J. Cifre

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.

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.

Growth of diamond by laser ablation of graphite

Thin film growth by laser ablation deposition has been established as a useful method for obtaining films of novel materials on different substrates. The high energy of the emitted particles in the ablation process could be used to obtain diamond thin films at relatively low substrate temperatures. We tried this technique to grow carbon films with a high amount of diamond phases from a pyrolytic graphite target. The laser is an excimer laser (ArF, λ = 193 nm) focused at a fluence of 3 J cm−2 and operating at 5 Hz repetition rate. The films were grown on (100) silicon substrates at 450 °C. The films were deposited under (a) vacuum conditions, (b) 1 mbar of hydrogen or (c) 1 mbar of helium to determine the influence of a gas atmosphere in the growth process. Scanning electron microscopy demonstrated the growth of diamond on silicon substrates with crystals as big as 20 μm. Raman spectroscopy studies of the films showed the peak at 1332 cm−1 characteristic of the diamond structure, even for vacuum deposition conditions.

Analysis of contamination in diamond films by secondary ion mass spectroscopy

Abstract Polycrystalline diamond films grown by hot filament chemical vapor deposition (HFCVD) have been obtained in a tungsten filament reactor, from gaseous mixtures of methane and hydrogen. The films have been analyzed for tungsten impurities by secondary ion mass spectroscopy (SIMS). Boron doping of the films has also been achieved by the addition of a boron nitride source within the reactor chamber. SIMS analysis shows the presence of tungsten or boron within diamond films. The concentration of both impurities are higher in films deposited at lower pressures and with lower methane concentrations. SIMS depth profiles show a maximum concentration of tungsten at the interface between film and substrate; this effect is not apparent in the boron depth profile.

Interfacial layer effects in the growth of CVD diamond

Abstract A comparative study of the formation of interfacial layers in the growth of diamond films was performed. Polycrystalline diamond films were grown on silicon and molybdenum substrates by two different methods: hot-filament and microwave-assisted chemical vapor deposition (CVD). Several sets of samples were prepared with increasingly short deposition times in order to investigate the early stages of growth. Silicon carbide and molybdenum carbide were the diamond film—substrate layers observed and their morphology, structure and composition were analyzed by different characterization techniques. Thicker and better-defined interfacial layers were found in films grown by hot-filament CVD than in those obtained by microwave CVD with the same deposition conditions. An increase in the amount of silicon carbide caused an increase in the delay time observed in the onset of diamond film growth.

Plasma-deposited silicon nitride films with low hydrogen content for amorphous silicon thin-film transistors application

Abstract A comparative study of the vibrational properties of PECVD amorphous silicon nitride films obtained from SiH 4 + NH 3 and SiH 4 + N 2 precursor gas mixtures has been performed by FT-IR transmission spectroscopy. The bonded hydrogen, calculated from the absorption spectra, shows important quantitative and qualitative differences depending on the precursor gas mixtures used. The hydrogen content of near-stoichiometric films obtained from SiH 4 + N 2 mixture is 10 times lower than that of films prepared from SiH 4 + NH 3 mixture. In addition, hydrogen is mainly bonded to nitrogen atoms in films from SiH 4 + NH 3 , whereas it is mainly bonded to silicon atoms in films from SiH 4 + N 2 . These low-hydrogenated silicon nitride films, obtained from mixtures containing N 2 , have been applied as insulator layers in the preparation of amorphous silicon thin-film transistors (a-Si TFTs). The TFTs were of normal staggered type composed of the structure Al/a-SiN:H/a-Si:H grown on NiCr source and drain electrodes deposited on glass substrates. TFTs with a 0.2 μm thick a-Si:H layer and 10 μm channel length have on-off current ratios of 5 × 10 4 , electron field-effect mobilities of about 1.5 cm 2 /V s and threshold voltages around 5 V.

P-doped polycrystalline silicon films obtained at low temperature by hot-wire chemical vapor deposition

Abstract P-doped polycrystalline silicon films were deposited over Corning 7059 substrates at a moderate temperature, 330°C, in a hot-wire reactor. The films were obtained from the decomposition of silane and hydrogen (10% SiH4, 90% H2) and different amounts of diborane. The structure and morphology of the samples were studied with X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). X-ray diffraction spectra show sharp diffraction peaks corresponding to silicon reflections, and Raman spectra show no evidence of amorphous phases and present a high intensity and narrow peak at 520 cm−1, which is the typical feature of crystalline silicon structure. The efficiency of boron incorporation was studied by secondary ion mass spectrometry (SIMS). The electrical properties of doped samples were also studied.

Boron incorporation effects in CVD diamond film growth

Abstract We report the p-doping of MWCVD diamond films with a novel gaseous source of boron: trimethylboron B(CH 3 ) 3 . Polycrystalline diamond films with a wide range of boron contents were obtained. Surface SEM analysis showed morphological differences related to the boron content in the films. Raman spectra of samples with boron content lower than 0.2 at% showed a decrease of the non-diamond phases found in undoped samples. The Raman peak of diamond disappeared in films of 0.7 at% of boron. The lattice parameter of diamond increased with increasing boron content. Structural and electrical analysis indicated the predominant substitutional or interstitial boron incorporation into the films, depending on the boron concentration.