Growth and in-plane undulations of GaAs/Ge superlattices on [001]-oriented Ge and GaAs substrates: formation of regular 3D island-in-network nanostructures

Abstract

Coherently strained pseudo-superlattices (PSLs) of 20-period GaAs/Ge have been epitaxially grown on [001]-oriented Ge and GaAs substrates by metalorganic chemical vapor deposition. High-resolution X-ray diffraction and dynamic simulation revealed an increase in the growth rate of Ge (R_Ge) and a decrease in that of GaAs (R_GaAs) when GaAs is used, instead of Ge, as the substrate under otherwise the same growth conditions. They also revealed an increase in R_Ge while a decrease in R_GaAs when the growth temperature is increased. These changes are related to atomic intermixing at the interfaces that resulted in interfacial (GaAs)_1−xGe_x alloys with lattice constants larger than that calculated assuming Vegard's law. Cross-sectional transmission-electron microscopy revealed regular in-plane undulations of the PSLs and energy-dispersive X-ray spectroscopy provided evidence that the Ge sublayers were conformally grown on GaAs while the GaAs sublayers were grown in isolated islands that initiated in the valleys of the Ge sublayers. The growth mechanism was interpreted based on surface chemical potentials that depend on the balance between the surface-energy and the local strain-relaxation energy. The three-dimensional periodic island-in-network nanostructures, as well as their optical and lattice dynamical properties, are of great importance not only for fundamental studies but also for fabricating thermoelectric devices.