Effect Of Free Carrier Screening Research Articles

Memristive switching in the surface of a charge–density–wave topological semimetal

Owing to the outstanding properties provided by nontrivial band topology, topological phases of matter are considered as a promising platform towards low-dissipation electronics, efficient spin-charge conversion, and topological quantum computation. Achieving ferroelectricity in topological materials enables the non-volatile control of the quantum states, which could greatly facilitate topological electronic research. However, ferroelectricity is generally incompatible with systems featuring metallicity due to the screening effect of free carriers. In this study, we report the observation of memristive switching based on the ferroelectric surface state of a topological semimetal (TaSe4)2I. We find that the surface state of (TaSe4)2I presents out-of-plane ferroelectric polarization due to surface reconstruction. With the combination of ferroelectric surface and charge-density-wave-gapped bulk states, an electric-switchable barrier height can be achieved in (TaSe4)2I-metal contact. By employing a multi-terminal-grounding design, we manage to construct a prototype ferroelectric memristor based on (TaSe4)2I with on/off ratio up to 103, endurance over 103 cycles, and good retention characteristics. The origin of the ferroelectric surface state is further investigated by first-principles calculations, which reveals an interplay between ferroelectricity and band topology. The emergence of ferroelectricity in (TaSe4)2I not only demonstrates it as a rare but essential case of ferroelectric topological materials, but also opens new routes towards the implementation of topological materials in functional electronic devices.

Electron-phonon interaction and phonon frequencies in two-dimensional doped semiconductors

Electron-phonon interaction and phonon frequencies of doped polar semiconductors are sensitive to long-range Coulomb forces and can be strongly affected by the screening effects of free carriers, the latter changing significantly when approaching the two-dimensional limit. We tackle this problem within a linear-response dielectric-matrix formalism, where the screening effects can be properly taken into account by generalized effective charge functions and the inverse scalar dielectric function, allowing for controlled approximations in relevant limits. We propose complementary computational methods to evaluate from first principles both effective charges---encompassing all multipolar components beyond dynamical dipoles and quadrupoles---and the static dielectric function of doped two-dimensional semiconductors and provide analytical expressions for the long-range part of the dynamical matrix and the electron-phonon interaction in the long-wavelength limit. As a representative example, we apply our approach to study the impact of doping in disproportionated graphene, showing that optical Fr\"ohlich and acoustic piezoelectric couplings, as well as the slope of optical longitudinal modes, are strongly reduced, with a potential impact on the electronic/intrinsic scattering rates and related transport properties.

Dynamics of exciton energy renormalization in monolayer transition metal disulfides

Fundamental understandings on the dynamics of charge carriers and excitonic quasiparticles in semiconductors are of central importance for both many-body physics and promising optoelectronic and photonic applications. Here, we investigated the carrier dynamics and many-body interactions in two-dimensional (2D) transition metal dichalcogenides (TMDs), using monolayer WS2 as an example, by employing femtosecond broadband pump-probe spectroscopy. Three time regimes for the exciton energy renormalization are unambiguously revealed with a distinct red-blue-red shift upon above-bandgap optical excitations. We attribute the dominant physical process in the three typical regimes to free carrier screening effect, Coulombic exciton–exciton interactions and Auger photocarrier generation, respectively, which show distinct dependence on the optical excitation wavelength, pump fluences and/or lattice temperature. An intrinsic exciton radiative lifetime of about 1.2 picoseconds (ps) in monolayer WS2 is unraveled at low temperature, and surprisingly the efficient Auger nonradiative decay of both bright and dark excitons puts the system in a nonequilibrium state at the nanosecond timescale. In addition, the dynamics of trions at low temperature is observed to be significantly different from that of excitons, e.g., a long radiative lifetime of ~ 108.7 ps at low excitation densities and the evolution of trion energy as a function of delay times. Our findings elucidate the dynamics of excitonic quasiparticles and the intricate many-body physics in 2D semiconductors, underpinning the future development of photonics, valleytronics and optoelectronics based on 2D semiconductors.

Unconventional active biosensor made of piezoelectric BaTiO3 nanoparticles for biomolecule detection

A smart biosensor based on piezoelectric and semiconducting properties of Barium titanate nanoparticles (BT NPs) by means of a piezoelectric nanogenerator (PNG) is reported for the first time. An Al/BT/ITO NG (Aluminum (Al)/Barium titanate (BT)/Indium Tin Oxide (ITO) nanogenerator) was devised as a self-powered biosensor for actively detecting glucose. The piezoelectric output generated from this NG has dual functions, as both an energy source and a biosensing signal. The biomolecule alters device conductance (charge-carrier density), providing a gate potential, and can vary the local work function and band alignment due to its adsorption on the surface of BT NPs (interfacial contact effect). Here, we tailored the piezoelectric and semiconducting properties of BT NPs using glucose molecules. The glucose molecules (lewis base) on the surface of BT NPs film (lewis acid) act as a gate potential, and the field effect eventually influence the charge carrier density (electrons) of BT NPs film, which varies the screening effect of free-carriers on the piezoelectric output. The novel self-powered glucose biosensor has good selectivity (∼6-fold increase in response vs. interferents) and the approach demonstrated here can serve as a prototype for the development of next-generation smart/self-powered nanosystems for theranostic applications.

