Ionel Solomon

Substrate selectivity in the formation of microcrystalline silicon: Mechanisms and technological consequences

We report the results of an in situ spectroscopic ellipsometry study concerning the substrate dependence of the evolution of microcrystalline silicon films deposited by alternating amorphous silicon deposition and hydrogen plasma treatment. The evolution of the composition of the films during growth, up to thicknesses of ∼100 nm, indicates that besides etching, the diffusion of atomic hydrogen efficiently promotes the growth (and/or nucleation) of buried crystallites. Moreover, the evolution of the films strongly depends on the nature of the substrate. This substrate selectivity is discussed in terms of initial growth processes. The effect of the hydrogen plasma well below the film surface, which produces the thickness‐dependent film composition, along with the substrate selectivity, may be of prime importance in technological applications of microcrystalline silicon.

Optical properties and hydrogen concentration in amorphous silicon

Abstract We have made a systematic study of the dependence of hydrogen concentration and optical properties on the deposition parameters for amorphous silicon prepared by r.f. glow discharge decomposition of silane. The hydrogen content and the optical gap decrease with increasing deposition temperature T s , but we also found that the application of an electrical bias to the substrate results in a large increase of the hydrogen content. The rate of increase of the optical gap with the hydrogen content varies with the deposition conditions. This is interpreted as the result of different modes of incorporation of the hydrogen in the material.

Physics of low density-of-states a-Si1−xCx films

Abstract We give a report on a systematic study of the deposition conditions for the production of low-density-of-states carbonated a-Si:H from SiH 4 -CH 4 mixtures. Two types of materials can be produced: in the “low powder-density regime”, there is no primary decay of the methane. The deposition rate is independent of the gas phase composition and the incorporation of carbon is low even at high methane concentration. In the “high power-density regime”, both gases are decomposed and silicon-carbon alloys in all proportions are obtained.

Influence of carbon incorporation in amorphous hydrogenated silicon

Abstract Glow-discharge a-Si1−x, Cx: H films prepared from mixtures of SiH4, and CH4 have been studied in the low carbon concentration range. The density of states at mid-gap, determined by measuring the space-charge limited current, was found to depend quadratically on the carbon concentration in the film. This suggests that the density of states increases because of the pairing of diluted defect precursors in the network. A tentative model is described and discussed in terms of the present knowledge of the structure of SiC alloys. Data on the composition dependence of the optical gap and the refractive index are also presented.

Surface Plasmon Resonance on Gold and Silver Films Coated with Thin Layers of Amorphous Silicon−Carbon Alloys

The paper reports on a novel surface plasmon resonance (SPR) substrate architecture based on the coating of a gold (Au) or silver (Ag) substrate with 5 nm thin amorphous silicon-carbon alloy films. Ag/a-Si(1-x)C(x):H and Au/a-Si(1-x)C(x):H multilayers are found to provide a significant advantage in terms of sensitivity over both Ag and Au for SPR refractive index sensing. The possibility for the subsequent linking of stable organic monolayers through Si-C bonds is demonstrated. In a proof-of-principle experiment that this structure can be used for real-time biosensing experiments, amine terminated biotin was covalently linked to the acid-terminated SPR surface and the specific streptavidin-biotin interaction recorded.

Electrochemistry and photoluminescence of porous amorphous silicon

Abstract A systematic study of the electrochemistry of hydrogenated amorphous silicon (a-Si:H) is presented in order to explain pore formation in this material. The similarity of the cyclovoltammograms of amorphous and crystalline silicon demonstrates that the basic electrochemical reactions are similar in both materials. A striking difference between the electrochemistry of these two materials is the existence of an instability which limits the maximum thickness of the porous amorphous material that can be obtained. We explain this instability by the large value of the resistivity of amorphous silicon in comparison with that of the electrolyte, and model this phenomenon by a linear stability analysis. We detected the same phenomenon during anodization of highly resistive p-type crystalline silicon. The photoluminescence intensity of porous a-Si:H is quite comparable with that of crystalline silicon of the same thickness.

Molecular monolayers on silicon as substrates for biosensors

(111) silicon surfaces can be controlled down to atomic level and offer a remarkable starting point for elaborating nanostructures. Hydrogenated surfaces are obtained by oxide dissolution in hydrofluoric acid or ammonium fluoride solution. Organic species are grafted onto the hydrogenated surface by a hydrosilylation reaction, providing a robust covalent Si-C bonding. Finally, probe molecules can be anchored to the organic end group, paving the way to the elaboration of sensors. Fluorescence detection is hampered by the high refractive index of silicon. However, improved sensitivity is obtained by replacing the bulk silicon substrate by a thin layer of amorphous silicon deposited on a reflector. The development of a novel hybrid SPR interface by the deposition of an amorphous silicon-carbon alloy is also presented. Such an interface allows the subsequent linking of stable organic monolayers through Si-C bonds for a plasmonic detection. On the other hand, the semiconducting properties of silicon can be used to implement field-effect label-free detection. However, the electrostatic interaction between adsorbed species may lead to a spreading of the adsorption isotherms, which should not be overlooked in practical operating conditions of the sensor. Atomically flat silicon surfaces may allow for measuring recognition interactions with local-probe microscopy.

Microcrystalline silicon growth by the layer-by-layer technique: long term evolution and nucleation mechanisms

The deposition of microcrystalline silicon films has been studied by in situ spectroscopic ellipsometry. We found that both the etching of the amorphous phase at the film growing surface and the rearrangement of the bulk induced by hydrogen diffusion are necessary to explain the long term evolution during deposition and nucleation mechanisms of microcrystalline silicon. The transition from amorphous to microcrystalline silicon deposition occurs through a highly porous phase. The independent control of the deposition and hydrogen plasma treatment times achieved in the layer-by-layer technique is very well adapted to overcome the substrate selectivity. In particular, we show that we can convert an amorphous film into the microcrystalline state.

The Physics of Plasma Deposition of Microcrystalline Silicon

The growth of microcrystalline silicon (μc-Si), deposited by a succession of silane and hydrogen plasmas, is investigated in situ by ellipsometry in the visible and near UV-range. It is found that the amorphous tissue is more affected by the hydrogen etching than the crystallites. The model of “selective etching” emerges from these measurements. Although this model is compatible with the “partial chemical equilibrium” of Vepek, it is somewhat more general and explains the porous nature of the (μc-Si) as well as the many atomic layers deposition-etching sequences.

Highly sensitive and reusable fluorescence microarrays based on hydrogenated amorphous silicon-carbon alloys

We have designed a new architecture of fluorescent microarrays based on a thin layer of hydrogenated amorphous silicon-carbon alloy (a-Si(0.85)C(0.15):H) deposited on an aluminium-on-glass back reflector. These substrates are modified with an organic monolayer anchored through Si-C bonds and terminated with carboxyl groups, allowing for the covalent immobilization of biological probes. The fluorescence yield is maximized by optimization of the a-Si(0.85)C(0.15):H layer thickness. This approach is assessed for DNA recognition, demonstrating an increase in sensitivity by over one order of magnitude as compared to commercial slides, and the possibility of following in situ the molecular recognition event (hybridization). The immobilization chemistry provides these substrates with a superior chemical stability toward ageing or long-term exposure to physiological buffers, which allows for many successive hybridization/dehybridization cycles without measurable changes in performance.