Two-dimensional Electron Hole Research Articles

Two-Dimensional Electron-Hole Plasma in Colloidal Quantum Shells Enables Integrated Lasing Continuously Tunable in the Red Spectrum.

Combining integrated optical platforms with solution-processable materials offers a clear path toward miniaturized and robust light sources, including lasers. A limiting aspect for red-emitting materials remains the drop in efficiency at high excitation density due to non-radiative quenching pathways, such as Auger recombination. Next to this, lasers based on such materials remain ill characterized, leaving questions about their ultimate performance. Here, we show that colloidal quantum shells (QSs) offer a viable solution for a processable material platform to circumvent these issues. We first show that optical gain in QSs is mediated by a 2D plasma state of unbound electron-hole pairs, opposed to bound excitons, which gives rise to broad-band and sizable gain across the full red spectrum with record gain lifetimes and a low threshold. Moreover, at high excitation density, the emission efficiency of the plasma state does not quench, a feat we can attribute to an increased radiative recombination rate. Finally, QSs are integrated on a silicon nitride platform, enabling high spectral contrast, surface emitting, and TE-polarized lasers with ultranarrow beam divergence across the entire red spectrum from a small surface area. Our results indicate QS materials are an excellent materials platform to realize highly performant and compact on-chip light sources.

Hot luminescence of two-dimensional electron hole systems in modulation-doped silicon

Modulation doping of silicon has great potential for miniaturization, surface passivation, and third generation photovoltaics (PV). At a modulation-doped silicon surface, we observe the formation of a 2D hole layer at the silicon surface at low temperatures by means of photoluminescence (PL) measurements. A line shape analysis of band–band and hot luminescence reveals the hole density (which is equal to the modulation-doped acceptor density). A high excitation intensity leads to a Fermi edge singularity of the band–band and hot PL emission. While the 2D layer can be characterized by the observed luminescence, the spectral region of twice the bandgap is fully dominated by emission from this surface layer, impeding the measurement of bulk hot luminescence, e.g., from Auger electrons or from nonthermalized carriers in a hot carrier PV device.

Spin-polarized two-dimensional electron/hole gas at the interface of nonmagnetic semiconducting half-Heusler compounds: Modified Slater-Pauling rule for half-metallicity at the interface

Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.

Two-Dimensional Electron-Hole System under the Influence of the Chern–Simons Gauge Field Created by the Quantum Point Vortices

In the present work the Chern-Simons(C-S) gauge field theory developed by Jackiw and Pi [1] and widely used to explain the fractional quantum Hall effects, was applied to describe the two-dimensional (2D) electron-hole (e-h) system in a strong perpendicular magnetic field under the influence of the quantum point vortices creating the Chern-Simons(C-S) gauge field. The composite particles formed by electrons and by holes with equal integer positive numbers of the attached quantum point vortices are described by the dressed field operators, which obey to the Fermi or to the Bose statistics depending on the even or odd numbers . It is shown that the phase operators as well as the vector and the scalar potentials of the C-S gauge field depend on the difference of the electron and of the hole density operators. They vanish in the mean field approximation, when the average values of the electron and of the whole densities coincide. Nevertheless, even in this case, the quantum fluctuations of the C-S gauge field lead to new physics of the 2D e-h system.

Metal-insulator transitions in bilayer electron-hole systems in transition metal dichalcogenides

We investigated metal-insulator transitions for double layer two-dimensional electron hole systems in transition metal dicalcogenides (TMDC) stacked on opposite sides of thin layers of boron nitride (BN). The interparticle interaction is calculated by including the screening due to the polarization charges at different interfaces, including that at the encapsultion and the substrate of experimental structures. We compute and compare the energies of the metallic electron-hole plasma and the newly proposed insulating exciton solid with fixed-node diffusion Monte Carlo simulation including the high valley degeneracy of the electron bands. We found that for some examples of current experimental structures, the transition electron/hole density is in an accessible range of g x 10^12 /cm*2 with g between 4.1 and 14.5 for spacer thicknesses between 2.5 and 7.5 nm. Our result raise the possibility of exploiting this effect for logic device applications.

Direct observation of polarization-induced two-dimensional electron/hole gases at ferroelectric-insulator interface

Two-dimensional electron gas or hole gas (2DEG or 2DHG) and their functionalities at artificial heterostructure interfaces have attracted extensive attention in recent years. Many theoretical calculations and recent experimental studies have shown the formation of alternating 2DEG and 2DHG at ferroelectric/insulator interfaces, such as BiFeO3/TbScO3, depending on the different polarization states. However, a direct observation based on the local charge distribution at the BiFeO3/TbScO3 interface has yet to be explored. Herein we demonstrate the direct observation of 2DHG and 2DEG at BiFeO3/TbScO3 interface using four-dimensional scanning transmission electron microscopy and Bader charge analysis. The results show that the measured charge state of each Fe/O columns at the interface undergoes a significant increase/reduction for the polarization state pointing away/toward the interface, indicating the existence of 2DHG/2DEG. This method opens up a path of directly observing charge at atomic scale and provides new insights into the design of future electronic nanodevices.

