J. Tauc

States in the gap

Abstract Experimental evidence about the states in the gap of chalcogenide glasses is discussed. The total concentration of states is estimated from the measurements of the magnetic susceptibility and their density distribution from the optical and photo-emission measurements. Possible models for the interpretation of the experimental facts are considered.

Picosecond interferometric technique for study of phonons in the brillouin frequency range

Abstract We describe a new technique - picosecond interferometry - for the study of phonons in the Brillouin frequency range in transparent materials. This method makes possible the measurement of longitudinal phonon velocity and attenuation under conditions such that the conventional approach is not applicable.

Raman scattering in amorphous Ge and III–V compounds

Abstract Raman spectra of amorphous Ge prepared by various methods, and several amorphous III–V compounds (InP, GaP, GaAs, InAs, InSb, AlSb) prepared by flash evaporation or sputtering are reported. The results give a semiquantitative picture of the density of phonon states which preserves the main features of the crystalline material. The Raman spectra of III–V compounds show the presence of V-V bonds whose intensity depends on the preparation of the films.

Temperature dependence of the absorption edge of liquid sulfur

Abstract The absorption coefficient of liquid sulfur was measured in the energy range of 1.2–2.6 eV. No discontinuous behavior in the temperature dependence of the optical edge was observed at the polymerization temperature if the increase in light scattering occurring at the onset of the polymerization process is taken into account. Therefore, the electronic states that determine the optical edge appear to be insensitive to higher-order structural correlations.

Elastic properties of silicon oxynitride films determined by picosecond acoustics

The sound velocity and elastic modulus of a series of amorphous silicon oxynitride films produced by reactive sputtering have been measured as a function of nitrogen content. The acoustic pulses were generated and detected with ultrashort light pulses. The sound velocity was found to depend linearly on the nitrogen content.

Photomodulation spectroscopy of dangling bonds in doped and undoped a-Si:H

Abstract The energy levels and the effective correlation energy U eff of dangling bond defects in doped and undoped a Si:H were determined by the steady state and transient photomodulation spectroscopy. We found that in undoped a Si:H the two dangling bond levels (D ± ) are symmetrically located around midgap with U eff of 0.5 ± 0.1 eV, whereas in P(B) doped a Si:H the states D - (D + ) are pushed away from the conduction (valence) band toward midgap and U eff = 0.4 ± 0.1 eV .

Spectrum of photoinduced optical absorption in a-Si: H☆

Abstract Photoinduced absorption below the band gap in a -Si: H is interpreted in terms of transitions between holes trapped in states near the trap quasi-Fermi level and the valence band.

Sound velocity and index of refraction of AlAs measured by picosecond ultrasonics

We use picosecond optics techniques to generate and detect acoustic pulses in an epitaxially grown film of AlAs. From the round trip time of the acoustic pulse we find a sound velocity in the [100] direction of 6.4×105 cm s−1. We also show how measurements of this type can be used to determine the refractive index.

Influence of interference on photoinduced changes in transmission and reflection

Abstract The determination of photoinduced changes in the absorption coefficient (Δα) and index of refraction (Δn) from the measured changes in transmission (ΔT) and reflection (ΔR) of a thin film is discussed. It is shown that on a film with a thickness gradient special optical thicknesses can be found at which Δα is proportional to ΔT, or Δn is proportional to ΔR. The analysis is illustrated on the decays of photoinduced transmission and reflection in a-Si:H in the picosecond time domain.

Heat Flow in Glasses on a Picosecond Timescale

The transient thermal properties of amorphous materials have been the subject of several recent investigations [1–4]. These experiments have mainly been designed to study the time-dependence of the specific heat at low temperature (T≤2K) on time scales ≥0.1µsec. In this paper we describe a new technique which enables measurements of thermal properties to be made on a time scale into the picosecond range, and we present preliminary results of measurements of this type.