals an incline pattern with the optical band gap energy, while the compressive stress shows opposite relation.

Study of aluminium doped zinc oxide nanofilms deposited by rf magnetron sputtering

The transparent conductive films can be widely applied into transparent electrode, liquid crystal displays, solar cells and various optoelectronic devices because of its excellent conductivity and transparency. In this paper, aluminium doped zinc oxide (ZAO) nanofilms are prepared by radio frequency magnetron sputtering. By analysing the results of X-ray diffraction (XRD) and scanning electron microscopy (SEM), it is found that the films prepared have preferred orientation. The properties of ZAO films prepared by magnetron sputtering are controlled by the dopant concentration. X-ray diffraction and SEM show that the increase in dopant concentration may lower the quality of crystallisation. Aluminium doped zinc oxide films can adjust band gaps over quite a large range. Instead of forming a new compound (e.g. Al2O3), Al doping just enables A13+ to replace Zn2+, causing distortion in the crystal lattice. Comparing the optical transmittance in visible light of ZAO samples annealed under 400°C with ones without annealing, no big difference is observed. The transmittance of both samples stay over 90%. The annealing process can improve the electrical properties of thin films, while the oxygen/argon ratio has less impact on the film transmittance. The results indicate that the conductivity of ZAO films have been improved significantly than that of pure zinc oxide films, thus improving the conductivity.

Influence of oxygen and argon flow on properties of aluminum-doped zinc oxide thin films prepared by magnetron sputtering

In this study, the influence of the oxygen/argon (O2/Ar) flow ratio on aluminum-doped zinc oxide (ZAO) films using a Zn1.22Al0.02O1.25 target was investigated systematically. Different samples were obtained by changing the O2/Ar flow ratio from 0.11 to 3. The grain size first decreased and then increased as the O2/Ar flow ratio increased, reaching a minimum size of 8.53nm at a flow ratio of 1. All films showed different average transmittances above 400nm because of different surface structures and film thicknesses; the thickness of films varied from 261 to 897nm. Moreover, the ZAO films exhibited different optical bandgaps between 3.22eV and 3.31eV. The resistivity first increased from 2.1×10−4Ωcm to 350×10−4Ωcm and then decreased to 220×10−4Ωcm with increasing O2/Ar flow ratio. Both the carrier concentration and Hall mobility first decreased from 5.6×1020cm−3 to 0.3×1020cm−3 and from 3.9cm2/Vs to 0.6cm2/Vs, respectively, and then increased to 0.9×1020cm−3 and 1.1cm2/Vs, respectively, with increasing O2/Ar flow ratio.

The Preparation of ZnO: Al Thin Films on Flexible Substrates by Magnetron Sputtering Method

ZnO: Al film was deposited on TPT substrate with SiO2 buffer layer by RF magnetron sputtering. The obtained film had a hexagonal structure and highly (002) preferred orientation. The lattice constant distortion of the film with buffer layer was decreased and the compressive stress was 0.779GPa. The carrier concentratio reached to 3.15×10+20/cm3. The resistivity of ZAO film with SiO2 buffer layer was about 9.2×10-3 Ω·cm and the average transmittance was over 72% in the range of 380~900nm.

Optimization of oxygen and pressure of ZnO:Al films deposited on PMMA substrates by facing target sputtering

ZnO:Al (ZAO) thin films have been deposited on PMMA substrates using facing target sputtering system at room temperature. The dependence of the properties on the oxygen partial pressure and the sputtering pressures was investigated. With increased oxygen partial pressure, the resistivity increases sharply, while the optical band gap and the carrier concentration decrease. The intrinsic band gap of 3.40eV was obtained according to the Burstein–Moss (BM) widening. The lowest resistivity of 2.4×10−3Ωcm and the figure of merit value of 3.42×10−3Ω−1 were achieved when the oxygen partial pressure and sputtering pressure was 0% and 0.7Pa, respectively. For ZAO films prepared at various sputtering pressure, carrier concentration changes slightly whereas Hall mobility increases almost linearly with increasing grain size. The carrier transport of the films is mainly limited by grain boundary scattering.

