Empirical Tight-binding Calculations Research Articles

(GaAs/InAs)–GaAsSb digital alloy type-II superlattice for extended short-wavelength infrared detection

GaInAs–GaAsSb type-II superlattices (T2SLs) on an InP substrate are promising candidates for an optical absorption layer in the extended short-wavelength region (2–3 μm), offering more flexibility in designing a cutoff wavelength compared to strained GaInAs bulk material. However, T2SL-based photodetectors inherently suffer from lower quantum efficiency (QE) due to the reduced overlap of the wavefunctions of the conduction and valence bands in the optical matrix element of the T2SL. To improve QE, a (GaAs/InAs)–GaAsSb digital alloy T2SL, which replaces the GaInAs random alloy layer in the GaInAs–GaAsSb T2SL with a GaAs/InAs digital alloy, has been proposed recently by an empirical tight-binding calculation. This paper presents a demonstration of a fabricated photodetector using the (GaAs/InAs)–GaAsSb digital alloy grown on an InP substrate by molecular beam epitaxy and shows that the average QE in the wavelength region of 2.3–2.6 μm is approximately 1.6 times higher than that of a conventional GaInAs–GaAsSb T2SL photodetector. Furthermore, the dark-current density of the digital alloy photodetector is lower than that of the GaInAs–GaAsSb T2SL photodetector despite having a longer cutoff wavelength.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

Topological phase transition of single-crystal Bi based on empirical tight-binding calculations

The topological order of single-crystal Bi and its surface states on the (111) surface are studied in detail based on empirical tight-binding (TB) calculations. New TB parameters are presented that are used to calculate the surface states of semi-infinite single-crystal Bi(111), which agree with the experimental angle-resolved photoelectron spectroscopy results. The influence of the crystal lattice distortion is surveyed and it is revealed that a topological phase transition is driven by in-plane expansion with topologically non-trivial bulk bands. In contrast with the semi-infinite system, the surface-state dispersions on finite-thickness slabs are non-trivial irrespective of the bulk topological order. The role of the interaction between the top and bottom surfaces in the slab is systematically studied, and it is revealed that a very thick slab is required to properly obtain the bulk topological order of Bi from the (111) surface state: above 150 biatomic layers in this case.

Crystalline spin–orbit interaction and the Zeeman splitting in Pb1−xSnxTe

The ratio of the Zeeman splitting to the cyclotron energy (), which characterizes the relative strength of the spin–orbit interaction in crystals, is examined for the narrow gap IV–VI semiconductors PbTe, SnTe, and their alloy Pb1−xSnxTe on the basis of the multiband theory. The inverse mass α, the g-factor g, and M are calculated numerically by employing the relativistic empirical tight-binding band calculation. On the other hand, a simple but exact formula of M is obtained for the six-band model based on the group theoretical analysis. It is shown that M < 1 for PbTe and M > 1 for SnTe, which are interpreted in terms of the relevance of the interband couplings due to the crystalline spin–orbit interaction. It is clarified both analytically and numerically that M is not a quantized value but a continuous one, and M = 1 is obtained just at the band inversion point, where the transition from trivial to nontrivial topological crystalline insulator occurs. By using this property, one can detect the transition point only with the bulk measurements. It is also proposed that M is useful to evaluate quantitatively a degree of the Dirac electrons in solids.

Structure and optical properties of capped and uncapped CdS nanoparticles prepared in aqueous medium

We have prepared the hexagonal structure of CdS nanoparticles in an aqueous solution with different sizes and varied surface compositions by using fixed molar ratio of the starting precursors in the presence of capping molecules. In addition, we have prepared uncapped CdS nanoparticles by cadmium chloride and thiourea at low temperature. We showed that the environmental conditions and the type of the aqueous medium are the effective parameters for the exchange of the nanoparticle size. The prepared nanoparticles have sizes in the range from 25 to 100 A. We have compared the experimentally determined size of CdS nanoparticles with that determined by theoretical calculations. The comparison showed that the size determined by Scherrer’s equation is fitted well with the empirical tight binding calculations, and that the effective mass approximation yields size values is in good agreement with the size estimated by high resolution transmission electron microscopy. Photoluminescence spectroscopy revealed that the nanoparticles with stoichiometries composition S/Cd ~ 1 have a high intensity band edge emission in the blue region for the capped nanoparticles and green emission for the uncapped nanoparticles.

