Variation Of Carrier Density Research Articles

The picosecond laser ablation mechanism of monocrystalline silicon by coupling two-temperature model (TTM)-Molecular dynamic (MD)

Monocrystalline silicon is the most widely used material in microchips, microsensors and photovoltaic systems, where ultrashort pulse laser has been applied in grooving and holing. This paper investigates the ablation mechanism of monocrystalline silicon by picosecond laser, where a TTM-MD (Two-temperature model-Molecular dynamics) model was established based on the interaction theory between ultrashort pulse laser and Si. The effects of plasma shielding and surface reflectivity on the laser irradiation energy are investigated to analyze the variation of carrier temperature, lattice temperature and carrier density on the monocrystalline silicon surface. The occurrence of material melting, bubble nucleation and phase explosion are simulated to explore the material removal mechanism and the effect of laser fluence on the ablation behavior. The simulation model was finally verified by the ablation threshold experiments where a high-speed camera was applied to monitor the ablation process.

Theoretical study of susceptibility and effective mass in semiconductor quantum well-wires

The susceptibility and effective mass of electrons and hole vary as functions of carrier density and temperature in parabolic lateral barrier quantum-well wires (QWWRs). To investigate these properties, we utilize the leading-order dynamically screened Coulomb interaction, wherein the electron self-energy is calculated within the plasmon-pole approximation (PPA). All findings for GaAs QWWRs reveal a substantial difference between susceptibility and effective mass at low and high temperatures for varying carrier densities. These results contribute to illuminate of how Coulomb interactions and exchange–correlation energies influence the susceptibility and effective mass of GaAs semiconductors across a broad range of temperature and carrier density conditions.

Graphene on SiO2/Si and Al2O3 under thermal annealing and electric current: Competition of dopant desorption and conformation to substrate

In this work, we study phonon and electronic properties of graphene on SiO2/Si and Al2O3 by simultaneous Raman and electrical measurements in the temperature range from room temperature to 550 °C, or at voltages from 20 to −20 V. The dependencies of G and 2D peak parameters and electrical resistance on temperature and voltage made it possible to observe in situ a competition between the p-type adsorbate removal from graphene surface and substrate-induced doping due to graphene-substrate conformality increase, both stimulated by either ambient or Joule heating. The analyzed parameters were dominated by the conformality increase, with the hole density increasing significantly and unidirectionally, while resistance and I-V curves fluctuated due to the competition. Having calculated Raman peak shift temperature coefficients, resistance temperature coefficients, total variations of carrier density, resistance and strain, we show that Al2O3 substrate can be used to reduce the desorption barrier, the overall doping, the impact on graphene resistance and on phonon anharmonicity – however, it should be used with regard to the possibility of introducing strain or stronger doping after longer treatments. The conformality effects should be taken into account when performing annealing, as well as graphene applications for sensors or strong electric currents.

Experimental and theoretical studies into longitudinal spatial hole burning as a power limit in high-power diode lasers at 975 nm

Spatial-hole-burning as a limit to the continuous-wave (CW) output power of GaAs-based diode lasers is experimentally studied. For 90 μm stripe lasers with 6 mm resonator length and 0.8% front facet reflectivity, spontaneous emission (SE) intensity data show that the carrier density in the device center rises rapidly at the rear facet with bias and falls at the front, consistent with simulation. At the front, the carrier density at the edge of the laser stripe also rises rapidly with bias (lateral carrier accumulation, LCA), consistent with previous observations of increased local current flow. Devices with 20% front facet reflectivity for a flat longitudinal optical field profile show smaller variation in the local carrier density. Weak variation is seen in the carrier density outside the stripe; hence, current spreading is not a power limit. SE wavelength data show higher temperatures at the front with a twofold higher increase in temperature for 0.8% than for 20% front facet. The increased front temperature likely triggers lateral spatial-hole-burning and LCA in this region, limiting power. Finally, pulsed threshold current is more strongly temperature dependent for devices with 0.8% than 20% front facets, attributed to the higher rear facet carrier density. The temperature dependence of slope in pulsed is comparable for both devices at low bias but is more rapid for 0.8% at 20 A, likely due to non-clamping at the back. The temperature dependence of slope for CW is strong with 0.8% facets, likely due to the high temperature and LCA at the front but reduced for 20% facets.

