Ga-doped ZnO Films Research Articles

Synergistic effect of concentration and annealing on structural, mechanical, and room-temperature thermoelectric properties of n-type Ga-doped ZnO films

The development of Ga-doped Zinc Oxide (GZO) films from nontoxic, abundant elements using a cost-effective and scalable approach is crucial for the profitability and sustainability of thermoelectric applications. Challenges in formulating GZO film inks have led to extensive experimentation to enhance their thermal, structural, mechanical, and room-temperature thermoelectric properties by adjusting ink concentration and annealing treatment. Increasing the GZO nanoparticle concentration reduced the melting temperature and crystallinity, whereas annealing at 200 °C decomposed the polyethylene oxide (PEO) binder. TEM analysis revealed the polycrystalline structure of the GZO nanoparticles and their interaction with the binder, while XRD patterns confirmed the characteristic peaks of the GZO films; annealing effectively eliminated the PEO diffraction peaks. The GZO films from the concentrated 1.24M ink exhibited minimal grain growth, reduced lattice strains, uniform elemental distribution, and enhanced surface texture and conductivity, which were further improved by annealing. Increasing the GZO nanoparticle concentration facilitated the formation of a conductive network, while annealing enhanced the conductivity by promoting the formation of a cohesive, interconnected network through impurity removal, nanoparticle redistribution, and coalescence. Consequently, the annealed 1.24M film demonstrated the highest nanohardness of 791 MPa and a thermoelectric power factor of 1.78 nW/m∙K2 at room temperature, which were attributed to enhanced electrical conductivity and Seebeck coefficient through concentration and annealing synergies.

Thin Ga-doped ZnO Film with Enhanced Dual Visible Lines Emission

Introduction: In this study, a simple and facile route was employed to prepare a highly transparent and luminescent ultra-thin gallium doped ZnO film (GZO). Methods: The thin GZO film has been deposited using the simultaneously ultrasonic vibration and sol-gel spin-spray coating technique. The structural and optical properties of pure and doped thin films were investigated by various methods, such as X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), scanning electron microscopy (SEM), UV-Vis, and PL spectroscopy. Results: XRD results indicated that both pure and doped ZnO films had a hexagonal wurtzite structure with (101) preferred orientation. XPS and EDX studies confirmed the incorporation and presence of Ga ions into the ZnO lattice structure. The doped sample showed nearly 90% of transparency, and a strong blue-green emission in the visible region. Conclusion: The obtained results proved that the prepared thin film could be a novel candidate for optoelectronic applications.

3D printing flexible Ga-doped ZnO films for wearable energy harvesting: thermoelectric and piezoelectric nanogenerators

The 3D printing of energy harvesters using earth-abundant and non-toxic elements promotes energy sustainability and market competitiveness. The semiconducting behavior and non-centrosymmetric wurtzite crystal structure of gallium-doped zinc oxide (GZO) films make them attractive for thermoelectric and piezoelectric nanogenerators. This study investigates the thermal, structural, mechanical, thermoelectric, and piezoelectric properties of 3D-printed GZO nanocomposite films. Thermal analysis demonstrates the stability of the nanocomposite film up to 230 °C, making it suitable for wearable energy harvesters. The crystalline structure of the nanocomposite film aligns with the hexagonal wurtzite structure of ZnO and displays a bulk-like microstructure with a uniform distribution of elements. The presence of Ga 2p, Zn 2p, O 1 s, and C 1 s core levels confirms the development of the nanocomposite film, characterized by a fine granular structure and a conductive domain compared to the neat resin film. The inclusion of GZO nanofillers tailors the stress–strain behavior of the nanocomposite film, enhancing flexibility. The 3D-printed GZO nanocomposite films demonstrate a promising thermoelectric power factor and piezoelectric power densities, along with mechanical flexibility and thermal stability. These advancements hold significant potential for wearable and hybrid energy generation technologies.

