Effect of oxygen flow ratio on the performance of RF magnetron sputtered Sn-doped Ga2O3 films and ultraviolet photodetector

This work explored the properties of RF magnetron sputtered Sn-doped Ga₂O₃ films grown on sapphire substrates at different oxygen flow ratios from 0.0 to 2.5%. The in situ optical emission spectroscopy was conducted to monitor the plasma radicals generated during the films’ deposition. All the films deposited at room temperature show amorphous structures with some nanoparticles. The deposition rate decreased monotonically with increasing oxygen flow ratio. The proposed conductive mechanism of the films can be mainly attributed to the changes in the ratio of substitutional Sn (Sn⁴⁺ valance state) atoms replacing lattice Ga sites (Ga³⁺ valance state) and the SnO₂ phase in the films. Metal–semiconductor–metal solar-blind photodetectors were developed and analyzed to illustrate the effect of oxygen flow ratio. A high performance photodetector with a low dark current of 1.14 pA, high on/off ratio of 812 and short rise/decay time of 0.05 s/0.12 s was realized at an optimization growth condition. The elaboration of the conductive mechanism and effect of oxygen flow ratio on the performance of Sn-doped Ga₂O₃ films and their photodetectors is crucial for the preparation of high-quality Sn-doped Ga₂O₃ films and its application in optoelectronic devices.     

Influence of substrate temperature on the properties of (AlGa)2O3 thin films prepared by pulsed laser deposition

(AlGa)₂O₃ thin films were deposited on (0001) sapphire substrates by pulsed laser deposition at different substrate temperatures. The influence of substrate temperature on surface morphology, optical properties, and crystal quality has been systematically investigated by atomic force microscope, transmission spectra, X-ray diffraction, and Raman spectroscopy. The results reveal that all the (AlGa)₂O₃ films have smooth surface and high transmittance. The (AlGa)₂O₃ film with the good crystal quality can be obtained at a substrate temperature of 400 °C. Our results provide an experimental basis for realizing the Ga₂O₃-based quantum well.     

Annealing temperature controlled crystallization mechanism and properties of gallium oxide film in forming gas atmosphere

Gallium oxide (Ga₂O₃) films had been fabricated on Al₂O₃(0001) substrate by employing pulsed laser deposition (PLD) and annealed at different temperatures under forming gas (FG) atmosphere (95% N₂ + 5% H₂). The influence of annealing temperature on the structural, optical, chemical composition, and surface morphological properties of the Ga₂O₃ thin films was investigated comprehensively. The annealing processes with hydrogen gas play a crucial role in the characteristics of Ga₂O₃ thin films. A crystallization mechanism of Ga₂O₃ films controlled by annealing temperature has been proposed firstly and analyzed systematically, which contains three kinds of competitive mechanism, namely the thermal enhanced crystallization, the enhanced H₂ dissociative adsorption on Ga₂O₃ surfaces, and the high-temperature decomposition of Ga₂O₃. Both Ga⁺ and Ga³⁺ oxidation valence states were presented in all samples, which indicated lattice oxygen deficiency in Ga₂O₃ films. The variation of the non-lattice oxygen proportion of Ga₂O₃ films related to the crystallization mechanism firstly increased and then decreased with the increase of annealing temperature. The detailed crystallization mechanism of PLD-Ga₂O₃ films annealed in FG offers a guideline and references for the further fabrication of high-quality Ga₂O₃ films and their applications in high-performance devices.     

