Andrew Steigerwald

Semiconductor point defect concentration profiles measured using coherent acoustic phonon waves

Coherent acoustic phonon interferometry is used to quantitatively measure depth-dependent point defect concentrations in semiconductor systems with a depth range of the order of tens of microns. Using time-resolved pump-probe techniques, the optical response of ion-beam irradiated GaAs crystals is analyzed as a function of defect concentration ranging over four orders of magnitude. Varying the ion dose quantitatively relates changes in the optical response to local defect concentrations. Thermal annealing is shown to reduce the effect on the optical response, indicating recovery of the crystal lattice through self-interstitial-vacancy recombination.

Determination of optical damage cross-sections and volumes surrounding ion bombardment tracks in GaAs using coherent acoustic phonon spectroscopy

We report the results of coherent acoustic phonon spectroscopy analysis of band-edge optical modification of GaAs irradiated with 400 keV Ne++ for doses between 1011–1013 cm−2. We relate this optical modification to the structural damage density as predicted by simulation and verified by ion channeling analysis. Crystal damage is observed to cause optical modification that reduces the amplitude of the optoacoustic signal. The depth-dependent nature of the optoacoustic measurement allows us to determine optical damage cross-sections along the ion track, which are found to vary as a function of position along the track. Unexpectedly, we find that this optical modification is primarily dependent on the structural damage density and insensitive to the specific defect configuration along the ion track, suggesting that a simple model of defect density along the track is sufficient to characterize the observed optical changes. The extent of optical modification is strongly probe frequency-dependent as the freque...

Ion implantation induced modification of optical properties in single-crystal diamond studied by coherent acoustic phonon spectroscopy

Single-crystal CVD diamond specimens were implanted with 1-MeV He+ ions at fluences ranging from 1014 to 1016 cm−2 and analyzed using coherent acoustic phonon spectroscopy. The coherent acoustic phonon response varies greatly with implantation fluence and provides depth-dependent information about the implantation defect-induced modification of diamonds optical characteristics. The results indicate an increase in the real and imaginary refractive index, as well as a sign reversal of the photoelastic coefficients at higher levels of implantation damage. These studies provide insight into the application of ion implantation to the fabrication of diamond-based photonic devices.

Mechanical and electronic properties of ferromagnetic Ga 1 − x Mn x As using ultrafast coherent acoustic phonons

Ultrafast two-color pump-probe measurements, involving coherent acoustic phonon (CAP) waves, have provided information simultaneously on the mechanical properties and on the electronic structure of ferromagnetic GaMnAs. The elastic constant C11 of Ga1-xMnxAs (0.03<x<0.07) are observed to be systematically smaller than those of GaAs. Both C11 and Vs of GaMnAs are found to increase with temperature (78 K<T<295 K), again in contrast to the opposite behavior in GaAs. In addition, the fundamental bandgap (at E0 critical point) of Ga1-xMnxAs is found to shift slightly to higher energies with Mn concentration.

The effect of hydrogen desorption kinetics on thermionic emission from polycrystalline chemical vapor deposited diamond

Hydrogen influences many properties of diamond films, such as invoking negative electron affinity, inducing increased electron emission from diamond thermionic emitters. However, the thermionic emission diminishes at temperatures exceeding 750 °C. In this work, we observed the isothermal thermionic emission decrease followed first-order rate kinetics. Arrhenius examination indicated an activation energy consistent with values for the H-C bond at the surface derived from other works. Results obtained in this study establish a direct link between the presence of hydrogen and the degree of thermionic emission from diamond and is information relevant to the development of higher thermal emission from diamond.

Annealing effect in boron-induced interface charge traps in Si/SiO2 systems

Boron-induced charge traps near the interface of Si/SiO2 systems are investigated by time-dependent second harmonic generation (TD-SHG), a technique which is sensitive to interface electric fields. Using this approach, we monitored the modification of the charge state of the traps after systematic annealing in H2, Ar, and 1 mTorr vacuum at the temperatures of about 200 °C and 800 °C, and in 100 °C deionized-water. The initial decreasing TD-SHG signals were found to be significantly diminished upon annealing in the non-oxygen environments. We attribute the observed TD-SHG experimental results to neutralization of the built-in boron-induced charge traps and discuss possible mechanisms.

Experimental and theoretical determination of the opto-acoustic spectrum of silicon

Knowledge of the photon energy-dependent photo-elastic (PE) response for a wide range of semiconductors is essential for the development of opto-electronic platforms. Coherent acoustic phonon spectroscopy provides a novel approach to measure depth-dependent optical properties in semiconductor materials with high resolution. Here we report measurements and calculations for bulk silicon in the photon energy range 1.5–3.4 eV. First-principles calculations of the PE spectra show remarkable agreement with the opto-acoustic measurements. These results show promise that this combined experimental and theoretical approach can be effective in characterizing defect induced modification in PE response as a function of depth and defect concentration.