Controlling a Nanowire Quantum Dot Band Gap Using a Straining Dielectric Envelope

We tune the emission wavelength of an InAsP quantum dot in an InP nanowire over 200 meV by depositing a SiO(2) envelope using plasma-enhanced chemical vapor deposition without deterioration of the optical quality. This SiO(2) envelope generates a controlled static strain field. Both red and blue shift can be easily achieved by controlling the deposition conditions of the SiO(2). Using atomistic empirical tight-binding calculations, we investigate the effect of strain on a quantum dot band structure for different compositions, shape, and crystal orientations. From the calculations, we estimate the applied strain in our experiment. This enables engineering of the band gap in nanowires with unprecedented possibilities to extend the application range of nanowire devices.

Tight-binding calculation with up to the 3rd nearest neighbor coupling for small-radius carbon nanotubes

We present the empirical tight-binding calculation of band structures of small-radius single-walled carbon nanotubes (SWCNTs). It incorporates the curvature effect as well as the electronic hopping up to the 3rd nearest neighbor and, as a result, closes the discrepancy between previous tight-binding calculations and first-principle studies. The calculation produces a non-monotonous variation of band gaps, where the gap increases initially and then decreases, when the tube radius decreases. In particular, in the zigzag (6, 0) case, it yields a vanishing band gap. We conclude that the inclusion of the 3rd nearest neighbor hopping is of paramount importance in the tight-binding calculation of small-radius SWCNTs.

Thermoelectric transport in periodic one-dimensional stacks of InAs/GaAs quantum dots

We investigate the effect of the narrow electronic minibands of periodic one-dimensional stacks of disk-shaped InAs quantum dots (QDs) in GaAs on their electronic transport characteristics by employing an empirical tight-binding calculation and a continuum model of the electronic structure. Our model includes both the minibands and the continuum of the host conduction band. The rate of the electron-acoustic-phonon scattering is found using Boltzmann's semiclassical transport theory. The electric conductivity, the Seebeck coefficient and the thermoelectric figure-of-merit for $n$-doped QD arrays are then analyzed as a function of the donor concentration and temperature. For QDs several nanometers in height, the figure-of-merit at temperatures below 100 K as a function of doping is richly structured, reflecting the miniband electron energy spectrum of a QD stack. Certain windows of concentration are revealed, where QD arrays display a geometry-controlled enhanced efficiency as thermoelectric converters. For optimizing the peak values of the figure-of-merit attainable for donor concentrations within the experimentally accessible range, a very thin spacer layer between the QDs $(\ensuremath{\lesssim}5\text{ }\text{nm})$ is found to be most suitable. Assuming that the lattice thermal conductivity can be reduced below $0.5\text{ }\text{W}/(\text{m}\phantom{\rule{0.2em}{0ex}}\text{K})$, a figure-of-merit larger than 2 appears within reach.

Armchair nanoribbons of silicon and germanium honeycomb structures

We present a first-principles study of bare and hydrogen passivated armchair nanoribbons of the puckered single layer honeycomb structures of silicon and germanium. Our study includes optimization of atomic structure, stability analysis based on the calculation of phonon dispersions, electronic structure and the variation of band gap with the width of the ribbon. The band gaps of silicon and germanium nanoribbons exhibit family behavior similar to those of graphene nanoribbons. The edges of bare nanoribbons are sharply reconstructed, which can be eliminated by the hydrogen termination of dangling bonds at the edges. Periodic modulation of the nanoribbon width results in a superlattice structure which can act as a multiple quantum wells. Specific electronic states are confined in these wells. Confinement trends are qualitatively explained by including the effects of the interface. In order to investigate wide and long superlattice structures we also performed empirical tight binding calculations with parameters determined from \textit{ab initio} calculations.

Ab initio calculation of band edges modified by (001) biaxial strain in group IIIA–VA and group IIB–VIA semiconductors: Application to quasiparticle energy levels of strained InAs/InP quantum dot

Results of first-principles full potential calculations of absolute position of valence and conduction energy bands as a function of (001) biaxial strain are reported for group IIIA–VA (InAs, GaAs, InP) and group IIB–VIA (CdTe, ZnTe) semiconductors. Our computational procedure is based on the Kohn–Sham form of density functional theory (KS DFT), local spin density approximation (LSDA), variational treatment of spin-orbital coupling, and augmented plane wave plus local orbitals method (APW+lo). The band energies are evaluated at lattice constants obtained from KS DFT total energy as well as from elastic free energy. The conduction band energies are corrected with a rigid shift to account for the LSDA band gap error. The dependence of band energies on strain is fitted to polynomial of third degree and results are available for parameterization of biaxial strain coupling in empirical tight-binding models of IIIA–VA and IIB–VIA self-assembled quantum dots (SAQDs). The strain effects on the quasiparticle energy levels of InAs/InP SAQD are illustrated with empirical atomistic tight-binding calculations.

