Homoepitaxial growth of (100) Si-doped β-Ga2O3 films via MOCVD

Homoepitaxial growth of Si-doped β-Ga₂O₃ films on semi-insulating(100) β-Ga₂O₃ substrates by metalorganic chemical vapor deposition(MOCVD) is studied in this work. By appropriately optimizing the growth conditions, an increasing diffusion length of Ga adatoms is realized, suppressing 3D island growth patterns prevalent in(100) β-Ga₂O₃ films and optimizing the surface morphology with [010] oriented stripe features. The slightly Si-...     

Preparation and photodetection performance of high crystalline quality and large size β-Ga2O3 microwires

Ultrawide band gap semiconductors are promising solar-blind ultraviolet(UV) photodetector materials due to their suitable bandgap, strong absorption and high sensitivity. Here, β-Ga₂O₃ microwires with high crystal quality and large size were grown by the chemical vapor deposition(CVD) method. The microwires reach up to 1 cm in length and were single crystalline with low defect density. Owing to its high crystal quality, a metal–semiconductor–metal photodetector fabricated f...     

Large-area β-Ga2O3 Schottky barrier diode and its application in DC–DC converters

We demonstrate superb large-area vertical β-Ga₂O₃ SBDs with a Schottky contact area of 1 × 1 mm² and obtain a high-efficiency DC–DC converter based on the device. The β-Ga₂O₃ SBD can obtain a forward current of 8 A with a forward voltage of 5 V, and has a reverse breakdown voltage of 612 V. The forward turn-on voltage(VF) and the on-resistance(Ron) are 1.17 V and 0.46 Ω, respectively. The conversion efficiency of the β-Ga₂O_3<...     

High-temperature annealing of (■01) β-Ga2O3 substrates for reducing structural defects after diamond sawing

A commercial epi-ready(■01) β-Ga₂O₃ wafer was investigated upon diamond sawing into pieces measuring 2.5 ×3 mm². The defect structure and crystallinity in the cut samples has been studied by X-ray diffraction and a selective wet etching technique. The density of defects was estimated from the average value of etch pits calculated, including near-edge regions, and was obtained close to 109 cm^-2. Blocks with lattice orientation deviated by angles of 1-3 ...     

High-speed performance self-powered short wave ultraviolet radiation detectors based on κ(ε)-Ga2O3

High-speed solar-blind short wavelength ultraviolet radiation detectors based on κ(ε)-Ga₂O₃ layers with Pt contacts were demonstrated and their properties were studied in detail.The κ(ε)-Ga₂O₃ layers were deposited by the halide vapor phase epitaxy on patterned GaN templates with sapphire substrates.The spectral dependencies of the photoelectric properties of structures were analyzed in the wavelength interval 200-370 nm.The maximum photo to dark curre...     

Physical properties of hematite α-Fe_2O_3 thin films:application to photoelectrochemical solar cells

The physical properties and photoelectrochemical characterization of aluminium doped hematiteα-Fe₂O₃, synthesized by spray pyrolysis,have been investigated in regard to solar energy conversion.Stable Al-doped iron（Ⅲ） oxide thin films synthesized by a spray pyrolysis technique reveals an oxygen deficiency,and the oxide exhibits n-type conductivity confirmed by anodic photocurrent generation.The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate.The deposited iron oxide thin films show the single hematite phase with polycrystalline rhombohedral crystal structure with crystallite size 20-40 nm.Optical analysis enabled to point out the increase in direct band-gap energy from 2.2 to 2.25 eV with doping concentration which is attributed to a blue shift.The dielectric constant and dielectric loss are studied as a function of frequency.To understand the conduction mechanism in the films,AC conductivity is measured.The conduction occurs by small polaron hopping through mixed valences Fe^2+/3+ with an electron mobility 300 K of 1.08 cm²/（V·s）.Theα-Fe₂O₃ exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as a photoanode for various electrolytes using white light to obtainⅠ-Ⅴcharacteristics.The Al-doped hematite exhibited a higher photocurrent response when compared with undoped films achieving a power conversion efficiency of 2.37%at 10 at%Al:Fe₂O₃ thin films along with fill factor 0.38 in NaOH electrolyte.The flat band potential V_fb(-0.87 V_SCE) is determined by extrapolating the linear part to C^-2= 0 and the slope of the Mott-Schottky plot.     

A large-area multi-finger β-Ga2O3 MOSFET and its self-heating effect

The self-heating effect severely limits device performance and reliability. Although some studies have revealed the heat distribution of β-Ga₂O₃ MOSFETs under biases, those devices all have small areas and have difficulty reflecting practical conditions. This work demonstrated a multi-finger β-Ga₂O₃ MOSFET with a maximum drain current of 0.5 A. Electrical characteristics were measured, and the heat dissipation of the device was investigated through inf...     

