Heavy-hole Wave Functions Research Articles

Photoluminescence properties of GaAsBi single quantum wells with 10% of Bi

A set of single quantum well (SQW) samples of GaAs1-xBix with x ~ 0.1 and p-doped GaAs barriers grown by molecular beam epitaxy was investigated by the temperature-dependent photoluminescence (PL) spectroscopy. Those GaAsBi SQW structures showed a high crystalline quality, a smooth surface and sharp interfaces between the layers and exhibited a high PL intensity and a lower than 100 meV PL linewidth of QW structures. Temperature dependence of the optical transition energy was S-shape-free for all investigated structures and it was weaker than that of GaAs. An analysis of the carrier recombination mechanism was also carried out indicating that the radiative recombination is dominant even at room temperature. Moreover, numerical calculations revealed that a higher Be doping concentration leads to an increased overlap of the electron and heavy hole wave functions and determines a higher PL intensity.

Supercoupling between heavy-hole and light-hole states in nanostructures

Spintronics, quantum computing and quantum communication science utilizing cubic semiconductors rely largely on the properties of the hole states, composed of light and heavy hole wavefunction components. The admixture of light-hole (LH) into ground hole state predominately by the heavy hole (HH) would induce unique features of LH in optical transitions, spin relaxation, and spin polarization. We point to an unexpected source of HH-LH mixing in quantum dots, arguing that in contrast with current models the mixing does not reflect the strain between the dot and its matrix and does not scale inversely with the energy splitting between the bulk HH and LH states. Instead, we show via atomistic pseudopotential calculations on a range of strained and unstrained dots of different symmetries that the HH-LH mixing is enabled by the presence in the QD of a dense ladder of intermediate states between the HH and LH states which amplifies and propagates this interaction and leads to "supercoupling" (analogous to super-exchange in magnetism). This explains a number of outstanding puzzles regarding the surprising large coupling seen in unstrained QD (GaAs/AlAs) of ideal shapes and the surprising fact that in strained QD (InAs/GaAs) the coupling is very strong despite the fact that the 12-fold increase in bulk HH-LH splitting overrides the ~4 fold enhancement of the coupling matrix element by strain in comparison with unstrained GaAs QDs.

Cubic Rashba spin-orbit interaction of a two-dimensional hole gas in a strained-Ge/SiGe quantum well.

The spin-orbit interaction (SOI) of a two-dimensional hole gas in the inversion symmetric semiconductor Ge is studied in a strained-Ge/SiGe quantum well structure. We observe weak antilocalization (WAL) in the magnetoconductivity measurement, revealing that the WAL feature can be fully described by the k-cubic Rashba SOI theory. Furthermore, we demonstrate electric field control of the Rashba SOI. Our findings reveal that the heavy hole (HH) in strained Ge is a purely cubic Rashba system, which is consistent with the spin angular momentum m(j) = ± 3/2 nature of the HH wave function.

Influence of Strain-Reducing Layer on Strain Distribution of Self-Organized InAs/GaAs Quantum Dot and Redshift of Photoluminescence Wavelength

A systematic investigation about the strain distributions around the InAs/GaAs quantum dots using the finite element method is presented. A special attention is paid to influence of an In0.2Ga0.8 As strain reducing layer. The numerical results show that the horizontal- and vertical-strain components and the biaxial strain are reinforced in the InAs quantum dot due to the strain-reducing layer. However, the hydrostatic strain in the quantum dot is reduced. In the framework of eight-band k · p theory, we study the band edge modifications due to the presence of a strain reducing layer. The results demonstrate that the strain reducing layer yields the decreasing band gap, i.e., the redshift phenomenon is observed in experiments. Our calculated results show that degree of the redshift will increase with the increasing thickness of the strain-reducing layer. The calculated results can explain the experimental results in the literature, and further confirm that the long wavelength emission used for optical fibre communication is realizable by adjusting the dependent parameters. However, based on the calculated electronic and heavy-hole wave function distributions, we find that the intensity of photoluminescence will exhibits some variations with the increasing thickness of the strain-reducing layer.

Design of a novel periodic asymmetric intra-step-barrier coupled double strained quantum well electroabsorption modulator at 1.55 μm

A novel structure for electroabsorption modulator at 1.55μm with In(1−x−y)Ga(x)Al(y)As periodic asymmetric intra-step-barrier coupled double strained quantum well (AICD-SQW) is proposed. The intra-step-barrier in the structure provides a high optical saturation power due to delayed red shift. The asymmetric strained well layers reduce the oscillator strength at zero field, and as a result a lower insertion loss is obtained. This is due to the deep separation of electron and heavy hole wave functions. This structure shows that electroabsorption modulator (EAM) properties such as large change in absorption, high extinction ratio, large Stark shift, very low insertion loss, zero chirp, and higher figures of merit are possible to be achieved simultaneously as compared with intra-step quantum well (IQW). In numerical analysis, the exciton equation in momentum space is solved numerically, using Gaussian quadrature method (GQM), to obtain the exciton binding energy and oscillator strength. The asymmetric quantum well structure Hamiltonian is numerically solved by transfer matrix method (TMM), to obtain the electron and hole subband energy levels, taking the strain into account. The electroabsorption coefficient is calculated for different applied electric field for TE input light polarization. The EAM parameters such as extinction ratio, insertion loss, chirp and the figures of merit are calculated and the performances of AICD-SQW are compared with IQW.

