M.C. Polo

Polycrystalline silicon films obtained by hot-wire chemical vapour deposition

Silicon films were deposited at moderate substrate temperatures (280–500° C) from pure silane and a silane-hydrogen mixture (10% SiH4, 90% H2) in a hotwire CVD reactor. The morphology, structure and composition of the samples were studied with scanning electron microscopy, transmission electron microscopy, transmission electron diffraction, X-ray diffraction, Raman spectroscopy and secondary ion mass spectrometry. The sample deposited at 500° C with pure silane has an amorphous structure, whereas the samples obtained from silane diluted in hydrogen have a polycrystalline structure, even that grown at the lowest temperature (280° C). Polycrystalline samples have a columnar structure with 0.3–1 μm crystallite sizes with preferential orientation in [220] direction. Deposition rates depend on the filament-substrate distance and range from 9.5 to 37 Å/s for the polycrystalline samples. The high quality of the polycrystalline samples obtained makes the hot-wire technique very promising. Moreover, it is expected to be easily scaled up for applications to large-area optoelectronic devices and to photovoltaic solar cells.

Internal stress and strain in heavily boron-doped diamond films grown by microwave plasma and hot filament chemical vapor deposition

The internal stress and strain in boron‐doped diamond films grown by microwave plasma chemical vapor deposition (MWCVD) and hot filament CVD (HFCVD) were studied as a function of boron concentration. The total stress (thermal+intrinsic) was tensile, and the stress and strain increased with boron concentration. The stress and the strain measured in HFCVD samples were greater than those of MWCVD samples at the same boron concentration. The intrinsic tensile stress, 0.84 GPa, calculated by the grain boundary relaxation model, was in good agreement with the experimental value when the boron concentration in the films was below 0.3 at.%. At boron concentrations above 0.3 at.%, the tensile stress was mainly caused by high defect density, and induced by a node‐blocked sliding effect at the grain boundary.

Preparation of BCN thin films by r.f. plasma assisted CVD

Abstract Boron-carbon-nitrogen (BC x N y ) films were grown on silicon substrates heated at 300 C by r.f. plasma assisted chemical vapour deposition from CH 4 -N 2 -B 2 H 6 gas mixtures. Dense and smooth films with different x:y composition ratios were obtained by varying the flow rate of the precursor gases. The analysis by X-ray photoelectron and infrared spectroscopies of the films revealed the formation of an hybrid B-C-N phase. Microhardness measurements performed with a nanoindenter showed that the mechanical properties of the BC x N y films depended on their composition and some of them presented a hardness higher (13 GPa) than that of hexagonal boron nitride films (12 GPa).

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Abstract Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm3) and sp3 content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Youngs modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Youngs modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm2, producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.

Pulsed laser deposition of diamond from graphite targets

Diamond crystals of 1 μm mean size were grown on (100) silicon substrates by ArF (193 nm) laser ablation of graphite in a hydrogen atmosphere with a laser power density of 1.3×108 W/cm2 at relatively low substrate temperature (450 °C). Raman spectroscopy analysis confirmed the diamond cubic structure of the crystals by the presence of a sharp peak at 1332 cm−1. When a KrF (248 nm) laser was used instead of the ArF no diamond phases were detected in the deposited films and the Raman spectra showed only the two bands centered at 1340 and 1600 cm−1 characteristic of amorphous carbon. The results demonstrated that the laser wavelength is a determinant parameter in the growth of diamond by laser ablation of graphite.

Trimethylboron doping of CVD diamond thin films

Abstract Trimethylboron (B(CH3)3) has been used to obtain p-doped CVD diamond films in a microwave CVD reactor. Diamond films were grown from mixtures of methane, hydrogen and variable quantities of B(CH3)3 diluted in helium. We obtained samples with boron contents in the range 0.03–9 at.%. Raman analysis showed that samples with boron levels up to 0.2 at.% present an increase of film quality in terms of preserving diamond phase and decreasing the graphitic content. For higher boron concentrations the diamond Raman peak vanishes, and X-ray diffraction analysis shows an important expansion of the diamond crystalline network. Electrical measurements showed that, in samples with a boron content up to 0.2 at.%, the electrical conductivity increases by five orders of magnitude. For higher boron concentrations, the conductivity does not increase further. Using the temperature dependence of conductivity an activation energy of 0.1 eV and 0.17 eV was calculated in films with boron contents of 0.15 at.% and 0.03 at.% respectively. C–V measurements of the lowest doped sample, containing 0.03 at.% of boron, gave an acceptor density of 3 × 1016cm−3.

