L. Calcagno

Structural modification of polymer films by ion irradiation

Abstract The atomic and electronic structure of polymer films undergoes deep modifications during high energy (keV-MeV) ion irradiation, from molecular solid to amorphous material. At low energy density (1022–1024 eV cm 3 ) typical effects include chain scissions, crosslinks, molecular emission and double bonds formation. In hydrocarbon polymer (polystyrene, polyethylene) the main effect of irradiation is the formation of new bonds as detected by molecular weight distribution, solubility and optical measurements. Moreover the concentration of trigonal carbon (sp2) in the polymer changes with ion fluence (1011–1014 ions cm 2 ) and stabilizes to a value of 20% independently on the initial chemical structure of the irradiated sample. Photoemission spectroscopy shows an evolution of valence band states from localized to extended states. At high energy density (1024–1026 eV cm 3 ) the irradiated polymer continues to evolve showing spectroscopic characteristics close to those of hydrogenated amorphous carbon. Trigonal carbon concentration changes with ion fluence (1014–1016 ions cm 2 ) reaching the steady state value of 60% and the hydrogen concentration decreases to 20%. Moreover the values of the optical gap (2.5–0.5 eV) suggest the presence of medium range order in the obtained hydrogenated amorphous carbon. These values are consistent with the formation of graphitic clusters, whose size goes from 5 A to 20 A by changing the ion fluence (or energy density).

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.

Molecular weight distribution and solubility changes in ion-bombarded polystyrene

Among the chemical and physical modifications induced by ion bombardment on polymers, solubility changes have attracted considerable attention owing to the technological interest for microlithography purpose. The solubility changes are due to occurrence of cross-linking and scission at molecular level with heavy modification of the molecular weight distribution too. The molecular weight distribution of implanted polystyrene shows considerable changes upon bombardment. These changes have been followed on bombarded nearly-monodisperse PS samples. The use of samples with known molecular weight distribution allows to apply the Gel Theory for determining the chemical yields. This method is a direct, relatively simple tool for following the chemical modifications in bombarded polymers.

Density enhancement in ion implanted polymers

Ion beam irradiation of polystyrene and polyimide films induces a strong densification of the starting material. The density increases to about 70% of the initial value after bombardment with heavy ions (Ar, Kr, Xe etc) in the energy range 25–250 key. The density change does not depend on the chemical structure and seems to be a general property of the irradiated polymers. The energy deposited along the ion track is the key parameter of the investigated process.

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.

Structural and electrical properties of Ni∕Ti Schottky contacts on silicon carbide upon thermal annealing

The evolution of the structural and electrical properties of Ni∕Ti∕SiC Schottky contacts upon thermal treatments was investigated. The samples were prepared by sequentially evaporating titanium and nickel layers onto silicon carbide (6H-SiC) substrates and were annealed in vacuum in the temperature range 400–650°C. Above 450°C a solid state reaction sets in, giving rise to the formation of nickel silicides (i.e., Ni31Si12 and Ni2Si). During reaction, by increasing annealing temperatures, the electrical characteristics of the contacts showed an increase of the Schottky barrier, along with a decrease of the device leakage current. An inversion of this trend was observed at around 600°C, which can be attributed to the inhomogeneity of the nickel silicide/SiC barrier. The scenario of the reaction of the Ni∕Ti∕SiC system is presented. The physical information obtained from the study of this bilayer can be extremely important in the control of the electrical properties of Schottky barriers for advanced devices ...

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...

New Achievements on CVD Based Methods for SiC Epitaxial Growth

The results of a new epitaxial process using an industrial 6x2” wafer reactor with the introduction of HCl during the growth have been reported. A complete reduction of silicon nucleation in the gas phase has been observed even for high silicon dilution parameters (Si/H2>0.05) and an increase of the growth rate until about 20 µm/h has been measured. No difference has been observed in terms of defects, doping uniformity (average maximum variation 8%) and thickness uniformity (average maximum variation 1.2 %) with respect to the standard process without HCl.

Effects of annealing temperature on the degree of inhomogeneity of nickel-silicide/SiC Schottky barrier

The electrical characteristics of nickel-silicide Schottky contacts on silicon carbide have been measured by using current-voltage technique in the temperature range of 120–700K. Thermal annealing at 873K formed the nickel silicide. The electrical behavior of the contact showed a deviation from linearity at low temperatures. Annealing at high temperature (1223K) produces deep modifications in the electrical characteristics at low bias and low temperatures, which are consistent with the formation of an inhomogeneous barrier. The description of the experimental results by using Tung’s model [R. T. Tung, Phys. Rev. B 45, 13509 (1992)] allowed us to determine the values of the average barrier height of 1.62 and 1.14eV for the diode annealed at 873 and 1223K, respectively.