Carrier‐Modulated Anomalous Electron Transport in Electrolyte‐Gated SrTiO3

The anomalous transport phenomena and underlying mechanism of SrTiO3 have been long term concerned, including the Kondo physics and anomalous Hall effects to be discussed herein. Nevertheless, a clarification of the physical origins for these phenomena and their intertransitions upon varying carrier density remain to be an issue, due to the limited modulation of carrier density. Herein, the intriguing transport behaviors of electrolyte‐gated SrTiO3 in the field‐effect transistor geometry are investigated, which allows a relatively wide carrier density window. It is revealed that the remarkable Kondo‐like resistance upturn, identified at relatively low carrier density, becomes gradually disappeared and replaced by the emerging nonlinear Hall resistance with increasing gating voltage, indicating the interesting gating‐controlled transport behaviors. The detailed discussions on the underlying physics suggest that the gating‐controlled transport can be well described by the multiband transport model taking into account magnetic scattering in the channel layer, a comprehensive scenario for the emergent transport effects in carrier‐doped SrTiO3. The present work provides a promising platform for exploring novel quantum materials phenomena in SrTiO3 and analogous systems.

Extraction of Mobility from Quantum Transport Calculations of Type-II Superlattices

Type-II superlattices (T2SLs) are being investigated as an alternative to traditional bulk materials in infrared photodetectors due to predicted fundamental advantages. Subject to significant quantum effects, these materials require the use of quantum transport methodologies, such as the nonequilibrium Green's function (NEGF) formalism to fully capture the relevant physics without uncontrolled approximations. Carrier mobility is a useful parameter that affects carrier collection in photodetectors. This work investigates the application of mobility extraction methodologies from quantum transport simulations in the case of T2SLs exemplified using an InAs/GaSb midwave structure. In a resistive region, the average velocity can be used to calculate an apparent mobility that incorporates both diffusive and ballistic effects. However, the validity of this mobility for predicting device properties is limited to cases of diffusive limited transport or when the entire device can be included in the simulation domain. Two methods that have been proposed to extract diffusive limited mobility, one based on approximating the ballistic component of transport and the other which considers the scaling of resistance with simulation size, were also studied. In particular, the resistance scaling approach is demonstrated to be the method most physically relevant to predicting macroscopic transport. We present a method for calculating the mobility from resistance scaling considerations that accounts for carrier density variation between calculations, which is particularly relevant in the case of electrons. Finally, we comment on the implications of applying the different mobility extraction methodologies to device property predictions. The conclusions of this study are not limited to T2SLs, and may be generally relevant to quantum transport mobility studies.

Numerical Modeling of Branching-Streamer Propagation in Ester-Based Insulating Oil Under Positive Lightning Impulse Voltage: Effects From Needle Curvature Radius

Ester-based insulating oil provides the possibilities for controlling pollutant emissions and enhancing transformer safeties. A 2-D needle-plate electrode branching-streamer model has been established to numerically investigate the propagation characteristics of initial streamer branches and the effects from needle radius in ester-based insulating oil under lightning impulse (LI) voltage. The results indicate that by adding the source term that affects charge carrier density variation to the current continuity equation, the revised model could better capture the branching-streamer morphology in the experiment than the existing model. The accumulation of positive ions in the streamer branches promotes the electric field distortion, introducing the maximum electric field at the head of the streamer branches, which causes the appearance of the branching streamer due to the localized charge carrier surge (CCS). The electric field at the needle zone is affected by the curvature radius, and the smaller curvature radius enhances the electric field, allowing more space charge to be generated, which leads to easier distortion of the electric field at the head of the streamer branches and promotes forward propagation of the streamer branches. In addition, the smaller curvature radius contributes to the generation of the secondary streamer.

The Investigation of Carrier Mobility Effect on the Performance Characteristics of the InGaN-Based Vertical Cavity Surface Emitting Laser (VCSEL) by Solving the Rate Equations

Background: Among the parameters that play an important role in describing the performance of many devices is carrier mobility which is a criterion for the easy movement in semiconductor crystals. Objective: The effect of carrier mobility on the performance characteristics of InGaN quantum well vertical-cavity surface-emitting laser was analytically investigated. Methods: By solving the Poisson’s equation, current density equation, charge concentration continuity equation and carrier and photon rate equations, the variation of current density and carrier density with respect to the position and time and the effects of carrier mobility and temperature on these parameters were investigated. Furthermore, the effect of mobility on the variation of output power versus the injection current and on the time variation of photon and carrier density and the output power was investigated. Results: By increasing the carrier mobility, the threshold current is reduced and the output power is increased. In studying the effect of temperature on the desired parameters, the variation of carrier density with respect to time and position was affected by the temperature change. This phenomenon is due to the dependence of these parameters on the diffusion coefficients and consequently on the mobility of the carriers and the dependence of mobility on temperature. Conclusions: The output power increased, and the time delay in accruing the laser decreased. Consequently, the carrier recombination increased, further resulting in a rapid laser operation.

