Cohesive energies of cubic III-V semiconductors

Growth and Fabrication of High-Quality Single Nanowire Devices with Radial p-i-n Junctions

Nanowires (NWs) with radial p-i-n junction have advantages, such as large junction area and small influence from the surface states, which can lead to highly efficient material use and good device quantum efficiency. However, it is difficult to make high-quality core-shell NW devices, especially single NW devices. Here, the key factors during the growth and fabrication process that influence the quality of single core-shell p-i-n NW devices are studied using GaAs(P) NW photovoltaics as an example. By p-doping and annealing, good ohmic contact is achieved on NWs with a diameter as small as 50-60 nm. Single NW photovoltaics are subsequently developed and a record fill factor of 80.5% is shown. These results bring valuable information for making single NW devices, which can further benefit the development of high-density integration circuits.

Effect of Hartree–Fock pseudopotentials on local density functional theory calculations

Optimal choice of the element-specific pseudopotential improves the band gap.

Selective area heteroepitaxy of GaSb on GaAs (001) for in-plane InAs nanowire achievement

The growth of in-plane GaSb nanotemplates on a GaAs (001) substrate is demonstrated combining nanoscale patterning of the substrate and selective area heteroepitaxy. The selective growth of GaSb inside nano-stripe openings in a SiO₂ mask layer is achieved at low temperature thanks to the use of an atomic hydrogen flux during the molecular beam epitaxy. These growth conditions promote the spreading of GaSb inside the apertures and lattice mismatch accommodation via the formation of a regular array of misfit dislocations at the interface between GaSb and GaAs. We highlight the impact of the nano-stripe orientation as well as the role of the Sb/Ga flux ratio on the strain relaxation of GaSb along the [110] direction and on the nanowire length along the [1-10] one. Finally we demonstrate how these GaSb nanotemplates can be used as pedestals for subsequent growth of in-plane InAs nanowires.

Polarity-Driven Quasi-3-Fold Composition Symmetry of Self-Catalyzed III-V-V Ternary Core-Shell Nanowires

A quasi-3-fold composition symmetry has for the first time been observed in self-catalyzed III-V-V core-shell nanowires. In GaAsP nanowires, phosphorus-rich sheets on radial {110} planes originating at the corners of the hexagonal core were observed. In a cross section, they appear as six radial P-rich bands that originate at the six outer corners of the hexagonal core, with three of them higher in P content along ⟨112⟩A direction and others along ⟨112⟩B, forming a quasi-3-fold composition symmetry. We propose that these P-rich bands are caused by a curvature-induced high surface chemical potential at the small corner facets, which drives As adatoms away more efficiently than P adatoms. Moreover, their polarity related P content difference can be explained by the different adatom bonding energies at these polar corner facets. These results provide important information on the further development of shell growth in the self-catalyzed core-shell NW structure and, hence, device structure for multicomponent material systems.

Evaluation of electronic correlation contributions for optical tensors of large systems using the incremental scheme

A new method is developed to calculate the optical tensors of large systems based on available wave function correlation approaches (e.g., the coupled cluster ansatz) in the framework of the incremental scheme. The convergence behaviors of static first- and second-order polarizabilities with respect to the order of the incremental expansion are examined and discussed for the model system Ga(4)As(4)H(18). The many-body increments of optical tensors originate from the dipole-dipole coupling effects and the corresponding contributions to the incremental expansion are compared among local domains with different distances and orientations. The weight factors for increments of optical tensors are found to be tensorial in accordance with the structural symmetry as well as the polarization and the external electric field directions. The long-term goal of the proposed approach is to incorporate the sophisticated molecular correlation methods into the accurate wave function calculation of optical properties of large compounds or even crystals.

Influence of electronic correlations on the ground-state properties of cerium dioxide

The electron-correlation effects on the ground-state properties of CeO(2) are studied by ab initio quantum-chemical methods. For this purpose the method of increments is applied. It combines Hartree-Fock calculations for periodic systems with correlation calculations requiring only information of the corresponding finite-cluster calculations. Using the coupled-cluster approach for the evaluation of the individual increments, we recover 93% of the experimental cohesive energy. The lattice constant and bulk modulus are found to be in good agreement with experimental values. For comparison also the results obtained with density functional methods are presented.

The extension of the fragment molecular orbital method with the many-particle Green’s function

By using the many-particle Green's function (GF) the extension of the fragment molecular orbital (FMO) method by Kitaura et al. [Chem. Phys. Lett. 313, 701 (1999)] is proposed. It is shown that the partial summation of the cluster expansion of GF reproduces the same extrapolation formula as that of FMO. Therefore we can determine the excitation energy, the transition moment, and the linear response of a molecule from GF approximated with the FMO procedure. It is also shown that no wave function exists which is consistent to the FMO results. The perturbation expansion in which the self-consistent charge approximation defines the unperturbed state is reported. By using it the three-body effects missing in the pair approximation of FMO are analyzed and the corrections to the energy and the reduced density matrices are proposed. In contrast to the previous works these new corrections are not expressed as the addition or the subtraction of the energies of fragments. They are size extensive and require only the quantities available by the FMO calculation. The accuracy of these corrections is validated with the extended Hubbard model and the several test molecules.

