Research

Researchers: Zhiheng Liu, Xiong Lu, Travis Wade

Past efforts to achieve selective bond scission by vibrational excitation have been thwarted by energy thermalization. However, we have observed and are now exploring resonant photodesorption of hydrogen from a Si(111) surface using tunable infrared radiation. The wavelength dependence of the desorption yield peaks at 0.26 electron volts, the energy of the Si-H vibrational stretch mode. The desorption yield is quadratic with the infrared intensity. A strong H/D isotope effect rules out thermal desorption mechanisms, and electronic effects are not applicable in this low-energy regime. A molecular mechanism accounting for the desorption event remains elusive.

Below: Typical curves of desorbed H2 signal as a function of time at three wavelengths. The background pressure has been subtracted. The FEL fluence was 0.8 J/cm2 per macropulse. The inset shows the fine structure of a desorption curve. The step rises are due to desorption by individual FEL macropulses. The 33-ms interval corresponds to the 30-Hz repetition rate of the macropulse.

Video (AVI, MP43) - Depiction of proposed desorption mechanism

• Infrared photons excite hydrogen surface states
• Excitations migrate and combine until two neighboring hydrogen atoms are excited enough to leave the surface
• The surface is not appreciably heated during the process

Researchers: Jingbo Qi, Andrew Steigerwald, Ying Xu

Spin-based electronics (spintronics) offer radical alternatives to conventional electronics, promising:

• Decreased power consumption
• Increased data processing speed
• Increased integration densities

We perform time-resolved Kerr rotation and MSHG measurements on semiconductor and magnetic semiconductor heterostructure systems to study coherent spin dynamics.

(a) Kerr rotation measurements for Ga1-xMnxAs (x = 0.035) excited by linearly-polarized and circularly-polarized light (σ+ and σ-) at a temperature of 20 K. The photon energy was 1.56 eV. Oscillations due to magnetization precession are superimposed on the decay curves. (b) Oscillation data (open circles) for linear excitation. The solid line is the fitted result. (c) Fourier transformation profile for the oscillation data in (b).

Magnetically-induced second harmonic generation measurements employing pump-probe techniques are used to study important spin-related issues, such as spin relaxation and spin transport in detail.

Researchers: Ying Xu, Jingbo Qi, Andrew Steigerwald

We are performing first studies on long-lived oscillations in optical pump-probe measurements on GaSb-GaAs heterostructures. The oscillations arise from a photogenerated coherent longitudinal acoustic phonon wave, which travels from the top surface of GaSb across the interface into the GaAs substrate, providing information on the optical properties of the material as a function of time/depth. Wavelength-dependent studies of the oscillations near the bandgap of GaAs indicate strong correlations to the optical properties of GaAs.

Above: Total pump-probe response of GaSb(20 nm)/GaAs at 890 nm at 300 K.
Inset: The subtracted oscillatory response (open diamond) fitted with a damped oscillation simulation (solid line)

Below: Oscillatory responses at different wavelengths with 500 nm GaSb top layer. The dash line indicates when the strain wave travels from GaSb layer into the GaAs substrate.

• High-k Dielectrics and SOI

  Researchers: Heungman Park, Elissa Jennings

  Utilizing optical second harmonic generation (SHG), we are studying the interfaces of semiconductor and oxide ZrO2 and HfO2, as well as the interfaces of silicon-on-insulator (SOI) wafers. The experiments show that SHG intensity depends on the roughness of interface, thermal processing and introduced metallic contaminations (Ni, Cu, Fe) at the interface. The SHG measurement can provide the quality of the buried oxide interface and the presence of metallic contaminations. SHG has potential applications for non-invasive characterization of SOI wafers.

• Stress Effects

  Researchers: Justin Gregory, Elissa Jennings

  Much previous work has explored the properties of the Si/SiO2 interface and (other interfaces) while under stress using SHG. However, these studies typically focus on samples where the stress is a result of lattice mismatch in epitaxial growth. As such, these experiments are limited in the actual stresses that can be applied by the natural crystal lattice dimensions. Utilizing a unique four-point stressing device (see diagram below), we are able to observe SHG from thin samples under external mechanical stress, allowing for the stress to be selected from a continuum of values.