Chomani Gaspe1,Christopher Richardson1
Laboratory for Physical Sciences1
Chomani Gaspe1,Christopher Richardson1
Laboratory for Physical Sciences1
2D holes in planar Ge/SiGe heterostructures are an emerging material platform for future superconductor-semiconductor hybrid devices and spin-based quantum devices. Because of lighter effective mass and absence of valley degeneracy, quantum confined holes in compressively strained Ge offer extremely high mobilities thus long carrier life times.<br/>Fully compressively strained Ge with well controlled quantum well (QW) thickness is crucial to lift the valence band degeneracy between HH and LH bands such that participating holes confine in the ground state of HH band that exhibits light effective mass. The Ge QW is compressively strained to the in- plane lattice constant of relaxed Si<sub>0.2</sub>Ge<sub>0.8</sub> buffer layer.<br/>In this experimental study, compressively strained Ge quantum well with strain-relaxed Si<sub>0.2</sub>Ge<sub>0.8</sub> barrier layers were grown by molecular beam epitaxy (MBE) on Si (001) substrate via engineered abrupt and graded Ge/Si<sub>1-x</sub>Ge<sub>x</sub> buffer layers. Si and Ge fluxes were created with electron beam evaporation solid sources controlled by feedback stabilized electron impact emission spectroscopy. High quality abrupt metamorphic and graded buffer layers were achieved with low temperature growth and high temperature cyclic annealing. Strain relaxation in the Ge/SiGe heterostructure was analyzed and the fully strained Ge QW was confirmed by reciprocal space maps. The thickness of the thin Ge QW and the Si<sub>0.2</sub>Ge<sub>0.8</sub> top barrier layer was determined by the x-ray reflectivity measurements. The threading dislocation density at the QW, abrupt and graded buffer layers was analyzed by selective etch pit density using iodine-based acid etch and plan-view SEM. Improved threading dislocation density and background doping density were realized by optimization of abrupt and graded buffer layer growth process. Growth details, and structural and electrical characterization of MBE grown SiGe heterostructures are discussed.