Reversible and Nonvolatile Manipulation of the Spin-Orbit Interaction in Ferroelectric Field-Effect Transistors Based on a Two-Dimensional Bismuth Oxychalcogenide

The spin-orbit interaction (SOI) offers a nonferromagnetic scheme to realize spin polarization through utilizing an electric field. Electrically tunable SOIs through electrostatic gates have been investigated; however, the relatively weak and volatile tunability limits their practical applications in spintronics. Here, we demonstrate the nonvolatile electric field control of the SOI via constructing ferroelectric Rashba architectures, i.e., two-dimensional ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}/\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$-${\mathrm{PbTiO}}_{3}$ ferroelectric field-effect transistors. The experimentally observed weak antilocalization (WAL) cusp in ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ films implies the Rashba-type SOI that arises from the asymmetric confinement potential. Significantly, taking advantage of the switchable ferroelectric polarization, the WAL-to-weak-localization-transition trend reveals the competition between spin relaxation and the dephasing process, and the variation of carrier density leads to a reversible and nonvolatile modulation of the spin-relaxation time and the spin-splitting energy of ${\mathrm{Bi}}_{2}{\mathrm{O}}_{2}\mathrm{Se}$ films by this ferroelectric gating. Our work provides a scheme to achieve nonvolatile control of the Rashba SOI with the utilization of ferroelectric remanent polarization.

Facet-engineering enhanced room-temperature ferroelectric control of magnetism in semiconducting β-FeSi2″

The electric-field control of the magnetism of magnetic semiconductors is highly desirable for spintronic-integrated circuits with ultralow power consumption. However, the low Curie temperature, small magnetic moments and the limited screening thickness hinder the practical application of the devices. Herein, we propose a novel strategy to enhance electrical control of magnetic semiconductors at room temperature by facet engineering. Taking β-FeSi2 as a prototype material, which has strong facet-induced room-temperature ferromagnetism, significant nonvolatile room-temperature control of magnetism is realized at the interface between ferromagnetic β-FeSi2 and ferroelectric P(VDF-TrFE). Because β-FeSi2 is ferromagnetic at the surface and non-magnetic in the bulk, the surface ferromagnetism can be successfully modulated through the remarkable carrier density variation at the interface caused by large remanent ferroelectric polarization. The facet-engineering enhanced room-temperature ferroelectric control of magnetism paves the way towards future nonvolatile and low-power-consumption spintronic devices.

Zero-field superconducting diode effect in small-twist-angle trilayer graphene

The critical current of a superconductor can be different for opposite directions of current flow when both time-reversal and inversion symmetry are broken. %When time-reversal and inversion symmetry are simultaneously broken, the critical current of a 2D superconductor is expected to depend on the directions of current flow. Such nonreciprocal behavior in superconducting transport, which creates a superconducting diode, has recently been demonstrated experimentally by breaking these symmetries with an applied magnetic field \cite{Ando2020diodes} or by construction of a magnetic tunnel junction \cite{Diez2021magnetic}. Here we report an intrinsic superconducting diode effect which is present at zero external magnetic field in mirror symmetric twisted trilayer graphene (tTLG). Such nonreciprocal behavior, with sign that can be reversed through training with an out-of-plane magnetic field, provides direct evidence of the microscopic coexistence between superconductivity and time-reversal symmetry breaking. In addition to the magnetic-field trainability, we show that the zero-field diode effect can be controlled by varying carrier density or twist angle. In accordance with these experimental controls, a natural interpretation for the origin of the intrinsic diode effect is an imbalance in valley occupation of the underlying Fermi surface, which likely leads to finite-momentum Cooper pairing and nematicity in the superconducting phase.

