Properties Of Semiconductor Alloys Research Articles

Alloy composition fluctuations and percolation in semiconductor alloy quantum wells

Fluctuations in local alloy composition on small length scales may have a significant effect on device performance, particularly when there is a large disparity in the properties such as atomic size of the constituent alloy components. In particular, a random alloy is subject to a percolation threshold, above which an infinitely connected network of the minority alloy component exists. While these percolation thresholds are well known for ideal 2D and 3D lattices, they are unknown for the intermediary “2.5D” case, appropriate for quantum well structures. This letter presents calculations of the percolation threshold for 2.5D quantum well-like hexagonal, diamond/silicon and body-centred cubic lattices that are directly relevant to many semiconductor alloys, and enables further experimental inquiry into the effect of percolation on the properties of semiconductor alloys.

Localization and percolation in semiconductor alloys: GaAsN vs GaAsP.

Tradition has it that in the absence of structural phase transition, or direct-to-indirect band-gap crossover, the properties of semiconductor alloys (bond lengths, band gaps, elastic constants, etc.) have simple and smooth (often parabolic) dependence on composition. We illustrate two types of violations of this almost universally expected behavior. First, at the percolation composition threshold where a continuous, wall-to-wall chain of given bonds (e.g., Ga-N-Ga-N...) first forms in the alloy (e.g., $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$), we find an anomalous behavior in the corresponding bond length (e.g., Ga-N). Second, we show that if the dilute alloy (e.g., $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ for $x\ensuremath{\rightarrow}1$) shows a localized deep impurity level in the gap, then there will be a composition domain in the concentrated alloy where its electronic properties (e.g., optical bowing coefficient) become irregular: unusually large and composition dependent. We contrast $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ with the weakly perturbed alloy system $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ having no deep gap levels in the impurity limits, and find that the concentrated $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ alloy behaves normally in this case.

Dependence of Optical Properties of Semiconductor Alloys on Long Range Order, Strain and Pressure

AbstractMany III – V semiconductor alloys exhibit spontaneous (111) alternate monolayer ordering when grown from the vapor phase. We have studied theoretically the ordering induced changes in the optical properties of the semiconductor alloys. We describe (i) the change of the band gap ΔE9, and the valence band splitting ΔE12 as functions of the long range order parameter η, (ii) the consequence of coexistence of (001) epitaxial strain and (111) chemical ordering on the optical properties, (iii) optical anisotropy and spin polarization effects in the ordered alloy, (iv) theory of reflectance-difference spectroscopy in ordered alloy, and (v) the ordering-induced changes on high energy E1 and E2 transitions. Specific, experimentally testable predictions are listed in the summary section.