Properties Of Ga-doped ZnO Films Research Articles

Variation in electrical and structural properties of Ga-doped ZnO films caused by oxygen out-diffusion

Variation in electrical and structural properties of Ga-doped ZnO films caused by oxygen out-diffusion

Effects of the thermal annealing on the thermoelectric properties of Ga-doped ZnO thin films

We deposited successfully Ga-doped ZnO (GZO) thin films by using magnetron Dcsputtering technique, followed by annealing. The effects of the thermal annealing on thermoelectric properties of GZO films were investigated. The obtained results showed that due to annealing, the thermoelectric properties of the GZO films were significantly enhanced: (1) power factor increased with an increase of electron mobility due to high film crystallinity; (2) The figure of merit ZT values of the GZO film annealed at 500 oC (ZT = 0.114) was one order higher the asdeposited GZO film (ZT = 0.012). The room temperature photoluminescence (PL) spectra depicted various kinds of point defects which controlled thermoelectric properties and both oxygen vacancies VO and zinc interstitial Zni played an important role.

Influence of ZnO buffer layer on the electrical, optical and surface properties of Ga-doped ZnO films

The influence of the ZnO buffer layer thickness on the structural, electrical, optical and surface properties of Ga-doped ZnO (GZO) films deposited on glass substrates by RF magnetron sputtering were investigated. X-ray diffraction results showed the obtained films had highly c-axis oriented with hexagonal (002) structures and GZO film with 20 nm buffer layer had the best crystalline quality. The resistivity in GZO/ZnO bi-layer films decreased significantly than that in GZO film without a ZnO buffer layer, and GZO film with 20 nm buffer layer showed the lowest resistivity of 4.09 × 10−4 Ω cm. The bi-layer films exhibited the highest transmittance of over 80% in the visible light range and displayed a low near infrared transmittance. The correlation between surface morphology and wettability was studied and GZO/ZnO bi-layer films exhibited hydrophobic property with contact angle of θ from 104° to 108.5°, indicating acceptable property of environmental durability.

Effects of oxygen plasma post-treatment on the structural, electrical and optical properties of Ga-doped ZnO films

The effects of O2 plasma post-treatment on the electrical behavior of Ga-doped ZnO (GZO) films were characterized. GZO films were spin-coated onto glass and post-treated in an O2 plasma at a 0- to 100-W radio-frequency (RF) power and a 0- to 60-s process times in a capacitively-coupled plasma system. Atomic force microscopy, X-ray diffraction, Hall, UV-Vis spectroscopy, photoluminescence, and photocurrent measurements were used to study the influence of the O2 plasma post-treatment on the surface morphological, electrical, and optical properties of the GZO films. With increasing RF power during the O2 plasma post-treatment, the electrical properties of the GZO films improved significantly. The carrier concentration of the GZO films increased by a factor of approximately 52 from 5.89 × 1017 to 3.08 × 1019 cm -3 for a 30-s O2 plasma exposure at 100-W RF power. The electrical improvement was attributed to the GZO films’ high crystallinity, caused by the O2 plasma post-treatment reducing the number of oxygen defects. The plasma treatment had little effect on the transmittance of the GZO films. The optical band gap of the film increased with increasing RF power. An enhanced UV photocurrent was obtained for the GZO film after a 30-s O2 plasma post-treatment at a 100-W RF power, and the recovery was slow.

Preparation of GZO/Pt/GZO Multi-Layered Transparent Electrodes and Their Application to Touch Sensor Devices.

We have proposed, prepared, and characterized Pt-inserted Ga-doped ZnO (GZO) transparent electrodes of GZO/Pt/GZO (GPG) multilayers on glass and flexible plastic substrates. Pt-inserted GZO electrodes showed remarkably decreased resistivity, even though the thickness of the Pt layer was only a few nm. However, the optical transmittance of the GPG electrodes was degraded with increasing thickness of the Pt layer. The structural, optical, and electrical properties of GZO film and GPG multilayered transparent electrodes on glass substrates were largely affected by post- annealing conditions such as the ambient and temperature. Post-annealing in a N2 ambient was the most effective treatment for achieving high optical transmittance and low electrical resistivity of the GZO films and GPG structure. Good optical transmittance of 78% and electrical resistivity of 3.5 x 10(-3) Ω·cm of the GZO electrode were obtained for the 4-nm-thick Pt insertion layer and post-annealing at 400 °C. In addition, touch sensor devices were fabricated by using GPG structures and flexible plastic substrates. The touch sensor devices exhibited a capacitance ratio of 1.4 between the two states before and after fingertip touching.

