The supercell approach to defects and alloys has circumvented the limitations of those methods that insist on using artificially high symmetry, yet this step usually comes at the cost of abandoning the language of E versus k band dispersion. Here we describe a computational method that maps the energy eigenvalues obtained from large supercell calculations into an effective band structure (EBS) and recovers an approximate E( k) for alloys. Making use of supercells allows one to model a random alloy A 1−x B x C by occupying the sites A and B via a coin-toss procedure, affording many different local environments (polymorphic description) to occur. We present the formalism and implementation details of the method and apply it to study the evolution of the impurity band appearing in the dilute GaN:P alloy. We go beyond the perfectly random case, realizing that many alloys may have nonrandom microstructures, and investigate how their formation is reflected in the EBS. It turns out that the EBS is extremely sensitive in determining the critical disorder level for which delocalized states start to appear in the intermediate band. In addition, the EBS allows us to identify the role played by atomic relaxation in the positioning of the impurity levels.

Random substitutional A(x)B(1-x) alloys lack formal translational symmetry and thus cannot be described by the language of band-structure dispersion E(k(→)). Yet, many alloy experiments are interpreted phenomenologically precisely by constructs derived from wave vector k(→), e.g., effective masses or van Hove singularities. Here we use large supercells with randomly distributed A and B atoms, whereby many different local environments are allowed to coexist, and transform the eigenstates into an effective band structure (EBS) in the primitive cell using a spectral decomposition. The resulting EBS reveals the extent to which band characteristics are preserved or lost at different compositions, band indices, and k(→) points, showing in (In,Ga)N the rapid disintegration of the valence band Bloch character and in Ga(N,P) the appearance of a pinned impurity band.

The intermediate-band solar cell ͑IBSC͒ concept has been recently proposed to enhance the current gain from the solar spectrum whilst maintaining a large open-circuit voltage. Its main idea is to introduce a partially occupied intermediate band ͑IB͒ between the valence band ͑VB͒ and conduction band ͑CB͒ of the semiconductor absorber, thereby increasing the photocurrent by the additional VB→ IB and IB→ CB absorptions. The confined electron levels of self-assembled quantum dots ͑QDs͒ were proposed as potential candidates for the implementation of such an IB. Here we report experimental and theoretical investigations on In y Ga 1−y As dots in a GaAs 1−x P x matrix, examining its suitability for acting as IBSCs. The system has the advantage of allowing strain symmetrization within the structure, thus enabling the growth of a large number of defect-free QD layers, despite the significant size mismatch between the dot material and the surrounding matrix. We examine the various conditions related to the optimum functionality of the IBSC, in particular those connected to the optical and electronic properties of the system. We find that the intensity of absorption between QD-confined electron states and host CB is weak because of their localized-to-delocalized character. Regarding the position of the IB within the matrix band gap, we find that, whereas strain symmetrization can indeed permit growth of multiple dot layers, the current repertoire of GaAs 1−x P x barrier materials, as well as In y Ga 1−y As dot materials, does not satisfy the ideal energetic locations for the IB. We conclude that other QD systems must be considered for QD-IBSC implementations.

The influence of cluster size and of cluster-substrate interaction on the magnetic properties of Co clusters of 1-10 atoms on Pt(111) and Au(111) is studied by fully relativistic ab initio calculations. The focus is on systematic trends of the spin and orbital magnetic moments, the exchange coupling, and the crossover temperature. The spin magnetic moments of Co clusters are larger for the Pt substrate than for the Au substrate, while the reverse is true for the orbital magnetic moments. The local magnetic moments of Co atoms generally increase if the number of Co neighbours decreases. The exchange coupling constants J i j depend on the cluster size and on the location of respective atoms. The crossover temperature increases monotonically with cluster size and is larger for clusters on Pt than for clusters on Au.