B. Remaki

Measurement of porous silicon thermal conductivity by micro-Raman scattering

We present a noncontact and nondestructive method to measure thermal conductivity in layered materials using micro-Raman scattering. This method was successfully applied to monocrystalline silicon whose thermal conductivity was found to be 63 W/m K at about 550 °C and then applied to porous silicon layers. For a 50 μm thick layer with 50% porosity, we found a thermal conductivity of 1 W/m K confirming the thermal insulating properties of this material.

Thermal conductivity of thick meso-porous silicon layers by micro-Raman scattering

We report here a theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized meso-porous silicon layers. The model is in good agreement with experimental measurements performed by micro-Raman scattering on the layers surface. For the first time, thermal conductivity inhomogeneity along the porous layer thickness of 100 μm is studied. Direct correlation between the thermal conductivity and morphology variations along the layer thickness is brought to the fore. A new approach to estimate local porosity of the porous layers based on thermal conductivity and Si nanocrystallite size measurements is also proposed.

On mechanical properties of nanostructured meso-porous silicon

Mechanical properties of meso-porous silicon are studied using topographic measurements and finite element simulations. Our approach is based on an original analysis of the strain at the free surface of porous silicon tub embedded in bulk Si regions allowing the determination of the Young’s modulus of the porous layers. In particular, the internal stress in the porous Si region is evaluated from the corresponding deformation of the monocrystalline Si adjacent region which mechanical parameters are well known. Moreover, a mechanical anisotropy of the columnar nanostructured porous Si is brought to the fore from the characteristic shape of the strained porous layer profile. Moderately oxidized, 70% in porosity, porous silicon patterns were investigated. Correlation of our measurements with x-ray data reported early in literature shows the macroscopic strain being close to the silicon lattice relative increase revealing an elastic deformation regime. The porous layers exhibit an unexpected low and strongly a...

Electrical barrier properties of meso-porous silicon

Abstract We present a contribution dealing with the study of the barrier properties of meso-porous silicon (PS) with metals and p + -Si crystalline silicon. Metal/PS/p + -Si and p + -Si/PS/p + -Si structures with different thickness (1–10 μm) of PS are investigated by means of current–voltage and capacitance–voltage characteristics combined with thermal stimulation in the 150–350 K temperature range. These experiments allowed a clear separation of bulk and contacts contributions to the electrical impedance. The barrier properties (nature and heights) of metal/PS contacts and PS/Si interfaces are then evaluated. The p + -Si/PS/ p + -Si structures exhibit ohmic contacts allowing space charge limited current (SCLC) ensured by carriers injection in the PS layers. From this analysis, the electrical behavior of the metal/PS/p + -Si structures is interpreted in terms of a Schottky barrier biased through a semi-insulating PS layer.

Space charge analysis in doped zinc phthalocyanine thin films

We present an improved method for the determination of the space charge density in organic semiconductors used as active layers in Schottky barriers. These measurements provide a powerful tool for the interpretation of basic properties such as the rectifying effect, doping process and carrier trapping mechanisms of films together with a way to assess the potential for sensor applications. Metal/molecular semiconductor Schottky junctions were prepared on zinc phthalocyanine layers doped by a controlled exposure to the ambient air. The organic material is deposited on aluminium or heavily doped silicon substrates, in order to make a Schottky barrier (film thickness around 1 μm). An ohmic contact is obtained by a gold deposition on the strongly doped side of the molecular material. We have investigated the current-voltage and capacitance-voltage characteristics. The results are interpreted in terms of a space charge region at the interface with the substrate, followed by an extended semi-insulating layer.The contribution of these two regions to the total impedance is analyzed in well improved conditions of measurements.