An insulated-gate bipolar transistor model based on the finite-volume charge method

A finite-volume charge method has been proposed to simulate PIN diodes and insulated-gate bipolar transistor (IGBT) devices using SPICE simulators by extending the lumped-charge method. The new method assumes local quasi-neutrality in the undepleted N− base region and uses the total collector current, the nodal hole density and voltage as the basic quantities. In SPICE implementation, it makes clear and accurate definitions of three kinds of nodes — the carrier density nodes, the voltage nodes and the current generator nodes — in the undepleted N− base region. It uses central finite difference to approximate electron and hole current generators and sets up the current continuity equation in a control volume for every carrier density node in the undepleted N− base region. It is easy to increase the number of nodes to describe the fast spatially varying carrier density in transient processes. We use this method to simulate IGBT devices in SPICE simulators and get a good agreement with technology computer-aided design simulations.

Carrier density variation and linewidth enhancement factor in optically injection locked semiconductor lasers

We theoretically investigate the behavior of carrier density variation in optically injected semiconductor lasers when varying the linewidth enhancement factor. The variation of carrier appears independent of the LEF in the positive detuning side, while decreased in the negative detuning side with any growth in the LEF. This variation is found to be enhanced when the injection level is boosted.

Modulation doping of the FeSe monolayer on SrTiO3

The discovery of higher-temperature superconductivity in FeSe monolayers on ${\mathrm{SrTiO}}_{3}$ substrates has sparked a surge of interest in the interface superconductivity. One point of agreement reached to date is that modulation doping by impurities in the substrate is critical for the enhanced superconductivity. Remarkably, the universal doping about 0.1 electrons per Fe, i.e., so-called ``magic'' doping, has been observed on a range of Ti oxide substrates, which concludes that there likely is some important interaction limiting the FeSe doping. Our study discovers that the polarization change at the interface Se because of the close proximity to the substrate from that in the free-standing FeSe film significantly amplifies the total potential difference at the interface above and beyond the work function difference for charge transfer. Additionally, the titanate substrate with a large number of free electrons basically serves as an ``infinite'' charge reservoir, which leads to the saturated FeSe doping with the complete removal of the interface potential gradient. Our work has developed the theory for modulation doping in the van der Waals materials/oxides heterostructure, providing a solution to the puzzle of magic doping in FeSe monolayers on titanates. The information also presents experimental pathways to accommodate a variable carrier density of FeSe monolayers via modulation doping.

Carrier Density and Thickness Optimization of InxGa1-xN Layer by Scaps-1D Simulation for High Efficiency III-V Solar CelL

In this study, the indium gallium nitride (InxGa1-xN) p-n junction solar cells were optimized to achieve the highest conversion efficiency. The InxGa1-xN p-n junction solar cells with the whole indium mole fraction (0 £ x £ 1) were simulated using SCAPS-1D software. Optimization of the p- and n-InxGa1-xN layer's thickness and carrier density were also carried out. The thickness and carrier density of each layer was varied from 0.01 to 1.50 µm and 1015 to 1020 cm-3. The simulation results showed that the highest conversion efficiency of 23.11% was achieved with x = 0.6. The thickness (carrier density) of the p- and n-layers for this In0.6Ga0.4N p-n junction solar cell are 0.01 (1020) and 1.50 μm (1019 cm-3), respectively. Simulation results also showed that the conversion efficiency is more sensitive to the variations of layer's thickness and carrier density of the top p-InxGa1-xN layer than the bottom n-InxGa1-xN layer. Besides that, the results also demonstrated that thinner p-InxGa1-xN layer with higher carrier density offers better conversion efficiency.

Manipulating electron waves in graphene using carbon nanotube gating

Graphene with its dispersion relation resembling that of photons offers ample opportunities for applications in electron optics. The spacial variation of carrier density by external gates can be used to create electron waveguides, in analogy to optical fiber, with additional confinement of the carriers in bipolar junctions leading to the formation of few transverse guiding modes. We show that waveguides created by gating graphene with carbon nanotubes (CNTs) allow obtaining sharp conductance plateaus, and propose applications in the Aharonov-Bohm and two-path interferometers, and a pointlike source for injection of carriers in graphene. Other applications can be extended to Bernal-stacked or twisted bilayer graphene or two-dimensional electron gas. Thanks to their versatility, CNT-induced waveguides open various possibilities for electron manipulation in graphene-based devices.

Design and prediction of PIT devices through deep learning.

