Gallium Oxide Thin Films Research Articles

Balancing Carrier Dynamics in Oxygen-Vacancy-Tuned Amorphous Ga2O3 Thin-Film Self-Powered Photoelectrochemical-Type Solar-Blind Photodetector Arrays for Underwater Imaging.

Underwater imaging technology plays a pivotal role in marine exploration and reconnaissance, necessitating photodetectors (PDs) with high responsivity, fast response speed, and low preparation costs. This study presents the synergistic optimization of responsivity and response speed in self-powered photoelectrochemical (PEC)-type photodetector arrays based on oxygen-vacancy-tuned amorphous gallium oxide (a-Ga2O3) thin films, specifically designed for solar-blind underwater detection. Utilizing a low-cost one-step sputtering process with controlled oxygen flow, a-Ga2O3 thin films with varying oxygen vacancy (VO) concentrations are fabricated. By balancing the trade-offs among electrocatalytic reactions, charge transfer, carrier recombination, and trapping, both the responsivity and response speed of a-Ga2O3-based self-powered PEC-PDs are simultaneously improved. Consequently, the optimized PEC-PDs demonstrated exceptional performance, achieving a responsivity of 33.75mA W-1 and response times of 12.8ms (rise) and 31.3ms (decay), outperforming the vast majority of similar devices. Furthermore, a pronounced positive correlation between anomalous transient photocurrent spikes and the concentration of VO defects is observed, offering compelling evidence for VO-mediated indirect recombination. Finally, the proof-of-concept solar-blind underwater imaging system, utilizing an array of self-powered PEC-PDs, demonstrated clear imaging capabilities in seawater. This work provides valuable insight into the potential for developing cost-effective, high-performance a-Ga2O3 thin-film-based PEC-PDs for advanced underwater imaging technology.

Plasma surface treatment of amorphous Ga2O3 thin films for solar-blind ultraviolet photodetectors

Recently, amorphous gallium oxide (a-Ga2O3) thin films are gaining more and more attention due to their advantages of low cost and low temperature growth. However, a-Ga2O3 solar-blind ultraviolet photodetectors are plagued by issues such as slow response speed and high dark current. In this work, radio frequency magnetron sputtering was used for a-Ga2O3 deposition, and a novel approach of plasma treatment (including oxygen, argon and hydrogen plasmas) was employed to tailor the device performance using a plasma-enhanced chemical vapor deposition system. It is demonstrated that: (i) the oxygen atoms and ions in the oxygen plasma effectively reduce the oxygen vacancy concentration and improve the respond speed of a-Ga2O3 photodetectors; (ii) the argon ions in the argon plasma create additional defects (such as dangling bonds and oxygen vacancies) and improve the responsivity of a-Ga2O3 photodetectors; (iii) the hydrogen atoms in the hydrogen plasma may incorporate into the film and form an ultra-thin passivation layer near the surface, which gives rise to a balance between responsivity and response speed. Finally, through the detailed analysis of the optical emission spectra and photoelectric performance, the underlying physical mechanism based on the energy band diagram has been proposed to interpret the obtained experimental results.

An Aerosol-Assisted Chemical Vapor Deposition Route to Tin-Doped Gallium Oxide Thin Films with Optoelectronic Properties.

Gallium oxide is a wide-bandgap compound semiconductor material renowned for its diverse applications spanning gas sensors, liquid crystal displays, transparent electrodes, and ultraviolet detectors. This paper details the aerosol assisted chemical vapor deposition synthesis of tin doped gallium oxide thin films using gallium acetylacetonate and monobutyltin trichloride dissolved in methanol. It was observed that Sn doping resulted in a reduction in the transmittance of Ga2O3 films within the visible spectrum, while preserving the wide bandgap characteristics of 4.8 eV. Furthermore, Hall effect testing revealed a substantial decrease in the resistivity of Sn-doped Ga2O3 films, reducing it from 4.2 × 106 Ω cm to 2 × 105 Ω cm for the 2.5 at. % Sn:Ga2O3 compared to the nominally undoped Ga2O3.

Raman Spectroscopy as an Effective Tool for Assessment of Structural Quality and Polymorphism of Gallium Oxide (Ga2O3) Thin Films.