Illumination-dependent free carrier screening effect on the performance evolution of ZnO piezotronic strain sensor

Performance modulation of ZnO optoelectronic devices in the presence of proper piezoelectric polarization charges has been widely reported, whereas relatively less work has been performed about the influence of photoexcitation on piezotronics. In this study, we experimentally investigated the performance evolution of ZnO piezotronic strain sensor under various 365 nm UV irradiation densities. The device demonstrated a response ratio of ~200 under no illumination and under −0.53% compressive strain, and the response time is approximately 0.3 s. However, tremendous performance degradation was observed with the increase in the illumination density, which is attributed to the UV-modulated change in the free electron concentration and Schottky barrier height. It was observed that increased carrier density intensifies the screening effect and thus, the modulation ability of piezo-polarization charges weakens. Meanwhile, the deterioration of rectifying behavior at the interface under UV illumination also jeopardizes the device performance.

Computational Study of Effects of Surface Roughness and Impurity Scattering in Si Double-Gate Junctionless Transistors

Electron transport in Si double-gate junctionless transistors (JLTs) is simulated on the basis of the multisubband Monte Carlo method, considering acoustic phonons, intervalley phonons, ionized impurities (IIs), and surface roughness (SR) scattering. It is demonstrated that JLTs can actually minimize the mobility degradation caused by SR scattering and improve the current drivability. On the other hand, II scattering is confirmed to degrade the current drivability compared with conventional MOSFETs with an intrinsic channel. However, the performance degradation in JLTs due to II scattering is shown to be insignificant, owing to the screening effect of free carriers and the forward scattering properties of high-speed carriers. Furthermore, by extracting a backscattering coefficient, the screening effect and the forward scattering properties are shown to more mitigate II scattering as the gate voltage increases.

Highly stable piezo-immunoglobulin-biosensing of a SiO2/ZnO nanogenerator as a self-powered/active biosensor arising from the field effect influenced piezoelectric screening effect.

Highly stable piezo-immunoglobulin-biosensing has been realized from a SiO2/ZnO nanowire (NW) nanogenerator (NG) as a self-powered/active biosensor. The piezoelectric output generated by the SiO2/ZnO NW NG can act not only as a power source for driving the device, but also as a sensing signal for detecting immunoglobulin G (IgG). The stability of the device is very high, and the relative standard deviation (RSD) ranges from 1.20% to 4.20%. The limit of detection (LOD) of IgG on the device can reach 5.7 ng mL(-1). The response of the device is in a linear relationship with IgG concentration. The biosensing performance of SiO2/ZnO NWs is much higher than that of bare ZnO NWs. A SiO2 layer uniformly coated on the surface of the ZnO NW acts as the gate insulation layer, which increases mechanical robustness and protects it from the electrical leakages and short circuits. The IgG biomolecules modified on the surface of the SiO2/ZnO NW act as a gate potential, and the field effect can influence the surface electron density of ZnO NWs, which varies the screening effect of free-carriers on the piezoelectric output. The present results demonstrate a feasible approach for a highly stable self-powered/active biosensor.

Biomolecule-adsorption-dependent piezoelectric output of ZnO nanowire nanogenerator and its application as self-powered active biosensor

Self-powered active biosensor has been realized from ZnO nanowire (NW) nanogenerator (NG). The piezoelectric output generated by ZnO NW NG can act not only as a power source for driving the device, but also as a biosensing signal. After immersing in 10−3gml−1 human immunoglobulin G (IgG), the piezoelectric output voltage of the device under compressive deformation decreases from 0.203±0.0176V (without IgG) to 0.038±0.0035V. Such a self-powered biosensor has higher response than transistor-type biosensor (I–V behavior). The response of self-powered biosensor is in a linear relationship with IgG concentration (logarithm, 10−7–10−3gml−1) and the limit of detection (LOD) on IgG of the device is about 6.9ngml−1. The adsorption of biomolecules on the surface of ZnO NWs can modify the free-carrier density, which vary the screening effect of free-carriers on the piezoelectric output. The present results demonstrate a feasible approach for actively detecting biomolecules by coupling the piezotronic and biosensing characteristics of ZnO NWs.

