Ga-doped ZnO Films Research Articles

X-RAY PHOTOELECTRON SPECTROSCOPIC STUDY ON HIGH-ELECTRON-MOBILITY GALLIUM AND HYDROGEN CO-DOPED ZINC OXIDE THIN FILMS

In this study, gallium and hydrogen co-doped ZnO (HGZO) thin films were investigated. The films were deposited by sputtering from Ga-doped ZnO (GZO) ceramic target in hydrogen and argon plasma. The as-deposited HGZO films possess enhanced electron mobility of 48.6 cm2/Vs as compared to that of 39.4 cm2/Vs of GZO films, sputtered from the same target. Because of insignificant variation in crystallinity, this improvement is attributed to roles of hydrogen in crystalline lattice structure of the films. X-ray photoelectron spectroscopy (XPS) is employed as an essential technique for quantitative analyses and chemical binding states of films constituent elements. The roles of hydrogen are clarified through the binding states of Zn 2p, O 1s and Ga 3d. Obtained results suggest that the films are deposited more effectively in hydrogen plasma. Some point defects such as oxygen vacancies (VO), dangling bonds can be passivated in form of H+VOHO and O–H bonds. As a result, the reduction of scattering centers is indicated as a reason for the mobility improvement of the HGZO films.

Resistivity reduction in Ga-doped ZnO films with a barrier layer that prevents Zn desorption

The improvement of electrical properties by thermal annealing of Ga-doped ZnO (GZO) films deposited by sputtering has been investigated. The resistivity of a GZO film deposited at room temperature decreases as the vacuum annealing temperature is increased up to 400 °C. A further increase in annealing temperature increases the resistivity, resulting from a decrease in carrier density. On the other hand, the formation of a SiO2 layer covering the surface of the GZO film eliminates this resistivity upturn, causing its decrease from 1.26 × 10−3 Ω cm before annealing to 4.1 × 10−4 Ω cm at 600 °C. A comparison between the films annealed at different vacuum pressures have revealed that electrical resistivity is improved by preventing atomic migration between the GZO lattice and the surrounding atmosphere, i.e., oxygen absorption to or Zn desorption from the GZO lattice. The optical transmittance of a SiO2-capped GZO film is higher than that of a bare GZO film after thermal annealing.

Influence of the spin acceleration time on the properties of ZnO:Ga thin films deposited by sol–gel method

A detailed study of the structural, morphological, optical, and electrical properties of the spin-coated Ga-doped ZnO (GZO) films in which these properties are altered by varying the spin acceleration time (tacc) was undertaken. The results showed that, by varying tacc, there is no significant variation in crystal size, and band gap energy of the films, while some physical parameters such as strain $$\left( \varepsilon \right)$$ , surface roughness (RMS), and porosity (P) are remarkably varied. By increasing tacc from 0 to 8 s, $$\varepsilon$$ , RMS, and P, were increased by 230, 83, and 49%, respectively. Electrical data were presented to demonstrate unambiguously that the variation in the films resistivity (ρ) is closely connected to the change in these parameters. To explain these findings, a simple formula was proposed in the investigated range of tacc, which demonstrated that ρ is more related to $$\varepsilon$$ and P, and then to RMS.

Ga-doped ZnO films magnetron sputtered at ultralow discharge voltages: Significance of controlling defect generation

In magnetron sputtered oxide thin films, both the structure and physical properties are affected by bombardment-induced defects and their annihilation. Ga-doped zinc oxide (GZO) films with low Ga content (~1.7 at.%) were magnetron sputtered at various discharge voltages (70–220 V) and the growth temperatures (130 °C and 380 °C). The microstructure was characterized by X-ray Diffractometry, Raman Spectroscopy, and Transmission Electron Microscopy. Meanwhile, the electrical and optical properties were measured by Hall-effect measurement and Spectroscopic Ellipsometry, respectively. We found that reducing the discharge voltage led to higher structural quality and better electrical properties, independent of the growth temperature. The benefit, from using a high temperature (i.e., 380 °C), of the structural improvement, can only be achieved when the discharge voltage has been decreased to ultralow (i.e., 70 V). Our results suggest that for high quality GZO films deposition, controlling the defect generation should be preferable to simply increasing the growth temperature.

