Self-assembled quantum dots in a nanowire system for quantum photonics.

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.

Surface origin of high conductivities in undoped In2O3 thin films.

The microscopic cause of conductivity in transparent conducting oxides like ZnO, In{2}O{3}, and SnO{2} is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for O-vacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In{2}O{3} samples under high-temperature equilibrium conditions. However, nominally undoped thin-films of In{2}O{3} can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O{2} partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In{2}O{3} thin films.

Fine structure of negatively and positively charged excitons in semiconductor quantum dots: electron-hole asymmetry.

We present new understanding of excitonic fine structure in close-to-symmetric InAs/GaAs and InGaAs/GaAs quantum dots. We demonstrate excellent agreement between spectroscopy and many-body pseudopotential theory in the energy splittings, selection rules and polarizations of the optical emissions from doubly charged excitons. We discover a marked difference between the fine structure of the doubly negatively and doubly positively charged excitons. The features in the doubly charged emission spectra are shown to arise mainly from the lack of inversion symmetry in the underlying crystal lattice.

Design rules to achieve high-T(C) ferromagnetism in (Ga, Mn)As alloys.

The Curie temperature T(C) of ferromagnetic semiconductor alloys depends not only on the alloy composition, but also on the spatial configuration of the magnetic impurities. Here we use a set of first-principle-calculated Curie temperatures to uncover-via a statistical, 'data mining' approach-the rules that govern the dependence of T(C) on the configuration of Mn substitutional impurities in GaAs. We find that T(C) is lowered (raised) when the average number of first (third and fourth) nearest-neighbour Mn pairs increases, suggesting simple atom-by-atom strategies to achieve high T(C) in (Ga, Mn)As alloys.

Impact ionization can explain carrier multiplication in PbSe quantum dots.

The efficiency of conventional solar cells is limited because the excess energy of absorbed photons converts to heat instead of producing electron-hole pairs. Recently, efficient carrier multiplication has been observed in semiconductor quantum dots. In this process, a single, high-energy photon generates multiple electron-hole pairs. Rather exotic mechanisms have been proposed to explain the efficiency of carrier multiplication in PbSe quantum dots. Using atomistic pseudopotential calculations, we show here that the more conventional impact ionization mechanism, whereby a photogenerated electron-hole pair decays into a biexciton in a process driven by Coulomb interactions between the carriers, can explain both the rate (<<1 ps) and the energy threshold ( approximately 2.2 times the band gap) of carrier multiplication, without the need to invoke alternative mechanisms.

First-principles combinatorial design of transition temperatures in multicomponent systems: the case of Mn in GaAs.

The transition temperature TC of multicomponent systems--ferromagnetic, superconducting, or ferroelectric--depends strongly on the atomic arrangement, but an exhaustive search of all configurations for those that optimize TC is difficult, due to the astronomically large number of possibilities. Here we address this problem by parametrizing the TC of a set of approximately 50 input configurations, calculated from first principles, in terms of configuration variables ("cluster expansion"). Once established, this expansion allows us to search almost effortlessly the transition temperature of arbitrary configurations. We apply this approach to search for the configuration of Mn dopants in GaAs having the highest ferromagnetic Curie temperature. Our general approach of cluster expanding physical properties opens the way to design based on exploring a large space of configurations.

Magnetism without magnetic ions: percolation, exchange, and formation energies of magnetism-promoting intrinsic defects in CaO.

We investigate theoretically the prospects of ferromagnetism being induced by cation vacancies in nonmagnetic oxides. A single Ca vacancy V(0)(Ca) has a magnetic moment due to its open-shell structure but the ferromagnetic interaction between two vacancies extends only to four neighbors or less. To achieve magnetic percolation on a fcc lattice with such an interaction range one needs a minimum of 4.9% vacancies, or a concentration 1.8 x 10(21) cm(-3). Total-energy calculations for CaO show, however, that due to the high vacancy formation energy even under the most favorable growth conditions one can not obtain more than 0.003% or 10(18) cm(-3) vacancies at equilibrium, showing that a nonequilibrium vacancy-enhancement factor of 10(3) is needed to achieve magnetism in such systems.