Effects of oxygen gas flow rates and Ga contents on structural properties of Ga-doped ZnO films prepared by ion-plating with a DC arc discharge

Structural properties of highly c-axis oriented polycrystalline Ga-doped ZnO (GZO) films prepared by ion-plating with a DC arc discharge were studied in terms of the oxygen gas flow rate (FO2) introduced into the chamber during the deposition process and the Ga2O3 content in the GZO sintered pellet. The X-ray diffraction (XRD) measurements revealed that the GZO films have the residual compressive stress along the a-axis direction (in-plane) and the tensile stress along the c-axis direction (out-of-plane). The increase in FO2 or the increase in Ga2O3 content was effective for relaxing the in-plane compressive stress induced by the so-called atomic peening effect. The positive correlation between the carrier concentration (n) and the primitive cell volume (V) would be due to the incorporation of Ga atoms substituting Zn sites (GaZns) together with the generation of n-type intrinsic defects or complex defects.

Effect of main gas and carrier gas on ZnO thin films deposited by atmospheric pressure plasma jet

The effect of main gas flow rate and gas type of carrier gas on the electrical and optical properties of Ga-doped ZnO films deposited by atmospheric pressure plasma jet was investigated. Adding Ar and H2 in the carrier gas reduces the sheet resistance of the films from 230 to 90Ω/□ and increases the deposition rate by 38% due to the incorporation of hydrogen as a shallow donor and the lower breakdown potential of Ar, respectively. The effect of the main gas flow rate is less significant compared with the gas type of the carrier gas, but a proper flow rate is required to produce the film with lower resistivity. The lowest resistivity of 1.25×10−3Ωcm was achieved with a carrier gas mixture percentage of 75% and a main gas flow rate of 30slm at a substrate temperature of 180°C in open air. All the samples presented in this study exhibit average transmittance above 80% in the visible region and low haze below 5.5%.

Effects of film thickness and thermal treatment on the structural and opto-electronic properties of Ga-doped ZnO films deposited by sol–gel method

In this work, Ga-doped ZnO (GZO) films were prepared using the sol–gel dip coating method and the effects of film thickness (230–480nm), heating temperature (400–600°C) and atmosphere (air/argon) on the system structural and opto-electronic performances were investigated. The results show that highly c-axis oriented crystals could be easily formed in GZO coatings, which origins from a preferred nucleation and gets fully developed during the structure evolution. The increase of film thickness prolongs the sol–gel preheating process and promotes nucleation, resulting in a more crystalline film. Higher temperature annealing (600°C) in air could degrade the Ga doping efficiency and decrease the carrier concentration, but the enlarged grain size helps enhance the carrier mobility and improve the film conductivity. Under the modified annealing process in argon, favorable carrier density (3.376×1019cm−3) and mobility (15.74cm2(Vs)−1) can be obtained at the same time, resulting a minimum film resistivity of 1.18×10−2Ωcm. The studied GZO coatings keep transparent in the visible and near infrared range, leading to high emissivity values (>0.738). We firstly prove that the carrier concentration could play a more dominant role than its mobility in determining the thermal emittance performance of thin films.