High energy electron irradiation effects on Ga-doped ZnO thin films for optoelectronic space applications

Gallium-doped ZnO (GZO) thin films of thickness 394 nm were prepared by a simple, cost-effective sol–gel spin coating method. The effect of 8 MeV electron beam irradiation with different irradiation doses ranging from 0 to 10 kGy on the structural, optical and electrical properties was investigated. Electron irradiation influences the changes in the structural properties and surface morphology of GZO thin films. X-ray diffraction analysis showed that the polycrystalline nature of the GZO films is unaffected by the high energy electron irradiation. The grain size and the surface roughness were found maximum for the GZO film irradiated with 10 kGy electron dosage. The average transmittance of GZO thin films decreased after electron irradiation. The optical band gap of Ga-doped ZnO films was decreased with the increase in the electron dosage. The electrical resistivity of GZO films decreased from 4.83 × 10−3 to 8.725 × 10−4 Ω cm, when the electron dosage was increased from 0 to 10 kGy. The variation in the optical and electrical properties in the Ga-doped ZnO thin films due to electron beam irradiation in the present study is useful in deciding their compatibility in optoelectronic device applications in electron radiation environment.

Abrupt conversion of the conductivity and band-gap in the sputter grown Ga-doped ZnO films by a change in growth ambient: Effects of oxygen partial pressure

GZO films were sputter grown under the different O2/Ar + O2 ratios and the systematic investigation has been tried in an effort to get a better understanding on the correlations between the involved defects and the film properties. The abrupt conversion of the resistivity was observed from the highly conductive, 2.52 ⅹ10−4 Ω cm, at pure Ar ambient to resistive, 0.295 Ω cm, with an oxygen addition of 1% in the ambient, accompanying the significant reduction in the electron density and mobility. Hall measurement at 10 K–300 K and in-depth XPS analysis suggested that the higher Hall mobility and carrier density in the film grown in pure Ar ambient may be due to a lower compensation ratio and the compensation via Gazn-Vzn complexes may play a major role in the sudden reduction in the electron density and the reduced density of the Gazn, substitutional donors induced by oxygen addition in growth ambient.

Influence of crystallographic polarity on the opto-electrical properties of polycrystalline ZnO thin films deposited by magnetron sputtering

Herein, the influence of crystallographic polarity on the properties of ZnO polycrystalline thin films deposited by magnetron sputtering, has been studied. The electrical and optical properties of undoped ZnO and heavily Ga-doped ZnO thin films showed significant differences between Zn polarity and O polarity. The Hall mobility and carrier concentration of the undoped ZnO thin films with Zn polarity were higher than those with O polarity. The ZnO film with Zn polarity showed a dominant near-band-edge emission, while that with O polarity showed relatively intense deep-level emission in the visible wavelength region. It was suggested that acceptor-type defects tend to be formed in the ZnO films with O polarity. The Ga-doped ZnO films deposited on the Zn-polar ZnO layer showed a higher carrier concentration and Hall mobility as compared to deposition on O-polar ZnO. Crystallographic polarity is thus a very important factor for controlling the properties of polycrystalline ZnO-based thin films deposited on glass substrates.

Strong enhancement of emission efficiency in GaN light-emitting diodes by plasmon-coupled light amplification of graphene

Recently, we have demonstrated that excitation of plasmon-polaritons in a mechanically-derived graphene sheet on the top of a ZnO semiconductor considerably enhances its light emission efficiency. If this scheme is also applied to device structures, it is then expected that the energy efficiency of light-emitting diodes (LEDs) increases substantially and the commercial potential will be enormous. Here, we report that the plasmon-induced light coupling amplifies emitted light by ∼1.6 times in doped large-area chemical-vapor-deposition-grown graphene, which is useful for practical applications. This coupling behavior also appears in GaN-based LEDs. With AuCl3-doped graphene on Ga-doped ZnO films that is used as transparent conducting electrodes for the LEDs, the average electroluminescence intensity is 1.2–1.7 times enhanced depending on the injection current. The chemical doping of graphene may produce the inhomogeneity in charge densities (i.e., electron/hole puddles) or roughness, which can play a role as grating couplers, resulting in such strong plasmon-enhanced light amplification. Based on theoretical calculations, the plasmon-coupled behavior is rigorously explained and a method of controlling its resonance condition is proposed.

