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Highly mismatched semiconductor alloys (HMAs) offer unusual combinations of bandgap and lattice constant, which are attractive for myriad applications. Dilute borides, such as BGa(In)As, are typically assumed to be HMAs. BGa(In)As can be grown in higher alloy compositions than Ga(In)NAs with comparable bandgaps, potentially enabling routes to lattice-matched telecom lasers on Si or GaAs. However, BGa(In)As remains relatively unexplored, especially with large fractions of indium. Density functional theory with HSE06 hybrid functionals was employed to study BGaInAs with 4%–44% In and 0%–11% B, including atomic rearrangement effects. All compositions showed a direct bandgap, and the character of the lowest conduction band was nearly unperturbed with the addition of B. Surprisingly, although the bandgap remained almost constant and the lattice constant followed Vegard's law with the addition of boron, the electron effective mass increased. The increase in electron effective mass was higher than in conventional alloys, though smaller than those characteristics of HMAs. This illustrates a particularly striking finding, specifically that the compositional space of BGa(In)As appears to span conventional alloy and HMA behavior, so it is not well-described by either limit. For example, adding B to GaAs introduces additional states within the conduction band, but further addition of In removes them, regardless of the atomic arrangement. 

Tensile-strained pseudomorphic Ge 1–x–y Sn x C y was grown on GaAs substrates by molecular beam epitaxy using carbon tetrabromide (CBr 4 ) at low temperatures (171–258 °C). High resolution x-ray diffraction reveals good crystallinity in all samples. Atomic force microscopy showed atomically smooth surfaces with a maximum roughness of 1.9 nm. The presence of the 530.5 cm −1 local vibrational mode of carbon in the Raman spectrum verifies substitutional C incorporation in Ge 1–x–y Sn x C y samples. X-ray photoelectron spectroscopy confirms carbon bonding with Sn and Ge without evidence of sp 2 or sp 3 carbon formation. The commonly observed Raman features corresponding to alternative carbon phases were not detected. Furthermore, no Sn droplets were visible in scanning electron microscopy, illustrating the synergy in C and Sn incorporation and the potential of Ge 1–x–y Sn x C y active regions for silicon-based lasers. 

Ab-initio simulations of dilute germanium carbides (Ge:C) using hybrid functionals predict a direct bandgap with <;1%C. Growth of dilute Ge:C shows reduced direct gap consistent with the model, with no structural defects detected. Ge:C may enable lasers and compact modulators on Si.