H. D. Fuchs

The origin of visible luminescencefrom “porous silicon”: A new interpretation

Abstract Luminescence and vibrational properties (infrared and Raman) of anodically oxidized (“porous”) silicon and of chemically synthesized siloxene (Si 6 O 3 H 6 ) and its derivates are compared. Based on the quantitative agreement of these two types of materials it is concluded that the origin of the strong room temperature luminescence in “porous” silicon can be traced to siloxene derivates present in “porous” silicon.

Luminescence and optical properties of siloxene

Abstract We review the optical and luminescence properties of siloxene (Si 6 O 3 H 6 ). The preparation and basic structural properties of siloxene are described, and theoretical results concerning optical transitions in different modifications of siloxene are discussed. The dominant structural building blocks in as-prepared siloxene are two-dimensional Si planes which gave rise to a direct band gap in the visible energy range. The optical properties of annealed siloxene originate from isolated Si 6 rings which act as radiative recombination centers. The luminescence of annealed siloxene and porous silicon are compared in detail. In particular, new experimental results based on optically detected magnetic resonance provide microscopic information about radiative states in porous Si which is incompatible with the conventional quantum confinement model.

Quality of AlAs‐like and InSb‐like interfaces in InAs/AlSb superlattices: An optical study

Two short‐period InAs/AlSb superlattices, grown with an AlAs‐like interface and an InSb‐like interface, respectively, were studied with Raman spectroscopy, x‐ray diffraction, and ellipsometry. Our measurements show that the InSb‐like interface grows perfectly pseudomorphically, whereas the sample with the AlAs‐like interface shows indications of relaxation and As interdiffusion. This different interface quality seems to be a fundamental problem, rather than the result of the growth technique.

Optical Characterization of the Visible Photoluminescence from Porous Silicon

The visible photoluminescence of electrochemically etched silicon wafers is studied by cw- and pulsed-laser excitation. Raman data and infrared transmission measurements on the same samples prove the presence of oxygen and hydrogen in different bonding configurations in the luminescent layers. Identical optical properties are found for chemically synthesized siloxene (Si 6 O 3 H 6 ) and its derivates. We present evidence that the origin of the strong room temperature luminescence in “porous” silicon can be traced to siloxene derivates present in the samples.

New growth technique for luminescent layers on silicon

We report the growth of siloxene on crystalline silicon (111) surfaces based on the chemical transformation of a calcium disilicide layer. The siloxene films obtained show a prominent luminescence in the green, with intensities comparable to the luminescence observed from porous silicon. The structural properties of the siloxene films are studied with infrared transmission and TEM micrographs.

Temperature dependence of luminescence in porous silicon and related materials

We have investigated the temperature dependence of the visible luminescence of porous silicon and other Si-based luminescent materials (as-prepared and annealed siloxene, amorphous Si:O:H alloys) at temperatures between 10 and 359 K. In all samples, the decrease of the luminescence intensity, I(T), at temperatures > 100 K can be described by the relation I0/I(T) = 1 + exp(T/T0). At temperatures < 100 K, some of the strongly luminescent samples show a decrease of the luminescence intensity with decreasing temperature. We have found this decrease to be related to a “fine structure” in the luminescence spectra of yet unknown origin.

Visible luminescence from porous silicon and siloxene

Infrared transmission and Raman spectra of siloxene (Si6O3H6) and porous Si have been analyzed in terms of the structural composition. Siloxene consists of Si6-rings interconnected by oxygen and terminated by hydrogen, and corrugated Si(111)-planes saturated by OH and H. The pronounced photoluminescence of siloxene is mainly related to the Si6-rings. Porous Si shows similar luminescence and almost identical vibrational properties as siloxene, which makes this class of materials a possible candidate for the origin of the luminescence in p-Si. Annealing at temperatures below 300 °C and moderate illumination of siloxene induces the transformation from the Si-plane-like structure to the Si-ring-like structure. Anneal temperatures above 300 °C and intense illumination of both siloxene and p-Si leads to an effusion of hydrogen and an irreversible oxidation of the samples.

Isotope effects on the electronic excitations and phonons in semiconductors

Changes in the average isotopic mass, M , produce a trivial effect in the phonon spectra of monoatomic solids, i.e., the variation of phonon frequencies such as M −1/2 . More sophisticated effects also appear. Effects on the frequencies of phonons and electronic excitations due to changes in the mean squared amplitude, 〈 u 2 〉, which also varies like M −1/2 , and effects of the mass fluctuations in isotopically mixed crystals are discussed. The latter can be represented by a complex self-energy whose real part corresponds to a frequency shift and its imaginary part to an increase in phonon linewidth.

Correlation between the luminescence and Raman peaks in quantum-confined systems

Abstract Two of the main models for the explanation of strong visible-light emission in silicon, namely physical quantum confinement of electrons in nanometer-size “wires” or spherical crystallites, and chemical quantum confinement to subnanometer-size silicon particles due to the isolating effect of oxygen atoms, are considered with regard to a possible correlation between Raman shift and photoluminescence (PL) peak position. The physical confinement model predicts opposite shifts with changing size of the confinement. In the chemical confinement model, the shift of the gap is not a size effect; it occurs owing to chemical substitution of the bond terminators of the silicon atoms. This is accompanied by a parallel shift in the Raman frequency. The calculation of the vibration spectra of small size wires and spheres allows the correct assignment of experimentally observed Raman peaks. With the help of this assignment, the analysis of the observed spectra shows a parallel shift of Raman and PL peaks. The calculated frequencies for siloxene derivatives (a known manifestation of chemical quantum confinement) are lower than those observed in most porous silicon samples: still the parallel shift favors the idea of chemical as against physical quantum confinement.

Electronic and structural properties of porous silicon

Abstract We review recent results concerning the structural properties and the origin of visible luminescence in porous silicon. The preparation of porous silicon is briefly described and current models for radiative recombination in porous Si are discussed. Special attention is given to a comparison between the properties of porous Si and those of siloxene and amorphous Si:O:H alloys, based on vibrational, optical, and spin resonance spectroscopy.