In-situ analyses on the reactive sputtering process to deposit Al-doped ZnO films using an Al-Zn alloy target

The kinetic energies of generated ions were investigated during the reactive sputtering process to deposit Al-doped ZnO (AZO) films using an Al-Zn alloy target. The sputtering system was equipped with specially designed double feedback system to stabilise the reactive sputtering processes and analysis was performed with a quadrupole mass spectrometer combined with an energy analyser. Negative ions O⁻, O₂⁻, AlO⁻ and AlO₂⁻ with high kinetic energies corresponding to cathode voltage are generated at the partially oxidised target surface, after which some of the ions undergo subsequent charge exchange and/or dissociation. Positive ions O⁺, Ar⁺, Zn⁺ and Al⁺ with lower kinetic energies (around 10 eV) are generated by charge exchange of sputtered neutral O, Ar, Zn and Al atoms, respectively. As the target surface oxidises, cathode voltage decrease, the flux of high-energy negative ions increases and the electrical properties of the AZO degrade by ion bombardment as well as the AZO films that are deposited by conventional magnetron sputtering using an AZO target.     

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.     

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.     

Characterization of transparent conducting oxide thin films deposited on ceramic substrates

In this work, we investigate the optical and electrical properties of various transparent conductive oxide (TCO) thin films deposited on insulating ceramics for emerging optoelectronic applications. Thin films investigated include indium tin oxide (ITO), ruthenium oxide (RuO₂), and iridium oxide (IrO₂) on Al₂O₃ ceramic substrates. The conducting films have been deposited by various techniques including RF magnetron sputtering and low-cost spray pyrolysis. The morphological characteristics of the films were carried out using high magnification optical microscopy and atomic force microscopy (AFM). Optical and electrical characterization was carried out by optical absorbance/transmittance, van der Pauw, current-voltage (I-V), and Hall effect measurements. The results are presented in this paper.     

Discharge power dependence of structural and electrical properties of Al-doped ZnO conducting film by magnetron sputtering (for PDP)

In order to investigate the possible application of ZnO films as a transparent conducting oxide (TCO) electrode for AC PDP, ZnO:Al films were prepared by DC magnetron sputtering method. The effects of discharge power and doping concentration on the structural and electrical properties of ZnO films were mainly studied experimentally. Five-inch PDP cells using either a ZnO:Al or indium tin oxide (ITO) electrode were also fabricated separately under the same manufacturing conditions. The luminous properties of both the PDP cells were measured and compared with each other.By doping the ZnO target with 2 wt% of Al₂O₃, the film deposited at a discharge power of 40 W resulted in the minimum resistivity of 8.5 × 10⁻⁴ Ω-cm and a transmittance of 91.7%. However, a high doping concentration of 3 wt% of Al₂O₃ and excessive sputtering power over 40 W may induce high defect density and limit the growth of small grains. Although the luminance and luminous efficiency of the cell using ZnO:Al are lower than those of the cell with the ITO electrode by about 10%, these values are sufficient enough to be considered for the normal operation of AC PDP.     

Angular distribution and sputtering yield of Al and Al2O3 during 40 key argon ion bombardment

The angular distribution and sputtering yield of Al and Al₂O₃ during 40 keV argon ion bombardment have been measured by collecting the sputtered particles on a semi-cylindrical collector and analysing them by Rutherford backscattering spectrometry (RBS). It was found that due to the relatively high residual pressures (1–4 · 1O^?4Pa) in the sputtering chamber not only aluminium but also oxygen atoms are deposited on the collector through the mechanism of reactive sputtering both when sputtering the oxide and the metal target. The angular distribution of the collected aluminium and oxygen atoms in the case of pure aluminium sputtering follows a cosine law while in the case of Al₂O₃ sputtering a considerable deviation from the cosine law is observed. This deviation is explained by a preferred orientation (texture) of the crystallites in the polycrystalline oxide targets. It was found that the very thin films deposited on the collector when sputtering both types of targets have a composition close to AlO₂. The sputtering yield of Al and Al₂O₃ by 40 keV argon ions has been determined. On the basis of the obtained values an estimation of the productivity of the reactive sputter deposition of Al₂O₃ films from oxidized and non-oxidized targets is made.     