Hot-Electron Microwave Noise and Energy Relaxation in (Be)MgZnO/ZnO Heterostructures

Pulsed hot-electron microwave noise measurements of the (Be)MgZnO/ZnO heterostructures are presented in this work. The heterostructures of different barrier thicknesses and different bulk electron densities in ZnO layer are compared. Capacitance–voltage (C–V) measurements reveal the decrease in the two-dimensional electron gas (2DEG) peak in electron density profile at the Zn-polar BeMgZnO/ZnO interface as the BeMgZnO barrier layer thickness decreases. For thin-barrier heterostructures, the peak disappears and only the bulk electron density is resolved in C–V measurements. The excess noise temperature at ∼10 GHz in thick-barrier heterostructures is noticeably higher (∼10 times) compared to thin-barrier heterostructures, which is attributed to the strong noise source in the contacts of the former. In the case of thin-barrier heterostructures, at electric fields above ∼10 kV/cm and electron density ≳1×1017cm−3, strong noise source is resolved, which was also observed earlier in the Ga-doped ZnO films due to the formation of self-supporting high-field domains. However, for the low electron densities (≲6 ×1016 cm−3), the aforementioned noise source is not observed, which suggests the importance of a deep ZnO/GaN interface with 2DEG for power dissipation. The hot-electron temperature dependence on the dissipated power of those low-electron-density heterostructures is similar to that of O-polar ZnO/MgZnO. The estimated electron energy relaxation time in ZnO/MgZnO is ∼0.45 ps ± 0.05 ps at dissipated electrical power per electron of ∼0.1 nW/el and approaches ∼0.1 ps as the dissipated power is increased above ∼10 nW/el.

Optimization of photoelectric properties of transparent conductive B and Ga co-doped ZnO films for electrochromic applications

To improve the performance of electrochromic devices, it is urgently necessary to develop transparent indium-free conductive electrodes with high conductivity and transparency. In this study, thin films of Ga-doped ZnO (GZO) and B–Ga co-doped ZnO (BGZO) were prepared on glass substrates using the plasma-assisted atomic layer deposition (PEALD) technique, and their structural and optoelectronic properties were characterized. It is found that all thin films were consistent with hexagonal fibrous ZnO structure, and the surface morphology of the membrane was homogenous at nanoscale. UV spectroscopy analysis showed that the doping of B and Ga elements could increase the transparency of ZnO films. The electrical properties can be remarkably enhanced by doping with B and Ga elements. Further more, tungsten oxide (WO3) was deposited on the as-prepared BGZO films using magnetron sputtering method. We activated the sample by applying voltage and observed a color transition from transparent to dark blue. It has a modulation range of 65 % in the 633 nm spectrum and a coloring efficiency of 37.8 cm2C-1. This work opens up remarkable new opportunities for developing high-performance transparent conductive electrode in electrochromic devices.

Improved sputtering method to deposit high conducting doped ZnO films without substrate heating

Magnetron sputtering deposition with Zn supply was utilized to deposit Ga-doped ZnO (GZO) films to minimize acceptor-like crystalline defects. This deposition technique significantly increased the carrier concentration of GZO films. In addition, the impact of partial oxygen pressure on the deposition atmosphere on carrier concentration was remarkably reduced. As a result, the resistivity of the films decreased to as low as 4 × 10−4 Ωcm without the need for intentional substrate heating. Consequently, the deposition with Zn supply shows great potential for producing ZnO-based transparent conducting films with practically low resistivity on polymer substrates that have lower heat tolerance.

Relationship between electrical and optical characterization of Ga-doped ZnO thin films deposited by magnetron sputtering

Introduction: Transparent conducting films have received much attention in energy conversion applications. To replace high-cost indium-tin-oxide (ITO), Ga-doped ZnO (GZO) film is considered due to its high conductivity, good transparency, low cost, and low toxicity. Methods: GZO and pure ZnO films were deposited on glass substrates by dc magnetron sputtering. The crystalline structure of the samples was verified by using X-ray diffraction. In particular, the relationship between the electrical and optical characterization of the GZO film was investigated through the plasma wavelength obtained from transmittance and reflectance spectra. Meanwhile, carrier transport was directly confirmed by Hall effect-based measurements. Results: The GZO film shows a hexagonal wurtzite structure, with successful incorporation of Ga into the ZnO lattice. Ga doping increases the carrier concentration, leading to a decrease in the resistivity of the film. This study also discusses the correlation of carrier transport obtained from Hall effect-based measurements and optical spectroscopies. Here, we extracted the optical carrier concentration and optical mobility from the plasma wavelength and compared them with the Hall data. Conclusion: The dependence of carrier transport on ionized impurity scattering can be pointed out. It proposes an effective way to qualitatively predict the electrical characteristics and transport properties of thin films via optical transmittance and reflectance analysis.

