Ga2O3 Thin Films Research Articles

Band alignments at Ga2O3 heterojunction interfaces with Si and Ge

Amorphous Ga2O3 thin films were deposited on p-type (111) and (100) surfaces of silicon and (100) germanium by atomic layer deposition (ALD). X-ray photoelectron spectroscopy (XPS) was used to investigate the band alignments at the interfaces using the Kraut Method. The valence band offsets were determined to be 3.49± 0.08 eV and 3.47± 0.08 eV with Si(111) and Si(100) respectively and 3.51eV± 0.08 eV with Ge(100). Inverse photoemission spectroscopy (IPES) was used to investigate the conduction band of a thick Ga2O3 film and the band gap of the film was determined to be 4.63±0.14 eV. The conduction band offsets were found to be 0.03 eV and 0.05eV with Si(111) and Si(100) respectively, and 0.45eV with Ge(100). The results indicate that the heterojunctions of Ga2O3 with Si(100), Si(111) and Ge(100) are all type I heterojunctions.

Twin-induced phase transition from β-Ga2O3 to α-Ga2O3 in Ga2O3 thin films

We deposited a 300-nm-thick Ga2O3 thin film on an amorphous SiO2/Si substrate via pulsed laser deposition. X-ray diffraction patterns revealed the formation of β-Ga2O3 phase at a substrate temperature of 700 °C. X-ray photoelectron spectra indicated that the degree of oxidation increased after annealing at 700 °C. Further annealings at higher temperatures led to a transition of the β-Ga2O3 phase to the α-Ga2O3 phase; this transition was caused by the twin structure formed during the crystallinity improvement process. In addition, we discuss the mechanism of the transition from the β phase to the α phase in the β-Ga2O3 thin films.

Ultrahigh Performance of Self-Powered β-Ga2O3 Thin Film Solar-Blind Photodetector Grown on Cost-Effective Si Substrate Using High-Temperature Seed Layer

We demonstrated an ultrahigh-performance and self-powered β-Ga2O3 thin film solar-blind photodetector fabricated on a cost-effective Si substrate using a high-temperature seed layer (HSL). The polycrystalline β-Ga2O3 thin film deposited with HSL shows high performance in the solar-blind region in comparison to the amorphous Ga2O3 thin film deposited without HSL. The zero-bias digitizing sensor prototype with an HSL produces a digitized output bit with deep UV (DUV) light that exhibits a high on/off (I254 nm/Idark) ratio of >103, a record-low dark current of 1.43 pA, and high stability and reproducibility over 100 cycles even after >2100 h. The photodetector shows minimum persistent photoconductivity and fast response in milliseconds. The photodetector yields a responsivity of 96.13 A W–1 with an external quantum efficiency of 4.76 × 104 at 5 V for 250 nm monochromatic light. The photodetector shows a high response to even a rare weak signal of DUV (44 nW/cm2). These values are the highest reported to date...

The influence of sputtering power on the structural, morphological and optical properties of β-Ga2O3 thin films

In this work, Ga2O3 thin films were grown on Al2O3 (0001) substrate by radio frequency magnetron sputtering. The influence of sputtering power on crystalline structure, morphology, transmittance, band gap, and photoluminescence were investigated in detail. X-ray diffraction results showed that the β-Ga2O3 films were oriented along (2¯01) plane, and the crystalline quality improved with increasing sputtering power. Scanning electron microscope images revealed that the proportion of large-size grain increased with increasing sputtering power. The β-Ga2O3 films displayed very high transmittance close to 100% in visible region. Photoluminescence spectra showed that all the films exhibited intense blue and green emission centered at approximately 430 nm and 550 nm originated from the donor-acceptor pair recombination.

The structure and magnetic properties of β-(Ga0.96Mn0.04)2O3 thin film * * Project supported by the National Natural Science Foundation of China (Nos. 11404029, 51572033, 51172208) and the Fund of State Key Laboratory of Information Photonics and Optical Communications (BUPT).

