Simona Lorenti

Fabrication of miniaturised Si-based electrocatalytic membranes

The increasing interest for light and movable electronic systems, cell phones and small digital devices, drives the technological research toward integrated regenerating power sources with small dimensions and great autonomy. Conventional batteries are already unable to deliver power in more and more shrunk volumes maintaining the requirements of long duration and light weight. A possible solution to overcome these limits is the use of miniaturised fuel cell. The fuel cell offers a greater gravimetric energy density compared to conventional batteries. The micromachining technology of silicon is an important tool to reduce the fuel cell structure to micrometer sizes. The use of silicon also gives the opportunity to integrate the power source and the electronic circuits controlling the fuel cell on the same structure. This paper reports preliminary results concerning the micromachining procedure for fabricating a Si-based electrocatalytic membrane for miniaturised Si-based proton exchange membrane fuel cells (PEMFC).

Fabrication of miniaturized Si-based electrocatalytic membranes

Abstract The increasing interest for lightweight and portable electronic systems, cellphones and small digital devices is driving technological research towards integrated regenerating power sources with small dimensions and great autonomy. Conventional batteries are already unable to deliver power in ever smaller volumes while maintaining the requirements of long duration and light weight. A possible solution to overcome these limits is the use of miniaturized fuel cells. The fuel cell offers a greater gravimetric energy density compared to conventional batteries. The micromachining technology of silicon is an important tool to reduce the fuel cell structure to micron sizes. The use of silicon also gives the opportunity to integrate the power source and the electronic circuits controlling the fuel cell on the same structure. This article reports preliminary results concerning the micromachining process for fabricating a silicon-based electrocatalytic membrane for miniaturized Si-based proton-exchange membrane (PEM) fuel cells.

A look underneath the SiO2/4H-SiC interface after N2O thermal treatments

Summary The electrical compensation effect of the nitrogen incorporation at the SiO2/4H-SiC (p-type) interface after thermal treatments in ambient N2O is investigated employing both scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). SSRM measurements on p-type 4H-SiC areas selectively exposed to N2O at 1150 °C showed an increased resistance compared to the unexposed ones; this indicates the incorporation of electrically active nitrogen-related donors, which compensate the p-type doping in the SiC surface region. Cross-sectional SCM measurements on SiO2/4H-SiC metal/oxide/semiconductor (MOS) devices highlighted different active carrier concentration profiles in the first 10 nm underneath the insulator–substrate interface depending on the SiO2/4H-SiC roughness. The electrically active incorporated nitrogen produces both a compensation of the acceptors in the substrate and a reduction of the interface state density (D it). This result can be correlated with the 4H-SiC surface configuration. In particular, lower D it values were obtained for a SiO2/SiC interface on faceted SiC than on planar SiC. These effects were explained in terms of the different surface configuration in faceted SiC that enables the simultaneous exposition at the interface of atomic planes with different orientations.

Ion Implantation Defects in 4H-SiC DIMOSFET

In this paper the influence of point defects generated by the ion implantation process in 4H-SiC DIMOSFET has been studied in detail. The point defects generated by the source or body implantation process have been detected by micro-photoluminescence and the effect of these defects on the electrical characteristics of the DIMOSFET has been studied. In particular it has been observed that a reduction of the source ion implanted dose produces a large reduction of point defects in the source region and a considerable improvement of the electrical characteristic of the DIMOSFET.

Characterization of Embedded Rhomboidal Microchannels Formation on Silicon (100) Surface

Several techniques are routinely used to fabricate arrays of microchannels embedded in crystalline silicon. They are commonly based on plasma or anisotropic etch of silicon and on subsequent wafer bonding to close the cavities on top. These techniques are quite complex and usually work on (110) oriented silicon wafers. In this paper we present an innovative microchannel formation process, entirely based on front side wafer silicon bulk micromachining, which allows us to produce rhomboidal microchannels embedded on (100) silicon wafers. Compared to the traditional techniques, the proposed process is very compatible with the standard microelectronics silicon technology. The kinetics of rhomboidal microchannel formation are monitored by cyclic voltammetry measurements and the results are compared with a detailed structural characterization performed by scanning electron microscopy. The effectiveness of this process is discussed in view of the possible applications in the microfluidic field.

