Ga-doped ZnO Thin Films Research Articles

WITHDRAWN: Changes in electrical and optical properties of polycrystalline Ga-doped ZnO thin films due to thermal desorption of zinc

WITHDRAWN: Changes in electrical and optical properties of polycrystalline Ga-doped ZnO thin films due to thermal desorption of zinc

Effects of post-annealing on the structural and nanomechanical properties of Ga-doped ZnO thin films deposited on glass substrate by rf-magnetron sputtering

In this study, the effects of post-annealing on the structure, surface morphology and nanomechanical properties of ZnO thin films doped with a nominal concentration of 3at.% Ga (ZnO:Ga) are investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) and nanoindentation techniques. The ZnO:Ga thin films were deposited on the glass substrates at room temperature by radio frequency magnetron sputtering. Results revealed that the as-deposited ZnO:Ga thin films were polycrystalline albeit the low deposition temperature. Post-annealing carried out at 300, 400 and 500°C, respectively, has resulted in progressive increase in both the average grain size and the surface roughness of the ZnO:Ga thin film, in addition to the improved thin films crystallinity. Moreover, the hardness and Young's modulus of ZnO:Ga thin films are measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option. The hardness and Young's modulus of ZnO:Ga thin films increased as the annealing temperature increased from 300 to 500°C, with the best results being obtained at 500°C.

Growth and electrical properties of nonpolar and polar Ga-doped ZnO thin films on LaAlO3 and SrTiO3 substrates

Ga-doped ZnO (GZO) thin films were grown on LaAlO3 (LAO) and SrTiO3 (STO) substrates by using a pulsed laser deposition (PLD) technique at various substrate temperatures. It was found that the nonpolar a-plane GZO thin films were grown on LAO (100) substrates. However, the polar c-plane or the c-plane coexistent with the a-plane GZO films were obtained on STO (100) substrates. The in-plane orientational relationship between the GZO films and substrates was also investigated. The electrical properties of the films were deeply affected by the growth orientation. The GZO films with a-axis oriented phase showed higher resistivity than with c-oriented phase, which can be ascribed to the lower mobility due to the increase in grain boundary scattering.

Pulsed laser deposited Ga doped ZnO/SiO x/Si(100) thin films and their field emission behavior

Pulsed laser deposition was used for growing undoped ZnO and Ga (3 and 5 at. wt.%) doped ZnO thin films on Si (100) substrates with native oxide. X-ray diffraction studies reveal highly c-axis oriented, single phase growth of wurtzite ZnO and Ga doped ZnO thin films. X-ray photoelectron spectroscopy study shows that gallium exists in the Ga 3+ oxidation state replacing Zn 2+ in the ZnO matrix. It, hence, acts as a donor, increasing the charge carrier density, which was confirmed by Hall measurement. Significant change in the surface morphology of the thin films, such as increased surface roughness, presence of nanostructures, was observed in the Ga doped ZnO thin films. Surface morphology was also influenced by the Ga doping concentration. Higher charge carrier density with the modified surface morphology vindicated its use for field emission studies. Reduction in the threshold field defined for obtaining an emission current density of 0.1 μA/cm 2, increased field enhancement factor are features of Ga doped ZnO thin films. The stability of field emission current density of the undoped ZnO and Ga doped ZnO thin films was studied at the preset current density of 2.5 and 2.0 μA/cm 2 respectively, for two and half hours.

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al 2O 3 substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E 2 High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B 1 modes (B 1 Low, 2B 1 Low and B 1 High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.

Effect of annealing temperature on the microstructural and electrical properties of epitaxial Ga-doped ZnO film deposited on c-sapphire substrate

4 wt. % Ga-doped ZnO (GZO) film has been prepared on c (0001)—sapphire (Al2O3) substrate by Pulse Laser Deposition method. The effect of annealing temperature on the microstructural and electrical properties of the GZO thin film was investigated. X-ray diffraction and High-Resolution Transmission electron microscopy (HR–TEM) studies showed that the electrical and defects properties of GZO thin film were greatly influenced by annealing temperature. The crystallinity and electrical properties of the film can be improved by annealing at 800 °C. However, at a too high annealing temperature of 1,000 °C, the newly processing defects such as extended dislocations and a new nanoscale stacking fault were formed. Consequently, the crystallinity and electrical properties of the film annealed at 1,000 °C were degraded. The study was useful to acquire optimal annealing conditions to improve the properties of film.