Characterization of Bi2O3 thin films for doping photoluminescent Er3+ ions

Undoped and Er³⁺-doped Bi₂O₃ thin films were sputter-deposited on Si(100) substrates. Sufficiently oxidized Bi₂O₃ films with refractive indices between 2.17?2.23 were obtained at a wavelength of 633 nm; these values are comparable to those of bulk Bi₂O₃ crystals. While the film composition was stable for deposition temperatures between room temperature (RT) and 450 °C, the refractive index steeply decreased above 450 °C and reached 1.4 at 600 °C. The lowering of the optical transmittance spectra indicated aggregation of metallic Bi and darkening of the film. All films exhibited X-ray diffraction patterns of α-Bi₂O₃. The direct and indirect bandgap energies derived from the Tauc plots were 3.4–3.7 eV and 1.9–2.5 eV, respectively, depending on the O₂ flow rate and deposition temperature. Upon excitation of Er³⁺-doped Bi₂O₃ films at 532 nm, Er³⁺ emissions peaking at 1537 and 1541 nm appeared, and the photoluminescence spectra included fine structures reflecting crystal-field splitting. Resonant excitation of Er³⁺ 4f levels and indirect excitation via the defect levels of Bi₂O₃ followed by energy transfer to Er³⁺ contributed to the emission. The films deposited at RT with Er concentrations of 2 at.% had the emission intensity of Er³⁺, but concentration quenching strongly suppressed the Er³⁺ emission because the doped Er³⁺ ions stayed inside the Bi₂O₃ crystals. At deposition temperatures above 400 °C, the concentration quenching was mitigated possibly because out-diffusion of Er³⁺ ions reduced the effective number of Er³⁺ ions in the Bi₂O₃ crystalline domains.     

Characterization of non-stoichiometric Ga2O3-x thin films grown by radio-frequency powder sputtering

We report the synthesis and characterization of non-stoichiometric Ga₂O_3-x thin films deposited on sapphire (0001) substrates by radio-frequency powder sputtering. The chemical and electronic states of the non-stoichiometric Ga₂O_3-x thin films were investigated. By sputtering in an Ar atmosphere, the as-grown thin films become non-stoichiometric Ga₂O_2.7, due to the difference in sputtering yield between Ga and O species of the Ga₂O₃ target. The electronic states of the thin films consist of ~85% Ga³⁺ and ~15% Ga¹⁺, corresponding to Ga₂O₃ and Ga₂O, respectively. The films have the electrical characteristics of a semiconductor, with electrical conductivity of approximately 5.0 × 10^-4 S cm^-1 and a carrier concentration of 4.5 × 10¹⁴ cm^-3 at 300 K.     

Effect of substrate temperature on microstructure and optical properties of nanocrystalline alumina thin films

Aluminum oxide (Al₂O₃) thin films were deposited on silicon (100) and quartz substrates by pulsed laser deposition (PLD) at an optimized oxygen partial pressure of 3.0×10^?3 mbar in the substrate temperatures range 300–973 K. The films were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, spectroscopic ellipsometry, UV–visible spectroscopy and nanoindentation. The X-ray diffraction studies showed that the films deposited at low substrate temperatures (300–673 K) were amorphous Al₂O₃, whereas those deposited at higher temperatures (≥773 K) were polycrystalline cubic γ-Al₂O₃. The transmission electron microscopy studies of the film prepared at 673 K, showed diffuse ring pattern indicating the amorphous nature of Al₂O₃. The surface morphology of the films was examined by atomic force microscopy showing dense and uniform nanostructures with increased surface roughness from 0.3 to 2.3 nm with increasing substrate temperature. The optical studies were carried out by ellipsometry in the energy range 1.5–5.5 eV and revealed that the refractive index increased from 1.69 to 1.75 (λ=632.8 nm) with increasing substrate temperature. The UV–visible spectroscopy analysis indicated higher transmittance (>80%) for all the films. Nanoindentation studies revealed the hardness values of 20.8 and 24.7 GPa for the films prepared at 300 K and 973 K respectively.     

Recrystallization behavior,oxygen vacancy and photoluminescence performance of sputter-deposited Ga2O3 films via high-vacuum in situ annealing

Ga₂O₃ films were deposited on Si substrates through radio-frequency magnetron sputtering at room temperature and were annealed in situ in a high-vacuum environment. The as-deposited Ga₂O₃ film exhibited an island-like surface morphology and had an amorphous microstructure, with a few nanocrystalline grains embedded in it. After high-temperature in situ annealing, the films recrystallized and exhibited coalesced surfaces. Because of the thermally driven diffusion of Ga, the interfacial layer between Si and Ga₂O₃ was composed of SiGaO_x. Compared with ex situ annealing in air, in situ annealing in high vacuum is more advantageous because it enhances surface mobility and improves the crystallinity of the Ga₂O₃ films. The higher oxygen vacancy concentration of in situ annealed films revealed that oxygen atoms were easily released from the Ga₂O₃ lattice during high-vacuum annealing. Photoluminescence (PL) spectra exhibited four emission peaks centered in ultraviolet, blue, and green regions, and the peak intensities were significantly enhanced by thermal annealing at >600 °C. This work elucidates the effect of the in situ annealing treatment on the recrystallization behavior, interfacial microstructure, oxygen vacancy concentration, and PL performance of the Ga₂O₃ films, making it significant and instructional for the further development of Ga₂O₃-based devices.     

