Pierre Joubert

Mode of growth and microstructure of polycrystalline silicon obtained by solid‐phase crystallization of an amorphous silicon film

The structure and the morphology of crystallized amorphous silicon (α‐Si) films which were deposited on glass and annealed in a conventional furnace or by rapid thermal process (RTP) are studied using transmission electron microscopy (TEM). The ellipsoidal shape of the grains is attributed to the fast solid‐state crystallization along the two mutually perpendicular 〈112〉 and 〈110〉 crystallographic directions. The growth is solely based on the twin formation. The stability of the microtwins was studied by RTP and in situ TEM heating experiments. The effect of the film thickness on the preferred orientation of the grains is discussed. Very thin films exhibit (111) preferred orientation due to the strongly anisotropic rate of growth of the nuclei, which imposes an orientation filtering due to a growth velocity competition. The mode of growth of these films is compared with poly‐Si films grown by low‐pressure chemical‐vapor deposition.

Effect of grain boundaries on the formation of luminescent porous silicon from polycrystalline silicon films

Luminescent porous silicon can be obtained by electrochemical etching under illumination from n‐type polycrystalline silicon which has been fabricated by solid phase crystallization of in situ doped amorphous films deposited by low pressure chemical vapor deposition. The analyses of the nanostructures by microscopy show that the pore orientations mainly follow the current flow which is channeled by macropores located at grain boundaries. The luminescence results obtained from this porous polycrystalline silicon are comparable to those obtained from porous samples of single‐crystals silicon.

Optical loss study of porous silicon and oxidized porous silicon planar waveguides

We have studied optical losses as a function of the wavelength for planar waveguides formed from porous silicon or oxidized porous silicon. Scattered light from the surface of samples was also observed. This observation reveals the influence of porous silicon dissolution front fluctuations called waviness on propagation. After oxidation, the measured losses decreased strongly and attained a value equal to 0.5 dB/cm in the near infrared. Surface and volume scattering losses were modeled in order to determine their principal contributions to overall losses. For porous silicon waveguides obtained from a P+ silicon substrate, the losses were mainly due to absorption by the material; whereas, for oxidized porous silicon waveguides, the principal contribution depends on the used wavelength. In the visible spectrum, losses due to volume scattering were predominant while in the near infrared, surface scattering was responsible for most of the losses.

Pressure dependence of in situ boron‐doped silicon films prepared by low‐pressure chemical vapor deposition. I. Microstructure

The effects of silane pressure and temperature on the in situ boron incorporation and resistivity of low‐pressure chemical vapor deposited polycrystalline silicon films were studied in the ranges of 2.5×10−3–1 Torr and 515–700 °C. By lowering the silane pressure, the boron concentration increases (up to 1×1022 cm−3) and the resistivity decreases down to about 2×10−3 Ω cm without annealing. For high deposition pressure (≥0.1 Torr), the resistivity decreases as the temperature is lowered. In this latter case the secondary‐ion mass spectrometry profiles reveal a boron accumulation at the layer‐substrate interface, which is always observed independently of the substrate nature.

Porosity Gradient Resulting from Localised Formation of Porous Silicon: The Effect on Waveguiding

Porous silicon formation on patterned substrates leads to a depth-dependent porosity. These porosity variations depend on the anodisation parameters and on the size of the open windows in the masking layer. During the anodisation at a constant current intensity, the interfacial reaction area increases and consequently the porosity decreases. Moreover, the porous silicon growth rate depends on crystallographic directions and induces a porosity gradient along the core/cladding interface. These porosity gradients could be crucial in some applications such as optical waveguides. Oxidised porous silicon waveguides were fabricated through a masking layer by applying two constant current intensities during anodisation. The measured near field distribution reveals that the light propagation is localised near the core/cladding interface. These observations confirm that a porosity gradient exists along vertical cross section of waveguides. This study deals with the porosity gradient estimations resulting from the electrochemical etching through an opened window in a masking layer.

Effect of crystallographic directions on porous silicon formation on patterned substrates

Abstract Growth kinetics of porous silicon, formed by electrochemical anodisation of (100) and (111) n+ and p+ substrates, was measured. We show that the growth rate along 〈100〉 is always higher than along 〈111〉. The 〈111〉/〈100〉 growth rate ratio induces the etching profiles and the final shapes of the porous silicon regions obtained on patterned Si substrates. The profile is sharper in the case of the n+ than that of the p+ (100) substrates. For n+ (111) Si substrates, the etching profile is different and depends on the alignment of the mask patterns according to the axis of symmetry of the silicon crystal.

Poly(p phenylene vinylene)/porous silicon composites

Abstract Composites made by mixing porous silicon grains with poly(p phenylene vinylene) or PPV were studied by optical and electrical characterization. Infrared, UV–vis absorption, Raman and photoluminescence measurements were performed on films with different silicon concentrations and the results were compared to those obtained separately from porous and PPV samples. The optical spectra showed that porous silicon was incorporated into the polymer without significant change in the polymer structure. In contrast, porous silicon was oxidized and aged by the conversion process of the polymer. Light emitting diodes fabricated with the composites have low turn-on voltage as compared to devices using PPV. The improvement was explained by the enhancement of the conductivity and by the increase in the contact area between the film and the electrode due to the change in morphology of the surface of the film.

Substrate Effects on the Kinetics of Solid Phase Crystallization In a -Si

The effect of substrate nature on the solid phase crystallization at 600 °C of a -Si deposited by low pressure chemical vapor deposition is investigated by x-ray diffraction and transmission electron microscopy. The nucleation rate varies slightly resulting to a weak variation in the final grain sizes as a function of the substrate type. In all cases the grain growth mode is found to be three dimensional. In contrary, a drastic effect of the substrate is observed for films deposited by plasma enhanced CVD. Fast crystallization is obtained on indium tin oxide (ITO) resulting to small grain poly-Si, whereas the crystallization is retarded on glass leading to an increase in the grain size.

N-TYPE POLYCRYSTALLINE SILICON FILMS OBTAINED BY CRYSTALLIZATION OF IN SITU PHOSPHORUS-DOPED AMORPHOUS SILICON FILMS DEPOSITED AT LOW PRESSURE

The low‐pressure chemical‐vapor deposition of phosphorus‐doped silicon film on glass at 550 °C was investigated as a function of silane pressure (1–100 Pa) and phosphine/silane mole ratio ranging between 4×10−6 and 4×10−4. At this low temperature the film is homogeneous in thickness and the silicon is amorphous except for low pressure (1 Pa). Phosphorus concentration varies linearly with mole ratio in amorphous deposited films. The resistivity of films annealed at 600 °C decreases while the incorporation of phosphorus (mole ratio) increases, and varies with phosphorus concentration from 101 to 10−3 Ω cm. For the same phosphorus content, the resistivity is lower if the silicon film is amorphous deposited and subsequently crystallized, than if the film is polycrystalline deposited. Carrier concentration and mobility are measured using the Hall method. Doping efficiency and electrical properties are discussed.

Porous Silicon Micromachining to Position Optical Fibres in Silicon Integrated Optical Circuits

Low-loss optical fibre connections require deep grooves etched in silicon substrate for accurate fibre positioning. As shown in this paper these grooves can be obtained by using localised formation of porous silicon on patterned substrates. Cr-Au masking layer with a duration in HF solution longer than 30 min is used to fabricate grooves with a depth higher than 75 μm. N+-type silicon provides grooves with a pseudo-V shape which is compatible with accurate fibre alignment. By using this technology, arrays of optical fibres are positioned with an accuracy higher than 1 μm.