Shallow Quantum Wells Research Articles

Bound state continuum resonance transition in a shallow quantum well

Bound state continuum resonance transition in a shallow quantum well

Mobility exceeding 100 000 cm2/V s in modulation-doped shallow InAs quantum wells coupled to epitaxial aluminum

The two-dimensional electron gas residing in shallow InAs quantum wells coupled to epitaxial aluminum is a widely utilized platform for exploration of topological superconductivity. Strong spin-orbit coupling, a large effective $g$ factor, and control over proximity-induced superconductivity are important attributes. Disorder in shallow semiconductor structures plays a crucial role for the stability of putative topological phases in hybrid structures. We report on the transport properties of 2DEGs residing 10 nm below the surface in shallow InAs quantum wells in which mobility may exceed 100 000 ${\mathrm{cm}}^{2}/\mathrm{V}$ s at 2DEG density ${n}_{2\mathrm{DEG}}\ensuremath{\le}1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ at low temperature.

Hot exciton relaxation in coupled ultra-thin CdTe/ZnTe quantum well structures

The photoluminescence (PL) and PL excitation (PLE) spectra of CdTe/ZnTe asymmetric double quantum well (QW) structures are studied on a series of samples containing two CdTe layers with nominal thicknesses of 2 and 4 monolayers (ML) in the ZnTe matrix. The samples differ in the thickness of the ZnTe spacer between CdTe QWs which is 45, 65 and 75 ML. It has been found that at above-barrier excitation the PL from a shallow QW at sufficiently weak excitation intensities is determined by recombination of hot excitons. It is shown that under these conditions, when PL is excited by lasers with different wavelengths, the ratio of the PL intensities from shallow and deep QWs decreases exponentially with an increase of the initial kinetic energy of hot excitons. It is found that energy relaxation of hot excitons with LO phonon emission determine the shape of the PLE spectrum of shallow QW in the range of exciton kinetic energies up to more than 20 LO phonons above ZnTe bandgap. We have shown that the results obtained are well described by the model of charge and energy transfer between QWs.

Repairing the surface of InAs-based topological heterostructures

Candidate systems for topologically-protected qubits include two-dimensional electron gases (2DEGs) based on heterostructures exhibiting a strong spin–orbit interaction and superconductivity via the proximity effect. For InAs- or InSb-based materials, the need to form shallow quantum wells to create a hard-gapped p-wave superconducting state often subjects them to fabrication-induced damage, limiting their mobility. Here, we examine scattering mechanisms in processed InAs 2DEG quantum wells and demonstrate a means of increasing their mobility via repairing the semiconductor–dielectric interface. Passivation of charged impurity states with an argon–hydrogen plasma results in a significant increase in the measured mobility and reduction in its variance relative to untreated samples, up to 45 300 cm2/(V s) in a 10 nm deep quantum well.

Charge scattering mechanisms in shallow InAs quantum wells

We studied the charge scattering mechanisms present in In0.2Al0.8Sb/InAs/Al0.8Ga0.2Sb wells placed in close proximity to the surface of the heterostructures, at depths from 7 nm to 15 nm. The heterostructures were either unintentionally doped, doped from below the channel, or from above the channel. Measurements of sheet and Hall resistances were performed at T = 2 K in a variable magnetic field and under illumination with wavelengths of 400 nm up to 1300 nm. The charge density dependencies of the Hall mobility and quantum scattering time were used to infer the dominant scattering mechanisms. We found that the surface proximity induces significant band bending and an asymmetric placement of the charge distribution in the well. The result is an increase in interface roughness scattering, which reduces the mobility and the quantum scattering time values. In addition, the quantum scattering time is sensitive to scattering off charged impurities, remote or close to the well. Top doping restores the band profile symmetry and improves the transport. A symmetric profile, however, lowers the expectations for a strong spin–orbit coupling and spintronic applications.

Quantum transport in high-quality shallow InSb quantum wells

InSb is one of the promising candidates to realize a topological state through proximity induced superconductivity in a material with strong spin–orbit interactions. In two-dimensional systems, thin barriers are needed to allow strong coupling between superconductors and semiconductors. However, it is still challenging to obtain a high-quality InSb two-dimensional electron gas in quantum wells close to the surface. Here, we report on a molecular beam epitaxy grown heterostructure of InSb quantum wells with substrate-side Si-doping and ultrathin InAlSb (5 nm, 25 nm, and 50 nm) barriers to the surface. We demonstrate that the carrier densities in these quantum wells are gate-tunable and electron mobilities up to 350 000 cm2(V s)−1 are obtained from magnetotransport measurements. Furthermore, from temperature-dependent magnetoresistance measurements, we obtain an effective mass of 0.02 m0 and find Zeeman splitting compatible with the expected band edge g-factor.

Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology

AbstractBuried‐channel semiconductor heterostructures are an archetype material platform for the fabrication of gated semiconductor quantum devices. Sharp confinement potential is obtained by positioning the channel near the surface; however, nearby surface states degrade the electrical properties of the starting material. Here, a 2D hole gas of high mobility (5 × 105 cm2 V−1 s−1) is demonstrated in a very shallow strained germanium (Ge) channel, which is located only 22 nm below the surface. The top‐gate of a dopant‐less field effect transistor controls the channel carrier density confined in an undoped Ge/SiGe heterostructure with reduced background contamination, sharp interfaces, and high uniformity. The high mobility leads to mean free paths ≈ 6 µm, setting new benchmarks for holes in shallow field effect transistors. The high mobility, along with a percolation density of 1.2 × 1011cm−2, light effective mass (0.09me), and high effective g‐factor (up to 9.2) highlight the potential of undoped Ge/SiGe as a low‐disorder material platform for hybrid quantum technologies.

Multicomponent Electron–Hole Liquid in Si/SiGe Quantum Wells

The density functional theory is used to calculate the energy of an electron–hole liquid in Si/Si1–xGex/Si quantum wells. Three one-dimensional nonlinear Schrodinger equations for electrons and light and heavy holes are solved numerically. It is shown that, in shallow quantum wells (small x), both light and heavy holes exist in the electron–hole liquid. Upon an increase in the Ge content, a transition to a state with one type of holes occurs, with the equilibrium density of electron–hole pairs decreasing by more than a factor of 2.

Gate-controlled quantum dots and superconductivity in planar germanium

Superconductors and semiconductors are crucial platforms in the field of quantum computing. They can be combined to hybrids, bringing together physical properties that enable the discovery of new emergent phenomena and provide novel strategies for quantum control. The involved semiconductor materials, however, suffer from disorder, hyperfine interactions or lack of planar technology. Here we realise an approach that overcomes these issues altogether and integrate gate-defined quantum dots and superconductivity into germanium heterostructures. In our system, heavy holes with mobilities exceeding 500,000 cm2 (Vs)−1 are confined in shallow quantum wells that are directly contacted by annealed aluminium leads. We observe proximity-induced superconductivity in the quantum well and demonstrate electric gate-control of the supercurrent. Germanium therefore has great promise for fast and coherent quantum hardware and, being compatible with standard manufacturing, could become a leading material for quantum information processing.

Quasi-Two-Dimensional Electron–Hole Liquid in Shallow SiGe/Si Quantum Wells

An analytical expression is obtained for the energy of a quasi-two-dimensional electron-hole liquid (EHL) in shallow quantum wells. It is shown that in the Si/Si1–xGex/Si structures with small x, the EHL contains light and heavy holes. With increasing x, the transition of EHL to a state with heavy holes occurs, and the equilibrium density of electron-hole pairs strongly decreases. The effect of an external electric field on the EHL properties is studied.

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.

Open-Circuit Voltage in AlGaAs Solar Cells With Embedded GaNAs Quantum Wells of Varying Confinement Depth

We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0–3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage $V_{{{\rm OC}}}$ in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), $V_{{\rm{OC}}}$ degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), $V_{{\rm{OC}}}$ does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved $V_{{\rm{OC}}}$ of 0.23 V, which is beyond the detailed balance limit of standard solar cells. $V_{{\rm{OC}}}$ dependence is explained by analyzing the current–voltage (I–V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of $V_{{{\rm OC}}}$ .

Improving the internal quantum efficiency of green InGaN quantum dots through coupled InGaN/GaN quantum well and quantum dot structure

The InGaN quantum dot (QD) is promising for use in green light-emitting diodes and laser diodes owing to its small strain and weak quantum-confined Stark effect. However, its small carrier capture cross section still sets a limit to its internal quantum efficiency (IQE). Tunneling-enhanced carrier transfer in a coupled InGaN/GaN quantum well (QW) and quantum dot structure has been studied on the basis of temperature-dependent and time-resolved photoluminescence. It is found that carriers can tunnel from a shallow QW to deep QDs at room temperature. Compared with the conventional single-QD layer, the IQE of the QDs can be enhanced more than two times to about 45%.

Kinetics of accumulation of excess holes under photoexcitation and their relaxation in GaAs/AlGaAs shallow quantum wells

The kinetics of accumulation of long-lived excess holes that appear in GaAs/Al0.05Ga0.95As shallow quantum wells under above-barrier photoexcitation and their relaxation is studied by time-resolved photoluminescence spectroscopy. The establishment of a steady state in the nonequilibrium electron-hole system under various combinations of above-barrier and intrawell excitation is also investigated. It is found that the temperature dependence of the excess-hole relaxation time (their lifetime in the quantum wells) exhibits activation behavior with two activation energies. It is established that excitons produced by nonresonant intrawell excitation undergo efficient cooling as they scatter off accumulated long-lived holes.

