The Spatial Correlation and Anisotropy of ß-(AlxGa1-x)2O3 Single Crystal.

The long-range crystallographic order and anisotropy in ß-(AlxGa1-x)2O3 (x = 0.0, 0.06, 0.11, 0.17, 0.26) crystals, prepared by optical floating zone method with different Al composition, is systematically studied by spatial correlation model and using an angle-resolved polarized Raman spectroscopy. Alloying with aluminum is seen as causing Raman peaks to blue shift while their full widths at half maxima broadened. As x increased, the correlation length (CL) of the Raman modes decreased. By changing x, the CL is more strongly affected for low-frequency phonons than the modes in the high-frequency region. For each Raman mode, the CL is decreased by increasing temperature. The results of angle-resolved polarized Raman spectroscopy have revealed that the intensities of ß-(AlxGa1-x)2O3 peaks are highly polarization dependent, with significant effects on the anisotropy with alloying. As the Al composition increased, the anisotropy of Raman tensor elements was enhanced for the two strongest phonon modes in the low-frequency range, while the anisotropy of the sharpest Raman phonon modes in the high-frequency region decreased. Our comprehensive study has provided meaningful results for comprehending the long-range orderliness and anisotropy in technologically important ß-(AlxGa1-x)2O3 crystals.

Effective Suppression of MIS Interface Defects Using Boron Nitride toward High-Performance Ta-Doped-ß-Ga2O3 MISFETs.

ß-Ga2O3 is considered an attractive candidate for next-generation high-power electronics due to its large band gap of 4.9 eV and high breakdown electrical field of 8 MV/cm. However, the relatively low carrier concentration and low electron mobility in the ß-Ga2O3-based device limit its application. Herein, the high-quality ß-Ga2O3 single crystal with high doping concentration of ∼3.2 × 1019 cm-3 was realized using an optical float-zone method through Ta doping. In contrast to the SiO2/ß-Ga2O3 gate stack structure, we used hexagonal boron nitride as the gate insulator, which is sufficient to suppress the metal-insulator-semiconductor (MIS) interface defects of the ß-Ga2O3-based MIS field-effect transistors (FETs), exhibiting outstanding performances with a low specific on-resistance of ∼6.3 mΩ·cm2, a high current on/off ratio of ∼108, and a high mobility of ∼91.0 cm2/(V s). Our findings offer a unique perspective to fabricate high-performance ß-Ga2O3 FETs for next-generation high-power nanoelectronic applications.

Anisotropic luminescence and third-order electric susceptibility of Mg-doped gallium oxide under the half-bandgap edge.

Strong anisotropy of photoluminescence of a (100)-cut ß-Ga2O3 and a Mg-doped ß-Ga2O3 single crystals was found in UV and visible spectral range, the bands of which were attributed to different types of transitions in the samples. Green photoluminescence in the Mg-doped sample was enhanced approximately twice. A remarkable enhancement of two-photon absorption and self-focusing in ß-Ga2O3 after doping was revealed by 340-fs laser Z-scanning at 515 nm. The absolute value of complex third order susceptibility χ(3) determined from the study increases by 19 times in [001] lattice direction. Saturable absorption and associated self-defocusing were found in the undoped crystal in the [010] direction, which was explained by the anisotropic excitation of F-centers on intrinsic oxygen defects. This effect falls out of resonance in the Mg-doped crystal. The χ(3) values which are provided by a decrease of bandgap in Mg-doped ß-Ga2O3 are χ(3) [001] = 1.85·10-12 esu and χ(3) [010]=χ(3)yyyy = 0.92·10-12 esu. Our result is only one order of magnitude lower than the best characteristic in green demonstrated by a Mg-doped GaN, which encourages subsequent development of Mg-doped ß-Ga2O3 as an effective nonlinear optical material in this region.

Fabrication of a Nb-Doped ß-Ga2O3 Nanobelt Field-Effect Transistor and Its Low-Temperature Behavior.

For the first time, we report the successful fabrication of well-behaved field-effect transistors based on Nb-doped ß-Ga2O3 nanobelts mechanically exfoliated from bulk single crystals. The exfoliated ß-Ga2O3 nanobelts were transferred onto a purified surface of the 110 nm SiO2/Si substrate. These Nb-doped devices showed excellent electrical performance such as an ultrasmall cutoff current of ∼10 fA, a high current on/off ratio of >108, and a quite steep subthreshold swing (SS, ∼120 mV/decade). Furthermore, we investigated the temperature dependence down to 200 K, providing insightful information for its operation in a harsh environment. This work lays a foundation for wider application of Nb-doped ß-Ga2O3 in nano-electronics.