Precision excimer laser annealed Ga-doped ZnO electron transport layers for perovskite solar cells.

Organic–inorganic hybrid perovskite solar cells (PSCs) continue to attract considerable attention due to their excellent photovoltaic performance and low cost. In order to realize the fabrication of PSCs on temperature-sensitive substrates, low-temperature processing of all the components in the device is required, however, the majority of the high-performance PSCs rely on the electron transport layers (ETLs) processed at high temperatures. Herein, we apply excimer laser annealing (ELA) to treat ETLs (Ga-doped ZnO, GZO) at room temperature. A synergetic improvement in optical transparency and electrical conductivity is achieved after ELA treatment, which in turn improves light absorption, enhances electron injection, and depresses charge recombination. Devices fabricated with ELA treated GZO ETL acheived a power conversion efficiency (PCE) of 13.68%, higher than that of the PSCs utilizing GZO with conventional high-temperature annealing (12.96%). Thus, ELA is a promising technique for annealing ETLs at room temperature to produce efficient PSCs on both rigid and flexible substrates.

Investigation on influence of seeding layer on morphological properties of ZnO-based nanostructure prepared by hydrothermal process

In this present work, bare ZnO low-dimensional structures were synthesized by hydrothermal process with crystal growth assistance using seed layers. Different seeding layer species including ZnO and Ga-doped ZnO films prepared by sol-gel dip coating were introduced onto the substrate before the hydrothermal growth of ZnO-based nanostructures. Zn(NO3)2 and GaN3O9 were utilized as Zn and Ga source during hydrothermal process. The influence of seeding layer types on morphological and shapes of the hydrothermally grown nanostructures have been extensively scrutinized by Field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD) and optical spectroscopy. It was revealed that different morphologies and exceptional shapes in form of nanorods of ZnO-based nanostructures were obtained as different seeding layers were introduced. More detailed studies for clarification of seed layer effect on the growth of ZnO-based nanostructures are mentioned and discussed.

Effects of the carrier concentration on polarity determination in Ga-doped ZnO films by hard x-ray photoelectron spectroscopy

Core level (CL) and valence band (VB) spectra of heavily Ga-doped ZnO (GZO) films with carrier concentrations (Ne) ranging from 1.8×1020 to 1.0×1021cm−3 were measured by high-resolution Al Kα (hν=1486.6eV) x-ray photoelectron spectroscopy (XPS) and Cr Kα (hν=5414.7eV) hard x-ray photoelectron spectroscopy (HAXPES). The CL spectra of the GZO films measured by XPS had little dependence on Ne. In contrast, clear differences in asymmetric broadening were observed in the HAXPES spectra owing to the large probing depth. The asymmetry in the Zn 2p3/2 and O 1s HAXPES spectra is mainly attributed to the energy loss of the conduction electron plasmon caused by the high Ne of the GZO films. Similar asymmetry was also observed in the VB spectra of these GZO films. It was found that such asymmetry plays a crucial role in the determination of crystal polarity. With increasing Ne, the intensity of the sub-peak at a binding energy Eb of about 5eV in the VB spectrum decreased and the sub-peak became indistinguishable. We clarified the limitation of the criterion using the sub-peak and proposed an alternative method for polarity determination.

Characteristics of the orientation distribution and carrier transport of polycrystalline Al-doped ZnO films prepared by direct current magnetron sputtering

We investigated the influence of Al contents on the structural, electrical and optical properties of Al-doped ZnO (AZO) polycrystalline films with the columnar grain structures. In addition, we demonstrated an effective way for achieving improved Hall mobility AZO films at any given Al contents. We deposited 500-nm-thick AZO films on glass substrates at a substrate temperature of 200°C by direct-current magnetron sputtering with a direct-current power of 200W. The oxide targets were high-density sintered AZO targets with Al2O3 contents from 0.5 to 3.0wt%. Analysis of the data obtained by X-ray diffraction and Hall effect measurements showed that an increase in the Al2O3 contents resulted in an increase in volume fraction of (0001) orientation (V(0001)) from 85 to 98% and in carrier concentration of the AZO films. The use of a 10-nm-thick Ga-doped ZnO film deposited by ion plating with direct-current arc discharge as an interface layer between the glass substrate and a thick AZO film drastically changed the above behavior of the orientation distribution limited by the Al2O3 contents. We found the V(0001) of about 99% at any given Al2O3 content. The resulting well-defined (0001) crystallographic orientation led to very small contribution of grain boundary scattering to carrier transport. Hall mobility of AZO films with the very thin interface layer was improved with enhanced carrier concentration as a result.