Changes in the elemental composition and microstructure of an YBa2Cu3O7 − δ target during magnetron sputtering

YBa₂Cu₃O_{7 ? δ} high-temperature superconductor targets subjected to ion-plasma sputtering in a magnetron-sputtering system are studied by X-ray diffraction analysis and secondary ion-mass spectrometry. It is found that dc magnetron sputtering is accompanied by diffusive oxygen depletion and restructuring of the target surface caused by radiation and heat effects of bombarding ions. It is assumed that these changes in the target are responsible for a decrease in the growth rate of YBa₂Cu₃O_{7 ? δ} films.     

Low-cost ZnO:Al transparent contact by reactive rotatable magnetron sputtering for Cu(In,Ga)Se2 solar modules

Sputtering ZnO as transparent front contact (TCO) is standard in today's industrial scale Cu(In,Ga)Se₂ (CIGS) module manufacturing. Although innovative concepts like rotatable magnetron sputtering from ceramic targets have been realised, costs are still high due to expensive ceramic targets. Significant cost reductions are expected by using reactive sputtering of metallic targets.Therefore, ZSW and industrial partners investigated the reactive sputtering of Al-doped zinc oxide (ZAO) as TCO on CIGS absorbers of high quality and industrial relevance. The reactive DC sputtering from rotatable magnetron targets is controlled in the transition mode by adjusting oxygen flow and discharge voltage. Optimisation leads to ZAO films with a TCO quality nearly comparable to standard films deposited by DC ceramic sputtering. Scanning electron microscopy, X-ray diffraction, and Hall analyses of the ZAO films are performed.Medium-size CIGS modules are coated with reactively sputtered ZAO, resulting in 12.8% module efficiency and surpassing the efficiency of the ceramic witness device. Cd-free buffered devices are also successfully coated with reactive TCO. Damp heat stability according to IEC61646 is met by all reactively sputtered devices.     

Natively textured surface aluminum-doped zinc oxide transparent conductive layers for thin film solar cells via pulsed direct-current reactive magnetron sputtering

Natively textured surface aluminum-doped zinc oxide (ZnO:Al) layers for thin film solar cells were directly deposited without any surface treatments via pulsed direct-current reactive magnetron sputtering on glass substrates. Such an in-situ texturing method for sputtered ZnO:Al thin films has the advantages of efficiently reducing production costs and dramatically saving time in photovoltaic industrial processing. High purity metallic Zn-Al (purity: 99.999%, Al 2.0 wt.%) target and oxygen (purity: 99.999%) were used as source materials. During the reactive sputtering process, the oxygen gas flow rate was controlled using plasma emission monitoring. The performance of the textured surface ZnO:Al transparent conductive oxides (TCOs) thin films can be modified by changing the number of deposition rounds (i.e. thin-film thicknesses). The initially milky ZnO:Al TCO thin films deposited at a substrate temperature of ~ 553 K exhibit rough crater-like surface morphology with high transparencies (T ~ 80-85% in visible range) and excellent electrical properties (ρ ~ 3.4 × 10^− 4 Ω cm). Finally, the textured-surface ZnO:Al TCO thin films were preliminarily applied in pin-type silicon thin film solar cells.     

Structural evolution,electrical and optical properties of AZO films deposited by sputtering ultra-high density target

Aluminum-doped zinc oxide (AZO) target was fabricated using AZO nanopowders synthesized by co-precipitation method and then the AZO films with different thicknesses were deposited on glass by d.c. magnetron sputtering at room temperature. AZO target is nodules free and shows homogeneous microstructure, ultra-high density and low resistivity. ZnAl₂O₄ phase appears in AZO target and disappears in AZO films. All AZO films show c-axis preferred orientation and hexagonal structure. With increasing film thickness from 153 to 1404 nm, the crystallinity was improved and the angle of (002) peak was close to 34.45°. The increase in grain size and surface roughness is due to the increase in film thickness. The decrease of resistivity is ascribed to the increases of carrier concentration and Hall mobility. The lowest resistivity is 9.6 × 10^?4 Ω·cm. The average transmittance of AZO films exceeds 80%, and a sharp fundamental absorption edge with red-shifting is observed in the visible range. The bandgap decreases from 3.26 to 3.02 eV.     

