M. Rosenbauer

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.

Optical and structural investigation of stain‐etched silicon

We report on optical and vibrational properties of porous silicon (por‐Si) layers grown on p‐type Si wafers by electroless etching technique. The results indicate a correlation between the photoluminescence (PL) intensity and the multihydride complexes (SiHn with n≥2). However, similar correlation was also found for monohydride species from the layers containing no multihydrides. It is shown that the increase in the amount of oxidation is responsible for broadening of the PL emission band. Furthermore, a new IR absorption band is observed at 710 cm−1 and assigned to multihydrides suggesting a new local bonding environment for hydrogen atoms in these layers.

Properties of sputtered a-SiOx:H alloys with a visible luminescence

Abstract Thin films of amorphous silicon-oxygen-hydrogen alloys have been prepared by sputtering of silicon in water vapour. The obtained samples were characterized by infrared absorption, Rutherford backscattering, elastic recoil detection, optical transmission, and luminescence measurements. The appearance of a strong, visible (red) luminescence at room temperature for oxygen contents around 40–50 at% is discussed on the basis of microscopic evidence deduced from optically detected magnetic resonance.

Structural and luminescence studies of stain-etched and electrochemically etched germanium

Luminescence from stain-etched and electrochemically etched germanium samples was observed in the blue part of the visible spectrum. Although it is weaker than the luminescence of porous silicon, its origin is of scientific interest as a parallel to the luminescence of porous silicon. Stain-etched germanium was studied in more detail. Under visible laser excitation the luminescence band is located around 525 nm. Another luminescence band around 400 nm is observed with UV excitation. The luminescence lifetime in the UV-excited band is of the order of 1 ms, compared with about 100 ns in the band excited in the visible. The luminescence intensity either passes through a maximum or slowly decreases as the temperature is increased from 8 to 300 K. This behaviour is similar to that observed in porous silicon. The IR spectra of stain-etched Ge show the presence of GeO2, -OH groups and water. In the Raman spectra the unmistakable bands of trigonal crystal GeO2 are observed. A band at 760 cm−1 in the Raman spectra and 752 cm−1 in the IR spectra could be assigned to substoichiometric regions and oxygen-vacancy complexes in GeO2. The chemical composition of the samples is characterized by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The XP spectra give evidence for GeO2 and GeO. SIMS data show the presence of both hydrogen and -OH groups. On the basis of these data we propose that the luminescence of stain-etched and electrochemically etched germanium is due to defects in GeO2.

Transient photoluminescence decay in porous silicon and siloxene

Abstract The photoluminescence decay after laser pulse excitation is studied in porous silicon and siloxene as a function of sample temperature, detection wavelength, laser intensity, and pulse length. The time dependence in all samples is characterized by a nonexponential decay directly after the laser excitation and a single exponential decay for long times after the laser pulse. The exponential decay is identical for the porous silicon samples and annealed siloxene and depends only on detection wavelength and sample temperature. As prepared siloxene exhibits the same decay characteristics. However, the nonexponential decay is more pronounced and the single exponential decay is a factor of 2–5 faster compared with annealed siloxene. The decay of the photoluminescence is another indication of the identical origin of the strong visible luminescence in porous Si and in annealed siloxene.

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.

Temperature-dependent photoluminescence in porous amorphous silicon

Abstract Having shown by transmission electron microscopy that a suitable electrolytic processing of boron-doped amorphous hydrogenated silicon (a-Si:H) yields porous amorphous films with a nanometre-scale microstructure similar to that of highly porous p-type crystalline silicon layers (PcSL), we report and discuss the qualitative similarities and quantitative differences between the temperature dependence of the time-resolved and steady-state photoluminescence (PL) in both materials.

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.