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.     

Electrical and optical properties of Al-doped ZnO films deposited by hollow cathode gas flow sputtering

Al-doped ZnO (AZO) films were deposited on glass by hollow cathode gas flow sputtering using Zn-Al alloy targets. Sputtering power for all the depositions was fixed at 1500 W. Resistivities of 0.81-1.1 × 10^− 3 Ω cm were obtained for AZO films deposited at room temperature with an O₂ flow from 38 to 50 standard cubic centimetre/minute (SCCM), while static deposition rates were almost constant at 270-300 nm/min. On the other hand, lower resistivities of 5.2-6.4 × 10^− 4 Ω cm were obtained for AZO films deposited at 200 °C with an O₂ flow from 25 to 50 SCCM, while the static deposition rates were almost constant at 200-220 nm/min. Average transmittances in the visible light region were above 80% for both sets of films.     

Highly conductive alumina-added ZnO ceramic target prepared by reduction sintering and its effects on the properties of deposited thin films by direct current magnetron sputtering

To obtain a suitable sputtering target for depositing transparent conducting Al-doped ZnO (AZO) films by using direct current (DC) magnetron sputtering, this study investigates the possibility of using atmosphere controlled sintering of Al₂O₃ mixed ZnO powders to prepare highly conductive ceramic AZO targets. Experimental results show that a gas mixture of Ar and CO could produce a sintered target with resistivity in the range of 2.23 × 10^− 4 Ω cm. The fairly low resistivity was mainly achieved by the formation of both aluminum substitution (Al_Zn) and oxygen vacancy (V_O), thus greatly increasing the carrier concentration. Compared to usual air sintered target, the thin film deposited by the Ar + CO sintered target exhibited lower film resistivity and more uniform spatial distribution of resistivity. A film resistivity as low as 6.8 × 10^− 4 Ω cm was obtained under the sputtering conditions of this study.     

Deterioration and recovery in the resistivity of Al-doped ZnO films prepared by the plasma sputtering

A hot-cathode plasma sputtering technique was used for fabricating the highly transparent and conducting aluminum-doped zinc oxide (AZO) films on glass substrates from a disk-shaped AZO (Al₂O₃: 2 wt.%) target. Under particular conditions where the target voltage was V_T = −200 V and the plasma excitation pressure was P_S = 1.5 × 10⁻³ Torr, the lowest resistivity of 4.2 × 10⁻⁴ Ω cm was obtained at 400 nm, and this was associated with a carrier density of 8.7 × 10²⁰ cm⁻³ and a Hall mobility of 17 cm²/V s. From the annealing experiment of the AZO films in the oxygen and nitrogen gases of the atmospheric pressure it was revealed that both the oxygen vacancies and the grain boundaries in the polycrystalline AZO film played an important role in the electrical properties of the film.     

High-rate deposition of Sb-doped SnO2 films by reactive sputtering using the impedance control method

SnO₂ films doped with Sb (ATO) were deposited both on unheated glass substrates and on glass substrates that had been heated at 200 °C by reactive sputtering of an Sb-Sn alloy target with a plasma control unit (PCU) and mid-frequency (mf, 50 kHz) unipolar pulsing. The PCU feedback system monitors the oxidation states of target surface by detecting the sputtering cathode voltage (impedance control method). The mf pulse wave is approximately square-shaped; this helps to reduce arcing on the target when high power density is applied on the cathode. In case of the ATO depositions on the heated substrate at 200 °C in the “transition region” of reactive sputtering, the deposition rate was 280 nm/min, the lowest resistivity of the ATO films was 4.6 × 10^− 3 Ω cm and the optical transmittance was over 80% in the visible region of light.     

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.     

Negative ions in reactive magnetron sputtering

Negative ions are present in magnetron sputtering if electronegative elements are involved. The majority of the negative ions impinging on transparent conductive oxide (TCO) films during growth is O^? produced at the oxidised target surface while it is rather unimportant whether it is bulk oxide or a surface oxide formed in reactive sputtering. O^? bombards the film with energies equivalent to the target voltage and by far exceed 100 eV. It is hence apt to cause radiation damage in sensitive TCO films. This is shown by the lateral distribution of the highenergy O^? flux in planar magnetron sputtering that exhibits the same pattern imaging the erosion groove as the resistivity of TCO films. This lateral inhomogeneity strongly depends on the erosion groove depth. The emission of O^? further depends on the sputtered material, for TiO₂ deposition it is much less than for other TCO materials. The emission probability correlates to the secondary electron emission coefficient of the oxide.     

