The Experts below are selected from a list of 4824 Experts worldwide ranked by ideXlab platform
Rositsa Yakimova - One of the best experts on this subject based on the ideXlab platform.
-
sublimation epitaxial growth of hexagonal and cubic sic
Comprehensive Semiconductor Science and Technology, 2011Co-Authors: Mikael Syvajarvi, Rositsa YakimovaAbstract:Sublimation growth is the key growth process for silicon carbide. The advancements have come through from evolution from the first Acheson process to produce coarse crystalline silicon carbide, continued by the introduction of the Lely processes in which single-crystal silicon carbide was possible, to finally reach a modified Lely process with a seeding technique introduced by Tairov and Tsvetkov (1978), which made large-area single-crystal wafers available. This evolution created the platform for single-crystal growth of semiconductor grade silicon carbide for electronics. Even so, the growth is not completely understood. The polytype stability and their structural quality in bulk and epitaxial growth still needs better understanding of the growth processes. Here, we describe a sublimation growth process working at lower temperatures than the modified Lely growth process. The quality of the material in relation to growth mechanism and surface appearance as a consequence of step flow growth and step bunching is described. The stability of hexagonal Polytypes, and, in particular, the appearance of the cubic silicon carbide polytype is described.
-
sublimation epitaxial growth of hexagonal and cubic sic
Comprehensive Semiconductor Science and Technology, 2011Co-Authors: Mikael Syvajarvi, Rositsa YakimovaAbstract:Sublimation growth is the key growth process for silicon carbide. The advancements have come through from evolution from the first Acheson process to produce coarse crystalline silicon carbide, continued by the introduction of the Lely processes in which single-crystal silicon carbide was possible, to finally reach a modified Lely process with a seeding technique introduced by Tairov and Tsvetkov (1978), which made large-area single-crystal wafers available. This evolution created the platform for single-crystal growth of semiconductor grade silicon carbide for electronics. Even so, the growth is not completely understood. The polytype stability and their structural quality in bulk and epitaxial growth still needs better understanding of the growth processes. Here, we describe a sublimation growth process working at lower temperatures than the modified Lely growth process. The quality of the material in relation to growth mechanism and surface appearance as a consequence of step flow growth and step bunching is described. The stability of hexagonal Polytypes, and, in particular, the appearance of the cubic silicon carbide polytype is described.
F Bechstedt - One of the best experts on this subject based on the ideXlab platform.
-
polytypism in zns znse and znte first principles study
Physical Review B, 2014Co-Authors: F Boutaiba, M Ferhat, A Belabbes, F BechstedtAbstract:We report results of first-principles calculations based on the projector augmented wave (PAW) method to explore the structural, thermodynamic, and electronic properties of cubic (3C) and hexagonal (6H, 4H, and 2H) Polytypes of II-VI compounds: ZnS, ZnSe, and ZnTe. We find that the different bond stacking in II-VI Polytypes remarkably influences the resulting physical properties. Furthermore, the degree of hexagonality is found to be useful to understand both the ground-state properties and the electronic structure of these compounds. The resulting lattice parameters, energetic stability, and characteristic band energies are in good agreement with available experimental data. Trends with hexagonality of the polytype are investigated.
-
structure energetics and electronic states of iii v compound Polytypes
Journal of Physics: Condensed Matter, 2013Co-Authors: F Bechstedt, A BelabbesAbstract:Recently several hexagonal Polytypes such as 2H, 4H, and 6H have been discovered for conventional III–V semiconductor compounds in addition to the cubic 3C zinc-blende polytype by investigating nanorods grown in the [111] direction in different temperature regimes. Also III-mononitrides crystallizing in the hexagonal 2H wurtzite structure under ambient conditions can be deposited in zinc-blende geometry using various growth techniques. The polytypic crystal structures influence the local electronic properties and the internal electric fields due to the spontaneous polarization in non-cubic crystals.In this paper we give a comprehensive review on the thermodynamic, structural, and electronic properties of twelve Al, Ga, and In antimonides, arsenides, phosphides, and nitrides as derived from ab initio calculations. Their lattice parameters, energetic stability, and characteristic band structure energies are carefully discussed and related to the atomic geometries of the Polytypes. Chemical trends are investigated. Band offsets between Polytypes and their consequences for heterocrystalline structures are derived. The described properties are discussed in the light of available experimental data and previous computations. Despite several contradictory results in the literature, a unified picture of the III–V Polytypes and their heterocrystalline structures is developed.
