Gallium-doped Zinc Oxide Films Research Articles

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.

Gallium‐Doped Zinc Oxide/Tungsten‐Doped Indium Oxide Stacks with Enhanced Lateral Transport Capability for Efficient and Low‐Cost Silicon Heterojunction Solar Cells

To decrease the series resistance in front gallium‐doped zinc oxide (GZO) silicon heterojunction (SHJ) solar cells caused by high‐resistivity GZO films, stack films including tungsten‐doped indium oxide (IWO) films which have better lateral transport properties are used as the front transparent conductive oxide (TCO) to improve charge transport. The crystal structure and electrical and optical characteristics of GZO/IWO stacks with different thickness ratios are investigated, and the current–voltage performance of SHJ solar cells with front GZO/IWO stacks and rear GZO film are analyzed. The effective transmittance of the stacks is greater than 98% in the visible region. When the thickness of GZO/IWO is 50 nm:50 nm, the resistivity reaches 8.59 × 10−4 Ω cm, which is a significantly 70% reduction compared with that of a single GZO film. Meanwhile, the power conversion efficiency is improved to 23.8%, effectively reducing the efficiency gap by approximately 0.12% compared to a single IWO transparent electrode. More effective lateral transport lowers the series resistance of SHJ solar cells. By employing stacks with lower indium content in the front TCO of SHJ solar cells, the cost can be reduced without significantly affecting the efficiency, which is important for the large‐scale development of SHJ solar cells.

High transparent conductive Ga-doped ZnO-based multilayer thin films with embedded ultrathin TiN layer deposited in oxygen-containing atmosphere.

To avoid metal layer oxidation during the deposition of transparent conductive oxide (TCO)/metal/TCO multilayer films in an oxygen-containing atmosphere, the ultra-thin (<10 nm) titanium nitride (TiN) layer has been proposed to replace metal embedding in gallium-doped zinc oxide (GZO) film for the development of indium-free transparent electrodes. The effects of TiN thickness on the structure, morphology, electrical, and optical properties of GZO/TiN/GZO multilayer thin films deposited in argon-oxygen mixtures on glass substrates by magnetron sputtering are investigated. The experimental results reveal that multilayers with the 8 nm-thick TiN layer have the optimal performance (figure of merit of 2.75 × 10-1 Ω-1): resistivity of 4.68 × 10-5 Ω cm, and optical transmittance of above 91% in the visible region, which is superior to the sandwich film with the metal embedded layer.

Preparation, optimization and electrochromic properties of dual-band device with four modes

A dual-band electrochromic device with four modes was fabricated to selective modulate the transmittance from visible to near-infrared bands for the energy-saving field. The new LiClO4 + TBACl + AgNO3 electrolyte was prepared to respectively regulate the redox reactions of silver ion and lithium ion to prove the four modes of the device. The dual-band device with a sandwich structure was fabricated by the titanium dioxide electrochromic film, the gallium doped zinc oxide ion storage film and the LiClO4 + TBACl + AgNO3 electrolyte. The titanium dioxide and gallium doped zinc oxide films were prepared by a nanoparticle deposition system, which is an environmentally friendly dry deposition technique. By controlling the supplied potential, the redox reactions of silver ion and lithium ion present four modes, including transparent mode, warm mode, cold mode and full block mode. For warm mode, the localized surface plasmon resonance effect was excited by generating silver nanoparticles using two-stage potential. Moreover, the titanium dioxide film with high surface roughness shown the large light scattering effect, the transmittance of ∼0% at whole wavelengths was obtained in full block mode. The electrochromic device shown the large transmittance modulation (71%) and excellent cyclic stability (600 cycles). Therefore, it is proved that the transmittance can be adjusted at a specific wavelength by the electrochromic device, which provides an effective way for developing dual-band smart devices to reduce energy consumption.

High-Antimicrobial Gallium-Doped Zinc Oxide Thin Films on Bio-Based Poly(Ethylene Furanoate) Substrates for Food Packaging Application

Thin films of gallium-doped zinc oxide (GZO), with a thickness of around fifty nanometers were deposited on bio-based poly(ethylene furanoate) (PEF) substrates by radio-frequency sputtering. By optimizing the Ga concentration in the target, the optics, water vapor barrier and antibacterial properties of PEF/GZO composite films can be adjusted. The highest visible light transmittance of the samples was around 85.1%. Furthermore, by introducing some GZO films with typical concentrations, the water vapor barrier and antibacterial properties of PEF films were improved. The optimized water vapor permeability of PEF/GZO composite film was 5.3 × 10-12 g·m/m2·s·Pa, and the highest antibacterial rate can reach 99.85% after 4 h. By XPS analysis, the antibacterial mechanism in the samples is envisaged to be mainly due cytotoxicity of Ga ions. The above results indicate that PEF/GZO films have great potential in the field of antibacterial food packaging.