Anderson Localization in a Two-Dimensional Electron–Hole System

Anderson Localization in a Two-Dimensional Electron–Hole System

Modulation characteristics of uncooled graphene photodetectors

We report on the modulation characteristics of the uncooled terahertz (THz) and infrared (IR) detectors using uniform graphene layers (GLs) and perforated graphene layers (PGL). Our analysis shows that the PGL detectors could have more than an order of magnitude smaller dark current compared to more conventional GL reversed biased photodetectors operating in the same frequency range. The modulation mechanism for the simulated photodetectors is the variation of the density and effective temperature of the two-dimensional electron–hole plasma with the varying intensity of the THz or IR radiation. Our simulations predict a reasonable modulation depth up to the modulation frequencies of 50 GHz for the impinging radiation frequencies up to the mid-IR range.

Imaging confined and bulk p-type/n-type carriers in (Al,Ga)N heterostructures with multiple quantum wells

The current state of dopant assessment for the optimization of the III-nitride-based heterostructures for high frequency, high power, and light emission applications relies heavily on quantitative chemical analysis techniques. In such complex heterostructures, determination of p-type carrier density of the cap layer, control of background concentration, and assessment of polarization induced confined carriers are necessary for the realization of optimal devices. None of these can be completely inferred from chemical analysis owing to several material and growth issues including poor activation of Mg, presence of O impurities, and amphoteric nature of carbon impurities. Here, as regions of interest require nanometer resolution, especially near the interfaces featuring triangular quantum wells and exhibiting electron/hole confinement, exploitation of the behavior of the nanosize metal–semiconductor junction formed between the metallic scanning probe microscopy probe and the III-nitride surface is promising for carrier determination. By combining two techniques sensitive to local change in capacitance and rectifying characteristic of conduction at the nanoscale, the nature of free carriers originating from extrinsic n-type and p-type dopants and polarization induced confined carriers, two-dimensional electron gas and hole gas, were eventually revealed across III-nitride heterostructures.

Enhancing Performance of a GaAs/AlGaAs/GaAs Nanowire Photodetector Based on the Two-Dimensional Electron-Hole Tube Structure.

Here, we design and engineer an axially asymmetric GaAs/AlGaAs/GaAs (G/A/G) nanowire (NW) photodetector that operates efficiently at room temperature. Based on the I-type band structure, the device can realize a two-dimensional electron-hole tube (2DEHT) structure for the substantial performance enhancement. The 2DEHT is observed to form at the interface on both sides of GaAs/AlGaAs barriers, which constructs effective pathways for both electron and hole transport in reducing the photocarrier recombination and enhancing the device photocurrent. In particular, the G/A/G NW photodetector exhibits a responsivity of 0.57 A/W and a detectivity of 1.83 × 1010 Jones, which are about 7 times higher than those of the pure GaAs NW device. The recombination probability has also been significantly suppressed from 81.8% to 13.2% with the utilization of the 2DEHT structure. All of these can evidently demonstrate the importance of the appropriate band structure design to promote photocarrier generation, separation, and collection for high-performance optoelectronic devices.

Density functional theory of two-dimensional electron–hole complexes

The energy of the two-dimensional electron–hole complex has been calculated in the framework of the density functional theory. We show that the energy of a direct two-dimensional exciton, without taking into consideration the exchange–correlation interaction, is very different from the exact value. We find that the number of particles in the indirect electron–hole complexes decreases with increasing interlayer distance in a strong magnetic field.

Terahertz Cyclotron Photoconductivity in a Highly Unbalanced Two-Dimensional Electron–Hole System

Terahertz cyclotron-resonance photoconductivity in a two-dimensional electron–hole system under conditions where the cyclotron resonance occurs owing to the absorption of radiation by electrons whose density is one to three orders of magnitude lower than the hole density is experimentally investigated for the first time. Information on the behavior of the main parameters (i.e., amplitude and broadening) characterizing resonance photoconductivity as a function of wavelength, temperature, and electron density is obtained. On this basis, it is concluded that resonance photoconductivity in the system under study results from cyclotron resonance caused by transitions between the partially filled zeroth Landau level and the first Landau level of electrons, and resonance broadening is caused by scattering on a short-range screened impurity potential. It is found that a decrease in the electron density by an order of magnitude does not lead to a significant reduction of the photoconductivity signal; moreover, at a wavelength of 432 μm, the signal even grows slightly. This fact can be associated with the effective enhancement of the field of the incident radiation in the system under study.

