P. Musumeci

Structural and electrical characterisation of Titanium and Nickel silicide contacts on Silicon carbide

The interfacial reaction and phase formation as a function of the annealing temperature (600-1000°C) and time were investigated on both titanium and nickel thin films evaporated on n-type 6H-SiC (0001) substrate. The study was carried out employing a combination of Rutherford backscattering spectrometry, X-ray diffraction, transmission electron microscopy and sheet resistance measurements. A correlation has been found between the annealing process and the electrical measurements on transmission line method (TLM) structures and Schottky diodes. In the case of titanium, the formation of different phases in the analysed temperature ranges significantly changes the electrical properties. In fact, while a double layer of TiC and Ti5Si3 was formed at 900 and 950°C, the ternary phase Ti3SiC2 was observed only at 1000°C. With this high temperature process the specific contact resistance decreases from 10-4 to 6.7 × 10-5 Ωcm2. In the case of nickel silicide the only phase that has been observed between 600 and 950°C was the Ni2Si. The carbon of the consumed silicon carbide layer has been dissolved in the silicide film, during the reaction, forming carbon precipitates. The specific contact resistance reaches the lowest value (3.6 × 10-5 Ωcm2) after the highest temperature anneal. The Ni2Si/SiC Schottky diodes show almost ideal characteristics (n = 1.07) and a barrier height of ∼1.3 eV. From the electrical characterisation, a non-uniform Schottky barrier height seems to be formed.

Schottky--ohmic transition in nickel silicide/SiC-4H system: is it really a solved problem?

The transition from Schottky to ohmic contact in the nickel silicide/SiC system during annealing from 600 to 950 °C was investigated by measuring the electrical properties of the contact and by analyzing the microstructure of the silicide/SiC interface. The graphite clusters formed by carbon atoms during silicidation are uniformly distributed into the silicide layer after annealing at 600 °C and they agglomerate into a thin layer far from the silicide/SiC interface after annealing at 950 °C. At this temperature an increase of the Schottky barrier height was measured, while deep level transient spectroscopy evidences the absence of the 0.5 eV peak related to the carbon vacancies.

Amorphization and defect recombination in ion implanted silicon carbide

The damage produced in silicon carbide single crystals by ion implantation was investigated by Rutherford backscattering channeling and transmission electron microscopy techniques. Implantations were performed at liquid nitrogen and at room temperatures with several ions to examine the effect of the ion mass and of the substrate temperature on the damaging process. The damage accumulation is approximately linear with fluence until amorphization occurs when the elastic energy density deposited by the ions overcomes a critical value. The critical energy density for amorphization depends on the substrate temperature and is greatest at 300 K indicating that defects recombination occurs already at room temperature. Formation of extended defects never occurred and point defects and uncollapsed clusters of point defects were found before amorphization even in the case of light ion implantation. The atomic displacement energy has been estimated to be ∼12 eV/atom from the analysis of the damage process in dilute c...

Relaxation and crystallization of amorphous silicon carbide probed by optical measurements

Abstract Optical spectroscopy in the visible (300–1100 nm) and in the infrared (400–4000cm−1) regions was used to monitor the relaxation and crystallization processes of pure amorphous silicon carbide (a-SiC) thin films upon annealing at temperatures between 200 and 1000°C. These films were obtained by ion implantation of crystalline material with 200keVkr+ at a fluence of 2 × 10 ions cm−2. The refractive index n and the absorption index k were calculated from the ultraviolet-visible transmittance and reflectance, and information on the vibration modes of the Si-C bonds was detected from infrared transmittance. Thermal treatment changes the optical properties of a-SiC; in particular, annealing at temperatures lower than 800°C resulted in a continuous variation in both the refractive index and the absorption index and in a decrease in the infrared silicon-carbon peak width. Annealing at higher temperatures produces sudden variations in the shape of the refractive index and in the infrared silicon-carbon pe...

Strongly enhanced light trapping in a two-dimensional silicon nanowire random fractal array

We report on the unconventional optical properties exhibited by a two-dimensional array of thin Si nanowires arranged in a random fractal geometry and fabricated using an inexpensive, fast and maskless process compatible with Si technology. The structure allows for a high light-trapping efficiency across the entire visible range, attaining total reflectance values as low as 0.1% when the wavelength in the medium matches the length scale of maximum heterogeneity in the system. We show that the random fractal structure of our nanowire array is responsible for a strong in-plane multiple scattering, which is related to the material refractive index fluctuations and leads to a greatly enhanced Raman scattering and a bright photoluminescence. These strong emissions are correlated on all length scales according to the refractive index fluctuations. The relevance and the perspectives of the reported results are discussed as promising for Si-based photovoltaic and photonic applications.