High quality epitaxial single phase (Ga0.96Mn0.04)2O3 and Ga2O3 thin films have been prepared on sapphire substrates by using laser molecular beam epitaxy (L-MBE). X-ray diffraction results indicate that the thin films have the monoclinic structure with a preferable orientation. Room temperature (RT) ferromagnetism appears and the magnetic properties of β-(Ga0.96Mn0.04)2O3 thin film are enhanced compared with our previous works. Experiments as well as the first principle method are used to explain the role of Mn dopant on the structure and magnetic properties of the thin films. The ferromagnetic properties are explained based on the concentration of transition element and the defects in the thin films.

Interfacial Engineering of Nanoporous Architectures in Ga2O3 Film toward Self-Aligned Tubular Nanostructure with an Enhanced Photocatalytic Activity on Water Splitting.

The present work demonstrates the formation of self-aligned nanoporous architecture of gallium oxide by anodization of gallium metal film controlled at -15 °C in aqueous electrolyte consisting of phosphoric acid. SEM examination of the anodized film reveals that by adding ethylene glycol to the electrolyte and optimizing the ratio of phosphoric acid and water, chemical etching at the oxide/electrolyte interfaces can be controlled, leading to the formation of aligned nanotubular oxide structures with closed bottom. XPS analysis confirms the chemical composition of the oxide film as Ga2O3. Further, XRD and SAED examination reveals that the as-synthesized nanotubular structure is amorphous, and can be crystallized to β-Ga2O3 phase by annealing the film at 600 °C. The nanotubular structured film, when used as photoanode for photoelectrochemical splitting of water, achieved a higher photocurrent of about two folds than that of the nanoporous film, demonstrating the rewarding effect of the nanotubular structure. In addition, the work also demonstrates the formation of highly organized nonporous Ga2O3 structure on a nonconducting glass substrate coated with thin film of Ga-metal, highlighting that the current approach can be extended for the formation of self-organized nanoporous Ga2O3 thin film even on nonconducting flexible substrates.

Stabilization and enhanced energy gap by Mg doping in ε-phase Ga2O3 thin films

Mg-doped Ga2O3 thin films with different doping concentrations were deposited on sapphire substrates using laser molecular beam epitaxy (L-MBE) technique. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and ultraviolet-visible (UV-vis) absorption spectrum were used to characterize the crystal structure and optical properties of the as-grown films. Compared to pure Ga2O3 thin film, the Mg-doped thin films have transformed from the most stable β-phase into ε-phase. The absorption edge shifted to about 205 nm and the optical bandgap increased to ∼ 6 eV. These properties reveal that Mg-doped Ga2O3 films may have potential applications in the field of deep ultraviolet optoelectronic devices, such as deep ultraviolet photodetectors, short wavelength light emitting devices and so on.

Selectivity of the gas sensor based on the 50%In2O3–50%Ga2O3 thin film in dynamic mode of operation

The article considers the gas sensor with the sensitive layer based on the 50%In2O3 –50%Ga2O3 thin film. The temperature and concentration dependencies of gas-induced resistance response of this sensor and the dynamical dependencies of its resistance response on the test gases in air are investigated. The test gases were ethanol, acetone, ammonia and liquefied petroleum gas. The information parameters of the sensor in the dynamical mode of operation were considered to improve its selectivity. The presented results show that the selectivity of the sensor in this mode may be improved by using the following information parameters: gas-induced resistance response in steady state, activation energy of the response and pre-exponential factor of the temperature dependence of the response time constant.

Molecular engineering of Ga-ketoiminates: synthesis, structure and evaluation as precursors for the additive-free spin-coated deposition of gallium oxide thin films

New gallium ketoiminate precursors with high solubility, favourable hydrolysis and decomposition route enabling additive free processing of Ga2O3 thin films.