Raman spectroscopy, a versatile and nondestructive technique, was employed to develop a methodology for gallium oxide (Ga2O3) phase detection and identification. This methodology combines experimental results with a comprehensive literature survey. The established Raman approach offers a powerful tool for nondestructively assessing phase purity and detecting secondary phases in Ga2O3 thin films. X-ray diffraction was used for comparison, highlighting the complementary information that these techniques may provide for Ga2O3 characterization. Few case studies are included to demonstrate the usefulness of the proposed spectroscopic approach, namely the impact of deposition conditions such as metal-organic vapor-phase epitaxy and pulsed electron deposition (PED), and extrinsic elements provided during growth (Sn in the case of PED) on Ga2O3 polymorphism. In conclusion, it is shown that Raman spectroscopy offers a quick, reliable, and nondestructive high-resolution approach for Ga2O3 thin film characterization, especially concerning phase detection and crystalline quality.

PLD‐Grown Semi‐Insulating Ga2O3 Thin Film‐Based Optoelectronic Artificial Synaptic Devices for Neuromorphic Computing Applications

AbstractDevelopment of artificial opto‐electronic synaptic devices plays a crucial role for the practical implementation of energy‐efficient, parallel processing of human brain. In this article, two terminal inter‐digitated devices are fabricated on Gallium oxide (Ga2O3) thin films grown on sapphire substrates by pulsed laser deposition (PLD) technique to study its ability to mimic biological synaptic behaviors. The layers are found to exhibit long persistent photo‐conductivity (PPC) effect, which is identified to be the key parameter to replicate the behavior of biological synapses. Channel resistance and PPC time constants should also be optimized to improve the efficiency of response and energy consumption of synaptic devices. It has been observed that both conductivity and the PPC decay time of Ga2O3‐films can be controlled by varying oxygen pressure and growth temperature (TG). These devices demonstrate their ability to perform paired pulse facilitation (PPF) at very low applied bias in mV‐range. They can mimic biological synapses showing short‐to‐long‐term memory transition (STM‐to‐LTM) and learning‐forgetting behavior. One of these devices is found to show synaptic behavior with the energy consumption of as low as 71fJ electrical and 21nJ optical per synaptic event. These findings thus strengthen the candidature of Ga2O3 films for the development of next‐generation opto‐electronic neuromorphic devices and systems.

Temperature modulation enables in situ growth of gallium oxide nanostructures on thin films exhibiting diverse morphologies

In this study, we successfully synthesized three gallium oxide thin films on (0001)-faceted Al2O3 substrates using hydrogen-reduced chemical vapor deposition (CVD) at different deposition temperatures, resulting in distinct morphologies and crystalline phases. These films consist of two pure-phase β-Ga2O3 films and one mixed-phase film comprising both β- and ε-Ga2O3 phases. The mixed-phase samples were subsequently annealed at high temperatures to obtain pure-phase β-Ga2O3 thin films. Furthermore, by precisely controlling the growth temperature, we achieved in-situ formation of Ga2O3 nanostructures (nanowires and nanoribbons) on the surface of the β-Ga2O3 thin films. The presence of different dominant crystal planes in the samples was confirmed through X-ray diffraction analysis (XRD) for verification purposes. In directly grown β-Ga2O3 films, the (−201) plane emerged as the primary crystallographic plane, while both the (−201) and (401) planes dominated in β-Ga2O3 films obtained via ε-Ga2O3 film annealing. Raman spectroscopy findings further supported these observations regarding the crystalline phase of the films as observed through XRD analysis. Scanning electron microscopy (SEM) images revealed distinct film morphologies resulting from variations in deposition temperatures. A flatter surface was observed when a higher proportion of ε-Ga2O3 was present in the film, however, regular cracking occurred after annealing. Atomic Force Microscope (AFM) images demonstrated that at a deposition temperature of 800 °C, regular steps were present on the film's surface. Mixed-phase films exhibited hillock morphology that disappeared after annealing, leading to a decrease in the surface roughness in root mean square (RMS) value. By elevating the growth temperature, a significantly enhanced density of gallium oxide nanoribbons can be seamlessly integrated onto the film surface. The transferred nanoribbons exhibit an ultra-thin profile with a minimum thickness of merely 12 nm, rendering them more slender and economically advantageous in comparison to conventional mechanical stripping methods. This study presents a CVD-based approach for growing gallium oxide thin films with varying morphologies by employing temperature modulation and demonstrates an innovative one-step method for growing nanostructures on gallium oxide films in situ, providing valuable insights for future device development involving gallium oxide.

