M. F. Cerqueira

Macrocrystalline silicon thin films prepared by RF reactive magnetron sputter deposition

Abstract Hydrogenated microcrystalline silicon (μc-Si:H) thin films with Cu as a dopant material (about 2 wt.%) were deposited by RF planar magnetron sputtering in an argon/hydrogen plasma. The composition and microstructure of the films were analysed by SEM, ERD/RBS, X-ray diffraction and Raman spectroscopy. These techniques revealed a columnar film structure, each column consisting of several small (nano) crystals with a lateral dimension up to 100 A. The crystals are oriented, generally with the (111) plane parallel to the sample surface. The hydrogen content of the thin films is about 27–33 at.%. Low deposition rates and low sputter gas pressures favour crystallisation and grain growth. The behaviour can be understood in terms of the diffusion or relaxation length Λ of the deposited Si-atoms.

Structural studies and influence of the structure on the electrical and optical properties of microcrystalline silicon thin films produced by RF sputtering

Microcrystalline silicon thin films were produced by reactive magnetron sputtering on glass substrates under several different conditions (RF power and gas mixture composition). The film structure was studied by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and Raman spectroscopy, allowing the determination of crystal sizes, crystallinity and mechanical strain. These parameters were evaluated by fitting a pseudo-Voigt function to the X-ray data, and by the application of the strong phonon confinement model to the Raman spectra. The degree of crystallinity and the presence of single crystals or crystal agglomerates, which was confirmed by TEM, depends on the preparation conditions, and strongly affects the optical spectra and the electrical transport properties.

Fabrication of GeSn-multiple quantum wells by overgrowth of Sn on Ge by using molecular beam epitaxy

We report on the fabrication and structural characterization of epitaxially grown ultra-thin layers of Sn on Ge virtual substrates (Si buffer layer overgrown by a 50 nm thick Ge epilayer followed by an annealing step). Samples with 1 to 5 monolayers of Sn on Ge virtual substrates were grown using solid source molecular beam epitaxy and characterized by atomic force microscopy. We determined the critical thickness at which the transition from two-dimensional to three-dimensional growth occurs. This transition is due to the large lattice mismatch between Ge and Sn (≈14.7%). By depositing Ge on top of Sn layers, which have thicknesses at or just below the critical thickness, we were able to fabricate ultra-narrow GeSn multi-quantum-well structures that are fully embedded in Ge. We report results on samples with one and ten GeSn wells separated by 5 and 10 nm thick Ge spacer layers that were characterized by high resolution transmission electron microscopy and X-ray diffraction. We discuss the structure and material intermixing observed in the samples.

Effect of grain size and hydrogen passivation on the electrical properties of nanocrystalline silicon films

The properties of mixed-phase (nanocrystalline/amorphous) silicon layers produced by reactive RF-sputtering are described. The chemical composition and nanostructure (i.e., Nanocrystal (NC) size and volume fraction) of the films were studied by Rutherford Backscattering Spectroscopy (RBS) and micro-Raman spectroscopy, respectively. Samples with different fractions of the nanocrystalline phase and NC mean size were produced by changing the deposition parameters, without post-growth annealing. The electrical conductivity of the films, measured as function of temperature, is discussed in relation to their nanostructure.

Photoluminescence of erbium doped microcrystalline silicon thin films produced by reactive magnetron sputtering

Microcrystalline silicon thin films doped with erbium were produced by RF sputtering and their structural, chemical and optical properties were studied by X-ray diffractometry at grazing incidence, Rutherford back scattering (RBS) and optical transmission spectroscopy. The samples exhibit a sharp photoluminescence (PL) spectrum from the Er centres with the strongest peak positioned at 1.536 μm with a full width at half maximum of about 8 nm. When the temperature varies between 5 and 300 K, the photoluminescence decreases only 5-fold, in contrast to the behaviour reported for monocrystalline silicon.