Surface photovoltage and photoluminescence spectroscopy of self-assembled InAs/InP quantum wires

The optical properties of InAs/InP multi-layer quantum wire (QWR) structures of various spacer thicknesses have been investigated by means of room temperature surface photovoltage and photoluminescence spectroscopy. Combined with empirical tight binding calculations, the spectra have revealed transitions assigned to QWR families with heights equal to integer number of 5, 6 and 7 monolayers. From the comparison of the experimental and theoretical results the atomic concentration of phosphorus in the wires has been estimated.

Fabrication of ordered arrays of quantum wires through hole patterning

We present empirical tight-binding calculations of the electronic structure of an array of mechanically connected silicon nanopillars. Structural parameters are chosen so that electrons are confined within each pillar, obtaining an array of effectively decoupled one-dimensional states. We address fabrication issues and present results demonstrating the suitability of the process.

Spin-orbit splitting, Fermi surface topology, and charge-density-wave gapping in2H−TaSe2

Empirical tight-binding calculations on the layer compound $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{Se}}_{2}$ have been performed and are found to replicate the basic phenomenology of Fermi surface topology and charge-density-wave gapping as revealed by angle-resolved photoemission spectroscopy (ARPES). The recently discovered piece of Fermi surface having a ``dog's bone'' shape is a consequence of the large spin-orbit splitting of $\mathrm{Ta}\phantom{\rule{0.2em}{0ex}}5d$ levels. The $3\ifmmode\times\else\texttimes\fi{}3$ reconstruction, simulated here by bond strengthenings and weakenings, gives rise to gapping, predominantly on the $K(H)$-centered Fermi surface pocket in agreement with ARPES data. The concept of nesting is not used in the analysis.

Angle-resolved photoemission spectroscopy of perovskite-type transition-metal oxides and their analyses using tight-binding band structure

Nowadays it has become feasible to perform angle-resolved photoemission spectroscopy (ARPES) measurements of transition-metal oxides with three-dimensional perovskite structures owing to the availability of high-quality single crystals of bulk and epitaxial thin films. In this article, we review recent experimental results and interpretation of ARPES data using empirical tight-binding band-structure calculations. Results are presented for SrVO3 (SVO) bulk single crystals and La1− x Sr x FeO3 (LSFO) and La1− x Sr x MnO3 (LSMO) thin films. In the case of SVO, from comparison of the experimental results with calculated surface electronic structure, we concluded that the obtained band dispersions reflect the bulk electronic structure. The experimental band structures of LSFO and LSMO were analyzed assuming the G-type antiferromagnetic state and the ferromagnetic state, respectively. We also demonstrated that the intrinsic uncertainty of the electron momentum perpendicular to the crystal surface is important for the interpretation of the APRES results of three-dimensional materials.

SiH2 adsorption on the single dimer vacancy of the Si(100) surface

The energetic and structural properties ofSiH2 adsorbed on the single dimer vacancy (SDV) of theSi(100)-c(4 × 2) surface have been studied by using empirical tight-binding (ETB) total energy calculations. The adsorptionsof SiH2,SiH2 with a hydrogenadatom (SiH2+H)and SiH2 with twohydrogen adatoms (SiH2+2H) on the Si(100) surface with SDV structure have been studied. Threepossible sites (A, B and C) are found in each case. Adsorbing ofSiH2 or H on the SDV makes the structure of the SDV change. When a unit ofSiH2 is adsorbed on the SDV, the A site is more stable than the B site. IfSiH2+H orSiH2+2H is adsorbed on the SDV, the relative stability of the A site and B site reverses. The C siteis found to be the least stable site in all cases.