Enhancement of Electrical Properties of ZnO: Al Thin Films on Transparent TPT Substrates by SiO2 Buffer Layers

The electrical properties need to be improved, although Aluminum doped ZnO thin films (ZnO: Al) have been successfully deposited on transparent TPT substrates by our group. In this paper, ZnO: Al film was deposited on TPT substrate with SiO2 buffer layer by RF magnetron sputtering. The obtained film had a hexagonal structure and highly (002) preferred orientation. Compared with ZAO film without buffer layer, the lattice constant distortion of the film with buffer layer was decreased and the compressive stress was decreased by 9.2%, reaching to 0.779GPa. The carrier concentration and hall mobility of the film with buffer layer were both increased; especially the carrier concentration was enhanced by two orders of magnitude, reaching to 2.65×10+20/cm3. The resistivity of ZAO film with SiO2 buffer layer was about 7.6×10-3 Ω·cm and the average transmittance was over 70% in the range of 450~900nm.

Preparation and Properties of ZnO/ZAO Double-Layers Thin Films on the Substrate of Glass

Aluminium zinc oxide(ZAO) thin films were deposited on soda-Lime-Silica glass substrate by middle frequency power magnetron sputtering. Then zinc oxide(ZnO) thin films were deposited above ZAO thin films by electrochemical deposition method at different time. ZAO thin films and ZnO/ZAO double-Layers thin films were characterized by X-Ray diffraction(XRD) and scanning electron microscope(SEM). A four-Point probe was used to determine the resistivity of the films. The optical transmittance of ZAO films and ZnO/ZAO films was measured by UV-Visible spectrum. The results represent that the transmittance of ZAO/ZnO thin films decreases gradually with deposition time increasing. When the deposition time is 5 minutes, the maximum transmittance of ZnO/ZAO films reaches to 85% at wave length from 400nm to 600nm, and the thickness and resistivity of thin film are 610nm and 2.04×10-3Ω•cm, respectively. However, the thickness and resistivity are highest when the deposition time is 20 minutes, which reaches to 808nm and 1.2×10-2Ω•cm. Meanwhile, the lattice constants a and c of ZAO/ZnO thin films demonstrate an expansion with deposition time increasing. In essence, good-Quality double-Layers thin films of ZnO/ZAO play an important role in CIGS solar cells.

RF magnetron sputtering of ZnO and Al-doped ZnO films from ceramic and nanopowder targets: a comparative study

Undoped zinc oxide (ZnO) and aluminium-doped zinc oxide (ZAO) thin films have been prepared on glass substrates by RF magnetron sputtering from ceramic and nanopowder targets at room temperature (RT). The effects of target nature on the properties of the films have been studied. The X-ray diffraction (XRD) patterns show that ZnO and Al-doped ZnO thin films are highly textured along the c-axis perpendicular to the surface of the substrate from either nanopowder or ceramic targets. From the SEM images, it appears that the surface morphologies of ZAO films exhibit difference from that of undoped ZnO films. All films exhibit a transmittance higher than 80% in the visible region. The optical band gap (Eg) of ZnO and ZAO films obtained from nanopowder target is higher than those prepared using ceramic target. In two cases, Al-doping leads to a larger optical band gap (Eg) of the films.

Preparation of highly transparent conductive Al-doped ZnO thin films and annealing effects on properties

Aluminum-doped ZnO (ZAO) thin films were deposited on fused quartz substrates by radio frequency sputtering in pure argon ambient at 450 °C. Effects of in situ annealing temperature and annealing atmosphere on microstructure, electrical and optical properties of ZAO films have been investigated. Results showed that as-grown film without annealing treatments attained lowest resistivity of 1.1 × 10−4Ω cm. And all films performed high average transmittance greater than 90% in visible region. X-Ray diffraction (XRD), photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) were utilized to characterize the microstructure properties of films. XRD results indicated that as-grown film had higher crystalline quality and larger grain size than annealed films. Al atoms replaced Zn efficiently to provide electrons stable in all samples. PL spectra revealed that high annealing temperature and oxygen atmosphere would generate more Zn vacancy (VZn) and oxide antisite defect (OZn), respectively and composition content results from XPS provided supports to this.

