Hole Wave Functions Research Articles

Improved performance of InGaN/GaN MQW LEDs with trapezoidal wells and gradually thinned barrier layers towards anode

We design and evaluate the performance of three InGaN/GaN multiple quantum well blue LEDs – A. rectangular quantum wells with a fixed barrier width, B. trapezoidal quantum wells with a fixed barrier width, and C. trapezoidal quantum wells with a decreasing barrier width towards the anode end – in terms of efficiency droop and power output. We obtain band diagram, electric field, emission spectra and carrier concentration using well calibrated APSYS simulation. Use of trapezoidal quantum wells increases better overlapping between electron and hole wavefunctions thereby increasing radiative recombination events. Furthermore decreasing barrier width from n- to p- regions shortens hole transport path which results in better hole transport and distribution in the wells and hence larger radiative recombination rate. Our proposed structure C exhibits efficiency droop reduction of 2.1% and enhancement of optical power of 236.7% compared to conventional rectangular quantum well structure at injection current of 120 mA.

Dynamical Shiba states from precessing magnetic moments in ans-wave superconductor

We study theoretically the dynamics of a Shiba state forming around precessing classical spin in an s-wave superconductor. Utilizing a rotating wave description for the precessing magnetic impurity, we find the resulting Shiba bound state quasi-energy and the spatial extension of the Shiba wavefunction. We show that such a precession pertains to dc charge and spin currents flowing through a normal STM tip tunnel coupled to the superconductor in the vicinity of the impurity. We calculate these currents and find that they strongly depend on the magnetic impurity precession frequency, precession angle, and on the position of the Shiba energy level in the superconducting gap. The resulting charge current is found to be proportional to the difference between the electron and hole wavefunctions of the Shiba state, being a direct measure for such an asymmetry. By dynamically driving the impurity one can infer the spin dependence of the Shiba states in the absence of a spin-polarized STM tip

Advantages of InGaN/InGaN quantum well light emitting diodes: Better electron-hole overlap and stable output

Shift of the emission spectrum and the efficiency loss with the operating current are the major challenges in the conventional InGaN/GaN QW LEDs. Several approaches are being tried by different research groups to overcome this hurdle, including, introduction of In in the barriers for which a rigorous study seemed essential. In this paper, our comprehensive and systematic studies on InGaN/InGaN quantum well light emitting diodes reveal that the overlap of electron and hole wave functions can be increased even at low operating currents by the introduction of In in the barriers. The changes of output emission for different In content in the barrier and various doping at different currents are reported, which show that the shift of the energy can be minimized for suitable choice of parameters. The emission and the overlap of electron and hole wave functions have a significant dependence on the doping in the barrier layers which may be used advantageously. Computations have been performed through the self-consistent solutions of Schrödinger and Poisson equations.

Manipulation of structural and optical properties in ZnO/ZnS type-II and ZnS/ZnO inverted type-II core/shell nanocrystals: tight-binding theory

Based on the atomistic tight-binding theory (TB), I report here the results of my theoretical investigation into the electronic structures and optical properties of ZnO/ZnS type-II and ZnS/ZnO inverted type-II core/shell nanocrystals, with special emphasis on the single-particle spectra, atomistic orbital characters, optical band gaps, overlaps of ground electron and hole wave functions, ground-state oscillation strengths, electron–hole interactions and Stokes shift under the variation of the coated shell thicknesses. The underlying mechanism from an analysis of the computations is sensitive to the band alignments and growth shell thickness. In the presence of the terminated shell, the optical band gaps of ZnO/ZnS core/shell nanocrystals are reduced, while those of ZnS/ZnO core/shell nanocrystals are increased. With the increasing monolayers of the growth shell, the optical band gaps of ZnO/ZnS and ZnS/ZnO core/shell nanocrystals are decreased because of quantum confinement. It is expected that selecting ZnS as a shell for ZnO nanocrystals, with its large core and growth shell sizes, leads to the formation of the lower band gaps into the visible light spectrum. High optical behaviour is observed in ZnO/ZnS core/shell nanocrystals. As described by electron–hole coulomb interaction, a strong confinement between electrons and holes is found in ZnS/ZnO core/shell nanocrystals. The energies of electron–hole coulomb interaction are reduced with the increasing thickness of the coated shell. The Stokes shift of ZnS/ZnO core/shell nanocrystals is greater than that of ZnO/ZnS core/shell nanocrystals as elucidated by the electron–hole exchange interaction. The reduction of Stokes shift is presented with the increasing passivated shell monolayers. Finally, the present systematic study on the structural and optical properties mainly exposes a new viewpoint to understand the shell-size- and band-profile-dependent behaviours of ZnO/ZnS type-II and ZnS/ZnO inverted type-II core/shell nanoparticles to provide detailed information for use in designing electronic nanodevices.

