Quantum-well States Research Articles

Gilbert damping in magnetic layered systems

The Gilbert damping constant present in the phenomenological Landau-Lifshitz-Gilbert equation describing the dynamics of magnetization is calculated for ferromagnetic metallic films as well as Co/nonmagnet (NM) bilayers. The calculations are done within a realistic nine-orbital tight-binding model including spin-orbit coupling. The convergence of the damping constant expressed as a sum over the Brillouin zone is remarkably improved by introducing finite temperature into the electronic occupation factors and subsequent summation over the Matsubara frequencies. We investigate how the Gilbert damping constant depends on the ferromagnetic film thickness as well as on the thickness of the nonmagnetic cap in Co/NM bilayers $(\text{NM}=\text{Cu}$, Pd, Ag, Pt, and Au). The obtained theoretical dependence of the damping constant on the electron-scattering rate, describing the average lifetime of electronic states, varies substantially with the ferromagnetic film thickness and it differs significantly from the dependence for bulk ferromagnetic metals. The presence of nonmagnetic caps is found to largely enhance the magnetic damping in Co/NM bilayers in accordance with experimental data. Unlike Cu, Ag, and Au a particularly strong enhancement is obtained for Pd and Pt caps. This is attributed to the combined effect of the large spin-orbit couplings of Pd and Pt and the simultaneous presence of $d$ states at the Fermi level in these two metals. The calculated Gilbert damping constant also shows an oscillatory dependence on the thicknesses of both ferromagnetic and nonmagnetic parts of the investigated systems which is attributed to quantum-well states. Finally, the expression for contributions to the damping constant from individual atomic layers is derived. The obtained distribution of layer contributions in Co/Pt and Co/Pd bilayers proves that the enhanced damping which affects the dynamics of the magnetization in the Co film originates mainly from a region within the nonmagnetic part of the bilayer. Such a nonlocal damping mechanism, related to spin pumping, is almost absent in other investigated bilayers: Co/Cu, Co/Ag, and Co/Au.

The effects of optical phonon on the binding energy of bound polaron in a wurtzite ZnO/MgxZn1−xO quantum well

An improved Lee-Low-Pines intermediate coupling method is used to study the energies and binding energies of bound polarons in a wurtzite ZnO/MgxZn1−xO quantum well. The contributions from different branches of long-wave optical phonons, i.e., confined optical phonons, interface optical phonons, and half-space optical phonons are considered. In addition to electron-phonon interaction, the impurity-phonon interaction, and the anisotropy of material parameters, such as phonon frequency, electron effective mass, and dielectric constant, are also included in our computation. Ground-state energies, binding energies and detailed phonon contributions from various phonons as functions of well width, impurity position and composition are presented. Our result suggests that total phonon contribution to ground state and binding energies in the studied wurtzite ZnO/Mg0.3Zn0.7O quantum wells varies between 28–23 meV and 62–45 meV, respectively, which are much larger than the corresponding values (about 3.2–1.8 meV and 1.6–0.3 meV) in GaAs/Al0.3Ga0.7As quantum wells. For a narrower quantum well, the phonon contribution mainly comes from interface and half-space phonons, for a wider quantum well, most of phonon contribution originates from confined phonons. The contribution from all the phonon modes to binding energies increases slowly either when impurity moves far away from the well center in the z direction or with the increase in magnesium composition (x). It is found that different phonons have different influences on the binding energies of bound polarons. Furthermore, the phonon contributions to binding energies as functions of well width, impurity position, and composition are very different from one another. In general, the electron-optical phonon interaction and the impurity center-optical phonon interaction play an important role in electronic states of ZnO-based quantum wells and cannot be neglected.

Mechanism of strain-influenced quantum well thickness reduction in GaN/AlN short-period superlattices

We report on the mechanism of strain-influenced quantum well (QW) thickness reduction in GaN/AlN short-period superlattices grown by plasma-assisted molecular beam epitaxy. Density functional theory was used to support the idea of a thermally activated exchange mechanism between Al adatoms and Ga surface atoms that is influenced by the strain state of the GaN QWs. These ab initio calculations support our experimentally observed reduction in QW thickness for different intrinsic strains.