Surface free-carrier screening effect on the output of a ZnO nanowire nanogenerator and its potential as a self-powered active gas sensor

The output of a piezoelectric nanogenerator (NG) fabricated using ZnO nanowire arrays is largely influenced by the density of the surface charge carriers at the nanowire surfaces. Adsorption of gas molecules could modify the surface carrier density through a screening effect, thus, the output of the NG is sensitive to the gas concentration. Based on such a mechanism, we first studied the responses of an unpackaged NG to oxygen, H2S and water vapor, and demonstrated its sensitivity to H2S to a level as low as 100 ppm. Therefore, the piezoelectric signal generated by a ZnO NWs NG can act not only as a power source, but also as a response signal to the gas, demonstrating a possible approach as a self-powered active gas sensor.

Impact of active layer design on InGaN radiative recombination coefficient and LED performance

The relative roles of radiative and nonradiative processes and the polarization field on the light emission from blue (∼425 nm) InGaN light emitting diodes (LEDs) have been studied. Single and multiple double heterostructure (DH) designs have been investigated with multiple DH structures showing improved efficiencies. Experimental results supported by numerical simulations of injection dependent electron and hole wavefunction overlap and the corresponding radiative recombination coefficients suggest that increasing the effective active region thickness by employing multiple InGaN DH structures separated by thin and low barriers is promising for LEDs with high efficiency retention at high injection. The use of thin and low barriers is crucial to enhance carrier transport across the active region. Although increasing the single DH thickness from 3 to 6 nm improves the peak external quantum efficiency (EQE) by nearly 3.6 times due to increased density of states and increased emitting volume, the internal quantum efficiency (IQE) suffers a loss of nearly 30%. A further increase in the DH thickness to 9 and 11 nm results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably due to degradation of the InGaN material quality and reduced electron-hole spatial overlap. Increasing the number of 3 nm DH active regions separated by thin (3 nm) In0.06Ga0.94N barriers improves EQE, while maintaining high IQE (above 95% at a carrier concentration of 1018 cm−3) and showing negligible EQE degradation up to 550 A/cm2 in 400 × 400 μm2 devices due to increased emitting volume and high radiative recombination coefficients and high IQE. Time-resolved photoluminescence measurements revealed higher radiative recombination rates with increasing excitation due to screening of the internal field and enhanced electron and hole overlap at higher injection levels. To shed light on the experimental observations, the effect of free-carrier screening on the polarization field at different injection levels and the resulting impact on the quantum efficiency were investigated by numerical simulations.

CALCULATION OF THE ELECTRON DRIFT MOBILITY IN Cr2+:ZnS AND Cr2+:ZnSe MATERIALS BY RODE ITERATION MODEL

The results of electron drift velocity in Cr2+:ZnS , and Cr2+:ZnSe are calculated for different temperatures, free-electron concentrations and compositions. The two-mode nature of the polar optic phonons is considered jointly with deformation potential acoustic, piezoelectric, alloy and ionized-impurity scattering. Band non-parabolocity, admixture of p functions, arbitrary degeneracy of the electron distribution, and the screening effects of free carriers on the scattering probabilities are incorporated. The Boltzmann equation is solved by an iterative technique using the currently established values of the material parameters. The iterative results are in fair agreement with other recent calculations obtained using the relaxation-time approximation and experimental methods.

LOW-FIELD ELECTRON TRANSPORT PROPERTIES IN ZINCBLENDE AND WURTZITE GaN STRUCTURES USING AN ITERATION MODEL FOR SOLVING BOLTZMANN EQUATION

An iteration calculation has been carried out to study electron transport properties in zincblende and wurtzite GaN materials. The two-mode nature of the polar optic phonons is considered jointly with deformation potential acoustic, piezoelectric, ionized impurity scattering. Band non-parabolicity, admixture of p functions, arbitrary degeneracy of the electron distribution, and the screening effects of free carriers on the scattering probabilities are incorporated. Electron drift mobility in both zincblende and wurtzite GaN crystal structures are calculated for different temperature and doping dependencies. It is found that the electron mobility decreases monotonically as the temperature increases from 100 K to 600 K. The low temperature value of electron mobilty increases significantly with increasing doping concentration. The agreement of iterative results with the available experimental data is found to be satisfactory.

Thermoelectric power and low-field electron mobility in Al_xGa_{1-x}N lattice-matched to GaN

The results of thermoelectric power and electron drift mobility in Al_xGa_{1-x}N lattice-matched to GaN are calculated for different temperatures, free-electron concentrations and compositions. The two-mode nature of the polar optic phonons is considered jointly with deformation potential acoustic, piezoelectric, alloy and ionized-impurity scattering. Band non-parabolicity, admixture of p functions, arbitrary degeneracy of the electron distribution and the screening effects of free carriers on the scattering probabilities are incorporated. The Boltzmann equation is solved by an iterative technique using the currently established values of the material parameters. The iterative results are in fair agreement with other recent calculations obtained using the relaxation-time approximation and experimental methods.

