Two-dimensional Semiconductor Structures Research Articles

Quantum chaotic features of the spin-orbit coupled excitons in disordered two-dimensional insulators.

We study the synergy between disorder (phenomenologically modeled by the introduction of Riesz fractional derivative in the corresponding Schrödinger equation) and spin-orbit coupling (SOC) on the exciton spectra in two-dimensional (2D) semiconductor structures. We demonstrate that the joint impact of "fractionality" and SOC considerably modifies the spectrum of corresponding "ordinary" (i.e., without fractional derivatives) hydrogenic problem, leading to the non-Poissonian statistics of the adjacent level distance distribution. The latter fact is strong evidence of the possible emergence of quantum chaotic features in the system. Using analytical and numerical arguments, we discuss the possibilities to control the above chaotic features using the synergy of SOC, Coulomb interaction, and "fractionality," characterized by the Lévy index μ.

Spin orbit coupling enhanced thermionic cooling in two-dimensional semiconductor structures

We theoretically investigate the effect of Rashba spin–orbit coupling (RSO) on thermionics properties of two-dimensional semiconductor. Due to the hybridization of parabolic and linear energy dispersion, the thermal energy equipartition is modified. The thermionic emission from the lower spin branch is enhanced and that from the upper spin branch is reduced. For a double quantum well cooling structure under lower doping, the net heat transported through the structure is enhanced by RSO. The coefficient of performance (COP) is improved by about 20% in the low bias regime and around 3% in the high bias regime. Since the RSO can be further tuned by an applied field, the COP can be further optimized.

To the Theory of Dimensional Quantization in Narrow-Gap Crystals

This article discusses studies of size quantization phenomena in zero-, one-, and two-dimensional semiconductor structures. The main attention is paid to the mechanisms of photon-kinetic effects in these structures. Despite many studies of the physical properties of low-dimensional systems of current carriers, the size quantization of energy spectra in narrow-gap semiconductors and the associated photonic-kinetic effects are still insufficiently studied. Therefore, this study focuses on the quantum mechanical study of size quantization in certain cases using Kane's multiband model. The insolvability of the 8×8 matrix Schrödinger equation in the Kane model for a potential well of arbitrary shape is analyzed. The dependence of the energy spectrum on the two-dimensional wave vector is studied for various cases. In particular, the energy spectra for InSb and GaAs semiconductors are considered, depending on the band parameters and the size of the potential well. Conclusions are presented on the analysis of various cases of size quantization in narrow-gap crystals with cubic or tetrahedral symmetry in the three-band approximation. It is shown that the energy spectrum corresponds to a set of size-quantized levels that depend on the Rabi parameter, band gap, and well size. The size-quantized energy spectra of electrons and holes in InSb and GaAs semiconductors are analyzed in a multiband model.

Hydrogen storage performance enhancement and bandgap opening of M-Decorated (M = Li, Na and K) III4–V4 monolayer by fluorine functionalization

The effects of fluorine functionalization on the hydrogen storage capability of alkaline decorated III4–V4 monolayers is studied. This structure can store up to two hydrogen molecules per alkaline atom. Here, we demonstrate that functionalizing alkaline decorated monolayer with a high electronegative element of fluorine can significantly enhance both binding energy and the maximum number of the stored hydrogen molecules. In this regard, the hydrogen molecule storage capability is notably improved from two to four and when absolute binding is considered. In addition, F-functionalization of M-decorated III4–V4 can demonstrate bandgap opening effect, introduce semiconducting characteristics and forming a new two-dimensional semiconductor structure. The bandgap for Li–Al4P4–F is 1.23 eV which is very close to the solar peak. The resulted bandgap in the M–III4–V4-F structure is even significantly larger than that of pristine III4–V4 monolayer. The binding of the hydrogen molecule to the alkaline atom is also improved from 0.114 to 0.272 eV by the fluorine functionalization.

Influence of a quantizing magnetic field on the Fermi energy oscillations in two-dimensional semiconductors

In this article we investigated the effects of quantizing magnetic field and temperature on Fermi energy oscillations in nanoscale semiconductor materials. It is shown that the Fermi energy of a nanoscale semiconductor material in a quantized magnetic field is quantized. The distribution of the Fermi-Dirac function is calculated in low-dimensional semiconductors at weak magnetic fields and high temperatures. The proposed theory explains the experimental results in two-dimensional semiconductor structures with a parabolic dispersion law.