Topological transition and edge states in HgTe quantum wells from first principles

(HgTe)${}_{N}$(CdTe)${}_{M}$(110) and (001) superlattices are studied by means of $ab$ $initio$ calculations versus the thickness of the HgTe quantum wells (QWs). The used approximate quasiparticle theory including spin-orbit coupling (SOC) gives the correct band ordering, band gap, and SOC splitting for bulk HgTe and CdTe. The resulting band discontinuities indicate confinement also for occupied states. In agreement with earlier $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ calculations and experiments we find a topological transition from the topological nontrivial quantum spin Hall state into a trivial insulator with decreasing QW thickness. The spatial localization near the interfaces and the spin polarization are demonstrated for the edge states for QWs with thicknesses near the critical one. They do not depend on the QW orientation and are therefore topologically protected. Below the critical QW thickness, the trivial insulator exhibits drastic confinement effects with a significant gap opening. We show that the inclusion of inversion symmetry, the nonaxial rotation symmetry of the QWs, and the real QW barriers lead to some agreement but also significant deviations from the predictions within toy models. The deviations concern the critical thickness, the number and localization of edge states, and the possibility to find QW subbands between edge states.

Influence of free carriers on exciton ground states in quantum wells

The influence of free carriers on the ground state of the exciton at zero magnetic field in a quasi-two-dimensional quantum well that contains a gas of free electrons is considered in the framework of the random phase approximation. The effects of the exciton–charge-density interaction and the inelastic scattering processes due to the electron–electron exchange interaction are taken into account. The effect of phase-space filling is considered using an approximate approach. The results of the calculation are compared with the experimental data.

Voltage control of magnetic anisotropy in Fe films with quantum well states

The influence of a gate voltage on magnetic anisotropy is investigated in a thin Fe film epitaxially grown on a Ag(1,1,10) substrate and covered by MgO. Oscillations in step-induced magnetic anisotropy due to quantum well states (QWS) confined in the Fe film are observed and shown to persist up to room temperature at low Fe thicknesses. By systematically examining the voltage and thickness dependence of the magnetic hysteresis loop characteristics, we identify two distinct effects by which an applied voltage modifies the magnetic anisotropy. The first effect is due to voltage-induced changes to interfacial perpendicular magnetic anisotropy which, due to the vicinal geometry, leads to changes in the effective in-plane uniaxial magnetic anisotropy. A second effect is observed at lower film thicknesses and shows nonmonotonic voltage-induced effects on magnetic anisotropy. This nonmonotonic behavior coincides with the onset of significant QWS-induced effects on magnetic anisotropy and suggests a link between QWS- and voltage-induced anisotropy changes.

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.

Observation of spin-dependent quantum well resonant tunneling in textured CoFeB layers

We report the observation of spin-dependent quantum well (QW) resonant tunneling in textured CoFeB free layers of single MgO magnetic tunnel junctions (MTJs). The inelastic electron tunneling spectroscopy spectra clearly show the presence of resonant oscillations in the parallel configuration, which are related with the appearance of majority-spin Δ1 QW states in the CoFeB free layer. To gain a quantitative understanding, we calculated QW state positions in the voltage-thickness plane using the so-called phase accumulation model (PAM) and compared the PAM solutions with the experimental resonant voltages observed for a set of MTJs with different CoFeB free layer thicknesses (tfl = 1.55, 1.65, 1.95, and 3.0 nm). An overall good agreement between experiment and theory was obtained. An enhancement of the tunnel magnetoresistance with bias is observed in a bias voltage region corresponding to the resonant oscillations.

Finite size effects on helical edge states in HgTe quantum wells with the spin—orbit coupling due to bulk- and structure-inversion asymmetries

There is a quantum spin Hall state in the inverted HgTe quantum well, characterized by the topologically protected gapless helical edge states lying within the bulk gap. It has been found that for a strip of finite width, the edge states on the two sides can couple together to produce a gap in the spectrum. The phenomenon is called the finite size effect in quantum spin Hall systems. In this paper, we investigate the effects of the spin—orbit coupling due to bulk- and structure-inversion asymmetries on the finite size effect in the HgTe quantum well by means of the numerical diagonalization method. When the bulk-inversion asymmetry is taken into account, it is shown that the energy gap Eg of the edge states due to the finite size effect features an oscillating exponential decay as a function of the strip width of the HgTe quantum well. The origin of this oscillatory pattern on the exponential decay is explained. Furthermore, if the bulk- and structure-inversion asymmetries are considered simultaneously, the structure-inversion asymmetry will induce a shift of the energy gap Eg closing point. Finally, based on the roles of the bulk- and structure-inversion asymmetries on the finite size effects, a way to realize the quantum spin Hall field effect transistor is proposed.