Impact of the cap layer on the electronic structures and optical properties of self-assembledInAs∕GaAsquantum dots

Cap layer impact on the electronic structures and optical properties of self-assembled $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots is theoretically studied within the framework of Burt and Foreman's eight-band effective-mass Hamiltonian. A numerically stable finite difference scheme for this nonsymmetrized Hamiltonian and an efficient implementation of Jacobi-Davidson eigensolver for the resulting matrix are proposed. Our theoretical results show that as the cap layer thickness increases, the photoluminescence (PL) peak position exhibits a monotonous blueshift and the PL intensity enhances. These results are accounted for by the strain modified band edges and the space separation of electron and heavy-hole wave functions in the growth direction. Dot shape and size effects are also discussed. Our calculations are in good agreement with recent experimental findings.

Novel ZnTe‐based green light emitters grown on ZnTe substrates

Lattice matched ZnCdSeTe/(ZnMgSeTe quasi-quaternary) pn-junction green light emitting diodes (LED) grown on (001) ZnTe substrates were studied. The ZnMgSeTe quasi-quaternary layers, which consisted of stacked layers of very thin ZnSe and ZnMgTe layers, were introduced in order to obtain high carrier concentrations in the n-type layers. Two layer designs near the ZnCdSeTe active layers in the LEDs were compared to optimize the emission properties. Electrical properties from two LED structures showed no difference. On the other hand, total emission intensity from the modified LED structure was drastically improved by a factor over 30 under same operation conditions. Momentums of the electron and heavy hole wavefunctions inside the two LED structures were simulated to understand an experimentally observed difference of the emission properties. Results of the simulations showed that the modified LED structure has more efficient confinements of electrons and holes inside the ZnCdSeTe active layers by introducing thicker ZnSe and ZnMgTe layers neighboring the ZnCdSeTe active layer. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of interfacial atomic segregation and intermixing on the electronic properties of InAs/GaSb superlattices

Abrupt InAs/GaSb superlattices have In-Sb and Ga-As interfacial chemical bonds that are not present in the constituent materials InAs and GaSb. We study the effect of interfacial atomic mixing on the electronic structure of such superlattices, including electron and hole energies and wave function localization, interband transition energies, and dipole matrix elements. We combine an empirical pseudopotential method for describing the electronic structure with two different structural models of interfacial disorder. First, we use the ``single-layer disorder'' model and change in a continous way the composition of the interfacial bonds. Second, we study interfacial atomic segregation using a layer-by-layer kinetic model of molecular beam epitaxy growth, fit to the observed scanning tunneling microscopy segregation profiles. The growth model provides a detailed structural model of segregated InAs/GaSb superlattices with atomic resolution. The application of the empirical pseudopotential method to such structures reveals remarkable electronic consequences of segregation, among them a large blueshift of the band gap. This result explains the surprising gap increase with growth temperature observed for similar structures. In particular we find that (i) superlattices with only In-Sb interfacial bonds have lower band gaps (by 50 meV) than superlattices with only Ga-As interfacial bonds. (ii) Heavy-hole--to--electron transition energies increase with the number of Ga-As interfacial bonds more than light-hole--to--electron transition energies. (iii) The heavy-hole $\mathrm{hh}1$ wave functions show a strong localization on the In-Sb interfacial bonds. The heavy-hole wave functions have very different amplitudes on the Ga-As interface and on the In-Sb interface. (iv) Sb segregates at InAs-on-GaSb growth, whereas As and In segregate at GaSb-on-InAs growth, but Ga does not segregate. (v) The segregation of Sb and In induces a blueshift in the band gap. (vi) There is an in-plane polarization anisotropy due to the low symmetry of the no-common-atom InAs/GaSb superlattice. This anisotropy is reduced by interfacial segregation.

Effects of interfacial atomic segregation on optical properties of InAs/GaSb superlattices

Largely because of the lack of detailed microscopic information on the interfacial morphology, most electronic structure calculations on superlattices and quantum wells assume abrupt interfaces. Cross-sectional scanning tunneling microscopy (STM) measurements have now resolved atomic features of segregated interfaces. We fit a layer-by-layer growth model to the observed STM profiles, extracting surface-to-subsurface atomic exchange energies. These are then used to obtain a detailed simulated model of segregated InAs/GaSb superlattices with atomic resolution. Applying pseudopotential calculations to such structures reveals remarkable electronic consequences of segregation, including a blueshift of interband transitions, lowering of polarization anisotropy, and reduction of the amplitude of heavy-hole wave functions at the inverted interface.