Influence of oxygen on the optical, electrical, and heating properties of gallium-doped zinc oxide (GZO) films

Thin-film transparent heaters (TFTHs) are gaining popularity in optoelectronics and a variety of domestic applications, including smart windows, car defrosters, and other devices. The deposition and characterization of TFTHs made of gallium-doped zinc oxide (GZO) are presented in this work. GZO thin films were deposited via pulsed laser deposition on glass substrates with varying oxygen partial pressures from 0 to 10 mTorr during deposition. The samples demonstrated very low sheet resistance values between 5 and 17 Ω/sq from 0 to 10 mTorr, respectively. UV/vis transmission spectra revealed that TFTHs have a high optical transparency above 80%. GZO-based TFTHs demonstrated a consistent and repeatable joule heating effect, with temperatures reaching 76 °C with a low input voltage of 10 V. This research could guide the future use of GZO as a transparent conducting oxide material for many potential cost-effective applications from low-powered electronics to lightweight and wearable devices.

Study of Gallium-Doped Zinc Oxide Thin Films Processed by Atomic Layer Deposition and RF Magnetron Sputtering for Transparent Antenna Applications.

Gallium-doped zinc oxide (GZO) films were fabricated using RF magnetron sputtering and atomic layer deposition (ALD). The latter ones demonstrate higher electrical conductivities (up to 2700 S cm-1) and enhanced charge mobilities (18 cm2 V-1 s-1). The morphological analysis reveals differences mostly due to the very different nature of the deposition processes. The film deposited via ALD shows an increased transmittance in the visible range and a very small one in the infrared range that leads to a figure of merit of 0.009 Ω-1 (10 times higher than for the films deposited via sputtering). A benchmarking is made with an RF sputtered indium-doped tin oxide (ITO) film used conventionally in the industry. Another comparison between ZnO, Al:ZnO (AZO), and Ga:ZnO (GZO) films fabricated by ALD is presented, and the evolution of physical properties with doping is evidenced. Finally, we processed GZO thin films on a glass substrate into patterned transparent patch antennas to demonstrate an application case of short-range communication by means of the Bluetooth Low Energy (BLE) protocol. The GZO transparent antennas' performances are compared to a reference ITO antenna on a glass substrate and a conventional copper antenna on FR4 PCB. The results highlight the possibility to use the transparent GZO antenna for reliable short-range communication and the achievability of an antenna entirely processed by ALD.

Influence of oxygen partial pressure on the strain behaviour of reactively co-sputtered Ga doped ZnO thin films

Gallium doped ZnO films (GZO) were deposited by reactive co-sputtering of Zn and GaAs at different partial pressures of oxygen in Ar-O2 mixture. The GZO films were deposited at 375 °C, with GaAs area coverage of 0 - 3 % and 4 - 10 % O2 in sputtering atmosphere. The composition of GZO films was studied by X-ray photoelectron spectroscopy, which showed that the films deposited at 4 % and 5 % O2 display Ga/Zn ratio of ∼ 0.1 and contain oxygen vacancies, while those deposited at 6 % and 7 % O2 display Ga/Zn ratio of ∼ 0.01 and contain chemisorbed/interstitial oxygen. X-ray diffraction measurements supported by Raman studies revealed that undoped ZnO films are strongly c-axis oriented and display decrease of crystallite size along c-axis, along with substantial increase of hydrostatic tensile strain with increase of O2 percentage, which are attributed to the presence of interstitial oxygen. In sharp contrast, GZO films deposited at 4 % and 5 % O2 under limited oxygen availability, display smaller and mis-oriented crystallites along c-axis and large compressive strain, which is also hydrostatic, owing to the incorporation of Ga. The GZO films deposited at 6 % and 7 % O2 display improved crystallinity and decrease of compressive strain, which are attributed to reduced Ga content of the films and sufficient availability of oxygen during deposition. This study reveals the critical influence of the partial pressure of oxygen on the stoichiometry, doping concentration, crystallinity and the nature of strain in reactively sputtered GZO films.