Symmetry properties and electronic band structure of ordered Zn0.5Cd0.5Se alloys

Along the [001] direction of a zincblende semiconductor we find a sequence of cation–anion planes. In the case of a random zincblende ternary alloy of the type A1−xBxC the A and B cations are randomly distributed in each cation plane. In the particular case, when x=0.5, another possible arrangement is the sequence A–C–B–C–A–C…; we can describe this crystal as an (AC)1(BC)1 superlattice, however, it is not a true superlattice, it is one of the possible ordered structures that we can obtain with A0.5B0.5C composition. This ternary alloy has the simple tetragonal structure, in contrast with the fcc crystalline structure of the random alloy with the same composition. Here, we present the results for the electronic band structure of the ordered Zn0.5Cd0.5Se alloy qualitatively determined by means of a comparative analysis of the symmetry properties of both crystals. The volume reduction of the first Brillouin zone (1BZ) of the tetragonal alloy, in relation to the cubic alloy, gives place to band folding effects. They are advantageously employed to describe the band structure of the ordered alloys in terms of the random one. Band gap reduction, removal of degeneracy at some symmetry points of the 1BZ and the doubling of the number of bands is concluded for the ordered alloy compared to the ordered one. A quite good agreement is obtained when comparing the ordered band structure obtained by symmetry arguments with an empirical tight binding calculation.

Electronic and optical properties of GaAsN/GaAs quantum wells: A tight-binding study

We present empirical tight-binding (TB) calculations of the electronic structure of GaAs1−xNx/GaAs (001) quantum wells (QWs) with small N concentrations (0&amp;lt;x&amp;lt;0.045). We use a recently developed TB model for the electronic structure of dilute GaAs1−xNx [Shtinkov et al., Phys. Rev. B 67, 081202R (2003)] without introducing any additional parameters apart from the valence band offset (VBO) between GaAs and GaAs1−xNx. The dependences of the bound states energies on the QW width and on the N concentration x are investigated and the nature of the lowest-energy optical transitions is analyzed, showing that the knowledge of the first two optical transitions is sufficient to determine the value of the VBO. Our results are compared with experimental data from the literature, revealing good agreement for x⩽0.02. For larger concentrations, we find that the agreement is greatly improved if the concentrations determined from x-ray diffraction data are corrected for deviations from Vegard’s rule. The comparison with experimental results suggests that the unstrained GaAs/GaAs1−xNx VBO is close to zero, in agreement with other studies.

Giant birefringence in zinc-blende-based artificial semiconductors

We use extended-basis empirical tight-binding calculations and examine the anisotropy of the refractive index in ultrashort-period superlattices of materials sharing no common atom. We find that a strong birefringence can be engineered in these articial semiconductors, allowing phase matching for frequency difference generation. The prominent role of epitaxial constraint and bond-length alternation is evidenced.

Empirical tight-binding calculations of the electronic structure of dilute III–V–N semiconductor alloys

We present empirical tight-binding (TB) calculations of the electronic structure of GaP1−xNx and InyGa1−yAs1−xNx alloys with low nitrogen content (x&amp;lt;0.05) over the entire Brillouin zone. Following the method recently developed for GaAs1−xNx [Shtinkov et al., Phys. Rev. B 67, 081202 (2003)], we add to the TB basis an additional anion s orbital (sN) in order to account for the N-induced change of the electronic structure. The band structures of GaP1−xNx and InyGa1−yAs1−xNx are calculated using an sp3d5s*sN TB parametrization. Our TB results are in excellent agreement with experimental and other theoretical data without introducing any additional fitting parameters, demonstrating that the developed method is a promising tool for modeling a wide range of dilute nitride materials and heterostructures.

Structures and Energetics of Regioisomers of C60 Dimer and Trimers

Ten different regioisomers of C60 trimers with intact cages were studied using the empirical tight-binding (TB) total energy calculation and the semiempirical PM3, AM1, and MNDO methods. We determined the stable atomic structure and energies of 10 (C60)3 regioisomers with ETBTEC, PM3, AM1, and MNDO calculations. The dimerization of C60 is exothermic according to the density-functional-based nonorthogonal tight-binding (DF-TB) method; the semiempirical PM3, AM1, and MNDO; and the ab initio 3-21G LDA and HF calculations, but it is endothermic in both the TB and the ab initio B3LYP6-31G*//4-21G B3LYP calculations. Therefore, according to the TB calculations, the most stable regioisomer of C60 trimers is equatorial. The energies of the trans-3(C2), trans-1(D2h), trans-2(C2), trans-4(Cs), cis-1 (C3ν), cis-2(Cs), cis-3(C2), cis-1(Cs), and cis-2(C3ν) regioisomers relative to the equatorial regioisomer are 13,19, 20, 30, 66, 99, 169, 215, and 673 meV, respectively.