The impact of oxygen on Ga doped ZnO film.

The Ga doped ZnO (GZO) film is one of the promising alternative films to replace ITO film, but its properties suffer from degradation when it is deposited under oxygen-rich conditions. This degradation has been investigated by depositing the films under different oxygen partial pressures. XRD results showed that all GZO films had wurtzite structure and the lattice parameter-c contracted when oxygen was introduced into the argon deposition atmosphere, but the parameter-c nearly remained constant when oxygen partial pressures were further increased. The contraction of parameter-c was caused by the increasing concentrations of VZn (Zn vacancy). It was the first time to observe that the impurity phase of Ga2Zn6O9 appeared and disappeared in GZO films during the increase of oxygen partial pressures. Analogously, conductivity decayed and optical bandgap decreased abruptly as oxygen was introduced, which enhanced self-compensation of donors and acceptors. The energy band structures of GZO and ZnO films were determined by using UPS, and the results showed that oxygen had little effect on the electron affinity of the GZO film, but a significant difference in electron affinity between the ZnO and GZO films was observed. This result indicated that although the electron affinity of ZnO could be effectively tuned by doping with Ga, it remained quite stable for GZO under oxygen-rich conditions.

Influence of neutron/gamma irradiation on damage and scintillation of Ga-doped ZnO thin films

As a new type of inorganic scintillator, ZnO crystal doped with Ga has been used for Ultra-fast scintillating detectors with fast response to detect X-ray, gamma, neutron, and charged particles. Moreover, due to the large bandgap energy and high threshold displacement energy, ZnO: Ga has a strong radiation resistance in theory. However, there is a lack of experimental studies on the change of scintillation properties of ZnO: Ga under an intense irradiation environment. This study deposited thin Ga-doped ZnO films on α-Al2O3(0001) substrate by dual-target reactive magnetron sputtering. The ZnO: Ga samples were irradiated by neutrons from 1010 to 7.1 × 1014 neutrons/cm2 (1 MeV neutron equivalent fluence) and γ rays from 500 to 10,000 Gy, respectively. The structural and optical properties of all irradiated samples were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), optical transmittance (TS), and effective light output at room temperature. The results indicated that all the irradiated samples are still in both hexagonal wurtzite structure and c-axis orientation. Compared with unirradiated samples, it is found that the samples' optical transmittance, energy resolution, and effective light output have little change due to irradiation, which has fully proved that the ZnO: Ga scintillation films have good radiation resistance under neutron and gamma irradiation. Hence, ZnO: Ga is an attractive candidate for radiation detectors with ultra-fast responses under extreme radiation environments.

Pressure and Thickness Dependence of Physical Properties of ZnO:Ga Thin Films by Radio Frequency Magnetron Sputtering

Indium-free transparent conducting oxide films have attracted extensive attention in the field of optoelectronics. Ga-doped ZnO (GZO) thin films are deposited by radio frequency magnetron sputtering on glass substrates at a temperature of 200 °C with ZnO:Ga2O3 (3 wt%). The structural, electrical, and optical properties of the GZO thin films were investigated in terms of deposition pressure and film thickness variations. X-ray diffraction analysis showed that all the prepared GZO films exhibited hexagonal wurtzite crystal structure with a (002) preferential orientation along the c-axis, regardless of pressure and thickness. The average visible transmittance (including the glass substrate) in a wavelength range of 400–700 nm decreased with increasing thickness but varied less with pressure. The highest average visible transmittance reached 88.4% at the thickness of 150 nm and the pressure of 5 mTorr. The optical band gap of the GZO films calculated using Tauc’s method was in the range of about 3.6–3.9 eV. The resistivity of GZO thin films decreased with decreasing deposition pressure and increasing film thickness, and the minimum resistivity obtained at 5 mTorr and 1000 nm was 3.36 × 10-4 Ω-cm. The maximum figure of merit (FOM) of 3.09 × 10-2 Ω-1 was achieved at 5 mTorr and 1000 nm. The superior optical and electrical properties and high FOM show that the prepared GZO thin films are suitable for transparent conducting films and optoelectronic devices.

