Valley-mixing Effects Research Articles

Temperature effect on the magnetic oscillations in 2D materials

We study the magnetic oscillations (MO) in 2D materials with a buckled honeycomb lattice, considering a perpendicular electric and magnetic field. At zero temperature the MO consist of the sum of four sawtooth oscillations, with two unique frequencies and phases. The values of these frequencies depend on the Fermi energy and electric field, which in turn determine the condition for a beating phenomenon in the MO. We analyse the temperature effect in the MO by considering its local corrections over each magnetization peak, given by Fermi–Dirac like functions. We show that the width of these functions is related to the minimum temperature necessary to observe the spin and valley properties in the MO. In particular, we find that in order to observe the spin splitting, the width must be lower than the MO phase difference. Likewise, in order to observe valley mixing effects, the width must be lower than the MO period. We also show that at high temperatures, all the maxima and minima in the MO shift to a constant value, in which case we obtain a simple expression for the MO and its envelope. The results obtained show unique features in the MO in 2D materials, given by the interplay between the valley and spin.

Tight Binding Simulation of Quantum Transport in Interband Tunneling Devices

We have studied quantum transport in both Si and GaAs interband tunneling diodes (ITD's). In the simulation, a non-equilibrium Green's function method based an empirical tight binding theory has been used to take into account evanescent-wave matching at interfaces and realistic band structures. Comparison has been made between the results of our multiband (MB) model and those of conventional two-band (2B) model. As a result, it is found that the current–voltage (I–V) characteristics of the Si ITD have considerably smaller peak current density than the conventional 2B model, since our MB model reflects correctly the indirect gap band structure. On the other hand, in the GaAs ITD, there is small difference between the two models, because tunneling occurs between the conduction band and the valence band at F point. It is also found that the matching of evanescent electron modes is essentially necessary to include the valley-mixing effects at the tunneling interfaces.

Full multiband simulation of quantum electron transport in resonant tunneling devices

We present a formulation and calculations of multiband quantum transport of electrons in resonant tunneling diodes (RTDs) based on a non-equilibrium Green function theory. In the formulation we have used an empirical tight-binding method to include a realistic band structure, where evanescent-wave matching at heterointerfaces, Γ– X valley-mixing effects, and space charge effect are duly taken into account. Our results show that the current–voltage characteristics of a GaAs/AlAs double-barrier RTD has a larger current density than the conventional single band (SB) model since the latter is found to overestimate the decay constant in the barriers. Note that matching of evanescent electron modes essentially requires the inclusion of valley-mixing effects for GaAs/AlAs heterostructures owing to breaking of lattice-translational symmetry occurring at interfaces.

Multi-band simulation of quantum transport in resonant interband tunneling devices

We have analyzed quantum transport in InAs/AlSb/GaSb/AlSb/InAs (type II) double-barrier resonant interband tunneling diodes. Our procedure uses a tight-binding non-equilibrium Green's function method where an evanescent wave matching at heterointerfaces as well as a realistic band structure, i.e., the conduction-valence-band-mixing effects are duly taken into account. We have compared our results with experimental results and those of a two-band model which considers only the lowest conduction and the light-hole states. Our results show that current–voltage characteristics have extra current due to the significant heavy-hole mixing effects in the GaSb quantum well especially when the well width (Lw) is less than 12 mono layers. It should be noted that the matching of evanescent electron modes is essentially necessary to include the valley-mixing effects for the heterostructures, since breaking of a lattice-translational symmetry occurs at the interfaces.

Valley-Mixing Effects in AlAs/GaAs (001) Superlattices

The effects of ΓX-valley mixing in (AlAs)n(GaAs)n (001) superlattices (SLs) on the electronic properties are theoretically investigated versus the SL period (n = 1–25) and the band offsets. The calculations are based on the empirical sp3s* tight-binding model, which includes only nearest-neighbour interactions. The results show that the highest state of the valence band (VB) is always confined to the GaAs slabs, whereas the bottom state of the conduction band (CB) shows different behaviours as it is sensitive to band-mixing effects. It is due to these effects in the ultrathin-layer SLs (n ≤ 8) that the electrons become localized in the AlAs X-valley and the heterostructure becomes of type-II. Whereas, for thick-layer SLs (n ≥ 9), the GaAs wells become completely separated. As a result, the electrons are localized in the GaAs slabs and the SL is of type-I. The estimated critical layer thickness is about 26 Å (nc = 9), which is consistent with the photoreflectance experiments. The valley-mixing effects are shown to be efficient only for small conduction band offset (CBO ≤ 200 meV) and are essential for the existence of type-II behaviour. The relevance of our work to the photonic device applications is furtherly discussed.

Multiband quantum transport with Γ– X valley-mixing via evanescent states

Calculations of quantum transport of electrons through heterostructures are presented based on an empirical tight-binding model where an evanescent-wave matching at a heterointerface as well as the Γ– X valley-mixing effects are duly taken into account. Our results show, in particular, that current–voltage ( I– V) characteristics of a GaAs/AlAs/GaAs single barrier diode have extra structures associated with the X valley tunneling, which can illustrate the experimental results. It should also be noted that the matching of evanescent electron modes is essentially necessary to include the valley-mixing effects for GaAs/AlAs heterostructures, since a lattice-translational symmetry is lacking in the growth direction in such structures.

Valley mixing effects on electron tunneling transmission in GaAs/AlAs heterostructures

We propose a set of boundary conditions for electron envelope functions at GaAs/AlAs (001) heterointerfaces so as to take into account Γ- X mixing in the effective mass method. The proposed conditions enable one to obtain the dependence of the mixing effect upon the parity of monomolecular layer numbers in (GaAs) N (AlAs) M superlattices. The electron transmission spectra of GaAs(AlAs) M GaAs single barrier structures are calculated in the generalized effective-mass approximation. It is shown that the spectral fine structure depends essentially upon the parity of M. The low-temperature d.c. current-voltage characteristics of the single-barrier structure is derived taking into account the camel-back X-band structure in bulk AlAs and GaAs.

Extension of the Kronig–Penney model for Γ-X mixing in superlattices

An extension of the Kronig–Penney model to treat mixings of different valleys in superlattices is presented. The value of the mixing parameter α used in the model is deduced by comparison with more sophisticated theory and various experiments for the GaAs/AlAs material system. It is shown that with the use of a single value for the mixing parameter, this simple and efficient model can reasonably reproduce the important aspects of valley mixing effects in GaAs/AlAs superlattices over a wide range of layer thicknesses.

Valley-mixing effects in (GaAs)l(AlAs)m superlattices with microscopically imperfect interfaces.

We report the first full-scale calculation of the subband structure along the cubic axes of GaAs-AlAs superlattices with microscopically imperfect interfaces. We show that the valley-mixing effects can be used as a sensitive probe of the interface disorder whose characteristic dimension is of order of the bulk lattice constant. It would appear that our predictions concerning the effect of disorder on the optical spectra of small-period structures are in close agreement with recent experimental data.

Valley-mixing effects in short-period superlattices.

La structure de la sous-bande de (GaAs) N /(AlAs) M le long de la direction de la croissance [001] a travers le centre de la zone de Brillouin et a travers le point de joint de zone a (100), est calculee en utilisant une methode de liaison forte des seconds voisins