Insights into Cu2O thin film absorber via pulsed laser deposition

Cuprous oxide materials are of growing interest for optoelectronic devices and were produced by several chemical and physical methods. Here, we report on the structural, optical, and electrical properties of Cu_xO thin films prepared by the pulsed laser deposition technique. The substrate temperature, as well as the oxygen partial pressure in the deposition chamber, were varied to monitor the copper to oxygen ratio within the deposited films. The growth conditions were carefully optimized to provide the highest conductivity and mobility. Thus, 100 nm thick cuprous oxide films (Cu₂O) deposited at 750 °C exhibited a resistivity of 16 Ω?cm, high mobility of 30 cm²/(V?s), and a bandgap of around 2 eV. The film deposited at the optimized deposition parameters on Nb:STO (001) substrate with Au top electrode showed a photovoltaic response with an open circuit voltage of 0.56 V. These results path the way to efficient solar cells made with Cu₂O films via the pulsed laser deposition technique.     

Raman enhancement by graphene-Ga2O3 2D bilayer film

2D β-Ga₂O₃ flakes on a continuous 2D graphene film were prepared by a one-step chemical vapor deposition on liquid gallium surface. The composite was characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy (XPS). The experimental results indicate that Ga₂O₃ flakes grew on the surface of graphene film during the cooling process. In particular, tenfold enhancement of graphene Raman scattering signal was detected on Ga₂O₃ flakes, and XPS indicates the C-O bonding between graphene and Ga₂O₃. The mechanism of Raman enhancement was discussed. The 2D Ga₂O₃-2D graphene structure may possess potential applications.     

Enhanced thickness uniformity of large-scale α-Ga2O3 epilayers grown by vertical hot-wall mist chemical vapor deposition

Smooth surface morphology and high thickness uniformity heteroepitaxy of corundum-structured (α-) gallium oxide (Ga₂O₃) crystalline thin films on 100-mm diameter c-plane sapphire substrates were successfully demonstrated using vertical hot-wall mist chemical vapor deposition (CVD). The growth rate and surface morphology of the epitaxial layers were numerically and experimentally found to be dependent on the diameter of the precursor-diluted microdroplets approaching the substrate surface. Since the microdroplet is gradually evaporated while traveling through the furnace, the growth variables such as temperature, mist-flow velocity, and substrate position were tuned to obtain a suitable diameter of microdroplets approaching the substrate. In this study, the diameter of the approaching microdroplet was ≈2 μm, which was optimal for the smooth surface (root mean square roughness ≈1 nm) of α-Ga₂O₃ epitaxial layers with a growth rate of ≈230 nm/h. Due to the even flow of mist in the vertical furnace, high thickness uniformity of the α-Ga₂O₃ epitaxial layer is guaranteed on large-scale substrates, with a standard deviation of thickness as small as 28 nm, paving the way for highly reliable Ga₂O₃-based electric and optoelectronic devices.     

Structure and conducting properties of La1-xSrxCoO3-δ films

La_1-xSr_xCoO_3-δ (LSCO) films have been deposited on LaAlO₃ (LAO), La_1-xSr_xGa_1-yMg_yO_3-δ/LaAlO₃ (LSGM/LAO) and yittria-stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The optimum conditions for deposition were determined for the different substrates in an ambient of 80–310 mTorr oxygen pressure and at a substrate temperature range of 450 to 750°C. The films structures were analyzed by XRD, RBS and SEM. Epitaxial LSCO films were grown with (110) preferred orientation on YSZ, and with (100) orientation on LAO and LSGM/LAO. The electrical resistivity of the epitaxial LSCO films ranged from 10⁻² to 10⁻⁴ Ω cm, depending on the deposition temperature and substrate. The ionic conducting behavior of the LSCO film on YSZ was investigated by impedance measurement.     

