Magneto-absorption Experiments Research Articles

Dirac energy spectrum and inverted bandgap in metamorphic InAsSb/InSb superlattices

A Dirac-type energy spectrum was demonstrated in gapless ultrashort-period metamorphic InAsSb/InSb superlattices by angle-resolved photoemission spectroscopy (ARPES) measurements. The Fermi velocity value of 7.4 × 105 m/s in a gapless superlattice with a period of 6.2 nm is in good agreement with the results of magnetoabsorption experiments. An “inverted” bandgap opens in the center of the Brillouin zone at higher temperatures and in the superlattice with a larger period. The ARPES data indicate the presence of a surface electron accumulation layer.

In-plane spectrum in superlattices

We show that the existing theory does not give correct in-plane spectrum of superlattices at small in-plane momentum. Magnetoabsorption experiments demonstrate that the energy range of the parabolic region of the spectrum near the electron sub-band bottom is lower by an order of magnitude than the value predicted by the traditional approach. We developed a modified theory according to which the energy range of the parabolic region and carrier in-plane effective masses are determined by the effective band gap of the superlattice rather than by the bulk band gaps of the superlattice layers. The results of this theory are consistent with the experiment.

Determination of effective mass in InN by high-field oscillatory magnetoabsorption spectroscopy

We report on oscillatory magnetoabsorption experiments on several indium nitride (InN) epilayers under a pulsed magnetic field up to 56 T. This optical technique is uniquely suited to probe the bulk electronic structure in the vicinity of the fundamental band edge by Landau level spectroscopy. We discuss these results in light of two recent studies on the InN band structure by high-field magnetotransport [Goiran et al., Appl. Phys. Lett. 96, 052117 (2010)] and magneto-photoluminescence [Pettinari et al., Phys. Rev. B 79, 165207 (2009)]. In particular, we evidence a strong nonparabolicity of the conduction band and demonstrate that the electron effective mass ${m}_{e\ensuremath{\perp}0}^{*}=0.055{m}_{0}$ measured by Shubnikov--de Haas oscillations is confirmed by the magneto-optical experiments.

Motion-enhanced magnetic moments of excitons in ZnSe

Recent studies of excitons in wide quantum wells have shown that the magnetic properties are strongly affected as the excitons acquire kinetic energy. In the center of mass approximation, these motion-induced changes are ascribed to mixing between the $1S$ exciton ground state and the higher lying $nP$ states. The origin of the mixing is due to the dispersion curves for the valence band not being of simple parabolic form. Detailed previous studies of excitons in CdTe have resulted in excellent agreement between experiment and the predictions of this model. One consequence of the mixing is that the magnetic moment of the exciton is not simply the sum of the magnetic moments of the electron and hole, but contains motion-induced contributions, which can easily dominate the contributions from the individual charge carriers. To confirm the validity of the model, we have carried out detailed investigations of the magnetic properties of center of mass excitons in a second semiconductor, ZnSe, for which the magneto-optical properties of the individual charge carriers are completely different from those of CdTe. Excellent agreement is obtained between theory and experiment with a choice of the Luttinger parameter ${\ensuremath{\gamma}}_{3}=0.98$, in close agreement with the value determined independently by two-photon magnetoabsorption experiments. The success of the model when applied to both materials provides strong evidence that motion-induced changes in magnetism are a universal feature in zinc-blende semiconductors.

Long-Range Coulomb Interaction in Bilayer Graphene

We report on our studies of interacting electrons in bilayer graphene in a magnetic field. We demonstrate that the long-range Coulomb interactions between electrons in this material are highly important and account for the band asymmetry in recent optical magneto-absorption experiments [E. A. Henriksen, Phys. Rev. Lett. 100, 087403 (2008)10.1103/PhysRevLett.100.087403]. We show that in the unbiased bilayer (where both layers are at the same electrostatic potential), the interactions can cause mixing of Landau levels in moderate magnetic fields. For the biased bilayer (when the two layers are at different potentials), we demonstrate that the interactions are responsible for a change in the total spin of the ground state for half-filled Landau levels in the valence band.

