Carrier Dynamics at Si-Dielectric Interfaces Studied by Second-Harmonic-Generation

Abstract

This dissertation describes research on the application of intense, tunable, ultrafast lasers to studies of the silicon-dielectric system. Recently developed and now commercially available ultra-fast lasers are excellent tools for studying nonlinear optical effects in materials. Second-harmonic-generation (SHG) has come to be recognized as a uniquely useful nonlinear optical effect. For reasons of symmetry, SHG is sensitive to interfaces between materials with inversion symmetry. This technique yields rich information on electronic structure, local fields, symmetry and carrier dynamics at interfaces. It also has the advantages of being contactless and non-intrusive. It is therefore a promising tool for the investigation of mature Si-dielectric interfaces. In this thesis, we present first SHG measurements of the Si-high-k dielectric material system. Our data show features that are drastically different from the Si-SiO2 system. Unlike the Si-SiO2 system, where a rapid initial rise in SHG intensity is followed by a gradual increase until saturation, SHG from Si/ZrSiOx gradually decreases to a stable level after an initial quick rise. We attribute this difference to different carrier dynamics in these two systems. While in the SiO2 system, electron injection dominates the process, holes and electrons have a much more equal role in ZrSiOx. This arises from characteristically different band offset relationships in these two systems. Wavelength-dependent SHG measurements reveal a threshold of about 1.4 eV. A band diagram at the Si/ZrSiOx interface is then constructed with a conduction band offset of about 2.8 eV, which is well above the minimum 1 eV value determined by many calculations to prevent carrier tunneling. This lends strong support to ZrSiOx as a candidate for gate oxide in future generation semiconductor devices.