Band gap tuning of Ga2O3–Al2O3 ceramics

Translucent ceramics comprising monoclinic Ga₂O₃ and non-cubic Ga₂O₃–Al₂O₃ solid solution with a composition of Ga_1.5Al_0.5O₃, GaAlO₃ and Ga_0.4Al_1.6O₃ were fabricated through two-step spark plasma sintering by using calcinated commercial mixture oxide ceramic powders. All sintered ceramics were nearly fully densified, with a relative density exceeding 99.2%. The average grain sizes decreased with increasing Al content. Band gap was tuned from 4.08 to 6.37 eV. The highest total transmittance (73.6% at 1100 nm) was measured in Ga_0.4Al_1.6O₃ ceramics with both hexagonal (rhombohedral) and monoclinic crystal structures and a minimum average grain size of 0.288 ± 0.067 μm. Defect related photoluminescence, measured under excitation by a 254 nm UV lamp in the GaAlO₃ and Ga_0.4Al_1.6O₃ ceramics, lasted more than 15 s after removal of the UV source. These translucent ceramics from Ga₂O₃–Al₂O₃ solid solution provide an alternative form of a potential transparent conducting material.     

Enhancement of the radiation resistance of cerium-containing fluorophosphate glasses through codoping with Sb2O3 and Bi2O3

The growing demand for radiation-resistant optical glasses for space and nuclear radiation applications has attracted significant research interest. However, radiation-resistant fluorophosphate glasses have been poorly studied. In this work, we report on the tailoring and performance of radiation-resistant fluorophosphate glasses that contained cerium through codoping with Sb₂O₃ and Bi₂O₃. The physical properties, optical properties, microstructure, and defects of fluorophosphate glasses were investigated using transmittance measurements, absorption measurements, as well as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. The results showed that the radiation resistance of all codoped fluorophosphate glasses was better than the undoped cerium-containing fluorophosphate glasses after 10–250 krad(Si) irradiation. Especially in glasses doped with Bi₂O₃, the optical density increment at 385 nm was only 0.1482 after 250 krad(Si) irradiation. The CeO₂ prevented the development of phosphate-related oxygen hole center (POHC) defects, whereas further codoping with Bi₂O₃ suppressed the formation of oxygen hole center (OHC) and POEC defects, reducing the breaking of phosphate chains caused by CeO₂. Bi³⁺ is more likely than Sb³⁺ to change the valence, affecting the transition equilibrium of intrinsic defects and reducing the concentration of defects produced by irradiation. When codoping with Sb₂O₃ and Bi₂O₃, Bi₂O₃ does not enhance radiation resistance owing to the scission effect of Sb₂O₃ on the phosphate chain, which is not conducive to the radiation resistance of glasses. This indicates that the cerium-containing fluorophosphate glasses doped with Bi₂O₃ can effectively suppress the defects caused by irradiation and improve the radiation resistance of the glasses.     

Synthesis of polycrystalline gallium oxide solar-blind ultraviolet photodetector by Aerosol Deposition

An aerosol deposition method was used to fabricate a solar-blind photodetector (for UV-C) using thin films of β-Ga₂O₃, which is a wide-bandgap oxide material. The Ga₂O₃ films deposited at room temperature presented a polycrystalline structure and a thickness of approximately 4 µm and showed a high transmittance of approximately 70–80 % in the visible region; the transmittance was approximately 60–80 % even after heat treatment up to a 800 °C. The Ga₂O₃ films that were post-annealed at a temperature of 800 °C showed an I_photo/I_dark ratio of approximately 40,000 in the solar-blind region with a light source of 254 nm, together with very good light detection characteristics (initial rising and decay times of 0.45 s and 0.13 s, respectively). Because of the good performances observed for the Ga₂O₃ thin films even at extreme conditions, they exhibit a high potential for use as photodetectors in several applications.     

Influence of deposition temperature on luminescent efficiency of Y2O3:Eu3+ thin films deposited on quartz fabric by EBE

Y₂O₃:Eu³⁺ thin films were grown on quartz fabric substrate by electron beam evaporation (EBE) at different deposition temperatures. It was found that an increase of deposition temperature from room temperature (R.T.) to 250 °C results in improved morphologies of the films, such as reduced defects, spherical particle shape and dense surface topography. A change in the predominant orientation of Y₂O₃:Eu³⁺ thin films was detected from (222) at low temperatures of R.T.–150 °C to (400) at higher temperatures of 200–250 °C. The luminescent intensity of the films was gradually improved with an increase in deposition temperature and the optimal brightness was observed when the films were grown at 250 °C and improved by 32.67% in comparison with that of the films grown at R.T. The results reveal that the improved morphologies and effective crystallization can contribute to the enhanced luminescent properties of the Y₂O₃:Eu³⁺ thin films.     