Effects of growth conditions on the microstructures and photoluminescence properties of Ga-doped ZnO films

ZnO has a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, which is recognized as one of the promising semiconductors for optoelectronic device applications. However, ZnO generally displays visible defect-related deep-level emission and/or UV near-band-edge emission, which is strongly dependent on the growth method and condition. It has been reported that doping with IIIA elements can improve the optical properties of ZnO. Among them, Ga doping is considered not to induce large lattice distortion of ZnO due to the fact that the bonding lengths of Ga-O and Zn-O are similar and ionic radii of Ga3+ and Zn2+ are also similar. The gallium related compounds such as triethylgallium, gallium nitrate and gallium oxide are used as the Ga doping sources. It has been proved that ZnO film can be grown directly by the thermal oxidation of ZnS substrate. In this research, the Ga doping is adopted in the growth of ZnO film by applying the molten gallium to the surface of ZnS substrate and performing the subsequent thermal oxidation in the air at 650 and 700 °C for 3 and 8 h, respectively. The effects of growth condition on the microstructures and photoluminescence properties of the Ga-doped ZnO film are investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and photoluminescence at room temperature. In addition, the relationship among the oxidation temperature, oxidation time, Ga doping content and photoluminescence properties is discussed. The results show that the Ga-doped ZnO films grown under different growth conditions exhibit various amounts of Ga content and the gallium is present in the ZnO matrix as Ga3+ by partially substituting Zn2+. The Ga doping affects the microstructure and photoluminescence property by changing the defect type and level, stoichiometric ratio, and crystal quality of ZnO film. As the oxidation temperature increases, the amount of Ga doping content increases. In addition, the grain size of the Ga-doped ZnO film increases and becomes uniform, and the ratio of ultraviolet emission intensity to visible emission intensity increases. However, as the oxidation time increases, the amount of Ga doping content decreases, the grain size of the Ga-doped ZnO film becomes non-uniform, and the ratio of ultraviolet emission intensity to visible emission intensity decreases.

Carbon monoxide gas sensing properties of Ga-doped ZnO film grown by ion plating with DC arc discharge

Abstract. The carbon monoxide (CO) gas sensing properties of low-resistance heavily Ga-doped ZnO thin films were evaluated. The ZnO films with a thickness of 50 nm were deposited at 200 °C by ion plating. The electrical properties of the ZnO films were controlled by varying the oxygen assist gas flow rate during deposition. The CO gas sensitivity of ZnO films with Au electrodes was investigated in nitrogen gas at a temperature of 230 to 330 °C. CO gas concentration was varied in the range of 0.6–2.4% in nitrogen gas. Upon exposure to CO gas, the current flowing through the film was found to decrease. This response occurred even at the lowest temperature of 230 °C, and is thought to be the result of a mechanism different than the previously reported chemical reaction.

Growth and electro-optical properties of Ga-doped ZnO films prepared by aerosol assisted chemical vapour deposition

Abstract Transparent conductive Ga-doped ZnO thin films were deposited onto glass substrates by a low-cost aerosol assisted chemical vapour deposition technique and the effect of gallium content on the ZnO film growth behaviour and opto-electronic properties was systematically investigated. It is found that, upon increasing Ga addition, the ZnO film crystallinity exhibits a continuous reduction in quality associated with the preferential orientation transformed from (002) to (102). The (002) oriented samples had a microstructure of parallel columnar grains while the (102) oriented coating was thickened by overlapping particles. The ZnO:Ga coatings exhibit high carrier concentration (up to 4.1 × 1020 cm− 3) but low carrier mobility (up to 0.8 cm2 V− 1 s− 1), resulting in a minimum resistivity value of 2.3 × 10− 2 Ω cm. The inferior carrier mobility performance could result from a profound ionized and neutral impurity scattering effect. Good visible transmittance (≈ 70–80%) is observed in these ZnO:Ga films and samples with higher carrier density present better infrared reflection performance (up to 37.2% at 2500 nm).

