We investigated the relationship between Hall sensitivity, temperature stability, and frequency response of III-nitride two-dimensional electron gas-based Hall-effect sensors. For this study, we utilized three different heterostructure designs, each with varying percentages of "Al" content in the (Al)GaN barrier layer. The relationship among Hall sensitivity, temperature stability, and frequency response was investigated by varying the 2DEG carrier mobility, sheet density, and sheet resistance of the Hall device. The investigation demonstrated and explained a trade-off of Hall sensitivity of the order of 50% to obtain a sensor with a 16% reduction in sensitivity over the temperature range from room temperature to 377 degrees C with a frequency bandwidth of approximately 5.8 MHz. However, in terms of input voltage utilization, AlN/GaN retains its advantages, offering high sensitivity together with enhanced frequency response.

In this work, we present a study of the temperature- and frequency-dependent AC conductance in CVD-grown GeSn on Ge/Si substrates, complemented by nanoscale Kelvin probe force microscopy and scanning capacitance microscopy. The AC conductivity in GeSn films follows Jonscher's universal power law, sigma(omega)similar to omega(s), with 0.15 < s< 0.50 in the low-temperature region (85-130 K). The weak temperature dependence of s below similar to 110 K suggests the dominance of quantum mechanical tunneling, while its increase with temperature reflects the influence of other mechanisms, such as variable range hopping and/or phonon-assisted tunneling on the overall AC conductivity. We identify the dominant hopping-based AC transport mechanisms and correlate them with film morphology, localized defect states, and spatial variations in carrier concentration and surface potential. These results reveal the decisive role of disorder-induced localized states in low-temperature transport, providing critical insights for optimizing their performance in advanced optoelectronic devices.

Temperature-dependent charge transport in epitaxial GeSiSn films with varying thicknesses and Si and Sn content, grown on Ge/Si(001) substrates, was investigated using admittance spectroscopy. The conductance exhibits a crossover from Mott variable-range hopping at low temperatures to thermally activated conduction at higher temperatures. Below 150 K, carriers move via localized states in the band tails, with hopping parameters governed by the density and spatial distribution of disorder-induced states. Increasing nominal Sn concentration and thickness enhances structural disorder and Sn segregation, leading to a higher density of states and reduced hopping length. Scanning capacitance microscopy reveals variations in charge-carrier concentration in high-Sn films, indicating the presence of coexisting p-type and n-type regions at the microscale, consistent with compositional fluctuations, Sn segregation, and microstrain. These results demonstrate that transport in GeSiSn alloys is primarily dominated by disorder-assisted hopping at low temperatures, establishing a quantitative link between microscopic disorder and macroscopic electrical response in metastable group-IV semiconductors.

GaAsSb is a useful ternary semiconductor compound alloy and heterostructures made of GaSbxAs1-x facilitate versatile opto-electronic device applications. Thus, it is of significant importance to study carrier dynamics in GaAsSb and GaAsSb-containing heterostructures. This work investigated carrier localization mechanisms in GaAsSb and their influence on luminescence in both a single quantum well (QW) of 10 nm GaSb0.1As0.9 sandwiched between GaAs and a 250 nm bulk-like GaSb0.1As0.9 film on GaAs. Photoluminescence measurements reveal that for both samples the emission is impacted by exciton localization, however this is associated with different mechanisms in each sample. In the bulk film excitons become localized around Sb-composition fluctuations, while in the QW they localize along interface imperfections. Further spectral characterization demonstrates that both localized exciton (LE) and free exciton (FE) emission are indirect transitions in the QW. However, in bulk GaAsSb FE emission is direct, while LE emission is indirect. Therefore, the luminescence characteristics reflect different carrier dynamics that are correlated to the nature of the sample, i.e., depending on whether it is a QW or the bulk material.

A hybrid nanostructure is constructed with self-assembled GaSb/GaAs quantum dots (QDs) coupling to an In0.125Ga0.875As/GaAs quantum well (QW) through a 4.5 nm GaAs thin spacer. This quantum dot-well (QDW) hybrid structure retains the characteristics of a type II band alignment while enabling flexibility in engineering the hybrid energy levels by independently controlling the configuration of the QDs, the QW, and the spacer layer. Photoluminescence measurements prove that this structure has several advantages in comparison with a structure containing QDs only. These advantages include a reduction in the effective band gap and an enhancement of the carrier generation as well as collection efficiency due to carrier injection from the QW. Thus, it holds promise for photovoltaic and photodetector applications. However, there is also carrier localization and emission in the wetting layer (WL) of the QDs, which competes for carriers with the dots. Thus, the QDW system should be carefully optimized to reduce the carrier localization impact while it is applied to improve the ultimate performance of photovoltaic and photodetector devices.