Spin qubit manipulation of acceptor bound states in group IV quantum wells

The large spin–orbit coupling in the valence band of group IV semiconductors provides an electric field knob for spin-qubit manipulation. This fact can be exploited with acceptor based qubits. Spin manipulation of holes bound to acceptors in engineered SiGe quantum wells depends very strongly on the electric field applied and on the heterostructure parameters. The g-factor is enhanced by the Ge content and can be tuned by shifting the hole wave-function between the heterostructure constituent layers. The lack of inversion symmetry induced both by the quantum well and the electric fields together with the g-factor tunability allows the possibility of different qubit manipulation methods such as electron spin resonance, electric dipole spin resonance and g-tensor modulation resonance. Rabi frequencies up to hundreds of MHz can be achieved by electric field manipulation of heavy-hole qubits, and of the order of GHz with light-hole qubits.

Quantum-Confined Stark Effect of Individual Defects in a van der Waals Heterostructure.

The optical properties of atomically thin semiconductor materials have been widely studied because of the isolation of monolayer transition metal dichalcogenides (TMDCs). They have rich optoelectronic properties owing to their large direct bandgap, the interplay between the spin and the valley degree of freedom of charge carriers, and the recently discovered localized excitonic states giving rise to single photon emission. In this Letter, we study the quantum-confined Stark effect of these localized emitters present near the edges of monolayer tungsten diselenide (WSe2). By carefully designing sequences of metallic (graphene), insulating (hexagonal boron nitride), and semiconducting (WSe2) two-dimensional materials, we fabricate a van der Waals heterostructure field effect device with WSe2 hosting quantum emitters that is responsive to external static electric field applied to the device. A very efficient spectral tunability up to 21 meV is demonstrated. Further, evaluation of the spectral shift in the photoluminescence signal as a function of the applied voltage enables us to extract the polarizability volume (up to 2000 Å3) as well as information on the dipole moment of an individual emitter. The Stark shift can be further modulated on application of an external magnetic field, where we observe a flip in the sign of dipole moment possibly due to rearrangement of the position of electron and hole wave functions within the emitter.

Composition manipulation of near infrared InAsxSb1−x nanocrystals: Atomistic tight-binding theory

Based on a successful atomistic tight-binding model in the conjunction with an empirical bowing parameter and the widely used virtual crystal approximation, the theoretical investigations of near infrared InAsxSb1−x nanocrystals with the experimentally synthesized sizes and As compositions (x) are reported. Under various experimental As compositions (x), the single-particle spectra, charge densities, density of states (DOS), overlaps of ground electron and hole wave functions, optical spectra, atomistic electron-hole interactions and stokes shift are numerically computed. I report the correlation of the structural and optical properties of InAsxSb1−x nanocrystals with different alloy compositions (x). With the increasing compositions (x), the single-electron energies are increased, while the single-hole energies are reduced, thus introducing the wider optical band gaps. The atomistic tight-binding model reproduces very well the change in the band gap values with the compositions observed in the experimental reports. The As compositions (x) of alloy InAsxSb1−x nanocrystals are used to propel photonic and optoelectronic device performance in a broad range of the near infrared spectrum with the wave length from 825 to 990 nm. With the increasing content (x), the optical intensities are reduced, whereas atomistic electron-hole interactions and stokes shift are progressively increased. Finally, the present systematic study of alloy InAsxSb1−x nanocrystals is one of the most important milestones on the road to provide the understanding of the composition-dependent structural and optical properties and a complete tactic to design a facile band gap modulation method of preparing the interesting near infrared emitting devices and detectors.

The implications of grading on the emission line width of core–shell nanocrystals

An excitonic coupling with phonons leads to broadening of the emission line. The narrow excitonic emission line width observed in CdZnS/ZnS alloy core graded shell nanocrystals (NCs) with varying compositions is studied using temperature-dependent photoluminescence measurements. Contrary to the consensus that a narrow emission line width is observable with a reduction in size (due to the increased exciton–acoustic phonon coupling coefficient σ), an increased value is noted with reduced size. Based on a theoretical approach to graded core–shell NCs, the relationship between the electron–hole wave function overlap and exciton lifetime is invoked to understand this anomaly. Smaller alloy core–shell NCs (CdZnS/ZnS-I) have a longer lifetime than larger NCs (CdZnS/ZnS-II), indicating reduced electron–hole wave function overlap for CdZnS/ZnS-I NCs and hence a larger ‘effective size’ of NCs, even though the actual size is smaller. The experimental findings demonstrate that graded core–shell NCs reveal an additional functionality, facilitating control of the emission line width of NCs via minimal interaction with the solid state environment.

