Occupied Subbands Research Articles

Magneto-photoluminescence in a type-II broken-gap n-GaInAsSb/p-InAs heterojunction

Magneto-photoluminescence in a single type-II broken-gap n-Ga0.94In0.06As0.13Sb0.87/p-InAs heterostructure with a 2D electron channel at the heterointerface containing two occupied electron subbands has been studied in the spectral range 0.3–0.8 eV in high magnetic fields of up to 10 T at low temperatures (T = 7 K). At photon energies in the range 0.5–0.8 eV, bulk photoluminescence from the layer of the n-GaInAsSb alloy was observed. In the low-energy part of the spectrum (0.3–0.45 eV), three narrow emission bands with photon energies hνa = 0.419 eV, hνb = 0.404 eV, and hνc = 0.384 eV and full widths at half-maximum FWHM = 4–7 meV were observed. These bands are due to radiative transitions of 2D electrons localized in the quantum well on the InAs side near the type-II heterointerface. The electron effective mass in the occupied subband E2 was estimated to be m2 = 0.027m0, which is close to the effective mass at the bottom of the InAs conduction band.

Type of the second hole subband on the Si(110) surface: Spin splitting anisotropy

Magnetoresistance measurements have been performed for the cases of one and two occupied size-quantization hole subbands in silicon field-effect transistors prepared on the Si (110) surface. In both cases, Shubnikov-de Haas oscillations exhibit very weak sensitivity to the in-plane component of the magnetic field. This indicates that both lowest subbands are formed by heavy holes. This conclusion disagrees with the wide-spread opinion that the second subband is the ground subband of light holes in the system under consideration.

Investigation of filling factor in In0.53Ga0.47As/In0.52Al0.48As quantum wells with two occupied subbands

The magnetic field dependence of filling factors has been investigated on InP based In053Ga047As/In052Al048As quantum well samples with two occupied subbands by means of magnetotransport measurements at the temperature of 15K in a magnetic field range of 0 to 13T. Under the condiction that Laundau-level broadening is larger than the spin splitting of each subband, filling factors are even when the splitting energy of two subbands is an integer multiple of the cyclotron energy, i.e.ΔE21=kωc. If the splitting energy of two subbands is half of an odd interger multiple of the cyclotron erergy, i.e.ΔE21=(2k+1)ωc/2, the filling factor is odd.

Electron transport properties of In0.53Ga0.47As/In0.52Al0.48As quantum wells with two occupied subbands

Magnetotransport properties of two-dimensional electron gas have been investigated for three In0.53Ga0.47As/In0.52Al0.48As quantum well samples having two occupied subbands with different well widths. When the intersubband scattering is considered, we have obtained the subband density, transport scattering time, quantum scattering time and intersubband scattering time, respectively, by analyzing the result of fast Fourier transform of the first derivative of Shubnikov-de Haas oscillations. It is found that the main scattering mechanism is due to small-angle scattering, such as ionized impurity scattering, for the first subband electrons.

Magnetosubbands of semiconductor quantum wires with Rashba spin-orbit coupling

We have investigated the electronic structure of Rashba spin-split quantum wires in a magnetic field. For our numerical calculations, a harmonic confinement was assumed. We find that wire structures with several occupied one-dimensional subbands still exhibit a beating pattern in the magnetoresistance. The wire width turns out to strongly affect the magnetic field values at which nodes occur in the beating pattern. In the limit of narrow wires, the beating pattern would vanish altogether because spin-split subbands become populated equally.

Bernstein modes in tunneling-coupled quantum wells

We investigate intraband plasmon excitations in modulation-doped tunneling-coupled quantum wells which can be tuned by a gate voltage. Utilizing far infrared transmission spectroscopy and grating coupler techniques we are able to observe the optical plasmon, i.e., the in-phase-oscillation of the electrons in the two subbands of the tunneling-coupled system. In a magnetic field we find a strong interaction with the harmonics of the cyclotron resonance which varies with the asymmetry of the density in the two subbands. Surprisingly one finds, in certain regimes, instead of an expected stronger splitting, that the splitting is even smaller than expected in a two-valley model with equally occupied subbands.

Transport Properties of 2DEGs in AlGaN/GaN Heterostructures: Spin Splitting and Occupation of Higher Subbands

To study the electronic transport properties of two-dimensional electron gases confined at the interfaces of AlGaN/GaN heterostructures, Shubnikov–de Haas (SdH) and Hall measurements were performed with structures covering a wide range of sheet carrier concentrations from 2.25 × 1012 to 1.83 × 1013 cm—2. For samples with sheet carrier concentrations above 1.7 × 1013 cm—2, the occupation of a second subband was observed. Fourier transformation was used to separate the contributions of both occupied subbands to the electronic transport. Quite similar quantum scattering times and effective masses of the electrons in the first and second subband were determined. In samples with lower sheet carrier concentration traces of a spin splitting could be found by investigating the angle dependence of the SdH-oscillations.

