Modulation-doped Quantum Research Articles

Intensity-dependent nonlinear optical properties in a modulation-doped single quantum well

In the present work, the changes in the intersubband optical absorption coefficients and the refractive index in a modulation-doped quantum well have been investigated theoretically. Within the envelope function approach and the effective mass approximation, the electronic structure of the quantum well is calculated from the self-consistent numerical solution of the coupled Schrödinger–Poisson equations. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results GaAs/Al x Ga 1− x As are presented for typical modulation-doped quantum well system. The linear, third-order nonlinear and total absorption and refractive index changes depending on the doping concentration are investigated as a function of the incident optical intensity and structure parameters, such as quantum well width and stoichiometric ratio. The results show that the doping concentration, the structure parameters and the incident optical intensity have a great effect on the optical characteristics of these structures.

The effect of hydrostatic pressure on subband structure and optical transitions in modulation-doped quantum well

Within the framework of effective-mass approximation, we have calculated theoretically the effects of hydrostatic pressure and doping concentration on subband structure and optical transitions in modulation-doped GaAs/Al x Ga 1− x As quantum well for different well widths. The electronic structure of modulation-doped quantum well under the hydrostatic pressure is determined by solving the Schrödinger and Poisson equations self-consistently. The results obtained show that intersubband transitions and the subband energy levels in the modulation-doped quantum well can be significantly modified and controlled by the well width, donor concentration and hydrostatic pressure.

G-factor anisotropy in a GaAs/AlxGa1−xAs quantum well probed by electron spin resonance

The anisotropy of electron $g$-factor is investigated for several GaAs/Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As heterostructures using an electrically detected electron spin resonance technique at liquid helium temperature. For a modulation-doped 25-nm single quantum well with electron density $n=4\ifmmode\times\else\texttimes\fi{}{10}^{11}$ cm${}^{\ensuremath{-}2}$ we extracted an out-of-plane $g$-factor value of $|{g}_{\mathit{zz}}|=0.410$ and in-plane values of $|{g}_{\mathit{yy}}|=0.359$ and $|{g}_{\mathit{xx}}|=0.289$. In addition, linear in magnetic field corrections to the $g$-factor components were also extracted and strong anisotropy in their values was established.

Hysteretic magnetoresistance and thermal bistability in a magnetic two-dimensional hole system

Colossal negative magnetoresistance and the associated field-induced insulator-to-metal transition, the most characteristic features of magnetic semiconductors, are observed in n-type rare earth oxides and chalcogenides, p-type manganites, n-type and p-type diluted magnetic semiconductors (DMS) as well as in quantum wells of n-type DMS. Here, we report on magnetostransport studies of Mn modulation-doped InAs quantum wells, which reveal a magnetic field driven and bias voltage dependent insulator-to-metal transition with abrupt and hysteretic changes of resistance over several orders of magnitude. These phenomena coexist with the quantised Hall effect in high magnetic fields. We show that the exchange coupling between a hole and the parent Mn acceptor produces a magnetic anisotropy barrier that shifts the spin relaxation time of the bound hole to a 100 s range in compressively strained quantum wells. This bistability of the individual Mn acceptors explains the hysteretic behaviour while opening prospects for information storing and processing. At high bias voltage another bistability, caused by the overheating of electrons10, gives rise to abrupt resistance jumps.

Asymmetric heterostructure for photovoltaic InAs quantum dot infrared photodetector

A photovoltaic InAs quantum dot-under-a-well photodetector is reported with a peak responsivity at 7 μm wavelength. In this structure, we implement an improved injection scheme, which allows a controlled feeding of the quantum dots through a modulation-doped InGaAs quantum well injector. A thin Al0.3Ga0.7As barrier significantly reduces the dark current and, at the same time, is responsible for the photovoltaic behavior. At 4 K and no applied bias, a responsivity of 2.5 mA/W and a detectivity of D∗=2.3×1010 cm Hz1/2/W in the dark is measured. The TBLIP of the device is 60 K and the D∗ at this temperature is 2×109 cm Hz1/2/W.

Inter-sub-band transitions and binding energies of donor impurities in a modulation-doped quantum well in the presence of electric field

In this study, we have investigated theoretically the effects of the electric field and doping concentration on the optical transitions in a modulation-doped GaAs-AlGaAs quantum well for different well widths. The binding energies of the donor have also been computed using a trial wave function with two parameters in the framework of an effective-mass approximation. The electronic structure of a modulation-doped quantum well under the electric field is determined by solving the Schrödinger and Poisson equations self-consistently in the effective-mass approximation. The results obtained show that inter-sub-band transitions and the energy levels in the modulation-doped quantum well can be significantly modified and controlled by the well width and donor concentration. The sensitivity to the well widths of the absorption coefficient can be used in various optical semiconductor devices’ applications.

Engineering artificial graphene in a two-dimensional electron gas

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Ultrahigh-mobility electrons with linearly dispersing bands might open new venues for the studies of Dirac-fermion physics in semiconductors.

