Semiconductor Transition Research Articles

Influence of a Thin V2O3 Spacer on Interlayer Interactions in Fe-Ni/V2O3/FeNi Film Structures

In this paper, we explore the suggestions of the results of experimental studies on low-dimensional layered systems in FeNi/V2O3/FeNi film structures. The multifunctional material V2O3 is used as an interlayer between the magnetically active FeNi layers. The films were obtained by ultrahigh vacuum magnetron sputtering on a glass substrate with a base size of 10−10 Torr. It has been found that for V2O3 films, the decrease in the metal–semiconductor transition temperature increases significantly. Magnetic characteristics were studied on the MPMS-XL SQUID magnetometer. The exchange effect occurs both in the region where the oxide has a magnetic order and in the paramagnetic region. The latter is due to the effect of the magnetic panel in the oxide package. The phenomenon of oscillation of the exchange field occurs depending on the phenomenon of intermediate observation observed experimentally.

Out-of-Band Fibre-Optic Time and Frequency Transfer Using Asymmetric and Symmetric Opto-Electronic Repeaters.

Signal repeaters for fibre-optic communication can be realized with back-to-back connected transceivers. This configuration can provide high gain (≈30 dB) at low cost, and the needed semiconductor lasers and modulators can be realized for practically any relevant wavelength. Unfortunately, for time and frequency transfer the uncorrelated wavelength drifts in the transceiver lasers can compromise transfer stability, and device replacement due to failure may result in large time offsets that have to be measured via global navigation satellite services. This work demonstrates that good results can nevertheless be obtained with standard telecom DWDM tranceivers over long time periods (years). More importantly, a simple wavelength-symmetric repeater is proposed that can be used to cancel the detrimental effects of wavelength drifts and which lessens the need for link re-calibrations after transceiver replacements. In a proof-of-concept test setup, timing drift due to wavelength drift of a repeater laser is reduced by approximately two orders of magnitude.

Strain engineering of electronic properties, quantum capacitance, and photocatalytic properties of Zr2CO2 MXene

The electronic, optical, and photocatalytic properties of Zr2CO2 MXene under biaxial strain are investigated by hybrid HSE06 functional. The quantum capacitance is also studied. The tensile strain limit of Zr2CO2 is about 17%. Zr2CO2 under +8% strain undergoes the indirect-direct semiconductor transition, which is due to blueshift of Zr-d and O-p states. Absorption coefficient is more sensitive to the tensile strain, and tensile strain can effectively improve the solar energy utilization of Zr2CO2 MXene. Zr2CO2 under strains are all potential cathode materials in aqueous and ionic/organic systems. Zr2CO2 under +2% and +3% strains can perform photocatalytic water splitting in acid environment. Tensile strain can improve the CO2 photocatalysts and enrich the photocatalytic CO2 reduction products. The N2 reducing ability of Zr2CO2 is enhanced under tensile strain, but weakened under compressive strain. Zr2CO2 under strain cannot reduce N2 to N2H and N2H2 at pH=0 and 7. The hydrogen evolution reaction and oxygen evolution reaction are further evaluated.

Understanding the growth mechanisms of ultrasmall silver selenide quantum dots for short-wave infrared detectors

Colloidal quantum dots (CQDs) allow for wavelength-tunable, economic, and elegant access to the infrared (IR) spectrum by utilizing inter- and intraband excitonic transitions in narrow bandgap semiconductors. With the emergence of new applications for IR detection, developing competitive non-toxic and environmentally benign alternatives to heavy metal-containing semiconductors is of increasing importance. Over the last decade, silver selenide (Ag2Se) has emerged as an alternative to lead- and mercury-based QDs, especially with large, intraband Ag2Se CQDs showing promising performance in the mid-wave IR. Herein we explored the critical nucleation and growth mechanisms of ultrasmall (2.5–3.5 nm) interband absorbing Ag2Se CQDs operating in the near to short-wave IR. Classical nucleation and growth was observed at most reaction conditions, with a growth temperature of 140 °C and a Ag:Se precursor molar ratio of 1:2 providing the most robust control over the IR absorption spectrum. We proceeded to perform an in-depth study of the impact of surface ligand chemistry of CQD thin-films on resulting optoelectronic properties. By tuning the functional groups and optimizing ligand exchange parameters, we obtained IR responsivity values of ∼25 mA/W in photoconductor devices and ∼1.8 mA/W in photodiode devices in the range 800 – 1250 nm.

