Hole trapping in self-assembled SiGe quantum nanostructures

The electronic behaviour of boron-doped single germanium layers grown in a silicon matrix by solid source MBE has been studied by deep level transient spectroscopy (DLTS) and photoluminescence (PL). The structures result from the Si-capping of nine monolayers of germanium on (100) silicon at different substrate temperatures (650, 550 and 450 °C). Due to the growth procedure, the buried germanium structures consist of a wetting layer (WL) (typical of the Stranski–Krastanov growth mode of germanium in silicon) plus the self-assembled structure under the silicon cap. The detailed morphology of the structure depends on the growth temperature. Emission and trapping behaviours of the layer have been studied by DLTS and PL measurements, while Transmission Electron Microscope (TEM) observations were used to characterise the actual film morphology. We first show that the major electrical signal arising from the structure comes from the WL. The corresponding ground state depth clearly follows the trend imposed by a typical quantum confinement: the thinner the well, the higher the level. We also show that the electrical signal is maintained in the low-temperature-grown sample even if light emission from the WL is non-existent. The growth procedure coupled with PL results also allowed us to confirm the high level of elastic relaxation inherent to dome islands nucleation. The hole trapping behaviour of the WLs have also been investigated. A surprising point-defect-like behaviour is observed, suggesting that the WL capture rate is charge state independent.