Effects of deposition temperature on structure and properties of mo-doped indium oxide by radio frequency magnetron sputtering

Transparent conductive oxide (TCO) thin films of Mo-doped In₂O₃ (IMO) were prepared on glass substrates by radio frequency magnetron sputtering from the 2 wt% Mo-doped In₂O₃ ceramic target. The depositions were carried out under an oxygen-argon atmosphere by varying the deposition temperature from 200 °C to 350 °C. The crystal structure and thickness of IMO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of deposition temperature on the electrical and optical transmittance properties of IMO thin films were investigated by four-point probe Hall system and UV-VIS-NIR spectrophotometer separately. The optimum deposited IMO thin films were obtained with resistivity of 6.9 × 10⁻⁴ Ω cm and carrier mobility 45 cm²v⁻¹s⁻¹ at 350 °C. The average optical transmittance of IMO films on glass substrates are over 80% in the near-infrared region.     

Transparentes leitfähiges TiO2:Nb – ein TCO‐Material der Zukunft?

Transparent conductive TiO₂:Nb – a TCO material of the future? Niobium doped titania is a widely unknown TCO material. We present the results of TiO₂:Nb thin films deposited by DC and pulse DC sputtering from a ceramic titania target with a content of approximately 6 wt.‐percent Nb in a pilot scale in‐line sputtering plant. With DC sputtering on Borofloat 33 substrate a 100 nm thin film show after annealing at 450 °C a resistivity of 7.2 × 10^?4 Ωcm with a low extinction coefficient of 0.02 and a transmittance in the visible range of 74.8%.     

New natively textured surface Al-doped ZnO-TCOs for thin film solar cells via magnetron sputtering

Natively textured surface aluminum doped zinc oxide (ZnO:Al) thin films were directly deposited via pulsed direct current (DC) reactive magnetron sputtering on glass substrates. During the reactive sputtering process, the oxygen gas flow rate was varied from 8.5 sccm to 11.0 sccm. The influences of oxygen flow rate on the structural, electrical and optical properties of naturally textured ZnO:Al TCO thin films with milky surface were investigated in detail. Gradual oxygen growth (GOG) technique was developed in the reactive sputtering process for textured ZnO:Al thin films. The light-scattering ability and optical transmittance of the natively textured ZnO:Al TCO thin films can be improved through gradual oxygen growth method while maintaining a low sheet resistance. Typical natively textured ZnO:Al TCO thin film with crater-like surface exhibits low sheet resistance (R_s ～ 4 Ω), high transmittance (T_a > 85%) in visible optical region and high haze value (12.1%).     

Effect of Thickness on the Properties of Ga-doped Nano-ZnO Thin Films Prepared by RF Magnetron Sputtering

Nano transparent conductive oxide (TCO) Ga-doped ZnO (GZO) thin films with thickness from 260 nm to 620 nm were prepared on glass substrates by RF magnetron sputtering from a powder target with 3 at.% Ga₂O₃. The substrate temperature was kept at 300 °C. The effect of thickness on the structural, electrical, and optical properties of GZO thin films was investigated. It shows that the nano-GZO films are dense and flat, and have polycrystalline structure with preferentially in the (002) orientation. With the increase of thickness, the crystallinity and the grain sizes of the films are improved, meanwhile the carrier concentration increases and the lowest resistivity of 3.685×10⁻³ Ω cm occurs in the 620 nm thick GZO film. The average optical transmittance of all the films is over 80% in the visible range. Decreasing the thickness, the optical transmission of the films increase, and the absorption edge shifts to shorter wavelength, which means the optical band gap is broadened.     

Formation process of Al2O3 thin film by reactive sputtering

Al and Al₂O₃ films were deposited by RF magnetron sputtering using a mixed gas of Ar and O₂. The surface of the Al target was changed from the metallic mode to the oxide mode at a critical O₂ flow ratio of 8%. The atomic ratio of sputtered Al atoms to supplied oxygen atoms was found to be approximately 2:3 at the critical O₂ flow ratio. The oxide layer thickness formed on the Al target was estimated to be 5-7 nm at an O₂ flow ratio of 100% by ellipsometry.     

Post-deposition treatment behaviour of zinc oxide thin films in hydrogen peroxide solution

Abstract

Zinc oxide thin films with c axis orientation were grown on Si (100) by radio frequency magnetron sputtering in mixture of argon and oxygen environment using Zn target. These films are treated in hydrogen peroxide (H₂O₂) solution at room temperature for one hour and two hours. During this study it has been found that post-deposition treatment of ZnO films improves the film quality. Structural, electrical and optical properties have been compared before and after H₂O₂ treatment by X-ray diffraction analysis, ac conductivity, band gap and refractive index. The full width at half maximum decreases after post-deposition process, which improves the crystal quality. The relief in stress has been observed but films are not fully stress free. The film after treatment becomes highly insulating having resistivity of the order of 10¹⁴ Ω cm^?1 and can be used for piezoelectric applications. The increase in the band gap and refractive index, near to bulk value, has been observed after post-deposition treatment indicating the increase in grain size and crystal quality.     