Infrared surface-plasmon-resonance attenuator for broadly controllable effective radiant temperature

We describe a controllable infrared plasmonic laser attenuator with application to infrared detector characterization. The device promises a broad range of effective radiant temperature and fine temperature control/resolution near ambient. The enabling mechanism is controllably-frustrated surface-plasmon-resonance using a Kretschmann prism coupler. The predicted wide dynamic range depends on optical and geometric tolerances for the coupler. To investigate these, a mid-wave-infrared coupler comprising a conducting Ga-doped ZnO film deposited directly on a right-angle sapphire prism was fabricated, tested, and compared with theory. The attenuation resonance was observed by measuring specular reflection of a p-polarized quantum cascade laser beam at 4.45 μm wavelength as a function of internal incidence angle from the coated prism face. The predicted resonance for comparison was based on ellipsometrically-obtained optical constants and film thickness. Near perfect match between theory and experiment was achieved after adjusting for experimental uncertainties in optical parameters. The results quantify the accuracy and precision with which optical constants and geometrical parameters must be known to achieve the predicted performance.

Effect of GZO cap-layer thickness and post-annealing treatment on GZO/HGZO bi-layer films

Hydrogen and Ga-codoped ZnO (HGZO) films caped with different thickness of Ga-doped ZnO (GZO) films (0–80 nm) were prepared by RF magnetron sputtering at room temperature, and the influences of GZO layer thickness and post-annealing treatment on structural, electrical and optical properties of the GZO/HGZO bi-layer films were investigated. The results show that increasing GZO layer thickness weakens the (002) preferred orientation, decreases the crystallize size and increases compressive stress of the films. The electrically conductive properties of the films first enhance and then degrade with increase of GZO layer thickness, and the best conductive properties can be obtained at GZO layer thickness of 20 nm. As GZO layer thickness increases, transmittance in visible range (TVis) and bandgap (Eg) of the films show a decreased trend, but their Urbach energy (Eu) increases. The crystallite size of the films first greatly and then slightly increases with increasing annealing temperatures to 400 °C, and annealing atmosphere (air or vacuum) and time (10 min or 2 h) have little effect on the crystallite size at the same annealing temperature. With increasing annealing temperature, compressive stress greatly decreases firstly and then slightly decreases under vacuum annealing or converts to tensile stress under air annealing. The conductive properties of the films degrade after vacuum annealing for 2 h, but they degrade significantly even after air annealing for 10 min. Increasing GZO layer thickness can retard the decline of conductive properties of the films under vacuum annealing of 200–400 °C for 2 h or air annealing of 200–300 °C for 10 min. With increasing annealing temperature, the TVis of the films basically shows a trend of first increase and then decrease, and the Eg and Eu of the films basically decrease. After annealing at 400 °C, the decreased Eg and Eu values of films under air annealing are close for different annealing time, but they are larger than those under vacuum annealing. Basically, thicker GZO layer is more likely to restrain the decrease of Eg, but leads to obvious decrease of Eu. In addition, effect of hydrogen doping and out-diffusion, crystallize growth and stress relaxation mechanism under annealing, and carrier transport and Eg shift mechanisms are also discussed.

