Giampiero Amato

High-quality porous-silicon buried waveguides

This letter reports a method to produce porous-silicon waveguides by means of a laser local oxidation process. The estimated losses of the waveguides are below 1 dB/cm. This demonstrates the applicability of this material in integrated optics and telecommunications. Moreover, our results disclose the opportunity to integrate optoelectronic devices onto Si substrates. The laser writing method is achievable at low laser power, thus it is highly efficient and achievable with the standard equipment present in most laboratories. Another advantage is that oxidation is achieved without heating the complete chip, thus simplifying the integration process, i.e., the oxidation is inherently local through the direct-write process. This method opens the opportunity to build microstructures, like channel and membrane filters, in a flexible manner by R&D laboratories.

Micromachining of silicon with a proton microbeam

Abstract In the recent years the fabrication of sensors and actuator devices on a microscopic scale and their integration with electronic devices and micro-electromechanical systems (MEMS) has become an area of considerable commercial and technological interest, with huge development potentialities. High energy ion microbeam is a suitable tool for such purposes. In this paper we present an alternative way to exploit the lithographic properties of micro ion beams based on the selective damage of silicon to produce porous silicon microstructures. We used a 2 MeV proton microbeam to irradiate definite areas of silicon samples in order to produce damaged layers localised at the end of the proton trajectories. By performing an electrochemical etching in a suitable HF solution, a porous silicon pattern, complementary to the irradiated one, is always formed. The main effect of the damage on the porous silicon formation is to reduce the velocity of formation. To interpret this, such dead layers can be seen to be more or less opaque to the migration of free holes. Consequently the patterned region can be more or less revealed according to the formation time. The procedure allows for the production of microstructures of porous silicon whose unique properties are of great interest for applications. Preliminary results obtained on silicon samples, with different doping levels (p+, p, n+) and irradiating regions with different areas (from 200×200 μm 2 to 25×25 μm 2 ) are presented in order to evaluate the most suitable range of exposure and aspect-ratio of the microstructures.

Direct fabrication of three-dimensional buried conductive channels in single crystal diamond with ion microbeam induced graphitization

Abstract We report on a novel method for the fabrication of three-dimensional buried graphitic micropaths in single crystal diamond with the employment of focused MeV ions. The use of implantation masks with graded thickness at the sub-micrometer scale allows the formation of conductive channels which are embedded in the insulating matrix at controllable depths. In particular, the modulation of the channels depth at their endpoints allows the surface contacting of the channel terminations with no need of further fabrication stages. In the present work we describe the sample masking, which includes the deposition of semi-spherical gold contacts on the sample surface, followed by MeV ion implantation. Because of the significant difference between the densities of pristine and amorphous or graphitized diamond, the formation of buried channels has a relevant mechanical effect on the diamond structure, causing localized surface swelling, which has been measured both with interferometric profilometry and atomic force microscopy. The electrical properties of the buried channels are then measured with a two point probe station: clear evidence is given that only the terminal points of the channels are electrically connected with the surface, while the rest of the channels extends below the surface. IV measurements are employed also to qualitatively investigate the electrical properties of the channels as a function of implantation fluence and annealing.

Electrical Properties of Mesoporous Silicon: From a Surface Effect to Coulomb Blockade and More

Since the publication of the paper of [V. Lehmann, F. Hofmann, F. Moeller, and U. Gruening, Thin Solid Films, 255, 20 (1995)], a great deal of effort has been produced to understand the basic mechanisms ruling the electron transport in Si mesostructures and how these phenomena are affected by the external environment. After more than 10 years of studies on mesoporous silicon in interaction with gas molecules, the latest experimental evidences and physical insights have been revealed, such as gas sensitivity, chemisorption phenomena, coulomb blockade, and glassy dynamics at room temperature. But by reading that former paper, the feeling of an extraordinary comprehension and intuition of the physical mechanisms occurring in this fascinating material continuously accompanied the reader. A review of these major results, starting from the first evidence of a strong interaction with nitrogen dioxide, to the in situ Fourier transform IR and electron paramagnetic resonance spectroscopy studies, and to the more recent electronic transport experiments on this material was reported, which follows Lehmanns intuitions in his paper.

Patterning of porous silicon by electron-beam lithography

Electron-beam (E-beam) lithography has been applied to porous silicon substrates. In contrast with optical lithography, the whole process is fully compatible with the material, without any need for particular previous treatments. Porous silicon behaves as a low density substrate, allowing fine writing even at moderate electron energies, with a negligible proximity effect. High quality structures are expected if films deposited on porous silicon are defined by the E-beam, as in the case of freestanding membranes. Patterns written at lower resolution have been successfully transferred to porous silicon using plasma etching techniques. This allows the possibility of direct lateral structuring of porous silicon, a key factor in the realization of high quality devices for photonics.