In-situ analysis of positive and negative energetic ions generated during Sn-doped In2O3 deposition by reactive sputtering

Using a quadrupole mass spectrometer combined with an energy analyser, we have investigated the in-situ energy distribution of highly energetic ions generated during reactive sputtering of In-Sn alloy (IT) targets and non-reactive sputtering of Sn-doped In₂O₃ (ITO) ceramic targets. Ar⁺, In⁺, O⁺, O⁻, O₂⁻, InO⁻ and InO₂⁻ ions with kinetic energies greater than 40 eV were clearly observed. Upon increasing the O₂ flow ratio for reactive sputtering, the surface of the IT target changes from metal (metal mode) to oxide (oxide mode) via a state of mixed metal and oxide (transition region). O⁻ ions with the kinetic energy corresponding to cathode voltage are generated at the oxide layer, which expands upon the target surface with increasing O₂ flow ratio in the metal mode and the transition region. In contrast, the flux of 60-eV Ar⁺ ions decreases with increasing O₂ flow ratio. The presence of 125- and 200-eV In⁺ ions is attributed to the dissociation of InSnO₂⁻ and InO₂⁻ with the kinetic energy corresponding to cathode voltage, respectively, while the presence of 40- and 150-eV O⁺ ions is attributed to the dissociation of InO₂⁻ and O₂⁻ with the kinetic energy corresponding to cathode voltage, respectively.     

Low temperature preparation of transparent, antireflective TiO2 films deposited at different O2/Ar ratios by microwave electron cyclotron resonance magnetron sputtering

A serial of crystalline titanium oxide ceramic films were deposited at low temperature using microwave electron cyclotron resonance (MW-ECR) magnetron sputtering with different O₂/Ar ratios. The influences of O₂/Ar ratio on the deposition rate, morphology, crystalline nature, optical adsorption property of the obtained titanium oxide thin films were investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and UV-Vis spectra. Therefore, the optimum O₂/Ar ratio for deposition of anatase TiO₂ thin films on unheated glass substrate was realized in a MW-ECR magnetron sputtering process. The as-deposited anatase TiO₂ films were transparent and were antireflective in the visible region.     

A study of crystallographic phases in non-stoichiometric (oxygen deficiency) indium oxide thin films

Indium oxide is a well-known transparent conductive oxide (TCO) in its stoichiometric composition (In₂O₃). Its electrical and optical properties are strongly influenced by the chemical composition. This work focuses on an experimental investigation of the crystallographic phases in non-stoichiometric (oxygen deficiency) compositions of indium oxide thin films. The thin films were deposited at 300 °C by reactive sputtering of pure indium target at different oxygen gas flow rates on Si substrates. Two different phases are identified only in the non-stoichiometric compositions: metallic indium- and crystalline indium-rich oxide. The metallic indium phase appears as nano-crystals, a few nano-meters in diameter, evenly dispersed and occupies only 1 vol. % of the film. These metallic nano-particles have a negligible effect on the optical transparency and electrical conductivity of the films. The indium-rich oxide (In_xO_y) phase which occupies about 99 vol. % of the film has the bixbyite crystallographic structure and average grain size of about 50 nm. This phase has a pronounced effect on improving the TCO figure-of-merit (FM) relative to stoichiometric crystalline In₂O₃ films due to a higher increase of the electrical conductivity than the decrease of the optical transparency.     

Advantages of transparent conducting oxide thin films with controlled permittivity for thin film photovoltaic solar cells

Our recent investigations have identified a pathway to produce transparent conducting oxide (TCO) films that demonstrate higher infrared transparency. The technique involves controlling the dielectric permittivity of the TCO film such that the electrical properties are maintained, but the plasma frequency (ω_p) is shifted to longer wavelength. This has the effect of reducing free-carrier absorption in the visible and near-infrared spectral region, thus producing a TCO film with higher optical transmission. The technique has been demonstrated for sputtered films of indium tin oxide by adding small amounts of ZrO₂ to a ceramic sputtering target, and for SnO₂:F films deposited by chemical vapor deposition using a metalorganic Zr source.