Deterministic modeling of hybrid nonlinear effects in epsilon-near-zero thin films

In nonlinear optics, significant effort is concentrated on improving the strength and efficiency of interactions; however, experimentally investigating nonlinear materials is a complex, time-consuming, and costly investment. Moreover, it is often challenging to isolate, study, and optimize material parameters in an experiment due to complexities in the growth process. Recently, epsilon-near-zero materials have received a great deal of attention as promising nonlinear optical materials, but like many up-and-coming materials, the ability to explore and optimize their properties has been challenging. Here, we establish a framework to rapidly evaluate the performance of nonlinear epsilon-near-zero materials for both inter- and intraband effects in silico, requiring only an energy–momentum (E–k) diagram, linear optical properties, and experimental conditions. Measured nonlinear reflection and transmission in gallium-doped zinc oxide films are compared to the numerical framework for both intra- and interband excitation to verify accuracy across wavelength and irradiance while two figures of merit (FoMs) are introduced to quickly evaluate the performance of films without a full numerical framework. This capability is used to predict the performance of highly doped gallium nitride, cadmium oxide, zinc oxide, and indium tin oxide films, and efficient intra- and interband operation conditions are identified. Through this numerical framework and the FoMs, the exploration of unstudied epsilon-near-zero materials is enabled without the need for a nonlinear experiment, thereby accelerating the search for more efficient nonlinear materials and excitation conditions.

Spray deposited gallium doped zinc oxide (GZO) thin film as the electron transport layer in inverted organic solar cells

The electric field-assisted spray deposition of highly conducting and transparent gallium doped zinc oxide (GZO) films, with noticeable improvement in optical, structural, and electrical properties, is reported for application in organic photovoltaics with superior device performance. The GZO films deposited with applied voltages of 0 and 2000V to the nozzle exhibited an improved electrical resistivity of ∼1.8 × 10−3 Ω-cm and a transmittance (in the visible region) of over 80% for the films deposited under electric field, which can be used as electron transport layer (ETL) in inverted organic solar cells (IOSCs- ITO/GZO/PTB7:PC71BM/MoO3/Ag). Improved photovoltaic performance (power conversion efficiency: 6.5%) is obtained for the IOSC devices fabricated GZO ETL with the applied voltage of 2000V in comparison to the device fabricated using GZO ETL with the applied voltage = 0V (power conversion efficiency: 5.8%).

Gallium doped zinc oxide thin films as transparent conducting oxide for thin-film heaters

The addition of suitable metallic dopants into the indium or zinc oxide matrix is essential to obtain transparent conducting oxide (TCO) thin films for high-performance optoelectronics devices. However, scarcity of indium is one of the major challenges for the common use of indium doped tin oxide (ITO) as a TCO material for future state-of-the-art devices. To overcome the challenge, doped zinc oxide is used an alternative material for traditional ITO and retains both high transparency and electrical conductivity. One such potential material is gallium-doped zinc oxide (GZO). GZO thin films were deposited onto glass as well as Kapton substrates using the pulsed laser deposition technique. Structural, optical, and electro-thermal properties of these films were studied to assess the performance of the films as thin-film transparent heaters. The samples show a good transmittance value greater than 85% in the visible range of the electromagnetic spectrum. At room temperature, the electrical resistivity of GZO films showed a value of 110.46 × 10−4 Ω cm on glass and 2.90 × 10−4 Ω cm on the Kapton substrate, followed by a Joule heating effect, with temperatures reaching more than 120 °C at an applied voltage of ∼12 V. This high transparency, cost-effectiveness, low sheet resistance, and small surface roughness make GZO a unique and potential candidate for various practical applications not only as a transparent electrode but also as an indium free thin-film transparent heater and an affordable transparent conducting oxide in displays.

Structural and optical properties of ZnO:Ga thin films deposited on ito/glass substrates for optoelectronic applications

Doped (with GaCl3), undoped ZnO and ITO/ZnO:Ga nanostructured thin films are synthesized using the spray pyrolysis method. The doped ZnO thin films are synthesized at the atomic ratio of Ga/Zn added in the starting solution fixed at 1, 2, 3, and 5. Gallium-doped ZnO films synthesized on glass/ITO substrates are annealed at 4500C in different environments: vacuum, oxygen, and hydrogen. X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and current–voltage (I–V) measurements are applied to characterize the structural properties, composition, surface morphology, and electrical properties of ZnO:Ga nanostructured thin films. X-ray diffraction analysis shows that ZnO:Ga films deposited on glass substrates have a dense and homogeneous surface with a hexagonal structure. The ZnO:Ga films deposited on glass/ITO substrates are composed of two phases, namely, hexagonal ZnO and cubic ITO. The I–V characteristics show the presence of good ohmic contacts between Al and In metals and ZnO:Ga thin films regardless of the nature of the substrate and the annealing atmosphere.

