Amorphous Ga2O3 Research Articles

Solution-processed amorphous Ga2O3:CdO TFT-type deep-UV photodetectors

Amorphous Ga2O3:CdO thin films and their thin-film transistor (TFT) photodetectors were fabricated via a simple and low-cost spin-coating method. The film optical bandgap and TFT electrical parameters can be effectively regulated via changing the Cd content. The optimized Ga2O3:CdO TFT photodetector exhibited a responsivity of 2.17 A/W and a high UV/vis. rejection of 1.88 × 104 under 260 nm at small working biases, which are comparable or even superior to the recently reported results. Systematic comparison between metal-semiconductor-metal- and TFT-type photodetectors further confirmed that our TFT photodetectors had superior detective performance and lower electrical power consumption.

Matching vacancy formation energy and defect levels with the density of amorphous Ga2O3

In this study, we present six structural models for amorphous Ga2O3 (a-Ga2O3) with different mass densities from ab initio molecular dynamics simulations. The detailed local structural order, non-perfect coordination number, electronic and vibrational properties of a-Ga2O3 as well as the energetics and stability of O vacancy with various charge states have been systemically investigated. As the density increases, the first peak in the radial distribution function is weakened and the range of vibrational frequencies is narrowed, while the average coordination number and band gap gradually increase. Density also plays an important role in oxygen vacancy formation in a-Ga2O3. In the low-density a-Ga2O3, oxygen vacancy is more stable and the average formation energy of oxygen vacancy (VO) with neutral state is 3.32–3.70 eV lower than that in crystalline β-Ga2O3 (4.20 eV). In the high-density a-Ga2O3, VO in the 2 + charge state may form spontaneously in the case of lower Fermi energy and the thermodynamic transition energy for VO was found to shift to higher Fermi energy. Hence, the electron trap levels observed in experiment might be correlated with local structural distortion and non-perfect coordination number. Moreover, highly localized valence tail has been observed in all the six a-Ga2O3 systems. Thus, it would be hard to shift the Fermi level to the valence band, leading to rarely observed p-type conductivity in amorphous Ga2O3. These results not only establish a fundamental picture on the origin of a wide range of defect levels in Ga2O3 thin films deposited under different conditions, but also provide useful evidence to identify them with oxygen vacancy states.

Influence of deposition temperature on amorphous Ga2O3 solar-blind ultraviolet photodetector

Solar-blind ultraviolet photodetectors have potential applications in space communication, ozone hole monitoring and missile tracking. Amorphous Ga2O3 (a-Ga2O3) films are deposited by a simple radio frequency magnetron sputtering at different deposition temperatures. Fully transparent devices on a quartz substrate are fabricated with high responsivity, wide detection range and good repeatability. With the increase of Ga2O3 deposited temperature, the concentration of oxygen vacancy increases accordingly, leading to a wide detection range from 250 to 325 nm and high responsivity (138 A W−1 at 5 V bias). The underlying mechanism has been discussed and analyzed. Our results should advance the application of a-Ga2O3-based ultraviolet photodetectors and other relevant devices.

Room-Temperature Ozone Sensing Capability of IGZO-Decorated Amorphous Ga2O3 Films.

In present work, we report a room-temperature ozone sensor using InGaZnO (IGZO)-decorated amorphous Ga2O3 (a-Ga2O3) thin films. The gas sensing tests demonstrate that the topmost IGZO modification can significantly promote the sensors' responsivity. Intriguingly, the sensing capability presents a first increasing and then decreasing tendency as the surface morphology of IGZO develops from dispersed particles to a continuous film. Finite difference time domain (FDTD) simulation results prove that IGZO nanoparticles can remarkably increase the surface density of photogenerated electrons, clearly manifesting a strong dependence on IGZO particle size and contributing to the boosted responsiveness. However, once IGZO particles coalesce into a thin film, the ozone sensor's responsivity starts to decrease even though the number of photogenerated carriers still increases. The smaller specific surface area of IGZO thin film is believed to be responsible for this phenomenon. The proposed ozone gas sensors own the merits of low cost, room-temperature detection, easy integration, and mass production, significantly expanding the application fields of a-Ga2O3 thin film.

