Analysis of Terahertz-Emitting SiGe Quantum Cascade Structures by Transmission Electron Microscopy

We have performed a detailed transmission electron microscopy study of SiGe quantum cascade devices designed to operate in the THz band. Test structures have been grown onto SiGe virtual substrates, with the first being to identify the effect on quantum well morphology of increasing Ge concentration (or strain). We have used a combination of energy dispersive X-ray microanalysis, electron energy-loss imaging and scanning transmission electron microscopy to measure the compositions of the various quantum wells. These are subsequently correlated with the roughness of final quantum-well morphology. We find that in virtual substrates with a 20% Ge concentration within the SiGe alloy of the surface buffer layer, quantum wells with a Ge concentration up to similar to35% can be grown before morphological buckling occurs.