Time-of-flight secondary ion mass spectrometry fragment regularity in gallium-doped zinc oxide thin films

Time-of-flight secondary ion mass spectrometry fragment analysis remains a challenging task. The fragment appearance regularity (FAR) rule is particularly useful for two-element compounds such as ZnO. Ion fragments appearing in the form of ZnxOy obey the rule 2x ge 2y + 1 in the positive secondary ion spectrum and 2x le 2y + 1 in the negative spectrum where the valence of Zn is + 2 and that of O is − 2. Fragment analysis in gallium-doped ZnO (GZO) films can give insights into the bonding of the elements in this important semiconductor. Fragment analysis of 1 and 7 wt% GZO films shows that only the negative ion fragments obey the FAR rule where ZnO‒, 66ZnO‒, 68ZnO‒ and ZnO2‒ ion fragments appear. In the positive polarity, subdued peaks from out-of-the-rule ZnO+, 66ZnO+ and 68ZnO+ ion fragments are observed. The Ga ion peaks are present in both the positive and negative spectra. The secondary ion spectra of undoped ZnO also shows consistency with the FAR rule. This implies that Ga doping even in amounts that exceed the ZnO lattice limit of solubility does not affect the compliance with the FAR rule.

Refractive index and salinity sensors by gallium-doped zinc oxide thin film coated on side-polished fibers

This work presents a high-sensitivity refractive index and salinity sensor by using fiber-optic side-polishing and electron-beam evaporation techniques. Thin film coated on the flat surface of side-polished fibers can generate a lossy mode resonance (LMR) effect. A gallium-doped zinc oxide (GZO) thin film was prepared by an electron-beam evaporation with the ion assisted deposition method. The residual thickness of the side-polished fiber was 76.5 μm, and GZO film thickness of 69 nm was deposited on the flat surface of the side-polished fiber to fabricate LMR-based fiber sensors. The variation in the optical spectrum of LMR-based fiber sensors was measured by different refractive index saline solutions. The LMR wavelength shift is caused by the refractive index change, which is nearly proportional to the salinity. The corresponding sensitivity of the proposed fiber-optic sensor was 3059 nm/RIU (refractive index unit) for the refractive index range of 1.333 to 1.398. To evaluate the sensitivity of LMR salinity sensors, the saline solution salinities of 3.6%, 7.3%, 10.9%, 14.6%, 18.2% and 21.9% were measured in this work. The experimental result shows that the sensitivity of the proposed salinity sensor is 9.94 nm/%.

Kramers-Kronig analysis of the optical linearity and nonlinearity of nanostructured Ga-doped ZnO thin films

Pure zinc oxide and gallium-doped ZnO films have been deposited on a glass substrate using a sol-gel assisted spin coating technique. The prepared films were characterized by structural, morphological, optical, and the effect of doping is studied. XRD analyses of the films confirm amorphous nature. Raman studies give support to XRD and confirm secondary phases are absent in the film. The AFM images give the grain size, which decreases with an increase in doping concentration from 116 nm to 81 nm. Roughness, Skewness, and Kurtosis factor are varying from 45.694 nm to 29.153 nm 1.085 to 0.453, 3.583 to 2.605, respectively. Kramers-Kronig equations are the most accurate methodology for optical thin-film technology for the calculations of optical linearity and nonlinearity parameters. The bandgap values are decreasing with increasing thickness from 2.97 to 2.79, respectively. The doped films are more transparent compared to the pure ZnO films. The enhancement in the linear optical properties such as refractive index, absorption index, and dielectric constant are also observed with an increase in the doping concentration. The nonlinear optical properties such as χ(1), χ(3) and n(2) are varying in the range of 0.288 esu to 0.252 esu, 1.1 × 10−12 esu to 7 × 10−13 esu, 1.92 × 10−11 esu to 1.30 × 10−11 esu, respectively. The enhancement in optical properties with increasing the doping concentration suggests the present films are useful for various optical device applications.

Gallium-doped zinc oxide films with diverse nanomorphologies grown via sol–gel united spin coating technique

Some gallium-doped zinc oxide nanofilms (GZONFs) with diverse morphologies were produced on the p-type Si (100) substrate by the sol–gel united spin coating. The thermally annealed nanofilms were characterized using different techniques to determine the influence of various Ga contents (0–5%) on their structures, morphologies, optical and electrical characteristics. The XRD patterns of the as-prepared nanofilms displayed the existence of the single-phase polycrystalline particles with varying sizes (55–36 nm). The band gap values of these GZONFs were ranged from 3.227–3.269 eV. The nanofilm produced with 1% of Ga exhibited the minimal electrical resistivity ≈ 4.6429 × 10–3 Ω cm, highest carrier density ≈ 1.37648 × 1020 cm−3 and Hall mobility ≈ 9.779 cm2/V s. It was shown that the overall properties of the proposed GZONFs can be customized by adjusting the concentration of Ga doping.