Ion bombardment-induced enhancement of the properties of indium tin oxide films prepared by plasma-assisted reactive magnetron sputtering

Research on tin doped indium oxide (ITO) has for many years been stimulated by the need to simultaneously optimize the electrical, optical and mechanical properties, and by new challenges related to the deposition of transparent conducting oxides on flexible plastic substrates. In the present work, we investigate the growth and optical, electrical, and mechanical (hardness, elastic modulus and stress) properties of ITO films deposited by plasma assisted reactive magnetron sputtering (PARMS) from an indium-tin alloy target. PARMS achieves an effective control of bombardment by reactive species (e.g., O₂⁺, O⁺) on the surface of the growing film by varying the bias voltage, V_B, induced by a radiofrequency power applied to the substrate. Stress-free films possessing high transparency (> 80% — film on glass) and low resistivity (4 × 10^− 4 Ω cm) can be deposited by PARMS under conditions of intense ion bombardment (≤ 600 eV).     

Al-doped ZnO thin films deposited by reactive frequency magnetron sputtering: H2-induced property changes

Al-doped ZnO (AZO) transparent conductive thin films have been prepared by radio-frequency magnetron sputtering with a ceramic target (98 wt.% ZnO, 2 wt.% Al₂O₃) in different Ar + H₂ ambient at a substrate temperature of 200 °C. To investigate the influence of H₂-flow on the properties of AZO films, H₂-flow was changed during the growth process with a fixed Ar-flow of 60 sccm. The results indicate that H₂-flow has a considerable influence on the transparent conductive properties of AZO films. The low resistivity in the order of 10^− 4 Ω cm and the high average transmittance more than 92% in the visible range were obtained for the samples prepared in the optimal H₂-flow range from 0.4 sccm to 1.0 sccm. In addition, the influence of H₂-flow on the structure and composition of AZO films have also been studied.     

High-rate deposition of Ta-doped SnO2 films by reactive magnetron sputtering using a Sn-Ta metal-sintered target

Ta-doped SnO₂ films were deposited on glass substrate (either unheated or heated at 200 °C) by reactive magnetron sputtering with a Sn-Ta metal-sintered target using a plasma control unit (PCU) and mid-frequency (mf, 50 kHz) unipolar pulsing. The PCU feedback system precisely controlled the flow of the reactive and sputtering gases (O₂ and Ar, respectively) by monitoring either discharge impedance or the plasma emission of the atomic O* line at 777 nm. The planar target was connected to the switching unit, which was operated in unipolar pulse mode. Power density on the target was maintained at 4.4 W cm^− 2 during deposition. The lowest obtained resistivity for the films deposited on heated substrate was 6.4 × 10^− 3 Ωcm, where the deposition rate was 250 nm min^− 1.     

Al-doped ZnO (AZO) films deposited by reactive sputtering with unipolar-pulsing and plasma-emission control systems

Al-doped ZnO (AZO) films were deposited on fused silica glass substrates unheated or heated at 200 °C by reactive dc sputtering using a Zn-Al alloy target with mid-frequency pulsing (50 kHz) and the plasma control unit with a feedback system of the optical emission intensity of the atomic O* line at 777 nm to control oxygen gas flow. The stable and reproducible depositions were successfully carried out in the transition region. The deposition rates attained in this study were about 10-20 times higher than the one by conventional sputtering using oxide targets. The AZO films with the lowest resistivity of 3.8 × 10^− 4 Ω cm was deposited on the substrate heated at 200 °C with a sputter power of 4 kW, where the deposition rate was 385 nm/min.     

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.     

Direct current reactive sputtering of aluminium

The influence of the oxygen content in the gas flow on the discharge current and on the chemical composition of sputtered AlO_x was investigated. Unlike SiO₂, Al₂O₃ films are formed on the substrate only for oxygen contents sufficient for full oxidation of the target surface. The minimum oxygen content can be determined by the sharp change in discharge current at constant working voltage and total gas pressure. Infrared spectra confirming this statement are given. The kinetics of the processes of forming and sputtering off of an oxide layer on the aluminium target surface were also investigated. Some speculations about the interrelation between oxide formation on the substrate and on the target for direct current reactive sputtering are given.     

