R. Glosser

Direct evidence for the amorphous silicon phase in visible photoluminescent porous silicon

We report on micro‐Raman spectroscopy studies of porous silicon which show an amorphous silicon Raman line at 480 R cm−1 from regions that emit visible photoluminescence. A Raman line corresponding to microcrystalline silicon at 510 R cm−1 is also observed. X‐ray photoelectron spectroscopy data is presented which shows a high silicon‐dioxide content in porous silicon consistent with an amorphous silicon phase.

Phonon mode study of near-lattice-matched InxGa1-xAs using micro-Raman spectroscopy

We identify the four allowable phonon modes in InxGa1−xAs on InP:InAs‐like transverse optical (TO) (225±2 cm−1), InAs‐like longitudinal optical (LO) (233±1 cm−1), GaAs‐like TO (255±2 cm−1), and GaAs‐like LO (269±1 cm−1), using the selectivity of first‐order Raman scattering off the (100) normal surface and the (011) cleaved plane and detailed line‐shape analysis employing a sequential simplex optimization procedure. Raman scattering off the (011) cleaved plane was achieved for the first time in thin‐film InGaAs using microprobing capabilities (∼1 μm). We also identify another phonon mode R* at 244 cm−1 which is attributed to an alloy disorder mode in these films. For the five identified phonon modes, a linear relationship between the Raman frequencies and composition determined from x‐ray diffraction was determined for near‐lattice‐matched conditions (0.42<1−x<0.52).

Pressure concentration isotherms of thin films of the palladium‐hydrogen system as modified by film thickness, hydrogen cycling, and stress

We describe the results of a novel adaptation of the volumetric technique to measurement of the pressure concentration isotherms for thin films of the palladium‐hydrogen system. The isotherm shapes are comparable to bulk being steep in the alpha and beta phases and flat in the mixed phase region. However, it is noted that the concentration onset of the beta phase is smaller than bulk and drops markedly for thicknesses less than 670 A. Comparison is made with quartz crystal microbalance results. We find a thickness dependence to the number of absorption‐desorption cycles required to obtain reproducible isotherms.

Raman investigation of ion beam synthesized β-FeSi2

The Raman spectra of ion beam synthesized (IBS) β-FeSi2 are investigated and evidence for the presence of a net tensile stress is presented. Possible origins of the observed stress are suggested and a simple model is proposed in order to calculate a value of the observed stress. A correlation between the tensile stress, the nature of the band gap, and the resulting light emitting properties of IBS β-FeSi2 is suggested.

ORGANOMETALLIC COMPOUNDS AS SINGLE-SOURCE PRECURSORS TO NANOCOMPOSITE MATERIALS : AN OVERVIEW

Abstract Molecularly doped silica xerogels are prepared by adding either main-group or transition metal organometallic compounds containing bifunctional ligands to conventional sol–gel formulations. These bifunctional ligands contain distal (alkoxy or hydroxy)silyl groups, so that the dopant molecules become covalently incorporated into the silica xerogel matrix as it is being formed. Subsequent thermal treatment under reducing or oxidizing-then-reducing conditions leads to the decomposition of molecular precursor and to the formation of a nanoparticulate material with precise stoichiometry highly dispersed throughout the xerogel matrix.

Diamond growth in turbulent oxygen‐acetylene flames

Turbulent premixed oxygen‐acetylene flames have been used to synthesize polycrystalline diamond films on molybdenum substrates at temperatures ranging from 500 to 1300 °C and facetted single crystals on mm‐sized natural diamond substrates at temperatures of 1200–1300 °C. Turbulence was achieved by increasing the torch’s orifice diameter and/or the flow velocity; the presence of turbulence was confirmed by observations of changes in the flame shape, measurements of the flame’s noise spectrum, and calculations of the Reynolds number. The optical emission spectra of several diamond‐growing turbulent flames were also compared to the spectra of laminar flames. The variation in diamond quality with temperature and oxygen acetylene flow ratio was studied with one or more of the following techniques: Raman spectroscopy, scanning and transmission electron microscopy, infrared spectroscopy, and photoluminescence spectroscopy. Crystals grown on molybdenum at temperatures of 600–1100 °C were observed to be transparen...

Anomalous photoluminescence behavior of porous Si

We have investigated the photoluminescence (PL) behavior of porous Si and have found that the PL intensity rises with time under exposure to continuous laser light at high power densities (≳50 W/cm2). Samples exposed to lower power density laser light exhibit the well‐known degradation of its PL intensity. As the power density is raised, the PL intensity increases with time and shows a ‘‘window’’ effect in which the PL intensities will again degrade at higher power density. Micro‐Raman studies were performed on the sample and no apparent change in crystalline structure could be found for differing power densities. We correlate this anomalous PL effect with porous Si samples etched with different HF acid concentrations and show that this effect occurs only for porous Si samples etched with a relatively high acid concentration (≳30%).

Correlation between the photoreflectance response at E1 and carrier concentration in n- and p-GaAs

We investigated the photoreflectance (PR) response of metalorganic chemical vapor deposition‐grown n‐ and p‐type GaAs at the higher‐energy transition E1(≊2.9 eV). The doping level range of interest was 2×1016–4×1018 cm−3 for Si:GaAs and 6×1016– 1×1019 cm−3 for Zn:GaAs. Both the position and the broadening parameter, Γ1, of the E1 transition within the doping concentration range were investigated. The evaluation of Γ1’s, based on the curve fitting of the PR response and the Kramers–Kronig analyzed data reveal a nearly linear relation between Γ1 and the logarithm of the carrier concentration. This observation has the potential application for contactless determination of carrier concentration in moderate and heavily doped nanoscale films. Secondary‐ion mass spectroscopy measurements show that there is relatively large compensation in Si:GaAs samples, but it does not correlate with the broadening of the E1 structure.

Excitonic transitions in β-FeSi2 epitaxial films and single crystals

Photoreflectance spectra were obtained from an epitaxial film and a bulk single crystal of β-FeSi2 at low temperatures (T⩽180 K). A model based on the results of low-temperature absorption [M. Rebien et al., Appl. Phys. Lett. 74, 970 (1999)] was used to describe the main features of the spectra. In agreement with the absorption results, transitions corresponding to the ground state and first excited state of the free exciton were observed in both the epitaxial film and single crystal. However, additional subband gap features are revealed in the photoreflectance spectra of the thin film. It is suggested that these may be related to impurity transitions or an impurity transition plus a bound exciton resonance. From the analysis of the spectra taken on the thin film, over a temperature range of 12–180 K, we extract a free exciton binding energy of (0.009±0.002) eV and a direct energy gap at T=0 K of (0.934±0.002) eV.

Optical characterization of n- and p-doped 4H–SiC by electroreflectance spectroscopy

We have studied the electroreflectance (ER) spectra of n- and p-type 4H–SiC polytype samples from 3 to 6.5 eV. The fundamental band gap and higher lying critical points are measured at room temperature. For this polytype, we observe band-gap narrowing in one of the structures with higher doping concentration.