Presence Of Free Carriers Research Articles

Phonon-assisted Auger decay of excitons in doped transition metal dichalcogenide monolayers.

The competition between the radiative and nonradiative lifetimes determines the optical quantum yield and plays a crucial role in the potential optoelectronic applications of transition metal dichalcogenides (TMDCs). Here, we show that, in the presence of free carriers, an additional nonradiative decay channel opens for excitons in TMDC monolayers. Although the usual Auger decay channel is suppressed at low doping levels by the simultaneous momentum and energy conservation laws, exciton-phonon coupling relaxes this suppression. By solving a Bethe-Salpeter equation, we calculate the phonon-assisted Auger decay rates in four typical TMDCs as a function of doping, temperature, and dielectric environment. We find that even for a relatively low doping of 1012 cm-2, the nonradiative lifetime ranges from 16 to 165ps in different TMDCs, offering competition to the radiative decay channel.

Polarization Saturation in Multilayered Interfacial Ferroelectrics.

Van der Waals polytypes of broken inversion and mirror symmetries have been recently shown to exhibit switchable electric polarization even at the ultimate two-layer thin limit. Their out-of-plane polarization has been found to accumulate in a ladder-like fashion with each successive layer, offering 2D building blocks for the bottom-up construction of 3D ferroelectrics. Here, it is demonstrated experimentally that beyond a critical stack thickness, the accumulated polarization in rhombohedral polytypes of molybdenum disulfide saturates. The underlying saturation mechanism, deciphered via density functional theory and self-consistent Poisson-Schrödinger calculations, point to a purely electronic redistribution involving: 1.Polarization-induced bandgap closure that allows for cross-stack charge transfer and the emergence of free surface charge; 2. Reduction of the polarization saturation value, as well as the critical thickness at which it is obtained, by the presence of free carriers. The resilience of polar layered structures to atomic surface reconstruction, which is essentially unavoidable in polar 3D crystals, potentially allows for the design of new devices with mobile surface charges. The findings, which are of general nature, should be accounted for when designing switching and/or conductive devices based on ferroelectric layered materials.

Direct measurement of ferroelectric polarization in a tunable semimetal

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe2, a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables measurement of the spontaneous polarization as free carriers are added to the system. Here we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe2 is thus shown to be a fully tunable ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.

Structure and optical properties of Er-doped transparent glass–ceramics based on ZnO nanocrystals with anomalous light scattering

This paper reports a study of the structure and optical spectra of glass–ceramics of the potassium–zinc–aluminosilicate system doped with Er3+ ions and containing ZnO nanocrystals. The volume fractions and sizes of the crystals are determined, the optical-density spectra are recorded, and the extinction-coefficient spectra are obtained for glass–ceramics created as a result of a series of successive isothermal heat treatments at 720°C and 750°C. The extinction coefficient is resolved into two terms, one of which describes the extinction caused by the presence of free carriers in the ZnO semiconductor crystals, while the second term describes the scattering and intrinsic absorption of light by these crystals. It is shown that the light scattering has an anomalous character: the dependence of the scattering coefficient on the inverse wavelength is described by a power function with an exponent that substantially exceeds the Rayleigh value of 4, while the measured scattering coefficient is appreciably less than that calculated for independent Rayleigh scatterers.

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

AbstractRapid advances in perovskite photovoltaics have produced efficient solar cells, with stability and duration improving thanks to variations in materials composition, including the use of layered 2D perovskites. A major reason for the success of perovskite photovoltaics is the presence of free carriers as majority optical excitations in 3D materials at room temperature. On the other hand, the current understanding is that in 2D perovskites or at cryogenic temperatures insulating bound excitons form, which need to be split in solar cells and are not beneficial to photoconversion. Here, a tandem spectroscopy technique that combines ultrafast photoluminescence and differential transmission is applied to demonstrate a plasma of unbound charge carriers in chemical equilibrium with a minority phase of light‐emitting excitons, even in 2D perovskites and at cryogenic temperatures. The underlying photophysics is interpreted as formation of large polarons, charge carriers coupled to lattice deformations, in place of excitons. A conductive polaron plasma foresees novel mechanisms for LEDs and lasers, as well as a prominent role for 2D perovskites in photovoltaics.