Temperature and thickness-dependent growth behaviour and opto-electronic properties of Ga-doped ZnO films prepared by aerosol-assisted chemical vapour deposition

Ga-doped ZnO thin films were deposited onto glass substrates by aerosol-assisted chemical vapour deposition of zinc and gallium acetylacetonates in methanol. The effect of deposition temperature and thickness on the film growth behaviour and functional properties was investigated. It is found that the film's preferred deposition and growth sites on the glass surface change with the substrate temperature. The resulting ZnO:Ga coatings are mainly composed of c-axis oriented crystallites and this (002) texture tends to be weakened by increasing thickness, although their crystallinity exhibits a continuous improvement associated with the emergence of columnar grain structures. The increase of deposition temperature transforms the wedge-shaped film morphology into round-shaped particles and enhances the specimen charge carrier density from 1.263–2.790 × 1020 cm−3 to 4.095–5.282 × 1020 cm−3. Due to the improved carrier mobility with system crystallinity, the film resistivity reduces with thickness and the minimum value obtained is 6.51 × 10−3 Ω cm. High visible transmittance (≈70–90% at 550 nm) and favourable infrared reflection (up to 45.5% at 2500 nm) are also observed in these ZnO:Ga coatings, arising from their wide band gaps and high carrier densities, as well as carrier numbers.

Carrier transport and photoluminescence properties of Ga-doped ZnO films grown by ion-plating and by atmospheric-pressure CVD

Ga-doped ZnO (GZO) films with carrier concentrations ranging from 5.2×1017 to 2.9×1020cm−3 were grown on r-plane sapphire substrates by atmospheric-pressure CVD (AP-CVD). The gradients of Hall mobility (μ) — temperature (T) curves (denoted by Δμ/ΔT) for the GZO films grown by AP-CVD (CVD-GZO films) plotted as a function of carrier concentration n obey those for the GZO films deposited by ion plating with dc arc discharge (IP-GZO films). This suggests that the dominant carrier scattering mechanism limiting the carrier transport is common between the CVD-GZO and IP-GZO films at any given n. The CVD-GZO films with low n exhibited the Ga-related neutral donor bound exciton and two-electron satellite (TES) lines at low temperature. With increasing n, the above two characteristic lines shifted higher energies accompanied by broadening. The IP-GZO films exhibited two emission lines with opposite n dependences; with increasing n, one line shifted towards higher energies, whereas another line shifted towards lower energies. With an increase in n caused by the donor doping, the relaxation of the momentum-conservation law brought the remarkable changes to the PL and PLE spectra of both the IP-GZO and CVD-GZO films with n of more than 4.0×1019cm−3 that is close to the Mott density.

Microstructure, optical and photoluminescence properties of Ga-doped ZnO films prepared by pulsed laser deposition

Ga-doped zinc oxide (GZO) films were prepared on Si(100) and fused silica substrates by pulsed laser deposition (PLD). The influence of gallium doping on the morphology, microstructure, optical and photoluminescence properties of GZO films were investigated by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–visible spectrophotometer and photoluminescence spectra. It was found that the Ga doping concentration altered the microstructure and crystallite size of GZO films. X-ray diffraction study revealed that the undoped ZnO and GZO films exhibited a preferred orientation along the (002) plane and the Ga atoms doped into ZnO film deteriorated the film crystallinity. The Ga doping resulted in the decrease of surface roughness and crystallite size of GZO films, the increase in transmittance in the visible region and the increase of optical energy gap of the GZO films. The room-temperature photoluminescence spectra demonstrated that the films exhibited near-band-edge (NBE) emission, while deep-level (DL) emission was also observed in the GZO films deposited at Ga concentrations over 2at%. The intensity ratio of NBE-to-DL emission in the PL spectra of the GZO films was strongly dependent on the Ga concentrations.

Temperature dependence of electrical properties of Ga-doped ZnO films deposited by ion plating with DC arc discharge

Ga-doped ZnO (GZO) films with carrier concentration n ranging from 3×1018 to 1.04×1021cm−3 were deposited by ion plating with DC arc discharge. The results of Hall effect measurements of the GZO films revealed that the gradients of Hall mobility (μ)-temperature (T) curves (denoted by Δμ/ΔT) plotted as a function of n can be divided into three regions: (1) Region I (3×1018<n<4×1019cm−3); Δμ/ΔT>0 and μ decreasing with increasing n, (2) Region II (4×1019<n<3×1020cm−3); μ independent of T (Δμ/ΔT=0) and (3) Region III (n>3×1020cm−3); Δμ/ΔT<0. For all GZO films, the activation energies extracted from the 1000/T–ln (μT) curves were lower than the thermal energy kBT at 300K, indicating that the carrier electrons can overcome the potential barriers at grain boundaries. Moreover, comparison of the calculated mean free path l of the carrier electrons and the depletion layer width at the grain boundaries showed that the grain boundary scattering mechanism plays a minor role in the carrier transport compared with the intra-grain scattering mechanism for GZO films with n higher than 3×1018cm−3. The dependence of Δμ/ΔT on n demonstrates the continuous transformation of the dominant intra-grain scattering mechanism from the ionized impurity scattering mechanism to the phonon scattering mechanism with increasing n.