Aerosol-assisted route to low-E transparent conductive gallium-doped zinc oxide coatings from pre-organized and halogen-free precursor.

Thermal control in low-emission windows is achieved by the application of glazings, which are simultaneously optically transparent in the visible and reflective in the near-infrared (IR). This phenomenon is characteristic of coatings with wide optical band gaps that have high enough charge carrier concentrations for the material to interact with electromagnetic radiation in the IR region. While conventional low-E coatings are composed of sandwiched structures of oxides and thin Ag films or of fluorinated SnO2 coatings, ZnO-based glazing offers an environmentally stable and economical alternative with competitive optoelectronic properties. In this work, gallium-doped zinc oxide (GZO) coatings with properties for low-E coatings that exceed industrial standards (Tvisible > 82%; R2500 nm > 90%; λ(plasma) = 1290 nm; ρ = 4.7 × 10−4 Ω cm; Rsh = 9.4 Ω·□−1) are deposited through a sustainable and environmentally friendly halogen-free deposition route from [Ga(acac)3] and a pre-organized zinc oxide precursor [EtZnOiPr]4 (1) via single-pot aerosol-assisted chemical vapor deposition. GZO films are highly (002)-textured, smooth and compact without need of epitaxial growth. The method herein describes the synthesis of coatings with opto-electronic properties commonly achievable only through high-vacuum methods, and provides an alternative to the use of pyrophoric ZnEt2 and halogenated SnO2 coatings currently used in low-emission glazing and photovoltaic technology.

Lossy Mode Resonance Sensors Fabricated by RF Magnetron Sputtering GZO Thin Film and D-Shaped Fibers

We demonstrate a new refractive index (RI) and salinity sensor based on a lossy mode resonance (LMR) effect which combines fiber-optic side-polishing and radio-frequency (RF) sputtering techniques. The side-polished fiber can enhance optical fibers to generate an evanescent field in sensing applications. Gallium-doped zinc oxide (GZO) thin films produce a high attenuation lossy mode resonance effect that permits a highly sensitive refractive index and salinity fiber sensor. GZO thin film was prepared by an RF magnetron sputtering method. The thickness of the D-shaped fiber sensing device was 74.7 μm, and a GZO film thickness of 67 nm was deposited on the polished surface of the D-shaped fiber to fabricate LMR type liquid salinity sensors. The sensitivity of 3637.8 nm/RIU was achieved in the RI range of 1.333 to 1.392. To investigate the sensitivities of LMR salinity sensors, the NaCl solution salinities of 0%, 50%, 100%, 150%, 200%, and 250% were measured in this work. The experimental result shows that the sensitivity of the salinity sensor is 0.964 nm per salinity unit (SU).

Enhancement of Electrical Conductivity of Transparent Ga -Doped Zinc Oxide Films via Quench Reduction in an Atmospheric Pressure Plasma Jet

Transparent conducting oxides such as gallium-doped zinc oxide (GZO) are of increasing importance for displays and photovoltaic applications. For manufacturing, an atmospheric-pressure plasma jet (with no vacuum chamber) is suitable for continuous inline processing; however, deposition becomes more susceptible to oxygen quenching, reducing the plasma's reactivity and the formation of oxide. Here the authors show that the electrical conductivity and deposition rate of GZO films are both enhanced by using a different nozzle feature, to lessen oxygen quenching. These findings may be extended to other areas where vacuum-based technologies are not applicable, such as biomedical applications.

Influences of Ga Doping on Crystal Structure and Polarimetric Pattern of SHG in ZnO Nanofilms.

The second-harmonic generation (SHG) in gallium doped ZnO (GZO) nanofilms was studied. The Ga doping in GZO nanofilms influenced the crystal structure of the films, which affected SHG characteristics of the nanofilms. In our experiments, a strong SHG response was obtained in GZO nanofilms, which was excited by 790 nm femtosecond laser. It was observed that the Ga doping concentrations affected, not only the intensity, but also the polarimetric pattern of SHG in GZO nanofilms. For 5.0% doped GZO films, the SHG intensity increased about 70%. The intensity ratio of SHG between the incident light polarization angle of 90° and 0°changed with the Ga doping concentrations. It showed the most significant increase for 7.3% doped GZO films, with an increased ratio of c/a crystal constants. This result was attributed to the differences of the ratios of d33/d31 (the second-order nonlinear susceptibility components) induced by the crystal distortion. The results are helpful to investigate nanofilms doping levels and crystal distortion by SHG microscopy, which is a non-destructive and sensitive method.