Francesco La Via

Richardson's constant in inhomogeneous silicon carbide Schottky contacts

The electrical characterization of nickel silicide Schottky contacts on silicon carbide (4H–SiC) is reported in this article. In spite of the nearly ideal behavior of the contact at room temperature (n=1.05), the electrical behavior monitored in a wide temperature range exhibited a deviation from the ideality at lower temperatures, thus suggesting that an inhomogeneous barrier has actually formed. A description of the experimental results by the Tung’s model, i.e., considering an effective area of the inhomogeneous contact, provided a procedure for a correct determination of the Richardson’s constant A**. An effective area lower than the geometric area of the diode is responsible for the commonly observed discrepancy in the experimental values of A** from its theoretical value in silicon carbide. The same method was applied to Ti/4H–SiC contacts.

OHMIC CONTACTS TO SIC

In this chapter, the most significant results obtained in the last decade in the field of ohmic contacts to SiC are reviewed. First, the basic concepts related to the physics of ohmic contacts and to the contact resistance measurement techniques are briefly reported. Then, some aspects concerning the formation of low resistance (10-5-10-6 Ωcm2) ohmic contacts on n-type and for p-type SiC are discussed, focusing on Ni-based and Al/Ti-based contacts. Examples of innovative applications on practical devices are also reported, as the simultaneous formation of ohmic contacts on n- and p-type SiC for vertical power MOS devices, obtained adding Al to a standard Ni contact, and a single-metal technology for ohmic and rectifying contacts in MESFETs, using Ti or Ni without post-deposition annealing.

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.

A kinetic Monte Carlo method on super-lattices for the study of the defect formation in the growth of close packed structures

A modified Kinetic Lattice Monte Carlo model has been developed to predict growth rate regimes and defect formation in the case of the homo-epitaxial growth of close packed crystalline structures. The model is an improvement over standard Monte Carlo algorithms, which usually retain fixed atom positions and bond partners indicative of perfect crystal lattices. Indeed, we extend the concepts of Monte Carlo growth simulations on super-lattices containing additional sites (defect sites) with respect to those of the reference material. This extension implies a reconsideration of the energetic mapping, which is extensively presented, and allows to describe a complex phenomenology that is out of accessibility of standard stochastic approaches. Results obtained using the Kawasaki and the Bond-Counting rules for the transition probability of the Monte Carlo event are discussed in details. These results demonstrate how the defect types (local or extended), the formation mechanisms and the defect generation regimes can be characterized using our approach.

SiC-4H epitaxial layer growth using trichlorosilane (TCS) as silicon precursor

4H-SiC epitaxial layers have been grown using trichlorosilane (TCS) as the silicon precursor source together with ethylene as the carbon precursor source. A higher C/Si ratio is necessary compared with the silane/ethylene system. This ratio has to be reduced especially at higher Si/H2 ratio because the step-bunching effect occurs. From the comparison with the process that uses silane as the silicon precursor, a 15% higher growth rate has been found using TCS (trichlorosilane) at the same Si/H2 ratio. Furthermore, in the TCS process, the presence of chlorine, that reduces the possibility of silicon droplet formation, allows to use a high Si/H2 ratio and then to reach high growth rates (16 *m/h). The obtained results on the growth rates, the surface roughness and the crystal quality are very promising.

Structural properties of fluorinated SiO 2 thin films

Abstract Fluorinated SiO2 (SiOF) films, prepared by plasma enhanced chemical vapour deposition from SiH4, N2O and CF4 precursors, have been analysed by infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to extract chemical and structural information. Notwithstanding XPS reveals that fluorine concentrations are quite low (less than 4 at.%), the analysis of the Si–O–Si vibration modes in the IR spectra indicates that CF4 addition involves a deeper modification of the film structure, than the simple formation of Si–F bonds. In particular, by increasing the F concentration in the oxides, the stretching frequency of the Si–O–Si bonds increases, while the bending frequency decreases. On the basis of the central force model, both observations are consistent with the occurrence of a Si–O–Si bond angle relaxation phenomenon, the importance of which increases with the fluorine concentration in the films.

Tailoring the TI/4H-SiC Schottky barrier by ion irradiation

The effects of ion irradiation on the Ti∕4H–SiC Schottky barrier are discussed. The Ti∕SiC interfacial region was modified by irradiating Schottky diodes with 8 MeV Si+4 ions at fluences between 1×109 and 1×1012ions∕cm2. By increasing the ion fluence, an increase of the Schottky barrier ΦB occurs, from the value of 1.05 eV after preparation to the value of 1.21 eV after irradiation at a fluence of 1×1012ions∕cm2, without substantial changes in the ideality factor (n=1.09). Along with the barrier height increase, a decrease of the leakage current of about two orders of magnitude was observed after irradiation. The results were interpreted in terms of the structural and electrical modification of the interfacial region.

Roughness of thermal oxide layers grown on ion implanted silicon wafers

We have studied by atomic force microscopy (AFM) the surface morphology of SiO2 layers grown by thermal oxidation of silicon wafers implanted with As, B, Ge, or Si ions. In order to grow oxides having comparable thickness at different temperatures, we have employed both dry and wet processes. Roughness values up to 0.4 nm have been measured on the surface of these oxides. Surface morphology is not influenced by the oxidation ambient, while temperature-related effects are predominant. For low temperature oxidation (920 °C), the predominating effects are due to the behavior of the implanted species, and mainly consist in segregation phenomena, that are the result of a complex competition among different factors, including the segregation coefficient, the relative diffusion rates in the oxide and silicon, and the oxidation rate. At higher temperature (1100 °C), impurity segregation is inhibited or considerably reduced by the increased diffusivity, and defects formation, due to the evolution of the radiation ...

High Quality Single Crystal 3C-SiC(111) Films Grown on Si(111)

We have developed a high-quality growth process for 3C-SiC on on-axis (111)Si substrates with the ultimate goal to demonstrate high quality and yield electronic and MEMS devices. A single-side polished 50 mm (111)Si wafer was loaded into a hot-wall SiC CVD reactor for growth. The 3C-SiC process was performed in two stages: carbonization in propane and hydrogen at 1135°C and 400 Torr followed by growth at 1380°C and 100 Torr. X-ray diffraction rocking curve analysis of the 3C-SiC(222) peak indicates a FWHM value of 219 arcsec. This is a very interesting result given that the film thickness was only 2 µm, thus indicating that the grown film is of very high quality compared with published literature values. X-ray polar figure mapping was performed and it was observed that the micro twin content was below the detection limit. Therefore TEM characterization was performed in plan view to allow assessment of the stacking fault density as well as confirmation of the very low micro twin concentration in this film. TEM analysis indicates a low concentration of stacking faults in the range of 104 cm-1.

Structural properties of SiO2 films prepared by plasma-enhanced chemical vapor deposition

Abstract SiO2 thin films have been prepared by plasma-enhanced chemical vapor deposition from SiH4 and N2O precursors by using different values of the N2O/SiH4 flow ratio (γ). Rutherford backscattering spectrometry has been employed to obtain the O/Si atomic ratio of the films. Infrared spectroscopy has demonstrated that oxides having the same O/Si atomic ratio are characterized by a different structure. Indeed, from the analysis of the Si–O–Si stretching peaks, we have found that the peak frequency and full-width at half-maximum (FWHM) are dependent on γ. Peak position and FWHM have been used to calculate the bond angle distribution of the films. The results have demonstrated the occurrence of a Si–O–Si bond angle relaxation phenomenon in films deposited by using a larger excess of N2O.