Integration of graphene with GZO as TCO layer and its impact on solar cell performance

In this study, we investigated the impact of incorporating graphene with Ga-doped ZnO (GZO) when employing them as a TCO layer on Si-based solar cell. GZO thin films with various thicknesses (50–450 nm) were fabricated by the sputtering method using a single target. The aim here was to determine the GZO film with the optimum thickness to incorporate it with single layer graphene as TCO. This thickness was found to be 350 nm as that was the best crystalline quality found in the XRD pattern. Further, this sample had the lowest sheet resistance and highest transmission values as confirmed by electrical (sheet resistance), and optical characterizations (transmission). Topographic (SEM and AFM), electrical (resistivity and carrier concentration) measurements were also conducted on the same sample. The graphene film grown on copper in a CVD system was then transferred on top of this sample to fabricate the hybrid TCO structure. We found that graphene integrated GZO hybrid TCO film showed higher sheet resistance due to high sheet resistance of graphene and similar optical properties thanks to high optical transmission of graphene. Employing graphene-based TCO layer in the solar cell resulted in higher open-circuit voltage, consequently improving the conversion efficiency from 10.0% to 11.2%.

Characteristics of GZO-based multilayer transparent conducting films

Ga-doped ZnO (GZO)/metal/GZO structures were fabricated on glass substrates to be the transparent conducting layers in this study. GZO films and metal films were deposited at room-temperature by a radio-frequency sputter and a thermal evaporator, respectively. The GZO/Ag/GZO (GAG) structures had poor electrical and optical properties due to the formation of Ag islands on the GZO layer. A 1-nm Cu seed layer was deposited on the GZO layer to fabricate the GZO/Ag/Cu/GZO (GACG) structure to improve its electrical and optical properties. The GACG structure had sheet resistance of 9 [Formula: see text], average visible transmittance of 86% and figure of merit of [Formula: see text] [Formula: see text]. In addition, the sheet resistance of the GACG structure kept almost the same after annealing at [Formula: see text]C in atmosphere for more than 5 h, which showed good thermal stability.

Performances of thin film transistors with Ga-doped ZnO source and drain electrodes

In this paper, indium-gallium-zinc-oxide thin film transistors (TFTs) were successfully fabricated by the pulsed laser deposition Ga-doped ZnO (GZO) films as source/drain (S/D) electrodes. And the GZO electrodes involved were prepared at room temperature. It was found that with the increase of Ga doping content, the electrical properties of TFTs increased first and then decreased. When the doping amount of Ga2O3 was 2 wt.%, the TFT showed excellent performance with a μ sat of 12.22 cm2V−1 s−1, an I on/I off of 6.4 × 107, a V on of 0 V and a SS of 0.082 V decade−1. At this time, the contact between the S/D electrodes and the active layer was ohmic and the TFT had a very low contact resistance with an R SD-eff of 0.064 Ω cm2. In addition, the device exhibited good electrical stability, and the drift of V on is only 2/−0.2 V at a positive/negative bias gate voltage of +10 V/−10 V with a duration of 5400 s under dark condition.

Defect species in Ga-doped ZnO films characterized by photoluminescence

Photoluminescence (PL) spectra of Ga-doped ZnO (GZO) films were investigated with reference to those of undoped ZnO films to elucidate the effect of high-level Ga3+ doping. A transition from ZnO-like to GZO-like spectra occurred at a Ga content around 2 at. %. The room temperature PL spectra of sufficiently oxidized GZO films exhibited band edge and violet components, while emissions at wavelengths longer than 480 nm were sharply cut off. The close resemblance of the spectral shapes of the GZO and Zn-rich ZnO films indicated disordering of the ZnO lattice by excess Ga dopants. Deposition under a reducing atmosphere at 100 and 200 °C produced oxygen-deficient GZO films with additional emission signals corresponding to oxygen vacancy (VO) defects between 480 and 600 nm. For GZO films with Ga content larger than 4 at. %, increasing the deposition temperature above 400 °C or postannealing at 500 °C smeared out deep-level emission signals, suppressed the near-band edge emission, and deactivated the donor role of Ga3+. These changes can be ascribed to outdiffusion of Ga3+ from the cation sites and rearrangement of the ZnO crystal network. Argon plasma treatment of GZO films generated a VO-related emission band through preferential sputtering of oxygen atoms. Hydrogen donors trapped at the resulting VOs would be the origin of a slight increase in carrier concentration, by 1 × 1020 cm−3.