Spin polarization of quantum-well and interface states of ultrathin films of Bi on w(110) with Ag interlayers

The electronic and spin structure of quantum-well and interface states, formed in the system, consisting of bilayer of Bi on 1 ML Ag/W(110) was investigated by angle- and spin- resolved photoelectron spectroscopy. It has been shown that interface states are formed in local surface-projected gap of W(110) and are characterized by spin polarization and spin-orbit splitting, corresponding to surface resonances with high density spin-polarized states near Fermi edge.

The role of quantum-well states and carrier scattering times in discontinuities of opto-electrical characteristics of SCH lasers

Drift-diffusion computer simulations model available in Synopsys’ Sentaurus TCAD is used to study electrical, I-V, and optical, I-L, characteristics of separate-confinement heterostructure laser based on AlGaAs. We investigate the role of the width and depth of Quantum Well (QW) active region, below and above the lasing threshold. The device properties depend on both, the number of bound QW states and on closeness of the highest bound states to conduction or valence band offset. The lasing action may not exist at certain widths or hights of QW, and the threshold current is a discontinuous function of these parameters. The effects are more pronounced at low temperatures. Discontinuities in characteristics are found, at certain conditions, in temperature dependencies as well. The carriers scattering time on QW is shown to have the crucial role on amplitude of discontinuities.

Effect of spin–orbit coupling on atomic-like and delocalized quantum well states in Au overlayers on W(110) and Mo(110)

The spin structure of quantum well states (QWSs) in three-monolayer-thick gold overlayers on W(110) and Mo(110) is studied experimentally by spin- and angle-resolved photoelectron spectroscopy and theoretically by ab initio calculations. The spin–orbit coupling effects manifest themselves differently in the spin structure of atomic-like d and delocalized sp QWSs depending on how strongly the states are influenced by the substrate. The QWSs of a d character show a strong spin polarization with an almost identical structure for W(110) and Mo(110), suggesting a weak interaction with the substrate, whereas for sp QWSs, the interaction is much stronger, which to a large extent determines their splitting and spin polarization. The theoretical model yields a qualitative agreement with the experiment and explains the observed behavior.

Quantum confinement in oxide quantum wells

Abstract

Tuning gap states at organic-metal interfaces via quantum size effects

Organic-metal interfaces are key elements in organic-based electronics. The energy-level alignment between the metal Fermi level and the molecular orbital levels determines the injection barriers for the charge carriers at the interfaces, which are crucial for the performance of organic electronic devices. Dipole formation at the interfaces has been regarded as the main factor that affects the energy-level alignment. Several models have been proposed for the mechanism of dipole formation in the context of the interface between organic molecules and a bulk metal crystal surface, at which surface states were mostly used to probe the interfacial properties. Here we report that when the bulk metal crystal is replaced by a uniform metal thin film, the resulting two-dimensional quantum-well states will be able to not only probe but also modify the interfacial electronic structures, such as gap states, that have no counterpart at the organic-bulk crystal interface.

Controlled Interaction of Surface Quantum-Well Electronic States

We report on the construction of well-defined surface quantum well arrangements by combining self-assembly protocols and molecular manipulation procedures. After the controlled removal of individual porphyrin molecules from dense-packed arrays on Ag(111), the surface state electrons are confined at the bare silver patches. These act as quantum wells that show well-defined unoccupied bound surface states. Scanning tunneling spectroscopy and complementary boundary element method calculations are performed to characterize the interaction between the bound states of adjacent quantum wells and reveal a hybridization of wave functions resulting in bonding and antibonding states. The interwell coupling can be tuned by the deliberate choice of the molecules acting as potential barriers. The fabrication method is shown to be ideally suited to engineer specific configurations as one-dimensional chains or two-dimensional artificial molecules.

