Electronic and vibrational properties of interstitial clusters in degenerately boron-doped silicon

Degenerate boron-doped silicon is prone to interstitial clustering, which adversely impacts key electrical properties, such as carrier concentration and mobility. Clustering becomes increasingly important with device miniaturisation, due to the high boron concentrations involved. Here we use vibrational and Compton electron energy-loss spectroscopy (EELS) in a (scanning) transmission electron microscope to measure changes in the vibrational and electronic properties of the silicon host lattice due to degenerate boron doping (1020cm-3hole concentration). A broad phonon defect band centred at 1064 cm^-1wavenumber was detected. Subtle changes in bonding anisotropy due to boron doping along [110] and [100] directions were also observed. Density-functional-theory modelling showed that boron acceptors had very little effect on the phonon and bonding properties. Instead boron interstitial clusters two to three atoms in size produce changes that agree more closely with experiment. However, the limited vibrational EELS energy resolution and background thermal diffuse scattering artefacts in Compton profiles do not allow a precise identification of the numerous cluster configurations that could potentially form. The results nevertheless suggest the potential of using high-spatial-resolution EELS for the detection of clustering phenomena at the device level.