Low-Optical-Loss Transparent Conductive Ga-Doped ZnO Films for Plasmonics in the Near-Infrared Spectral Range

Low-Optical-Loss Transparent Conductive Ga-Doped ZnO Films for Plasmonics in the Near-Infrared Spectral Range

Ga-doped ZnO films by magnetron sputtering at ultralow discharge voltages: Effects of defect annihilation

Preparation of high quality transparent conductive oxide (TCO) films by sputter deposition involves an intricate balance of defect generation by the highly energetic negative oxygen ions (depending on the discharge voltage) and the concomitant annihilation of these defects during film growth. Ga-doped ZnO films with a low Ga content (1.7at.%) were deposited to investigate the effects of defect annihilation on the microstructure evolutions as well as the optical and electrical properties. To achieve this aim, we prepared the GZO films by magnetron sputtering at ultralow discharge voltages (~70V) to minimize the defect generation, and varied the substrate temperature (from 130°C to 380°C) to adjust the annihilation rates. The microstructure was systematically characterized by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Raman Spectroscopy. The electrical and optical properties were obtained by a Hall-effect measurement system and Spectroscopic Ellipsometry (SE), respectively. It was found that (i) even under the condition of highly controlled defect generation from the bombardment of negative oxygen ions, a sufficient annihilation of the defects cannot be realized without externally heating the substrate; and (ii) there existed a threshold temperature, above which both the structural quality and the electrical properties were improved with the increased temperature over the temperature range we examined. These results reveal that the growth temperature during the GZO film deposition has played an important role in effective annihilation of the irradiation-induced structural defects, which may be due to the higher diffusion barrier of Ga atoms in our GZO films.

Pulse repetition rate dependent structural, surface morphological and optoelectronic properties of Ga-doped ZnO thin films grown by pulsed laser deposition

Transparent conductive Ga-doped ZnO (GZO) thin films were grown on glass substrates by pulsed laser deposition using 2 at% Ga2O3 doped ZnO target. A Substrate temperature of 400 °C and oxygen pressure of 7 × 10−3 Torr was kept constant during the deposition. The effects of laser pulse repetition rate from 3 to 15 Hz on crystallographic, surface morphological, optical and electrical properties of the GZO films were investigated. All the samples show the hexagonal wurtzite phase with a dominant c-axis orientation and the optimal crystallization of the GZO film occurs at 10 Hz; which is slightly better that of the film deposited at 5 Hz. AFM and FE-SEM analysis revealed that the film growth is profoundly influenced by the pulse repetition rate and the film thickness increases with an increase in the pulse repetition rate due to enhancement in deposition rate and the island density. Hall-effect measurements confirmed that all samples are heavily doped degenerate semiconductors with n-type electrical conductivity. The electrical transport properties of the film are found to be largely dependent on the crystallinity; while their optical transmittance is majorly dominated by the film thickness and roughness. The GZO film deposited at 5 Hz shows the highest figure of merit (76.11 × 10−3/Ω) as a result of its optimum sheet resistance (3.63 Ω/□) and optical transmittance (∼88%).

The investigation of Ga-doped ZnO as an interlayer for ohmic contact to Cd1−xZnxTe films

In this work, high quality Cd1−xZnxTe films were prepared on fluorine doped tin oxide (FTO) glass substrates by close-spaced sublimation (CSS) method. A low resistivity sputtered Ga-doped ZnO (GZO) film was used as an interlayer between Au electrodes and Cd1−xZnxTe films try to reduce the contact resistance and contribute to bring about a better Ohmic contact. Circular transmission line model (CTLM) was adopted to investigate the effects of GZO intermediate layer on the contact properties of Au/GZO/Cd1−xZnxTe structure. The results show a low contact resistivity of 0.37Ωcm2 for Au/GZO contacts on Cd1−xZnxTe films. Cd1−xZnxTe film radiation detectors were also fabricated using Au/GZO contacts and an energy resolution of about 28% was obtained from a 60KeV 241Am γ-ray source for the first time.

