Ultrasmall Si encapsulated in SiO2 (talk)

UlClassical impurity doping of deeply nanoscale silicon in very large scale integration faces great challenges impairing further device miniaturization. These challenges include dopant out-diffusion, self-purification and inactivation due to increased ionization energies. Based on Density function theory calculations we reported a nanoscale electronic structure shift induced by anions (in particular, oxygen and nitrogen termination) at surfaces (NESSIAS) in ultra small group IV-crystals in previous publications. Depending on the terminating anion at the surface of Si nano crystals the electronic structure is shifted towards higher or lower energies with respect to the bulk levels, resulting in a p-type behavior (nitrogen-termination) or n-type behavior (oxygen-termination), respectively. The predictions of the calculations are supported by measurements performed at Elettra Sincrotrone Trieste using X-ray absorption spectroscopy (XAS), to determine the highest unoccupied state, and ultraviolet photoelectron spectroscopy (UPS), to determine the lowest unoccupied state. The measurements were carried out on 2-dimensional Si nanowells (Nwell) with Si thicknesses in the range of 1.1 nm to 5.0 nm embedded in silicon dioxide or silicon nitride, respectively. While the NESSIAS effect leads to shifting the electronic structure to smaller energies with decreasing Si thickness, quantum confinement counteracts this resulting in an upshift of the electronic structure. Interestingly, the two competing effects yield a minimum at a thickness around 2 nm which is equivalent to a rather large n-type doping concentration. We present our XAS and UPS measurements and an approach to determine the charge carrier density and carrier mobility with electronic transport measurements in ultrathin SiO2-embedded Si Nwells using van der Pauw measurements.