Multi-stacks of epitaxial GeSn self-assembled dots in Si: Structural analysis

We report on the growth and structural and morphologic characterization of stacked layers of self-assembled GeSn dots grown on Si (100) substrates by molecular beam epitaxy at low substrate temperature T = 350 °C. Samples consist of layers (from 1 up to 10) of Ge0.96Sn0.04 self-assembled dots separated by Si spacer layers, 10 nm thick. Their structural analysis was performed based on transmission electron microscopy, atomic force microscopy, and Raman scattering. We found that up to 4 stacks of dots could be grown with good dot layer homogeneity, making the GeSn dots interesting candidates for optoelectronic device applications.

Optical modulation spectroscopy of hydrogenated microcrystalline silicon

The properties of microcrystalline silicon thin films prepared by RF sputtering were investigated by optical modulation spectroscopy at room temperature and the results were correlated with Raman and conductivity measurements. For comparative purposes, a number of good quality PECVD c-Si:H samples were also investigated. For PECVD samples the OMS signal is very weak, and only measurable for probe beam energies comparable to the gap of amorphous silicon. This indicates the absence of gap states and therefore a very high crystalline fraction, as confirmed by Raman and TEM measurements. In what concerns RF-sputtered samples, different behaviors can be distinguished: Some samples show a low OMS signal which can be attributed either to a high crystallinity (low density of gap states) or to high recombination rates. These two cases can be distinguished by electrical conductivity values and analysis of the Raman spectra. Other samples exhibit a OMS signal similar to a-Si:H and have low conductivity values, consistent with a Raman spectrum typical of a-Si:H.

Structural characterization of mC-Si:H films produced by R.F. magnetron sputtering

Microcrystalline silicon thin films were produced by R.F. magnetron sputtering. The microstructure of these films has been studied by X-ray diffraction, Transmission Electron Microscopy (TEM) and Raman spectroscopy. Average values of crystalline size and strain obtained by the different techniques used are critically compared and the reasons for the differences are discussed.

Role of counter-ion and helper lipid content in the design and properties of nanocarrier systems: a biophysical study in 2D and 3D lipid assemblies

There is a direct correlation between the physicochemical properties of nanocarrier systems and their biological performance, including stability under physiological conditions, cellular internalization and transfection efficiency. Therefore, understanding the biophysical aspects that affect self-assembled nanocarriers is determinant for a rational design of efficient formulations. In this study, a comprehensive evaluation of the effects of each component on the molecular organization of aggregates formed by the cationic lipids dioctadecyldimethylammonium bromide and chloride (DODAB and DODAC) and the neutral lipid monoolein (MO) was made. Specifically, the effects of the helper lipid content (MO) and the role of the counter-ion of the cationic lipids were evaluated in 2D and 3D assemblies by Langmuir surface pressure–molecular area (π–A) isotherms, Brewster Angle Microscopy (BAM), infrared reflection absorption spectroscopy (IRRAS), confocal Raman microscopy, and Small Angle X-ray Scattering (SAXS). The results show that MO has a different distribution on the DODAC and DODAB bilayers, and a fluidizing effect dependent on the MO content. For low MO molar ratios, the fluidizing effect was more pronounced in DODAC : MO mixtures, indicating a more homogeneous distribution of MO in DODAC than in DODAB bilayers. For high MO molar ratios, packing of membranes was similar for both cationic lipids, and the effect of the counter-ion is attenuated. The distribution of MO in the two cationic systems is closely related with the efficiency of the counter-ions in the screening of the charged group.

Electron confinement in nanocrystals embedded in random media: Andersen localization effects

The effect of Anderson localization of electrons in nanocrystals (NC’s) embedded in an amorphous matrix of the same semiconductor material is investigated. A simple condition of localization in the disorder potential produced by the amorphous matrix around a perfectly crystalline core is formulated, determined by the reflection from the inhomogeneous random barrier at the NC/matrix interface. It is found that there are confined states in the NC’s, arising from an almost complete reflection of the electron wavefunction by the barrier, which can be characteristic of either strong or weak localization. The local density of states has been calculated numerically using parameters of Si NC’s embedded in amorphous silicon, which contains information concerning the states confined in the NC’s. These states, with energies depending on the NC size, are resonant and have a lifetime decreasing with the increase of the energy.