Carrier dynamics, in particular, carrier localization and its impacts on carrier recombination and emission, were investigated via photoluminescence (PL) for an InGaAsSb/GaAs multiple quantum-well (MQW) sample. Composition segregation and interface diffusion were observed, suggesting formation of localized energy states (LESS) in this quaternary alloy heterostructure. Steady-state PL measurements at 10 K revealed additional emission from LESs alongside the quantum-well energy state (QES) emission. A pronounced blue-shift of approximate 100 meV, exhibiting a cubic-root law of dependence on excitation power, indicated a type-II band alignment and corresponding indirect transition recombination for carriers in both LESs and QESs. With increasing temperature, thermal activation and redistribution of carriers between LESs and QESs were observed, while the QESs gained carriers from the LESs and became the dominant transition pathway at elevated temperature. Time-resolved PL (TRPL) measurements unveiled transient spectral features of both LES and QES emissions, along with shifts in the ensemble MQW PL peak. Rise and decay times showed strong wavelength dependence. These transient behaviors are attributed to the type-II band alignment, band bending effects, and dynamic carrier transfer between QESs and LESs. The characteristics obtained from steady-state and transient luminescence provide valuable insights for understanding carrier dynamics in the InGaAsSb/GaAs MQW heterostructure.

Semiconductor quantum dots (QDs) offer a rich landscape for spin control and quantum light emission. While most studies have focused on type-I band alignment, the potential of type-II systems remains underexplored. Here, we report low-field optical polarization in type-II In(Ga)As/GaAsSb QDs, enabled by hyperfine-induced mixing between bright and dark excitons via level anticrossing under magnetic fields as low as 0.17 T. The weak-field regime arises from the suppressed wave function overlap, yielding a reduced electron-hole exchange interaction. A theoretical model based on the spin Hamiltonian and the spin-split state populations accurately captures the observed mirror-symmetric luminescence helicity, reproducing the experimental polarization response. Additionally, polarization recovery measurements confirm the role of nuclear spin interactions in mediating the in-plane electron spin precession. Our work demonstrates an alternative route for light polarization control using weak magnetic fields and nonresonant linear excitation, establishing type-II QDs as promising platforms for compact sources of circularly polarized light.

The spin orientation of optical emissions in GaAs1-xSbx/GaAs quantum wells (QWs) is investigated through magneto-photoluminescence measurements. Notably, spin-dependent intensity oscillations emerge in high magnetic fields, originating from interband transitions between Landau levels in the GaAs barriers and described by the magnetoabsorption effect. These spin-resolved absorption resonances, when coupled with coherent relaxation toward the ground-state optical transition in the type-II QWs lead to pronounced spin polarization peaks. A theoretical framework based on rate equations for spin-split states demonstrates that such polarization is only achieved when spin-flip times are comparable to or exceed the optical recombination time, ensuring that spin orientation is maintained from carrier excitation to optical recombination. This prediction is experimentally validated though time-resolved measurements with circularly polarized excitation, where a spin lifetime of 3.5 ns is determined, confirming the role of magnetoabsorption in sustaining spin coherence.

In this study, the breaking of time reversal symmetry in monolayers of ultrathin films was probed using the z-scan technique. Graphene and nickel ultrathin films forming monolayered system were fabricated using electron beam evaporation on glass and silicon substrates. The nonlinear optical behavior of the fabricated films was characterized by using the z-scan apparatus pumped with linearly polarized beam at 650 nm. The interactions between the monolayers and linearly polarized light in two configurations (horizontal and vertical polarizations) were traced using the output of the open aperture scans. The results showed different responses related to changes in the absorption-emission of light in ultrathin films. The interplay between these processes marked a difference in time reversal symmetry of nonlinear optical response of the monolayers in the films.

The optical properties of a heterostructure containing GaSb/GaAs quantum dots (QDs) have been systematically investigated via photoluminescence (PL) measurements to gain insights into carrier dynamics. The QD and wetting layer (WL) emissions exhibit a complementary dependence on the excitation intensity and temperature, reflecting the interplay between carrier localization in the WL and carrier relaxation from the WL to the QDs. Carrier dynamics related to localization, injection, and recombination are further validated by time-resolved photoluminescence (TRPL). These findings highlight the necessity of carefully optimizing GaSb/GaAs QD structures to mitigate the impact of carrier localization, thereby enhancing the ultimate performance of devices utilizing these QDs as active region materials.

