Conduction Subbands Research Articles

Radiative phonon-assisted and Auger recombination in Si nanocrystals

Recent analysis of the literature shows that the photoluminescence (PL) of Si nanocrystals and porous silicon is caused by phonon-assisted exciton radiative recombination, as well as by direct radiative electron transfer from the second to the first conduction sub-band, which is related to the Auger recombination. The PL decay curve for porous silicon after excitation with ultraviolet laser pulse has been established experimentally. We have constructed continuity equations for the first and the second conduction sub-bands, including radiative phonon-assisted exciton recombination, Auger recombination and direct radiative transition from the second to the first conduction sub-band. The solution of these equations yields the theoretical PL decay curve. These equations contain adjustable parameters. We have estimated their values by using the least-squares fit of the theoretical curve to the experimental data. We have shown that in our case both radiative components are approximately equal at the beginning, but the relative part of the phonon-assisted recombination increases with time and finally tends to 100%. Using the obtained values of the fit parameters, we have constructed the decay curves at different excitation intensities. We conclude that, at a small excitation power, the decay curve is exponential, the phonon-assisted radiative exciton recombination is dominating, and quantum efficiency is close to one. At a high excitation power, the decay curve is approximately stretched-exponential, the direct radiative transitions from the second to the first conduction sub-band are dominating, and quantum efficiency is about 1/3 for porous silicon sample.

Multiband model of the valence-band electronic structure in cylindrical GaAs nanowires

We compute the hole states in the GaAs free-standing nanowires, and in the GaAs/(Al,Ga)As core-shell nanowires of type I-s, which are grown along the [100] direction. The hole states are extracted from the 4-band Luttinger-Kohn Hamiltonian, which explicitly takes into account mixing between the light and heavy holes. The axial aproximation is adopted, which allowed classification of states according to the total angular monentum (fz when expressed in units of the Planck constant). The envelope functions are expanded in Bessel functions of the first kind. The dispersion relations of the subbands E(kz) obtained by the devised method do not resemble parabolas, which is otherwise a feature of the dispersion relations of the conduction subbands. Furthermore, the energy levels of holes whose total orbital momentum is fz=1/2 are shown to cross for a free-standing wire. The low energy fz=1/2 states are found to anticross, but these anticrossings turn into crossings when the ratio of the inner and outer radius of the core-shell wire takes a certain value. The influence of the geometric parameters on the dispersion relations is considered for both free standing and core-shell nanowires.

New methods for enhancing ZT value of nanowires

We report an important modulating factor, degeneracy, for ZT value enhancement of nanowire. Three approaches to design nanowire of large ZT value by degeneracy effects are proposed: (1) controlling growth direction, (2) increasing the number of near degenerate conduction (valence) subbands induced by quantum confinement and (3) searching for bulk materials with two (or more) sets of near degenerate conduction (or valence) bands to fabricate nanowires.

Application of band theory to experimental eigen-state energies of In0.53Ga0.47As quantum wells lattice-matched to InP

Nonparabolic subband structure of InGaAs/InAlAs quantum wells (QWs) was studied theoretically and experimentally. In this paper, nonparabolic effective masses of electrons and band offset of InAlAs barriers were experimentally deduced from eigen-states of conduction subbands confined within the two-dimensional InGaAs QWs. As the dispersion relations for electron, light hole and hole subbands were obtained using the envelope function approximation taking into account band nonparabolicity, a simple way to design was proposed.

Spin-sensitive intersubband dynamics of optically generated carriers in semiconductor quantum wells

A theoretical analysis of the intersubband dynamics in undoped quantum wells is presented where spin-oriented carriers are initially generated by circularly polarized interband excitations. Subsequent resonant intersubband excitations induce Rabi rotations between the subbands resulting in a fast periodic modulation of the spin orientation in each subband. It is investigated whether pure spin modulations can be performed where the charge density of the subbands is kept constant when initially equal occupations of the two lowest conduction subbands with opposite spin orientations have been prepared. While this is the expected behavior for a three-band model of noninteracting particles it turns out that when the Coulomb interaction is taken into account the spin modulations are typically accompanied by corresponding modulations of the subband occupations. It is demonstrated that under realistic conditions it should nevertheless be feasible to realize a pure spin rotation in a given subband provided the intersubband excitations are sufficiently short or the carrier density is sufficiently low. Successive spin rotations are shown to decrease the degree of spin polarization even when spin-relaxation processes are neglected.