Calculation of the Fermi–Dirac Function Distribution in Two-Dimensional Semiconductor Materials at High Temperatures and Weak Magnetic Fields

This article investigated the effects of a quantizing magnetic field and temperature on Fermi energy oscillations in nanoscale semiconductor materials. It is shown that the Fermi energy of a nanoscale semiconductor material in a quantizing magnetic field is quantized. The distribution of the Fermi–Dirac function is calculated in low-dimensional semiconductors at weak magnetic fields and high temperatures. The proposed theory explains the experimental results in two-dimensional semiconductor structures with a parabolic dispersion law.

МОДЕЛИРОВАНИЕ ТЕМПЕРАТУРНОЙ ЗАВИСИМОСТИ КВАНТОВЫХ ОСЦИЛЯЦИОННЫХ ЭФФЕКТОВ В 2D ПОЛУПРОВОДНИКОВЫХ МАТЕРИАЛАХ

For the first time, a mathematical model was developed for determining the effect o f temperature and a quantizing magnetic field on oscillations o f the Fermi energy in nanoscale semiconductor structures with a parabolic dispersion law. A mathematical expression is derived for calculating the dependence o f the distribution o f the Fermi-Dirac function on the magnetic field, on the thickness o f the quantum well and on the temperature in low-dimensional semiconductor materials. The possibility o f calculating the Fermi energy oscillations in two-dimensional electron gases at high temperatures and weak magnetic fields is shown for the first time. The proposed theory explains the experimental results in two-dimensional semiconductor structures with a parabolic dispersion law.

Tunable Spectral Properties of Photodetectors Based on Quaternary Transition Metal Dichalcogenide Alloys MoxW(1-x)Se2yS2(1-y)

The ability to control the bandgap in two-dimensional graphene-like semiconductors is an essential task for the development of optoelectronic and nanoelectronic devices. Complex compositions alloys of transition metal dichalcogenides, such as Mo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2y</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2(1-y)</sub> , are the most optimal candidates for this purpose. Here we present spectrally selective photodetectors based on such quaternary transition metal dichalcogenides monolayers. It is shown that the spectral selectivity of optical detectors can be changed in a wide range if the composition of these quaternary monolayer transition metal dichalcogenides varies. This effect is directly related to the significant influence of the composition of such two-dimensional semiconductors on their bandgap. A theoretical model for estimation of quaternary transition metal dichalcogenides optical coefficients and optical detectors on their basis is proposed. The optical absorption simulation in created detectors was made, the results of which qualitatively coincided with the detectors performance. Thus, the obtained results can contribute to the development of valleytronics for two-dimensional semiconductor structures.

On Derivation of Dresselhaus Spin-Splitting Hamiltonians in One-Dimensional Electron Systems

Two-dimensional (2D) semiconductor structures of materials without inversion center (e.g. zinc-blende ${\rm A^{III}B^V}$) possess the zero-field conduction band spin-splitting (Dresselhaus term), which is linear and cubic in wavevector $k$, that arises from cubic in $k$ splitting in a bulk material. At low carrier concentration the cubic term is usually negligible. However, if we will be interested in the following dimensional quantization (in 2D plane) and the character width in this direction is comparable with the width of 2D-structure, then we have to take into account $k^3$-terms as well (even at low concentrations), that after quantization leads to comparable contribution that arises from $k$-linear term. We propose the general procedure for derivation of Dresselhaus spin-splitting Hamiltonian applicable for any curvilinear 1D-structures. The simple examples for the cases of a quantum wire (QWr) and a quantum ring (QR) defined in usual [001]-grown 2D-structure are presented.

Chaotization of internal motion of excitons in ultrathin layers by spin-orbit coupling.

We show that Rashba spin-orbit coupling (SOC) can generate chaotic behavior of excitons in two-dimensional semiconductor structures. To model this chaos, we study a Kepler system with spin-orbit coupling and numerically obtain a transition to chaos at a sufficiently strong coupling. The chaos emerges since the SOC reduces the number of integrals of motion as compared to the number of degrees of freedom. Dynamically, the dependence of the exciton energy on the spin orientation in the presence of SOC produces an anomalous spin-dependent velocity resulting in chaotic motion. We observe numerically the critical dependence of the dynamics on the initial conditions, where the system can return to and exit a stability domain through very small changes in the initial spin orientation. This chaos can have a strong influence on the lifetime of optically injected carriers in semiconductors and organometallic perovskites. Hence, this effect should be taken into account while designing structures for photovoltaic and optical spintronics applications, where excitons play a significant role.

Viscous magnetoresistance of correlated electron liquids

We develop a theory of magnetoresistance of two-dimensional electron systems in a smooth disorder potential in the hydrodynamic regime. Our theory applies to two-dimensional semiconductor structures with strongly correlated carriers when the mean free path due to electron-electron collisions is sufficiently short. The dominant contribution to magnetoresistance arises from the modification of the flow pattern by the Lorentz force, rather than the magnetic field dependence of the kinetic coefficients of the electron liquid. The resulting magnetoresistance is positive and quadratic at weak fields. Although the resistivity is governed by both viscosity and thermal conductivity of the electron fluid, the magnetoresistance is controlled by the viscosity only. This enables extraction of viscosity of the electron liquid from magnetotransport measurements.