Properties of Al Doped ZnO Thin films by DC Reaction Magnetron Sputtering

The high quality ZAO thin films were successfully produced by DC reaction magnetron sputtering technology. The XRD，electrical and optical properties of films are particular investigated. The results show that ZAO films are polycrystalline hexagonal wurtzite structure，and Al2O3 crystal phase are not found. At the same time，the high quality ZAO films with the minimum resistivity of 4.5x10-4Ω•㎝, the transmittance in visible region above 80% and the reflectivity in IR region above 70% are gained.

Large lateral photovoltaic effect observed in nano Al-doped ZnO films

Zinc oxide (ZnO), including a variety of metal-doped ZnO, as one kind of most important photoelectric materials, has been widely investigated and received enormous attention for a series of applications. In this work, we report a new finding which we call as lateral photovoltaic effect (LPE) in a nano Al-doped ZnO (ZAO) film based on ZAO/SiO2/Si homo-heterostructure. This large and stable LPE observed in ZAO is an important supplement to the existing ZnO properties. In addition, all data and analyses demonstrate ZAO film can also be a good candidate for new type position-sensitive detector (PSD) devices.

Inhomogeneous optoelectronic and microstructure property distribution across the substrate of ZnO:Al films deposited by room temperature magnetron sputtering

Aluminum doped zinc oxide (ZAO) films were deposited by direct current (DC) reactive magnetron sputtering from a ZnO:Al2O3 (3wt.% Al2O3) ceramic target at room temperature. In order to explore the inhomogeneous property distribution across the substrate, the films were deposited with varied substrate–target distances (Ds) ranging from 2cm to 9cm. The experimental results obtained from four-point probe, spectrophotometer, scanning electron microscope, X-ray diffractometer and Auger electronic spectrometer were analyzed to explore the nonuniform property distribution of the obtained ZAO films. The results confirmed that the films’ optoelectronic properties, crystallinity and surface morphology, etc., which were obtained from different substrate areas facing the target were remarkably different. It was revealed that the inhomogeneous property distribution was noticeably dependent on the Ds. It was also suggested that the great difference of electrical conductivity among films from different substrate areas could not be ascribed to the difference of chemical composition, but might be explained by the distinctive crystallinity correspondingly. Films from different substrate regions with distinctive electrical characteristics were either (002) or (110) textured.

Thickness effect on structure and properties of ZAO thin films by RF magnetron sputtering at different substrate temperatures

Al-doped ZnO (ZAO) films, having film thickness of about 50–1200 nm, were deposited at substrate temperature of 100–300 °C by radio-frequency magnetron sputtering. Structural, electrical, and optical properties of as-deposited ZAO films have been studied as a function of film thickness and substrate temperature. The investigation of X-ray diffraction indicates that the crystalline quality of the film improves and its stress relaxes with increasing film thickness or substrate temperature, and preferred (0 0 2) orientation is found at substrate temperature above 100 °C. By the observation under scanning electron microscope, columnar structure can be obviously observed from cross-section of the films with increasing film thickness at substrate temperature above 100 °C; the surface morphology can exhibit nanocrystalline, honeycomb, or hillock structure, depending on film thickness and substrate temperature. Hall effect measurements reveal that the decrease of resistivity of the film is generally accompanied by the increase of carrier concentration and mobility with increasing film thickness or substrate temperature. It is speculated that the main scattering mechanism in as-deposited ZAO films is intercrystallite boundary scattering. The transmission spectra measurements of ZAO films indicate that average transmittance between 400 and 800 nm decreases from about 86% to 70% with increase in film thickness. The obtained energy gap ( E g ) of the films at different substrate temperatures is found in order of 200 °C>300 °C>100 °C, which can be attributed to high carrier concentration and compressive stress at 200 °C, but markedly low carrier concentration at 100 °C. With increase in film thickness, the decrease in compressive stress and/or increase in crystallite size result in the decreased or unchanged tendency of E g of the films although the carrier concentration increases.

