J. von Behren

Quantum confinement in nanoscale silicon: The correlation of size with bandgap and luminescence

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.

Preparation and characterization of ultrathin porous silicon films

Light emitting porous silicon thin films with thicknesses from ∼0.1 to ∼100 μm were produced by electrochemical etching and subsequently lifted off the silicon wafer by an electropolishing step. The structural integrity of the thinner layers was maintained by deposition on sapphire windows where they remain attached by van der Waals or electrostatic forces. The procedure for manufacturing high quality layers and their structural and optical properties is discussed.

Correlation of Photoluminescence and Bandgap Energies with Nanocrystal Sizes in Porous Silicon

We have measured the crystallite sizes, the bandgap energies, and the photoluminescence (PL) energies in porous silicon (PSi) samples having a wide range of porosities and kept in different ambient conditions. The dependence of the bandgap energy on the crystallite size agrees with theory. For PSi samples exposed to air and containing crystallites smaller than 5 nm, the PL intensity increases by several orders of magnitude and the PL peak energy shifts from the near infrared to the red, in agreement with the quantum confinement model for the PL. For crystallites smaller than 3 nm, there is a Stokes shift between the excitonic bandgap and PL energies, which increases to several hundreds of meV for sizes ∼2 nm, indicating that, in PSi exposed to air, the PL is not due to free excitons. Before exposure to air, very high porosity PSi samples emit at shorter wavelengths than after exposure to air, suggesting that the Stokes shift depends on the surface chemistry.

The femtosecond optical response of porous, amorphous and crystalline silicon

Abstract The carrier dynamics in porous silicon have been investigated using femtosecond time-resolved spectroscopy. The results are compared to those obtained under identical conditions on a-Si:H, crystalline silicon and GaAs. Porous silicon is an indirect bandgap material with a free carrier absorption cross section that is lower than that of a-Si:H and comparable to that of crystalline silicon. Our measurements indicate carrier trapping within 200 fs followed by a slower decay on a subnanosecond time scale that speeds up at elevated injected densities. This effect is interpreted as efficient Auger recombination of carriers generated in, and confined to, nanometer sized crystallites.

Optical Properties of Free-Standing Ultrahigh Porosity Silicon Films Prepared by Supercritical Drying

Highly luminescent free-standing porous silicon thin films of excellent optical quality have been manufactured by using electrochemical etching and lift-off steps combined with supercritical drying. One to 50 μm thick free-standing layers made from highly (p + ) and moderately (p) Boron doped single crystal silicon (c-Si) substrates have been produced with porosities (P) up to 95 %. The Fabry-Perot fringes observed in the transmission and photoluminescence (PL) spectra are used to determine the refractive index. At the highest P the index of refraction is below 1.2 from the IR to 2 eV. The absorption coefficients follow a nearly exponential behavior in the energy range from 1.2 eV and 4 eV. The porosity corrected absorption spectra of free-standing films made from p type c-Si substrates are blue shifted with respect to those prepared from p + substrates. For P > 70 % a blue shift is also observed in PL. At equal porosities the luminescence intensities of porous silicon films made from p + and p type c-Si are different by one order of magnitude.

Properties of ultrathin films of porous silicon

We have produced films of light emitting porous silicon (LEPSi) thinner than 1 μm, lifted them off the silicon wafer by an electropolishing step, and deposited them onto sapphire windows where they remain attached by van der Waals or electrostatic forces. Although free‐standing LEPSi films had been obtained before, our films are one order of magnitude thinner, luminesce strongly, and have excellent mechanical properties because of the sapphire substrate. The important steps in this procedure are discussed, and the structural, chemical, and optical properties of these films as measured using a variety of probes are reported. These films are semitransparent in the visible and thus make several new optical measurements possible. In particular, the results of photoinduced absorption measurements performed with 100 fs time resolution are presented.

Preparation, Properties and Applications of Free-Standing Porous Silicon Films

Using special electrochemical etching and lift-off steps, we have fabricated large-area freestanding porous silicon films in the thickness range from 0.1 μm to 50 μm. Their transmission is near 100% in the near infrared which is indicative of very high porosity/low index of refraction films. These optically flat and homogeneous films exhibit no surface and bulk scattering, despite the fact that they did not undergo supercritical drying. The relationship between the absorption coefficient, the luminescence spectrum, and the chemical and structural properties is examined as a function of preparation and post-treatment conditions. Because of their superior optical properties, these films are suitable for many device applications.