Thermally activated resonance tunneling in asymmetric systems of CdSe/ZnSe double quantum wells with self-assembled quantum dots

In the luminescence study of double quantum wells formed by depositing two CdSe layers with different nominal thicknesses into a ZnSe matrix, a heavy dependence of the photoluminescence spectrum on the thickness of the ZnSe barrier separating the quantum wells, the excitation photon energy, and temperature is observed. The photoluminescence spectra are studied at barrier widths of 34, 50, and 63 monolayers, excitation photon energies of 3.06, 2.71, and 2.54 eV, and temperatures T in the range of 5–200 K. Upon above- (3.06 eV) and below-barrier (2.71 eV) excitation, the photoluminescence spectrum exhibits two bands, I1(T) and I2(T), corresponding to the annihilation of excitons localized in the quantum dots of the shallow and deep quantum wells. An increase in temperature to ∼50 K yields only a slight decrease in the total integrated emission intensity of both bands IPL(T) and the intensities of each of the two bands, I1(T) and I2(T). A further increase in temperature results in substantial redistribution of the photoluminescence intensity between the two wells, which is attributed to the tunneling of excitons from the QD (quantum-dot) states of the shallow well to states of the deep well. This process is of the activation character and manifests itself as a sharp decrease in the integrated emission intensity related to the shallow quantum well, I1(T), and a simultaneous increase in the integrated emission intensity of quantum dots of the deep quantum well, I2(T). The experimentally detected effect is most profound in the range of temperatures T = 110–130 K and in the samples with a barrier thickness of 50 monolayers. It is most likely that the tunneling is of a resonance nature. This inference follows from the fact that the barrier width is much larger than the well widths for both wells, which predetermines only slight penetration of the wave functions into the neighboring well, and the effect of tunneling itself is only slightly supressed, as the barrier thickness is increased. At the same time, the activation energy is at least three time higher that the optical phonon energy, which cannot be explained on the basis of existing theory.

Modeling of color-coded III-nitride LED structures with deep quantum wells

We present, to the best of our knowledge, the first successful simulation of color-coded III-nitride light-emitting diodes (LEDs) incorporating in their active regions shallow and deep InGaN quantum wells (QWs). Dichromatic violet–aquamarine semipolar LEDs grown in Ga-polar and N-polar crystallographic orientations (Kawaguchi et al. in Appl Phys Lett 100:231110–231114, 2012) were used as an experimental benchmark. Opposite interface polarization charges in Ga-polar and N-polar LEDs provide different conditions for carrier transport and account for different shape of color-coded emission spectra. To reproduce experimentally observed trends, several effects specific for deep III-nitride QWs were essential in our modeling including \((1)\) strongly non-equilibrium character of active QW populations, \((2)\) dynamic carrier overshoot of narrow QW layers, and \((3)\) Auger-assisted QW depopulation.

Acceleration of carrier recovery in a quantum well semiconductor optical amplifier due to the tunneling effect

In this paper, we theoretically demonstrate that carrier recovery can be accelerated through the tunneling effect in a novel (to our knowledge) quantum well (QW) semiconductor optical amplifier. In the active region, we design the repeated element, including a shallow QW and a following deep QW. Through numerical calculation, we find this novel structure is helpful for improving the dynamic characteristics. In the single element, the shallow QW acts as a perfect carrier reservoir, while the deep QW acts as a “real” active region. Gain recovery time is shortened significantly.

Emission enhancement in GaN-based light emitting diodes with shallow triangular quantum wells

GaN-based light emitting diodes (LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QW structure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure.

Accumulation of the excess of one type of charge carriers and the formation of trions in GaAs/AlGaAs shallow quantum wells

The dynamics of excitons and trions in GaAs/AlGaAs heterostructures with shallow quantum wells is studied in time-resolved photoluminescence experiments carried out with different repetition rates of picosecond pump pulses for the cases of intrawell, above-barrier, and“two-color” excitation. It is established that excess charge carriers of one type accumulated in the quantum wells under above-barrier excitation play a key role in the formation and dynamics of the exciton-trion system and determine its composition and kinetic properties. The lifetime of excess charge carriers in the quantum wells, estimated from the experimental data, exceeds 10 μs.

The Over-Barrier Resonant States and Multi-Channel Scattering in Multiple Quantum Wells

We demonstrate an explicit numerical method for accurate calculation of the scattering matrix and its poles, and apply this method to describe the multi-channel scattering in the multiple quantum-wells structures. The S-matrix is continued analytically to the unphysical region of complex energy values. Results of calculations show that there exist one or more S-matrix poles, corresponding to the over-barrier resonant states critical for the effect of the absolute reflection of holes in the energy range where only the heavy ones may propagate over barriers in a structure. Light- and heavy-hole states are described by the Luttinger Hamiltonian matrix. In contrast to the single quantum-well case, at some parameters of a multiple quantum-wells structure the number of S-matrix poles may exceed that of the absolute reflection peaks, and at different values of parameters the absolute reflection peak corresponds to different resonant states. The imaginary parts of the S-matrix poles and hence the lifetimes of resonant states as well as the widths of resonant peaks of absolute reflection depend drastically on the quantum-well potential depth. In the case of shallow quantum wells there is in fact a long-living over-barrier resonant hole state.