Subband Edge Research Articles

Conductance quantization in strongly disordered graphene ribbons

We present numerical studies of conduction in graphene nanoribbons with different types of disorder. We find that even when defect scattering depresses the conductance to values two orders of magnitude lower than $2{e}^{2}/h$, equally spaced conductance plateaus occur at moderately low temperatures due to enhanced electron backscattering near subband edge energies if bulk vacancies are present in the ribbon. This work accounts quantitatively for the surprising conductance quantization observed by Lin et al. [Phys. Rev. B 78, 161409(R) (2008)] in ribbons with such low conductances.

Spin Hall effect due to intersubband-induced spin-orbit interaction in symmetric quantum wells

We investigate the intrinsic spin Hall effect in two-dimensional electron gases in quantum wells with two subbands, where a new intersubband-induced spin-orbit coupling is operative. The bulk spin Hall conductivity sigma(z)(xy) is calculated in the ballistic limit within the standard Kubo formalism in the presence of a magnetic field B and is found to remain finite in the B=0 limit, as long as only the lowest subband is occupied. Our calculated sigma(z)(xy) exhibits a nonmonotonic behavior and can change its sign as the Fermi energy (the carrier areal density n(2D)) is varied between the subband edges. We determine the magnitude of sigma(z)(xy) for realistic InSb quantum wells by performing a self-consistent calculation of the intersubband-induced spin-orbit coupling.

Spin Hall Effect in Symmetric Wells with Two Subbands

We investigate the spin Hall conductivity σxyz of a clean 2D electron gas formed in a two-subband well. We determine σxyz as arising from the inter-subband induced spin–orbit (SO) coupling η (Calsaverini et al., Phys. Rev. B 78:155313, 2008) via a linear-response approach due to Rashba. By self-consistently calculating η for realistic wells, we find that σxyz presents a non-monotonic (and non-universal) behavior and a sign change as the Fermi energy varies between the subband edges. Although our σxyz is very small (i.e., ≪``e/4π″), it is non-zero as opposed to linear-in-k SO models.

Spin-32physics of semiconductor hole nanowires: Valence-band mixing and tunable interplay between bulk-material and orbital bound-state spin splittings

We present a detailed theoretical study of the electronic spectrum and Zeeman splitting in hole quantum wires. The spin-$\frac{3}{2}$ character of the topmost bulk-valence-band states results in a strong variation in subband-edge $g$ factors between different subbands. We elucidate the interplay between quantum confinement and heavy-hole--light-hole mixing and identify a certain robustness displayed by low-lying hole-wire subband edges with respect to changes in the shape or strength of the wire potential. The ability to address individual subband edges in, e.g., transport or optical experiments enables the study of hole states with nonstandard spin polarization, which do not exist in spin-$\frac{1}{2}$ systems. Changing the aspect ratio of hole wires with rectangular cross section turns out to strongly affect the $g$ factor of subband edges, providing an opportunity for versatile in situ tuning of hole-spin properties with possible application in spintronics. The relative importance of cubic crystal symmetry is discussed, as well as the spin splitting away from zone-center subband edges.

Orientation-dependent excitations of lattice soliton trains with hybrid nonlinearity

We demonstrate selective excitation of soliton trains residing in different gaps or within the same Bloch band of a new type of photonic lattice merely by changing the orientation of an input probe beam. A self-focusing and -defocusing hybrid nonlinearity as established in a nonconventionally biased photorefractive crystal leads to controlled soliton transitions from different band edges or subband edges, in good agreement with our theoretical analysis.

Landé-like formula for thegfactors of hole-nanowire subband edges

We have analyzed theoretically the Zeeman splitting of hole-quantum-wire subband edges. As is typical for any bound state, their $g$ factor depends on both an intrinsic $g$ factor of the material and an additional contribution arising from a finite bound-state orbital angular momentum. We discuss the quantum-confinement-induced interplay between bulk-material and orbital effects, which is nontrivial due to the presence of strong spin-orbit coupling. A compact analytical formula is provided that elucidates this interplay and can be useful for predicting Zeeman splitting in generic hole-wire geometries.

Tailoring hole spin splitting and polarization in nanowires

Spin splitting in p-type semiconductor nanowires is strongly affected by the interplay between quantum confinement and spin-orbit coupling in the valence band. The latter’s particular importance is revealed in our systematic theoretical study presented here, which has mapped the range of spin-orbit coupling strengths realized in typical semiconductors. Large controllable variations of the g factor with associated characteristic spin polarization are shown to exist for nanowire subband edges, which therefore turn out to be a versatile laboratory for investigating the complex spin properties exhibited by quantum-confined holes.