Temperature-Dependent Crystallization of Ga2O3 for Ultraviolet Photodetectors

As an n-type oxide semiconductor, Ga2O3 shows great promise in solar-blind ultraviolet (UV) photodetectors due to its adequate band gap, high absorption coefficient, high thermal and chemical stability, high breakdown voltage, and high sensitivity to UV light. In order to investigate the photoresponse of the Ga2O3 photodetector with different crystallization status, we fabricated a series of β-Ga2O3 thin films on a sapphire substrate via pulsed laser deposition with different deposition temperatures from 250°C to 650°C. X-ray diffraction and scanning electron microscopy result showed crystallization enhancement due to the increasing of deposition temperature, and the crystallization procedure started at ∼ 450°C. Optical analysis indicated a blue shift present at the absorption edge (from 4.6 eV to 5.1 eV) with temperature increasing from 450°C to 650°C, showing modulation feasibility in the band gap due to temperature dependence. In addition, all samples showed high transmittance (over 90%) over the entire visible spectrum. Film detectors fabricated by these samples showed high photoresponse and high light/dark current ratio (ILight/IDark) on Ga2O3 deposited at 450°C. Transmission electron microscopy result for film deposited at 450°C showed that the top and bottom regions of films contained both amorphous and crystalline Ga2O3, while there was no crystallization in the middle area of the film, indicating that a combined amorphous-crystalline Ga2O3 film with adequate ratio of crystalline state can significantly enhance the photoresponse while restraining the dark current.

Fast Response Solar-Blind Photodetector with a Quasi-Zener Tunneling Effect Based on Amorphous In-Doped Ga2O3 Thin Films.

A high-performance solar-blind photodetector with a metal–semiconductor–metal structure was fabricated based on amorphous In-doped Ga2O3 thin films prepared at room temperature by radio frequency magnetron sputtering. The photodetector shows a high responsivity (18.06 A/W) at 235 nm with a fast rise time (4.9 μs) and a rapid decay time (230 μs). The detection range was broadened compared with an individual Ga2O3 photodetector because of In doping. In addition, the uneven In distribution at different areas in the film results in different resistances, which causes a quasi-Zener tunneling internal gain mechanism. The quasi-Zener tunneling internal gain mechanism has a positive impact on the fast response speed and high responsivity.

Ni5Ga3 catalysts for CO2 reduction to methanol: Exploring the role of Ga surface oxidation/reduction on catalytic activity

A δ-Ni5Ga3/SiO2 catalyst, which is highly active and stable for thermal CO2 hydrogenation to methanol, was investigated to understand its surface dynamics during reaction conditions. The catalyst was prepared, tested and characterized using a multitude of techniques, including ex-situ XRD (X-ray Diffraction), TEM (Transmission Electron Microscopy), H2-TPR (Temperature Programmed Reduction), CO chemisorption, along with in-situ ETEM (Environmental Transmission Electron Microscopy), APXPS (Ambient Pressure X-ray Photoelectron Spectroscopy) and HERFD-XAS (High Energy Resolution Fluorescence Detected X-Ray Absorption Spectroscopy). Upon air exposure Ga migrates from the subsurface region to the surface of the nanoparticles forming a Ga-oxide shell surrounding a metallic core. The oxide shell can be reduced completely only at high temperatures (above 600 °C); the temperature of the reducing activation treatment plays a crucial role on the catalytic activity. HERFD-XAS and APXPS measurements show that an amorphous Ga2O3 shell persists during catalysis after low temperature reductions, promoting methanol synthesis.

3D Solar‐Blind Ga2O3 Photodetector Array Realized Via Origami Method

AbstractA 3D solar‐blind photodetector array is realized from amorphous Ga2O3 films grown on polyethylene terephthalate substrates via an origami route. The photodetector cells exhibit a dark current of 0.17 nA, and the peak responsivity is about 8.9 A W−1 at 250 nm with a quantum efficiency of 4450%. The photodetector shows a distinct cut‐off wavelength at 268 nm with a solar‐blind ratio of more than two orders of magnitude (photocurrent ratio between 250 nm/300 nm). The photodetector cells reveal excellent electrical stability after thousands of bending cycles. All the photodetector cells of the 3D photodetector array have a highly consistent performance. In addition, the device can execute the functions of capturing a real‐time light trajectory and identifying multipoint light spatial distribution, which cannot be achieved in all the previously reported 2D solar‐blind photodetectors. The results suggest new pathways to fabricate 3D photodetectors from conventional semiconductor films, which may find potential applications in optical positioning, tracking, imaging and communications, etc.

