Conduction Subbands Research Articles

Magneto-optical spectrum of Mn-doped ZnO nanorods

Based on the framework of the effective-mass theory, the electronic structure of ZnO nanorods doped with manganese ions in the magnetic field is calculated by the Bessel function expanding method; the conduction and valence subbands are found to be non-degenerate in ZnO nanorods doped with manganese ions. The 10 lowest possible transitions are presented and the optical circularly polarized properties of each transition are also labeled. Through the analysis, the order of the valence subbands will change when the concentration of manganese ions varies, while the optical circularly polarized properties of the lowest transitions do not change when the magnetic field and the concentration of manganese ions increase. Meanwhile, each of the radiative intensities will increase with the increase of the wave vector.

Spintronic properties of one-dimensional electron gas in graphene armchair ribbons

We have investigated, using effective mass approach (EMA), magnetic properties of a one-dimensional electron gas in graphene armchair ribbons when the electrons occupy only the lowest conduction subband. We find that magnetic properties of the one-dimensional electron gas may depend sensitively on the width of the ribbon. For ribbon widths Lx=3Ma0, a critical point separates ferromagnetic and paramagnetic states while for Lx=(3M+1)a0 paramagnetic state is stable (M is an integer and a0 is the length of the unit cell). These width-dependent properties are a consequence of eigenstates that have a subtle width-dependent mixture of K and K′ states, and can be understood by examining the wavefunction overlap that appears in the expression for the many-body exchange self-energy. Ferromagnetic and paramagnetic states may be used for spintronic purposes.

Magneto-optical spectrum of Mn-doped CdS nanorods

The electronic structure of CdS nanorods doped with manganese ions in the magnetic field is calculated based on the effective-mass theory, the conduction and valence subbands are found to be non-degenerate in Mn-doped CdS nanorods, the split energies of the valence subbands increase as the increment of the concentration of manganese ions. The possible transitions between the six lowest conduction subbands and the eight highest valence subbands are presented, the optical circularly polarized properties of each transition are also labelled. Through the calculation, it is found that the optical transitions can be influenced by the concentration of manganese ions and the applied magnetic field, and the radiative intensity of the transition in Mn-doped CdS nanorods increases with the increase in the manganese ions.

Valley polarized electronic transmission through a line defect superlattice of graphene

The band structure of a one-dimensional superlattice formed by patterning graphene lattice with a periodic line defect array is calculated. The lowest conduction subband of such a superlattice connects two inequivalent Dirac points with flat dispersion, which is reminiscent of the flat-bottomed subband of a zigzag-edged graphene nanoribbon (ZEGNR). According to such a subband characteristic, a finite length line defect superlattice in graphene, instead of a ZEGNR, can be utilized to realize valley filtering function. The calculated electronic transmission spectrum of such a structure shows the nearly perfect valley polarization for low energy incident electrons.

Application of multi-subband self-consistent energy balance method to terahertz quantum cascade lasers

We present simulation results for two resonant phonon terahertz quantum cascade lasers using a self-consistent energy balance model, which determines the electron temperature for each conduction subband. These temperatures, along with the electron populations and scattering rates, are determined in a manner similar to previously published models. However, the presented model is able to converge through the use of an algorithm that appears to be robust. The predicted individual subband electron temperatures, population densities and scattering rates are compared to previously published Monte Carlo and experimental studies for both lasers, where subband temperature variations were observed. These quantities were chosen since they provided the only comparison to modeling results from other studies.

Transverse laser dressing effects on the subband density of states in a 20-nm-wide GaAs/Al0.3Ga0.7As quantum well wire

The electronic subband structure of a relatively thick cylindrical quantum well wire irradiated by a non-resonant laser field is calculated within the one-band effective-mass approximation by using a finite element method. For the first time, the problem is solved for a large number of transverse electron modes in the wire. Exact laser-dressing effects on the lateral confinement potential are considered by using an analytical function. Important changes of the electron localization probability under intense laser field are described. The study reveals a competition between the quantum confinement and the laser field which breaks down the cylindrical symmetry of the wire and splits the degenerate electronic levels. A proper analysis of these results is accomplished by calculating the density of states function. Our central prediction is the non-uniform blueshift of the conduction subband structure which may be valuable for an active control of the optoelectronic properties related to intersubband and interband transitions in semiconductor quantum wires.