Resonant optical properties of AlGaAs/GaAs multiple-quantum-well based Bragg structure at the second quantum state

An AlGaAs/GaAs multiple-quantum-well based resonant Bragg structure was designed to match the optical Bragg resonance with the exciton-polariton resonance at the second quantum state in the GaAs quantum wells. The sample structure with 60 periods of AlGaAs/GaAs quantum wells was grown on a semi-insulating GaAs substrate by molecular beam epitaxy. Angle- and temperature-dependent photoluminescence, optical reflectance, and electro-reflectance spectroscopies were employed to study the resonant optical properties of the Bragg structure. Broad and enhanced optical and electro-reflectance features were observed when the Bragg resonance was tuned to the second quantum state of the GaAs quantum well excitons, manifesting a strong light-matter interaction. From the electro-optical experiments, we found the electro-reflectance features related to the transitions of x(e2-hh2) and x(e2-hh1) excitons. The excitonic transition x(e2-hh1), which is prohibited at zero electric field, was allowed by a DC bias due to the brake of symmetry and increased overlap of the electron and hole wave functions caused by the electric field. By tuning the Bragg resonance frequency, we have observed the electro-reflectance feature related to the second quantum state up to room temperature, which evidences a robust light-matter interaction in the resonant Bragg structure.

Interband Optical Transition Energies in Type-II PbSe/PbS Core/Shell Quantum Dots

Electronic and optical properties of Type-II lead based core/shell semiconducting quantum dots are reported. Binding energies of electron–hole pair, optical transition energies and the absorption coefficients are investigated taking into account the geometrical confinement in PbSe/PbS core/shell quantum dot nanostructure. The energies are obtained with the increase of shell thickness for various inner core radii. The probability densities of electron and hole wave functions of radial coordinate of the core PbSe and PbS shell quantum dots are presented. The optical transition energy with the spatial confinement is brought out. The electronic properties are obtained using variational approach whereas the compact density matrix method is employed for the nonlinear optical properties. The results show that (i) a decrease in binding energy is obtained when the shell thickness increases due to more separation of electron–hole pair and (ii) the energy band gap decreases with the increase in the shell thickness resulting in the reduction of the higher energy interband transitions.

Constructive and comprehensive studies on the advantages of using staggered InxGa1-xN/InyGa1-yN QWs in LEDs

A hurdle encountered in the InGaN/GaN LED, is, that the overlap of electrons and holes decreases significantly under certain conditions which deteriorates the performance of the LED. Several methods are being studied for increasing the overlap of electrons and holes of which introduction of a staggered InGaN/GaN QW, in place of the rectangular QW seems promising. In this paper we have studied comprehensively, different forms of the staggered QW; single sided and symmetric, with different Indium compositions in the well. Computations have been carried out through the self-consistent solutions of Schrödinger and Poison equations. We have studied the band structures, fields, the carrier distributions, the transition energies and the overlap of the electron and hole wave functions with current densities. It has been found that the overlap of electrons and holes increases significantly for the staggered QWs at the same current. The best results are obtained for the symmetrically staggered QWs, where the overlap increases even up to three times that of the rectangular QW and using this structure the operating current may be decreased by more than two orders of magnitude to obtain the same transition energy. The theoretical and computational results will be presented with suitable discussions.

Performance improvement of AlGaN-based deep-ultraviolet light-emitting diodes via asymmetric step-like AlGaN quantum wells

Characteristics of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) with light-emitting wavelength around 265 nm via step-like AlGaN quantum wells (QWs) have been investigated. Simulation approach yields a result that, there is significant enhancement of light output power (LOP) for DUV-LEDs with two-layer step-like AlGaN QWs compared to that with conventional one. The location and thickness of AlGaN layer with higher Al-content in the step-like QWs are confirmed to significantly affect the distributions and overlap of electron and hole wavefunctions. The best material characteristic is obtained when the step-like QW is designed as an asymmetric structure, such as Al0.74Ga0.26N (1.8 nm)/Al0.64Ga0.36N (1.2 nm), where AlGaN with higher Al-content layer is set to be located nearer from n-side and be thick as far as possible. The key factors for the performance improvements for this specific design is the enhanced hole transport and mitigated auger recombination.