Investigation of Band Alignment for Hybrid 2D-MoS2/3D-ß-Ga2O3 Heterojunctions with Nitridation.

Hybrid heterojunctions based on two-dimensional (2D) and conventional three-dimensional (3D) materials provide a promising way toward nanoelectronic devices with engineered features. In this work, we investigated the band alignment of a mixed-dimensional heterojunction composed of transferred MoS2 on ß-Ga2O3([Formula: see text]01) with and without nitridation. The conduction and valence band offsets for unnitrided 2D-MoS2/3D-ß-Ga2O3 heterojunction were determined to be respectively 0.43 ± 0.1 and 2.87 ± 0.1 eV. For the nitrided heterojunction, the conduction and valence band offsets were deduced to 0.68 ± 0.1 and 2.62 ± 0.1 eV, respectively. The modified band alignment could result from the dipole formed by charge transfer across the heterojunction interface. The effect of nitridation on the band alignments between group III oxides and transition metal dichalcogenides will supply feasible technical routes for designing their heterojunction-based electronic and optoelectronic devices.

Investigation of the Mechanism for Ohmic Contact Formation in Ti/Al/Ni/Au Contacts to ß-Ga2O3 Nanobelt Field-Effect Transistors.

The issue of contacts between the electrode and channel layer is crucial for wide-bandgap semiconductors, especially the ß-Ga2O3 due to its ultra-large bandgap (4.6-4.9 eV). It affects the device performance greatly and thus needs special attention. In this work, the high-performance ß-Ga2O3 nanobelt field-effect transistors with Ohmic contact between multilayer metal stack Ti/Al/Ni/Au (30/120/50/50 nm) and unintentionally doped ß-Ga2O3 channel substrate have been fabricated. The formation mechanism of Ohmic contacts to ß-Ga2O3 under different annealing temperatures in an N2 ambient is systematically investigated by X-ray photoelectron spectroscopy. It is revealed that the oxygen vacancies at the interface of ß-Ga2O3/intermetallic compounds formed during rapid thermal annealing are believed to induce the good Ohmic contacts with low resistance. The contact resistance (Rc) between electrodes and unintentionally doped ß-Ga2O3 reduces to ∼9.3 Ω mm after annealing. This work points to the importance of contact engineering for future improved ß-Ga2O3 device performance and lays a solid foundation for the wider application of ß-Ga2O3 in electronics and optoelectronics.

Recent Advances in ß-Ga2O3-Metal Contacts.

Ultra-wide bandgap beta-gallium oxide (ß-Ga2O3) has been attracting considerable attention as a promising semiconductor material for next-generation power electronics. It possesses excellent material properties such as a wide bandgap of 4.6-4.9 eV, a high breakdown electric field of 8 MV/cm, and exceptional Baliga's figure of merit (BFOM), along with superior chemical and thermal stability. These features suggest its great potential for future applications in power and optoelectronic devices. However, the critical issue of contacts between metal and Ga2O3 limits the performance of ß-Ga2O3 devices. In this work, we have reviewed the advances on contacts of ß-Ga2O3 MOSFETs. For improving contact properties, four main approaches are summarized and analyzed in details, including pre-treatment, post-treatment, multilayer metal electrode, and introducing an interlayer. By comparison, the latter two methods are being studied intensively and more favorable than the pre-treatment which would inevitably generate uncontrollable damages. Finally, conclusions and future perspectives for improving Ohmic contacts further are presented.

Diode-pumped continuous wave and Q-switched operation of Nd:CaYAlO4 crystal.

The continuous wave (CW) and passively Q-switched performances of Nd:CaYAlO(4) crystal with both a- and c-cut were demonstrated. The CW output powers of 1.15 W and 1.26 W were obtained under the pump power of 8.96 W with slope efficiencies of 15.2% and 16.8% for a- and c-cut samples, respectively. As a result, new dual-wavelength all-solid-state lasers at 1080 nm and 1081 nm were achieved with c-cut crystal. By using Cr(4+):YAG wafer as saturable absorber, we performed Q-switching experiments. The highest average output powers and shortest pulse widths were measured to be 0.798 W, 10.6 ns and 0.537 W, 9.6 ns for a- and c-cut samples, respectively.