Magneto-optical spectrum of ZnO nanorods

Based on the framework of the effective-mass theory, the Hamiltonians of the electron and hole states of the wurtzite structure in the magnetic field are derived. The obtained Hamiltonians can then be solved by the Bessel function expanding method in which the electron and the hole are assumed to be confined in the infinitely high potential barrier. The conduction and valence subbands are denoted after solving the Schrödinger equation. Several possible low interband transitions are calculated, and the magnetic field is found to split the eight transitions to 18 individual transitions. The optical circularly polarized properties of each transition are also labeled when the magnetic field is applied along the z axis.

Molecular beam epitaxial growth and optical properties of highly mismatched ZnTe1−xOx alloys

Highly mismatched ZnTe1−xOx(ZnTeO) alloys have been grown by molecular beam epitaxy. X-ray diffraction (XRD) analyses showed that a single-phase ZnTeO layer were grown with a substitutional O composition x up to 1.34% on ZnTe(001) substrate in this experiments. Optical transitions associated with the lower (E−) and upper (E+) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of ZnTe were clearly observed, and the dependence of the energy position of these bands on the O composition was consistent with the band anticrossing model.

Band structure and optical absorption in InAs/GaSb quantum well

An analysis of band structure, wave function distribution and absorption of linearly polarized light along the [110] direction in InAs/GaSb quantum well grown along the [001] direction is performed by the eight-band K-P model and finite difference method. Our study shows that the band structure and wave function distribution could be regulated effectively by changing the thickness of InAs or GaSb layer. When the bottom of conduction subband and the top of the valence subband are in resonance, the hybridization of ground electron and light-hole state at the zone-center is very weak, and the overlap between the wave function of the ground and the first-excited electron state is considerable, according to the theory of wave function engineering, so the transition rate between the ground and the first-excited electron state at the zone-center is larger than that when the bottom of conduction subband and the top of the valence subband are not in resonance. This is very important for designing advanced optoelectronic devices such as far-infrared or mid-infrared cascade lasers and detecters based on InAs/GaSb quantum wells.

Improving the RF Performance of Carbon Nanotube Field Effect Transistor

Compact model of single‐walled semiconducting carbon nanotube field‐effect transistors (CNTFETs) implementing the calculation of energy conduction subband minima under VHDLAMS simulator is used to explore the high‐frequency performance potential of CNTFET. The cutoff frequency expected for a MOSFET‐like CNTFET is well below the performance limit, due to the large parasitic capacitance between electrodes. We show that using an array of parallel nanotubes as the transistor channel combined in a finger geometry to produce a single transistor significantly reduces the parasitic capacitance per tube and, thereby, improves high‐frequency performance.

Excitation Dependence of Photoluminescence in the 1.5-1.6 μm Wavelength Region from Grown Dislocation-Rich Si Layers

Abstract We study photoluminescence (PL) in the 1.5-1.6 μm wavelength region from dislocation-rich Si layers grown using the oxidized Si surfaces. The obtained excitation dependences of the PL intensities are interpreted on the base of a model of the electronic structure that consists of the shallow dislocation-induced conduction sub-band and deep levels located about 0.3 eV above the valence band edge. At low temperature and low excitation conditions the dependences are described well by mean of the Shockley-Read-Hall statistics for the carrier recombination via deep states. The PL data at high excitation conditions suggest that the dominating process that limits the increase in the excess carrier concentration is Auger recombination in which one of three carriers involved into the recombination event is from the deep states. A blue shift of the dislocation-related PL peak with increasing excitation intensity is observed, which is associated with changes in the occupancy of the deep states.