A complete laboratory for transport studies of electron-hole interactions in GaAs/AlGaAs ambipolar bilayers

We present GaAs/AlGaAs double quantum well devices that can operate as both electron-hole (e-h) and hole-hole (h-h) bilayers, with separating barriers as narrow as 5 nm or 7.5 nm. With such narrow barriers, in the h-h configuration, we observe signs of magnetic-field-induced exciton condensation in the quantum Hall bilayer regime. In the same devices, we can study the zero-magnetic-field e-h and h-h bilayer states using Coulomb drag. Very strong e-h Coulomb drag resistivity (up to 10% of the single layer resistivity) is observed at liquid helium temperatures, but no definite signs of exciton condensation are seen in this case. Self-consistent calculations of the electron and hole wavefunctions show this might be because the average interlayer separation is larger in the e-h case than the h-h case.

Boron-based TADF emitters with improved OLED device efficiency roll-off and long lifetime

A class of four-coordinate boron complexes is reported that contain both electron-rich and electron-poor functional groups. Judicious selection of donor and acceptor moieties with the use of a boron atom as a separating node yields charge transfer complexes capable of thermally activated delayed fluorescence (TADF). Complexes were prepared by a modular method providing access to a wide range of emission colors. The singlet (S1) and triplet (T1) energies are independently tuned to achieve a small S1-T1 gap. Raising and lowering of S1 and T1 states can be predicted using cyclic voltammetry, NTO analysis, and spin density distribution as determined using Density Functional Theory; separation of the hole and electron wavefunction for S1 excitation and delocalization of spin density distribution in the T1 state can help in achieving negligible S1-T1 gap. Although photoluminescent quantum yields of the boron complexes in a host matrix are less than 65%, OLED device external quantum efficiencies of up to 8.1% have been achieved at a luminance of 1000 cd/m2. Selection of a boron emitter with a gap of less than 0.01 eV between the singlet and triplet excited state enables the fabrication of a device with low efficiency roll-off and long lifetime.

Effectivegfactor in black phosphorus thin films

We theoretically investigate the effective $g$ factor in the black phosphorus (BP) thin films (TFs) based on a multiband $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ theory. We demonstrate that the effective single particle $g$ factor in pristine BP TF is anisotropic arising from its anisotropic band structure with ${g}_{xx}^{*}\ensuremath{\approx}{g}_{yy}^{*}\ensuremath{\approx}2.0$ and ${g}_{zz}^{*}$ sensitively depending on the interband coupling and the band gap. The ${g}_{zz}^{*}$ approaches 2.0 with increasing hole doping density and gate electric field since both of them minish the interband coupling by reducing the overlap integral between the electron and hole wave functions. We also estimate the exchange interaction enhancement on the effective single particle $g$ factor by using the screened Hartree-Fock approximation. The exchange interaction enhanced $g$ factor (${g}_{\text{ex}}$) shows maxima (minima) at odd (even) filling factors. The effective $g$ factor (${g}^{*}$) oscillates with the increase of magnetic field and sensitively depends on the Landau level broadening as well as the gate electric field since both of them affect the interband coupling and the electron-electron interactions.

Elucidating the role of surface passivating ligand structural parameters in hole wave function delocalization in semiconductor cluster molecules.

This article describes the mechanisms underlying electronic interactions between surface passivating ligands and (CdSe)34 semiconductor cluster molecules (SCMs) that facilitate band-gap engineering through the delocalization of hole wave functions without altering their inorganic core. We show here both experimentally and through density functional theory calculations that the expansion of the hole wave function beyond the SCM boundary into the ligand monolayer depends not only on the pre-binding energetic alignment of interfacial orbitals between the SCM and surface passivating ligands but is also strongly influenced by definable ligand structural parameters such as the extent of their π-conjugation [π-delocalization energy; pyrene (Py), anthracene (Anth), naphthalene (Naph), and phenyl (Ph)], binding mode [dithiocarbamate (DTC, -NH-CS2-), carboxylate (-COO-), and amine (-NH2)], and binding head group [-SH, -SeH, and -TeH]. We observe an unprecedentedly large ∼650 meV red-shift in the lowest energy optical absorption band of (CdSe)34 SCMs upon passivating their surface with Py-DTC ligands and the trend is found to be Ph- < Naph- < Anth- < Py-DTC. This shift is reversible upon removal of Py-DTC by triethylphosphine gold(i) chloride treatment at room temperature. Furthermore, we performed temperature-dependent (80-300 K) photoluminescence lifetime measurements, which show longer lifetime at lower temperature, suggesting a strong influence of hole wave function delocalization rather than carrier trapping and/or phonon-mediated relaxation. Taken together, knowledge of how ligands electronically interact with the SCM surface is crucial to semiconductor nanomaterial research in general because it allows the tuning of electronic properties of nanomaterials for better charge separation and enhanced charge transfer, which in turn will increase optoelectronic device and photocatalytic efficiencies.

