Electronic Subband Structure Research Articles

Homogeneous dual-site P lattice doping in CdS quantum rods for visible-light photocatalytic water splitting

Dual-site lattice doping in semiconductor materials is highly promising for photocatalysis as it can tune the sub-band electronic structure and charge distribution of the photocatalyst. However, it is challenging to precisely regulate the band structure by dual-site doping using the same heteroatom. Herein, we report the homogeneous dual-site P lattice doping in CdS quantum rods, i.e. P is homogeneously doped into both substitutional site of S and interstitial site, leading to negative shift of valence and conduction bands with extraordinary visible-light excitation efficiency. The electrons generated in the newly designed photocatalyst have strong redox potential in reducing H+ into H2 in photocatalysis. By introducing nontrivial co-catalyst Ni3C quantum dots onto the surface of P-CdS quantum rods, the apparent quantum efficiency of the catalyst achieves 15.7% at 435 nm, which is 5 times higher than that of pristine CdS.

Persistent spin textures and currents in wurtzite nanowire-based quantum structures

We explore the spin and charge properties of electrons in wurtzite semiconductor nanowires where radial and axial confinement leads to tubular or ring-shaped quantum structures. Accounting for spin-orbit interaction induced by the wurtzite lattice as well as a radial potential gradient, we analytically derive the corresponding low-dimensional Hamiltonians. It is demonstrated that the resulting tubular spin-orbit Hamiltonian allows to construct spin states that are persistent in time and robust against disorder. We find that these special scenarios are characterized by distinctive features in the optical conductivity spectrum, which enable an unambiguous experimental verification. In both types of quantum structures, we discuss the dependence of the occurring persistent charge and spin currents on an axial magnetic field and Fermi energy which show clear fingerprints of the electronic subband structure. Here, the spin-preserving symmetries become manifest in the vanishing of certain spin current tensor components. Our analytic description relates the distinctive features of the optical conductivity and persistent currents to bandstructure characteristics which allows to deduce spin-orbit coefficients and other band parameters from measurements.

Quantum Transport and Sub-Band Structure of Modulation-Doped GaAs/AlAs Core-Superlattice Nanowires.

Modulation-doped III-V semiconductor nanowire (NW) heterostructures have recently emerged as promising candidates to host high-mobility electron channels for future high-frequency, low-energy transistor technologies. The one-dimensional geometry of NWs also makes them attractive for studying quantum confinement effects. Here, we report correlated investigations into the discrete electronic sub-band structure of confined electrons in the channel of Si δ-doped GaAs-GaAs/AlAs core-superlattice NW heterostructures and the associated signatures in low-temperature transport. On the basis of accurate structural and dopant analysis using scanning transmission electron microscopy and atom probe tomography, we calculated the sub-band structure of electrons confined in the NW core and employ a labeling system inspired by atomic orbital notation. Electron transport measurements on top-gated NW transistors at cryogenic temperatures revealed signatures consistent with the depopulation of the quasi-one-dimensional sub-bands, as well as confinement in zero-dimensional-like states due to an impurity-defined background disorder potential. These findings are instructive toward reaching the ballistic transport regime in GaAs-AlGaAs based NW systems.

The Effective Mass of Dirac Fermions and Spin-Dependent Thermodynamic Properties of Monolayer Ferromagnetic MoS2 in the Presence of Rashba Spin-Orbit Coupling

The k.p Hamiltonian model at low-energy regime and Green’s function technique are carried out to study the effect of Rashba spin-orbit coupling (RSOC) on the spin-dependent thermodynamic properties in monolayer molybdenum disulphide. In particular, we have studied electronic band structure (EBS), electronic heat capacity (EHC), and magnetic susceptibility (MS) of spin-up and spin-down subbands by variation of RSOC strength. An enhancement of the spin splitting can be achieved by increasing the Rashba coupling strength. Electronic subband structure shows that the effective mass of carriers decreases with Rashba coupling while the spin-up and spin-down subband gaps remain constant. The reduced effective mass leads to the increase of both EHC and MS as reasonable results. Spin splitting can be also obtained from the EHC and MS findings. At very strong Rashba couplings, two Dirac-like points appear for spin-up subbands which we can claim that the system gets a new quantum anomalous Hall phase.

Research Update: Conductivity and beyond at the LaAlO3/SrTiO3 interface

In this review, we focus on the celebrated interface between two band insulators, LaAlO3 and SrTiO3, that was found to be conducting, superconducting, and to display a strong spin-orbit coupling. We discuss the formation of the 2-dimensional electron liquid at this interface, the particular electronic structure linked to the carrier confinement, the transport properties, and the signatures of magnetism. We then highlight distinctive characteristics of the superconducting regime, such as the electric field effect control of the carrier density, the unique tunability observed in this system, and the role of the electronic subband structure. Finally we compare the behavior of Tc versus 2D doping with the dome-like behavior of the 3D bulk superconductivity observed in doped SrTiO3. This comparison reveals surprising differences when the Tc behavior is analyzed in terms of the 3D carrier density for the interface and the bulk.