Theoretical and experimental study of heat conduction in as-prepared and oxidized meso-porous silicon

Recently measured low thermal conductivity of as-prepared and slightly oxidized meso-porous silicon (meso-PS) offers new possibility to apply this promising material for thermal isolation in microsensors and microsystems. We report here a theoretical model describing specific mechanisms of heat transport in as-prepared and oxidized meso-PS. The model is in good agreement with experimental data obtained earlier. In order to compare the thermal conductivity values of meso-PS layers oxidized at different temperatures with each other and with thermal conductivity of monocrystalline Si, a series of photoacoustic measurements was carried out. Evolution of the thermal conductivity along with oxidized fraction of Si in meso-PS has the same dynamics as that described by the theoretical model.

Conductive polyethylene as sensitive layer for gas detection

Abstract Experimental results on the gas-sensing properties of composite polymer thin films are presented. This is the first study on such a composite in a sensor device application. Films are elaborated from a low-density polyethylene powder containing 1.5 wt.% of crystalline tetrathiofulvalinium-tetracyanoquinodimethane (TTF-TCNQ) organic salts as conducting additives. Samples are cast onto ITO glass substrates at 80 °C using the so-called reticulate doping technique, and exhibit transversal conductivities of the order of 10 −9 Ω −1 cm −1 . D.c. measurements are performed to investigate the gas-detection properties of the film in the presence of CO 2 , NO 2 , O 2 and humidity species in an argon gas flow. Several interesting responses are observed at room temperature, such as reversible (CO 2 , O 2 ), non-reversible (NO 2 ) or partially reversible (humidity). Response times from 5 min up to 1.5 h, and sensitivities from 15 to 50% by absorption of ‘polluting’ gases in the concentrations usually encountered, are evaluated. Absorption results and the stability of the mechanical properties of the films are rather encouraging for the future of these composite polymer structures in the domain gas detection.

High-density oxidized porous silicon

We have studied oxidized porous silicon (OPS) properties using Fourier transform infraRed (FTIR) spectroscopy and capacitance–voltage C–V measurements. We report the first experimental determination of the optimum porosity allowing the elaboration of high-density OPS insulators. This is an important contribution to the research of thick integrated electrical insulators on porous silicon based on an optimized process ensuring dielectric quality (complete oxidation) and mechanical and chemical reliability (no residual pores or silicon crystallites). Through the measurement of the refractive indexes of the porous silicon (PS) layer before and after oxidation, one can determine the structural composition of the OPS material in silicon, air and silica. We have experimentally demonstrated that a porosity approaching 56% of the as-prepared PS layer is required to ensure a complete oxidation of PS without residual silicon crystallites and with minimum porosity. The effective dielectric constant values of OPS materials determined from capacitance–voltage C–V measurements are discussed and compared to FTIR results predictions.

Integrated RF micro-coils on porous silicon

Porous silicon (PS) of up to 400 /spl mu/m with high porosity (70%) has been studied to integrate high-performance micro-coils on heavily doped Si substrates (7-15 m/spl Omega/cm). 2 /spl mu/m-thick Au spiral inductors with L/spl sim/15 nH were fabricated; they demonstrate a quality factor Q up to 8 in the 0,1 to 2 GHz frequency range. Substrate loss is greatly lowered, down to /spl sim/12 /spl Omega/ at 400 MHz, which offers attractive potentialities for high resolution RMN (80-500 MHz) spectroscopy integrated devices.

Investigation of the electrical properties of the metal−calixarene−semiconductor structures

Abstract The research made to use organic materials as active elements in the electronic components and sensors has attracted much attention during the last decade. One of the interesting properties of these materials is the possibility to easily prepare stable and homogeneous thin films. In this work, we are interested in the study of the electrical properties of calixarene thin films using a metal-oligomer-semiconductor structure. The electrical study of such a structure can contribute to provide complementary information about the nature of the electrical conduction of the material and to elucidate the interface properties. The aim of this work is to study the different functioning parameters of ion selective field effect transistors and thin film transistors based on calixarenes. We present here the study of I-V characteristics and the behaviour of capacitance and conductance characteristics versus frequency. The annealing effects on the films are also investigated.