Theory of plasmonsin carbon nanotube bundles

We develop a suitable theoretical framework for studyingplasmons in nanotube bundles. In the plane perpendicular to thetubes, the nanotubes form a two-dimensional lattice. We use asimple model of free electron gas confined to arrays ofcylindrical surfaces, however the theoretical framework can alsobe applied to more sophisticated models for carbon nanotubes.Both intrasubband (classical) and intersubband (quantum)plasmons in such nanotube bundle systems are studied. Analytical solutions have been obtained for the case where onlyone subband is occupied, while numerical solutions have beenobtained for the case of many occupied subbands. IntertubeCoulomb coupling is found to play an intricate role, as it canboth harden and soften plasmon modes in the same nanotubebundle. Intertube Coulomb coupling is also responsible forsizable plasmon dispersions in the transverse plane. Allplasmons are found to be undamped by the corresponding singleparticle type electron-hole pair excitations. The classicalplasmon exhibits three-dimensional character in the longwavelength limit, and one-dimensional character in the shortwavelength limit. For quantum plasmons, the plasmon energy canbe significantly larger than the corresponding single particleexcitation energy between subbands. This feature is similar toquantum plasmons in semiconductor quantum wire systems.

From two-dimensional to three-dimensional quantum dots

Laterally defined quantum dot structures have been fabricated on the basis of Al x Ga 1− x As parabolic quantum wells which allow the occupation of more than one subband in growth direction. Magneto-Coulomb oscillations allow the determination of a gate parameter regime where the states of the second subband are occupied in the quantum dot. The occupation of the second subband in the dot comes along with fluctuations in the conductance peaks at zero magnetic field which we characterize with time dependent measurements. We discuss the possibility that the fluctuations are an intrinsic property of the dot at the threshhold to two occupied subbands.

Optical study of the one-dimensional electron gas in cleaved-edge-overgrown semiconductor quantum wires

Cleaved-edge overgrown semiconductor quantum wires containing a one-dimensional electron gas with few occupied subbands are probed by optical experiments. Inter-sub-band excitation modes are observed by resonant inelastic light scattering at energies that agree well with theoretical estimates obtained within the Fermi-liquid model. Narrow spectral line-shapes indicate high uniformity and long electron scattering lengths that are consistent with recent transport measurements.

Calculation of plasmonic dispersion in quantum wires: angular degeneracy and shape-confinement effects

We present a random-phase approximation calculation of the collective plasmon excitation spectrum of a one-dimensional electron gas confined in cylindrical and rectangular semiconductor quantum wires. The plasmon spectrum for the cylindrical case is calculated considering up to three occupied subbands. Taking into account the twofold angular degeneracy of the single-particle energy spectrum, we derive different expressions for the electron-electron interaction when opposite angular quantum numbers are involved. That occurrence leads, even in a two-subband system, to the presence of new undamped plasmon modes for q → 0 not exhibited in previous work. Comparing plasmon modes between cylindrical and rectangular quantum wires, we find an investigable shape-confinement effect for the intrasubband plasma excitation in the lowest subband.

Quasi-two-dimensional Shubnikov-de Haas effect

The theory of the Shubnikov-de Haas effect is generalized to the case of two-dimensional systems with several occupied size-quantization subbands. Possible interlevel scattering is taken into account. It is shown that the relative amplitudes of the Shubnikov-de Haas oscillations are determined not only by the occupancy of the subbands but also by the intensity of intersubband transitions.

Collective interband excitations in the Raman spectra of quantum wires

A unifying theory for the collective interband excitations of the interacting electrons in a quantum wire with several occupied subbands is developed by using the bosonization method. Interband spin- and charge-density excitations for two subbands are treated in detail. The results are used to clarify the physical nature of the interband modes detected in recent Raman scattering experiments, especially the so-called single-particle excitations.

Landau-level interplay in InGaAs double quantum wells

Shubnikov–de Haas (SdH) and Hall measurements have been used to investigate a pair of adjacent two-dimensional electron gases (2DEGs) which were formed in two n0.53Ga0.47As quantum-wells, separated by a thin In0.52Al0.48As barrier, grown lattice-matched on InP. This double quantum-well system consists of two asymmetric InGaAs quantum wells, 9 nm and 7 nm respectively, separated by a 4.5 nm InAlAs barrier. The existence of two occupied electronic subbands with differing electron densities can clearly be identified by beating effects in the SdH oscillations. By applying a substrate bias the electron densities can be tuned and the beating is shifted. In the simultaneously performed Hall measurements additional features can be observed: Hall measurements with different total electron densities reveal plateaus for integer filling factors ν (with ν=ν1+ν2, ν1and ν2both integers, corresponding to the two subbands). Some even filling factors become suppressed and recover with changing electron density. Also, for some densities an odd filling factor is observed. The systematic tuning of the electron densities via the application of a bias voltage to the front gate reveals two Landau fans, one for each electronic system, respectively, crossing each other. The electron densities for both electronic systems can be identified by analysing the SdH spectra. As a function of the front-gate voltage, these densities seem to show evidence for an anticrossing of the two electronic states and therefore for a strong coupling between the states.