Heteroepitaxial growth of Ga2O3 thin films on nickel-nanodot-induced buffer layers for solar-blind ultraviolet photodetector applications

Heteroepitaxial growth of gallium oxide (Ga2O3) thin films has been conducted on nickel (Ni)-nanodot-induced buffer layers using a mist chemical vapor deposition. The separation degree and droplet size of the Ni nanodots on sapphire substrates were controlled through a thermal annealing process, thereby being used as templates for the Ga2O3 thin-film growth. The growth of the inverted cone-shaped ε-Ga2O3 polymorph was observed on the nanodots while the α-Ga2O3 polymorph was grown on the sapphire substrate without nanodots, ultimately a thin film with a mixture of ε and α polymorphs can be achieved. By comparing photoresponses of metal–semiconductor–metal photodetectors fabricated on the thin film with the mixed polymorphs and on a single-crystalline α-Ga2O3 film, a promising template for using solar-blind photo-detecting applications has been verified.

Flexible and Self‐Powered Photoelectrochemical‐Type Solar‐Blind Photodetectors Based on Ag Nanowires‐Embedded Amorphous Ga2O3 Films

AbstractThe development of flexible electronics for detecting solar‐blind deep ultraviolet (UV‐C) light holds great promise for emerging technologies. Here, flexible photoelectrochemical photodetectors (PEC‐PDs) are demonstrated for self‐powered solar‐blind UV‐C sensing based on one‐step radio frequency sputtered amorphous gallium oxide (a‐Ga2O3) thin films on indium tin oxide‐coated polyethylene naphthalate (ITO/PEN) substrates in a simple and low‐cost manner. Surface functionalization of the ITO/PEN substrates with silver nanowires (Ag NWs) successfully embedded within the a‐Ga2O3 films significantly enhances the optoelectronic performance. Incorporating Ag NWs reduces charge transfer resistance and potential barrier, while enhancing built‐in electric field at the Ag NWs/a‐Ga2O3 interface, synergistically facilitating efficient electron transport. Consequently, the Ag NW‐modified PEC‐PDs exhibit approximately two‐fold increases in key metrics including responsivity (11.23 mA W−1) and response times (0.07/0.09 s rise/decay) under 254 nm illumination compared to pristine devices. Moreover, the Ag NW‐functionalized PEC‐PDs demonstrate superb mechanical flexibility and fatigue durability, maintaining ≈95% of the initial photoresponse current and responsivity after 500 bending cycles. This work provides valuable insights into designing high‐performance flexible self‐powered PEC‐PDs through ingenious photoanode engineering.

Planar defects in α-Ga2O3 thin films produced by HVPE

The defect structure of α-phase gallium oxide thin films was investigated using transmission electron microscopy (TEM). Epitaxial Ga2O3 films were grown via halide vapor-phase epitaxy on c-plane sapphire substrates. TEM analysis revealed a high density of extended planar defects within the films, primarily located along prismatic planes of {112¯0} type. Displacement vectors were determined using the invisibility criterion for stacking faults. The study encompassed both planar and cross-sectional views of the films. It is hypothesized that these defects form due to the motion of edge partial dislocations with the 13⟨11¯00⟩ Burgers vector. Various mechanisms of their formation have been explored.

