Sverre Froyen

Polarization fields and band offsets in GaInP/GaAs and ordered/disordered GaInP superlattices

Using the first‐principles pseudopotential method we have calculated band offsets between ordered and disordered Ga0.5In0.5P and between ordered GaInP2 and GaAs. We find valence band offsets of 0.10 and 0.27 eV for the two interfaces with the valence band maximum on ordered GaInP2 and GaAs, respectively. Using experimental band gaps these offsets indicate that the ordered/disordered Ga0.5In0.5P interface has type I band alignment and that the ordered GaInP2/GaAs interface has type II alignment. Assuming transitivity of the band offsets, these results suggest a type I alignment between disordered Ga0.5In0.5P and GaAs and a transition from type I to type II as the GaInP side becomes more ordered. Our calculations also show that ordered GaInP2 has a strong macroscopic electric polarization. This polarization will generate electric fields in inhomogeneous samples, strongly affecting the electronic properties of the material.

Surface dimerization induced CuPtB versus CuPtA ordering of GaInP alloys

Using a valence force field approach and ab initio pseudopotential calculations, we examine the role of subsurface strain in the ordering of Ga0.5In0.5P alloys. We show that depending on the orientation of the surface phosphorus dimers, these alloys can have (i) a CuPtA ordering for 1×2 or c(4×4) reconstruction; (ii) a CuPtB ordering for 2×1 or β2(2×4) reconstruction; and (iii) a triple period ordering for 2×3 or c(8×6) reconstruction. These results are in good agreement with recent experiments of Gomyo et al. [Phys. Rev. Lett. 72, 673 (1994); Jpn. J. Appl. Phys. 34, L469 (1995)].

Pressure coefficients of band gaps in semiconductors

Abstract A first principles pseudopotential method within the local density approximation is used to calculate the pressure coefficients for band gaps in several diamond and zincblende semiconductors. Despite the underestimation of the band gaps found in almost all local density calculations, the predictions for the linear pressure coefficients and the critical pressures for conduction band inversion are consistent with experiment for all cases considered.

Structural properties of NaCl and KCl under pressure

The pseudopotential method within the local-density approximation is used to investigate the static and structural properties of NaCl and KCl. Calculated values for the lattice constants and bulk moduli are in good agreement with experiment. The pressure phase transformation to the CsCl structure is studied, and the role of the C44 shear instability as a possible transformation path is investigated and dismissed. The tetragonal distortion of the CsCl phase, which has been found in other alkali halides at high pressures, appears not to exist in KCl. Finally the authors have calculated the electronic energy levels at symmetry points in the Brillouin zone as a function of volume.

Ordering in semiconductor alloys

A thermodynamic first‐principles theory of stability, including charge transfer, elastic forces, and atomic relaxations reveals the physical origins of stable and metastable ordering in bulk and epitaxial semiconductor alloys and superlattices.

Structure and thermodynamic stability of GaAs(001) surfaces

Abstract The surface energies for a set of structural models for the reconstructions occuring on GaAs(001) were calculated with the first principles pseudopotential method and the local density approximation. On the basis of these calculations we are able to rule out the single-dimer-vacancy model for the 2 × 4 surface as a possible equilibrium structure. In the As-rich limit we find the c (4 × 4) As addimer model to be energetically favorable. As the Ga chemical potential increases the surface is predicted to transform into the β2(2 × 4) structure having two As dimers in the top layer and a third As dimer in the third layer. The α(2 × 4) structure, with two As dimers above a complete layer of Ga is found to be stable in a very narrow range. The β(4 × 2) structure and the model recently proposed by Skala et al. for the (4 × 2) reconstruction were determined to be energetically unfavorable. In the Ga-rich limit the lowest energy structure we have obtained is the β2(4 × 2) model.

High pressure phases of III–V semiconductors: A microscopic theory

Abstract The static structural properties and the structural stability of III-V semiconductors are investigated as a function of pressure using the pseudopotential method. At high pressures we predict a metallic rocksalt phase as the most stable structure. This phase is more stable than the ordered β-Sn structure previously thought to be the most stable form.

Evidence for internal electric fields in two variant ordered GaInP obtained by scanning capacitance microscopy

Single and two variant ordered GaInP samples are studied in cross section with the scanning capacitance microscope. Our study shows significant differences in the electronic properties of single and two variant GaInP. In unintentionally doped, ordered two variant samples, both n and p‐type like domains are observed with the scanning capacitance microscope. In contrast, a spatially uniform capacitance signal is observed in unintentionally doped single variant ordered GaInP. These microscopic capacitance observations can be qualitatively explained by bend bending or internal electric fields.

Superlattice energetics and alloy thermodynamics of GaAs/Ge

We re-examine the validity of the three-dimensional bulk thermodynamic description for the zinc-blende-to-diamond transition observed in (GaAs)1−xGe2x alloys. We use interaction parameters extracted from first-principles local-density total-energy calculations for (GaAs)p/(Ge2)p superlattices and a cluster-variation solution to the corresponding Ising-like hamiltonian. The resulting phase diagram obtained with such a realistic energy model fails to account for the observed critical composition in the relevant temperature range, suggesting that, contrary to a previous view, the transition is not driven by bulk thermodynamics.

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.