Zeeman splitting in cylindrical hole quantum wires

We have investigated the effect of one-dimensional (1D) confinement on the Zeeman splitting of holes in cylindrical quantum wires for magnetic fields applied parallel to the wire axis. Our theoretical study is based on the Luttinger–Hamiltonian description for the valence band in the spherical approximation. We obtain exact results for energies and Lande g-factors of hole states at the quasi-1D subband edges. The g-factor values are universal in the sense that they only depend on the subband index but not on the wire dimensions. Strong fluctuations are observed among the g-factors for low-lying subband edges, and their specific values are unexpected for a situation when the magnetic field is applied parallel to an axis of high symmetry that could naively be equated with the quantisation axis of total angular momentum. It appears that the requirement of a single-valued hole wave function associated with cylindrical symmetry induces a strong mixing between heavy and light holes, which results in the observed behaviour.

Effect of interface roughness on the density of states of finite barrier height quantum wells

We calculate the density of states of a 2D electron gas in finite barrier height quantum wells with the explicit inclusion of the interface roughness effect. By using Feynman path-integral method, the analytic expression is derived. The results show that the 2D density of states is dependent on the RMS of the fluctuation potential. The interface roughness causes localized states below the subband edge. We also apply the theory to model the finite barrier height quantum wells in Al x Ga 1− x As/GaAs.

Quantum confinement effects on the spin splitting and polarization of quasi-1D holes

We study spin splitting and polarization of hole subband edges in nanowires with cylindrical and square cross-section in the presence of a magnetic field applied along the wire direction. A general feature of the interplay between quantum confinement and the strong spin–orbit coupling in the valence band is that the g-factors of the hole–wire bound states fluctuate strongly as a function of level index. In addition, they are suppressed in comparison to g-factors found in higher-dimensional systems. We analyze the hole–spin properties of the wire levels in terms of invariants of the spin- 3 2 density matrix and discuss effects of confinement and geometry on the hole–spin polarization and its correlation to the g-factor values.

Band-Structure Effects in Ultrascaled Silicon Nanowires

In this paper, we investigate band-structure effects on the transport properties of ultrascaled silicon nanowire FETs operating under quantum-ballistic conditions. More specifically, we expand the dispersion relationship epsiv(kappa) in a power series up to the third order in kappa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and generate the corresponding higher order operator to be used within the single-electron Hamiltonian for the solution of the Schrodinger equation. We work out a hierarchy of nonparabolic models accounting for the following: 1) the shift of the subband edges and the change in the transport effective masses; 2) the higher order Hamiltonian operator; and 3) the splitting of the fourfold unprimed subbands in nanometer-size FETs. We then compute the device turn-on characteristics, the threshold shift versus diameter, and the subthreshold slope (SS) versus gate length. By compensating for the different threshold voltages, i.e., by reducing the turn- on characteristics to the same leakage current at zero gate bias, it turns out that the current discrepancies between the most general model and the bulk-parabolic model are contained within 20%. Finally, it turns out that the nonparabolic band structure gives an improved SS at the lowest gate lengths due to a reduced source-drain tunneling, reaching up to 30% enhancement.

Large variations in the hole spin splitting of quantum-wire subband edges

We study Zeeman splitting of zone-center subband edges in a cylindrical hole wire subject to a magnetic field parallel to its axis. The g-factor turns out to fluctuate strongly as a function of wire-subband index, assuming values that differ substantially from those found in higher-dimensional systems. We analyze the spin properties of hole-wire states using invariants of the spin-3/2 density matrix and find a strong correlation between g-factor value and the profile of hole-spin polarization density. Our results suggest possibilities for confinement engineering of hole spin splittings.

Dynamic conductance of a nanowire cross junction

The dynamic conductance of a nanowire cross junction is investigated with the scattering matrix approach including the effect of the displacement current. The intrawire conductance is basically inductive, but it is capacitative near to a quasibound state. The interwire conductance is basically capacitative, except near a quasibound state where it is inductive. In 9–18nm wires, interwire transmission of terahertz signals is suppressed for Fermi energy near to the first subband edge and the quasibound state below the second subband, thus limiting the junction’s operating speed. For these Fermi energies, the maximum operation frequency decreases with the increase in the wire diameter.