Electronic and optical properties of InN nanowires

We have employed a multiband envelope function method to study wurtzite [0001] InN nanowires of cylindrical cross section. The electronic subband structure and optical absorption spectrum are calculated as a function of the nanowire radius. The energies of the Γ-point conduction band states show a monotonous increase with decreasing radius. On the other hand, the size dispersion of the valence band states is more involved, showing various crossing and anticrossing effects due to the interplay between the confinement and band mixing effects. Thus, for small (R<2nm) nanowires the highest valence band state has dominant pz-symmetry, but for R>2nm the highest state has dominant p⊥-symmetry, although it is optically inactive. The optical absorption spectra polarized along or perpendicular to the nanowire axis are different on account of the different transitions involved. The spectra of the smaller nanowires display a distinguishable sequence of peaks reflecting the one-dimensional density-of-states. The absorption peaks become denser for larger nanowires, and for radii larger than 10nm the spectrum starts to ressemble that of bulk InN.

GaN/AlN constant total effective radius multi-wells quantum rings: Physical properties under well number variation effects

In this study, effect of the number of wells and quantum ring thickness on subband electronic structure, ground state wave function, Fermi energy and linear refractive index changes of an AlN/GaN constant total effective radius multi-wells quantum ring have been studied. It is shown that, effect of increasing of the inner quantum ring radius R0 on the ground state energy and other physical properties are larger than increasing of the number of wells. This fact helps the simplification of the experiments, since making quantum ring with larger R0 is easier than making multiple-wells quantum rings. However, effect of the increasing of the R0 values in systems with larger number of wells is also larger. Increase of the R0 also leads to increase of the refractive index changes amplitudes. Resonant energy undergo a red-shift by going from single quantum well system to double quantum well system, and further increasing of the numbers of wells leads to a blue-shift. Finally, we propose number of wells as a tuning tool to select the position of the maximum probability of finding free carriers within the system.

Electronic properties and charge control of δ-doped GaAs/Al0.3Ga0.7As and In0.53Ga0.47As/InP core–shell nanowires

The electronic sub-band structure, potential profile and charge distribution of delta(δ)-doped GaAs/Al0.3Ga0.7As and In0.53Ga0.47As/InP core–shell nanowires have been investigated in the effective mass approximation by varying systematically the position and sheet density of the δ-layer. The one-dimensional electron gas charge distribution has a maximum at the core but not at the GaAs/Al0.3Ga0.7As or In0.53Ga0.47As/InP interfaces, as in the case of epitaxial layer heterojunctions, irrespective of the position of the δ-layer. In contrast, carriers can be confined in a quasi-triangular potential well occupying several sub-bands that fall below the Fermi level at an InP/In0.53Ga0.47As core–shell heterojunction with a wider bandgap core, resembling the case of epitaxial layer heterojunctions, or in an InP/InGaAs/InP multiple core–shell nanowire. We discuss the need for adjusting the doping level to avoid strong band bending causing a shift of the charge distribution to the core.

Electronic properties of core-shell nanowire resonant tunneling diodes.

The electronic sub-band structure of InAs/InP/InAs/InP/InAs core-shell nanowire resonant tunneling diodes has been investigated in the effective mass approximation by varying the core radius and the thickness of the InP barriers and InAs shells. A top-hat, double-barrier potential profile and optimal energy configuration are obtained for core radii and surface shells >10 nm, InAs middle shells <10 nm, and 5 nm InP barriers. In this case, two sub-bands exist above the Fermi level in the InAs middle shell which belongs to the m = 0 and m = 1 ladder of states that have similar wave functions and energies. On the other hand, the lowest m = 0 sub-band in the core falls below the Fermi level but the m = 1 states do not contribute to the current transport since they reside energetically well above the Fermi level. We compare the case of GaAs/AlGaAs/GaAs/AlGaAs/GaAs which may conduct current with smaller applied voltages due to the larger effective mass of electrons in GaAs and discuss the need for doping.