-
electronic bands of iii v semiconductor Polytypes and their alignment
Physical Review B, 2012Co-Authors: A Belabbes, J. Furthmüller, Christian Panse, F BechstedtAbstract:The quasiparticle band structures of four Polytypes 3C, 6H, 4H, and 2H of GaP, GaAs, GaSb, InP, InAs, and InSb are computed with high accuracy including spin-orbit interaction applying a recently developed approximate calculation scheme, the LDA-1/2 method. The results are used to derive band offsets $\ensuremath{\Delta}{E}_{c}$ and $\ensuremath{\Delta}{E}_{v}$ for the conduction and valence bands between two Polytypes. The alignment of the band structures is based on the branch-point energy ${E}_{\mathrm{BP}}$ for each polytype. The aligned electronic structures are used to explain properties of heterocrystalline but homomaterial junctions. The gaps and offsets allow to discuss spectroscopic results obtained recently for such junctions in III-V nanowires.
-
unit cell structure of crystal Polytypes in inas and insb nanowires
Nano Letters, 2011Co-Authors: Dominik Kriegner, Christian Panse, Bernhard Mandl, Kimberly A Dick, Mario Keplinger, Johan Mikael Persson, Philippe Caroff, Daniele Ercolani, L Sorba, F BechstedtAbstract:The atomic distances in hexagonal Polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H Polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tends to stretch the distances of atomic layers parallel to the c axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.
-
lattice dynamics of sic Polytypes within the bond charge model
Physical Review B, 1994Co-Authors: M Hofmann, A. Zywietz, K Karch, F BechstedtAbstract:We present a phemomenological approach to the lattice-dynamical properties of various SiC Polytypes. A generalized bond-charge model is applied to the cubic and hexagonal Polytypes 3C, 6H, 4H, and 2H. The long-range microscopic electric field of ions and bond charges is fully taken into account via an Ewald technique. The short-range elastic interactions are described by bending and stretching forces between ions and bond changes. The force constants and effective charges are fit to phonon frequencies known from Raman and luminescence measurements. The reliability of the model is tested not only for the frequencies but also for the eigenvectors by comparison with results of ab initio calculations for 3C- and 2H-SiC. We show that the anisotropy in the uniaxial hexagonal Polytypes is mainly due to the nonanalyticity of the Coulomb forces. The corresponding frequency splittings are related to slight changes of the bond charges in dependence on the bond orientation. Differences in the elastic forces parallel and nonparallel to the hexagonal axis are only necessary to stabilize the zone-boundary phonons. The trends of the resulting frequencies and density of states are discussed versus the percentage hexagonality. Consequences of the different phonon dispersions for the elastic properties of the 3C, 6H, 4H, and 2H Polytypes are considered. We speculate about possible mechanisms of polytype stabilization by the lattice vibrations.
Vishnu P Kamath - One of the best experts on this subject based on the ideXlab platform.
-
polytypism in sulfate intercalated layered double hydroxides of zn and m iii m al cr observation of cation ordering in the metal hydroxide layers
Inorganic Chemistry, 2013Co-Authors: S Radha, Vishnu P KamathAbstract:The as-precipitated sulfate-intercalated layered double hydroxide of Zn and Al crystallizes in the structure of the 3R1 polytype. On hydrothermal treatment, this 3R1 polytype transforms into the somewhat rare 3H and 3R2 Polytypes at different temperatures. Observation of the 3R2 polytype distinct from the 3R1 polytype is evidence for the lack of cation ordering in the [Zn–Al–SO4] system. The layered double hydroxide of Zn and Cr (polytype, 1H) on hydrothermal treatment in mother liquor yields a cation-ordered phase also having the structure of the 1H polytype. Direct evidence of cation ordering is found by the appearance of weak supercell reflections corresponding to a = √3 × ao (ao is the a parameter of the cation-disordered phase). The same precursor under other conditions yields the cation-disordered 3R1 polytype. In this work, the structures of both the cation-ordered and the cation-disordered phases with similar states of hydration are refined and compared.