Synthesis of cubic aluminum nitride by carbothermal nitridation reaction

Abstract Cubic aluminum nitride (AlN) was synthesized by the carbothermal nitridation reaction of aluminum oxide (Al 2 O 3 ). The effects of Al 2 O 3 particle size, reaction temperature and reaction time on the synthesis of cubic AlN were investigated, and the reaction mechanism was also analyzed. The results showed that cubic AlN could be formed at a lower temperature with fine Al 2 O 3 powder than with coarse Al 2 O 3 powder. The cubic AlN may be the product of Al 23 O 27 N 5 synthesized from Al 2 O 3 and hexagonal AlN, and transforms into hexagonal AlN at temperatures above 1800°C.

MICROMECHANICAL PROPERTIES OF BN AND B-C-N COATINGS OBTAINED BY R.F. PLASMA-ASSISTED CVD

Abstract We have obtained highly transparent and hard BN films in a capacitively coupled r.f. plasma-assisted CVD reactor from three different gas mixtures: B2H6–H2–NH3, B2H6–N2 and B2H6–N2–Ar. It was found that the films were smooth, dense, and had a textured hexagonal structure with the basal planes perpendicular to the film surface. The microhardness, friction coefficient and adhesion of these coatings were measured by nanoindentation and microscratching. BCxNy films were also prepared in the same plasma-assisted CVD reactor from B2H6–N2–CH4 gas mixtures. The carbon content in the films was varied by using different CH4 flow rates. These films had a less ordered structure. The mechanical properties of these films had been compared to those of hexagonal BN films. Microhardness measurements showed that there is a correlation between film composition and hardness of the BCN films.

Effects of gas pressure and r.f. power on the growth and properties of magnetron sputter deposited amorphous carbon thin films

Abstract We discuss the effects of both the electrical power supplied to an r.f. magnetron discharge and the Ar gas pressure on the growth of amorphous carbon (a-C) films on silicon substrates by sputtering of a graphite target. The power applied to the magnetron cathode was provided in a continuous wave mode as well as in a pulsed mode where the amplitude of the r.f. signal was square-wave modulated. In the continuous wave deposition mode the power was varied from 100 to 300 W at a fixed pressure of 0.2 Pa, and the pressure from 0.2 to 2 Pa at a constant power of 300 W. The pulsed mode processes were performed by varying the r.f. peak power from 200 to 400 W at 100 Hz of modulating frequency and 20% of duty cycle. At 0.2 Pa pressure the deposition rate increased from 1 to 6 nm/min with increasing power, and decreased to 2 nm/min as pressure was increased up to 2 Pa. The compressive stress was approximately 3 GPa for films grown at low pressure and low power, and decreased below 1 GPa at higher pressure or power. Raman analysis revealed that increasing pressure favours the growth of a-C films with more disordered sp 2 domains, whereas the increase in r.f. power first leads to a reduction and then to an increase in the number and clustering of sp 2 sites into ordered rings. The friction coefficient measured using a ball-on-disk tribometer ranged between 0.1 and 0.2, being the films deposited at higher power levels that possessed the lowest values. These results were discussed in terms of the effects induced by pressure and power on the energy and flux of the species impinging the film-growing surface.

Diamond and diamond-like carbon films

Abstract Polycrystalline diamond films are grown from low pressure gas mixtures, the deposition techniques are Microwave Plasma Chemical Vapour Deposition and Hot filament Chemical Vapour Deposition, in both techniques the deposition temperature is close to 900°C. The film growth process is strongly dominated by the initial nucleation stage, after this stage, the film grows at a rate of one micron per hour. The carbon atoms in the diamond film are fully fourfold (sp 3 ) co-ordinated and the film properties are close to those of single crystalline diamond: extremely hard, resistant and transparent from UV to IR. Diamond-like carbon (DLC) films are amorphous and contain a variable amount of hydrogen in their structure, the carbon atoms are partially threefold (sp 2 ) co-ordinated. Films are obtained at temperatures below 250°C and deposited on almost any substrate. Film composition, structure and functional properties are strongly dependent on the level of ionic bombardment of the film during growth. DLC films are very hard, have a low friction coefficient and good wear resistance, are chemically inert and are transparent in the IR.