Low temperature growth and optical properties of α-Ga2O3 deposited on sapphire by plasma enhanced atomic layer deposition

Plasma enhanced atomic layer deposition was used to deposit thin films of Ga2O3 on to c-plane sapphire substrates using triethylgallium and O2 plasma. The influence of substrate temperature and plasma processing parameters on the resultant crystallinity and optical properties of the Ga2O3 films were investigated. The deposition temperature was found to have a significant effect on the film crystallinity. At temperatures below 200 °C amorphous Ga2O3 films were deposited. Between 250 °C and 350 °C the films became predominantly α-Ga2O3. Above 350 °C the deposited films showed a mixture of α-Ga2O3 and ε-Ga2O3 phases. Plasma power and O2 flow rate were observed to have less influence over the resultant phases present in the films. However, both parameters could be tuned to alter the strain of the film. Ultraviolet transmittance measurements on the Ga2O3 films showed that the bandgaps ranges from 5.0 eV to 5.2 eV with the largest bandgap of 5.2 eV occurring for the α-Ga2O3 phase deposited at 250 °C.

Structural and Dynamic Properties of Gallium Alkoxides.

A comparison of chlorido-gallium functionalized alkoxides as precursors for aerosol-assisted chemical vapor deposition (AACVD) was carried out. Variable-temperature (VT)-NMR studies were used to probe the fluxional behavior of these alkoxides in solution, and hence their utility as precursors. The synthesis involved the initial isolation of the dimer [GaCl(NMe2)2]2 via a salt metathesis route from GaCl3 and 2 equiv of LiNMe2. This dimer was then reacted with 4 equiv of HOCH2CH2CH2NEt2, resulting in the formation of Ga[μ-(OCH2CH2CH2NEt2)2GaCl2]3 (1). Mass spectrometry and VT-NMR confirmed the oligomeric structure of 1. Tuning of the ligand properties, namely, the chain length and substituents on N, resulted in formation of the monomers [GaCl(OR)2] (R = CH2CH2NEt2, (2); CH2CH2CH2NMe2, (3)). VT-NMR studies, supported by density functional theory calculations, confirmed that the ligands in both 2 and 3 possess a hemilabile coordination to the gallium center, owing to either a shorter carbon backbone (2) or less steric hindrance (3). Both 2 and 3 were selected for use as precursors for AACVD: deposition at 450 °C gave thin films of amorphous Ga2O3, which were subsequently annealed at 1000 °C to afford crystalline Ga2O3 material. The films were fully characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, UV–visible spectroscopy, and energy dispersive X-ray analysis.

High performance, flexible and room temperature grown amorphous Ga2O3 solar-blind photodetector with amorphous indium-zinc-oxide transparent conducting electrodes

The quest for flexible and completely visible transparent solar-blind photodetector is the great motivation for next generation invisible security and photo-sensor application. Here, we have developed high performance and completely flexible solar blind photodetector using room temperature grown amorphous Ga2O3 thin film and unconventional amorphous indium-zinc-oxide (a-IZO) transparent conducting electrodes. The a-IZO transparent conducting electrodes are deposited using an optimized and unique RF superimposed DC sputtering which helps to achieve high optoelectronic properties with excellent mechanical flexibility. In contrast to conventional and highly reflective silver (Ag) metal electrodes, IZO offers transparency >85% in visible-IR. The solar-blind photodetector with a-IZO electrode shows high responsivity of 43.99 A W−1 and high external quantum efficiency of 2.18 × 104% in contrast to conventional Ag metal electrodes. The performance of the a-IZO/a-Ga2O3 photodetector is the highest among the reported amorphous Ga2O3 photodetector. Moreover, the flexibility of a-IZO electrode and conventional Ag metal electrode on a-Ga2O3 photodetector is investigated under repeated bending cycle. The a-IZO electrode shows excellent flexibility of the solar-blind photodetector and demonstrated a decrease in photocurrent by 28.6% under 30 cm bending radius and a 38.8% decrease in photocurrent even after 500 stretch/release cycles at 30 cm bending radius. However, the conventional Ag electrode shows complete failure on bending with 30 cm bending radius even on single cycle. The results demonstrated a-IZO thin film as a potential electrode for next generation flexible and visible transparent solar-blind photodetector.

