Electronic Structure Shift of Deeply Nanoscale Silicon by SiO2 versus Si3N4 Embedding as an Alternative to Impurity Doping

Abstract

Conventional impurity doping of deeply nanoscale silicon (dns-Si) used in ultra-large-scale integration (ULSI) faces serious challenges below the 14-nm technology node. We report on a fundamental effect in theory and experiment, namely the electronic structure of dns-Si experiencing energy offsets of approximately 1 eV as a function of SiO2 versus Si3N4 embedding with a few monolayers (MLs). An interface charge transfer (ICT) from dns-Si specific to the anion type of the dielectric is at the core of this effect and is arguably nested in the quantum-chemical properties of oxygen (O) and nitrogen (N) versus Si. We investigate the size up to which this energy offset defines the electronic structure of dns-Si by density-functional theory (DFT), considering the interface orientation, the embedding-layer thickness, and approximants featuring two Si nanocrystals (NCs), one embedded in SiO2 and the other in Si3N4. Working with synchrotron ultraviolet- (UV) photoelectron spectroscopy (UPS), we use SiO2- versus Si3N4-embedded Si nanowells (NWells) to obtain their energy of the top valence-band states. These results confirm our theoretical findings and gauge an analytic model for projecting maximum dns-Si sizes for NCs, nanowires (NWires), and NWells where the energy offset reaches full scale, yielding a clear preference for electrons or holes as majority carriers in dns-Si. Our findings can replace impurity doping for n- or p-type dns-Si as used in ultra-low-power electronics and ULSI, eliminating dopant-related issues such as inelastic carrier scattering, thermal ionization, clustering, out-diffusion, and defect generation. As far as majority-carrier preference is concerned, the elimination of those issues effectively shifts the lower size limit of Si-based ULSI devices to the crystallization limit of Si of approximately 1.5 nm and also enables them to work under cryogenic conditions.