Enhancement of the hydrogen gas sensitivity by large distribution of c-axis preferred orientation in highly Ga-doped ZnO polycrystalline thin films

The hydrogen gas sensing properties of highly Ga-doped ZnO (GZO) polycrystalline thin films deposited by radio-frequency magnetron sputtering have been studied. The relationship between the microstructural properties of preferred c-axis oriented thin films and the hydrogen gas sensing properties is described. The crystallite size and the preferred orientation distribution were characterized by X-ray diffraction. The crystallite size increased and the preferred orientation distribution decreased with increasing film thickness. In order to control the crystallite size and the c-axis orientation separately, a highly oriented ZnO template layer with different thickness was employed for deposition of 30-nm-thick GZO films. The c-axis orientation of these films were nearly comparable each other, while the crystallite size increased significantly with increasing thickness of the ZnO templates. The hydrogen gas sensitivity at an operating temperature of 330°C increased slightly with decreasing crystallite size, while the sensitivity was dramatically enhanced by increasing the preferred orientation distribution. It is therefore proposed that the c-axis orientation plays an important role in determining the sensitivity of the hydrogen gas sensor.

Characterization of transparent conductive ZnO and Ga-doped ZnO films on polyethylene naphthalate sheets aged for six years in ambient atmospheric

Transparent conductive ZnO and Ga-doped (GZO) films that had been deposited on polyethylene naphthalate (PEN) sheet substrates were aged in ambient atmosphere conditions for six years. The films then evaluated to characterize the degradation in their sheet resistance over the substrate surface due to moisture permeation. The conductive properties of the ZnO and GZO films deposited on PEN substrates were more degraded compared to those for films deposited on glass substrates, exhibiting a nonuniform distribution of resistance. While ZnO and GZO films thinner than 100 nm were particularly sensitive to moisture infiltration, the increase in sheet resistance relative to the initial value became less obvious as thickness increased. This observation indicates that moisture permeation is limited to a certain depth from the surface. While GZO films thicker than 100 nm were free from signs of aging at Ga concentrations corresponding to standard transparent electrodes, regions with Ga concentrations more than 10% suffered from aging. This phenomenon has been observed in GZO films on glass substrates. The signs of aging were more serious for the ZnO films compared to the GZO films, as the oxygen vacancies in the ZnO films acted as reaction centers with H2O molecules. Furthermore, ZnO films thicker than 200 nm cracked because of misfit strain between the film and the PEN substrate, and cracks were widened and deeper for the thicker films. The cracked films experienced far greater deterioration than the films without cracks because moisture could easily enter the cracks and reach deep regions of the films.

Stress-induced surface characterization by wavelet and fractal analysis in Ga-doped ZnO thin films

ABSTRACTThe Ga-doped ZnO (GZO) were deposited by magnetron reactive sputtering on glass substrates at room temperature with different deposited times to obtain various thickness. The root-mean-square (RMS) roughness obtained from the atomic force microscopy (AFM) images is observed to shift linearly with the deposited time, the fractal geometry and multi-resolution signal decomposition (MRSD) based on wavelet transform were applied on the surface profiles and the results does not synchronously changes as the thickness, which is related to the profile’s frequency. The calculated compressive in-plane stress of highly c-axis oriented GZO films also shows an irregular variation as the increase of film thickness, what’s more, the in-plane stress and fractal dimension exhibit a polynomial relationship and the two parameters can be used for describing the surface morphology.

Growth of Ga-doped ZnO films by thermal oxidation with gallium and their optical properties

Metal gallium was evaporated onto ZnS films by physical vapor deposition method and then the thermal oxidation in the air was subsequently performed for the growth of Ga-doped ZnO films. The microstructures, photoluminescence (PL) and optical absorption properties of the Ga-doped ZnO films prepared under different deposition and oxidation conditions were investigated. The results showed that certain Ga doping can decrease the defects level, improve the crystallinity of ZnO films, and it became more effective with the extension of oxidation time. As the oxidation time increased, the Ga-doped ZnO films became more compact and uniform, displaying higher crystallinity. In addition, the optical band gaps of the ZnO films increased, the PL intensity of the visible emission decreased, and the luminescent center of the visible emission changed. Among them, the 505 nm emission resulted from oxygen vacancy, while the 539 nm emission was associated with oxygen interstitial.