Exploration of edge-dependent optical selection rules for graphene nanoribbons

Optical selection rules for one-dimensional graphene nanoribbons are explored based on the tight-binding model. A theoretical explanation, through analyzing the velocity matrix elements and the features of the wavefunctions, can account for the selection rules, which depend on the edge structure of the nanoribbon, i.e., armchair or zigzag edges. The selection rule of armchair nanoribbons is ΔJ = Jc - Jv = 0, and the optical transitions occur from the conduction to the valence subbands of the same index. Such a selection rule originates in the relationships between two sublattices and between the conduction and valence subbands. On the other hand, zigzag nanoribbons exhibit the selection rule |ΔJ| = odd, which results from the alternatively changing symmetry property as the subband index increases. Furthermore, an efficient theoretical prediction on transition energies is obtained by the application of selection rules, and the energies of the band-edge states become experimentally attainable via optical measurements.

THz-driven quantum wells: Coulomb interactions and Stark shifts in the ultrastrong coupling regime

We investigate the near infrared interband absorption of semiconductor quantum wells driven by intense terahertz (THz) radiation in the regime of ultrastrong coupling, where the Rabi frequency is a significant fraction of the frequency of the strongly driven transition. With the driving frequency tuned just below the lowest frequency transition between valence subbands, a particularly interesting phenomenon is observed. As the THz power increases, a new peak emerges above the frequency of the undriven exciton peak, which grows and eventually becomes the larger of the two. This reversal of relative peak intensity is inconsistent with the Autler–Townes effect in a three-state system while within the rotating wave approximation (RWA). In the samples investigated, the Bloch–Siegert shift (associated with abandoning the RWA), exciton binding energy, the Rabi energy, and non-resonant ac Stark effects are all of comparable magnitude. Solution of a semiconductor Bloch model with one conduction and multiple valence subbands indicates that the ac Stark effect is predominantly responsible for the observed phenomenon.

Ultrafast electron dynamics of intersubband excitation concerning cross-phase modulation in an InGaAs/AlAs/AlAsSb coupled double quantum well

The ultrafast electron dynamics accompanying intersubband excitation in an InGaAs/AlAs/AlAsSb double quantum well has been investigated by femtosecond pump-probe spectroscopy. The photoinduced changes in the refractive index that causes cross-phase modulation is dominated by a decreased number as well as increased temperature of the electrons in the lower conduction subbands. The change in the number of conduction electrons decays monotonically, whereas the change in the electron temperature exhibits a maximum at around 0.9 ps after the photoexcitation. Heating of the conduction electrons is caused by their excess energy after intersubband longitudinal-optical phonon scattering.

Variational analysis of the Rashba splitting in III–V semiconductor inversion layers

A spin-dependent variational theory is used to analyze the Rashba spin-orbit splitting in two-dimensional electron gases formed in III--V semiconductor inversion layers. The spin split conduction subbands in CdTe/InSb, insulator/InAs, InP/InGaAs, InAlAs/InGaAs, and AlGaAs/GaAs heterojunctions are calculated. The theory, presented here in detail, is based on the 8 $\ifmmode\times\else\texttimes\fi{}$ 8 $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ Kane model and on the introduction of simple and convenient spin-dependent Fang-Howard trial functions, and leads to analytical expressions for the split subbands, as well as allows for a detailed knowledge of the Rashba spin-orbit coupling, including its explicit dependence on structure parameters and its decomposition into separate contributions. The Rashba coupling parameter and the population difference in the spin-split subbands, as experimentally determined from the beating pattern of the Shubnikov-de Haas (SdH) oscillations, are obtained as a function of the electron density (${n}_{s}$). The separate contributions to the particularly large Rashba splitting in CdTe/InSb heterojunctions are also computed and discussed. It is shown, for example, that due to the spin-dependent boundary conditions, the direct Rashba spin-orbit coupling term in the effective Hamiltonian dominates the splitting only for ${n}_{s}>{10}^{10}$ cm${}^{\ensuremath{-}2}$ while it is the barrier penetration kinetic energy term that gives the largest contribution to the Rashba effect at lower densities.

