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AlN/GaN superlattices (SLs) with different periods grown on GaN buffer layers were studied by infrared spectroscopic ellipsometry (IRSE), Raman scattering (RS) and highresolution reciprocal space mapping (RSM). The lattice parameters and the degree of strain in the GaN buffer and the SL constituents were determined. Phonon modes originating from the buffer layer and the SL sublayers were identified and their frequency shifts were correlated with the strain state of the films.
Pores in porous 6H–SiC were found to propagate first nearly parallel with the basal plane and gradually change direction and align with the c axis. As a consequence, well-defined columnar pores were formed. It was shown that the rate of change of propagation directions was influenced by the etching parameters, such as hydrofluoric acid concentration and current density. Larger currents resulted in formation of larger pores. Pore sizes were found to increase with depth due to a decrease of the acid concentration. In addition, due to chemical etching effects, larger pore sizes were obtained close to the sample surface.
We report ordinary <ε┴> and extra-ordinary <ε║> dielectric function data of 4H- and 6H-SiC from 0.7 to 9.0 eV. These data, which were obtained by with spectroscopic ellipsometry, are in good qualitative agreement with trends recently reported in ab initio calculations.
Porosity depth profiles in porous silicon were realized by time modulation of the applied current density during electrochemical etching of crystalline silicon. The samples were investigated by variable angle spectroscopic ellipsometry. Using a basic optical model based on isotropy assumptions and the Bruggeman effective medium approximation, deviations from an ideal profile in terms of an interface roughness between the silicon substrate and the porous silicon layer and a compositional gradient normal to the surface were revealed. Furthermore, optical anisotropy of the sample was investigated by generalized ellipsometry. The anisotropy was found to be uniaxial with the optic axis tilted from surface normal by about 25°. The material was also found to exhibit positive birefringence.
Porosity depth profiles with exponential or sinusoidal shape were fabricated electrochemically in crystalline silicon using time-variable current densities and studied employing variable angle spectroscopic ellipsometry. Since volume porosity in porous silicon depends on the current density, it was possible to electrochemically tailor porosity depth profiles, which in a first approximation resembled the time modulation of the applied current. Optical characterization of the samples were realized using multilayer optical models and the Bruggeman effective medium approximation allowing variations of the index of refraction according to the applied current density profiles. The analysis also revealed deviations from desired profiles in terms of in-depth inhomogeneities.
Intrinsic, n- and p-type a-Si:H films were deposited by dc magnetron sputtering and analyzed with several techniques. The films were synthesized in a reactive Ar-H2 atmosphere giving H contents in the range of 3-20 at %. The films were sputtered from pure silicon targets and doped silicon targets with 1 at % B or P. Doping by co-sputtering from composite Si/B4C targets was also explored. The doping concentrations were 3 × 1020 − 2 × 1021 cm-3 for the p-type films and 2.6-2.9 × 1019 cm-3 for the n-type films. The conductivity was in the range 10-2−10-4 Ω-1 cm-1 for p-doped films and 10-5 Ω-1 cm-1 for the best n-doped films. Band gap estimations were obtained from dielectric function data and showed an increase with hydrogen content. A comparison to device quality PECVD-samples was also made.
Thin porous silicon (PS) layers (30–1000 nm) have been produced by electrochemical anodization of highly p-doped silicon in a hydrofluoric acid electrolyte at constant current density. Variable angle spectroscopic ellipsometry was used for characterization in the spectral range 1.24–5.00 eV. Four multilayer models were developed for the PS layers. Ellipsometric spectra were fitted for three of these models by utilizing an effective medium approximation for each sublayer containing crystalline silicon and voids. A third constituent, amorphous silicon, was added in the fourth model. Excellent fits to experimental data were obtained in the entire spectral range and thickness and composition of each individual sublayer in the models were determined. The analyses revealed a compositional gradient normal to the surface. The porosity and the fraction of amorphous silicon decreased with film depth, whereas the fraction of crystalline silicon increased. The mean porosity of a PS layer was obtained as the thickness weighted average of the porosities of the sublayers. The proposed multilayer models allow a more detailed analysis of PS layers compared to single layer models.
Thin films of CeO2 deposited by rf magnetron sputtering on sapphire and silicon substrates have been characterized with variable angle spectroscopie ellipsometry, atomic force microscopy (AFM), transmission electron microscopy and x-ray diffraction. A novel multiple model was used successfully for determination of the optical properties and surface roughness of the CeO2films. AFM analysis showed that the CeO2 films have hillock-shaped facet morphology. It was found that the surface roughness of CeO2 increased with film thickness. For films on sapphire substrates, the surface morphology and the crystalline quality were improved by post-annealing, e.g. flat surfaces were obtained after annealing at 1100 °C.
Prototype memory cells of a proposed three-bit memory device, whose optical read-out is based on the ellipsometric principle, nas been fabricated by deposition of 5 nm layers of amorphous silicon and amorphous germanium on crystalline silicon substrates. The differences in the ellipsometric parameters of memory cells corresponding to different logical states were much larger than the instrumental resolution. The possibility to optimize with respect to photon energy and angle of incidence is demonstrated.
Amorphous Ge/Si superlattices have been grown by dual-target dc magnetron sputtering at ambient temperature under different negative substrate bias voltages Vs. The films were studied with cross-sectional transmission electron microscopy (XTEM). All films were amorphous, but for Vs ≤ 110 V the superlattices exhibited a columnar structure. Within each column, the compositional modulation was clearly resolved except in the vicinity of the column boundaries. For 110<VS≤250V, dense films with homogeneous and well-defined layers were obtained. High-resolution XTEM studies showed that superlattices with individual layer thicknesses less than 1.5 nm and smooth interfaces could be fabricated. Increasing Vs > 250 V resulted in ion induced intermixing such that at Vs ≥ 450 no compositional modulation was observed. The obtained structural results are discussed in terms of available models for ion-irradiation effects during low-temperature ballistic growth.
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