Transition of photoconductive and photovoltaic operation modes in amorphous Ga2O3-based solar-blind detectors tuned by oxygen vacancies**Project supported by the National Key Research and Development Project, China (Grant No. 2017YFB0403003), the National Natural Science Foundation of China (Grant Nos. 61774081, 61322403, and 91850112), the State Key Research and Development Project of Jiangsu Province, China (Grant No. BE2018115), Shenzhen Fundamental Research Project,

We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga2O3-based solar-blind photodetectors in metal–semiconductor–metal (MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga2O3 interface is realized by tuning the conductivity of amorphous Ga2O3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga2O3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity (PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA, a high detectivity of , a fast response time of , and a high ultraviolet C (UVC)-to-ultraviolet A (UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the sub-gap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.

Investigation of growth characteristics, compositions, and properties of atomic layer deposited amorphous Zn-doped Ga2O3 films

Diethylzinc and H2O were used as the precursors for the thermal atomic layer deposition (TH-ALD) of ZnO deposition while the trimethylgallium and O2 plasma were used as a reactant for the plasma-enhanced atomic layer deposition (PE-ALD) of Ga2O3, respectively. The Zn-doped Ga2O3 (ZGO) films were fabricated by a combination of PE-ALD of Ga2O3 and TH-ALD of ZnO at a low temperature of 200 °C. The results show that as-deposited ZGO films were amorphous while ZnO with a crystalline structure. XPS results indicate that the Zn content in ZGO films increased from 9.70 to 24.65 at.% with the cycle ration of Ga2O3 with respect to ZnO decreasing from 7:1 to 3:1 while the oxygen vacancy increased from 27.65% to 37.93%. The rise in Zn doping contents is also accompanied by significant variations in the morphological, electrical, and optical properties of the ZGO films, including a decrease of film density and resistivity, an increase of RMS roughness, a strong transmittance in the ultraviolet-visible (UV–vis) area, and a widening of the band gap from 4.64 to 5.25 eV. These findings help deposit ZGO films with desired structure and properties for electronic device applications.

High-performance UV detectors based on room-temperature deposited amorphous Ga2O3 thin films by RF magnetron sputtering

Room-temperature-fabricated amorphous Ga2O3 is an inexpensive and highly sensitive material for high-performance solar-blind ultraviolet (UV) (220–280 nm) detectors, which are extremely useful given the widespread use of solar-blind UV photoelectronic technology.

The Structural and Photoelectrical Properties of Gallium Oxide Thin Film Grown by Radio Frequency Magnetron Sputtering

Gallium oxide thin films were deposited by radio frequency magnetron sputtering technique. Structural, optical and photoelectrical properties of Ga2O3 thin films were investigated in detail. The as-grown Ga2O3 thin films were amorphous, while those annealed film were polycrystalline with monoclinic structure. Amorphous and polycrystalline gallium oxide metal-semiconductor-metal photodetectors were fabricated. However, annealed polycrystalline films have no photoresponse for deep ultraviolet light. The responsivity of the photodetector based on as-grown amorphous film is 122.7 μA/W at 256 nm and the ultraviolet (UV)-to-visible rejection ratios were 100, which indicates that the amorphous Ga2O3 thin films have potential application for solar-blind photodetector. High concentration of oxygen vacancies of amorphous film are considered to be responsible for photoresponse in amorphous gallium oxide thin films.

Correction: High-performance UV detectors based on room-temperature deposited amorphous Ga2O3 thin films by RF magnetron sputtering

Correction for ‘High-performance UV detectors based on room-temperature deposited amorphous Ga2O3 thin films by RF magnetron sputtering’ by Shun Han et al., J. Mater. Chem. C, 2019, 7, 11834–11844.

