The electronic structure of amorphous silicon layers has been calculated within the empirical tight binding approximation using the Wooten-Winer-Weaire atomic structure model. We predict an important blue shift due to the confinement for layer thickness below 3 nm and we compare with crystalline silicon layers. The radiative recombination rate is enhanced by the disorder and the confinement but remains quite small. The comparison of our results with experimental results shows that the density of defects and localized states in the studied samples must be quite small.

Current-voltage characteristics, photosensitivity, frequency dependence of impedance, and electron beam induced current were measured for Al/porous silicon/c-Si structures with porous silicon (PS) layers of 60 and 80% porosity. Al/PS/c-Si structures have photosensitivity spectra typical for Al/c-Si structures. The sign of open circuit voltage changes in these structures when the wavelength becomes less than approximately 0.4 μm. It was shown that photosensitivity of Al/PS/c-Si structures and impedance dependence on the frequency, reverse bias and thickness of PS layer are completely determined by the PS layer of high resistivity and by the space charge region in c-Si substrate on PS/c-Si heterojunction. The annealing at above 100° C leads to a decrease of the forward current at a small bias. The photosensitivity of annealed structures reaches 10 A/W for some of the structures. The experimental results lead to inevitable conclusion about opposite band bending from the two sides of the heterojunction which is caused by high concentration of charged defects at PS/c-Si heterointerface. PS photodetectors with improved device parameter were obtained.

We report on luminescence hole burning experiments, which prove that radiative recombination between quantum confined states is the only viable model for the mechanism of the light emission from porous silicon. We find that more than 90% of the luminescence originates from quantum confined states inside the Si nanocrystals.

We present a study of the infrared reflectance of porous silicon carbide (PSC) formed by the electrochemical dissolution of silicon carbide substrates of both 6H and 4H polytypes. The formation of porous silicon carbide from a 4H-SiC substrate is reported for the first time, as is the reflectance from n-PSC, both as-anodized and passivated. The passivation of PSC has been accomplished using a short thermal oxidation. Fourier transform infrared (FTIR) reflectance spectroscopy is employed ex situ after different stages of the thermal oxidation process. The characteristics of the reststrahlen band normally observed in bulk SiC are altered by anodization; further changes in the reflectance spectra occur following oxidation for different short periods of time. An effective medium theory model which includes air, SiC and SiO2 as component materials is shown to characterize the observed changes in the reflectance spectra after different stages of PSC oxidation.

We have investigated optical constants of free-standing porous silicon films by dispersive Fourier transform spectroscopy (DFTS) in the NIR-VIS range. This allows the spectral variation of both the absorption coefficient and the refractive index of a material to be determined from the measurements of the attenuation and phase shift imposed on an electromagnetic wave by its interaction with a specimen. Using these optical constants, we have studied the complex dielectric function and the complex conductivity. To avoid the additive error in the absorption spectra arising from the pseudocoherence, we measured the transmission spectra by conventional Fourier transform spectroscopy (FTS). Using the refraction spectrum derived from the DFTS measurements, we have corrected for reflection losses in calculation of the absorption spectrum from the FTS transmission spectrum. The changes in the absorption coefficient and the refractive index due to oxidation, which is the most common aging phenomenon in porous silicon, have been studied using samples with different types of oxidization.

The ion beam synthesis (IBS) of β-FeSi2 was examined by Rutherford backscattering spectroscopy (RBS) and x-ray diffractometry (XRD), and the structural characterization was carried out by Raman spectroscopy and scanning electron microscopy (SEM). We found that the IBS of β- FeSi2 is controlled by two different processes depending on the annealing temperature (Ta) and Fe surface concentration (Cs); (I) precipitation of β-FeSi2 on the surface in Cs˜30 at% and Ta⩾700° C and (II) phase transition from γ -FeSi2 to β-FeSi2 in Cs<∼20 at%and Ta⩾600°C. The precipitation process(1) created large sized (-10 μm) polycrystalline grains of β-FeSi2. The good crystalline β-FeSi2 obtained above 800°C showed a clear reflectance maximum at 0.88 eV due to the optical transition at the direct band-gap of 0.84 eV observed in the characteristic plot ((ahv)2 vs. hv) of the optical absorption.

Experimentally reported interfacial luminescence from silicon nano structures are studied by using a first-principles calculation of a Si(100) quantum slab covered with silicon oxide. When Si-OH bonds were introduced at the silicon/silicon-oxide interface, wave functions at the valence band top and near conduction band bottom localized vertically and laterally near the Si-OH bonds. Such strong localization is the result of the cooperation of the coupling of non-bonding 2p lone pair orbitals on interfacial 0 atoms and the strong dipole of OH. Furthermore, the localization is significant only in silicon nano structures. This localization of the wave functions can be the source for creating localized excitons, which can dramatically enhance the intensity of photoluminescence. Therefore, interfacial Si-OH bonds are a theoretically convincing possible source of reported interfacial luminescence.

Guided-wave absorption and fluorescence in epitaxial Er:BaTiO3 thin film channel waveguides on MgO are reported. Guided- wave absorption is strongly dependent on post- growth annealing. Stimulated emission over a 40 nm bandwidth (λcente - 1.54 μm) has been achieved in oxygen-annealed waveguides. At 1.54 μm the absorption is reduced from ∼ -2 dB/cm to -∼-1.3 dB/cm in a 2.2 mm long 5 μm wide channel waveguide due the presence of the pump light.

