Marcelo C Tosin

Enhancement of diamond nucleation using the solid-liquid-gas interface energy

We demonstrate the enhancement of diamond nucleation through the use of the equilibrium forces at the solid-liquid-gas interface on a substrate wetted with droplets of an oil of low vapor pressure. Such a process is shown to produce well-faceted grains with densities of roughly 107 nuclei cm−2 (boundary), 105 nuclei cm−2 (oil-coated area), and 104 nuclei cm−2 (uncoated area) without the need for scratching or seeding the substrate. Diamond deposition was undertaken on silicon using ethanol and hydrogen in the feed of a hot-filament chemical vapor deposition reactor. The oil-covered regions, in addition to showing higher nucleation densities, have the merit that the intergrain spaces are covered with diamond structures, while the parts uncovered with oil exhibit intergrain spaces covered with diamond-like carbon.

Nitrogen-doped diamond films

We found that very high concentrations (up to 20% vol) of nitrogen in the ethanol/hydrogen gas mixture do not prejudice the diamond quality as determined by Raman spectroscopy. Nitrogen addition also increases the diamond growth rate, as was previously reported at low nitrogen concentrations. We observed that after a second heating cycle in air at temperatures between 300 and 673 K the electrical resistance versus temperature curves of the as-grown films presented a bulk semiconductor behavior. This stabilization was due to the oxidation of the as-grown hydrogenated surface. The electrical ionization energy Ed was found to be in the range of 1.62–1.90 eV corresponding to films produced with 0 to 20% vol nitrogen in the feed. The room temperature photoluminescence spectra of films produced at low nitrogen concentration suggest that Ed results from pure electronic transitions in the nitrogen-vacancy neutral defects; for samples produced with nitrogen concentrations in the range 15–20% vol the Ed values may ...

Effects of argon dilution of an ethanol/hydrogen gas feed on the growth of diamond by hot-filament chemical vapor deposition

Abstract We studied the effects on the growth kinetics and properties of diamond and carbon-like diamond films obtained by the introduction of argon at low to high concentrations (0–85 vol.% Ar) into the feed mixture (ethanol and hydrogen) of a hot-filament chemical vapor deposition (CVD) reactor. Scanning electron microscopy (SEM) analysis revealed that the addition of argon induces an increase in the diamond grain size, and increases the flaws between the grains and the density of vacancy defects. Well-faceted diamond films of good quality (measured by Raman spectroscopy) have been obtained using up to 65 vol.% of argon in the gaseous mixture, while higher concentrations (85 vol.% Ar) produce diamond-like carbon or other complex carbon structures. We also observed an increase in the diamond growth rate with argon addition that was associated with an increase in carbon free radicals (up to ∼40 vol.% Ar) or to the increase in the filament temperature necessary to keep the substrate temperature constant at higher Ar concentrations (above ∼40 vol.% Ar). Room temperature photoluminescence spectroscopy also confirmed that argon addition increases the density of vacancy defects in the diamond structure.

Measurement of the substitutional nitrogen activation energy in diamond films

We show that the electrical properties of nitrogen-doped nominally undoped polycrystalline chemical vapor deposited diamond films are modified by post-deposition heating in an oxidizing atmosphere. We found that the first heating cycle in air in the temperature range of 300–673 K decreased the graphitization content still present in the diamond surface and that after the second heating cycle the electrical resistance versus temperature curves became stabilized. Using a flow of argon with residues of oxygen over the surface of the sample during the heating cycles, the stabilization of the resistance-temperature dependence also occurred but only after the fourth heating cycle. The results suggest the existence of an oxidation mechanism of the nondiamond carbon atoms present at the diamond surface. After stabilization, the deep donor ionization energy was found to be Ed=1.62±0.02 eV. All results brought together strongly suggest that this level is due to single nitrogen atoms that occupy substitutional latti...

Structure and properties of diamond films deposited on porous silicon

Abstract Thick porous silicon (PS) layers, made by anodic etching of crystalline Si wafers, have been coated with diamond films using the hot-filament chemical vapor deposition (CVD) technique. Ethanol diluted in hydrogen was used as the carbon source for diamond deposition. We observed that diamond nucleation occurs predominantly on the top of the PS spikes, creating individual islands of growth. Nucleation is apparently homogeneous over the PS surface and the islands grow independently until the complete coalescence of the diamond film. Although the diamond film is polycrystalline, it does not have the usual columnar structure of diamond deposited on c-Si. A good adherence between diamond and PS was observed in diamond–PS and diamond–PS–diamond structures. The problems of the intrinsic and thermal stresses have been addressed. Samples were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Raman and photoluminescence spectroscopy.