Stabilizing the metastable γ phase in Ga2O3 thin films by Cu doping

The metastable cubic γ phase Ga2O3 with a defect spinel structure presents several fascinating properties, which may be a more attractive material than the stable β form. Herein, we found that the metastable γ-Ga2O3 can be stabilized by copper (Cu) doping through a simple operation and low cost method of sol-gel. The γ-Ga2O3 thin films were prepared under the annealing of 700 °C in nitrogen atmosphere for Cu doping. The β phase Ga2O3 ones were obtained at the same condition without the Cu dopant. While the Cu doped γ phase Ga2O3 will transform to β phase for a high annealing temperature above 800 °C. The grain size of Cu doped γ-Ga2O3 and undoped β-Ga2O3 thin films increases with the increase of the annealing temperature. The UV–Vis absorption spectrum of the β-Ga2O3 thin film exhibits a sharp intrinsic absorption edge at ∼250 nm, whilst Cu doped γ-Ga2O3 thin film displays an obvious red-shift. The band gap decreases from 4.90 eV to 4.38 eV for the Cu doping. The photoluminescence intensity at the ultraviolet and blue region of Cu doped γ-Ga2O3 thin films are stronger than undoped β-Ga2O3, which can be attributed to the introduction of more defects such as oxygen vacancies in γ phase by Cu doping.

The impact of dopant contents on structures, morphologies and optical properties of Eu doped Ga2O3 films on GaAs substrate

We have fabricated the europium (Eu) doped Ga2O3 thin films on GaAs substrate by using pulsed laser deposition. The impact of Eu contents on structure, surface morphology, and optical properties are systematically investigated. We demonstrate that the Eu contents in Ga2O3 films can be controlled by adjusting the Eu contents in the targets. Moreover, all the films exhibited monoclinic Ga2O3 structure and smooth surface. We also clearly observe intense red emissions at 611nm for the Eu doped Ga2O3 films. It is believed that this work paves the way for the development of GaAs-based red electroluminescence devices by using wide bandgap Ga2O3 as the host materials for Eu3+ ions.

(Invited) Gallium Oxide Power Devices Fabricated By Novel Processes

Innovative electric devices have been derived from new alloy systems for example AlAs-GaAs-InAs, AlP-GaP-AlAs-GaAs and AlN-GaN-InN. These alloy systems are consisted of binary compounds of same crystal structure and small lattice mismatches among them. However, it was hard work for oxides to construct an alloy of same crystals with small lattice mismatches because an oxide often takes various crystal polymorph including meta-stable phases. On the other hand, it may lead not only to a gold mine which contains novel functional crystals but also to a trigger to realize new functional devices. We found a new alloy concept of corundum-structured oxide crystals in 2013 toward successful synthesis of meta-stable phased α-Ga2O3, α-In2O3 thin films on sapphire substrates1)-3). A series of corundum-structured α-M2O3 (M= Al, Ga, In, Fe, Cr, V, Ti, Rh, Ir) is an attractive alloy system because they have unique physical properties including novel p-type semiconductor of α-Ir2O3 4). An α-Ga2O3 is one of the “ultra wide band-gap” semiconductor with a band gap of 5.3 eV3). Furthermore, it is advantageous for devices operating at high voltage with low on-resistance. Attempts were made to fabricate Schottky barrier diodes (SBDs). Since an α-Ga2O3 layer can be lifted off from a sapphire substrate, it is possible to fabricate vertical-structured SBDs with low series resistance. The on-resistance and breakdown voltage were 0.1 mW×cm2 and 531 V (SBD sample1) or 0.4 mW×cm2 and 855 V (SBD sample2), respectively5). The on-resistance value is seventh part of a commercially available SBD fabricated by SiC (0.7 mW×cm2). Moreover, aiming at showing potential as device materials, efforts have been given to fabricate MOSFETs with α-In2O3. These were fabricated using amorphous Al2O3 as a gate insulating layer and Au as gate and source electrodes. The device showed saturated drain current characteristics and clear pinch-off with subthreshold swing of 1.83 V/dec, field-effect mobility of 187 cm2/Vs, and effective mobility of 240 cm2/Vs. The relatively high mobility values are attractive for future evolution. Meanwhile, one of the most difficult attempts on exploring oxides is a discovery of new p-type semiconductors. Corundum-structured α-Rh2O3 and α-Ir2O3 were reported showing p-type conductivity only by Seebeck effect measurements because of their very small hall voltages6)-8). It is expected to be derived from larger energy level of Fermi-level than a bottom of conduction band at E-k dispersion in α-Rh2O3. An alloy of α-Rh2O3 and α-Ga2O3 were fabricated on α-Al2O3 substrates aiming to tune the Fermi-level of a α-Rh2O3. An α-(Rh,Ga)2O3 thin film showed clear p-type conductivity by hall effect measurements with a hole mobility of 1.0 cm2/Vs and carrier concentration of 7.6×1017/cm3, respectively. On the other hand, single-phased α-Ir2O3 thin films were fabricated for the first time. These also showed p-type conductivity by hall effect measurements. The detail of crystal-structure and electrical properties will present on the day. Part of this work was supported by “Strategic Innovation Program for Energy Conservation Technologies” of the New Energy and Industrial Technology Development Organization (NEDO).