Inverse design of intermediate band solar cell via a joint drift-diffusion simulator and deep reinforcement learning scheme

In this work, we developed an efficient inverse design approach for optimal intermediate band solar cells (IBSC) device design given a target performance by using a joint drift-diffusion simulator and deep reinforcement learning scheme. The drift-diffusion simulator for IBSC simulation was constructed by using the semiconductor module and wave optics module of COMSOL MultiPhysics®. The deep deterministic policy gradient (DDPG) algorithm was chosen as the learning algorithm to optimize the specified device structure. A GaAs quantum well-embedded IBSC was used as the test candidate to verify the performance of the DDPG-based inverse design approach. A maximum efficiency of was reached for the device with optimal structure parameters searched by the DDPG agent, which exceeds the target efficiency of 30%. The subsequent optical analysis revealed that the electric field enhancement due to light absorption at the IB region with a wavelength between 450 nm and 600 nm is mainly contributing to the significantly increased short-circuit current for the optimized device. Meanwhile, a parameters correlation with target conversion efficiency evaluated by topological data analysis successfully identified all the positive and negative parameters with respect to the target parameter, indicating the physical soundness of the optimized structure parameters. Our work presented here demonstrates that a well-trained AI agent can fulfill the target efficiency by searching the optimal parameters for solar cell devices. The AI-based inverse design approach shows promising potential to serve as an efficient device design tool by greatly reducing the number of trial-and-error experiment demonstrations and replacing laborious human-guided device design workload.

Reliability of SiC MOSFET Power Modules under Consecutive H3TRB and Power Cycling Stress

Power semiconductor modules are subject to both thermo-mechanical stress and electro-chemical stress during field operation. Usually, those stressors are investigated separately and possible interaction of both degradation mechanisms is neglected. In this work, the effect of combined thermo-mechanical and electro-chemical stress is investigated by means of consecutive H3TRB and PCT testing. One test group had been subjected to power cycling before the H3TRB test was performed, while another test group had been exposed to H3TRB stress before the power cycling test. As a reference, devices without preconditioning were tested in both, H3TRB and power cycling and are also used to compare the H3TRB and power cycling performance of the SiC devices to similar silicon devices. The results show, that the SiC devices feature a significantly better H3TRB performance than comparable silicon devices, but are inferior in terms of power cycling performance. Furthermore, the results for both test groups of the combined tests indicate that the failure modes for the previously stressed devices were the same as for the pristine devices and no impact of either preceding stress on the devices' lifetime could be observed. Therefore, the results of this work suggest no interaction between both stressors, at least not for the devices used for this investigation.

Andreev reflection between aluminum and graphene across van der Waals barriers

We present planar aluminum superconductor–graphene junctions whose hybrid interface is engineered for couplings ranging from tunneling to the strongly coupled regime by employing an atomically thin van der Waals tunneling barrier. Without the vdW barrier, we find Al makes strongly coupled contacts with the fully proximities graphene channel underneath. Using a large band gap hexagonal boron nitride (hBN) barrier, we find the junctions always remain in the weak coupling regime, exhibiting tunneling characteristics. Using monolayer semi-conducting transition metal dichalcogenides (TMDs) such as MoS2, we realize intermediate coupling with enhanced junction conductance due to the Andreev process. In this intermediate regime, we find that junction resistance changes in discrete steps when sweeping a perpendicular magnetic field. The period of the resistance steps in the magnetic field is inversely proportional to the junction area, suggesting the physical origin of our observations is due to magnetic-field-induced vortex formation in the planar junction.

Synergistic Effect of TiS3 and Ti3C2Tx MXene for Temperature-Tunable p-/n-Type Gas Sensing

We propose a strategy for highly tunable gas sensors, in which a MXene is combined with a sacrificial material that could be controllably oxidized upon mild annealing to form oxide nanoparticles that alter the sensing response. A controlled annealing of such composite generally retains the integrity of MXene sheets while gradually converting the sacrificial material to a metal oxide that could form semiconductor heterojunctions with MXene, fine-tuning its sensor properties. This strategy is demonstrated using gas sensors based on Ti3C2Tx MXene mixed with TiS3, a semiconducting transition metal trichalcogenide. Compared to pristine MXene, the Ti3C2Tx-TiS3 composite exhibited a significantly improved sensor response to ethanol, which served as a model analyte, both at room temperature and upon annealing. Furthermore, as a less thermally stable material than the MXene, TiS3 oxidizes faster than Ti3C2Tx at elevated temperatures, producing TiO2 nanoparticles that strongly affect the sensing response. A pristine Ti3C2Tx exhibits a p-type sensor response to ethanol at room temperature. Upon annealing, Ti3C2Tx gradually degrades to TiO2, changing the sensor response to n-type above ∼300 °C. The addition of TiS3 allows for the general preservation of MXene as the sensor material, as the temperature of the p–n transition decreases to about 200 °C, at which Ti3C2Tx is generally stable. This approach can likely be applied to a great variety of combinations of various MXenes and sacrificial compounds with sensor properties that could be tuned via annealing for specific analytes or applications.