MAGNETO-OPTICS OF QUANTUM DOTS IN THE NEAR FIELD

We examine a quantum dot (QD) illuminated in the near field with subwavelength spatial resolution, while simultaneously it is subjected to a magnetic field of variable orientation and magnitude. The magnetic field orientation can conserve or destroy the zero-magnetic-field ("structural") symmetry. The asymmetry induced by the magnetic field -except for specific orientations along symmetry axes- can be uncovered in the near-field (NF) but not in the far-field (FF) spectra. We predict that NF magnetoabsorption experiments of realistic spatial resolution could reveal the QD symmetry. This exceptional symmetry-resolving power of the near-field optics, is lost in the far field.

Near-field magnetoabsorption of quantum dots

We investigate the effect of an external magnetic field of variable orientation and magnitude (up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{T}$) on the linear near-field optical absorption spectra of single and coupled III-V semiconductor quantum dots. We focus on the spatial as well as on the magnetic confinement, varying the dimensions of the quantum dots and the magnetic field. We show that the ground-state exciton binding energy can be manipulated utilizing the spatial and magnetic confinement. The effect of the magnetic field on the absorption spectra, increasing the near-field illumination spot, is also investigated. The zero-magnetic-field ``structural'' symmetry can be destroyed varying the magnetic field orientation and this affects the near-field spectra. The asymmetry induced (except for specific orientations along symmetry axes) by the magnetic field can be revealed in the near-field but not in the far-field spectra. We predict that near-field magnetoabsorption experiments, of realistic spatial resolution, will be in the position to bring to light the quantum dot symmetry. This exceptional symmetry-resolving power of the near-field magnetoabsorption is lost in the far field. The influence of the Coulomb interactions on the absorption spectra is also discussed. Finally, we show that certain modifications of the magnetoexcitonic structure can be uncovered using a realistically acute near-field probe of $\ensuremath{\approx}20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Effectiveg*factor in the diluted nitridesGa1−yInyNxAs1−x

From the magnetoabsorption experiments we determined the Zeeman splitting and effective ${g}^{*}$ factors for the conduction and valence bands in GaInNAs alloys as a function of the nitrogen content up to 1.6%. Incorporation of N enhances the effective ${g}^{*}$ factor for the ${E}_{\ensuremath{-}}$ conduction band. In addition, we found that for the new ${E}_{+}$ conduction band, the effective ${g}^{*}$ factor is equal to the free electron one within the experimental error $({g}^{*}=2.8\ifmmode\pm\else\textpm\fi{}0.8)$. The band structure of GaInNAs in external magnetic field is analyzed within the six-level $\mathbf{k}∙\mathbf{p}$ model. The Hamiltonian in this model includes the anticrossing interaction between GaAs conduction band states and localized ${A}_{1}$ symmetry states induced by presence of nitrogen. The band anticrossing interaction increases substantially the ${E}_{\ensuremath{-}}$ conduction band electron ${g}^{*}$ factor for GaInNAs in comparison with the GaInNAs of the same energy gap. Our model explains quantitatively the dependence of the conduction band ${g}^{*}$ factor in GaInNAs on N content as well as ${g}^{*}$ factor values for other bands in GaInNAs. We also compare the conduction band effective mass dependence on N content calculated within our model with existing experimental and theoretical data.

Far-infrared study of an InAs-GaSb quantum well

We present FIR magneto-absorption experiments on InAs-GaSb heterojunctions. The results are discussed in terms of interband transitions and cyclotron resonances of a single electron (and a single hole) energy level. Fitting of the data to this model yields the position of these levels relative to the bottom of the quantum well.