Ultrafast Coherent Electron Spin Flip in a Modulation-Doped CdTe Quantum Well

We report the experimental realization of coherent electron spin flip in a modulation-doped CdTe quantum well. Coherent spin rotation is realized with an off-resonant laser pulse, which induces a polarization-dependent optical Stark shift in the trion resonance. Complete electron spin flip is made possible by a laser pulse designed to avoid excessive excitations of nearby exciton resonances and minimizes detrimental many-body effects. These results demonstrate an effective approach for ultrafast optical spin control in a complex spin system.

Analysis of electric-field-induced spin splitting in wide modulation-doped quantum wells

We analyze the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation. We argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than the commonly used Rashba term dependent on $k$ and electric field. It is pointed out that the expectation value of the electric field in a subband is sometimes not unique because the expectation values can even have opposite signs for the spin-split subband components. Symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting are also discussed. We apply a well established multiband approach to wide modulation-doped InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.

Limit to two-dimensional mobility in modulation-doped GaAs quantum structures: How to achieve a mobility of 100 million

Considering scattering by unintentional background charged impurities and by charged dopants in the modulation doping layer as well as by GaAs acoustic phonons, we theoretically consider the practical intrinsic (phonons) and extrinsic (background and dopants) limits to carrier mobility in modulation-doped AlGaAs-GaAs two-dimensional (2D) semiconductor structures. We find that reducing background impurity density to ${10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}3}$ along with a modulation doping separation of $1000\text{ }\text{\AA{}}$ or above will achieve a mobility of $100\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$ at a carrier density of $3\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}2}$ for $T=1\text{ }\text{K}$. At $T=4(10)\text{ }\text{K}$, however, the hard limit to the 2D mobility would be set by acoustic phonon scattering with the maximum intrinsic mobility being no higher than $22(5)\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$. Detailed numerical results are presented as a function of carrier density, modulation doping distance, and temperature to provide a quantitative guide to experimental efforts for achieving ultrahigh 2D mobilities.

Electronic transport in modulation-doped InSb quantum well heterostructures

The mobility and carrier concentration of a number of InSb-based modulation-doped quantum well heterostructures are examined over a range of temperatures between 4.5 and 300 K. Wide well (30 nm) and narrow well (15 nm) structures are measured. The temperature dependent mobilities are considered within a scattering model that incorporates polar optical and acoustic phonon scatterings, interface roughness scattering, and scattering from charged impurities both in the three-dimensional background and within a distributed ``quasi-two-dimensional'' doping layer. Room temperature mobilities as high as $51\text{ }000\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$ are reported for heterostructures with a carrier concentration of $5.8\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}2}$, while low-temperature mobility (below 40 K) reaches $248\text{ }000\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$ for a carrier concentration of $3.9\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}2}$. A Schr\odinger--Poisson model is used to calculate band structures in the material and is shown to accurately predict carrier concentrations over the whole temperature range. Low-temperature mobility is shown to be dominated by remote ionized impurity scattering in wide well samples and by a combination of ionized impurity and interface roughness scattering in narrow well samples.

Xt–Xlvalley crossover and deformation potential in AlP/GaP quantum wells

We investigate the properties of quasi-two-dimensional electrons in a wide range of modulation-doped AlP quantum wells (between 3 and 15 nm) with GaP barriers by measuring cyclotron resonance, quantum Hall effect, and Shubnikov--de Haas oscillations. The degeneracy of the three $X$ valleys is lifted due to both strain and confinement: In wider wells the strain contribution dominates, and the lowest energy states are the transverse $X$ valleys ${X}_{t}$. On the other hand, in narrower wells, the quantum confinement energy dominates, and the lowest energy states are the single longitudinal valleys ${X}_{l}$. The transition between these two configurations occurs for wells between 5 nm and 4. The effective mass components for the AlP $X$ valleys are determined from the cyclotron resonance data to be ${m}_{t}=(0.30\ifmmode\pm\else\textpm\fi{}0.01){m}_{0}$ and ${m}_{l}=0.90{m}_{0}$. From these effective mass values and the degeneracy data, we calculate the deformation potential for AlP to be ${\ensuremath{\Xi}}_{u}^{\text{AlP}}=3.3\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{eV}$.

Band-bending effects on the electronic properties of square quantum wells

We present a theory of the band-bending effects on two-dimensional 2D carriers confined in a modulationdoped square quantum well. We develop a tractable variational evaluation of several physical quantities that are important in the theory of 2D systems. Analytic expressions of the envelope wave function and the 2D screening form factor allow us to compute various electronic properties such as electrical mobility, density of states in the presence or in the absence of magnetic fields, and Landau level broadening. We prove that in the case of the interface roughness scattering, the band-bending effects lead to a peak in the channel-width dependence of the mobility and a minimum in the Landau level broadening. Our modeling explains recent measurements for a 2D hole gas.