Magneto-optical oscillations in the photoluminescence of quantum wells and the resonant polaron effect

Magneto-optical oscillations in the intensity of photoluminescence transitions in AlGaAs/InGaAs/GaAs asymmetric quantum wells with two occupied electronic subbands have been studied. The minima of the oscillations for the first excited electronic subband occur at resonant polaron conditions (ℏωLO=Nℏωc) in which the emission of LO phonons is strongly enhanced and are due to an increase of the LO phonon scattering of electrons near the Fermi level, which is confirmed by the increase verified in the photoluminescence linewidth at resonant polaron conditions. This effect can also explain the small difference in phase observed between the magneto-optical and the Shubnikov–de Haas oscillations.

Weak localization in p-type quantum wells

A weak-localization theory is derived for quantum heterostructures with strong spin-orbit interaction that predicts anomalous magnetoresistance. This theory treats real quantum wells with a few occupied quantum-well subbands. It is shown that in the presence of intense elastic transitions between these subbands the parameters that define the conductivity in classically weak magnetic fields are averaged efficiently. In the opposite limiting case, all the subbands give independent contributions to the anomalous magnetoresistance. Relevant characteristic magnetic fields are calculated for arbitrary ratios between the times for phase breaking and intersubband transitions.

Multiple subband crossing in a one-dimensional hole gas with enhanced g-factors

We have measured the transport properties of one-dimensional ballistic constrictions defined in a two-dimensional hole gas. The conductance quantization of the constrictions switches between even and odd multiples of e 2/ h as a function of a parallel magnetic field, as spin-splitting causes the subbands of the 1D channel to cross repeatedly. The in-plane g-factors of the 1D subbands are measured from the low field data. We find them to increase as the number of occupied 1D subbands decreases and attribute this enhancement of the g-factor to Coulomb interactions. The quantization at high fields shows that the subbands have crossed four times; in this regime the system is highly magnetized and exchange effects should be of particular importance.

Closely separated one-dimensional wires:: coupled ballistic conduction, wave function hybridization and compressibility investigations

We report the experimental study of a system comprising two parallel quasi-one-dimensional (1D) ballistic wires formed in a GaAs-Al x Ga 1− x As double quantum well structure. A unique surface gate geometry allows the device to be tuned smoothly from a configuration comprising a pair of vertically aligned 1D ballistic wires, to a single 1D ballistic wire in either the upper or lower 2DES. We investigate coupled ballistic conduction in two samples with contrasting wire separations (35 and 300 Å). The strongly coupled sample exhibits a cross-over from double to single wire conduction as the number of occupied 1D subbands is reduced. The weakly coupled sample is used to observe directly variations in the compressibility of a single ballistic channel arising from the discrete 1D density of states.

The strong Stark effect of intersubband transitions in corrugated lateral surface superlattices

Intersubband electro-absorption and electro-refraction index spectra of GaAs/AlAs corrugated lateral surface superlattices (CLSSLs) are calculated. Large shifts of intersubband transition energies and Fermi energy levels in the electron occupied subbands induced by applied electric fields result in strong intersubband Stark effects and large refractive index variations in the CLSSLs that we considered with moderate electric fields . Strong intersubband absorptions with normally incident light due to the corrugated interfaces of the CLSSLs which break the in-plane translational symmetry of the systems are predicted. Broadenings of the intersubband electro-absorption and electro-refraction index spectra caused by structural fluctuations of the CLSSLs are discussed.

Theory of luminescence spectra fromδ-doping structures: Application to GaAs

A general procedure for calculating luminescence spectra from $\ensuremath{\delta}$-doping structures of semiconductors is developed. The electron and hole states are self-consistently calculated within the eight-band Kane model. Explicit results are obtained for $p$-type $\ensuremath{\delta}$-doping wells and superlattices in GaAs. For a prototype superlattice (SL) it is demonstrated how the luminescence spectra of $\ensuremath{\delta}$-doping structures depend on their self-consistent potentials, band structures, and oscillator strengths of radiative recombination processes between extended electron and confined hole states. Wave-vector conserving (direct) and nonconserving (indirect) transitions are considered. Luminescence spectra are calculated for a series of $p$-type $\ensuremath{\delta}$-doping SL's, varying their sheet doping concentrations, doping spreads, and periods. A comparison with experimental spectra shows that direct transitions may be ruled out. The indirect spectra are dominated by an emission band below the gap whose structures reflect the various occupied hole subbands. Increasing the temperature, the calculated hole emission bands become stronger, in contrast with experiment. This discrepancy is solved by means of a photoinduced electron confinement.