2.83-kV double-layered NiO/β-Ga2O3 vertical p-n heterojunction diode with a power figure-of-merit of 5.98 GW/cm2

This work demonstrates high-performance NiO/β-Ga₂O₃ vertical heterojunction diodes(HJDs) with double-layer junction termination extension(DL-JTE) consisting of two p-typed NiO layers with varied lengths. The bottom 60-nm p-NiO layer fully covers the β-Ga₂O₃ wafer, while the geometry of the upper 60-nm p-NiO layer is 10 μm larger than the square anode electrode. Compared with a single-layer JTE, the electric field concentration is inhibited by double-la...     

Exploring heteroepitaxial growth and electrical properties of α-Ga2O3 films on differently oriented sapphire substrates

This study explores the epitaxial relationship and electrical properties of α-Ga₂O₃ thin films deposited on a-plane, mplane, and r-plane sapphire substrates. We characterize the thin films by X-ray diffraction and Raman spectroscopy, and elucidate thin film epitaxial relationships with the underlying sapphire substrates. The oxygen vacancy concentration of α-Ga₂O₃ thin films on m-plane and r-plane sapphire substrates are higher than α-Ga₂O     

锐钛矿TiO2(101)面电子性质和光学性质的第一性原理研究

The TiO₂（101） surface was studied using the plane-wave ultrasoft pseudopotential method based on the density functional theory,with emphasis on the structure,surface energy,band structure,density of states, and charge population.The anatase TiO₂（101） crystal surface structure,whose outermost and second layers were terminated by twofold coordinated oxygen atoms and fivefold coordinated titanium atoms,was found to be much more stable.The surface energy of the 18-layer atoms model was 0.580 J/m2.The surface electronic structure was similar to that of the bulk and no surface state.Compared with the bulk structure,the band gap increased 0.36 eV, the Ti5c-02c bond lengths reduced 0.171（?） after relaxation,and the charges of the surface were transferred to the body.Analysis of the optical properties of the TiO₂（101） surface showed that it did not absorb in the low-energy region.An absorption edge in the ultraviolet region corresponding to the energy of 3.06 eV was found.     

Two-step growth of β-Ga2O3 on c-plane sapphire using MOCVD for solar-blind photodetector

In this work,a two-step metal organic chemical vapor deposition(MOCVD) method was applied for growing β-Ga₂O₃ film on c-plane sapphire.Optimized buffer layer growth temperature(T_B) was found at 700℃ and the β-Ga₂O₃ film with full width at half maximum(FWHM) of 0.66° was achieved.A metal-semiconductor-metal(MSM) solar-blind photodetector(PD)was fabricated based on the β-Ga₂O₃ film.Ultrahigh responsivity of 1422 A/W@254 n...     

MEASUREMENT OF DENSITY OF THE GAP STATES IN a-Si:H BY THE NORMALIZATION OF PHOTOCONDUCTIVITY

The spectra of the optical absorption coefficient in low absorption region are obtained by using anormalization procedure for the photoconductivity spectra.The results are explained in terms of theoptical transition of electrons from localized states in the exponential valence band tail and in danglingbond states 1.0 eV below the conduction band edge to extended conduction band states.Then thedensity of the gap states below the Fermi level E_F is obtained.From the investigation ofrecombination kineties,the average density of,the gap states over the range of(F_(F_n)-E_F)and thedensity of the gap states above the Fermi level E_F are obtained.These indicate that the width of theconduction band tail is smaller than that of the valence band tail.     

表面结构和生长方向对WO3纳米线电子结构影响的第一性原理研究

The effects of the surface and orientation of a WO₃ nanowire on the electronic structure are investigated by using first principles calculation based on density functional theory（DFT）.The surface of the WO3 nanowire was terminated by a bare or hydrogenated oxygen monolayer or bare WO₂ plane,and the[010]- and[001]-oriented nanowires with different sizes were introduced into the theoretical calculation to further study the dependence of electronic band structure on the wire size and orientation.The calculated results reveal that the surface structure, wire size and orientation have significant effects on the electronic band structure,bandgap,and density of states （DOS） of the WO₃ nanowire.The optimized WO₃ nanowire with different surface structures showed a markedly dissimilar band structure due to the different electronic states near the Fermi level,and the O-terminated[001] WO₃ nanowire with hydrogenation can exhibit a reasonable indirect bandgap of 2.340 eV due to the quantum confinement effect,which is 0.257 eV wider than bulk WO₃.Besides,the bandgap change is also related to the orientation-resulted surface reconstructed structure as well as wire size.     

Recent advances in NiO/Ga2O3 heterojunctions for power electronics

Beta gallium oxide(β-Ga₂O₃) has attracted significant attention for applications in power electronics due to its ultrawide bandgap of ～ 4.8 eV and the large critical electric field of 8 MV/cm. These properties yield a high Baliga’s figures of merit(BFOM) of more than 3000. Though β-Ga₂O₃ possesses superior material properties, the lack of p-type doping is the main obstacle that hinders the development of β-Ga₂O₃-based power device...     

