Ts. Marinova

Nickel based ohmic contacts on SiC

We have compared the chemical and structural properties of Ni/SiC and Ni2Si/SiC interfaces. In the case of Ni/SiC, the contact formation is initiated by the dissociation of SiC, due to the strong reactivity of nickel at 950 °C. Ni2Si is formed and carbon accumulates, both at the interface and throughout the metal layer. At the interface, many Kirkendall voids are observed by TEM. Despite this poor interface morphology, low contact resistances have been measured. But the presence of carbon in the contact layer and at the interface is a potential source of contact degradation at high temperature. In the case of Ni/Si multilayers evaporated on SiC instead of pure Ni, the contact formation is preceded by Ni and Si mutual diffusion in the deposited layer yielding Ni2Si. Therefore, a smaller amount of carbon is released from SiC. Low carbon segregation, abrupt interface and low contact resistance characterize this contact. The thermal stability of Ni2Si contacts is illustrated with ageing experiments carried out at 500 °C.

Electrochemical deposition of thin zirconia films on stainless steel 316 L

Zirconia films (ZrO2) have been deposited electrochemically on stainless steel SS 316 L in a non-aqueous electrolyte based on absolute ethyl alcohol and ZrCl4. Scanning electron microscopy (SEM) studies have shown that the film consists of crystallites with a spheroidal shape, forming agglomerates with a very large surface area. Their sizes vary within the range of 0.1–0.5 μm. The layer composition is very close to the stoichiometric ZrO2, as has been determined by X-ray photoelectron spectroscopy (XPS).

Adsorption and thermal evolution of acetylene on a Cu(100) surface

Acetylene adsorption on a Cu(100) surface is studied by high resolution electron energy loss spectroscpy (HREELS) in the temperature range 140–550 K. Acetylene is adsorbed molecularly at 140 K, but undergoes significant rehybridisation close to sp 3 . CCH species are found to be the main product of acetylene decomposition above room temperature. At 550 K these species decompose into CH fragments and carbon.

Interaction of oxygen with a clean Ir(111) surface

HREELS and XPS methods were used for the investigation of the interaction of oxygen with Ir(111). At temperatures of 180 and 300 K oxygen is adsorbed dissociatively, 0.52±0.08 oxygen atoms per one Ir atom corresponding to a saturated coverage. Adsorbed oxygen atoms are characterized by a v (Ir-O) mode at 550 cm −1 . With rising temperature (above 300 K) diffusion of oxygen atoms into the subsurface Ir layer begins. At 600 K and an oxygen pressure of 1×10 −5 Pa, formation of a surface oxide begins which characterizes the initial stage of Ir(111) oxidation. A v (Ir-O) peak at about 800 cm −1 in the HREEL spectrum corresponds to the surface oxide. More intensive oxidation of Ir(111) proceeds at 850 K and an oxygen pressure of 0.1 MPa, the surface oxide formed differing in composition from IrO 2 .

Chemical composition and corrosion resistance of passive chromate films formed on stainless steels 316 L and 1.4301

Abstract The chemical composition and corrosion behavior of the natural and formed by chemical treatment in chromium-containing solution passive films on 316 L and 1.4301 stainless steel surfaces have been investigated by means of X-ray photoelectron spectroscopy and electrochemical in situ method of anodic polarization curves. It have been established that the oxide films formed by the chemical treatment have different chemical composition (Cr-enriched), color and reduced corrosion resistance compared to the natural passive films on both steels. The results have shown that the lower part of the oxide layer represents a uniform modified passive film and the upper one is a porous Cr3+-enriched film with an island-like structure. The latter permits a direct contact of the solution with the modified passive film which controls the corrosion resistance by the dissolution of the Fe3+ oxides present in the lower thinner layer.

Characterization of ohmic and Schottky contacts on SiC

The interface chemistry of nickel and tungsten based contacts on SiC has been investigated by XPS on as-deposited samples and after contact formation. After annealing at 950 °C for 10 min, Ni/SiC and Ni/Si/SiC ohmic contacts are formed due to the chemical reactions, as a result of which Ni2Si appears. However, Ni/Si (instead of pure Ni) deposition on SiC leads to modification of the diffusion processes and formation of a contact layer free of carbon. After annealing at 1200 °C for 4 min, the WN (W)/SiC systems are characterized by strong interface reactions resulting in W5Si3 and W2C formation in the contact layer. The 800 °C annealed WN/SiC contact is characterized by a chemically inert interface, and is found to be of a Schottky type.

Adsorption of ethylene on Ir(111)

The interaction of protonated and deuterated ethylene with an Ir(111) surface is investigated by HRBELS at temperatures of 180–500 K. The formation of ethylidyne (≡C−CH 3 ) already at 180 K is confirmed. It is established that the decomposition of ethylidyne on the iridium surface begins at lower temperatures than in the case of platinum and rhodium. At 300–450 K the ethylidyne species and the products of their decomposition (C 2 H fragments and atomic hydrogen) are present on the surface, which makes the interpretation of the spectra difficult. Isotopic shifts (ω D /ω D ) are also used to assign the vibrational losses. The spectral data are compared with IR spectroscopy results and EEL spectra of ethylene on other platinum group metals.

Interface chemistry and electric characterisation of nickel metallisation on 6HSiC

Abstract The interface chemistry and the electrical properties of annealed Ni 6H-SiC ohmic contacts have been compared using X-ray photoemission spectroscopy, current-voltage and resistance measurements by a four-point probe method. Substrates of n-type SiC wafers (CREE Res) with N d = 1 × 10 18 cm −3 have been used. The XP spectra indicate that carbon is in the graphite state and silicon is bonded predominantly to Ni resulting in NiSi formation which is preceded by decomposition of SiC to silicon and carbon after annealing. One hour annealing leads to a degradation of the contact resistance by a factor of 1.5 as compared with the contacts annealed for 5 min. This seems to be due to an increased graphite precipitation in the subsurface region of the contact layer. Current-voltage characteristics are linear for the contacts annealed above 900°C, while the contact resistance reaches a minimum around 1000°C.

XPS study of surface composition of polycrystalline CuxCo3−xO4 (0⩽x<1) obtained by thermal decomposition of nitrate mixtures

The composition of surface layers of spinel oxides CuxCo3−xO4 (0⩽x<1), obtained by thermal decomposition of nitrate mixtures, has been studied by means of XPS or ESCA. The surface layer sampled by XPS cannot be described by the bulk formula: the density of Co cations is lower than in the bulk and correspondingly, the oxygen-to-metal ratio and copper-to-cobalt ratio are higher than the mean values for the bulk. The increase in the copper content is accompanied with a decrease of the oxygen-to-metal ratio and an increase in the amount of O- and/or OH- species on the surface of the mixed spinels.

Electrochemical growth of thin La2O3 films on oxide and metal surfaces

This work presents a new electrochemical method (i.e. new electrolyte composition and electrolysis regime) for direct formation of La2O3 on ZrO2 and other oxide and metal substrates from nonaqueous electrolytes, data on the kinetics of the electrochemical processes and scanning electron microscopy (SEM) and XPS studies of the morphology, structure, dispersion and chemical composition of La2O3.