Substrate Biasing Effect on the Physical Properties of Reactive RF-Magnetron-Sputtered Aluminum Oxide Dielectric Films on ITO Glasses

High dielectric constant (high-k) Al2O3 thin films were prepared on ITO glasses by reactive RF-magnetron sputtering at room temperature. The effect of substrate bias on the subband structural, morphological, electrode/Al2O3 interfacial and electrical properties of the Al2O3 films is systematically investigated. An optical method based on spectroscopic ellipsometry measurement and modeling is adopted to probe the subband electronic structure, which facilitates us to vividly understand the band-tail and deep-level (4.8-5.0 eV above the valence band maximum) trap states. Well-selected substrate biases can suppress both the trap states due to promoted migration of sputtered particles, which optimizes the leakage current density, breakdown strength, and quadratic voltage coefficient of capacitance. Moreover, high porosity in the unbiased Al2O3 film is considered to induce the absorption of atmospheric moisture and the consequent occurrence of electrolysis reactions at electrode/Al2O3 interface, as a result ruining the electrical properties.

Photoluminescence emission of nanoporous anodic aluminum oxide films prepared in phosphoric acid

The photoluminescence emission of nanoporous anodic aluminum oxide films formed in phosphoric acid is studied in order to explore their defect-based subband electronic structure. Different excitation wavelengths are used to identify most of the details of the subband states. The films are produced under different anodizing conditions to optimize their emission in the visible range. Scanning electron microscopy investigations confirm pore formation in the produced layers. Gaussian analysis of the emission data indicates that subband states change with anodizing parameters, and various point defects can be formed both in the bulk and on the surface of these nanoporous layers during anodizing.

Terahertz plasmon and surface-plasmon modes in hollow nanospheres

We present a theoretical study of the electronic subband structure and collective electronic excitation associated with plasmon and surface plasmon modes in metal-based hollow nanosphere. The dependence of the electronic subband energy on the sample parameters of the hollow nanosphere is examined. We find that the subband states with different quantum numbers l degenerate roughly when the outer radius of the sphere is r2 ≥ 100 nm. In this case, the energy spectrum of a sphere is mainly determined by quantum number n. Moreover, the plasmon and surface plasmon excitations can be achieved mainly via inter-subband transitions from occupied subbands to unoccupied subbands. We examine the dependence of the plasmon and surface-plasmon frequencies on the shell thickness d and the outer radius r2 of the sphere using the standard random-phase approximation. We find that when a four-state model is employed for calculations, four branches of the plasmon and surface plasmon oscillations with terahertz frequencies can be observed, respectively.

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.

Semimetal to semiconductor transition and thermoelectric properties of bismuth nanotubes

We have theoretically investigated the electronic subband structures and thermoelectric transport properties of Bi nanotubes. The calculated semimetal to semiconductor transition in Bi nanotubes is in good agreement with experiment results. The expressions for Seebeck coefficient and the optimized carrier concentration are first derived, and have been used to explain the calculated and experimental results. The effects of thickness, diameter, and growth direction of Bi nanotubes and carrier concentration on the thermoelectric figure of merit (ZT) are discussed. It was found that the Bi nanotube oriented along the trigonal direction has the highest ZT, and the optimized carrier concentration is found to be in a range from 1018 cm−3 to 4.0 × 1018 cm−3. The predicted enhancement in ZT is important for the anticipated applications of Bi nanotubes for thermoelectric devices.

Optical properties and electronic subband structures in In xGa 1−xN/GaN multiple quantum wells

In xGa 1−xN/GaN multiple quantum wells (MQWs) were grown on sapphire substrates by using metalorganic chemical vapor deposition. The optical and electrical properties of the MQWs were investigated photoluminescence (PL) and Hall measurements, respectively. The electronic subband energies, the wave functions, and the Fermi energy in the In 0.1Ga 0.9N/GaN MQWs were calculated by using a multi-band k → ⋅ p → theory and considering the strain due to the lattice mismatch and the spontaneous and piezoelectric polarizations. The potential due to free carriers observed in the Hall measurement was also taken into account in solving the Schrödinger equation and the Pöisson equation self-consistently. The calculated interband transition energy was compared with the PL data. The calculated electronic structure of the In 0.1Ga 0.9N/GaN MQWs showed that only one subband below the Fermi level was occupied by the electrons.

Pressure dependence of the electronic structure of a [311] piezoelectricGa0.85In0.15As/AlAssuperlattice

We have studied the electronic subband structure of a piezoelectric [311] ${\text{Ga}}_{0.85}{\text{In}}_{0.15}\text{As}/\text{AlAs}$ superlattice by means of high-hydrostatic pressure and excitation-power-dependent photoluminescence at 78 K. In particular, we unraveled the origin of two optical transitions at around 1.96 and 2 eV at ambient pressure, which were recently found to give rise to an unexpectedly strong resonant enhancement of the acoustic-phonon Raman scattering for such samples with permanent built-in piezoelectric fields [G. Rozas et al., Phys. Rev. B 77, 165314 (2008)]. Here we demonstrate that these transitions are doubly indirect, in real and reciprocal space, corresponding to radiative recombination processes between electrons at the X valleys of the AlAs barriers and heavy holes at the $\ensuremath{\Gamma}$ point of the Brillouin zone but confined to the GaInAs quantum wells. In addition, the partial screening of the piezoelectric field induced by carrier photoexcitation under illumination becomes largely suppressed for pressures above 1.1 GPa due to conduction-band $\ensuremath{\Gamma}\text{-X}$ crossover effects.