Magnetic field control of light-induced spin accumulation in monolayer MoSe2

Semiconductor transition metal dichalcogenides (TMDs) have equivalent dynamics for their two spin/valley species. This arises from their energy-degenerated spin states, connected via time-reversal symmetry. When an out-of-plane magnetic field is applied, time-reversal symmetry is broken and the energies of the spin-polarized bands shift, resulting in different bandgaps and dynamics in the K and K valleys. Here, we use time-resolved Kerr rotation to study the magnetic field dependence of the spin dynamics in monolayer MoSe2. We show that the magnetic field can control the light-induced spin accumulation of the two valley states, with a small effect on the recombination lifetimes. We unveil that the magnetic field-dependent spin accumulation is in agreement with hole spin dynamics at the longer timescales, indicating that the electron spins have faster relaxation rates. We propose a rate equation model that suggests that lifting the energy-degeneracy of the valleys induces an ultrafast spin-flip toward the stabilization of the valley with the higher valence band energy. Our results provide an experimental insight into the ultrafast charge and spin dynamics in TMDs and a way to control it, which will be useful for the development of new spintronic and valleytronic applications.

Anderson localization in doped semiconductors

We theoretically consider the problem of doping induced insulator to metal transition in bulk semiconductors by obtaining the transition density as a function of compensation, assuming that the transition is an Anderson localization transition controlled by the Ioffe-Regel-Mott (IRM) criterion. We calculate the mean free path, on the highly doped metallic side, arising from carrier scattering by the ionized dopants, which we model as quenched random charged impurities. The Coulomb disorder of the charged dopants is screened by the carriers themselves, leading to an integral equation for localization, defined by the density-dependent mean free path being equal to the inverse of the Fermi wave number, as dictated by the IRM criterion. Solving this integral equation approximately analytically and exactly numerically, we provide detailed results for the localization critical density for the doping induced metal-insulator transition.

Insight into tunable electronic and magnetic properties in 2D ferromagnetic/antiferromagnetic van der Waals heterostructure

Two dimensional (2D) van der Waals (vdW) heterostructures have recently emerged as attractive candidates in spintronic devices. Here, a field-effect spin filter based on the 2D antiferromagnetic (AFM)/ferromagnetic (FM) vdW heterostructure, MnPS3/MnCl3, is proposed. The ground state of the MnPS3/MnCl3 heterostructure is predicted to be a type-I AFM semiconductor with AFM Néel ordering in MnPS3 and MnCl3 monolayers. Interestingly, an AFM–FM transition is found for the MnPS3/MnCl3 heterostructure when reducing the interlayer distance to the range of d < 2.0 Å. Moreover, the MnPS3/MnCl3 heterostructure undergoes semiconductor, metal, and half-metal transitions with the interlayer distance decreasing. Moreover, a semiconductor–half-metal transition can also be observed for MnPS3/MnCl3 by applying an external electric field. Our study offers a potential way to design spintronic devices by 2D layered magnetic vdW heterostructure.

Electronic origin of half-metal to semiconductor transition and colossal magnetoresistance in spinel HgCr2Se4

Half-metals are ferromagnets hosting spin-polarized conducting carriers and crucial for spintronics applications. The chromium spinel HgCr2Se4 represents a unique type of half-metal, which features a half-metal to semiconductor transition (HMST) and exhibits colossal magnetoresistance (CMR) across the ferromagnetic-paramagnetic (FM-PM) transition. Using angle-resolved photoemission spectroscopy (ARPES), we find that the Fermi surface of n-type HgCr2Se4 (n-HgCr2Se4) consists of a single electron pocket which moves above the Fermi level (EF) upon the FM-PM transition, leading to the HMST. Such a Lifshitz transition manifests a giant band splitting which originates from the exchange interaction unveiled with a specific chemical nonstoichiometry. The exchange band splitting and the chemical nonstoichiometry are two key ingredients to the HMST and CMR, consistent with our ab-initio calculation. Our findings provide spectroscopic evidences of the electronic origin of the anomalous properties of HgCr2Se4, which address the unique phase transition in half-metals.