Influence of substrate temperature on the electrical and optical properties of Ga-doped ZnO thin films fabricated by continuous composition spread

The electrical and optical properties of Ga-doped ZnO (GZO) thin films deposited at different substrate temperatures have been studied by a continuous composition spread (CCS) method. The full range of GZO compositions deposited at different substrate temperatures was explored to find excellent electrical and optical properties. Optimized GZO thin films with a low resistivity of 9.6 × 10 −4 Ω cm and an average transmittance above 89% in the 400–700 nm wavelength regions were able to be formed at a substrate temperature of 100 °C. Optimized composition of the GZO thin film which had the lowest resistivity and highest transmittance was found at 0.8 wt% Ga 2O 3 doped ZnO.

Optical and electrical properties of transparent conducting B-doped ZnO thin films prepared by various deposition methods

B-doped ZnO (BZO) thin films were prepared with various thicknesses up to about 500 nm on glass substrates at 200 °C by dc or rf magnetron sputtering deposition, pulsed laser deposition (PLD), and vacuum arc plasma evaporation (VAPE) methods. Resistivities of 4–6 × 10−4 Ω cm were obtained in BZO thin films prepared with a B content [B/(B + Zn) atomic ratio] around 1 at. % by PLD and VAPE methods: Hall mobilities above 40 cm2/Vs and carrier concentrations on the order of 1020 cm−3. All 500-nm-thick-BZO thin films prepared with a resistivity on the order of 10−3–10−4 Ω cm exhibited an averaged transmittance above 80% in the wavelength range of 400–1100 nm. The resistivity in BZO thin films prepared with a thickness below about 500 nm was found to increase over time with exposure to various high humidity environments. In heat-resistance tests, the resistivity stability of BZO thin films was found to be nearly equal to that of Ga-doped ZnO thin films, so these films were judged suitable for use as a transparent electrode for thin-film solar cells.

Fast response ultraviolet Ga-doped ZnO based photoconductive detector

A metal–semiconductor–metal photoconductive detector was fabricated using high quality Ga-doped ZnO film epitaxially grown onto alumina substrate by spray pyrolysis. The photocurrent increases linearly with incident power density for more than two orders of magnitude. Reflectance and photocurrent measurements were carried out to study optoelectronic properties of Ga-doped ZnO thin film. Both spectra are consistent with each other showing good response in UV than visible region. Peak responsivity of about 1187 A/W at 5 V bias for 365 nm light was obtained in UV region.

The transparence comparison of Ga- and Al-doped ZnO thin films

The Ga-doped ZnO (GZO) and Al-doped ZnO (AZO) thin films were grown on quartz glass substrates by pulsed laser deposition under different oxygen partial pressures ( P O 2 ) . The transparent performances of films versus properties of structure and conductivity were discussed. With the increase of P O 2 , the transmittance of both GZO films and AZO films increased to maximum and then decreased which were in according with the change of crystallization quality. The transmittance of GZO films was higher than that of AZO films, which were not dominated by the impurity ions induced by doping. AFM images and surface roughness mean square coefficients showed that the surfaces of GZO films were smoother than that of AZO films, which were due to the dopant Ga acting as the surfactant and smoothed the GZO films surface.

Effect of annealing treatment on structural, electrical, and optical properties of Ga-doped ZnO thin films deposited by RF magnetron sputtering

The effect of annealing on structural, electrical, and optical properties of Ga-doped ZnO (GZO) films prepared by RF magnetron sputtering was investigated in air and nitrogen. GZO films are polycrystalline with a preferred 002 orientation. The resistivities of annealed films are larger than the as-deposited. The transmittance in the near IR region increases greatly and the optical band gap decreases after annealing. The photoluminescence spectra is composed of a near band edge emission and several deep level emissions (DLE) which are dominated by a blue emission. After annealing, these DLEs are enhanced evidently.

Fabrication and properties of p-type K doped Zn 1− xMg xO thin film

A series of K doped Zn 1− x Mg x O thin films have been prepared by pulsed laser deposition (PLD). Hall-effect measurements indicate that the films exhibit stable p-type behavior with duration of at least six months. The band gap of the K doped Zn 1− x Mg x O films undergoes a blueshift due to the Mg incorporation. However, photoluminescence (PL) results reveal that the crystallinity decreased with the increasing of Mg content. The fabricated K doped p-type Zn 0.95Mg 0.05O thin film exhibits good electrical properties, with resistivity of 15.21 Ω cm and hole concentration of 5.54 × 10 18 cm −3. Furthermore, a simple ZnO-based p–n heterojunction was prepared by deposition of a K-doped p-type Zn 0.95Mg 0.05O layer on Ga-doped n-type ZnO thin film with low resistivity. The p–n diode heterostructure exhibits typical rectification behavior of p–n junctions.