A design strategy for achieving more than 90% of the overlap integral of electron and hole wavefunctions in high-AlN-mole-fraction AlxGa1−xN multiple quantum wells

A strategy for increasing the square of an overlap integral of electron and hole wavefunctions (I2) in polar c-plane AlxGa1−xN multiple quantum wells (MQWs) is proposed. By applying quadratic modulation to AlN mole fractions along the c-axis, local bandgap energies and concentrations of immobile charges induced by polarization discontinuity are simultaneously controlled throughout the MQW structure, and optimized band profiles are eventually achieved. The I2 value can be substantially increased to 94% when the well width (Lw) is smaller than 4.0 nm. In addition, I2 greater than 80% is predicted even for thick MQWs with Lw of 10 nm.

Photoluminescence enhancement in double Ge/Si quantum dot structures

The luminescence properties of double Ge/Si quantum dot structures are studied at liquid helium temperature depending on the Si spacer thickness d in QD molecules. A seven-fold increase in the integrated photoluminescence intensity is obtained for the structures with optimal thickness d = 2 nm. This enhancement is explained by increasing the overlap integral of electron and hole wavefunctions. Two main factors promote this increasing. The first one is that the electrons are localized at the QD base edges and their wavefunctions are the linear combinations of the states of in-plane Δ valleys, which are perpendicular in k-space to the growth direction [001]. This results in the increasing probability of electron penetration into Ge barriers. The second factor is the arrangement of Ge nanoclusters in closely spaced QD groups. The strong tunnel coupling of QDs within these groups increases the probability of hole finding at the QD base edge, that also promotes the increase in the radiative recombination probability.

Klein tunneling in Weyl semimetals under the influence of magnetic field.

Klein tunneling refers to the absence of normal backscattering of electrons even under the case of high potential barriers. At the barrier interface, the perfect matching of electron and hole wavefunctions enables a unit transmission probability for normally incident electrons. It is theoretically and experimentally well understood in two-dimensional relativistic materials such as graphene. Here we investigate the Klein tunneling effect in Weyl semimetals under the influence of magnetic field induced by ferromagnetic stripes placed at barrier boundaries. Our results show that the resonance of Fermi wave vector at specific barrier lengths gives rise to perfect transmission rings, i.e., three-dimensional analogue of the so-called magic transmission angles in two-dimensional Dirac semimetals. Besides, the transmission profile can be shifted by application of magnetic field in the central region, a property which may be utilized in electro-optic applications. When the applied potential is close to the Fermi level, a particular incident vector can be selected by tuning the magnetic field, thus enabling highly selective transmission of electrons in the bulk of Weyl semimetals. Our analytical and numerical calculations obtained by considering Dirac electrons in three regions and using experimentally feasible parameters can pave the way for relativistic tunneling applications in Weyl semimetals.

Anisotropic nature of hole g‐factor in individual InAs quantum rings

AbstractThe in‐plane g‐factors of electron and hole spins (, ) confined in the individual InAs/GaAs quantum rings (QRs) were investigated by using experimental and theoretical approaches. From the measurements, we found that the experimentally obtained varies largely from QR to QR, while the variation in is small. In addition, the in‐plane () and the out‐of‐plane () anisotropies in hole g‐factor were obviously confirmed while the electron g‐factor exhibits isotropic natures in both cases. From the model calculations, the effects of the shape anisotropies and the uniaxial stress were examined. The shape anisotropy in QRs modifies the spatial distributions of hole wavefunctions. Thus, it brings the resultant changes in the degree of valence‐band mixing and , and combined with uniaxial stress, a larger modulation in was achieved. Although more detailed discussions are necessary at this stage, our findings will give valuable information for the g‐factor control in semiconductor nanostructures.