Investigation on the Mist Intensity to Deposit Gallium Oxide Thin Films by Mist Chemical Vapor Deposition

Investigation on the Mist Intensity to Deposit Gallium Oxide Thin Films by Mist Chemical Vapor Deposition

Gallium oxide cantilevered thin film-based solar-blind photodetector and its arc detection applications

The performance of gallium oxide (Ga<sub>2</sub>O<sub>3</sub>) thin film detector based on metal-semiconductor-metal (MSM) is highly dependent on the uniformity of the Ga<sub>2</sub>O<sub>3</sub> thin film, and the manufacturing process is quite sophisticated, which poses a challenge for the scale-up and mass production of thin film photodetectors. In this work, an MSM Ga<sub>2</sub>O<sub>3</sub> thin film solar-blind photodetector with five-finger interdigital electrodes is fabricated by physically depositing Ga<sub>2</sub>O<sub>3</sub> thin film on the surface of a mass-produced cantilevered thin film chip. Through the microelectromechanical system (MEMS) process, the cantilever electrode structure is prepared, which protects the internal circuit and the integrity of the thin film. The Ga<sub>2</sub>O<sub>3</sub> thin film treated by argon plasma at a low temperature is amorphous, but the photodetector still possesses considerable ultraviolet detection performance. At a bias voltage of 18 V, it approaches the detection performance of crystalline Ga<sub>2</sub>O<sub>3</sub> thin film, with a detectivity of 7.9×10<sup>10</sup> Jones, an external quantum efficiency of 1779%, rise time and decay time of 1.22 s and 0.24 s, respectively. Moreover, a system of arc detection is built in outdoor environments. Without any optical focusing system, this photodetector achieves sensitive detection of pulsed arc in an outdoor sunlight environments. For pulsed arcs with an output voltage of 100 kV, the photodetector is capable of sensitive detection at a distance of 25 cm. Besides, the maximum detection distance of 155 cm indicates that the photodetector will have a favorable potential application value in the field of solar-blind detection. This work develops a sensitive functional thin film deposition technology based on the cantilever electrode structure fabricated by the MEMS process, which avoids the influence of the large-area uniformity of the functional thin film on the etching circuit. It provides a new technical approach and process route for preparing MSM photodetectors.

Deposition and photoluminescence of zinc gallium oxide thin films with varied stoichiometry made by reactive magnetron co-sputtering

This paper reports on the deposition and photoluminescence of amorphous and crystalline thin films of zinc gallium oxide with Ga:Zn atomic ratio varied between 0.3 and 5.7. The films are prepared by reactive direct current magnetron co-sputtering from liquid/solid gallium/zinc targets onto fused quartz substrates; the temperature of the substrate is varied from room temperature (RT) to 800 °C. The sputtering process is effectively controlled by fixing the sputtering power of one of the targets and controlling the power of the other target by plasma optical emission spectroscopy. The method, in conjunction with oxygen flow adjustment, enables the production of near-stoichiometric films at any temperature used. The composition analysis suggests a few at% oxygen deficiency in the films. The resulting deposition rate is at least an order of magnitude higher compared to the commonly used radio-frequency sputtering from a ceramic ZnO:Ga2O3 target. Deposited onto unheated substrates, the films with Ga:Zn ≈ 2 are X-ray amorphous. Well-defined X-ray diffraction peaks of spinel ZnGa2O4 start to appear at a substrate temperature of 300 °C. The surface of the as-deposited films is dense and exhibits a fine-featured structure observed in electron microscopy images. Increasing the deposition temperature from RT to 800 °C eliminates defects and improves crystallinity, which for the films with Ga:Zn ratio close to 2 results in an increase in the optical band gap from 4.6 eV to 5.1 eV. Room temperature photoluminescence established the main peak at 3.1 eV (400 nm); a similar peak in Ga2O3 is ascribed to oxygen-vacancy related transitions. A prominent feature around 2.9 eV (428 nm) is attributed to self-activation center of the octahedral Ga-O groups in the spinel lattice of ZnGa2O4. It was found that photoluminescence from ZnGa2O4 depends significantly on the stoichiometric ratio between Ga and Zn and the deposition/annealing temperature.

Phase-Dependent Phonon Heat Transport in Nanoscale Gallium Oxide Thin Films.