This study investigates the manipulation of the electronic properties of GeSn alloys at the nanoscale by applying electric fields between an atomic force microscope (AFM) tip and the surface of GeSn films grown on Ge/Si substrates. Local changes in work function (WF) and resistivity were observed by Kelvin Probe Force Microscopy and Scanning Spread Resistance Microscopy and were associated with an increased Sn content in the near-surface region within patterns produced by the AFM tip. The effect is explained by the poor stability of GeSn layers, making possible the diffusion of Sn toward the surface, driven by high electric fields near the AFM tip under combined alternating current and direct current biases. The modified regions of GeSn films with increased Sn content exhibit a significant increase in local conductivity and a lower WF, potentially providing a suitable platform for nanoscale electronic devices based on group-IV materials.

InGaAs/GaAs surface quantum dot (SQD) heterostructures have long been viewed as having great potential for realizing environmental gas detection devices. The research has recently been steered into discovering ways to inject carriers into the available SQD energy states to facilitate readable devices. In the present research, carrier injection from buried QDs (BQDs) to SQDs was controlled by changing the interlayer coupling. Photoluminescence (PL) measurements show that, by increasing the stacking period of BQDs, the carrier collection efficiency and subsequently the luminescence intensity for SQDs can be effectively improved. A rate equation simulation of the carrier recombination process is used to quantitatively describe the luminescent quantum efficiency (IQE) for the SQDs, indicating an increase with additional BQD layers due to an increase in carrier injection. The PL spectra along with the rate equation simulations further indicated that for these SQD hybrid structures nonradiative recombination dominates, with Auger recombination becoming significant at high excitation intensities. These results may help to understand the carrier dynamics in coupled SQDs hybrid structures. In addition, they provide a path to manipulating the properties of InGaAs SQDs for the development of gas sensors.

In this study, the preparation of graphene-nickel composite nanofilms and their linear and linear optical properties under cw regime were investigated. The films were fabricated on glass and c-Si substrates in two steps by electron beam evaporation. The surface morphology of the films showed distinctive features with smooth and rough regions related to the formation of pristine and wrinkled graphene. The complex permittivity functions of the films were calculated using Maxwell-Garnett Theory. The nonlinear optical absorption coefficient and refractive index were calculated based on the open and closed aperture z-scan measurements. Enhanced nonlinear optical properties compared to the pure nickel film were obtained. The enhancement is explained in terms of the participation of the graphene's pi-electrons in the d-band plasmonic oscillation.

In the manufacture of semiconductor devices, cracking of heterostructures has been recognized as a major obstacle for their post-growth processing. In this work, we explore cracked GaN/AlN multi-quantum wells (MQWs) to study the influence of pressure on the recombination energy of the photoluminescence (PL) from the polar GaN QWs. We grow GaN/AlN MQWs on a GaN(0001)/sapphire template, which provides 2.4% tensile strain for epitaxial AlN. This strain relaxes through the generation and propagation of cracks, resulting in a final inhomogeneous distribution of stress throughout the film. The crack-induced strain variation investigated by micro-Raman spectroscopy and X-ray diffraction mapping revealed a correlation between the spacing of the cracks and the amount of strain between them. We have developed a 2D model that allows us to calculate the spatial variation of the in-plane strain in the GaN and AlN layers. The measured values of compressive in-plane strain in the GaN QWs vary from -0.4 % away from cracks, to -0.7 % near cracks. PL from the GaN QWs exhibits a clear correlation to the varying strain resulting in an energy shift of - 140 meV. As a result, we can experimentally calculate a pressure coefficient of PL energy of - -60.4 meV/GPa for the - 7 nm thick polar GaN QWs. This agrees well with the previously predicted theoretical results by Kaminska et al. in 2016 [DOI: 10.1063/ 1.4962282], which were demonstrated to break down for such wide QWs. We will discuss this difference with respect to the reduction in both the expected point defects and extended defects resulting from not doping and growth on a GaN template, respectively. As a result, our work indicates that cracks can be utilized for investigating some fundamental material properties related to strain effects.