Band hybridization and spin-splitting in InAs/AlSb/GaSb type II and broken-gap quantum wells

We present a detailed theoretical study on the features of band hybridization and zero-field spin-splitting in InAs/AlSb/GaSb quantum wells (QWs). An eight-band k⋅p approach is developed to calculate the electronic subband structure in such structures. In the absence of the AlSb layer, the hybridized energy gaps can be observed at the anticrossing points between the lowest electron subband and the highest heavy-hole subband in the InAs and GaSb layers respectively. In such a case, the position and magnitude of the gaps are spin-dependent. When a thin AlSb layer is inserted between the InAs and GaSb layers, we find that the lowest electron subband in the InAs layer is only hybridized with the highest light-hole subband which is also hybridized with the highest heavy-hole subband in the GaSb layer. The hybridized energy gaps and spin-splitting in the InAs/AlSb/GaSb QWs are reduced significantly. These results can be used to understand why electrons and holes can be well separated and why relatively high mobilities for electrons and holes can be achieved in InAs/AlSb/GaSb type II and broken-gap QWs. The present study is relevant to the applications of InAs/GaSb based QW structures as new generation of high-density and high-mobility electronic devices.

Electron subband structure in strained silicon UTB films from the Hensel–Hasegawa–Nakayama model – Part 1 analytical consideration and strain-induced valley splitting

Multi-gate FinFETs and ultra-thin silicon body SOI FETs are considered as perfect candidates for future technology nodes. Strong size quantization leads to a formation of quasi-two-dimensional subbands in carrier systems within thin silicon films. The employed Hensel–Hasegawa–Nakayama k·p Hamiltonian accurately describes the bulk structure up to the energies of 0.5–0.8eV and includes a shear strain component. Shear strain is responsible for effective mass modification and is therefore an important source of the electron mobility enhancement in ultra-thin silicon films. The influence of shear strain on the subband structure in thin silicon films is investigated.

Electron subband structure in strained silicon UTB films from the Hensel–Hasegawa–Nakayama model – Part 2 efficient self-consistent numerical solution of the k · p schrödinger equation

A self-consistent Schrödinger–Poisson model for the calculation of the electron subband structure of ultra-thin body (UTB) devices for arbitrary substrate orientation is presented. The proposed approach is based on a two-band k·p Hamiltonian and takes the band nonparabolicity and arbitrary strain into account. Despite its small matrix size compared to full-band approaches, an excellent description of the band structure over a wide range of the Brillouin zone is assured. Furthermore, emphasis is put on the efficiency and accuracy of the numerical, two-dimensional k-space integration of the subband distribution functions. For this purpose, the Clenshaw–Curtis method which is based on non-equidistant interpolation nodes is employed. Simulation results of (001) and (110) oriented silicon UTB double gate devices demonstrate the suitability of the proposed numerical method. For Si body thicknesses in the nanometer regime, the presence of band structure effects which are not captured by a one-band model are clearly demonstrated.

Simulation of electron transport in (0001) and (112¯0) 4H-SiC inversion layers

Monte Carlo simulations are used to investigate electron transport in the inversion layer of a 4H silicon carbide metal-oxide-semiconductor field-effect transistor (MOSFET). The electronic subband structure is solved self-consistently along with the perpendicular field at the semiconductor-oxide interface. Inversion channel scattering rates due to acoustic and polar optical phonons, ionized dopants, trapped charge, and interface roughness are considered. Transport within (0001) and (112¯0) oriented inversion layers are compared. Simulations of the MOSFET low-field mobility, incorporating previously published experimental results for threshold voltages and charge densities, are found to agree well with experimental results. The mobility of the (112¯0) channel is much larger (90 cm2/V s) than that of the (0001) channel (&amp;lt;40 cm2/V s) due to a reduction in interface states. Furthermore, the mobility has a temperature coefficient of approximately −3/2 for (112¯0) layers due to dominant phonon scattering and +1 for (0001) layers, where interface trap scattering dominates. Since the band structure is very similar, transport variations among the two crystal orientations are found to result largely from the enhanced interface trap density in the (0001)-oriented interfaces.