Fabrication of SiGe nanostructures for infrared devices
The MBE growth conditions for the Stranski-Krastanov (SK) growth mode of tensile strained Si on Ge substrates were investigated. These self-organized Si islands provide a confinement of delta2-electrons, which is of special interest for conduction-electron spin manipulation or for photoluminescence in the infrared range. Under tensile strain, the wetting layer (WL) thickness is increased and goes along with a rather unusual coexistence of SK growth and plastic strain relaxation already at an early stage of 3D growth. Modifying the surface energy with surfactants reduced the WL thickness but could not prevent the whole system from plastic strain relaxation within the islands. Further, the use of pre-structured Ge substrates offered preferred nucleation sites for the Si atoms and reduced the WL thickness. Finally, the growth of a tensile-strained modulation-doped pure Si channel on SiGe pseudo-substrates lead to very high electron mobility in the range of 420000cm2/Vs at low temperatures. The SK growth mode under tensile strain is astonishing in the Si/Ge material system, but these structures offer exciting new possibilities in the field of infra-red optical information technology.