Omer Donmez

We combine room-temperature photomodulated reflectance (PR) spectroscopy with electronic band-structure calculations to investigate post-growth rapid thermal annealing (RTA) in a two-step-grown Be-doped Al0.15Ga0.85As/GaAs0.96Bi0.04 single-quantum-well structure. PR spectra from the as-grown sample and samples annealed at 550, 600, and 650 °C were fitted with the third-derivative functional form (TDFF). The fitted resonance energies and linewidths were compared with optical transitions calculated using a one-dimensional, strain-aware envelope-function model with VCA/VBAC band edges. The 600 °C sample exhibits the greatest overall spectral separability, whereas annealing at 650 °C produces renewed broadening and spectral overlap. Defect-independent QW anchors yield an effective best-matching value of CBiM = 1.49 eV within the adopted model. The value CBiM,SO = 1.44 eV is retained only as a regularized working value under the assumed D1/D2 defect model and the 0.05 eV proximity condition; the present data do not determine it independently. Within the heuristic ETDFF ± Γ/2 assignment window, the PR resonances are consistent with overlapping allowed and weak confined-state transitions, conditional Bi-related defect-assisted contributions, and GaAs/AlGaAs bulk- or barrier-related responses. The analysis shows that annealing modifies spectral visibility and broadening without eliminating residual interface- or defect-related contributions, and that linewidth-aware, physically constrained assignments are required for low-temperature-grown GaAsBi/AlGaAs quantum wells.

This study systematically investigates the orientation-dependent structural, optical, and vibrational responses of unintentionally doped and Sn-doped bulk β-Ga2O3 single crystals with (001) and orientations. Using angle-resolved X-ray diffraction (XRD), absorption spectroscopy, and polarized Raman spectroscopy, we characterize the interplay between crystallographic orientation and heavy donor incorporation. XRD results confirm phase-pure monoclinic structures, with (001) oriented wafers exhibiting superior structural perfection and lower mosaic spread than orientation. Absorption measurements reveal a doping-dependent blue shift (up to 53 meV) of the fundamental edge, consistent with the Burstein-Moss effect, alongside increased sub-gap absorption. Angle-resolved polarized Raman spectroscopy, analyzed through the effective Raman tensor formalism, identifies pronounced mode-dependent anisotropy governed by selection rules and birefringence. While (001) maintains stable angular fingerprints, orientation exhibits significant doping-dependent redistribution of the anisotropic response. Notably, a distinct Sn-related local vibrational mode at 257 cm-1 is resolved exclusively in oriented samples, providing experimental validation of preferential Sn substitution at Ga(II) octahedral sites. These findings demonstrate that the physical properties of β-Ga2O3 are fundamentally orientation-dependent, establishing a comprehensive framework for substrate selection in next-generation ultrawide-bandgap electronics.

We investigate the power loss per electron mechanism of hot electrons generated under electric and magnetic fields in n-type In_xGa_1-xAs_1-yBi_y epitaxial layers. Acoustic phonons are generated under various electric fields to determine the hot-electron energy relaxation mechanisms at low temperatures. The hot electron temperatures are determined by theoretical calculation of the amplitude of the magnetoresistance oscillation. The power loss per degenerate electron is analytically modeled with possible scattering mechanisms. The modeling of the experimental results reveals that power dissipation occurs by employing deformation potential energy scattering for all the samples. The deformation potential energy increases by ∼ 2.14 eV/Bi% when Bi atoms are introduced into ternary InGaAs alloy and the increase in the deformation potential energy is found to be independent of the electron density, which indicates that power dissipation occurs in the equipartition regime.

The temperature-dependent transport properties of n-type InGaAsBi epitaxial alloys with various doping densities are investigated by conducting magnetoresistance (MR) and Hall Effect (HE) measurements. The electronic band structure of the alloys and free electron distribution were calculated using Finite Element Method (FEM). Analysis of the oscillations in the transverse (Hall) resistivity shows that quasi-two-dimensional electron gas (Q-2D) in the bulk InGaAsBi epitaxial layer (three-dimensional, 3D) forms at the sample surface under magnetic field even though there is no formation of the spacial two-dimensional electron gas (2DEG) at the interface between InGaAs and InP:Fe interlayer. The formation of Q-2D in the 3D epitaxial layer was verified by temperature and magnetic field dependence of the resistivity and carrier concentration. Analysis of Shubnikov-de Haas (SdH) oscillations in longitudinal (sample) resistivity reveals that the electron effective mass in InGaAsBi alloys are not affect by Bi incorporation into host InGaAs alloys, which verifies the validity of the Valence Band Anti-Crossing (VBAC) model. The Hall mobility of the nondegenerate samples shows the conventional 3D characteristics while that of the samples is independence of temperature for degenerated samples. The scattering mechanism of the electrons at low temperature is in long-range interaction regime. In addition, the effects of electron density on the transport parameters such as the effective mass, and Fermi level are elucidated considering bandgap nonparabolicity and VBAC interaction in InGaAsBi alloys.