First-principles predictions of yet-unobserved ordered structures in the Ag-Pd phase diagram.

The complexity of first-principles total energy calculations limits the pool of structure types considered for a ground-state search for a binary alloy system to a rather small, O(10), group of "usual suspects." We conducted an unbiased search of fcc-based Ag(1)-(x)Pd(x) structures consisting of up to many thousand atoms by using a mixed-space cluster expansion. We find an unsuspected ground state at 50%-50% composition-the L1(1) structure, currently known in binary metallurgy only for the Cu(0.5)Pt(0.5) alloy system. We also provide predicted short-range-order profiles and mixing enthalpies for the high temperature, disordered alloy.

Spatial correlations in GaInAsN alloys and their effects on band-gap enhancement and electron localization.

In contrast to pseudobinary alloys, the relative number of bonds in quaternary alloys cannot be determined uniquely from the composition. Indeed, we do not know if the Ga0.5In0.5As0.5N0.5 alloy should be thought of as InAs+GaN or as InN+GaAs. We study the distribution of bonds using Monte Carlo simulation and find that the number of In-N and Ga-As bonds increases relative to random alloys. This quaternary-unique short range order affects the band structure: we calculate a blueshift of the band gap and predict the emergence of a broadband tail of localized states around the conduction band minimum.

Evolution of III-V nitride alloy electronic structure: the localized to delocalized transition.

Addition of nitrogen to III-V semiconductor alloys radically changes their electronic properties. We report large-scale electronic structure calculations of GaAsN and GaPN using an approach that allows arbitrary states to emerge, couple, and evolve with composition. We find a novel mechanism of alloy formation where localized cluster states within the gap are gradually overtaken by a downwards moving conduction band edge, composed of both localized and delocalized states. This localized to delocalized transition explains many of the hitherto puzzling experimentally observed anomalies in III-V nitride alloys.

Origins of nonstoichiometry and vacancy ordering in Sc1-x squarexS.

Whereas nearly all compounds A(n)B(m) obey Dalton's rule of integer stoichiometry (n: m, both integer), there is a class of systems, exemplified by the rocksalt structure Sc1-x squarexS, that exhibits large deviations from stoichiometry via vacancies, even at low temperatures. By combining first-principles total energy calculations with lattice statistical mechanics, we scan an astronomical number of possible structures, identifying the stable ground states. Surprisingly, all have the same motifs: (111) planes with (112) vacancy rows arranged in (110) columns. Electronic structure calculations of the ground states (identified out of approximately 3x10(6) structures) reveal the remarkable origins of nonstoichiometry.

Indium-indium pair correlation and surface segregation in InGaAs alloys

In-In pair correlations and In surface segregation in In xGa 1-xAs alloys are studied by first-principles total-energy calculations. By calculating the substitution energy of a single In atom, we find that the near-surface energetics explains the observed In segregation on InGaAs(001)-beta2(2x4) surfaces. Indium surface segregation further enhances the In site selectivity, thus the long-range ordering. We find that the [110] and [001] In-In pair correlations are repulsive and nearly isotropic in bulk but are highly anisotropic near the (001) surface. The sign of the [110] In-In interaction energies vs the distance from the surface is oscillatory. These findings explain the recent puzzling cross-sectional x-STM results.

Microscopic origin of the phenomenological equilibrium "Doping limit Rule" in n-type III-V semiconductors

The highest equilibrium free-carrier doping concentration possible in a given material is limited by the "pinning energy" which shows a remarkable universal alignment in each class of semiconductors. Our first-principles total energy calculations reveal that equilibrium n-type doping is ultimately limited by the spontaneous formation of close-shell acceptor defects: the (3-)-charged cation vacancy in AlN, GaN, InP, and GaAs and the (1-)-charged DX center in AlAs, AlP, and GaP. This explains the alignment of the pinning energies and predicts the maximum equilibrium doping levels in different materials.