Photoluminescence (PL) stability of silicon suboxide (SiOx, 0<x<l.5) under light soaking and thermal processing has been investigated. As the oxygen content increases, the PL peak position shifts to the visible, with improved stability against measurement temperature and light soaking. With increasing annealing temperature, the PL spectra shifted to infrared, characteristic of a- Si:H. With annealing above 550 °C, visible PL is lost while infrared PL can be recovered by rehydrogenation of Si-dangling bond defects. Infrared absorption, transmission electron microscopy, visible absorption, and photocurrent indicate a disproportionation reaction resulting in a-Si phase segregation.

In this paper we discuss a novel approach to the band gap engineering of semiconductors Si, Ge, GaAs and AIN. We suggest that nanoporous polymorphs of these materials may exist which would offer a significant variation of the electronic band gap. Structurally, nanoporous semiconductors are related to the zeolitic nets, and a systematic procedure of generating these structures from the (4;2) nets is described. We use the ab initio total energy quantum molecular dynamics method Fireball96 to investigate the energetics and the electronic properties of these nanoporous or “expanded” semiconductor phases.

Nanometer-sized silicon clusters show photoluminescence, but some technique to embed them in bulk materials is needed to enable practical device applications. Thus we calculated the properties of silicon clusters in zeolite, whose framework constitutes cages as large as the clusters. We chose the hydrogenated silicon cluster Si10H16 embedded in dehydrated all-silica zeolite-A. We first performed molecular mechanics simulation to obtain a thermodynamically stable structure. The cluster was located near the center of the cage. Then we calculated the electronic structure and optical properties for this stable structure by density functional method. The absorption spectrum of this material was nearly equal to the superposition of the spectra of Si10H16 and zeolite-A, which indicates that the properties of silicon clusters are retained even in zeolite cages. These results suggest that silicon clusters in zeolite may have the potential to be used as optoelectronic materials.

The suitability of the (001) Si/ZnS system for optoelectronic applications is theoretically investigated and compared to the Si/Ge system. The band-structure calculations show that Si/ZnS is superior to Si/Ge in a number of respects: directness of band gap can be easily obtained; and optical intersubband transitions fall within the important 1.3–1.6 μm window.

Porous silicon obtained by the anodization of heavily doped n+-type silicon wafers was used as a sacrificial layer to micromachine silicon platforms. The effect of experimental parameters, such as the nature of the masking layer, current density and anodization time, on the geometry of the porous Si formed in patterned substrate is shown. Advantages of this method on the orientation-dependent chemical etching (ODCE), which is classically used for optical fiber alignment, are discussed.

Thermal decomposition of a metastable silane in nanoporous alumina leads to the formation of luminescent silicon nanostructures. While varying the pore size of the transparent membranes the luminescence can be shifted from 504 nm up to 537 nm by building up a sheet-like structure of siloxene on the inner pore surface.

The electronic structures of halogenated polysilanes (PSi), whose side chains are replaced by halogen (X) atoms, have been theoretically investigated based on the firstprinciples calculations. It was found that non-bonding (n) electrons localizing at the X atom produce an important orbital mixing with the u valence electrons delocalized in the direction of the PSi skeleton (a-n mixing). This a-n mixing splits the top of the valence bands, and creates unoccupied states in the band gap. This valence-band-splitting effectively narrows the band gap to the visible range, and the unoccupied state in the band gap has the potential to be an electron acceptor. Moreover, we also investigated several replacement patterns as well as the kind of replaced halogen species (X=F, Cl, Br, I). The dispersion and energy position of the unoccupied state(s) can be artificially tuned by these chemical modifications. Thus, halogenation can change PSi into optelectronic polymers with visible photoluminescence.

Absorption coefficient and refractive index measurements of stoichiometric As2Te3 thin films grown on Si and KBr substrates are presented. The films were grown by an organometallic deposition process where the sources for both As and Te react at reduced pressures and temperatures to produce stoichiometric films in the presence of a surface; no growth occurs otherwise. Films were grown at 300 and 373 K and had a thickness range of about 0.2–1.2 μm. The refractive index was sample independent and had strong dispersion near the optical gap, rapidly increasing from the value of 3.8 at 1.5 μm as the wavelength decreases. An interference technique was utilized to measure the absorption of the films since interference effects and substrate absorption bands hindered the application of conventional optical transmission techniques. The optical gaps, obtained from Tauc plots, were in the range 0.83–0.95 and the results confirm that the As2Te3 films behave as noncrystalline semiconductors, being either amorphous or partially crystalline. Amorphous films on Si had absorption in the range of optical transparency comparable to that of the substrate, films on KBr substrates had much higher absorption.

In order to form strain-free GaAs films on Si substrates, GaAs-on-Si structures have been formed by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) and subsequently annealed under ultrahigh pressure at 2 GPa. The samples were then reannealed at atmospheric pressure in order to investigate the regeneration effect of strain in the GaAs films. It was found from X-ray diffraction (XRD) analysis and photoluminescence (PL) measurement that in the reannealed samples the strain near the surface of GaAs film was smaller than that near the interface with Si. Finally, additional GaAs layers were grown using MBE on both as-grown and strain-free GaAs-on-Si structures and it was found that the residual strain in the GaAs layer on strain-free GaAs-on-Si structure was smaller than in the as-grown case.