Preparing pure C-axis oriented SrTiO3 film on CeO2 film by introducing a nano-Y0.5Gd0.5Ba2Cu3O7-δ layer

SrTiO₃ films were deposited using pulsed laser deposition (PLD) at substrate temperatures ranging from 300 °C to 700 °C on a CeO₂ layer and nano-Y_0.5Gd_0.5Ba₂Cu₃O_7-δ buffered CeO₂ layer, respectively. The effect of the substrate temperature and substrate film on the crystallinity, preferred orientation, and surface topography were investigated. On the CeO₂ layer, there were two preferred SrTiO₃ orientations, (00l) and (110). By introducing a nano-Y_0.5Gd_0.5Ba₂Cu₃O_7-δ layer, pure c-axis oriented SrTiO₃ films were obtained at each substrate temperature. Based on the lattice mismatch, three kinds of growth modes of SrTiO₃ grains were proposed that were proved by scanning electron microscopy (SEM).     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Gallium oxide films deposition by RF magnetron sputtering; a detailed analysis on the effects of deposition pressure and sputtering power and annealing

In this study, gallium oxide (Ga₂O₃) thin films were deposited on sapphire and n-Si substrates using Ga₂O₃ target by radio frequency magnetron sputtering (RFMS) at substrate temperature of 300 °C at variable RF power and deposition pressure. The effects of deposition pressure and growth power on crystalline structure, morphology, transmittance, refractive index and band gap energy were investigated in detail. X-ray diffraction results showed that amorphous phase was observed in all the as-deposited thin films except for the thin films grown at low growth pressure. All the films showed conversion to poly-crystal β-Ga₂O₃ phase after annealing process. When the deposition pressure increased from 7.5 mTorr to 12.20 mTorr, change in the 2D growth mode to 3D columnar growth mode was observed from the SEM images. Annealing clearly showed formation of larger grains for all the thin films. Lower transmission values were observed as the growth pressure increases. Annealing caused to obtain similar transmittance values for the thin films grown at different pressures. It was found that a red shift observed in the absorption edges and the energy band gap values decrease with increasing growth pressure. For as-deposited and annealing films, increasing sputtering power resulted in the increase refractive index.     

Effect of laser power density on the electrochromic properties of WO3 films obtained by pulsed laser deposition

Pulsed laser deposition (PLD) was used to prepare tungsten trioxide (WO₃) films on ITO substrates with a varying laser power density of 4.0–5.5 W/cm². XPS indicated that when the laser power density decreased, the peak positions of the W 4f and O 1s orbits shifted slightly to low energy due to the difference in oxygen vacancies. As the laser power density decreased, W⁶⁺ gradually replaced the lattice position of O^2?, increasing oxygen vacancies in the lattice. The transmittance modulated values (ΔT) were over 44% at 830 nm, indicating strong absorption by the WO₃ thin films in the near-infrared ray. The switching time of the WO₃ thin films between bleached states and coloured states decreased as the laser power density increased due to the amorphous structure, morphology, and lower oxygen deficiency at a high power density. The high ΔT and very fast switching time of t_b (1.09 s) and t_c (6.01 s) demonstrated the excellent electrochromic (EC) properties of the WO₃ films prepared by PLD.     

Fabrication of Ce:(Gd2Y)(Ga3Al2)O12 scintillator ceramic by oxygen-atmosphere sintering and hot isostatic pressing

A Ce-doped (Gd₂Y)(Ga₃Al₂)O₁₂ ceramic with good performance was fabricated by sintering in an oxygen atmosphere and hot isostatic pressing. The microstructure as well as the optical and scintillation characteristics of the ceramic was investigated. The ceramic exhibited a high transmittance of 78% in the range of 500–800 nm. The radioluminescence intensity of the ceramic reached up to 30 times that of a bismuth germinate single crystal, and the light yield within 750 ns shaping time was approximately 25800 photons/MeV under ¹³⁷Cs γ-ray irradiation.