Dressed excitons within an incoherent electron gas: Observation of a Mollow triplet and an Autler-Townes doublet

We demonstrate that the interaction between excitons and a sea of incoherent electrons does not preclude excitons dressing by light. We investigate if exciton-electron scattering plays some inhibiting role in the coherent light-exciton coupling by measuring the dynamical absorption spectrum of a modulation-doped CdTe quantum well that shows clear evidence for significant electron scattering of the excitonic states. We show the occurrence of dressed and correlated excitons by detecting quantum coherent interferences through excitonic Autler-Townes doublet and ac Stark splitting that evolves into a Mollow triplet with gain. We also evidence the partial inhibition of the electron-exciton scattering by exciton-light coupling. The absorption spectrum of semiconductor quantum wells is dominated by a strong exciton resonance. Excitons are bound, conduction electron valence-hole pairs and are quite often compared to a two level system as a hydrogen atom. The role of many body interactions and exciton correlations in the exciton dynamical absorption spectrum has attracted much interest for several decades. The response of the exciton absorption to an incoherent and thermalized electronhole population obtained through nonresonant pumping in the continuum corresponds mainly to a broadening of the exciton resonance that is dominated by collision broadening excitation-induced dephasing rather than by a bleaching due to the saturation of the oscillator strength caused by phase space filling. 1 The effect increases with electron-hole concentration. In the regime of coherent pumping at the ex

Novel mechanism for rapid spin flip with increasing in-plane wave vector in slightly asymmetric modulation-doped quantum wells

We calculate the electric-field-induced spin splittings in wide slightly asymmetric modulation-doped quantum wells. When spin subbands are anticrossing we demonstrate two-step spin flips as the in-plane wave vector along the [11] direction is increased by 0.002 nm-1. At the beginning of this interval the y-component flips, at the end the x-component. Simultaneously the energy separation stays roughly constant below 1 μeV and the wave functions are interchanged. A bias change of about 1 meV is sufficient to move the Fermi level from below to above the anticrossing region.

半导体量子阱中传播效应对子带间拉比振荡的影响

We investigate intersubband Rabi flopping in modulation-doped semiconductor quantum wells with and without the propagation effects, respectively. It is shown that propagation effects have a larger impact on Rabi flopping than the nonlinearities rooted from electron-electron interactions in multiple quantum wells. By using ultrashort \pi pulses, an almost complete population inversion exists if the propagation effects are not considered; while no complete population inversion occurs in the presence of propagation effects. Furthermore, the magnitude of the impact of propagation effects may be controlled by varying the carrier density.

Optical and electrical properties of modulation-doped n and p-type Ga x In1-x N y As1-y /GaAs quantum wells for 1.3 μm laser applications

We present a comprehensive study of spectral photoluminescence (PL), photoconductivity and Hall mobility in undoped, n and p-type modulation-doped quantum wells of Ga1-x In x N y As1-y /GaAs with varying nitrogen concentration. We show that the increasing nitrogen composition red shifts the energy gap and this red shift is accompanied with a reduction of the 2D electron mobility in the quantum wells. True temperature dependence of the band gap, free from errors associated with nitrogen induced exciton trapping effects, is observed because in the modulation doped QW samples PL emission is dominated by band-to-band recombination and the S-shape temperature dependence is eliminated. Excellent fit to semi-experimental Varshni equation is obtained and the temperature dependence of the band gap in the linear regime (dE/dT) is tabulated as a function of nitrogen concentration and the type of dopant.

Rapid spin flip in a spin subband at an anticrossing region in a slightly asymmetric modulation-doped quantum well

The spin splitting due to structure and bulk inversion asymmetry is calculated for electron subbands in wide slightly asymmetric InGaSb quantum wells. At anticrossings, rapid spin flips in two steps are found as the in-plane wave vector along the [1 1] direction is increased by 0.002 nm −1. First the y-component and then the x-component is flipped. A change of bias of about 1 meV across the quantum well is sufficient move the Fermi level across the anticrossing region.

Correlated states and spin transitions in nanofabricated AlGaAs/GaAs few-electron quantum dots probed by inelastic light scattering

Spin transitions and correlated few-electron states are investigated by resonant inelastic light scattering in dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching. We focus on QDs with four electrons. We show that at moderate magnetic fields, the ground state is a singlet with total spin S=0. A rich spectrum of distinct spin and charge inter-shell excitations is found, which cannot be described by a mean-field Hartree–Fock framework based on the quantum description of Fock–Darwin energy levels. Instead, the experimental results are well modeled by numerical evaluations within a full configuration interaction approach that highlights the impact of correlation effects in this configuration. This work demonstrates that the sensitivity reached by resonant inelastic light scattering enables the study of few-electron effects in QDs formed by state-of-the-art nanofabrication processes under the extreme conditions of low temperatures and high magnetic fields.

Magneto-optical properties of n-type modulation-doped (Cd,Cr)Te quantum well

Magneto-photoluminescence (PL) of n-type modulation-doped (Cd,Cr)Te quantum well is investigated comparing with that of CdTe and (Cd,Mn)Te systems. The magnetic field dependence of PL spectra due to the heavy hole band is very similar to that of the non-magnetic CdTe system in both circular polarizations (CPs). This outcome is completely different from the case of (Cd,Mn)Te system, in which the PL spectra are perfectly polarized even at low fields in the right CP due to the giant Zeeman splitting and are red-shifted with an increase in the magnetic field. Furthermore, for the (Cd,Cr)Te system, the PL spectra due to the light hole band are gradually polarized with the magnetic field in the left CP and their energy shift is very small.