Dynamic properties of a one-dimensional charge density wave compound in the presence of uncondensed-carriers: Insights from numerical experiences

In the present Letter, a numerical study of the dynamic properties of one-dimensional (1D) incommensurate charge density wave (CDW) compound in the presence of free carriers (not affected by the Peierls transition) is carried out. The results obtained showed that the CDW sliding state is very sensitive to this local effect. The CDW depinning threshold field, excess current normalized and amplitude of Narrow-Band-Noise (NBN) are affected by these single particle’s excitations. This latter can be viewed as a screening effect of the applied electrical field by free carriers which relax in order to reduce the CDW polarization and generate a homogeneous system response.

Temperature effect on charge density wave-free carriers interaction

In this work, we study numerically the temperature effect on charge density wave-free carriers interaction using the Fukuyama-Lee-Rice model. Then, we have shown the local variations of the charge density wave (CDW) phase, the sliding CDW current Jcdw and the threshold field ET are thermally affected in the presence of free carriers. The results are discussed in the context of damping mechanisms of the collective mode due to the screening process.

Optical Phonon Behaviors of Photocharged Nanocrystals: Effects of Free Charge Carriers.

For semiconductor nanocrystals (NCs), the precise knowledge of phonons in the presence of free carriers is important for understanding their electronic and photonic properties in device applications. With Raman spectroscopy, this study investigates the effects of free charge carriers on optical phonon behaviors of NCs. The adoption of the photocharging method allows us to introduce free charge carriers into NCs without inducing other side effects. In the photocharged ZnO NCs, lower longitudinal optical (LO) phonon frequencies and weaker LO overtones relative to the fundamentals were found, which was explained by the screening and band-filling effects caused by the induced free carriers. The free carrier effects on optical phonon behaviors of NCs, usually neglected in previous studies, should be taken into consideration when discussing the electronic and photonic properties of NC-based devices.

Critical Dependence of the Excitonic Absorption in Cuprous Oxide on Experimental Parameters

We study the modification of the exciton absorption in cuprous oxide by the presence of free carriers excited through above band gap excitation. Without this pumping, the absorption spectrum below the band gap consists of the yellow exciton series with principal quantum numbers up to more than n = 20, depending on the temperature, changing over to an about constant, only slowly varying absorption above the gap. Careful injection of free carriers, starting from densities well below 1 μm–3, leads to a reduction of the band gap through correlation effects. The excitons in the Rydberg regime above n = 10 remain unaffected until the band gap approaches them. Then they lose oscillator strength and ultimately vanish upon crossing with the band gap.

Why Does CuFeS2 Resemble Gold?

While several potential applications of CuFeS2 quantum dots have already been reported, doubts regarding their optical and physical properties persist. In particular, it is unclear if the quantum dot material is metallic, a degenerately doped semiconductor, or else an intrinsic semiconductor material. Here we examine the physical properties of CuFeS2 quantum dots in order to address this issue. Specifically, we study the bump that is observed in the optical spectra of these quantum dots at ∼500 nm. Using a combination of structural and optical characterization methods, ultrafast spectroscopy, as well as electronic structure calculations, we ascertain that the unusual purple color of CuFeS2 quantum dots as well the golden luster of CuFeS2 films arise from the existence of a plasmon resonance in these materials. While the presence of free carriers causes this material to resemble gold, surface treatments are also described to suppress the plasmon resonance altogether.

Feature-rich electronic excitations of silicene in external fields

We develop a generalized tight-binding model to investigate the Coulomb excitations in monolayer silicene. The atomic interactions, spin-orbit coupling, magnetic and electric fields, as well as the Coulomb interactions are simultaneously included in our calculations. The magnetic field induces interband plasmons with discrete frequency dispersions restricted to quantized energy states. An intraband plasmon, with a higher intensity and continuous dispersion relation, exists in the presence of free carriers. This mode is dramatically transformed into an interband plasma excitation when the magnetic field is increased, leading to abrupt changes in the value of the plasma frequency and its intensity. Specifically, an electric field may separate the spin and valley polarizations and create additional plasmon modes, a unique feature arising from the buckled structure and the existence of noteworthy spin-orbit coupling.