Mg-doped p-type β-Ga2O3 thin film for solar-blind ultraviolet photodetector

The p-type high insulating thin films were obtained by doping Mg into β-Ga2O3. Thin films with various Mg concentrations were deposited on (0001) c-plane Al2O3 substrate by radio frequency magnetron sputtering followed by post-annealing treatment. The crystal structure expanded due to the substitution of the trivalent Ga3+ with the divalent Mg2+ in a larger ion radius. The Fermi level (EF) of the Mg doped film is closer to the valence band, exhibiting a characteristic of weak p-type. The Mg doped Ga2O3 thin films were used to construct the metal/semiconductor/metal (MSM) structure, and the devices showed a high resistance (4.1pA at 10V), a high sensitivity (8.7×105%), a high responsivity (23.8mA/W) and a short decay time (0.02s) under 254nm UV light irradiation, suggesting a potential application in solar-blind photodetector.

Highly conductive homoepitaxial Si-doped Ga2O3 films on (010) β-Ga2O3 by pulsed laser deposition

Si-doped Ga2O3 thin films were fabricated by pulsed laser deposition on semi-insulating (010) β-Ga2O3 and (0001) Al2O3 substrates. Films deposited on β-Ga2O3 showed single crystal, homoepitaxial growth as determined by high resolution transmission electron microscopy and x-ray diffraction. Corresponding films deposited on Al2O3 were mostly single phase, polycrystalline β-Ga2O3 with a preferred (201¯) orientation. An average conductivity of 732 S cm−1 with a mobility of 26.5 cm2 V−1 s−1 and a carrier concentration of 1.74 × 1020 cm−3 was achieved for films deposited at 550 °C on β-Ga2O3 substrates as determined by Hall-Effect measurements. Two orders of magnitude improvement in conductivity were measured using native substrates versus Al2O3. A high activation efficiency was obtained in the as-deposited condition. The high carrier concentration Ga2O3 thin films achieved by pulsed laser deposition enable application as a low resistance ohmic contact layer in β-Ga2O3 devices.

Fast growth rate of epitaxial β–Ga2O3 by close coupled showerhead MOCVD

We report on the growth of epitaxial β–Ga2O3 thin films on c-plane sapphire substrates using a close coupled showerhead MOCVD reactor. Ga(DPM)3 (DPM=dipivaloylmethanate), triethylgallium (TEGa) and trimethylgallium (TMGa) metal organic (MO) precursors were used as Ga sources and molecular oxygen was used for oxidation. Films grown from each of the Ga sources had high growth rates, with up to 10μm/hr achieved using a TMGa precursor at a substrate temperature of 900°C. As confirmed by X-ray diffraction, the films grown from each of the Ga sources were the monoclinic (2¯01) oriented β–Ga2O3 phase. The optical bandgap of the films was also estimated to be ∼4.9eV. The fast growth rate of β–Ga2O3 thin films obtained using various Ga-precursors has been achieved due to the close couple showerhead design of the MOCVD reactor as well as the separate injection of oxygen and MO precursors, preventing the premature oxidation of the MO sources. These results suggest a pathway to overcoming the long-standing challenge of realizing fast growth rates for Ga2O3 using the MOCVD method.