Influence of Exchange Interaction on Phonon–Polaron Properties in Diluted Magnetic Semiconductors

The energy spectrum of a phonon–polaron effect with weak coupling has been obtained in the classical magnetic field for diluted magnetic semiconductors, which has a cubic crystal structure with a one-valley anisotropic energy spectrum of the band electrons. Analytical expressions for polaron binding energy and lowest polaron subband energy (i.e., polaron ground-state energy of the lowest conduction subband with spin ↓), and the transverse and longitudinal polaron effective masses of the electron in the dependence on the magnetic field were found. On the basis of quantitative calculations, it has been established that with increasing magnetic field, polaron binding energy is decreased owing to the exchange interaction. Also, it has been established that the lowest polaron subband energy is shifted down in energy; at the same time, the bottom of the lowest polaron subband energy is located much lower compared with the bottom of the polaron band for the standard spectrum.

Mechanisms of Strong Photoluminescence from Si Nanocrystals

Photoluminescence mechanisms (models) are reviewed and experimental data are analyzed based on our model, related to direct radiative transitions from the second conduction sub-band to the first one.

Theoretical study of InxAl1‐xN/ZnO and InxGa1‐xN/ZnO strained quantum well lasers

AbstractWe perform a theoretical study of InxAl1‐xN/ZnO and InxGa1‐xN/ZnO strained Quantum Well (QW) Lasers oriented along the non‐polar A‐plane axis. In order to determine the energy band‐offsets in both heterostructures as a function of alloy composition, we use the ab‐initio method to compute the total energies of these systems. Then, we use the k·P method within the envelop function formalism to study conduction and valence energy subband structures for various alloy compositions and well widths. Finally, we calculate the Laser gain for such strained QW heterostructures. Our results show that type I heterostructures can be made with two kind of systems: InxGa1‐xN/InxAl1‐xN/ZnO and InxGa1‐xN/InyGa1‐yN/ZnO where InxGa1‐xN is the QW in both cases. These heterostructures are lattice‐matched along A‐plane axis at x=32% in InxAl1‐xN/ZnO and x=17% in InxGa1‐xN/ZnO. However, because of the anisotropy of the built‐in strain at the interface, they are still slightly compressively strained along c‐axis. Pure tensile strain or pure compressive can be achieved only at In‐content values well below or well above these critical values, respectively. Our results are compared with various theoretical and experimental values, and quite good agreement is found. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gate voltage and structure parameter modulated spin splitting in AlGaN/GaN quantum wells

AbstractIn this paper, considerable magnitude of spin splitting for the conduction subband at the Fermi energy is obtained in AlGaN quantum wells (QWs) grown along the c‐axis. We have analyzed how the magnitude of spin splitting of the first electron subband in AlGaN QWs with different sheet carrier concentration changes as a function of applied gate voltage, well width, and Al content in the barrier. It is also found that the contribution to spin splitting from Dresselhaus term is much larger than that from Rashba term, the contribution of Dresselhaus term to the total spin splitting depends greatly on the carrier concentrations, the change of well width has little effect on total spin splitting, and the magnitude of spin splitting can be greatly modulated by Al content in the barrier, gate voltage, and sheet carrier concentration. The internal polarized electric field is crucial for considerable spin splitting in III‐nitride QWs. Moreover, the magnitude of total spin splitting calculated here is comparable with other theoretical and experimental values observed in III‐nitride heterostructures.

Performance of Silicon Ballistic Nanowire MOSFET with Diverse Orientations and Diameters

To investigate the performance of silicon ballistic nanowire MOSFET with diverse orientations and diameters, the subband structure of silicon nanowire has been calculated by using the tight binding simulator, and observed the changing of minimum energy in conduction and valence subband and effective mass of electron and hole. This paper also present a compact model, which is used in numerical evaluation of the current vs. voltage characteristics of ballistic devices. Finally, the drain saturation current of n- and p-channel device with different orientations and diameters is shown.