Effect of RF Power on Structural and Electrical Properties of Ga-Doped ZnO for Transparent Electrode of Thin Film Solar Cells

We have investigated the structural and electrical properties of Ga-doped ZnO (GZO) thin films deposited by an RF magnetron sputtering at various RF powers from 50 to 90W. All the GZO thin films are grown as a hexagonal wurtzite phase with highly c-axis preferred parameters. The structural and electrical properties are strongly related to the RF power. The grain size increases as the RF power increases since the columnar growth of GZO thin film is enhanced at an elevated RF power. This result means that the crystallinity of GZO is improved as the RF power increases. The resistivity of GZO rapidly decreases as the RF power increases up to 70 W and saturates to 90W. In contrast, the electron concentration of GZO increases as the RF power increases up to 70 W and saturates to 90W. GZO thin film shows the lowest resistivity of <TEX>$2.2{\times}10^{-4}{\Omega}cm$</TEX> and the highest electron concentration of <TEX>$1.7{\times}10^{21}cm^{-3}$</TEX> at 90W. The mobility of GZO increases as the RF power increases since the grain boundary scattering decreases due to the reduced density of the grain boundary at a high RF power. The transmittance of GZO thin films in the visible range is above 90%. GZO is a feasible transparent electrode for application as a transparent electrode for thin film solar cells.

Half wave rectification of inorganic/organic heterojunction diode at the frequency of 1 kHz

An inorganic/organic vertical heterojunction diode has been demonstrated with p-type Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) deposited by spin coating on n-type Ga-doped ZnO (GZO) thin films. Transparent conducting GZO thin films are deposited on glass substrate by rf-magnetron sputtering. Electrical properties of GZO thin films are investigated depending on the processing temperatures. The resistivity, mobility and carrier concentration of the GZO thin films deposited at processing temperatures of 500°C are measured to be about 3.6×10−4Ω cm, 23.8cm2/Vs and 7.1×1020cm3, respectively. The root mean square surface roughness of the GZO thin films is calculated to be ~0.9nm using atomic force microscopy. Current-voltage characteristics of the n-GZO/p-PEDOT:PSS heterojunction diode present rectifying operation. Half wave rectification is observed with the maximum output voltage of 1.85V at 1kHz. Low turn-on voltage of about 1.3V is obtained and the ideality factor of the n-GZO/p-PEDOT:PSS diode is derived to be about 1.8.

A study on the epitaxy nature and properties of 3 wt% Ga-doped epitaxial ZnO thin film on Al 2O 3 (0 0 0 1) substrates

3 wt% Ga-doped ZnO (GZO) thin films were deposited on Al 2O 3 (0 0 0 1) substrates by RF magnetron sputtering at different growth temperatures ranging from 350 to 750 °C. The crystallinity, microstructure, epitaxial nature, and optical and electrical properties of the GZO thin films were examined by X-ray diffraction (XRD), transmission-electron microscopy (TEM), UV–visible spectroscopy and Hall measurements. XRD and TEM showed that the GZO thin films deposited below a growth temperature of 450 °C grew epitaxially with an orientation relationship of ( 0 0 0 1 ) [ 1 1 2 ¯ 0 ] G Z O ‖ ( 0 0 0 1 ) [ 1 1 2 ¯ 0 ] A l 2 O 3 . However, the GZO thin films deposited above 550 °C were in polycrystalline hexagonal wurtzite phase with c-axis preferred. The crystallinity of the GZO thin films deteriorated with increasing growth temperature. The GZO thin film deposited at 350 °C showed the lowest electrical resistivity of 1.13×10 −4 Ω cm. The electrical properties of the GZO thin films also deteriorated with increasing growth temperature. UV–visible spectroscopy showed that the GZO thin films are highly transparent (from 75% to 90%) in the visible region. In addition, the band gap of the deposited thin films decreased from 3.5 to 3.2 eV with increasing growth temperature.

Etching characteristics and mechanism of Ga-doped ZnO thin films in inductively-coupled HBr/ X ( X = Ar, He, N 2, O 2) plasmas

We investigated the etch characteristics and mechanisms of Ga-doped ZnO (Ga–ZnO) thin films in HBr/ X ( X = Ar, He, N 2, O 2) inductively-coupled plasmas. The etch rates of Ga–ZnO thin films were measured as a function of the additive gas fraction in the range of 0–100% for Ar, He, N 2, and O 2 at a fixed gas pressure (6 mTorr), input power (700 W), bias power (200 W), and total gas flow rate (40 sccm). The plasma chemistry was analyzed using a combination of the global (zero-dimensional) plasma model and Langmuir probe diagnostics. By comparing the behavior of the etch rate and fluxes of plasma active species, we found that the Ga–ZnO etch process was not limited by ion-surface interaction kinetics and appeared in the reaction rate-limited etch regime. In the HBr/O 2 plasma, the etch kinetics were probably influenced by oxidation of the etched surface.