Nitrogenated diamond produced by introducing ammonia into the gas feed in hot-filament CVD

Abstract Nitrogenated diamond and carbon nitride films have been produced using ammnonia as the source of atomic nitrogen. A feed mixture of C 2 H 5 OH+NH 3 +H 2 was used in a hot filament CVD reactor. The deposition parameters were optimized for the growth of well-faceted crystals onto either SiO 2 or undoped diamond substrates. Film structure and morphology were studied using optical and atomic force microscopies, and Raman and photoluminescence spectroscopies. Three types of film, according to the deposition substrate used and the nitrogen doping, have been identified. All films presented semiconductor characteristics and their deep and shallow activation energies have been determined from resistance versus temperature measurements.

Effects of the addition of helium on the synthesis of diamond films

We present experimental results of chemical vapor deposition (CVD) diamond growth from ethanol vapor with the substitution of hydrogen by helium in concentrations varying from 0 to 95 vol.% He. Scanning electron microscopy (SEM) analysis revealed that the addition of helium to the gas phase influenced the growth rate and film structure. The deposition rate of diamond increases up to a helium concentration of 40 vol.% and then decreases. We attribute this behavior to an increase in the density of vacancies in the film or to a change in the reaction kinetics provoked by the chemical inertness of helium. Diamond films with well-defined crystalline facets and of good quality (measured by Raman spectroscopy) were obtained with the addition of up to 70 vol.% He. Use of helium also provokes an increase in film porosity, with the appearance of a strong room temperature photoluminescence with a Gaussian peak at approximately 635 nm. The intensity of the luminescence increases with the concentration of helium and is probably related to nanoporous defects introduced into the diamond structure.

Microcrystalline diamond deposition on a porous silicon host matrix

Abstract Porous silicon (PS) is a nanostructured material obtained by etching pores into crystalline Si wafers. In this paper, we report on the nucleation and growth of diamond on very thick PS films (130–220 μm) of very high porosity (10–50%). The edges of the pores were in the form of small crosses, which followed the original directions of the 〈100〉 c-Si. The diamond coating was made by chemical vapor deposition (CVD) in a hot-filament reactor. We observed that the diamond nucleation occurs mainly at the edges of the pores but relatively few nuclei follow a preferential orientation axis. As the nucleation density is very low, coalescence does not occur even after 11 h and 30 min of deposition. Using a pre-deposition ‘seeding’ process with diamond grains, it was possible to produce a complete diamond CVD coating. A cross-section analysis of the diamond/PS/c-Si structure by scanning electron microscopy (SEM), micro-Raman and photoluminescence spectroscopies revealed interesting results: the luminescence of the PS under the diamond layer is preserved. There is no diamond deposition inside of the pores, but a small permeation of carbon was identified which forms diamond-like phases at the bottom of the pores. The Raman analyses indicated also a small contamination of the diamond layer by Si nano-crystals.

Structural and photoluminescent properties of porous silicon with deep pores obtained by laser-assisted electrochemistry

Abstract Columnar porous silicon (PS) with deep pores has been prepared by laser-assisted electrochemistry of n- type c-Si wafers immersed in HF/C 2 H 5 OH/H 2 O mixtures of different proportions. Analysis of the PS by micro-Raman spectroscopy was undertaken simultaneously with micro-photoluminescence spectroscopy to enable correlation of the characteristics of the luminescence spectra with the probable emission structures. In addition, the cross-sectional surfaces of the samples were studied by micro-Raman and micro-photoluminescence spectroscopy to compare the luminescence of the structures present in the bulk of the PS with that of the structures of the top surface. The results suggest that the strong luminescence observed in these PS films originates from structures present in the bulk of the PS, and not from surface structures. Morphological data obtained by scanning electron microscopy (SEM) and atomic force microscopy (AFM) are also discussed.

Nitrogenation of diamond by glow discharge plasma treatment

Abstract The effect of nitrogen incorporation on the electrical properties of diamond is currently under intense study. In this work, diamond grown by hot filament chemical vapor deposition was exposed to a radiofrequency (40 MHz) glow discharge of pure nitrogen for times of between 5 min and 1 h. The effect of this exposure on the chemical and elemental composition of the samples was assessed by Raman and X-ray photoelectron spectroscopy. Changes in the electrical resistance with increasing film nitrogenation and temperature were measured. The scope and limitations of this approach to nitrogenation are evaluated.