Electronic structures of GaAs/AlGaAs concentric double rings in applied electric and magnetic field

The electronic structures of GaAs/Al 0.35Ga 0.65As concentric double rings are calculated based on the effective mass envelope function theory, with and without the applied electric and magnetic field along the growth direction. The Hamiltonian matrix elements are determined through the Fourier transform method. As the heterostructure evolves from a single ring to the concentric double rings, our simulation is performed on the bound state energies of the electron and the hole. The results show that the energy levels undulate with the evolution of the ring. The applied magnetic field increases the ground state energies both of the electron and of the hole, as well as the transition energy between the first conduction subband and valence subband. However, the electric field decreases the electronic energies linearly.

Design and analysis of a new structure of InAs/GaAs QDIP for 8–12 μm infrared windows with low dark current

Based on the effective mass Schrödinger equation and eight-band k.p method, we study optical absorption in the 8–12 µm infrared window through the conduction subbands of self-assembled InAs/GaAs pyramidal shape quantum dots (QDs) using the finite difference method. Considering homogeneous and inhomogeneous broadening effects, the absorption spectra of QDs are calculated. Regarding the simulation results, we design a QD infrared photodetector (QDIP) with a double barrier resonant tunnelling (DBRT) at λ = 10 µm. We calculate the responsivity, detectivity and dark current for the device, considering the coverage factor of QDs. To enhance the performance of the presented DBRT especially at near room temperature, we propose a modified QDIP with asymmetric multiple barrier resonant tunnelling structure. With the modified structure, the dark current reduces to about half an order of magnitude compared with DBRT–QDIP, and detectivity increases to 1.4 × 109 cm Hz1/2/W at 200 K.

Electronic band structure and optical properties of silicon nanoporous pillar array

Silicon nanoporous pillar array (Si-NPA) is fabricated by hydrothermally etching single crystal silicon ( c-Si) wafers in hydrofluoric acid containing ferric nitrate. Microstructure studies disclosed that it is a typical micron/nanometer structural composite system with clear hierarchical structures. The optical parameters of Si-NPA were calculated by general light-absorption theory and Kramers–Kronig relations based on the experimental data of reflectance and the variations compared with the counterparts of c-Si were analyzed. The features of the electronic band structure deduced from the optical measurements strongly indicate that Si-NPA material is a direct-band-gap semiconductor and possesses separated conduction sub-bands which accords with conduction band splitting caused by silicon nanocrystallites several nanometers in size. All these electronic and optical results are due to the quantum confinement effect of the carriers in silicon nanocrystallites.

Temperature dependence of the optical transitions in Ga0.64In0.36N0.046As0.954 multiquantum wells of various widths studied by photoreflectance

The temperature dependencies of optical transitions in as-grown Ga0.64In0.36N0.046As0.954 multiple quantum wells (QWs) of various widths (ranging from 3.9 to 8.1 nm) grown at the low temperature of 375 °C were studied by photoreflectance (PR). In addition to the 11H transition, which is the fundamental transition for this QW, the optical transitions between excited states (22H and 33H transitions, where the notation klH denotes the transition between the kth heavy-hole valence subband and the lth conduction subband) were clearly observed in the PR spectra. The temperature dependencies of the QW transition energies were analyzed using Varshni and Bose–Einstein expressions. It was found that with increasing temperature, both the ground-state and the excited-state transitions shift to the red without showing S-shape-type behaviors, which are typical for photoluminescence from this system. This shift does not depend on the QW width and amounts to ∼80 meV for the 11H transition and ∼100 meV for the 22H and 33H transitions in the temperature range of 10–300 K. These shifts are comparable to those of N-free QWs.

Spin states in InAs/AlSb/GaSb semiconductor quantum wells

We investigate theoretically the spin states in InAs/AlSb/GaSb broken-gap quantum wells by solving the Kane model and the Poisson equation self-consistently. The spin states in InAs/AlSb/GaSb quantum wells are quite different from those obtained by the single-band Rashba model due to the electron-hole hybridization. The Rashba spin splitting of the lowest conduction subband shows an oscillating behavior. The D'yakonov-Perel' spin-relaxation time shows several peaks with increasing the Fermi wave vector. By inserting an AlSb barrier between the InAs and GaSb layers, the hybridization can be greatly reduced. Consequently, the spin orientation, the spin splitting, and the D'yakonov-Perel' spin-relaxation time can be tuned significantly by changing the thickness of the AlSb barrier.