Absorption saturation studies of Landau levels in quasi-two-dimensional systems

We report the first far-infrared linear and non-linear (saturation) magneto absorption experiments of coupled subband-Landau level excitations in a series of modulation-doped double-coupled multiple quantum-well (MQW) structures. Non-linear spectroscopy is carried out with a free electron laser. A plot of energy versus magnetic field for 4 laser lines at low intensity and high intensity is compared with linear (Fourier transform) spectroscopy at low and high temperatures. An exact numerical solution of the Schrödinger equation for this coupled system is also obtained and compared with experiments. Results show that the feature observed at high laser intensity and high temperature is due to transitions from the 1st-excited state of the coupled system to the higher excited states. Based on this, a simple 2-level model is developed, and with a layer-by-layer analysis for MQW systems, the electron lifetime of the first-excited state is obtained.

Band alignments and offsets in In(As,Sb)/InAs superlattices

The band alignments and band offsets were investigated for In(As,Sb)/InAs superlattices of various periods and compositions. Magnetoabsorption experiments allowed identification of subband energies and in-plane reduced masses. Using an 8\ifmmode\times\else\texttimes\fi{}8 k\ensuremath{\cdot}p band-structure calculation which takes account of nonparabolicity, valence subband mixing, and strain effects, we are able to fit calculated absorption curves to transition energies and reduced masses. Comparison of fits on several samples confirm that the electrons lie in the In(As,Sb) layer and the holes in the InAs layer. By using the fitted curve offsets, we are able to calculate the fractional conduction-band-offset parameter, ${\mathrm{Q}}_{\mathrm{c}}$, which for a type-II heterostructure can be greater than 1 and which we found to be 2.06\ifmmode\pm\else\textpm\fi{}0.11.

Saturation spectroscopy of hot carriers in coupled double quantum well structures

We report the first combination of far-infrared linear and nonlinear (saturation) magnetoabsorption experiments on coupled subband Landau level excitations in a series of modulation-doped multiple coupled double quantum well (MCDQW) structures. Linear spectroscopy was carried out with a Fourier transform spectrometer over a range of temperature, and saturation spectroscopy was carried out with the free electron laser at UC Santa Barbara. Based on theoretical analysis, an electron lifetime of approximately 1.2 ns was obtained.

Influence of Surface Step Density on the Growth of Mercury Cadmium Telluride by Molecular Beam Epitaxy

ABSTRACTWe report for the first time the results of structural and optical characterizations performed on Mercury Cadmium Telluride (MCT) grown simultaneously by Molecular Beam Epitaxy (MBE) on (11 1)B 4°misoriented and (211)B CdZnTe substrates. These two Te-terminated surfaces differ only by the density and the nature of their surface steps. In MBE conditions we do not observe any difference of incorporation between the two orientations at low growth temperatures and only a slight difference in Te and Hg incorporations above 200°C. This study confirms that the growth mode is essentially the same on both orientations and the growth proceeds by step-flow. The higher step density on (211) surfaces allows broader tolerances on the substrate temperature for the growth of twin-free material. It also leads to higher crystalline quality for the lowest temperatures. These observations have been confirmed by magneto-absorption experiments on superlattices grown simultaneously on both orientations.

Band offsets in strained InGaAsP/InGaAsP quantum well optical modulator structures

Magneto-absorption experiments on a range on In 1-x Ga x As y P 1-y /InGaAsP multi-quantum well (MQW) structures, including both tensile and compressive strained wells, are presented. Estimates for the band offsets are made in a lattice matched and a strain-balanced structure, and a model to predict the band offsets as a function of strain in InGaAsP heterostructures is described

Exchange interactions in CdFeTe semimagnetic semiconductors

The exchange splitting of the free exciton in diluted Cd 1- x Fe x Te ( x < 0.04) is studied by magnetoabsorption experiments carried out at T = 1.8 K, up to B = 7T. The four Zeeman-like components of the exciton spectrum are observed in circular polarization for magnetic field oriented along the principal crystallographic directions. The comparison between the energy splittings and the magnetization data, analyzed in terms of the crystal field model, yields accurate values for the conduction and valence exchange parameters: N 0 α = 0.30 ± 0.04 eV, N 0 β = −1.27 ± 0.08 eV.