Electrical stimulation of non-classical photon emission from diamond color centers by means of sub-superficial graphitic electrodes.

Focused MeV ion beams with micrometric resolution are suitable tools for the direct writing of conductive graphitic channels buried in an insulating diamond bulk, as already demonstrated for different device applications. In this work we apply this fabrication method to the electrical excitation of color centers in diamond, demonstrating the potential of electrical stimulation in diamond-based single-photon sources. Differently from optically-stimulated light emission from color centers in diamond, electroluminescence (EL) requires a high current flowing in the diamond subgap states between the electrodes. With this purpose, buried graphitic electrode pairs, 10 μm spaced, were fabricated in the bulk of a single-crystal diamond sample using a 6 MeV C microbeam. The electrical characterization of the structure showed a significant current injection above an effective voltage threshold of 150 V, which enabled the stimulation of a stable EL emission. The EL imaging allowed to identify the electroluminescent regions and the residual vacancy distribution associated with the fabrication technique. Measurements evidenced isolated electroluminescent spots where non-classical light emission in the 560–700 nm spectral range was observed. The spectral and auto-correlation features of the EL emission were investigated to qualify the non-classical properties of the color centers.

Anisotropic resistivity of (100)-oriented mesoporous silicon

The resistivity of (100)-oriented mesoporous silicon has been studied using two different electrode configurations. The authors observed that the electronic transport along the longitudinal direction (parallel to the sample surface) is strongly inhibited at room temperature but not along the perpendicular direction. They show that such electrical anisotropy can be removed by heating the material, reporting an increase of six orders of magnitude of the longitudinal conductivity when the temperature rises from 20to100°C. These experimental findings are interpreted on the basis of the material morphology and nanostructuration, which determine the availability of percolative pathways for free charge carriers.

Band-gap states in unfilled mesoporous nc-TiO2: measurement protocol for electrical characterization

The characterization of the intrinsic electrical properties of large surface area materials such as mesoporous nanocrystalline (nc-TiO2), requires one to avoid sensor-like responses to external agents. Both an appropriate sample configuration and a suitable measurement protocol are mandatory. In this work, both the stack and planar contacts configurations were studied, the latter giving evidence which is potentially useful for space charge limited (SCL) current investigations in order to study the band-gap states (BGSs) properties in nc-TiO2. Moreover, in the absence of a suitable measurement protocol, standard dye sensitized solar cells (DSSCs) electrode films show apparent SCL current behaviour, as a consequence of the surface electrical transport contribution due to the physisorbed water on the hydroxylated metal oxide large surface area. This feature recalls the typical results of electrolyte filled systems, in which an exponential distribution of trap states is reported; it is not expected to be an intrinsic feature of the nc-TiO2 trap states distribution. In the absence of adsorbates, no deviation from an ohmic regime is observed in standard electrodes, while in planar configuration samples, the SCL regime is accessible and shows a different BGS signature. The nature of electrical contacts, ohmic in this situation, is also discussed.

Formation of buried conductive micro-channels in single crystal diamond with MeV C and He implantation

Abstract As demonstrated in previous works, implantation with a MeV ion microbeam through masks with graded thickness allows the formation of conductive micro-channels in diamond which are embedded in the insulating matrix at controllable depths [P. Olivero et al., Diamond Relat. Mater. 18 (5–8), 870–876 (2009)]. In the present work we report about the systematic electrical characterization of such micro-channels as a function of several implantation conditions, namely: ion species and energy, implantation fluence. The current–voltage (IV) characteristics of the buried channels were measured at room temperature with a two point probe station. Significant parameters such as the sheet resistance and the characteristic exponent (α) of the IV power-law trend were expressed as a function of damage density, with satisfactory compatibility between the results obtained in different implantation conditions.

Kinetics of defect formation in chemically vapor deposited (CVD) graphene during laser irradiation: The case of Raman investigation

The effect of laser irradiation on chemically vapor deposited (CVD) graphene was studied by analyzing the temporal evolution of Raman spectra acquired under various illumination conditions. The spectra showed that the normalized intensity of the defect-related peak increases with the square root of the exposure time and varies almost linearly with the laser power density. Furthermore, the hardness of graphene to radiation damage depends on its intrinsic structural quality. The results suggest that, contrary to the common belief, micro-Raman spectroscopy cannot be considered a noninvasive tool for the characterization of graphene. The experimental observations are compatible with a model that we derived from the interpretative approach of the Staebler–Wronski effect in hydrogenated amorphous silicon; this approach assumes that the recombination of photoexcited carriers induces the breaking of weak C–C bonds.