Current-induced spin polarization in the isotropic k -cubed Rashba model: Theoretical study of p -doped semiconductor heterostructures and perovskite-oxide interfaces

Using the Matsubara Green's function formalism we calculate the temperature dependence of the nonequilibrium spin polarization induced by an external electric field in the presence of spin-orbit coupling. The model Hamiltonian includes an isotropic k-cubed form of the Rashba spin-orbit interaction. Such a Hamiltonian captures the electronic and spin properties of two-dimensional electron (hole) gas at the surfaces or interfaces of transition metal oxides or in p-doped semiconductor heterostructures. The induced spin polarization is calculated for the nonmagnetic as well as magnetic electron/hole gas. Relation of the spin polarization to the Berry curvature is also discussed.

Heating and evaporation of a two-dimensional electron–hole liquid by heat pulses

The dynamics of low-temperature (T = 5 K) photoluminescence spectra of Si/Si1-xGex/Si heterostructures (x = 0.045) under the influence of a stream of nonequilibrium phonons (heat pulses) propagating in the structure is investigated. The rapid evaporation of the electron–hole liquid in the quantum well of the structure is observed as the liquid is heated by nonequilibrium phonons. It is established that an increase in the exciton-gas density in the quantum well is caused by the evaporation of the electron–hole liquid and by an increase in the rate of exciton capture by the quantum well. It is shown that the interaction with nonequilibrium phonons results in the dissociation of bound-exciton complexes in the Si layers, which is accompanied by an increase in the exciton concentration and lifetime.

Fine structure of the emission spectrum of a two-dimensional electron–hole liquid in SiGe/Si quantum wells

The fine structure of the emission spectrum of a quasi-two-dimensional electron–hole liquid in shallow SiGe/Si quantum wells is observed experimentally. This fine structure is explained by the occurrence of steps in the density of states resulting from the coexistence of light and heavy holes in the liquid phase and by the interaction of charge carriers with charge-density oscillations in the liquid.

On the phase diagram of a two-dimensional electron–hole system

A first order phase transition between a BCS phase and an insulating Mott phase for a gas of spatially separated electrons and holes with tunable Coulomb interaction and variable density is predicted. In the framework of a BCS-like mean-field approach and a Landau expansion in terms of the pairing order parameter the phase diagram is studied. This indicates several phases and phase transitions, including an electron-hole plasma at low density and weak interaction, an intermediate BCS phase with Cooper pairs and an electron-hole plasma at high density and weak interaction. The insulating Mott phase appears for the strong interaction and low temperatures. We briefly discuss the possibilities to realize these phases in realistic systems such as coupled quantum wells and graphene double layers.

Density Functional Theory of Composite Fermions**Supported by the National Basic Research Program of China under Grant No 2012CB921304, and the National Natural Science Foundation of China under Grant No 11325416.

We construct a density functional theory for two-dimensional electron (hole) gases subjected to both strong magnetic fields and external potentials. In particular, we are focused on regimes near even-denominator filling factors, in which the systems form composite fermion liquids. Our theory provides a systematic and rigorous approach to determine the properties of ground states in a fractional quantum Hall regime that is modified by artificial structures. We also propose a practical way to construct an approximated functional.

Exciton–Mott Physics in Two-Dimensional Electron–Hole Systems: Phase Diagram and Single-Particle Spectra

Exciton Mott physics in two-dimensional electron-hole (e-h) systems is studied in the quasiequilibrium, which is the crossovers or phase transitions between the insulating exciton gas and the metallic e-h plasma. By developing a self-consistent screened T-matrix approximation, we succeed in obtaining the "global" phase diagram on the plane of the e-h density and the temperature as a contour plot of the exciton ionization ratio. The detailed features of the exciton-Mott crossover at high temperature are figured out beyond the conventionally used concept of the Mott density. At low temperature, we find not only the region unstable toward the inhomogeneity but the pure Mott transition point characterized by the discontinuity in the ionization ratio. The single particle spectra also exhibit interesting features reflecting the excitonic correlations.

Nonlinear optical rectification in a two-dimensional system

Abstract The optical rectification (OR) in the two-dimensional electron–hole system is investigated theoretically. An analytical expression formula of the OR coefficient is obtained via the framework of compact density matrix approach and the iterative method. The results show that the OR coefficient depends strongly on the system׳s size and the relaxation time. A more obvious OR coefficient should be found by adjusting an appropriate choice of the system parameters.

Measurement of photoluminescence spectral linewidth of a GaAs quantum well in perpendicular electric fields: Evidence of a crossover from trions to an electron-hole gas

By measuring the photoluminescence linewidth of a GaAs quantum well under perpendicular electric fields, we have traced the variation of the effective radius ${a}^{*}$ of a charged exciton (trion) as a function of electron density. The ${a}^{*}$ increases sharply above a critical density ${n}_{s}=2\ifmmode\times\else\texttimes\fi{}{10}^{14}$ m${}^{\ensuremath{-}2}$, which is consistent with the decrease of the screening length predicted by nonlinear-screening theory. Our analysis shows that the crossover from trions to the two-dimensional electron gas plus hole is generated by Coulomb screening in the high-electron-density regime, eliminating the possible mechanism by the trion localization effect due to the single-electron localization.