Effect of oxygen concentration and system geometry on the current-voltage relations during reactive sputter deposition of titanium dioxide thin films

Current-voltage relations at different magnetron sputtering systems and gas mixtures were studied during reactive sputter deposition of titanium dioxide thin films. The main goal of this work was to investigate the influence of reactive gas mixture (Ar + O₂) and system geometry on the electrical characteristics of the discharge. The geometries utilized were the conventional magnetron sputtering, hollow cathode magnetron sputtering and triode magnetron sputtering. A change in the system geometry leads to a change in the electric field distribution, which alters the working range of the discharge voltage and magnetron efficiency. It is noticed that the discharge voltage at constant current can be reduced when the geometry is altered from conventional magnetron to hollow cathode magnetron or triode magnetron, at the same time the magnetron efficiency is increased when hollow cathode magnetron or triode magnetron are used instead of conventional magnetron sputtering.     

Mechanical properties of Al2O3-doped (2 wt.%) ZnO films

We report a new method of evaluating the adhesion of Al₂O₃-doped (2 wt.%) ZnO (AZO) thin films. The AZO films were deposited by DC reactive magnetron sputtering on plastic film (PET: polyethyleneterephthalate) at various sputtering pressures, power, and reactive gas-flow ratios. The adhesion test of the films was carried out using the nanoindentation system. The fracture point as determined by the load-displacement curve occurred at the time of separation between the thin film and the substrate. The integration value of load and displacement to the fracture point is defined as the degree of adhesion (S_W). The AZO films showed that adhesion increase as sputtering power increases and sputtering pressure decreases.     

Target voltage measurements during DC sputtering of silver in a nitrogen/argon plasma

During reactive sputtering, addition of the reactive gas results in a target voltage change. This effect finds its origin in the modification of the gas composition but also in the change of the target condition. In this paper, we focus on the target voltage changes during magnetron sputtering of silver in an argon/nitrogen plasma. In the first second during the nitrogen addition, we notice a decrease of the target voltage followed by an increase in target voltage. The target voltage decrease can be easily explained from the increased volume ionisation. The change of the target condition seems to be responsible for the target voltage increase. However, this effect cannot be explained from the formation of a silver nitride phase on the target surface as generally accepted during reactive sputtering of metal nitrides. Indeed, silver is a poor nitride former. To explain the target voltage increase, we have studied in this paper the influence of ion implantation of N₂⁺ ions on the target voltage during magnetron sputtering. The ions were implanted in situ in a silver target and the target voltage of this modified target was registered under the same conditions as during the sputtering experiments. The implantation of the N₂⁺ ions results in a target voltage increase. Hence, during sputtering of a silver target in an argon/nitrogen plasma, the target voltage increases by the presence of non-reacted N atoms in the target top surface layers.     

Structural, optical and electrical properties of AZO/Cu/AZO tri-layer films prepared by radio frequency magnetron sputtering and ion-beam sputtering

Highly conducting tri-layer films consisting of a Cu layer sandwiched between Al-doped ZnO (AZO) layers (AZO/Cu/AZO) were prepared on glass substrates at room temperature by radio frequency (RF) magnetron sputtering of AZO and ion-beam sputtering of Cu. The tri-layer films have superior photoelectric properties compared with the bi-layer films (Cu/AZO, AZO/Cu) and single AZO films. The effect of AZO thickness on the properties of the tri-layer films was discussed. The X-ray diffraction spectra show that all films are polycrystalline consisting of a Cu layer with the cubic structure and two AZO layers with the ZnO hexagonal structure having a preferred orientation of (0 0 2) along the c-axis, and the crystallite size and the surface roughness increase simultaneously with the increase of AZO thickness. When the AZO thickness increases from 20 to 100 nm, the average transmittance increases initially and then decreases. When the fixed Cu thickness is 8 nm and the optimum AZO thickness of 40 nm was found, a resistivity of 7.92 × 10⁻⁵ Ω cm and an average transmittance of 84% in the wavelength range of visible spectrum of tri-layer films have been obtained. The merit figure (F_TC) for revaluing transparent electrodes can reach to 1.94 × 10⁻² Ω⁻¹.     