Heavy-light hole mixing at zinc-blende (001) interfaces under normal incidence.

The reduced point symmetry ${\mathit{C}}_{2\mathit{v}}$ of a zinc-blende-based (001) interface allows mixing between heavy- and light-hole states even under normal incidence. We have generalized the envelope function approximation to take into account such a mixing by including off-diagonal terms into boundary conditions for the envelopes. The normal off-diagonal hole reflection from a GaAs/AlAs(001) heterointerface as well as \ensuremath{\Gamma}-point interband matrix elements in GaAs/AlAs multilayered structures have been calculated and the results have been compared with those obtained by pseudopotential and tight-binding calculations. The best fit with the numerical calculations gives for the dimensionless heavy-light hole mixing coefficient values ${\mathit{t}}_{\mathit{l}\mathrm{\ensuremath{-}}\mathit{h}}$=0.9 and 0.32. The theory of exchange splitting of excitonic levels in type II GaAs/AlAs superlattices has been extended to include not only the heavy-light hole mixing but also an admixture of spin-orbit-split states in the heavy-hole wave function. An agreement between theory and experiment for the anisotropic exchange splitting has been achieved for ${\mathit{t}}_{\mathit{l}\mathrm{\ensuremath{-}}\mathit{h}}$=0.5. A tight-binding model has been used to relate the microscopic parameters with coefficients in the boundary conditions for the hole envelope function. The tight-binding model estimation of ${\mathit{t}}_{\mathit{l}\mathrm{\ensuremath{-}}\mathit{h}}$=0.44 is in reasonable agreement with the other estimations of ${\mathit{t}}_{\mathit{l}\mathrm{\ensuremath{-}}\mathit{h}}$. \textcopyright{} 1996 The American Physical Society.

Stability of magnetic polaron states in two-dimensional semimagnetic heterostructures.

The problem of centrosymmetric-heavy-hole-state stability in a quantum well with semimagnetic barriers is considered. We show that the hole state centered in the middle of the well can become unstable during magnetic polaron formation. We found that in the case of an experimentally observed two-dimensional exciton magnetic polaron such a transition is prevented mainly due to the electron-hole Coulomb attraction. We established the fact that the criterion for centrosymmetric-hole-state stability is strongly influenced by value and orientation of the external magnetic fields applied to the system. A heavy-hole transition to the state attributed to one of two heterointerfaces is expected to occur in CdTe/(Cd,Mn)Te quantum wells at liquid-helium temperature in the presence of a magnetic field normal to the quantum-well plane. We found that the sensitivity of the heavy-hole wave function to the external electric field varies dramatically during magnetic polaron formation.

Spin-flip Raman scattering and photoluminescence in modulation doped CdTe/ Cd1−txMnxTe: In superlattices

Spin-flip Raman (SFR) scattering arising from the s-d exchange interaction between electrons trapped near interfaces and Mn 2+ ions in the Cd 1− x Mn x Te barrier has been observed in CdTe/Cd 1− x Mn x Te: In superlatti the SFR shift indicated a relatively small effect g-factor due to the finite penetration of the electron wavefunction into the barrier, but slow saturation behavior reflecting the high Mn 2+ concentration of the barrier layer. In contrast, the photoluminescence data show that the exciton recombination occurs basically from electrons and holes located near the center of the well where the wavefunctions of electrons and holes overlap most. In a superlattice having very narrow wells, the heavy hole wavefunction penetrates into the barrier and interacts with Mn 2+ ion leading to the large red shift in magnetic fields. The red shift shows an anisotropy as a consequence of strain and quantum confinement effect of holes. The intensity of the peak decreases rapidly as the hole moves toward the interface with increasing magnetic field.

Determination of the extension of the heavy-hole wave function in GaAs by photomodulation spectroscopy.

The k-space extension, \ensuremath{\Delta}k, of the heavy-hole wave function in GaAs is determined utilizing the photomodulation technique. In this technique, optical transitions between the usually full split-off and heavy-hole bands, are induced by continuously photogenerating electron-hole pairs. The observed absorption profile fits well a model that assumes that the heavy hole is weakly localized by charged centers. The localization length corresponding to \ensuremath{\Delta}${\mathit{k}}^{\mathrm{\ensuremath{-}}1}$ is 18 \AA{}.

Wannier-Stark wavefunction spectroscopy in GaAs/GaAlAs superlattices

We have studied the Wannier-Stark localization at intermediate electric field (∝15 kV/cm) in GaAs/BaAlAs superlattices by the photocurrent technique. In this field regime the electron wavefunction remains extended over several periods of the superlattice while the heavy hole wavefunction is already strongly localized in a single quantum well. This situation is most suitable to appreciate the electron coherence length and allows an electron wavefunction “spectroscopy”. We present also appropriate models explaining the relative ratio and field sensitivity of the varied transitions which are observed.