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

Controlling thickness to tune performance of high-mobility transparent conducting films deposited from Ga-doped ZnO ceramic target

Structure modification has been found to tune significantly the transparent-conducting performance, especially mobility and conductivity of hydrogenated Ga-doped ZnO (HGZO) films. The strong correlation between film thickness and mobility of the films is revealed. The mobility increases quickly with increasing the thickness from 350 to 900 nm, and then tends to be saturated at further thicknesses. A higher mobility than 50 cm2/Vs can be achieved, which is an extra-high value for polycrystalline ZnO films deposited by using the sputtering technique. The thickness-dependent mobility originates from scatterings on grain boundaries and dislocation-induced defects controlled by thin-film growth. Based on the Volmer-Weber model, an expansion model is built up to describe the thickness-dependent crystal growth of the HGZO films, especially at the thick films. As a result, the 800 nm-thick HGZO film obtains the highest performance with high mobility of 51.5 cm2/Vs, low resistivity of 5.3 × 10−4 Ωcm, and good transmittance of 83.3 %.

Effect of oxygen partial pressure on the behavior of Ga-doped ZnO/p-Si heterojunction diodes fabricated by reactive sputtering

A systematic investigation of Ga-doped ZnO (GZO) films deposited by radio frequency reactive magnetron co-sputtering of Zn and GaAs has been carried out toward the realization of n-GZO/p-Si heterojunction diodes. X-ray diffraction and X-ray photoelectron spectroscopy studies of these films have shown a strong dependence of the c-axis orientation of crystallites, Ga/Zn ratio, oxygen vacancies and chemisorbed oxygen species on the partial pressure (percentage) of oxygen in the Ar-O2 sputtering atmosphere. The GZO films deposited at low O2 percentage (≤ 6%) exhibit Ga/Zn ratio ~ 0.03 and consequently high conductivity ~ 103 Ω−1 cm−1 with carrier concentration > 1020 cm−3, leading to the formation of non-rectifying contact with p-Si. A significant finding of the present study is that the doping level and conductivity of GZO layer can be controlled by O2 percentage in sputtering atmosphere, which has been utilized to fabricate heterojunction diodes with GZO films deposited above 6% O2, which possess low Ga/Zn ratio ~ 0.01 and carrier concentration ≤1019 cm−3. In particular, the diode fabricated with 8% O2 displays ideality factor ~ 7 and reverse saturation current ~ 2 × 10–6 A, along with improved turn-on-voltage and series resistance. The nearly complete c-axis orientation, absence of oxygen vacancies and presence of chemisorbed oxygen at the grain boundaries in GZO layer appear to facilitate improved diode performance.

High performance GZO/p-Si heterojunction diodes fabricated by reactive co-sputtering of Zn and GaAs through the control of GZO layer thickness.

The effect of thickness of Ga doped ZnO (GZO) layer on the performance of GZO/p-Si heterojunctions fabricated by reactive co-sputtering of Zn–GaAs target is investigated. GZO films were deposited at 375 °C with 0.5% GaAs area coverage of Zn target and 5% O2 in sputtering atmosphere. X-ray diffraction and X-ray photoelectron spectroscopy show that c-axis orientation of crystallites, Ga/Zn ratio and oxygen related defects depend substantially on the thickness of films. The 200–350 nm thick GZO films display low carrier concentration ∼1017 cm−3, which increases to >1020 cm−3 for thicker films. The diodes fabricated with >500 nm thick GZO layers display non-rectifying behaviour, while those fabricated with 200–350 nm thick GZO layers display nearly ideal rectification with diode factors of 1.5–2.5, along with, turn-on voltage ∼1 V, reverse saturation current ∼10−5 A, barrier height ∼0.4 eV and series resistance ∼200 Ω. The drastically improved diode performance is attributed to small Ga/Zn ratio (∼0.01) and extremely low dopant activation (∼0.3%), owing to diffusion and non-substitutional incorporation of Ga in thin GZO layers, which cause self-adjustment of doping concentration. These factors, together with c-axis orientation and chemisorbed oxygen at grain boundaries, facilitate ideal diode characteristics, not reported earlier for GZO/p-Si heterojunctions.