High Efficiency Light Emission Devices in Silicon

We report on the fabrication and performances of the most efficient Si-based light sources. The devices consist of MOS structures with erbium (Er) implanted in the thin gate oxide. The devices exhibit strong 1.54 μm electroluminescence at 300K with a 10% external quantum efficiency, comparable to that of standard light emitting diodes using III-V semiconductors. Emission at different wavelenghts has been achieved incorporating different rare earths (Ce, Tb, Yb, Pr) in the gate dielectric. The external quantum efficiency depends on the rare earth ions incorporated and ranges from 10% (for an Tb doped MOS) to 0.1% (for an Yb doped MOS). RE excitation is caused by hot electrons impact and oxide wearout limits the reliability of the devices. Much more stable light emitting MOS devices have been fabricated using Er-doped SRO (Silicon Rich Oxide) films as gate dielectric. These devices show a high stability, with an external quantum efficiency reduced to 0.2%. In these devices Er pumping occurs part by hot electrons and part by energy transfer from the Si nanostructures to the rare earth ions, depending by Si excess in the film. Si/SiO 2 Fabry-Perot microcavities have been fabricated to enhance the external quantum emission along the cavity axis and the spectral purity of emission from the films that are used as active media to realize a Si based RCLED (resonant cavity light emitting diode). These structures are realized by chemical vapour deposition on a silicon substrate. The microcavities are tuned at different wavelengths: 540nm, 980nm and 1540nm (characteristic emission wavelengths respectively for Tb, Yb and Er). The reflectivity of the microcavities is of 97% and the quality factor ranges from 60 (for the cavity tuned at 980nm) to 95 (for the cavities tuned at 540nm and 1540nm).

Hydrogen Flux Influence on Homo-Epitaxial 4H-SiC Doping Concentration Profile for High Power Application

Doping incorporation and good uniformity along the wafer it is a mandatory for application in high voltage electronic devices. In this work the effect of the Hydrogen (H) flux position inside the reaction chamber on homo-epitaxial 4H-SiC growth process has been studied. Capacitance-Voltage and FT-IR analyses show as the different position of the gas injector affect the doping and thickness uniformity and profile. On the other hand, By Candela and AFM analyses no morphological or surface influence by Hydrogen flux position has been observed.

Voids-Free 3C-SiC/Si Interface for High Quality Epitaxial Layer

A study of the carbonization process and of a low temperature buffer layer on the Cubic Silicon Carbide (3C-SiC) epitaxial growth has been reported in this work. From this study it has been evidenced the importance of the C/H2 ratio and of the buffer layer process on the voids formation at the 3C-SiC/Si interface. From our study, the influence of the voids the wafer curvature is highlighted. It has been observed that decreasing the density of these voids, decreases the stress of the 3C-SiC film; consequently, the wafer curvature is reduced.

Characterization of SiO2/SiC Interfaces Annealed in N2O or POCl3

This paper reports a comparative characterization of SiO2/SiC interfaces subjected to post-oxide-deposition annealing in N2O or POCl3. Annealing process of the gate oxide in POCl3 allowed to achieve a notable increase of the MOSFET channel mobility (up to 108 cm2V-1s-1) with respect to the N2O annealing (about 20 cm2V-1s-1), accompanied by a different temperature behaviour of the electrical parameters in the two cases. Structural and compositional analyses revealed a different surface morphology of the oxide treated in POCl3, as a consequence of the strong incorporation of phosphorous inside the SiO2 matrix during annealing. This latter explained the instability of the electrical behaviour of MOS capacitors annealed in POCl3.

Si-based resonant cavity light-emitting devices

We report on the fabrication and characterization of Si/SiO2 Fabry-Perot microcavities. These structures are used to enhance the external quantum emission along the cavity axis and the spectral purity of emission from Rare earth doped and undoped SiOx (x <= 2)films that are used as active media to fabricate a Si based RCLED (Resonant Cavity Light emitting Devices). These structures are fabricated by chemical vapour deposition on a silicon substrate. The microcavities are tuned at different wavelengths: 540nm, 980nm, 1540nm, 780nm and 850nm (characteristic emission wavelength respectively for Tb, Yb and Er and Silicon Rich Oxide (SRO)). The reflectivity of the microcavities is of 97% and the factor quality ranges from 50 (for the cavity tuned at 540nm) to 95 (for the cavities tuned at 980nm and 1540nm) and 150 (for the cavity tuned at 780nm and 850 nm). These cavities have been characterized by TEM analysis to evaluate films uniformity, thickness and densification after annealing process for temperature ranging from 800° to 1100°C. The reflectivity and photoluminescence spectra show resonant wavelengths in agreement with the calculated values. A new structure to electrically pump the active media has been designed. The electrical properties of the active media have been analysed. An enhancement of the photoluminescence signal of twenty times have been achieved for the selected emission wavelength.