D. M. Wood

Structural phenomena in coherent epitaxial solids

Abstract We discuss theoretically a number of effects characteristic of coherent epitaxial (rather then bulk) solids, namely: (i) occurence in epitaxial form of inter-semiconductor ordered phases with no counterpart in the bulk phase diagram, (ii) reversal of the order of stability of two structural modifications of the same ordered phase, (iii) epitaxy-enhanced solid solubilities, (iv) epitaxially-induced changes of order-disorder transition temperatures, (v) composition-pinning (“lattice latching”) in epitaxial alloys, and (vi) changes in nearest-neighbor bond lengths in epitaxial versus bulk semiconductor alloys. First-principles total energy and cluster-variation calculations are used to illustrate these effects for a number of systems.

Ordering-induced changes in the optical spectra of semiconductor alloys

It is shown how the recently predicted and subsequently observed spontaneous long‐range ordering of pseudobinary A0.5B0.5C isovalent semiconductor alloys into the (AC)1(BC)1 superlattice structure (a CuAuI‐type crystal) gives rise to characteristic changes in the optical and photoemission spectra. We predict new direct transitions and substantial splittings of transitions absent in the disordered alloy.

Structural stability and selectivity of thin epitaxial semiconductors

It is shown how the availability of structural degrees of freedom in various ternary AnB4−nC4 adamantine semiconductors can lead to their energetic stabilization when grown epitaxially, and how the substrate strain can preferentially stabilize one structure over another even when the two are equally stable (or unstable) in bulk form.

Electronic structure of [110] Si‐Ge thin‐layer superlattices

First‐principles electronic structure calculations for SinGen superlattices ( for n=4, 6, and 8) grown epitaxially on a (110) Si substrate reveal a nearly direct band gap (to within ≊0.04 eV for n=4) despite the pronounced indirectness of its constituents. This is unlike superlattices grown in the [001] direction which are indirect when grown on Si and quasi‐direct only on substrates with larger lattice constants, e.g., Ge. Transition dipole matrix elements for the lowest energy direct transition vanish for all repeat periods n but are finite for several other new low‐energy transitions.

Electronic structure of ultrathin SinGen strained superlattices: The possibility of direct band gaps

Abstract We examine theoretically the structural and electronic properties of thin Si n Ge n superlattices for n = 1, 2, 4 and 6, grown on (001) and (110)-oriented substrates. The increased repeat distance along the growth direction leads to folding of conduction band states to the Γ point of the superlattice Brillouin zone, resulting in a significant reduction in the minimum direct band gap. Transitions to these folded-in states can have non-zero dipole matrix elements because of (i) atomic relaxation, leading to the accomodation of distinct SiSi and GeGe bond lengths and (ii) the superlattice ordering potential. Our calculations show that superlattices grown coherently on a (001) Si substrate remain indirect band gap materials, with a minimum gap from Γ to Δ (near the X point) of the f.c.c. Brillouin zone. We find, however, that increasing the lattice parameter a s of the substrate will further reduce the direct band gap. For a s ⩾ a , where a is the average of the lattice constants for silicon and germanium, we predict a nearly direct band gap: for Si 6 Ge 6 the indirect band gap for a s = a is only ∽ 0.01 eV smaller than the direct band gap. The lowest conduction band states in this case are localized on the silicon sublattice. For (110)-oriented substrates, a similar degree of directness in the band gap can be achieved even on silicon.