-
polytypism in the lithium aluminum layered double hydroxides the lial2 oh 6 layer as a structural synthon
Inorganic Chemistry, 2011Co-Authors: Sylvia Britto, Vishnu P KamathAbstract:The [LiAl2(OH)6]+ layer obtained from gibbsite–Al(OH)3 belongs to the layer group symmetry P-312/m. This layer satisfies the defining characteristics of a synthon in that it predicts all the polymorphic modifications of the layered double hydroxides of Li and Al. The various possible ways of stacking these layers can be derived by the systematic elimination of the principal symmetry elements comprising the layer group. This approach yields the complete universe of possible structures. When the 3 axis of the layer is conserved in the stacking, the resultant crystal adopts the structure of the 1H, 2H, or 3R Polytypes (H, hexagonal; R, rhombohedral). When the 3 axis is destroyed and the 2/m axis is retained, the crystal adopts monoclinic symmetry and crystallizes in the structures of the 1M1 or 1M2 (M, monoclinic) Polytypes; the two Polytypes differ only in their translational component. Experimentally, gibbsite-based precursors yield the 2H polytype, and bayerite-based precursors yield the 1M polytype. Faul...
-
polytypism and stacking disorders in nickel hydroxide a first principles study
Journal of The Electrochemical Society, 2010Co-Authors: Madhvendra Nath Tripathi, Umesh V Waghmare, T N Ramesh, Vishnu P KamathAbstract:We use first-principles pseudopotential-based density functional theory calculations to determine the electronic structure, energetics and relative stability of Polytypes, and stacking faults of nickel hydroxide. Our results show that the 1H polytype of nickel hydroxide is the most stable, which agrees with the experiment. The close-packed Polytypes 2 H2 and 3 R 2 are higher in energy by only ⼠0.01 eV per formula unit, thereby accounting for the observation of faulted structures. The stacking faults with 3 R2 motifs are associated with higher energy (22.2 mJ/ m2) compared to those with 2 H2 motifs (7.2 mJ/ m 2) accounting for the experimentally observed transformations in the faulted samples. © 2010 The Electrochemical Society.
Mikael Syvajarvi - One of the best experts on this subject based on the ideXlab platform.
-
sublimation epitaxial growth of hexagonal and cubic sic
Comprehensive Semiconductor Science and Technology, 2011Co-Authors: Mikael Syvajarvi, Rositsa YakimovaAbstract:Sublimation growth is the key growth process for silicon carbide. The advancements have come through from evolution from the first Acheson process to produce coarse crystalline silicon carbide, continued by the introduction of the Lely processes in which single-crystal silicon carbide was possible, to finally reach a modified Lely process with a seeding technique introduced by Tairov and Tsvetkov (1978), which made large-area single-crystal wafers available. This evolution created the platform for single-crystal growth of semiconductor grade silicon carbide for electronics. Even so, the growth is not completely understood. The polytype stability and their structural quality in bulk and epitaxial growth still needs better understanding of the growth processes. Here, we describe a sublimation growth process working at lower temperatures than the modified Lely growth process. The quality of the material in relation to growth mechanism and surface appearance as a consequence of step flow growth and step bunching is described. The stability of hexagonal Polytypes, and, in particular, the appearance of the cubic silicon carbide polytype is described.
-
sublimation epitaxial growth of hexagonal and cubic sic
Comprehensive Semiconductor Science and Technology, 2011Co-Authors: Mikael Syvajarvi, Rositsa YakimovaAbstract:Sublimation growth is the key growth process for silicon carbide. The advancements have come through from evolution from the first Acheson process to produce coarse crystalline silicon carbide, continued by the introduction of the Lely processes in which single-crystal silicon carbide was possible, to finally reach a modified Lely process with a seeding technique introduced by Tairov and Tsvetkov (1978), which made large-area single-crystal wafers available. This evolution created the platform for single-crystal growth of semiconductor grade silicon carbide for electronics. Even so, the growth is not completely understood. The polytype stability and their structural quality in bulk and epitaxial growth still needs better understanding of the growth processes. Here, we describe a sublimation growth process working at lower temperatures than the modified Lely growth process. The quality of the material in relation to growth mechanism and surface appearance as a consequence of step flow growth and step bunching is described. The stability of hexagonal Polytypes, and, in particular, the appearance of the cubic silicon carbide polytype is described.
