Band Gap Character Research Articles

Electronic structures and the spin polarization of Heusler alloy Co2FeAl surface

The electronic structures of the Heusler alloy Co2FeAl surface are studied theoretically via first-principles calculations. The space localization of the surface states is the key effect on the electronic structures of the Co2FeAl surface. At the surface, the lattice parameter shrinks to minimize the total energy, and the minority spin gap disappears and shows a metallic band gap character. However, with the depth increasing, the lattice parameter equals to that of bulk phase, and there shows an energy gap opening at the Fermi level in the minority spin states. As a result, the spin polarization at the surface is lower than that of the bulk Co2FeAl, while it is close to that of bulk phase beneath the surface. According to the calculations, it is clear that the half-metallic property fading of the Co2FeAl films is caused by the surface states. Therefore, it is important to minimize the lattice mismatch at the interface of Co2FeAl in order to obtain a high tunneling magnetoresistance.

Effects of the passivating coating on the properties of silicon nanocrystals

The effects of the hydrogen-coating of silicon nanocrystals (Si:H NCs) on the chemical and physical properties are theoretically investigated. The empirical tight-binding (TB) method, within the minimal sp 3-basis set and second nearest-neighbor interaction scheme, is employed to calculate the electronic structures, oscillator strength (OS) and recombination rates (RR). The coating is found to induce numerous effects: (i) the full chemical passivation of the dangling bonds existing on the surface of the silicon NCs; (ii) the charge-carrier quantum-confinement (QC) enhancement, which yields direct bandgap character, distinguished with strong and fast photoluminescence (PL) emissions. In this perspective, based on the modeling of the PL data, the QC rules are derived and found to be power-low like, similar to the case of a single particle confined in a 3D box; and (iii) the enhancement of the optical properties (i.e., OS and RR). Furthermore, to deepen our understanding of the coating effects, we have considered the Si 29NC under three different situations: (a) un-coated; (b) the surface-dangling bonds being partially hydrogenated and the rest being dimerized (i.e., Si 29H 24 NC); (c) all the surface-dangling bonds being fully hydrogenated (i.e., Si 29H 36 NC). Using the density-functional-theory (DFT), the total energy calculation has confirmed that the occurrence of to hydrogenization is more probable than the dimerization (i.e., Si 29H 36 has lower energy and is, thus, more stable than Si 29H 24). On one hand, these results corroborate the experimental findings presenting the enhancement of the optical efficiency with the increasing hydrogen content. On the other hand, the atomic relaxation is also shown to further enhance the optical properties and this should in turn corroborate the results of the experimental heat treatment of Si:H NC films, recently reported in literature.

Bandgap characters in GaAs‐based ternary alloys

AbstractThe existence and origins of the bowing character in the bandgap variation of GaAs‐based ternary alloys are theoretically investigated based on two different computational methods. Within the framework of the virtual crystal approximation (VCA), both the empirical sp3s * tight‐binding (TB) method with, and without, the inclusion of the spin‐orbit coupling effects, and the first‐principle full‐potential linear augmented plane wave (FP‐LAPW) technique are applied on both the common‐cation GaSbxAs1‐x and the common‐anion Ga1‐xInxAs alloys. These methods are used to calculate the bandgap energy, the partial and total densities of states and the constituent charge ionicity versus the composition x. The results show that the bowing behavior exists in the case of common‐cation alloys (GaSbxAs1‐x) as a manifestation of a competition between the anion atoms (As and Sb) in trapping the made‐available‐cationic charges. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (χanion). Consistent with this in the case of common‐anion alloys (Ga1‐xInxAs), as due to the lack of anion competition, the bowing is just absent and the variation of bandgap energy is found to be rather linear. The excellent agreement between our theoretical results and recent photoluminescence data has corroborated our claim. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasmonic Cu2−xS Nanocrystals: Optical and Structural Properties of Copper-Deficient Copper(I) Sulfides

Cu(2-x)S (x = 1, 0.2, 0.03) nanocrystals were synthesized with three different chemical methods: sonoelectrochemical, hydrothermal, and solventless thermolysis methods in order to compare their common optical and structural properties. The compositions of the Cu(2-x)S nanocrystals were varied from CuS (covellite) to Cu(1.97)S (djurleite) through adjusting the reduction potential in the sonoelectrochemical method, adjusting the pH value in the hydrothermal method and by choosing different precursor pretreatments in the solventless thermolysis approach, respectively. The crystallinity and morphology of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which shows that most of them might be of pure stoichiometries but some of them are mixtures. The obtained XRDs were studied in comparison to the XRD patterns of previously reported Cu(2-x)S. We found consistently that under ambient conditions the copper deficient Cu(1.97)S (djurleite) is more stable than Cu(2)S (chalcocite). Corroborated by recent computational studies by Lambrecht et al. and experimental work by Alivisatos et al. This may be the reason behind the traditionally known instability of the bulk Cu(2)S/CdS interface. Both Cu(2)S and the copper-deficient Cu(1.97)S have very similar but distinguishable electronic and crystal structure. The optical properties of these Cu(2-x)S NCs were characterized by UV-vis spectroscopy and NIR. All presented Cu(2-x)S NCs show a blue shift in the band gap absorption compared to bulk Cu(2-x)S. Moreover the spectra of these Cu(2-x)S NCs indicate direct band gap character based on their oscillator strengths, different from previously reported experimental results. The NIR spectra of these Cu(2-x)S NCs show a carrier concentration dependent plasmonic absorption.