Graphene material has excellent performance and unique variable carrier density characteristics, making it an excellent mid-infrared material. And deep learning makes it possible to quickly design mid-infrared band devices with good performance. A graphene nano-ring-symmetric sector-shaped disk array structure based on the PIT principle is proposed here for sensing. The influence of structural parameters and Fermi energy changes are studied. And its FOM (Figure Of Merit) can reach 28.7; the sensitivity is 574 cm-1 / RIU (Refractive Index Unit). At the same time, we designed a six-layer deep learning network that can predict structural parameters and curve predictions. When predicting structural parameters, its MAPE (Mean Absolute Percentage Error) converges to 0.5. In curve prediction, MSE (Mean Square Error) converges to 1.2. It shows that predictions can be made very well. This paper proposes a symmetrical sector disk array structure and a 6-layer deep learning network. And the deep neural network designed based on the device data has good prediction accuracy under the premise of ensuring the network is simple. This will lay a good foundation for future sensor design and device acceleration optimization design.

Enhanced spectral modulation of Cs WO3 nanocrystals through anionic doping for energy-efficient glazing

Spectrally selective materials have been widely used to shield near-infrared radiation for windows in the region with a cooling-demand climate. CsxWO3 nanocrystals which exhibit strong localized surface plasmon resonance (LSPR) effect and small polaron transfer in near-infrared radiation have attracted great attention for fabricating the spectrally selective coating. The enhancement of its optical performance remains a challenge in energy-efficient windows. Herein, F-doped CsxWO3 nanocrystals were successfully prepared by a solvothermal method, which demonstrate stronger near-infrared absorption performance than CsxWO3 nanocrystals. The introduction of fluorine can enhance the free carrier density of the nanocrystals, which can lead to a higher absorption coefficient. The absorption coefficient variation of LSPR effect and small polaron transfer was explained by the variation of free carrier density and carrier mobility. When the F/W molar ratio was 0.4, the free carrier density reached 9.25 × 1014 cm-3. The spectrally selective coating prepared by F-doped CsxWO3 nanocrystals exhibited superior spectral selectivity with TVis, TNIR, Tlum, and Tsol of 67.21%, 11.85%, 72.76% and 49.01%, respectively. This substitutional doping strategy provides a promising potential to improve the spectral modulation of CsxWO3 nanocrystals for practical application of energy-saving windows.

Direct comparison of three-terminal and four-terminal Hanle effects in the persistent photoconductor Al0.3Ga0.7As:Si

The electronic rendition of the Hanle effect, which is interpreted as the ensemble dephasing of a spin accumulation in the semiconductor under a perpendicular magnetic field, has been one of the most widely utilized and effective methods of measuring spin lifetime, spin accumulation, and spin transport in semiconductors. However, the origin of the Hanle magnetoresistance in the three-terminal (3T) setup has been intensively questioned both theoretically and experimentally; this is in contrast to the nonlocal four-terminal (NL-4T) measurement, which is accepted as reflecting spin accumulation and its spatial decay in metals and semiconductors alike. Here, we present results from 3T and NL-4T Hanle measurements on the same spin injection and detection devices with an ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}:\mathrm{Si}$ semiconducting channel. The use of ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}:\mathrm{Si}$, a persistent photoconductor, enables examination of the evolution of both types of Hanle signals with varying carrier density in the channel on one and the same device via in situ photodoping. We observe that the 3T and NL-4T Hanle signals exhibit similar Lorentzian line shapes, and thus yield similar spin lifetimes at all carrier densities. Moreover, the amplitudes of both types of Hanle signals are found to be consistent with each other, showing a similar exponential decrease with carrier density and in agreement with the Valet-Fert theory, in contrast to devices with artificial oxide barriers. These observations provide compelling evidence that in devices in which the spin injectors and detectors are engineered to minimize the presence of localized states, the 3T Hanle measurements provide a reliable probe of the spin accumulation and its dynamics in the semiconductor channel.

Simultaneous Local Capacitance-Voltage Profiling and Deep Level Transient Spectroscopy Using Time-Resolved Scanning Nonlinear Dielectric Microscopy

Abstract Scanning nonlinear dielectric microscopy (SNDM) is a scanning probe technique that measures changes in oscillation frequency between the probe tip and a voltage-biased sample. As the probe moves across the surface of a semiconductor device, the oscillation frequency changes in response to variations in dielectric properties, charge and carrier density, dopant concentration, interface states, or any number of other variables that affect local capacitance. Over the past few years, researchers at Tohoku University have made several improvements in dielectric microscopy, the latest of which is a digital version called time-resolved SNDM (tr-SNDM). Here they describe their new technique and present an application in which it is used to acquire CV, dC/dV-V, and DLTS data from SiO2/SiC interface samples.