Papers by Author: Andrea Severino

Abstract: Micro Raman characterization has been used to determine the stress status of 3C-SiC epilayer grown on pseudomorphic-Si thin layer on Si_1-xGe_x/Si(001). The strain conditions of the Si_1-xGe_x films grown on Si(001) have been determined by the analysis of additional Silicon Raman peaks, which Raman shifts are related to the lattice parameter. Through the analysis of the Raman spectra, the correlation between the Si_1-xGe_x film, the crystal quality and the stress relaxation of the 3C-SiC as a function of the Germanium fraction (x), have been evaluated. The increase of Germanium fraction determines the reduction of the voids density located at the 3C-SiC/Si interface and the relaxation of the stress within the epilayer. Moreover, the 3C-SiC crystal quality, monitored by the Full Width at Half Maximum of the TO Raman mode, remains unchanged for any Germanium fraction values.

Abstract: In this paper the epitaxial process with chloride precursors has been described. In particular it has been shown that the growth rate can be increased to about 100 μm/h but higher growth rate can be difficult to reach due to the limited surface diffusion at the usual temperature of SiC epitaxy. This process gives several advantages because it gives the opportunity to increase the throughput and consequently to reduce the cost of epitaxy, using new reactor structures, and to reduce several kind of defects (Basal Plane Dislocations, Stacking Faults, Threading Dislocations) and to decrease the surface roughness at the same time.

Abstract: 3C-SiC shows encouraging physical properties for the development of low cost high power compatible silicon based technology. The fundamental capability of grown 3C-SiC on silicon substrates leads to the possibility of a full integration of Si based process technologies. This is the driving force for the efforts for development a high quality heteroepitaxial film. The fundamental issue is the reduction of defects and stress due to the lattice mismatch between the 3C-SiC epilayer and the Silicon substrate. In this paper we show a way to reduce macroscopic structural features and to enhance the material quality and the surface quality by simply using a process based on a multilayer (ML) buffer structure with n++ and n doping alternation. This process leads to an evident improvement of both surface roughness, morphology and crystal quality.

Abstract: In this paper we study the surface morphology of <11-20> 4° degree off, silicon terminated, 4H Silicon Carbide (4H-SiC) in terms of growth parameters and post growth argon thermal annealing. We find that out-of-equilibrium conditions favor the reduction of the surface roughness. Furthermore, we find preliminary indications that the same growth parameters that lead to the reduction of the surface roughness promote also a reduction of (1,3) and (4,4) stacking faults density.

Abstract: Using several characterization techniques (μ-Raman, mechanical profilometer and microstructure deflections) together with a recent stress model[ ] we study the heteroepitaxial growth of cubic silicon carbide on silicon (100). We show that the observed inconsistency between experimental results might be the result of defects generated on the silicon substrate during the carbonization process. In such a situation wafer curvature techniques do not allow the determination of the stress field in the grown films neither quantitatively nor qualitatively.

Abstract: 3C-SiC lattice parameters, both in-plane and out-of-plane, have been studied as a function of the temperature (up to 773 K) by performing X-Ray Diffraction (XRD) measurements in coplanar and non-coplanar geometry during the thermal treatments. A tetragonal distortion of the 3C-SiC cell has been observed, with a=b≠c, resulting from a tensile stress status induced by the presence of Si substrate. A linear expansion coefficient of about 4.404 × 10^-6 K^-1 at 773 K has been obtained for a 15 μm thick 3C-SiC film grown on (100) Si substrate. The discrepancy with the value reported in literature of 5.05 × 10^-6 K^-1 at 800 K [Slack et al., Journal of Applied Physics 46, 89 (1975)] may be related to the different nature of samples used.

Abstract: In this work, we focus our attention on the characterization of 3C-SiC films, grown within a CVD reactor, on Si substrates. It will be shown how the growth procedures influence the SiC film structure and quality with the growth rate used during the growth used as example. Evaluation of crystal structure has been conducted by X-Ray Diffraction (XRD), Raman microscopy and Transmission Electron Microscopy (TEM). Overall film quality increases if films are grown under low growth rate conditions, thanks also to an important reduction in the density of micro-twins. The trend of the full widths at half maximum (FWHMs) of SiC rocking curves, considered good ‘quality indicator’ as their broadenings are affected by crystallographic defects, as a function of 3C-SiC thickness shows a saturated regime for very thick films, due to the saturation of stacking fault density after 50 μm of growth. This work wants to suggest a reasonable path for the characterization of the material structure that can be useful, anywhere and in any time, to assess if the morphology and microstructure of our films are satisfactory and to drive towards the desired improvement.

Abstract: SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). The hetero-epitaxial growth of 3C-SiC on silicon substrates allows one to overcome the traditional limitations of SiC micro-fabrication, but the high residual stress created during the film grow limits the development of the material for these applications. In this work, in order to evaluate the amount of residual stress released from the epi-film, different micro-machined structures were developed. Through the measurement of natural resonant frequencies and Raman shift analysis, a strong relationship between the mechanical proprieties of the material (Young’s modulus) and the film crystal quality (defect density) was observed.

Abstract: Using several types of surface analysis (Optical profilometers (OP), Atomic Force Microscopies (AFM), Scanning Electron Microscopies (SEM) and cross-sectional high-resolution Transmission Electron Microscopies (TEM)) we analyze the surface morphologies of misoriented 4H silicon carbide after pre-growth hydrogen etching and homo-epitaxial growths. We observed the characteristic self-ordering of nano-facets on any analyzed surface. This nano-faceting, which should not be confused with step bunching, can be considered as a close-to-equilibrium instability, for this reason can be hindered.

Abstract: Micro-Raman spectroscopy has been used to study the dependence between the carrier concentration and electrical mobility in n-doped 3C-SiC films grown on (111) and (100) Si oriented substrates. Local stacking variations observed on the (111) 3C-SiC surface lead to a worse crystal morphology compared to (100) 3C-SiC films resulting in a decrease of the average bulk mobility.