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The optical properties of silicon nanocrystallites of known sizes, present in supercritically dried porous silicon films of porosities as high as 92%, have been measured by a variety of techniques. The bandgap and luminescence energies have been measured as a function of size for the first time. The bandgap increases by more than 1 eV due to quantum confinement. The peak luminescence energy which also shifts to the blue is increasingly Stokes shifted with respect to the bandgap, as the size decreases. The measured bandgap is in agreement with realistic theories and the Stokes-shift between bandgap and luminescence energies coincides with the exciton binding energy predicted by these theories. These results demonstrate unambiguously and quantitatively the role of quantum confinement in the optical properties of this indirect gap semiconductor.

Microhardness measurements have been performed on porous silicon (PSi) films. The dependence of the hardness on porosity, morphology and the underlying silicon substrate has been established. A model which determines the intrinsic film hardness has been developed and experimentally validated. The load dependence of hardness was measured as an index of the disorder in porous silicon. Hardness analysis also provided information on the nature of the PSi surface. Finally, several PSi-polymer nanocomposites have been fabricated and characterized. An improvement in hardness is observed, with no apparent change in the PL characteristics.

Time-resolved photoluminescence (PL) measurements on spark-processed Si ( sp -Si) are compared with those on dry-oxidized porous Si ( p -Si). Both types of substances yield non-exponential decay times in the nanosecond region which are essentially independent of the detection wavelength. However, subtle differences between photoluminescing sp -Si and oxidized p -Si exist. Specifically, blue/violet emitting sp -Si has a peak wavelength near 410 nm (3eV) under steady state conditions whereas oxidized p -Si luminesces with a maximum centred around 460–480 nm (2.7 – 2.58eV). Further differences include the peak structures in the PL spectra, the decay dynamics, and certain features in the lifetime distribution. It is concluded from the data that sp -Si and p -Si derive their PL from somewhat different mechanisms. Moreover, differences in decay times between SiO 2 and sp -Si suggest that silica does not seem to be the major cause for PL in sp -Si.

The results of photoluminescence (PL) and electroluminescence (EL) studies from partially oxidized porous silicon (POPS) layers are presented. The PL from POPS is stable, peaks at 600-570 nm and its temperature dependence can be fitted by an exponential law with an activation energy Ea ≈ 10 meV. The current-voltage characteristics of Au-(POPS)-crystalline silicon (c-Si) structures follow a power law I ≈ Vn. When the index n becomes higher than 2, electroluminescence (EL) is found. The EL peaks at 760 nm and is stable for more than 100 hours of operation. The intensity of the EL is a linear function of current for all measured structures up to current density J ≈ 1 A/cm2. Our results suggest that partially oxidized porous silicon is more useful for device applications than freshly anodized porous silicon which has unstable properties or than fully oxidized porous silicon in which transport is poor.

We report second harmonic generation measurements in p -type GaAs/Al 0.5 Ga 0.5 As/AlAs stepped quantum wells using a free electron laser tunable from 3 to 6 μm. The peak enhancement of the second order susceptibility in singly resonant quantum wells over that of bulk GaAs is nearly one order of magnitude. The dominant contribution is from the term related to X xyz (2) and X zxy (2) , which is in contrast to n -type quantum wells, where X zzz (2) dominates. The enhancement peaks at 3.9 μm, which is attributed to a single resonance corresponding to the SO-HH1 transition.

High intensity femtosecond pulses have been used to inject free carriers in the extended states of a-Si:H where they are maintained for a time of the order of one picosecond. Their optical properties are measured by pump and probe femtosecond spectroscopy. We find that the momentum scattering time that enters in the Drude model and in the expression for the mobility is less than one femtosecond for electrons. The upper bound for the mobility of electrons above the mobility edge is 6 cm2/V sec.

We have used picosecond laser pulses to produce transiently molten silicon layers and to measure their dielectric function ϵ at 1064 nm by reflectivity measurements in a pump and probe configuration. Starting approximately 100 ps after excitation, ϵ is a constant equal to - 27.3 + j 48.7. Immediately following excitation but before ϵ becomes constant, the reflectivity changes, which is interpreted as evidence for superheating.