Effects on the amorphous Ga2O3 film surfaces by sub-IB-metal-nano-layers

AbstractThis work reports by the first time a method to control the geometry of Ga2O3 films nanocrystallites at 350 °C. The formation of controlled shaped nano-crystallites of γ-Ga2O3 from amorphous Ga2O3 films grown by RF-Sputtering at room temperature driven by nano-layers of group IB metals (Cu, Ag or Au) is studied. The reported results can be explained by the role of subsurface metal nano-layers and the non-equilibrium nature of the sputtering processes. To study the effects on the surface structure and their optical properties arrays of amorphous-Ga2O3/IB-metal/amorphous-Ga2O3 were annealed in dry N2 atmosphere at 350 °C by 50, 100 and 150 min. The experimental results can be explained by the evolution of the amorphous character of the films amorphous films towards the nanocrystalline γ-Ga2O3 phase driven by the metal nano-layer seed nature. As the annealing time was increased the transition from amorphous-Ga2O3 to the nanocrystalline γ-Ga2O3 phase was detected by X-ray diffraction analysis. The transition to the nanocrystalline γ-Ga2O3 is demonstrated by the formation of octahedral, triangle and ball shape nanocrystallites with sizes of ∼5 to 50 nm according to FE-SEM analysis. The influence of the metal nano-layer is clearly seen by the shift of the plasmon frequency resonance produced by the Ga2O3/IB-metal/Ga2O3 arrays in the region from 400 to 600 nm caused by the modification of the interface Ga2O3/IB-metal produced by the applied annealing stages.

Direct wafer bonding of Ga2O3–SiC at room temperature

Integration of Ga2O3 on SiC substrate with a high thermal conductivity is one of the promising solutions to reduce the self-heating of Ga2O3 devices. Direct wafer bonding of Ga2O3–SiC at room temperature was achieved by surface activated bonding (SAB) using a Si-containing Ar ion beam. An average bonding energy of ~2.31 J/m2 was achieved. Both the structure and the composition of the interface were investigated to understand the bonding mechanism. According to the interface analysis, a ~2.2 nm amorphous SiC layer and a ~1.8 nm amorphous β-Ga2O3 layer originating from the ion beam bombardment for surface activation were found at the interface. A slight diffusion at the interface might already happen at room temperature, which should contribute to the strong bonding. To confirm the diffusion at a low temperature and investigate the possible interfacial variation during device operation, an annealing process was carried out at 473 K. The same analysis was applied on the annealed bonding interface. The interfacial layer shrank by ~0.5 nm after annealing. The further diffusion of Ga and Si at the interface caused by the annealing was confirmed. Besides, the position of the Ar count peak inside the amorphous Ga2O3 layer shifted by ~0.5 nm toward SiC.

Mechanical Properties of Nanocrystalline and Amorphous Gallium Oxide Thin Films

Nanocrystalline and amorphous Ga2O3 films (≈200 nm) with variable structural quality are produced by sputter‐deposition by varying the substrate temperature (Ts = 25–700 °C). The effect of Ts is significant on the microstructure and mechanical behavior of Ga2O3 films. The variation in mechanical behavior studied by nano‐indentation and nano‐scratch testing reveal distinct trends, which are directly related to the structure and morphology of Ga2O3 films. All the Ga2O3 films deposited at Ts < 500 °C are amorphous; the amorphous‐to‐crystalline transformation occurs at Ts = 500 °C. Ga2O3 films deposited at Ts ≥ 500 °C are nanocrystalline, β‐phase. The corresponding mechanical characteristics, namely the hardness (H) and elastic modulus (Er), show a strong correlation with structural characteristics. The H and Er increases from 17 to 27 GPa and 250 to 290 GPa, respectively, with increasing Ts from 25 to 700 °C. The plasticity index, which is the ratio of H/Er, is almost constant for the Ga2O3 films. The strain‐rate sensitivity measurements performed to determine the applicability in practical applications indicate the best performance of size controlled, nanocrystalline Ga2O3 films, which can be mechanically regarded as nano‐composite structures. The mechanical characteristics, scratch behavior, and strain rate sensitivity indicate the role of microstructure on the mechanical performance of Ga2O3 films.

Investigation on Amorphous Ga2O3 Based Conductive-Bridge Resistive Switching Memory with Low-Power, High-Speed, High Reliability

In this work, Ga2O3 single layer serving as the resistive switching layer of a conductive bridge RAM device using a Cu-based supply layer was reported. Gallium oxide is a wide bandgap semiconductor with high dielectric constant and high breakdown voltages. By applying a Ga2O3 layer as the resistive switching layer, the excellent memory performances, such as low operation current (down to 100μA), notably high on/off resistance ratio (more than 105), and high switching endurance (>103 cycles) have been demonstrated in this study. Meanwhile, good stable retention property is also achieved (five decades of window margin are constantly maintained beyond 104 s at 85 ◦C). The novel Ga2O3-based CBRAM is a promising technology for potential future high density nonvolatile memory applications.