V. Krastev

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.

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.

Raman and X-ray photoelectron spectroscopy study of carbon nitride thin films

Abstract Carbon nitride thin films were deposited on Si(100) substrates by electron beam evaporation of graphite and simultaneous low energy nitrogen ion bombardment. They were analysed by Raman and X-ray photoelectron spectroscopy. The formed amorphous layers are tetrahedrally bonded and consist of sp3 carbon bonds with one nitrogen atom among its nearest neighbours. Substitution of the tetrahedrally bonded carbon atom by nitrogen leads to decrease of the percentage weight of the nanocrystalline diamond phase and formation of a CNx phase embedded in the amorphous carbon layer. By changing the deposition conditions, redistribution of sp2 and sp3 bonded C–N occurs.

Correlation between the photoluminescence and chemical bonding in porous silicon

Abstract The changes in the visible photoluminescence and Raman spectra of n-type porous silicon as a result of rapid thermal annealing and chemical treatment have been studied. Modification of the chemical bonds due to these treatments has been observed by X-ray photoemission spectroscopy. Our results show that the change in the luminescence spectra of porous silicon is related to the modification of the chemical bonds and that complexes such as the terminations in siloxene as well as silicate molecules play a role in determining the optical properties of this new photonic material. The surface containing siloxene-like bonding exhibits very intensive photoluminescence. The presence of silicates leads to a decrease in luminescence intensity and to the appearance of a weak high energy band in the photoluminescence spectra.

Photoluminescence and chemical bonding in porous silicon layers — dependence on the P concentration in the Si substrate

Correlation between the photoluminescence (PL), porosity, crystallite size and chemical bonds in porous silicon (PS) prepared by electrochemical etching of Si with different levels of phosphorus doping has been studied. PL is very strongly related to the chemical bonding and depends on the Si, O and C content which is different for PS prepared on Si substrates with different resistivities. The influence of the doping level of Si on the process of electrochemical etching is discussed.

XPS study of plasma treated carbon layers deposited on porous silicon

Abstract A carbon film deposited on porous silicon in nitrogen plasma has been investigated by XPS. The presence of different carbon–nitrogen bonds in the whole carbonitride layer has been established. Irrespective of the low percentage of nitrogen (about 6%), it is found to participate in cyanide groups, bonds characteristic of β -C 3 N 4 as well as in C–N and CN bonds.

Visible luminescence from C-containing silicon oxide films

Abstract Luminescent a-Si:O:C layers were obtained by magnetron co-sputtering and annealing. The maximum photoluminescence intensity is comparable with that of porous silicon. The luminescence maximum is between 1.8 eV and 2.2 eV. X-ray photoelectron spectroscopy finds both CSi and CC bonding and maximum luminescence is obtained when the amount of the two kinds of bonding is comparable.

Interfacial reactions and ohmic contact formation in the Ni/Al–6H SiC system

The interface chemistry and the electrical properties of annealed Ni/Al–6H SiC Ohmic contacts are studied by using x‐ray photoemission spectroscopy, current–voltage characteristics, and contact resistance measurements employing a four‐point method. All depositions and analysis were performed on the Si face of nitrogen doped, n‐type (1×1018 cm−3) wafers of 6H SiC (0001). The chemical reactions at the interface are revealed. Al at the SiC interface acts by reducing the SiOx and forming Al2O3. At elevated temperatures, SiC in the presence of Al dissociates with formation of Al4C3 which is a stable compound. At about 1000 °C the Si atoms at the interface bond to Al and Ni either in ternary compounds with different stoichiometry or in NiSi. In the subsurface region carbon is present in the graphite state of ∼5 at. %. The Ohmic behavior of the contact is related to the formation of the NiSi while the weak contact resistivity degradation is due to the low graphite precipitation. The improved temperature stabilit...

Study of a New Chemical Etchant for GaSb (100) and (111) Substrate Preparation for Epitaxial Growth

A new chemical etchant for ex situ GaSb substrate preparation for epitaxial growth is proposed. The etchant utilizes components water solutions of d,l-tartaric acid C 4 H 6 O 6 (5M, 20°C), H 2 O 2 (9.7M), and HF (23M). Various volume ratios of these components have been tested, resulting in controllable etch rates in the range from 0.5 to 4 μm/min. The obtained surfaces are mirror-like, smooth, and free from etch pits as revealed by scanning electron microscopy observations. The proposed etchant in 20 :10 :1 volume ratio is compared with the Br 2 -methanol and CH 3 COOH :HNO 3 :HF (40 :18 :2) etchants with respect to the content of residual Sb 2 O 2 and Ga 2 O 3 and their thickness using x-ray photoelectron spectroscopy. The C 4 H 6 O 6 :H 2 O 2 :HF (20 :10 :1) etchant appears to be superior to the CH 3 COOH :HNO 3 :HF (40 :18 :2) one due to the high reproducibility of the obtained surfaces.

X-ray photoelectron spectroscopy study of post-hydrogenated homogeneous chemically vapour-deposited amorphous silicon films

Abstract Thin films of hydrogenated amorphous silicon (a-Si:H) deposited by homogeneous chemical vapour deposition were studied by X-ray photoelectron spectroscopy. The binding energies of Si 2p electrons were determined before and after post-hydrogenation. Samples without and with a high hydrogen content were investigated. The measurements obtained by constant-photocurrent method were used to discuss the results.