Different phases of Ga2O3 have been regarded as superior platforms for making new-generation high-performance electronic devices. However, understanding of thermal transport in different phases of nanoscale Ga2O3 thin-films remains challenging, owing to the lack of phonon transport models and systematic experimental investigations. Here, thermal conductivity (TC) and thermal boundary conductance (TBC) of the α-, β-, and (001) κ-Ga2O3 thin films on sapphire are investigated. At ≈80nm, the measured TC of α (8.8W m-1 K-1) is ≈1.8 times and ≈3.0 times larger than that of β and κ, respectively, consistent with model based on density functional theory (DFT), whereas the model reveals a similar TC for the bulk α- and β-Ga2O3. The observed phase- and size-dependence of TC is discussed thoroughly with phonon transport properties such as phonon mean free path and group velocity. The measured TBC at Ga2O3/sapphire interface is analyzed with diffuse mismatch model using DFT-derived full phonon dispersion relation. Phonon spectral distribution of density of states, transmission coefficients, and group velocity are studied to understand the phase-dependence of TBC. This study provides insight into the fundamental phonon transport mechanism in Ga2O3 thin films and paves the way for improved thermal management of high-power Ga2O3-based devices.

Effect of substrate temperature on structural, morphological, and optical properties of Gallium Oxide thin films deposited by RF-Sputtering

In this paper, gallium oxide thin films at different substrate temperatures were deposited on Indium TinOxide (ITO), glass and p-type silicon wafers by radio-frequency magnetron sputtering (RFMS). The effect of substrate temperature at 8 mTorr deposition pressure on the structural, morphological and optical properties of films was investigated. X-ray diffraction (XRD) analysis of thin films deposited on p-Si substrate suggests that crystal structure characteristics of synthesized thin films depend on the substrate temperature. From SEM analyzes of Ga2O3 thin films grown on p-type silicon substrate, It was observed that when the temperature increased, a porous structure appeared and the particle size changed depending on the substrate temperature. Morover, optained results from the absorption measurements, the band gap energy of the Gallium Oxide thin films deposited on the p-Si substrate decreased with increasing substrate temperature.

Investigation on the Mist Intensity to Deposit Gallium Oxide Thin Films by Mist Chemical Vapor Deposition

In this study, a novel, simple, and robust mist chemical vapor deposition is demonstrated, compatible with existing industrial practices to deposit gallium oxide thin films and influence of mist intensity on the properties of gallium oxide. The intensity of the mist generation is optimized to obtain smooth and uniform thin films. The thin film deposited in this work is mixed phase polycrystalline gallium oxide. Ultraviolet–visible–near‐infrared spectroscopy and photo response of thin film unveil that gallium oxide thin film is responsive to ultraviolet wavelengths including deep ultraviolet‐C and ultraviolet‐B bands and the mist‐generation intensity has negligible influence on the bandgap of the thin film. Thickness of thin film can be altered by varying the mist intensity. It is observed that there is no appreciable impact on refractive index of varying mist intensity. Morphological studies prove the formation of ultrasmooth thin film with root mean square value of 0.628 nm; which is closer and/or better than conventional semiconductor thin‐film deposition processes used for depositing Ga2O3.

A comparative study of photoelectric performance of Ga2O3 solar-blind photodetectors with symmetric and asymmetric electrodes

Gallium oxide (Ga2O3) thin films were deposited on quartz substrates under different oxygen flow ratios (O2/[O2+Ar], 0%−5%) by radio-frequency (RF) magnetron sputtering technique followed by thermal annealing at 900 °C for 2 h under N2 atmosphere. The influence of the oxygen concentration on the structural and optical characteristics of Ga2O3 thin films was investigated. The Ga2O3 thin films prepared under an oxygen flow ratio of 1% exhibited optimum crystal quality and the lowest oxygen vacancy (VO) concentration according to X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) investigations. All as-prepared polycrystalline Ga2O3 thin film samples exhibited high average transmittance above 87.8% in the visible spectral region and the 0% and 1% thin films had optical bandgap energies of about 4.9 eV. We also fabricated metal−semiconductor−metal (MSM) photodetectors based on 1% Ga2O3 thin films by employing pairs of Ni−Ni symmetrical or Ni−Al asymmetrical interdigitated electrodes. The Ni−Ni photodetector had a high on/off current ratio (Ion/Ioff > 105), a responsivity of 88.7 mA/W, and specific detectivity of 4.81×1010 J at 10 V bias under ultraviolet light-C (UVC) illumination, and the Ni−Al photodetector exhibited self-driven behavior along with short rise and fall times (0.45 s and 0.69 s, respectively).