Full characterization of laser-accelerated ion beams using Faraday cup, silicon carbide, and single-crystal diamond detectors

Multi-MeV beams of light ions have been produced using the 300 picosecond, kJ-class iodine laser, operating at the Prague Asterix Laser System facility in Prague. Real-time ion diagnostics have been performed by the use of various time-of-flight (TOF) detectors: ion collectors (ICs) with and without absorber thin films, new prototypes of single-crystal diamond and silicon carbide detectors, and an electrostatic ion mass spectrometer (IEA). In order to suppress the long photopeak induced by soft X-rays and to avoid the overlap with the signal from ultrafast particles, the ICs have been shielded with Al foil filters. The application of large-bandgap semiconductor detectors (>3 eV) ensured cutting of the plasma-emitted visible and soft-UV radiation and enhancing the sensitivity to the very fast proton/ion beams. Employing the IEA spectrometer, various ion species and charge states in the expanding laser-plasma have been determined. Processing of the experimental data based on the TOF technique, including est...

Calorimetric measurements of MeV ion irradiated polyvinylidene fluoride

Abstract High energy ion irradiation of semicrystalline polyvinylidene fluoride (PVDF) films changes its melting behaviour. After irradiation with 9 MeV carbon beam, differential scanning calorimetry of PVDF flims exhibits a quite complex behavior in the temperature range 20–200°C. At low ion fluence irradiation ( 12 ions/cm 2 ) the enthalpy of melting is constant (58.0 J/g), but a broad transition is observed before the melting temperature (160°C). At high fluences (10 12 –10 13 ions/cm 2 ) the enthalpy of melting decreases exponentially and the PVDF evolves into a insoluble carbon network. The crystalline polymer component decreases from 50% to 2% as suggested by the thermal analysis and the X-ray diffraction measurements.

Optical defects in ion damaged 6H-silicon carbide

Abstract Damage production induced in 6H-SiC single crystal by ion implantation was investigated “in situ” by optical transmittance and reflectance measurements and “ex-situ” by Rutherford backscattering-channeling spectroscopy (RBS). Different ions (H, He, N, Ar, Kr) were implanted in the fluence range 10 11 –10 17 ions/cm 2 at room temperature. RBS data showed that the implantation induced disorder was limited to a thickness comparable with the ion range and for a critical value of fluence the crystal-amorphous transition occurred even for implantation of light ion like H and He. The energy threshold for amorphization was equal for the different ions and it has been determined that about 23 eV/atom must be deposited in elastic collisions in order to form an amorphous layer. The transmittance, measured at 633 nm, decreased continuously with ion fluence being the decrease rate higher at low fluence. The absorption coefficient of the damaged layer has been determined for the different ions: it increased of orders of magnitude with respect to the undamaged material and it saturated when amorphization sets in. The absorption coefficient increased with the defects density and the cross section of defects absorption resulted independent of the ion species.

CARBON CLUSTERING IN SI1-XCX FORMED BY ION IMPLANTATION

Abstract Silicon-carbon alloys were formed by multiple energy implantation of C+ ions in silicon and in Silicon on Sapphire (SOS). The ion fluence ranged between 5 × 1016 − 3 × 1017 ions/cm2 and the energy between 10–30 keV in order to obtain constant carbon concentration into a depth of 100 nm. The carbon atomic fraction (x) was in the range 0.22–0.59 as tested by Rutherford backscattering spectrometry (RBS). Thermal annealing of the implanted films induced a transition from amorphous to a polycrystalline structure at temperatures above 850°C as detected by Infrared spectrometry (IR) in the wavenumber range 600–900 cm−1. The optical energy gap and the intensity of the infrared signal after annealing at 1000°C depended on the film composition: they both increased linearly with carbon concentration reaching a maximum at the stoichiometric composition (x = 0.5). At higher carbon concentration the IR intensity saturated and the optical energy gap decreased from the maximum value of 2.2 to 1.8 eV. The behaviour at the high carbon content has been related to the formation of graphitic clusters as detected by Raman spectroscopy.

Crystallization process of amorphous silicon–carbon alloys

The crystallization behavior of amorphous silicon carbon alloys films was investigated using infrared spectroscopy and transmission electron microscopy. The films were prepared by plasma enhanced chemical vapor deposition and the thickness and composition were checked by Rutherford backscattering spectroscopy. Annealing processes were carried out in the temperature range of 750–1100 °C in a vacuum furnace. The changes in the infrared absorption band (400–1200 cm−1) after annealing indicate a transition from amorphous to a crystalline phase. The onset temperature for this transition depends on the alloy composition, and it increases from 800 to 950 °C with increasing carbon concentration. The microstructure of crystallized SiC is influenced by the film composition. Transmission electron microscopy (TEM) analysis shows that the crystallized stoichiometric alloy is composed by polycrystalline β-SiC grains. TEM and Raman spectroscopy evidenced that in silicon rich and carbon rich alloys, the β-SiC grains are surrounded by silicon grains and graphitic clusters, respectively.