J. Furthmüller - One of the best experts on this subject based on the ideXlab platform.
-
relation between spontaneous polarization and crystal field from first principles
Physical Review B, 2013Co-Authors: A Belabbes, J. Furthmüller, Friedhelm BechstedtAbstract:Recently several hexagonal Polytypes of III-V compounds have been discovered during growth of nanowires. They exhibit a spontaneous polarization. We calculate the polarization using the ab initio density functional theory within the Berry phase and the electrostatic method. We demonstrate its clear relationship to the crystal field characterized by internal-cell parameters and polytype hexagonality. Sign and magnitude of the polarization field in wurtzite follow the deviation of the internal-cell parameter from its ideal value, thereby explaining the opposite behavior of III-nitrides and the corresponding phosphides, arsenides, and antimonides.
-
electronic bands of iii v semiconductor Polytypes and their alignment
Physical Review B, 2012Co-Authors: A Belabbes, J. Furthmüller, Christian Panse, F BechstedtAbstract:The quasiparticle band structures of four Polytypes 3C, 6H, 4H, and 2H of GaP, GaAs, GaSb, InP, InAs, and InSb are computed with high accuracy including spin-orbit interaction applying a recently developed approximate calculation scheme, the LDA-1/2 method. The results are used to derive band offsets $\ensuremath{\Delta}{E}_{c}$ and $\ensuremath{\Delta}{E}_{v}$ for the conduction and valence bands between two Polytypes. The alignment of the band structures is based on the branch-point energy ${E}_{\mathrm{BP}}$ for each polytype. The aligned electronic structures are used to explain properties of heterocrystalline but homomaterial junctions. The gaps and offsets allow to discuss spectroscopic results obtained recently for such junctions in III-V nanowires.
-
properties of hexagonal Polytypes of group iv elements from first principles calculations
Physical Review B, 2002Co-Authors: C Raffy, J. Furthmüller, Friedhelm BechstedtAbstract:Results of ab initio calculations are reported for hexagonal Polytypes of C, Si, and Ge in equilibrium and under hydrostatic pressure. For each polytype $2H,$ $3C,$ $4H,$ and $6H,$ the atomic geometry, the energetics, and the electronic structure are studied. The resulting lattice parameters are in good agreement with measured values. While $3C$ is the most stable polytype for each element, pressure-induced phase transitions to hexagonal modifications are found to be possible. Silicon is the most favorable candidate in this respect. The results are interpreted within the axial next-nearest-neighbor Ising model. It simultaneously allows the derivation of formation energies for stacking faults in agreement with other calculations and measurements. We predict significant differences in the band structures between the hexagonal Polytypes and the diamond structure. This holds especially for the energy gaps and the location of the conduction-band minima. Trends with the hexagonality of the polytype and the element are derived.
-
Polytypism and properties of silicon carbide
Physica Status Solidi (B) Basic Research, 1997Co-Authors: Friedhelm Bechstedt, P. Käckell, A. Zywietz, K. Tenelsen, Brigitte Adolph, K Karch, J. FurthmüllerAbstract:The relationship between crystal structure and related material properties is discussed for the common 3C, 6H, 4H, and 2H Polytypes of SiC. The theoretical results are derived in the framework of well converged density-functional calculations within the local-density approximation and the pseudopotential-plane-wave approach. In the case of electronic excitations additionally quasiparticle corrections are included. The lattice-dynamical properties of the noncubic Polytypes are described within a bond-charge model. We focus our attention on the actual atomic structures, the accompanying lattice vibrations, thermodynamical properties, properties of layered combinations of Polytypes, optical spectra, and surface equilibrium structures. On the one hand, the influence of the polytype on the material properties is considered. On the other hand, indications for driving forces of the polytypism are extracted.
