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 compositionthreshold where a continuous, wall-to-wall chain of given bonds (e.g., Ga-N-Ga-N.cntdot..cntdot..cntdot) first forms in the alloy (e.g., GaAs1-xNx), we find an anomalous behavior in the corresponding bond length (e.g., Ga-N). Second, we show that if the dilute alloy (e.g., GaAs1-xNx for x.fwdarw.1) shows a localized deep impurity level in the gap, then there will be a composition domain in theconcentrated alloy where its electronic properties (e.g., optical bowing coefficient) become irregular: unusually large and composition dependent. We contrast GaAs1-xNx with the weakly perturbed alloy system GaAs1-xPx having no deep gap levels in the impurity limits, and find that the concentrated GaAs1-xPx alloy behaves normally in this case.