Structural, optical and electrical properties of reactively sputtered Ag2Cu2O3 films

Ag₂Cu₂O₃ thin films were deposited on glass substrates by RF magnetron sputtering of an equiatomic silver-copper target (Ag_0.5Cu_0.5) in reactive Ar-O₂ mixtures. The reactive sputtering was done at varying power, oxygen flow rate and deposition temperature to study the influence of these parameters on the deposition of Ag₂Cu₂O₃ films. The film structure was determined by X-ray diffraction, while the optical properties were examined by spectrophotometry (UV-vis-NIR) and photoluminescence. Furthermore, the film thickness and resistivity were measured by tactile profilometry and 4-point probe, respectively. Additional mobility, resistivity and charge carrier density Hall effect measurements were done on a few selected samples. The best films in terms of stoichiometry and crystallography were achieved with a sputtering power of 100 W, oxygen and argon flow rates of 20 sccm (giving a deposition pressure of 1.21 Pa) and a deposition temperature of 250 °C. The optical transmittance and photoluminescence spectra of films deposited with these parameters indicate several band gaps, most prominently, a direct one of around 2.2 eV. Electrical characterization reveals charge carrier concentrations and mobilities in the range of 10²¹-10²² cm^− 3 and 0.01-0.1 cm²/Vs, respectively.     

Raman scattering studies of Mn-doped ZnO thin films deposited under pure Ar or Ar + N2 sputtering atmosphere

This paper investigates the anomalous and specific Raman modes present in Mn-doped ZnO thin films deposited using the magnetron co-sputtering method. To trace these peaks, we prepared Mn-doped ZnO films with different Mn concentrations by altering the sputtering power of the Mn target in a pure Ar or Ar + N₂ sputtering atmosphere. A broad band observed in the Raman spectra of heavily Mn-doped ZnO films ranges from 500 to 590 cm^− 1. This band involves the enhanced A₁ longitudinal mode and activated silent modes of ZnO, as well as a characteristic mode of Mn₂O₃. Four anomalous Raman peaks at approximately 276, 510, 645 and 585 cm^− 1 are present in pure and Mn-doped ZnO films deposited under the Ar + N₂ sputtering atmosphere. The peaks at 276 cm^− 1 and 510 cm^− 1 may originate from the complex defects of Zn_i-N_O and Zn_i-O_i, respectively, while the peak at approximately 645 cm^− 1 could be due to a complex defect of Zn_i coupled with both the N and Mn dopants. The results of this study suggest classifying the origins of anomalous and specific Raman peaks in Mn-doped ZnO films into three major types: structural disorder and morphological changes caused by the Mn dopant, Mn-related oxides and intrinsic host-lattice defects coupled with/without the N dopant.     

Electrical mechanism analysis of Al2O3 doped zinc oxide thin films deposited by rotating cylindrical DC magnetron sputtering

Cost efficient and large area deposition of superior quality Al₂O₃ doped zinc oxide (AZO) films is instrumental in many of its applications, including solar cell fabrication due to its numerous advantages over indium tin oxide (ITO) films. In this study, AZO films were prepared by a highly efficient rotating cylindrical direct current (DC) magnetron sputtering system using an AZO target, which has a target material utilization above 80%, on glass substrates in argon (Ar) ambient. A detailed analysis on the electrical, optical, and structural characteristics of AZO thin films was performed for the solar cell, as well as display applications. The properties of films were found to critically depend on deposition parameters, such as sputtering power, substrate temperature, working pressure, and film thickness. A low resistivity of ~ 5.5 × 10^− 4 Ω cm was obtained for films deposited at 2 kW, keeping the pressure, substrate temperature and thickness constant at 3 mTorr, 230 °C and ~ 1000 nm respectively. This was due to an increase in carrier mobility and large grain size. Mobility is found to be controlled by ionized impurity scattering within the grains, since the mean free path of carriers is much smaller than the grain size of the films. The AZO films showed a high transparency of ~ 90% in the long wavelength region. Our results offer a cost-efficient AZO film deposition method that can fabricate films with significant low resistivity and high transmittance that can be applied in thin-film solar cells, as well as thin film transistor (TFT) and non-volatile memory (NVM).