Effect of the substrate temperature on the properties of Ga-doped ZnO films for photovoltaic cell applications deposited by a pulsed DC magnetron sputtering with a rotating cylindrical target

Effect of substrate temperature on the properties of Ga-doped ZnO (GZO) films was investigated by pulsed DC magnetron sputtering with a rotating cylindrical target with an aim to establish suitable process conditions for their photovoltaic (PV) cell applications. Without formation of undesirable secondary oxide phases such as Ga2O3 and ZnGa2O4, the GZO film having mixed orientation at lower deposition temperature evolved into the c-axis oriented one with increasing deposition temperature to 230 °C, which accompanied morphological evolution to vertically oriented dense columnar structure and improved doping efficiency. Correlated with this, crater-like surface texturing was possible only on the sample deposited at 230 °C. Electrical resistivity and diffuse surface reflectance over the spectral range of 200–1200 nm of this GZO film after surface texturing were 8.73 × 10−4 Ω cm and 3.32%, respectively, indicating that the film has application potential as anti-reflection coating and front electrode of PV cells. Morphological features, surface texturing behavior, electrical and optical properties of the GZO films in this study suggest that this novel technique would be applicable to the fabrication of anti-reflection coating and front electrode of PV cells only when substrate temperature is sufficiently high.

Effect of Vacuum Annealing Temperature on Properties of Ga-Doped ZnO Films Deposited by DC Magnetron Reactive Sputtering

The Ga-doped ZnO thin films (GZO) were deposited at room temperature by DC magnetron reactive sputtering. The influence of vacuum annealing temperature on the morphology, internal stress, optical and electrical properties were investigated. The results showed that with the rise of annealing temperature, the grain size increased, the preferred orientation and crystallization degree became better, the internal stress reduced, the resistivity firstly decreased slightly before 350°C and then increased sharply. The minimum resistivity of 1.32×10-3Ω•cm was obtained at 350°C. The annealing temperature had a very big influence on the transmittance, and the average transmittance was about 80% in the visible range.

Structural, electrical and moisture resistance properties of Ga-doped ZnO films

The structural, electrical and moisture resistance properties of Ga-doped ZnO (GZO) films with 200 nm thickness in terms of their dependence on oxygen gas flow rate ( fO 2) during deposition were studied. GZO films are deposited on glass substrates by ion plating with DC arc discharge. After a reliability test at a temperature of 60 °C and a relative humidity of 95% for 500 h, the percentage of resistivity change of GZO films decreased from 16–20% to 3–11% with increasing fO 2 from 6–12 to 14–25 sccm. The minimum percentage of the resistivity change was observed in the GZO films deposited at fO 2 of 21 sccm and the resistivity after the reliability test was 3.5 × 10 − 4 Ω cm. The effects of the intrinsic defects on the percentage of resistivity change are discussed on the basis of electrical and optical characteristics of GZO films.

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Structural, electrical and optical properties of Ga-doped ZnO films on PET substrate

The effects of O 2 plasma pretreatment on the properties of Ga-doped ZnO films on PET substrate were studied. Ga-doped ZnO films were fabricated by RF magnetron sputtering process. To improve surface energy and adhesion of PET substrate, O 2 plasma pretreatment process was used prior to GZO sputtering. With increasing O 2 plasma treatment time, the contact angle decreases and the RMS surface roughness increases significantly. The transmittance of GZO films on PET substrate in a wavelength of 550 nm was 70–84%. With appropriate O 2 plasma treatment, the resistivity of GZO films on PET substrate was 3.4 × 10 −3 Ω cm.