Anisotropy, Phonon Modes, and Free Charge Carrier Parameters in Monoclinic β-Gallium Oxide Single Crystals

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic β-Ga2O3 single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and (2̅01), are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent β-Ga2O3 Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with Au and Bu symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in β-Ga2O3. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in β-Ga2O3 and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes.We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for β-Ga2O3

Pulsed Large Signal RF Performance of Field-Plated Ga2O3 MOSFETs

Comparison between pulsed and CW large signal RF performance of field-plated β-Ga 2 O 3 MOSFETs has been reported. Reduced self-heating when pulse resulted in a power added efficiency of 12%, drain efficiency of 22.4%, output power density of 0.13 W/mm, and maximum gain up to 4.8 dB at 1 GHz for a 2-μm gate length device. Increased power dissipation for higher VDS and IDS resulted in a degradation in performance, which, thermal simulation showed, could be entirely explained by self-heating. Buffer and surface trapping contributions have been evaluated using gate and drain lag measurements, showing minimal impact on device performance. These results suggest that β-Ga 2 O 3 is a good candidate for future RF applications

Ferromagnetic nanoclusters hybridized in Mn-incorporated GaInAs layers during metal–organic vapour phase epitaxial growth on InP layers under low growth temperature conditions

We demonstrate the successful formation of ferromagnetic nanoclusters in Mn-incorporated GaInAs layers grown by metal–organic vapour phase epitaxy on InP(100) substrates under low growth temperature conditions below 450 °C. We find that MnAs nanoclusters with NiAs-type hexagonal crystallographic structures, which show ferromagnetic characteristics up to a relatively high temperature of about 305 K, are formed near the layer surfaces of Mn-incorporated GaInAs layers grown at 440 °C. After deposition of undoped InP layers on Mn-incorporated GaInAs layers, MnP nanoclusters with orthorhombic cubic crystallographic structures, in which 7% arsenic is incorporated, are formed in InP layers. The samples with MnP nanoclusters show strong ferromagnetic coupling up to about 305 K, although the Curie temperature of MnP bulk compounds is 291 K. Energy dispersive x-ray spectroscopy (EDS) indicates that Mn concentrations in InP and GaInAs layers surrounding MnP nanoclusters are almost negligible. MnAs nanoclusters are also formed in Mn-incorporated GaAs layers grown under low growth temperature conditions of 450 °C on GaAs(100) substrates. From the results of magnetic characterizations with respect to growth temperatures of the samples, we found that all the Mn-incorporated GaAs layers grown at temperatures below 450 °C showed ferromagnetic behaviour.http://www.iop.org

High-Efficiency InGaN/GaN Quantum Well-Based Vertical Light-Emitting Diodes Fabricated on β‑Ga₂O₃ Substrate

We demonstrate a state-of-the-art high-efficiency GaN-based vertical light-emitting diode (VLED) grown on a transparent and conductive (−201)-oriented (β-Ga₂O₃) substrate, obtained using a straightforward growth process that does not require a high-cost lift-off technique or complex fabrication process. The high-resolution scanning transmission electron microscopy (STEM) images confirm that we produced high quality upper layers, including a multiquantum well (MQW) grown on the masked β-Ga₂O₃ substrate. STEM imaging also shows a well-defined MQW without InN diffusion into the barrier. Electroluminescence (EL) measurements at room temperature indicate that we achieved a very high internal quantum efficiency (IQE) of 78%; at lower temperatures, IQE reaches ∼86%. The photoluminescence (PL) and time-resolved PL analysis indicate that, at a high carrier injection density, the emission is dominated by radiative recombination with a negligible Auger effect; no quantum-confined Stark effect is observed. At low temperatures, no efficiency droop is observed at a high carrier injection density, indicating the superior VLED structure obtained without lift-off processing, which is cost-effective for large-scale devices

Origin of red emission in β-Ga2O3 analysed by cathodoluminescence and photoluminescence spectroscopy

The spectroscopic techniques of cathodoluminescence (CL) and photoluminescence (PL) are used to study the origin of red emission in β-Ga 2O 3 grown using the edge-defined film-fed grown (EFG) method and hydride vapor phase epitaxy. Room-temperature CL shows red emission peaks from samples doped with Fe, Sn, and Si and from unintentionally doped (UID) samples. Narrow emission lines around 690 nm are seen strongly in the Fe and UID samples. Temperature-dependent PL analysis of the two prominent red emission lines reveals properties similar to the R lines in sapphire for all samples but with different levels of emission intensities. These lines are attributed to Cr 3+ ionic transitions rather than Fe 3+, as reported previously. The most likely origin of the unintentional Cr incorporation is the source material used in the EFG method