Effect of Deposition Temperature on Structural and Electrical Properties of Ga-Doped ZnO for Transparent Electrode of Thin Film Solar Cells

We have investigated the structural and optical properties of Ga-doped ZnO (GZO) thin films deposited by RF magnetron sputtering at various deposition temperatures from 100 to <TEX>$500^{\circ}C$</TEX>. All the GZO thin films are grown as a hexagonal wurtzite phase with highly c-axis preferred parameter. The structural and electrical properties are strongly related to deposition temperature. The grain size increases with the increasing deposition temperature up to <TEX>$400^{\circ}C$</TEX> and then decreases at <TEX>$500^{\circ}C$</TEX>. The dependence of grain size on the deposition temperature results from the variation of thermal activation energy. The resistivity of GZO thin film decreases with the increasing deposition temperature up to <TEX>$300^{\circ}C$</TEX> and then decreases up to <TEX>$500^{\circ}C$</TEX>. GZO thin film shows the lowest resistivity of <TEX>$4.3{\times}10^{-4}\;{\Omega}cm$</TEX> and highest electron concentration of <TEX>$1.0{\times}10^{21}\;cm^{-3}$</TEX> at <TEX>$300^{\circ}C$</TEX>. The mobility of GZO thin films increases with the increasing deposition temperature up to <TEX>$400^{\circ}C$</TEX> and then decreases at <TEX>$500^{\circ}C$</TEX>. GZO thin film shows the highest resistivity of 14.1 <TEX>$cm^2/Vs$</TEX>. The transmittance of GZO thin films in the visible range is above 87% at all the deposition temperatures. GZO is a feasible transparent electrode for the application to the transparent electrode of thin film solar cells.

LTCC 기판상에 증착한 GZO 가스 센싱 박막의 두께 의존 특성 연구

A novel design of gas sensor using Ga-doped ZnO (GZO) thin films which are deposited on low temperature co-fired ceramic (LTCC) substrates is presented. The LTCC substrates with thickness of 400 are fabricated by laminating 12 green tapes which consist of alumina and glass particle in an organic binder. The GZO thin films with different thickness are deposited on LTCC substrates, by RF magnetron sputtering method. The microstructure and sensing properties of GZO gas sensing films are analyzed as a function of the film thickness. The films are well crystallized in the hexagonal (wurzite) structure with increasing thickness. The maximum sensitivity of 3.49 is obtained at 100 nm film thickness and the fastest 90% response time of 27.2 sec is obtained at 50 nm film thickness for the operating temperature of to the gas.

Pretreatment of Polyethylene Terephthalate Substrate for the Growth of Ga-Doped ZnO Thin Film

The effect of the pretreatment of polyethylene terephthalate (PET) substrate on the growth of transparent conducting Ga-doped ZnO (GZO) thin film was investigated. Because of its high gas and moisture absorption and easy gas permeation, PET substrate was annealed at 100 degrees C in a vacuum chamber prior to the sputtering growth of GZO thin film for the outgassing of impurity gases. GZO thin film was deposited on the pretreated PET substrate by rf-magnetron sputtering and significantly improved electrical properties of GZO thin film was achieved. Electrical and structural characterizations of the GZO thin films were carried out by 4-point probe, Hall measurement, and scanning electron microscopy, and the effects of the pretreatment on the improved properties of GZO thin films were discussed. This result is not only useful to PET substrate, but also could be applicable to other plastic substrates which inevitably containing the moisture and impurity gases.

Thermally stable transparent conducting and highly infrared reflective Ga-doped ZnO thin films by metal organic chemical vapor deposition

Highly transparent conductive Ga-doped ZnO (GZO) thin films have been prepared on glass substrates by metal organic chemical vapor deposition. The effect of Ga doping on the structural, electrical and optical properties of GZO films has been systematically investigated. Under the optimum Ga doping concentration (∼4.9 at.%), c-axis textured GZO film with the lowest resistivity of 3.6 × 10 −4 Ω cm and high visible transmittance of 90% has been achieved. The film also exhibits low transmittance (<1% at 2500 nm) and high reflectance (>70% at 2500 nm) to the infrared radiation. Furthermore, our developed GZO thin film can well retain the highly transparent conductive performance in oxidation ambient at elevated temperature (up to 500 °C).