Dependence of the photoluminescence energy and carrier lifetime of the carrier density in nitride quantum well

We investigated the high-carrier screening of macroscopic polarization fields in GaN quantum wells (QWs) using a variational wave function for electrons and holes. In particular, we studied of the influence of free-carrier screening on the photoluminescence (PL) energy emission and carrier lifetime in the QW. We show that the energy transition between electrons and holes are explained well by the free-carrier screening effect that compensates for the built-in electric field in the QW.

Determination of relative internal quantum efficiency in InGaN∕GaN quantum wells

We have investigated the relative quantum efficiency in a series of InGaN∕GaN single quantum wells with differing indium concentration. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time-resolved photoluminescence studies. Using these complementary techniques we have extracted the low-temperature internal quantum efficiency of the recombination and observed the effect of free-carrier screening on the radiative and nonradiative processes in the quantum well samples. All the samples exhibit high quantum efficiency, with the maximum being observed in the 10% indium sample. In addition, we observe the appearance of a delayed phonon signal, which we correlate to the measured quantum efficiency of the samples.

Non‐radiative processes in gallium nitride based superlattices

We have investigated the effect of the internal electric fields on radiative and non-radiative processes in a series of AlN/GaN superlattices. The results of measurements involving direct detection of phonons emitted as a result of non-radiative recombination and carrier energy relaxation are compared with time-resolved photoluminescence studies. Using these complimentary techniques we have observed directly the effect of free carrier screening on the radiative and non-radiative recombination processes in the superlattice samples. We find that, at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the de-screening effect results in an increase in recombination via non-radiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of internal fields on radiative and nonradiative processes in AlN/GaN superlattices

We have investigated the effect of the internal electric fields on radiative and nonradiative processes in a series of AlN/GaN superlattices. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time resolved photoluminescence studies. Using these complementary techniques we have observed directly the effect of free carrier screening on the radiative and nonradiative recombination processes in the superlattice samples. We find that at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the descreening effect results in an increase in recombination via nonradiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements.

Influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN quantum wells

We studied the influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN multiple quantum wells (MQWs) mainly in relation to a quantum-confined Stark effect. We performed a systematic time-resolved photoluminescence measurement of MQWs for various carrier densities and three different well widths (2.5, 4.0, and 5.5 nm). We show that the energy shift and the change in carrier lifetime are explained well by the free carrier screening effect which compensates for the internal electric field.

Optical gain of strained GaAsSb/GaAs quantum-well lasers: A self-consistent approach

We present a self-consistent model for the band structure and optical gain spectrum of GaAs1−xSbx/GaAs quantum-well (QW) lasers with carrier population. Experimental data indicate that this material system has a type-II QW configuration. By fitting the experimental photoluminescence data from various groups using our proposed empirical model, which assumes that 90% of the band-gap bowing parameter (1.2 eV) appears in the valence band, we find that the unstrained valence band edge discontinuity ratio Qv0 is close to 0.9 for an arbitrary Sb mole composition x of GaAs1−xSbx/GaAs QWs and the QW structure becomes type-II when the compressive strain induced band-edge shifts are taken into account. We show that for type-II QWs the self-consistent solution, which solves the Schrödinger equation and Poisson equation simultaneously, is necessary. Due to the free-carrier screening effect in the self-consistent potential, the electrons are attracted toward the barrier–well interfaces by the holes in the wells, and, therefore, sufficient gain can be achieved for lasing action with type-II QWs. Our theoretical results are compared with experimental data of laser operation wavelength and modal gain, with reasonable agreement. Our model will be important for modeling of type-II QW lasers and useful for design of GaAsSb/GaAs QW vertical-cavity surface-emitting lasers.

Luminescence energy shift and carrier lifetime change dependence on carrier density in In0.12Ga0.88N/In0.03Ga0.97N quantum wells

To clarify the carrier density dependence of carrier lifetime and luminescence energy, we have performed a systematic study of time-resolved photoluminescence (PL) measurements of In0.12Ga0.88N/In0.03Ga0.97N multiple quantum wells for various carrier density. The carrier recombination rate and the PL energy, for the carrier density below 1.5×1018cm−3, are found to decrease nonlinearly with the decrease of the carrier density, although the carrier lifetime is constant and the PL energy shifts linearly for carrier density above this value. We show that the energy shift for the small carrier density and the change in the carrier lifetime are well explained by the free carrier screening effect which compensates the internal electric field. The linear energy shift for the high carrier density is attributed to the band-filling effect.