SiO2 Modulation Doping for Si: Acceptor Candidates

Conventional impurity doping of ultrasmall nanoscale silicon (Si) currently used in very-large-scale integration faces serious miniaturization challenges below the 14-nm technology node such as dopant out-diffusion and inactivation by clustering in Si-based field-effect transistors (FETs). For Si nanocrystals showing quantum confinement, self-purification and massively increased ionization energies cause doping to fail. Modulation doping is widely used in group III-group V materials in particular for optoelectronic applications, where high carrier densities at low scattering and nonradiative recombination rates are crucial. Setting out from our recent success to introduce modulation acceptors into SiO2 with aluminum to provide holes to Si [Konig et al., Sci. Rep. 7, 46703 (2017)], we follow our atomistic concept to investigate other possible modulation acceptors in SiO2. Using density functional theory and experimental verification of key candidates by capacitance-voltage and deep-level-transient-spectroscopy measurements, we elucidate the role of atomistic parameters that determine the ability of the dopant species to provide modulation-acceptor states to SiO2 and thus holes to Si. Modulation-doped SiO2 can replace conventional doping of ultrasmall nanoscale Si from the SiO2 coating or trench of a FET and have a high potential for carrier-selective tunneling contacts in Si-based heterojunction solar cells and tunnel FETs.