Effect of composition on properties of In2O3–Ga2O3 thin films

The In2O3–Ga2O3 mixed oxide polycrystalline thin films with various ratios of components were obtained by pulsed laser deposition. The effect of films composition on surface morphology, electrophysical and gas sensing properties and energies of adsorption and desorption of combustible gases was studied. The films with50%In2O3–50%Ga2O3 composition showed maximum gas response (∼25 times) combined with minimum optimal working temperature (∼530 °C) as compared with the other films. The optical transmittance of the films in visible range was investigated. For 50%In2O3–50%Ga2O3 films, the transmittance is higher in comparison with the other films. The explanation of the dependency of films behaviors on their composition was presented.The In2O3–Ga2O3 films were assumed to have perspectives as gas sensing material for semiconducting gas sensors.

β-Ga2O3 versus ε-Ga2O3: Control of the crystal phase composition of gallium oxide thin film prepared by metal-organic chemical vapor deposition

Gallium oxide thin films of β and ε phase were grown on c-plane sapphire using metal-organic chemical vapor deposition and the phase compositions were analyzed using X-ray diffraction. The epitaxial phase diagram was constructed as a function of the growth temperature and VI/III ratio. A low growth temperature and low VI/III ratio were beneficial for the formation of hexagonal-type ε-Ga2O3. Further structure analysis revealed that the epitaxial relationship between ε-Ga2O3 and c-plane sapphire is ε-Ga2O3 (0001) || Al2O3 (0001) and ε-Ga2O3 || Al2O3 . The structural evolution of the mixed-phase sample during film thickening was investigated. By reducing the growth rate, the film evolved from a mixed phase to the energetically favored ε phase. Based on these results, a Ga2O3 thin film with a phase-pure ε-Ga2O3 upper layer was successfully obtained.

An Investigation of Electrical and Dielectric Parameters of Sol–Gel Process Enabled $\beta $ -Ga2O3 as a Gate Dielectric Material

In this paper, $\beta $ -Ga2O3 thin films were grown on a p-Si substrate using the sol–gel method. Structural characterization of the films was performed using X-ray diffraction. Electrical parameters such as breakdown field, interface traps density ( ${D}_{\text {it}}$ ), and series resistance ( ${R}_{s}$ ) were investigated at room temperature. The interface trap density was found to be $10^{12}$ eV $^{-1}$ cm $^{-2}$ using the Hill–Coleman method. This result is valuable for MOS capacitor applications. Dielectric parameters were investigated in the wide frequency range (20 kHz–1 MHz) at room temperature. We observed that these parameters have a strong dependence on frequency and voltage.

Solar blind photodetector based on epitaxial zinc doped Ga2O3 thin film (Phys. Status Solidi A 5∕2017)

Solar blind photodetector based on epitaxial zinc doped Ga₂O₃ thin film (Phys. Status Solidi A 5∕2017)

Direct charge carrier injection into Ga2O3 thin films using an In2O3 cathode buffer layer: their optical, electrical and surface state properties

Conductive Ga2O3 thin films with an In2O3 buffer layer have been prepared on c-plane sapphire substrates using a laser molecular beam epitaxy technique. The effects of the In2O3 buffer layer on the structure and optical, electrical and surface state properties of the Ga2O3 films have been studied. The change in conductivity of the thin films is attributed to different thicknesses of the In2O3 buffer layer, which determine the concentration of charge carriers injected into the upper Ga2O3 layer from the interface of the bilayer thin films. In addition, the increase in flat band voltage shift and capacitance values as the In2O3 buffer layer thickens are attributed to the increase in surface state density, which also contributes to the rapid shrinkage of the optical band gap of the Ga2O3. With transparency to visible light, high n-type conduction and the ability to tune the optical band gap and surface state density, we propose that Ga2O3/In2O3 bilayer thin film is an ideal n-type semiconductor for fabrication of transparent power devices, solar cell electrodes and gas sensors.