Fermi polaron-polaritons in charge-tunable atomically thin semiconductors

The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultra-cold Fermi gases. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity-coupling favoring ultra-low mass polariton formation and exciton-electron interactions leading to polaron or trion formation. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe$_2$ exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive-exciton-polaron resonance to the lower-energy attractive-polaron manifold. Simultaneous observation of polariton formation in both attractive and repulsive branches indicate a new regime of polaron physics where the polariton impurity mass is much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose-Fermi mixtures.

Effect of doping on the far-infrared intersubband transitions in nonpolar m-plane GaN/AlGaN heterostructures

This paper assesses the effects of Si doping on the properties of nonpolar m-plane GaN/AlGaN quantum wells (QWs) designed for intersubband (ISB) absorption in the far-infrared spectral range. For doping levels up to 3 × 1012 cm−2, structural analysis reveals uniform QWs with abrupt interfaces and no epitaxially induced defects. Cathodoluminescence spectroscopy confirms the homogeneity of the multiple QWs along the growth direction. Increasing the doping density in the QWs from 1 × 1011 cm−2 to 3 × 1012 cm−2 induces a broadening of the photoluminescence as well as a reduction of the exciton localization energy in the alloy. Also, enhancement of the ISB absorption is observed, along with a blue shift and widening of the absorption peak. The magnitude of the ISB absorption saturates for doping levels around 1 × 1012 cm−2, and the blue shift and broadening increase less than theoretically predicted for the samples with higher doping levels. This is explained by the presence of free carriers in the excited electron level due to the increase of the Fermi level energy.

Structural, Optical and Magnetic Properties of Zn0.5CuxCo0.5-x Dilute Magnetic Semiconductors

The series of Zn0.5Cu0.5-xCox (where x= 0, 0.1, 0.2, 0.3, 0.4, 0.5) dilute magnetic semiconductors have been synthesized using double sintering route. The structural, microstructural, magnetic and optical properties have been carries out using XRD, SEM, VSM and UV-Visible spectroscopy respectively. The XRD analysis has confirmed that Cu and Co ions incorporated in ZnO lattice and replaces ZnO sites without changing wurzite structure. SEM micrographs have showed the clear hexagonal wurzite grains growth. It has been observed that magnetization of all samples reduces as concentration of Cu increases. The ferromagnetism in Cu and Co doped ZnO DMS is due to the secondary phases which is confirmed from XRD pattern there are some secondary phases of CuO, Cu2O in wurzite structure. Ferromagnetism in the DMS was due to the presence of free carriers (holes and electrons) from valance band and localized d-spins of transition metal ions. The Band gap energy decreases as the concentration of Cu increases, which shows that Cu ions have incorporated successfully in ZnO lattice sites.

Structure and Magnetic Properties of (Al, Co) Co-Doped ZnO Thin Films

In this study, Zn0.99Co0.01Al0.015O thin film has been prepared by sol-gel method. The structural and magnetic properties of the sample were investigated. X-ray diffraction spectroscopy analyses indicate that the Co and Al codoping can not disturb the structure of ZnO. No additional peaks are observed in the Zn0.99Co0.01AlxO and Al3+ and Co2+ substitute for Zn2+ without changing the wurtzite structure. The resistance measurements confirm that Al ions increase the free carriers concentration. Based on the above experiments we think the ferromagnetic behavior of the sample could not originate from Co nanoclusters. The presence of free carriers and localized d spins is a prerequisite for the appearance of ferromagnetism. As the result, the carriers generated by Al doping is considered a main factor to induce the ferromagnetic phenomenon.

Ultrawide Frequency Range Crosstalk Into Standard and Trap-Rich High Resistivity Silicon Substrates

Substrate crosstalk into standard and trap-rich high-resistivity silicon (HR-Si) substrates over a wide frequency range, from ultralow frequency (ULF) to extremely high-frequency band (EHF), is investigated using finite-element numerical simulations and experiments. It is demonstrated that low-frequency substrate crosstalk is strongly impacted by the presence of free carriers at the interface between the HR-Si substrate and the interconnection passivation layers. The efficiency of a trap-rich layer, a polysilicon layer thicker than 300 nm, placed at that interface to recover the nominal high-resistivity characteristic of the Si substrate is theoretically and experimentally demonstrated. Finally, the wideband crosstalk behavior of the HR-Si substrate with and without a trap-rich layer is modeled by means of a simple equivalent lumped-element circuit. The proposed model shows excellent agreement with finite-element numerical simulations and experimental data for frequencies above 100 kHz. Due to the introduction of a trap-rich layer, HR-Si substrate behaves as a lossless dielectric substrate. In that case, a purely capacitive electrical equivalent circuit is sufficient to properly describe the substrate crosstalk characteristics.