Strain reconstruction of the valence band in Ga 1 − xIn xSb/GaSb quantum wells

We report measurements of the dispersion relation for holes in the M j= 3 2 strain split valence band in Ga − x In x Sb/GaSb quantum wells. The strain decoupling of the M j= 3 2 and M j= 1 2 levels gives the M j= 3 2 ∼ 0.07 m 0) for in-plane motion. The band is found to be highly non-parabolic, and the change in mass has been modelled directly using a full Luttinger-Kohn Hamiltonian calculation. Good agreement with experiment is achieved. Inter-band magneto-absorption experiments agreed with the low hole masses observed in cyclotron resonance and extended the study to higher energies in both the conduction and valence band.

Magneto-optical evidence of the exchange interaction in a Hg1-xMnxTe-CdTe superlattice.

Far-infrared magnetoabsorption experiments performed from 1.5 to 10 K in a ${\mathrm{Hg}}_{0.96}$${\mathrm{Mn}}_{0.04}$Te-CdTe superlattice are presented. The results, which are interpreted from band-structure calculations including magnetic field, show both exchange interaction and superlattice effects in such semimagnetic heterostructures.

Far-infrared magnetoabsorption study of weakly bound electrons in GaAs/AlxGa

Far-infrared magnetoabsorption experiments have been performed at 4.2 K on $\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ multiple-quantum-well (MQW) heterostructures that were selectively doped with Si donors in the centers of the quantum wells and barrier layers. Data were obtained for several values of dopant concentration in the barriers. The results provide strong evidence for the binding of electrons in the wells to positively charged ions in the barriers.

Absorption spectroscopy on Ga0.47In0.53As/Al0.48LIn0.52As multi-quantum-well heterostructures. I. Excitonic transitions.

The physical properties of excitons (linewidth, binding energy, oscillator strength, reduced effective mass) in ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As multi-quantum-well heterostructures (MQWH's) as a function of the well width and thus of the quasi-two-dimensional (2D) character of the respective MQWH were investigated by absorption and magnetoabsorption experiments. Samples lattice matched to InP with well widths ranging from 2.3 to 13.8 nm were grown by molecular-beam epitaxy. Double-crystal x-ray diffraction was used to determine the structural perfection and the individual layer thicknesses of the MQWH. The intrinsic statistical composition fluctuation of the ternary ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As well material is found to be the dominant contribution to the linewidth of the excitonic transition. With decreasing well width the binding energy of the electron--heavy-hole exciton increases up to 9\ifmmode\pm\else\textpm\fi{}2 meV for a well width of 3.4 nm. Similarly we observe a strong increase of the excitonic oscillator strength and of the reduced effective mass. A quasi-2D carrier system in this MQWH is realized for well widths of 3 to 5 nm.

Absorption spectroscopy on Ga0.47In0.53As/Al0.48In0.52As multi-quantum-well heterostructures. II. Subband structure.

The subband structure of ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As multi-quantum-well heterostructures (MQWH's) lattice matched to InP, grown by molecular-beam epitaxy, was determined by absorption and magnetoabsorption experiments for well widths ranging from 2.3 to 13.8 nm. A comparison to theoretical models (parabolic single-band and coupled six-band envelope-function approximations) reveals the necessity of taking the nonparabolicity of the conduction band as well as of all the valence subbands into account. The equal values for the reduced effective mass and the electron effective mass, obtained by Shubnikov\char21{}de Haas and cyclotron-resonance experiments, show that the highest valence-subband mass is much larger than the theoretical assumption from decoupled valence subbands. In view of the complete subband structure the best theoretical description is given by using a conduction-band offset \ensuremath{\Delta}${E}_{c}$=500 meV as determined by capacitance-voltage measurements in the literature. For this value the calculated subband transition energies are about 10% higher than the experimental results.