Even order nonlinear optical processes such as second harmonic generation (SHG) are widely-recognized noninvasive interface-specific probes of buried solid-solid interfaces and exposed surfaces of centrosymmetric materials.1 Si(001) interfaces are among the most important and the simplest for nonlinear optical analysis. Their importance derives from their ubiquitous presence in MOS devices, and from the sensitivity of device performance and nonlinear optical properties to interfacial properties such as microroughness, strain, contamination. Their simplicity derives from the small number of tensor components required to describe the interfacial nonlinear optical response. Yet the exceptionally weak interfacial susceptibility c(2)s has strongly inhibited quantitative interface second harmonic spectroscopy of Si(001) interfaces in the past. With state-of-the-art unamplified Ti: sapphire femtosecond lasers, we overcome this barrier by enabling dramatically higher SHG efficiency than with previous laser sources with minimal interface heating.2
Dr. Downer's Femtosecond Spectroscopy Group has developed two applications of interface nonlinear optics with femtosecond lasers as a real-time, noncontact diagnostic of Column IV semiconductor interfaces during processing and growth: a) ex situ SHG to measure angstrom-scale microroughness, metal contamination, and strain at the technologically important Si(001)/SiO2 interface, and directly in MOS device structures. 3,4,5 b) in situ SHG to monitor epitaxial growth rate, precursor adsorption and desorption kinetics during CVD growth of silicon and silicon-germanium films;2,6 There are two over-arching goals to this work: 1) to understand the nonlinear spectroscopy of semiconductors interfaces at the most fundamental level, e.g. to understand physically how and why the SH spectum of silicon is sensitive to hydrogen coverage, doping, strain, microroughness, applied electric fields and other technologically important parameters. Recent observation of fourth harmonic generation at GaAs surfaces7 using a femtosecond source is an exciting breakthrough in fundamental surface nonlinear optics. 2) To develop practical SHG-based interface monitoring instruments and procedures useful to the microelectronics industry. In the latter vein, some of the group's work has received a U.S. Patent.8 This work involves close collaboration with, and partial financial support from, local microelectronics industries such as Advanced Micro Devices, Inc., as well as collaboration with faculty and students from Engineering and Chemistry Departments who participate in the NSF Science & Technology Center for Synthesis, Growth and Analysis of Electronic Materials.