Impact of Semiconductor and Interface-State Capacitance on Metal/High-k/GaAs Capacitance–Voltage Characteristics

The frequency dispersion of the maximum capacitance of GaAs metal-oxide-semiconductor (MOS) capacitors is studied. The frequency dispersion behavior observed in the capacitance-voltage characteristics of GaAs MOS capacitors is a result of low semiconductor capacitance and an interface-state capacitance that varies with frequency. The traditional approach for calculating interface-state capacitance does not result in frequency dispersion of the maximum capacitance. An interface-state capacitance model based on an extremely high number of interface states in a thin disordered interfacial region is presented to explain the frequency dispersion behavior. The model shows an excellent match with the experimental results. The model also suggests that maximum capacitance values in low-frequency capacitance-voltage characteristics may be determined by the capacitance associated with the interface traps rather than the semiconductor charge and does not necessarily indicate the presence of free carriers.

Polaronic pseudogap in the metallic phase ofLa0.625Ca0.375MnO3 thin films

The electronic density of states (DOS) ofLa0.625Ca0.375MnO3 (LCMO) strain-free epitaxial thin films with an insulator–metal transition temperature (TIM) of 250 K was probed using variable-temperature scanning tunneling microscopyand spectroscopy. We find a depression in the DOS with a finite zero biasconductance (ZBC) signifying a pseudogap in the 78–310 K temperature range.With cooling, the ZBC is found to increase, indicating an increased DOS nearEF. We interpret the pseudogap as a signature of Jahn–Teller polarons while the ZBC change,in agreement with the bulk insulator–metal transition, optical Drude peak andphotoemission experiments, indicates the presence of free carriers at the Fermi energy inthe metallic phase. The free carriers are discussed in terms of correlated polaronic states.

Temperature dependent tunneling study of La0.625Ca0.375MnO3 thin films

The tunneling measurements on La0.625Ca0.375MnO3 strain free epitaxial thin films with an insulator-metal transition temperature (TIM) of 250 K were performed using variable temperature scanning tunneling microscopy and spectroscopy (STM/S). We find a depression in the DOS with a finite zero bias conductance (ZBC) signifying the presence of a pseudogap (PG) at all temperatures. With cooling, the ZBC is found to increase together with an increase in the PG energy of about 0.2 eV near the transition. The PG is a signature of the polarons while the increasing ZBC, in agreement with the bulk insulator-metal transition indicates the presence of free carriers at the Fermi energy.

DILUTED FERROMAGNETIC SEMICONDUCTORS — ORIGIN OF MAGNETIC ORDERING AND SPIN-TRANSPORT PROPERTIES

In the first hour of the lecture the present understanding of the origin of exchange interaction and mechanisms leading to ferromagnetic order in diluted magnetic semiconductors will be presented.1 The lecture will start by discussing energy positions of relevant open magnetic shells, including the correlation energy and excitations within the magnetic ions. The origin and magnitude of sp–d exchange interactions will then be described. This will be followed by presenting the physics of indirect exchange interactions between localized spins contrasting magnetic characteristics in the absence and in the presence of free carriers. The Zener and RKKY models of ferromagnetism will be introduced and the role of confinement, dimensionality, and spin-orbit interaction in determining properties of the ferromagnetic phase will be outlined. The second lecture will be devoted to theory of spin transport in layered structures of diluted ferromagnetic semiconductors, emphasizing the issues important for perspective spintronics devices. A recently developed theory,2 which combines a multi-orbital empirical tight-binding approach with a Landauer–Büttiker formalism will be presented. In contrast to the standard kp method, this theory describes properly the interfaces and inversion symmetry breaking as well as the band dispersion in the entire Brillouin zone, so that the essential for the spin-dependent transport Rashba and Dresselhaus terms as well as the tunneling via k points away from the zone center are taken into account. The applicability of this model for the description of tunneling magnetoresistance (TMR), resonant tunneling spectra, spin-current polarization in Esaki-Zener diodes, and domain-wall resistance will be presented. Note from Publisher: This article contains the abstract only.