Quantum ballistic transport in nanowire junctions

The quantum ballistic transport of electrons through nanowire junctions formed by putting one nanowire (with square, rectangular, and circular cross sections) on top of another wire (the two wires are not lying in the same plane) is studied using tight-binding models and the Green's function approach. Different tight-binding models are considered to find the optimum number of tight-binding sites in the transverse plan for approximating the continuum model in a finite range of energy and also to find the minimal site number for qualitative description of the transport characteristics. Resonant dips and peaks found in the interwire and intrawire conductances can be explained by the formation of bound and quasibound states at the cross junction. How the conduction channels of the wires are coupled together in the formation of the bound and quasibound states are analyzed using the projected Green's functions. Quasibound states unbound in both wires give rise to resonant peaks in the interwire conductance as well as resonant dips in the intrawire conductance. Quasibound states unbound in only one wire give rise to only resonant dips in the intrawire conductance without corresponding resonant peaks in the interwire conductance. Some exception in the latter case is discussed. Reduction of interwire coupling strength is shown to suppress the conductance at energy far from the subband edges. In the weak interwire coupling regime, larger conductances are found at energies close to the subband edges. A comparison of the square and rectangular wire junctions studies is made. Increase in the dimension perpendicular to a square wire junction reduces the interwire conductance and enhances the intrawire conductance. On the other hand, degeneracy of subbands due to the higher symmetry of a square cross section enhances the interwire conductance. Comparison of circular wire junctions with square wire junctions show that some conductance features found in square wire junctions disappear in circular wire junctions owing to the weaker interwire coupling in circular wire junctions, which is the result of a smaller contact area between wires. The energies and probability densities of some bound and quasibound states are also determined and analyzed in the present study.

Band structure and intersubband absorption in modulation-doped V-groove quantum wires

A self-consistent nonparabolic calculation of the band structure and intersubband absorption in a V-groove-quantum-wire are presented and analyzed with respect to doping concentration, geometry, and temperature. A comparison with a parabolic flatband model shows that both the self-consistency and the nonparabolicity considerably affect subband edges and the intersubband absorption which are studied when both effects are taken into account. The absorption spectra exhibit a prominent peak in the terahertz region (wavelength range λX=55−95 μm) for the polarization in the direction of the weak confinement (X), while for polarization in the direction of the strong confinement (Y), it shows two groups of peaks, one in a slightly lower wavelength range than for the X polarization, and the second for λY=8−13 μm. Technological parameters considered in our analysis can be used to tailor and adjust the absorption spectra for various applications in the terahertz spectral range.

Intersubband Scattering in Double-Gate MOSFETs

Quantum mechanical features of even the most basic of semiconductor components can be expected to become of paramount importance for future nanoelectronics and quantum information technology. Here, we show that the conductivity of even a two-dimensionally extended double-gate silicon-on-insulator metal–oxide–silicon field-effect transistor can develop a distinct peak structure in its top-gate voltage dependence at low temperatures, due to the discreteness of subband edges resulting from quantum mechanical confinement in the vertical gating direction. Our findings are confirmed using low-temperature magnetic field measurements.

Many-body optical gain and intraband relaxation time of wurtzite InGaN∕GaN quantum-well lasers and comparison with experiment

The optical gain and the intraband relaxation time of wurtzite InGaN∕GaN QW lasers are investigated theoretically considering the non-Markovian gain model with many-body effects. Gain spectra are compared with those obtained from the experiment. The calculated intraband relaxation time is about 24fs at the subband edge and gradually increases with the energy. The intraband relaxation time is shown to be nearly independent of the sheet carrier density in an investigated range. The calculated gain spectra is in good agreement with the experiment. The intraband relaxation time τin obtained from fitting with experiment is 25fs. This value agrees well with the calculated value (∼24fs) at the subband edge, for which most of optical transitions occur.

Handling finite length signals borders in two-channel multirate filter banks for perfect reconstruction

Perfect reconstruction multirate filter banks theory was built on the assumption of infinite length signals. For processing limited support signals, e.g. an image, discontinuities exist at the signal border. The problem of which values to use at these boundaries cannot be ignored if perfect reconstruction is required. This paper presents our own investigation into the different techniques adopted to deal with this problem. It shows precisely what type of borders management should be applied and especially how, according to the underlying constraints to ensure perfect reconstruction two-channel multirate filter banks implementation on finite length signals with a zero-delay transfer function. This gives numerically the minimum number of extra samples to be added at the signal's and subband's edges; a vital issue for many filter-banks-based architectures like with the discrete wavelet transform (DWT).

Electron transport in crossed nano-wire junctions

Electron transport in a crossed nano-wire junction is studied using a tight-binding model. Increase in the inter-wire tunneling coupling strength suppresses the transmissions through the junction around the first subband edge. The junction's transport characteristics can be explained by the formation of bound or quasi-bound states at the junction.

Intraband relaxation time in InGaNAs quantum‐well lasers and comparison with experiment

AbstractThe intraband relaxation time is investigated theoretically for InGaNAs/InGaNAs/GaAs QW lasers. The result is compared with that obtained from comparison between the experimental gain spectra and theoretical gain. The InGaNAs/InGaNAs QW shows smaller intraband relaxation time than InGaAs/InGaAsP QW. This is due to the fact that the InGaNAs/InGaNAs structure has larger electron effective masses than the InGaAs/InGaAsP QW. The calculated gain spectra is in good agreement with the experiment. The intraband relaxation time τin obtained from fitting the experimental data is 60 fs. This value reasonably agrees well with the calculated value of 62 fs at the subband edge, considering that the electrons and the holes are mostly near the subband edge. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)