Warpage modeling for semiconductor power module manufacturing flow improvement

The hybrid and battery electric vehicle market is heavily increasing the demand for semiconductor power modules. The manufacturing of such products is more complex than discrete devices. Some parameters, such as module deformation (warpage), play a major role. Power modules have to comply with customer Geometric Dimensioning & Tolerancing (GD&T) targets in order to avoid coolant leakage during operation and to guarantee mechanical compatibility with the driving board. Compliance with an appropriate flatness tolerance reduces rejects during manufacturing processes such as ultrasonic welding, and the consequent impact on reliability. Due to these requirements, there is a need to implement dedicated countermeasures during manufacturing flows and to develop the proper methodologies to monitor flatness tolerances. Appropriate component selection at the design stage also limits flatness through the prediction of deformations during assembly flows using finite element model. This activity outlines the manufacturing flow for a direct cooled semiconductor power module and presents a method for product optimization in terms of flatness tolerance using a dedicated finite element model that calculates the warpage deformation induced by baseplate soldering and different ceramic substrate layouts. Furthermore, we describe experimental methodologies for measuring power module flatness and straightness across the manufacturing phases. Dedicated experiments were conducted for methodological and design validation.

On the fractional perturbation theory and optical transitions in bulk semiconductors: Emergence of negative damping and variable charged mass

This study is devoted to the construction of a fractional perturbation theory which represents the basic technique of approximation in quantum mechanics. Our analysis is based on the fractional actionlike variational approach (FALVA) implementation of fractionality. After discussing the basic properties of fractional harmonic oscillator through the fractional Feynman's path integral approach, we have discussed the Rabi oscillations within the context of two-level quantum systems for simulated optical transition. In particular the case α>1 is correlated to the emergence of negative damping and negative probabilities which have been obtained in several quantum out-of-equilibrium systems. The fractional Fermi Golden rule and the fractional Heisenberg's uncertainty principle have been also obtained and their main properties and features have been discussed accordingly. We have discussed optical transitions in bulk semiconductors. It was observed that the fundamental properties of transitions rates are recovered if the charged particle moving inside the semiconductor is variable with time.

Ultrafast Control of the Optical Transition in Type-II Colloidal Quantum Wells

Manipulating the optical transition in semiconductors at ultrashort timescales is of both fundamental interest and central importance for emerging photonic applications. Traditionally, this manipulation is realized by electrostatic gating via Stark effects or band-gap renormalizations. Here, we report an ultrafast and all-optical route to engineer an indirect transition in core–crown colloidal quantum wells (CQWs), namely, CdSe/CdTe, with a type-II band alignment. Following the intense laser pulse excitation, the indirect band transition energy exhibits a pronounced blueshift–redshift crossover on the picosecond timescale, stemming from the formation and dissipation of the transient electric field (E-field) that forms upon photoexcitation to compensate for the driving force provided by the band offsets. Both the energy shift and dynamics of the transient E-field can be modulated optically by tuning the laser pulse excitation fluence. Our finding demonstrates a strong analogy between the type-II heterojunction and a p–n junction with respect to carrier equilibrium processes, which holds promise to facilitate the integration of CQWs within optical switching networks.

Fermi velocity and effective mass of quasiparticles in bilayer phosphorene nanoribbons

Based on the Green’s function approach in combination with the tight-binding approximation, we have investigated the electronic properties of zigzag bilayer phosphorene nanoribbons (ZBLPNRs). A complete and fully reversible metal-to-semiconductor or insulator phase transition has been observed via tuning a perpendicular electric field. We explain the effect of the interlayer hopping parameter on the electronic characterizations, namely, the energy band-gap, Fermi velocity, and effective mass of carriers in biased ZBLPNRs. In the presence of the interlayer hopping term, the band structure of a ZBLPNR in the vicinity of the Fermi level has the form of two tilted Dirac cones, while, in the absence of it, the created cones are not tilted. Besides, the bands above and below the Fermi level are not symmetric, i.e., there is no electron–hole symmetry for both limits of the interlayer coupling. A remarkable tuning of both the Fermi velocity and the reciprocal effective mass can be observed by adjusting the external bias voltage. At certain critical bias voltages, the band edge reciprocal masses and carrier velocities become zero, i.e., the electrons exhibit localization behaviour. This phenomenon is indeed a transition from massive to massless Dirac fermions and vice versa. The possibility of simultaneous control of the thermal and magnetic properties using an electric agent, which can be realized experimentally by using a perpendicular potential difference, opens up possibilities for low-power-consuming thermomagnetic devices based on ZBLPNRs.