A terahertz time domain pulsed spectroscopy system is modified to provide fully polarimetric radiation and analysis. The operation of this polarimetry system is characterized using a birefringent, x-cut quartz crystal. The modification is based on rotating the photoconductive antennas such that both the emitted and detected polarizations are out of the plane of incidence. Subsequently, broadband wire grid polarizers are used to select the incident and detected direction of linear polarization to be either parallel with (vertical) or perpendicular to (horizontal) the plane of incidence with the sample surface. The experiments are conducted in both transmission and reflection. Depending on the frequency, the phase retardation of the incoming electric field components along the two perpendicular optical axes of the quartz crystal changes differently. This results in the polarization of the light exiting the crystal changing with frequency. As a result, multiple frequencies are identified where the crystal behaves as a near ideal quarter-, half-, or full-wave retarder. Additionally, due to the time-domain nature of the experiment, transmitted and reflected electric fields are measured after multiple consecutive reflections within the crystal. This leads to a further, complex control over the final polarization state of the signal. Finally, images of a resolution standard are obtained demonstrating the characteristics of the polarimetry system.

This paper describes an optical isolation solution for integrating into power modules operating at temperatures of 200°C. The suggested method uses a high-temperature base material created from low-temperature co-fired ceramic (LTCC) and Peek carbon to create an optically isolated gate driver system that will work in high-temperature environments. The suggested method has improvised isolation techniques with higher bandwidth and reduces the propagation delay of the gate driver system in the integrated power modules. The proposed system was integrated into a SiC half-bridge module and its switching performance evaluated, which resulted in a turn-on time of 81.2 ns and a turn-off time of 54.4 ns.

This research provides a flexible approach to manipulate formation of InGaAs nanostructures on the GaAs (100) surface by varying arsenic (As-4) beam equivalent pressure (BEP). By selecting the As-4/(In+Ga) BEP ratio to be 4, 8, 20, 50 and 100, we were able to obtain different quantum structures from quantum well (QW) to quantum dots (QDs), then to spatially ordered quantum dot chains (QD-chains), and finally to quantum wires (QWRs), respectively. This transformation of nanostructures was explained by anisotropic surface diffusion coupled with the strain relieving Stranski-Krastanov growth mode, while the anisotropy was modulated by increasing As-4 flux and subsequently enhanced by multilayer-stacking growth with a suitable spacer thickness. Photoluminescence characteristics show correlation to the nanostructure morphology for each sample. In particular, the formation of QD-chains and QWRs results in anisotropic features that offer potential device applications.

This work exploits carrier injection hybrid structures in which carriers are injected into a layer of In0.4Ga0.6As surface quantum dots (SQDs) from an adjacent In0.15Ga0.85As quantum well (QW) as a function of spacer thickness from 10 nm down to 2.5 nm. Photoluminescence (PL) measurements verify that all such hybrid structures indeed have carriers collected into the QW and subsequently obtain an enhancement for PL intensity over that of the reference SQDs. The hybrid structure with the 2.5 nm spacer obtains the best carrier injection efficiency, due to the strongest coupling between the QW and the SQDs, while a thicker spacer results in less carrier injection from decreased quantum tunneling. However, the carrier injection is less efficient than expected. This is due to the fact that the QW confined energy states line up with the broad wetting layer (WL) energy states of SQDs of our test samples, leading to resonant carrier tunneling from the QW to the WL. Thus, there is significant carrier loss through tunneling into the WL of SQDs and then to surface states via nonradiative recombination. This characteristic must be considered in the design of surface sensitive detection devices using SQD injection structures.

This paper investigates polarimetric terahertz time domain transmission imaging aiming at enhancing the detection of different regions on the same sample. Here, x-cut quartz crystal adjacent to quartz glass are utilized to compose a sample that hypothetically mimics breast tumor of cancerous and healthy regions, respectively. The obtained images establish a potential advantage of polarimetric terahertz imaging for future use in imaging excised breast cancer tumors.

Ohmic contacts to polarization doped, compositionally graded indium gallium nitride (InGaN) films have been studied. The strong spontaneous and piezoelectric polarizations exhibited by the III-nitride materials result in effective doping in the graded layers without the use of impurity dopants. Two samples were grown by grading the In composition from-5% to 21.5% or 28% over 90 nm on top of GaN by molecular beam epitaxy, resulting in carrier concentrations of 2.13 x 1018 cm-3 and 2.87 x 1018 cm-3, respectively. Both films of Ni and Cr metals were tested for contact quality. Transmission line measurements demonstrated a minimum specific contact resistance of 3.19 x 10-4 0 cm2 resulting from room temperature deposition of Cr. The methods and materials characterization will be discussed.