A theoretical study of laser structures based on dilute-nitride InAsN for mid-infrared operation

New dilute-nitride InAsN/GaSb laser diodes on an InAs substrate with a ‘W’ or ‘M’ design are theoretically investigated using a ten-band k ⋅ p model including valence, conduction and nitrogen-induced bands. For these laser diodes, designed to operate at 3.3 µm at room temperature, optical transition matrix elements for TE and TM modes between the valence sub-bands and the conduction sub-bands, modal gain and total threshold current densities are calculated. Under the hypothesis of a total loss coefficient α = 50 cm−1, non-conventional ‘W’ and ‘M’ multiquantum well laser structures present a calculated threshold current density Jth lower than 1.1 kA cm−2.

Spin-polarized edge and transport in graphene nanoscale junctions

By simulating spin transport through graphene nanoscale junctions, which have been fabricated recently, we show that the junction can conduct a spin-polarized current even without an external field. Conduction through the conduction subband results in an opposite type of spin filtering from conduction through the valence subband. The junction is magnetic with a nonzero total spin that depends on the junction geometry. The qualitative conclusions remain the same no matter a one-side armchair edge junction or a zigzag edge junction is considered.

Variational Rashba splitting in two-dimensional electron gases in III-V semiconductor heterojunctions

Control of the Rashba spin-orbit coupling in semiconductor two-dimensional electron gases (2DEGs) is of fundamental interest to the rapidly evolving semiconductor spintronics and depends on the detailed knowledge of the controversial interface and barrier penetration effects. Based on the $8\ifmmode\times\else\texttimes\fi{}8$ $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Kane model for the bulk, we propose a spin-dependent variational solution for the conduction subbands of III-V heterojuctions, which reveals analytically the different contributions to the Rashba splitting and its dependency on heterostructure and band parameters as the band offset and effective masses. Perturbation expansions are used to derive renormalized parameters for an effective, simple, and yet accurate one band model. Spin-dependent modified Fang-Howard trial functions, which satisfy the spin-dependent boundary conditions, are then introduced. The subband splitting is given as a function of the variational parameter which is obtained minimizing the total energy of the 2DEG. Our calculations applied to InAlAs/InGaAs heterojunctions, where a near 20% increase in the splitting is observed due to the barrier penetration, are in good agreement with both experiment and exact numerical calculations. Well-known expressions in the limit of a perfect insulating barrier are exactly reproduced.

Effect of a step quantum well structure and an electric-field on the Rashba spin splitting

Spin splitting of conduction subbands in Al0.3Ga0.7As/GaAs/AlxGa1-xAs/Al0.3Ga0.7 As step quantum wells induced by interface and electric field related Rashba effects is investigated theoretically by the method of finite difference. The dependence of the spin splitting on the electric field and the well structure, which is controlled by the well width and the step width, is investigated in detail. Without an external electric field, the spin splitting is induced by an interface related Rashba term due to the built-in structure inversion asymmetry. Applying the external electric field to the step QW, the Rashba effect can be enhanced or weakened, depending on the well structure as well as the direction and the magnitude of the electric field. The spin splitting is mainly controlled by the interface related Rashba term under a negative and a stronger positive electric field, and the contribution of the electric field related Rashba term dominates in a small range of a weaker positive electric field. A method to determine the interface parameter is proposed. The results show that the step QWs might be used as spin switches.

Phonon renormalization in doped bilayer graphene

We report phonon renormalisation in bilayer graphene as a function of doping. The Raman G peak stiffens and sharpens for both electron and hole doping, as a result of the non-adiabatic Kohn anomaly at the $\Gamma$ point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This results in a change of slope in the variation of G peak position with doping, which allows a direct measurement of the interlayer coupling strength.

Size-dependent impurity activation energy in GaN nanowires

The effect of the surrounding dielectric on the conductivity of GaN nanowires is measured experimentally. The two following configurations are considered: bare suspended and SiO2-coated nanowires. The measured conductivity is consistently fitted by two exponential terms with different activation energies, indicating multichannel conduction. The larger energy, attributed to activation of impurities into the conduction subband, shows essentially inverse dependence on nanowire radius, consistent with the dielectric confinement effect. This agrees with calculated values from finite element analysis. The smaller energy is independent of the nanowire radius, suggesting a surface conduction channel.