Analysis of ZAO Thin Films by DC Reaction Magnetron Sputtereing

The high quality ZAO thin films were produced by DC reaction magnetron sputtering technology. The XRD, electrical and optical properties of ZAO thin films were particularly investigated. The results show that ZAO films are polycrystalline hexagonal wurtzite structure，and Al2O3 crystal phase are not found. At the same time，the high quality ZAO films with the minimum resistivity of 4.5x×10-4Ω·㎝, the transmittance in visible region above 80% and the reflectivity in IR region above 70% are gained.

Damp heat stability of Al-doped zinc oxide films on smooth and rough substrates

Properly encapsulated chalcopyrite-based solar modules from a number of manufacturers have passed the accelerated ageing tests in damp heat. Non-encapsulated modules after damp heat exposure show an increased series resistance which is contributed significantly from the increase in lateral sheet resistance of the transparent Al-doped zinc oxide (ZAO) front contact. We have shown previously, that a rough substrate morphology (on a μm-scale) is a major influence in the ZAO degradation mechanism, due to local perturbations of the ZAO growth ( extended grain boundaries). To further model and examine the extended grain boundaries we present a study of the electrical properties of RF-sputtered ZAO films before and after damp heat, grown on rough quartz glass as well as on polished, texture-etched and patterned silicon. The pattern consisted of equidistant trenches and have a periodical two-dimensional geometry. The strongest decrease of ZAO conductivity occurs on the patterned silicon substrate perpendicular to the trenches. The conductivity parallel to the trenches, however, had the same trend as the conductivity of ZAO on smooth silicon. Measured activation energies of the conductivity of 40 meV or less after damp heat are not sufficient to explain the drastic decrease of conductivity in terms of a grain-boundary-barrier theory (as described, e.g., by Seto). We will summarise our current understanding of the extended grain boundaries and present a refined barrier model to explain our experimental observations.

Influence of Vacuum-Annealing Temperature on the Properties of Direct Current (DC) Magnetron Sputtered ZAO Thin Films

This study focused on the properties of optics and electrics as well as microstructure and morphology of the ZAO thin films which were annealed at different temperature. After vacuum-annealed at 160° or above, the films' quality can be significantly improved. The growth along (002) direction of ZAO thin films were found to be faster than the ZAO films which were not treated and reached a maximum at 330°. FWHM decreased to 0.74°at 330° and grain size grew to 12.02039nm at 400°. The shape of crystal grain changed from anomalous formation to roundness. At the same time, the resistivity of ZAO thin films decreased and reached a minimum of 9.36×10-4Ω•cm while the average transmittance increased to 82% in visible region.

Effects of Argon-Oxygen Ratio and Sputtering Power on Optical Properties of ZnO:Al Thin Films Fabricated by RF Magnetron Sputtering

Al-doped ZnO (ZAO) thin film with good performance was prepared by radio frequency (RF) magnetron sputtering technology under optimized process conditions. The crystallized characteristic and microstructure of the prepared samples were investigated by X-ray diffraction (XRD) pattern and atomic force microscopy (AFM). The optical transmittance spectra was employed to investigate the dependence of the optical performance of the ZAO film on the process conditions, such as argon-oxygen pressure ratios and sputtering powers, which revealed that the transmittance in the visible spectrum is first increased and then decreased as the argon/oxygen pressure ratios increased from 7/1 to 10/1. When the argon/oxygen pressure ratio is 9/1, the transmittance adds up to the maximum. In addition, as the RF sputtering power increases from 80W to 120W, the transmittance of the films is slightly declined with an average transmittance of above 80%; and with the increase of the sputtering power to 140W and 160W, the maximum transmittance of the film obviously decreases below 75%. Moreover, the electrical properties of ZAO thin films were measured by the four-probe tester. The resistivity of the film is less than 10-3 Ω • cm, demonstrating that it have a good electrical performance.