Reactive Sputter Deposition of Ga2O3 Thin Films Using Liquid Ga Target

Ga2O3 is a promising material in the optoelectronics and semiconductor industry. In this work, gallium oxide thin films were deposited via radio frequency (RF) sputtering, using a liquid Ga target. The reactive sputtering was carried out using different oxygen flow rates and DC target potentials induced via the RF power. The thickness of the samples varied between 160 nm and 460 nm, depending on the preparation conditions. The composition and the refractive index of the layers were investigated via energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry, respectively. It was found that, through the use of a lower DC target potential, a better film quality and higher oxygen content can be achieved. The reactive sputtering was modeled based on the Berg model, with the aim of determining the sputtering yields and the sticking coefficient. It was shown that an increase in DC target potential leads to the preferential sputtering of gallium.

Influence of White Light on the Photoelectric Characteristics of UV Detectors Based on β-Ga2O3

In this report, we consider the effect of radiation with a radiation energy below the β-Ga2O3 band-gap on the photoelectric characteristics of ultraviolet (UV) radiation detectors based on gallium oxide thin films. The gallium oxide film was deposited by the radio frequency magnetron sputtering. Interdigital Ti/V electrodes were formed with an inter electrode distance d = 30 μm. The structures exhibit high value of photocurrent under irradiation with wavelength of λ = 254 nm and intensity of 780 μW/cm2. The detectors exhibit persistent photoconductivity after exposure to UV radiation. The presence of persistent conductivity is explained by the high concentration of traps in the Ga2O3 film and their recharging under UV irradiation. Radiation with an energy quantum below the the β-Ga2O3 band-gap changes the charge state of trap centers in a gallium oxide film. This effect leads to an increase in photocurrent under UV exposure and increases the stability of detectors.

Hydrogen detecting characteristics of a palladium nanoparticle/indium gallium oxide based sensor

An investigation and analysis have been conducted on a newly developed chemiresistive-type sensor to detect hydrogen gas. The studied senor is synthesized with an indium gallium oxide (IGO) thin film formed by RF magnetron sputtering and palladium (Pd) nanoparticles (NPs) deposited by vacuum thermal evaporation. The utilize of Pd NPs can significantly improve the sensing response and sensing speed for hydrogen gas. Energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), high-resolution scanning electron microscopy (HRSEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) are employed for the analyses of the relevant structure, element, and composition properties. The sensing mechanism for hydrogen gas is elucidated in this work. In this experiment, the studied Pd NP/IGO sensor exhibits an ultra high sensing response SR of 1.39×107 under 1% H2/air gas at 300°C.. The studied sensor can be operated at room temperature with a significant SR of 304.3. Besides, the studied sensor shows good repetitive operation performance. Hence, the studied Pd NP/IGO structure offers a great potential for the development of hydrogen sensing applications with outstanding performance.

Short-range positional order in phase separated indium gallium oxide islands deposited in a reducing atmosphere

We report a spontaneous phase separation in indium gallium oxide thin films with a nominal composition of (In0.006Ga0.994)2O3 grown on sapphire (0001) substrates using a powder sputtering method in a reducing atmosphere. The In-rich (In0.24Ga0.76)2O3-x domains are non-stoichiometric and located underneath the surface islands, whereas the In-poor domains are nearly stoichiometric Ga2O3, forming a continuous flat surface, though both domains have a monoclinic crystal structure. Furthermore, the In-rich domains exhibit a short-range positional order with a correlation length of approximately 1 μm because the phase separation occurs along the binodal decomposition accompanying periodic concentration profiling of In atoms. The analysis of X-ray photoelectron spectroscopy confirmed that the non-stoichiometry of the thin films originated from the formation of Ga sub-oxide (Ga2O) and metallic Ga, whereas the chemical state of the In atoms was close to In2O3 rather than metallic In. Our results indicate that the spontaneous phase separation occurs in (InxGa1-x)2O3 thin films even at an In content of 0.6 at%. Also, the non-stoichiometry associated with oxygen vacancies is an important parameter for causing spontaneous phase separation via the segregation of In atoms.