Lithographic quantum dot for sensitive infrared photon detection

AbstractSemiconductor quantum dots (QDs) with discrete energy levels provide a promising solution for infrared photon detection. A lithographically defined QD with split conduction subbands has been utilized as a photo‐active floating gate in a phototransistor structure. Electrons inside the QD are excited by incident infrared photons via intersubband transition and thereby tunnel out of the QD, lowering its electrostatic potential. As a consequence, photo‐signal arises as an electrical current increment in the phototransistor channel. It has been found that this though simple scheme exhibits a high sensitivity which is sufficient for detecting individual infrared photons (λ ∼ 14 μm).With continuous detection operation, more electrons are driven out of the QD. Eventually, the system approaches saturation due to over‐bended energy band profiles, therefore degrading the photo‐sensitivity. To alleviate this photo‐saturation effect, the QD electrostatic potential is controlled through a tuneable electrostatic potential barrier. Under illumination, the interplay between photo‐excitation and the potential barrier justifies the QD charge states, electrostatic potential and hence the channel current. It is found that, when the QD potential is externally lifted‐up, the device photo‐response can be enhanced clearly. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ferromagnet/superconductor heterostructures in graphene

We investigate the unusual features of the proximity effect in graphene-based ferromagnet/super-conductor (F/S) heterostructures. We show that, for a weakly doped ferromagnetic graphene with an exchange energy h exceeding its Fermi energy E F , the Andreev reflection of massless Dirac fermions at the FS interface is accompanied with a Klein tunneling through an exchange field p–n barrier between two spin-split conduction and valence subbands. This spin Andreev–Klein process leads to an anomalous exchange-field-induced increase of the amplitude of Andreev reflection which has significant consequences for the subgap conductance and the shot noise power of an SF junction. For h > E F , we find that the exchange field enhances the subgap conductance to reach a maximum value which is greater than the corresponding value for non-ferromagnetic structure at bias voltages e V < Δ / 2 . We further demonstrate that, due to the spin Andreev–Klein process, the Josephson coupling in a ferromagnetic graphene Josephson junction can be long-ranged, and that it survives at very large exchange fields h ≫ E F . The possibility for transitions between 0 and π Josephson coupling by varying the doping of the ferromagnetic contact is also demonstrated.

Electromodulation spectroscopy of the ground and excited state transitions in GaInN/AlInN multi-quantum wells

Contactless electroreflectance (CER) has been applied to investigate energies and intensities of optical transitions in GaInN/AlInN multi-quantum wells (MQWs) with the various QW widths (1.55 and 1.8nm) and barrier thicknesses (3 and 6nm). In addition to the ground state transition, optical transitions related to excited states (2H–1C and 1H–2C transitions, where kH–lC denotes a transition between the kth valence and the lth conduction subbands) have been observed in CER spectra. It has been found that the intensities of 2H–1C and 1H–2C transitions (i.e., transitions which are forbidden for non-polar square-like QWs) are comparable with the intensity of the ground state transition. In addition, it has been observed that the relative intensities of QW transitions depend on both the QW width as well as the barrier thickness. This experimental observation has been confirmed by theoretical calculations performed in the framework of the electron effective mass approximation model taking into account polarization effects in wurtzite III-N alloys. The calculations clearly show that the intensity of QW transitions for GaInN/AlInN MQWs strongly varies with the QW width as well as the barrier thickness.

Andreev-Klein reflection in graphene ferromagnet-superconductor junctions

We show that Andreev reflection in a junction between ferromagnetic (F) and superconducting (S) graphene regions is fundamentally different from the common FS junctions. For a weakly doped F graphene with an exchange field $h$ larger than its Fermi energy ${E}_{F}$, Andreev reflection of massless Dirac fermions is associated with a Klein tunneling through an exchange field $p\text{\ensuremath{-}}n$ barrier between two spin-split conduction and valence subbands. We find that this Andreev-Klein process results in an enhancement of the subgap conductance of a graphene FS junction by $h$ up to the point at which the conductance at low voltages $eVl\ensuremath{\Delta}/\sqrt{2}$ is greater than its value for the corresponding nonferromagnetic junction. We also demonstrate that the Andreev reflection can be of retro or specular types in both convergent and divergent ways with the reflection direction aligned, respectively, closer to and farther from the normal to the junction as compared to the incidence direction.