A rank‐revealing preconditioner for the fast integral‐equation‐based characterization of electromagnetic crystal devices

AbstractA novel rank‐revealing shielded‐block preconditioner that accelerates the iterative integral‐equation‐based analysis of wave propagation in electromagnetic crystal (EC) devices is presented. The proposed shielded‐block preconditioner exploits the bandgap character of the background electromagnetic crystal in order to achieve both rapid convergence of the iterative solver as well as a low matrix‐vector multiplication cost. The versatility and computational efficiency of the shielded‐block preconditioner are demonstrated by its application to the analysis of wave propagation in a defectless electromagnetic crystal along an electromagnetic crystal waveguide and out of an electromagnetic crystal horn antenna array. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 783–789, 2006; Pubished online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/mop.21475

Visible light emission from Si/SiO 2 multilayers in planar microcavities

Bright quantum-confined photoluminescence (PL) at visible wavelengths can be obtained from ultrathin-layer Si/SiO 2 superlattices. We demonstrate that with the use of an optical resonator the PL peak wavelength and bandwidth can be modified selectively. The strong enhancement and subsequent decrease in the PL intensity in these superlattices with decreasing Si layer thickness has been investigated theoretically. Calculations of the band structure of a Si quantum well separated by crystalline SiO 2 barriers using a tight-binding method reveal that the confined conduction and valence bands along the [0 0 1] symmetry direction are essentially dispersionless, are strongly nested, and have a direct band-gap character. The enhanced band-edge density of states and the stronger electron–hole interaction across the low-dielectric barriers lead to a competition between several length scales and produce the PL intensity variation with well width observed.

Band gap variation in laser irradiated polytypic crystals of cadmium iodide

Highly purified single crystals of cadmium iodide obtained through repeated zone refining have been subjected to laser beam exposure (Argon ion laser), both for various time durations and to various beam intensities, and then subjected to band gap determination by UV spectroscopy. The band gap has been found to decrease gradually with increase in the laser beam intensity, whereas its variation with increase in time of exposure shows an unusual behaviour such that it initially falls sharply followed by a gradual rise back to its original value. The results have been analysed and interpreted in terms of indirect band gap character of the material and an unusual variation in phonon frequency. The interpretation is well supported by X-ray diffraction and scanning electron microscopy (SEM) studies.

Synthesis of laminar SnSe crystals by a chemical vapour transport technique

Laminar crystals of tin monoselenide were grown by the chemical vapour transport technique using ammonium chloride as a transporting agent. The crystals were found to be p-type semiconducting in nature with direct and indirect character of the optical band gap. The Hall effect measurements enabled the determination of both carrier mobility and carrier concentration.

Single crystal growth of layered tin monoselenide semiconductor using a direct vapour transport technique

Single crystals of layered tin monoselenide were grown by a direct vapour transport technique. Electron and X-ray diffraction techniques were used for the confirmation of single crystallinity and lattice parameter determination respectively. The crystals were found to be p-type semiconducting in nature with an indirect character of the optical band gap. The stability of the SnSe phase was investigated using differential thermal and thermogravimetric analyses. The measurements of thermoelectric power and resistivity enabled the determination of both carrier mobility and carrier concentration. The variation of carrier concentration and carrier mobility with temperature indicated the presence of deep acceptor levels and lattice or phonons as the dominant scattering mechanism in these crystals.

Relationships between the band gaps of the zinc-blende and wurtzite modifications of semiconductors

While the direct band gaps of wurtzite (W) and zinc-blende (ZB) structures are rather similar, the W and ZB gaps can differ enormously (e.g., \ensuremath{\sim}1 eV in SiC) in indirect gap materials. This large difference is surprising given that the structural difference between wurtzite and zinc blende starts only in the third neighbor and that total energy differences are only \ensuremath{\sim}0.01 eV/atom. We show that zinc-blende compounds can be divided into five types (I--V) in terms of the order of their ${\mathrm{\ensuremath{\Gamma}}}_{1\mathit{c}}$, ${\mathit{X}}_{1\mathit{c}}$, and ${\mathit{L}}_{1\mathit{c}}$ levels and that this decides the character (direct, indirect, pseudodirect) of the wurtzite band gap. The observation of small ${\mathit{E}}_{\mathit{g}}^{\mathrm{W}}$-${\mathit{E}}_{\mathit{g}}^{\mathrm{ZB}}$ differences in direct band-gap systems (``type II,'' e.g., ZnS), and large differences in indirect gap systems (``type IV,'' e.g., SiC) are explained. We further show that while both type-III systems (e.g., AIN) and type-V systems (e.g., GaP) have an indirect gap in the zinc-blende form, their wurtzite form will have direct and pseudodirect band gaps, respectively. Furthermore, a direct-to-pseudodirect transition is predicted to occur in type-I (e.g., GaSb) systems.

Characterization of ultrathin Ge epilayers on (100) Si

The understanding of the epitaxy of pure Ge layers on Si is an important step towards the synthesis of SimGen (m, n < 10 monolayers) short-period superlattices. The possibility of a direct band-gap character makes these structures extremely attractive. We have grown thin buried Gen ([Formula: see text] monolayers) films on (100) Si by molecular beam epitaxy and studied their structural properties by a variety of techniques including Raman scattering spectroscopy, glancing incidence X-ray reflection, Rutherford backscattering, transmission electron microscopy, and extended X-ray absorption fine structure analysis. All these techniques allowed detection of the thin Ge layers and provided information about the thickness, morphology, strain distribution, and interface sharpness of these heterostructures. The Ge„ films with [Formula: see text] had a two-dimensional nature and showed no sign of strain relaxation. Intermixing at the Si–Ge interfaces was present in all these films and estimated to be not more than two monolayers. This smearing at the interfaces may have contributed to the maintenance of that pseudomorphicity. A thicker Ge layer (n = 12) showed evidence of strain relaxation and clustering in three-dimensional islands.