Efficiency droop and incomplete carrier localization in InGaN/GaN quantum well light-emitting diodes

A direct correlation between efficiency droop and broadening of emission spectrum of InGaN/GaN quantum wells (QWs) with increasing current density is found. A model of incomplete carrier localization in InGaN/GaN QWs is proposed. At low injection, the strong carrier localization and high-energy cutoff of emission spectrum results from fast carrier energy relaxation due to carrier hopping between localized tail states in QWs. At high level injection, the energy relaxation rate decreases due to the partial filling of tail states and high energy slope of the spectrum starts to determine by Boltzmann occupancy of tail states. This results in the incomplete carrier localization and the efficiency droop.

Single junction a-Si:H solar cell with a-Si:H/nc-Si:H/a-Si:H quantum wells

A single junction hydrogenated amorphous silicon (a-Si:H) solar cell with incorporating hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H (nc-Si:H/a-Si:H) multiple quantum wells (MQW) in the intrinsic region is proposed. Its theoretical realization has been made on the basis of existing quantum well solar cell models to determine the influence of the incorporated MQW over the conversion efficiency of a-Si:H solar cell. The atomic energy states and density of states of nc-Si:H/a-Si:H quantum wells (QW) are simulated at various QW widths. Based on the simulated results the absorption coefficient of nc-Si:H/a-Si:H QW is estimated and these values along with the evaluated atomic energy states of QW are used to optimize the QW width for the designed quantum well solar cell. The optimum well width is found to be 10nm, where the absorption coefficient increases in three steps starting from 1.0×104cm−1 to 3.0×104cm−1 for photon energy of 1.2eV to 1.4eV, respectively, and thereafter remained constant for higher energies. The estimated absorption coefficient of 10nm QW is involved in the calculations of photocurrent density to study the effect of number of incorporated QW on short circuit current density (JSC), open circuit voltage (VOC) and efficiency (η) of a-Si:H solar cell. The JSC, VOC, and η are presented for up to 100 QW incorporated a-Si:H solar cell. The results show that the conversion efficiency of a-Si:H solar cell is directly related to the number of incorporated QW, which is mainly due to the growing short circuit current density with increasing number of QW. The current–voltage plot for 100-QW a-Si:H solar cell with intrinsic region of 5.5μm showed a peak power of 13.3mW/cm2.

Limits of the Transition Time Between Electron States in Metals and Quantum Wells Obtained from the Energy-Time Uncertainty Principle

Limits of the Transition Time Between Electron States in Metals and Quantum Wells Obtained from the Energy-Time Uncertainty Principle

Electron quantization in arbitrarily shaped gold islands on MgO thin films

Low-temperature scanning tunneling microscopy has been employed to analyze the formation of quantum well states (QWS) in two-dimensional gold islands, containing between 50 and 200 atoms, on MgO thin films. The energy position and symmetry of the eigenstates are revealed from conductance spectroscopy and imaging. The majority of the QWS originates from overlapping Au 6p orbitals in the individual atoms and is unoccupied. Their characteristic is already reproduced with simple particle-in-a-box models that account for the symmetry of the islands (rectangular, triangular, or linear). However, better agreement is achieved when considering the true atomic structure of the aggregates via a density functional tight-binding approach. Based on a statistically relevant number of single-island data, we have established a correlation between the island geometry and the gap between the highest-occupied and the lowest-unoccupied molecular orbital in the finite-sized islands. The linear eccentricity is identified as a suitable descriptor for this relationship, as it combines information on both island size and island shape. Finally, the depth of the confinement potential is determined from the spatial extension of QWS beyond the physical boundaries of the Au islands. Our paper demonstrates how electron quantization effects can be analyzed in detail in metal nanostructures. The results may help elucidating the interplay between electronic and chemical properties of oxide-supported clusters as used in heterogeneous catalysis.

Accurate solution of the eigenvalue problem for some systems of ODE for calculating holes states in polynomial quantum wells

We demonstrate an explicit numerical method for accurate solving the eigenvalue problem for some systems of ordinary differential equations, in particular, those describing electron and hole bound states in semiconductor quantum wells with polynomial potential profiles. Holes states are described by the Luttinger Hamiltonian matrix. For solving the eigenvalue problem we use the recurrent sequences procedure that makes possible to derive exact analytical expression for the eigenfunctions,. Hole bound states energies and corresponding wave functions are calculated in a finite parabolic quantum well as functions of the lateral quasimomentum component and parameters of the potential.