Berry phase mechanism of the anomalous Hall effect in a disordered two-dimensional magnetic semiconductor structure.

The anomalous Hall effect (AHE) arises from the interplay of spin-orbit interactions and ferromagnetic order and is a potentially useful probe of electron spin polarization, especially in nanoscale systems where direct measurement is not feasible. While AHE is rather well-understood in metallic ferromagnets, much less is known about the relevance of different physical mechanisms governing AHE in insulators. As ferromagnetic insulators, but not metals, lend themselves to gate-control of electron spin polarization, understanding AHE in the insulating state is valuable from the point of view of spintronic applications. Among the mechanisms proposed in the literature for AHE in insulators, the one related to a geometric (Berry) phase effect has been elusive in past studies. The recent discovery of quantized AHE in magnetically doped topological insulators - essentially a Berry phase effect - provides strong additional motivation to undertake more careful search for geometric phase effects in AHE in the magnetic semiconductors. Here we report our experiments on the temperature and magnetic field dependences of AHE in insulating, strongly-disordered two-dimensional Mn delta-doped semiconductor heterostructures in the hopping regime. In particular, it is shown that at sufficiently low temperatures, the mechanism of AHE related to the Berry phase is favoured.

Transport in two-dimensional modulation-doped semiconductor structures

We develop a theory for the maximum achievable mobility in modulation-doped 2D GaAs-AlGaAs semiconductor structures by considering the momentum scattering of the 2D carriers by the remote ionized dopants, which must invariably be present in order to create the 2D electron gas at the GaAs-AlGaAs interface. The minimal model, assuming first-order Born scattering by random quenched remote dopant ions as the only scattering mechanism, gives a mobility much lower (by a factor of 3 or more) than that observed experimentally in many ultrahigh-mobility modulation-doped 2D systems, establishing convincingly that the model of uncorrelated scattering by independent random remote quenched dopant ions is often unable to describe the physical system quantitively. We theoretically establish that the consideration of spatial correlations in the remote dopant distribution can enhance the mobility by (up to) several orders of magnitudes in experimental samples. The precise calculation of the carrier mobility in ultrapure modulation-doped 2D semiconductor structures thus depends crucially on the unknown spatial correlations among the dopant ions in the doping layer which may manifest sample to sample variations even for nominally identical sample parameters (i.e., density, well width, etc.), depending on the details of the modulation-doping growth conditions.

Mobility versus quality in two-dimensional semiconductor structures

We consider theoretically effects of random charged impurity disorder on the quality of high-mobility two-dimensional (2D) semiconductor structures, explicitly demonstrating that the sample mobility is not necessarily a reliable or universal indicator of the sample quality in high-mobility modulation-doped 2D GaAs structures because, depending on the specific system property of interest, mobility and quality may be controlled by different aspects of the underlying disorder distribution, particularly since these systems are dominated by long-range Coulomb disorder from both near and far random quenched charged impurities. We show that in the presence of both channel and remote charged impurity scattering, which is a generic situation in modulation-doped high-mobility 2D carrier systems, it is quite possible for higher (lower) mobility structures to have lower (higher) quality as measured by the disorder-induced single-particle level broadening. In particular, we establish that there is no reason to expect a unique relationship between mobility and quality in 2D semiconductor structures as both are independent functionals of the disorder distribution, and are therefore, in principle, independent of each other. Using a simple, but reasonably realistic, ``2-impurity'' minimal model of the disorder distribution, we provide concrete examples of situations where higher (lower) mobilities correspond to lower (higher) sample qualities. We discuss experimental implications of our theoretical results and comment on possible strategies for future improvement of 2D sample quality.

Short-range disorder effects on electronic transport in two-dimensional semiconductor structures

We study theoretically the relative importance of short-range disorder in determining the low-temperature 2D mobility in GaAs-based structures with respect to Coulomb disorder which is known to be the dominant disorder in semiconductor systems. We give results for unscreened and screened short-range disorder effects on 2D mobility in quantum wells and heterostructures, comparing with the results for Coulomb disorder and finding that the asymptotic high-density mobility is always limited by short-range disorder which, in general, becomes effectively stronger with increasing `carrier density' in contrast to Coulomb disorder. We also predict an intriguing re-entrant metal-insulator transition at very high carrier densities in Si-MOSFETs driven by the short-range disorder associated with surface roughness scattering.