Strained induced metallic to semiconductor transitions in 2D Ruddlesden Popper perovskites: A GGA + SOC approach

Based on RP perovskites, we put 2%, 4%, 6%, and 8% biaxial strains on 2D monolayers of phenyl-ammonium tin iodide (PH2SnI4) perovskites. Density functional theory (DFT) is used to study the structural distortion, octahedral tilting, band gap tuning, Bader charge analysis, and mechanical stability of the resulting configurations. Band gaps under the effects of compressive strain (Eg1), zero strain (Eg2), and tensile strain (Eg3) follow a decreasing order of Eg1(1.262 eV) < Eg2(1.329 eV) < Eg3(1.331 eV) for 6%-strain. The absence of trap states in energy band gaps and dominant cation and anion contributions at conduction and valence band edges supports the defect tolerance. All the structures analyzed are mechanically stable. The covalently bonded structures are ductile/brittle under tensile/compressive strain, with a prominent stretching mode. Strain improves the optical performance with dielectric constant increased (red-shifted) under compressive strain and reduced (blue-shifted) under compressive strain.

Towards a comprehensive theory of metal–insulator transitions in doped semiconductors

A review is given on the theory of metal–insulator transitions (MIT) in doped semiconductors. We focus in particular on reviewing theories of their anomalous magnetic properties, which emerge from the interplay of spin and charge correlations and disorder. Building on the review of these existing theories and experiments, we suggest that the finite temperature phase diagram can be structured into 1. a quantum spin liquid phase at subcritical doping and low temperature, as described by the Bhatt–Lee theory of random spin clusters, mostly random singlets. 2. a critical non-Fermi-liquid fan, originating at the MIT, which is dominated by random Kondo singlets with a universal tail of the distribution of their binding energies. This is caused by multifractality and results in an anomalous power law divergence of the magnetic susceptibility with a universal power and 3. a supercritical, low temperature phase. Rare events caused by the random placement of dopants do not allow to define strict phase boundaries. Remaining open problems are reviewed and outlined. Finally, the possibility of finite temperature delocalization transitions is reviewed, which are caused by the correlation induced temperature dependence of the spin scattering rate from magnetic moments. This review article is devoted to the memory of Konstantin B. Efetov.

Theoretical Study on the Electronic Properties of Two-Dimensional Covalent Triazine Frameworks/As van der Waals Heterostructures

The manuscript substantiates the structural and electronic properties of covalent triazine frameworks (CTF)/As van der Waals heterostructures (vdWh) employing the standard first-principles calculation method. The numerical results designate that the CTF/As vdWh has robust crystal structures, a type-II band alignment (BA), and an indirect bandgap of 1.44 eV. The calculated results demonstrate that the strain could lead to interesting indirect-direct semiconductor transitions, while the external electric field could give rise to type-II to type-I BA and semiconductor-metal transitions. The underlined outcomes present the workability of CTF/As vdWhs in unprecedented high-performance optoelectronic equipment.

Ultrashort pulse generation in erbium-doped fiber lasers in South America: a historical review

This paper reviews the scientific contributions of the last 30 years on erbium-doped fiber laser ultrashort pulse generation in South America. The focus is on contributions authored in South American institutions. It reveals a wide variety of laser cavities, ranging in length from a few centimeters to tens of kilometers, and optical pulses, from tens of femtoseconds to nanoseconds, with fully controlled average and peak powers. Furthermore, different approaches for ultrashort pulse generation using active (synchronous and asynchronous) and/or passive mode-locking techniques are discussed, as well as the employment of nonlinear optical phenomena (Kerr effect) and several two-dimensional nanomaterials as saturable absorbers, such as carbon nanotubes, graphene, graphite, graphene oxide, reduced-graphene oxide, semiconductor transition metal dichalcogenides, and black phosphorus. Finally, the paper revisits and discusses several advances, important milestones, and records on ultrashort pulse generation in erbium-doped fiber lasers, with many essential outcomes attesting to the high quality of research carried out in South America in the field of ultrafast fiber lasers.