An ab initio study of the fcc and hcp structures of helium

The hexagonal close packed (hcp) and face centered cubic (fcc) structures of helium are studied by using a new ab initio computational model for large complexes comprising small subsystems. The new model is formulated within the framework of the energy incremental scheme. In the calculation of intra- and intersystem energies, model systems are introduced. To each subsystem associated is a set of partner subsystems defined by a vicinity criterion. In the independent calculations of intra- and intersystem energies, the calculations are performed on model subsystems defined by the subsystems considered and their partner subsystems. A small and a large basis set are associated with each subsystem. For partner subsystems in a model system, the small basis set is adopted. By introducing a particular decomposition scheme, the intermolecular potential is written as a sum of effective one-body potentials. The binding energy per atom in an infinite crystal of atoms is the negative value of this one-body potential. The one-body potentials for hcp and fcc structures are calculated for the following nearest neighbor distances (d0): 4.6, 5.1, 5.4, 5.435, 5.5, 5.61, and 6.1 a.u. The equilibrium distance is 5.44 a.u. for both structures. The equilibrium dimer distance is 5.61 a.u. For the larger distances, i.e., d0 > 5.4 a.u., the difference of the effective one-body potentials for the two structures is less than 0.2 microE(h). However, the hcp structure has the lowest effective one-body potential for all the distances considered. For the smallest distance the difference in the effective one-body potential is 3.9 microE(h). Hence, for solid helium, i.e., helium under high pressure, the hcp structure is the preferred one. The error in the calculated effective one-body potential for the distance d0 = 5.61 a.u. is of the order of 1 microE(h) (approximately 0.5%).

Bayesian error estimation in density-functional theory

We present a practical scheme for performing error estimates for density-functional theory calculations. The approach, which is based on ideas from Bayesian statistics, involves creating an ensemble of exchange-correlation functionals by comparing with an experimental database of binding energies for molecules and solids. Fluctuations within the ensemble can then be used to estimate errors relative to experiment on calculated quantities such as binding energies, bond lengths, and vibrational frequencies. It is demonstrated that the error bars on energy differences may vary by orders of magnitude for different systems in good agreement with existing experience.

Local-MP2 electron correlation method for nonconducting crystals

Rigorous methods for the post-HF (HF-Hartree-Fock) determination of correlation corrections for crystalline solids are currently being developed following different strategies. The CRYSTAL program developed in Torino and Daresbury provides accurate HF solutions for periodic systems in a basis set of Gaussian type functions; for insulators, the occupied HF manifold can be represented as an antisymmetrized product of well localized Wannier functions. This makes possible the extension to nonconducting crystals of local correlation linear scaling On techniques as successfully and efficiently implemented in Stuttgart's MOLPRO program. These methods exploit the fact that dynamic electron correlation effects between remote parts of a molecule (manifesting as dispersive interactions in intermolecular perturbation theory) decay as an inverse sixth power of the distance R between these fragments, that is, much more quickly than the Coulomb interactions that are treated already at the HF level. Translational symmetry then permits the crystalline problem to be reduced to one concerning a cluster around the reference zero cell. A periodic local correlation program (CRYSCOR) has been prepared along these lines, limited for the moment to the solution of second-order Moller-Plesset equations. Exploitation of point group symmetry is shown to be more important and useful than in the molecular case. The computational strategy adopted and preliminary results concerning five semiconductors with tetrahedral structure (C, Si, SiC, BN, and BeS) are presented and discussed.

Quantum chemical ab initio calculations of correlation effects in complex polymers: Poly(para-phenylene)

Different quantum chemical approaches to the ground state correlation energy per unit cell of infinite poly(para-phenylene) (PPP) chains are presented. PPP is an organic polymer with interesting optical properties, due to its conjugated, aromatic pi system. The inclusion of correlation effects is crucial for a sound quantum chemical description of such a system. The correlation calculations were performed on the coupled cluster with single and double excitations (CCSD) level of theory using Dunning's spd correlation consistent polarized valence double-zeta basis sets. The correlation energy per unit cell is determined by means of the incremental method, which comprises series of CCSD calculations with partial excitation spaces. The resulting correlation energy per unit cell of PPP is -21.797 eV and compares well with that obtained by a simple but much more demanding cluster convergence approach (-21.775 eV). In addition, the accuracy and performance of the incremental scheme is discussed with respect to full CCSD benchmark calculations on PPP oligomers. Two variants are considered, the conventional one based on bond-type local units, and an extended one based on natural chemical subunits. Whereas it is difficult to reach "chemical" accuracy with the first variant, the second variant allows an accurate and efficient treatment with only a few individual CCSD calculations for a polymer with an aromatic pi system such as PPP.

Ab initio many-body investigation of structure and stability of two-fold rings in silicates

In this paper we present ab initio many-body calculations on the strain energy of W silica, taken as a model system for edge-sharing tetrahedral SiO(2) systems with respect to corner-sharing ones as in alpha quartz. The mean-field results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by the second-order Møller-Plesset perturbation theory and the coupled-cluster approach. Correlation contributions are found to play an important role to determine the stability of edge-sharing units. The most sophisticated method used in our calculation, i.e., the coupled-cluster approach with single and double excitations, yields a strain energy of 0.0427 a.u. per Si(2)O(4) unit with respect to alpha quartz, which is even smaller than the value obtained by a previous density functional theory calculation.