Time-dependent magnetotransport in semiconductor nanostructures via the generalized master equation

Transport of electrons through two-dimensional semiconductor structures on the nanoscale in the presence of perpendicular magnetic field depends on the interplay of geometry of the system, the leads, and the magnetic length. We use a generalized master equation (GME) formalism to describe the transport through the system without resorting to the Markov approximation. Coupling to the leads results in elastic and inelastic processes in the system that are described to a high order by the integro-differential equation of the GME formalism. Geometrical details of systems and leads leave their fingerprints on the transport of electrons through them. The GME formalism can be used to describe both the initial transient regime immediately after the coupling of the leads to the system and the steady state achieved after a longer time.

Features of electron-spin-resonance excitation in impure asymmetric two-dimensional structures

The band spin-orbit coupling, which makes it possible for the orbital motion of electrons to affect the spin dynamics, is known to be present in crystals with destroyed mirror symmetry. A framework for analyzing effects of the spin-orbit coupling on spin-dependent electron kinetics is suggested and applied to the electron-spin resonance on an electron gas in an impure asymmetric two-dimensional semiconductor structure. The general case of excitation of the resonance by both the electric and magnetic components of the microwave electromagnetic field is considered and the frequency-dependent tensors of the electric conductivity and spin susceptibility as well as the spin-current-correlation tensors, which additionally characterize the response of a broken-mirror-symmetry conducting media to an external electromagnetic field, are calculated. It is shown that the electric component of the resonant microwave field can excite the resonance more effectively than the magnetic component in spite of a small value of the spin-orbit coupling. The formalism presented allows one to consider the case when an external static magnetic field is arbitrary inclined with respect to the plane of the structure and the cyclotron frequency ${\ensuremath{\omega}}_{c}$ corresponding to the perpendicular component of the field can take any value less that the Fermi energy ${ϵ}_{F}$. It is found that the cyclotron motion not only modifies the spin relaxation time but also has an effect on the spin precession giving rise to a shift of the Larmour frequency. The shift can be positive or negative depending on the sign of $g$ factor of current carriers relative to the sign of their charge. It is shown that due to the cyclotron motion the spin resonance can also take place in the particular case of zero $g$ factor. It is also found that because of the spin-velocity correlations the absorption of the linear polarized radiation can change its value at the magnetic field reversal.

Mode coupling and evolution in broken-symmetry plasmas

The control of nonlinear processes and possible transitions to chaos in systems of interacting particles is a fundamental physical problem. We propose a new nonuniform solid-state plasma system, produced by the optical injection of current in two-dimensional semiconductor structures, where this control can be achieved. Due to an injected current, the system symmetry is initially broken. The subsequent nonequilibrium dynamics is governed by the spatially varying long-range Coulomb forces and electron-hole collisions. As a result, inhomogeneities in the charge and velocity distributions should develop rapidly, and lead to previously unexpected experimental consequences. We suggest that the system eventually evolves into a behavior similar to chaos.

Valley-dependent many-body effects in two-dimensional semiconductors

We calculate the valley degeneracy $({g}_{v})$ dependence of the many-body renormalization of quasiparticle properties in multivalley two-dimensional (2D) semiconductor structures due to the Coulomb interaction between the carriers. Quite unexpectedly, the ${g}_{v}$ dependence of many-body effects is nontrivial and nongeneric, and depends qualitatively on the specific Fermi-liquid property under consideration. While the interacting 2D compressibility manifests monotonically increasing many-body renormalization with increasing ${g}_{v}$, the 2D spin susceptibility exhibits an interesting nonmonotonic ${g}_{v}$ dependence with the susceptibility increasing (decreasing) with ${g}_{v}$ for smaller (larger) values of ${g}_{v}$ with the renormalization effect peaking around ${g}_{v}\ensuremath{\sim}1\char21{}2$. Our theoretical results provide a clear conceptual understanding of recent valley-dependent 2D susceptibility measurements in AlAs quantum wells.

Anisotropic plasmons in a two-dimensional electron gas with spin-orbit interaction

Spin-orbit coupling-induced anisotropies of plasmon dynamics are investigated in two-dimensional semiconductor structures. The interplay of the linear Bychkov-Rashba and Dresselhaus spin-orbit interactions drastically affects the plasmon spectrum: the dynamical structure factor exhibits variations over several decades, prohibiting plasmon propagation in specific directions. While this plasmon filtering makes the presence of spin-orbit coupling in plasmon dynamics observable, it also offers a control tool for plasmonic devices. Remarkably, if the strengths of the two interactions are equal, not only the anisotropy but all the traces of the linear spin-orbit coupling in the collective response disappear.