Density functional theory studies of the optical properties of a β-FeSi2 （100）/Si（001） interface at high pressure

High pressure has a significant influence onβ-FeSi₂ band gaps and optical absorption tuning.In this work,using density functional theory,we investigate the effect of high pressure on the optical absorption behavior of aβ-FeSi₂（100）/Si（001） interface with some Si vacancies.As the pressure increases,the optical absorption peak down-shifts firstly,reach minimum values,and then un-shifts slowly.The electronic orbital analysis indicates that the electronic transition between the highest occupied states and the lowest unoccupied states mainly originate from Fe atoms at the interface regions.Structural analysis discloses that the Si（001） slab partially offsets the pressure exerted on theβ-FeSi₂（100） interface,but this effect will become weaker with further increasing pressure,and this physical mechanism plays an important role in its optical absorption behavior.     

Vertical β-Ga2O3 power electronics

<正>β-Ga₂O₃ possesses a highly promising critical electric field of 8 MV/cm, allowing devices with improved performance compared with other wide bandgap materials^{[1, 2]}. The 4-inch wafers grown from a melt and over 10 μm of the epitaxial layers grown by Halide vapor phase epitaxy(HVPE) with highly controllable doping concentration, are commercially available, paving the way of vertical power devices.     

Evidence from spectral emissometry for conduction intraband transitions in the intrinsic regime for silicon

From emissometry measurements in lightly doped Si at elevated temperatures, we have observed an anomalous absorption band in the wavelength range of 1–5 μm. The wavelength at which the band peaks, λ≈2.3 μm, shows a negligible dependence on temperature while the peak intensity increases with temperature presumably as a result of the increasing intrinsic carrier concentration. Spitzer and Fan reported a similar absorption band in direct absorption measurements at room temperature for n-type Si with extrinsic electron concentrations of 10¹⁴ to 10¹⁹cm⁻³. No such structure was found in extrinsic p-type Si. Spitzer and Fan were unable to identify the mechanism for this anomalous absorption. In both the experiments, this absorption of free electrons is due to intraband transitions in the conduction band from the Δ₁ conduction band edge across an energy gap of E ∼ 0.5 eV to a higher lying Δ₂′ conduction band.     

Measurements of Carrier Confinement at β-FeSi2-Si Heterojunction by Electroluminescence

A Si p-π-n diode with β-FeSi2 particles embedded in the unintentionally doped Si (p－type) was designed for determining the band offset at β-FeSi2-Si heterojunction.When the diode is under forward bias,the electrons injected via the Si np－ junction diffuse to and are confined in the β-FeSi2 particles due to the band offset.The storage charge at theβ-FeSi2-Si heterojunction inversely hamper the further diffusion of electrons,giving rise to the localization of electrons in the p－Si near the Si junction,which prevents them from nonradiative recombination channels.This results in electroluminescence (EL) intensity from both Si and β-FeSi2 quenching slowly up to room temperature.The temperature dependent ratio of EL intensity of β-FeSi2 to Si indicates the loss of electron confinement following thermal excitation model.The conduction band offset between Si and β-FeSi2 is determined to be about 0.2eV.     

Anisotropic optical and electric properties of β-gallium oxide

The anisotropic properties and applications of β-gallium oxide(β-Ga₂O₃) are comprehensively reviewed. All the anisotropic properties are essentially resulted from the anisotropic crystal structure. The process flow of how to exfoliate nanoflakes from bulk material is introduced. Anisotropic optical properties, including optical bandgap, Raman and photoluminescence characters are comprehensively reviewed. Three measurement configurations of angle-resolved polarized Raman spe...     

Electronic structures and NIR-II optical properties of black phosphorus using first principles

Near infrared-II (NIR-II, 1 000—1 700 nm) imaging with high penetration tissue depth and signal-noise ratio has attracted wide interest in biomedicine. As a two-dimensional (2D) material with narrow band gap, the band structure of layered black phosphorus, as an important characteristic of electronic structure, determines the electronic transport and infrared optical properties, which show great potential in NIIR-II imaging. Here, the electronic structure and NIR-II optical properties of black phosphorus have been investigated in detail by employing the generalized gradient approximation + U (GGA+U) correction based on density functional theory (DFT). First, we performed the band structure and density of states for different layers of black phosphorus. From the electronic structures, the location of valence band maximum didn’t shift obviously, and the position of conduction band minimum shifted downward gradually, inducing the band gaps decreased gradually with the increasing layer number. While the layer number increased to 5, the behaviour of electronic structure was very similar to that of the bulk black phosphorus. Then, we calculated the NIR-II optical properties, and found the optical band gap of black phosphorus also showed layer dependent properties. From a single layer to 5 layers, the optical band gap changed from 1.71 eV to 0.92 eV. It is noting that black phosphorus also showed the significant optical absorption in NIR-IIa (1 300—1 400 nm) and NIR-IIb (1 500—1 700 nm) windows. Especially, the NIR-II optical absorption can be enhanced with increasing the layer number to 5, indicating promising photoresponse materials in NIR-II imaging.