F.C. Marques

Coefficient of thermal expansion and elastic modulus of thin films

The coefficient of thermal expansion (CTE), biaxial modulus, and stress of some amorphous semiconductors (a-Si:H, a-C:H, a-Ge:H, and a-GeCx:H) and metallic (Ag and Al) thin films were studied. The thermal expansion and the biaxial modulus were measured by the thermally induced bending technique. The stress of the metallic films, deposited by thermal evaporation (Ag and Al), is tensile, while that of the amorphous films deposited by sputtering (a-Si:H, a-Ge:H, and a-GeCx:H) and by glow discharge (a-C:H) is compressive. We observed that the coefficient of thermal expansion of the tetrahedral amorphous thin films prepared in this work, as well as that of the films reported in literature, depend on the network strain. The CTE of tensile films is smaller than that of their corresponding crystalline semiconductors, but it is higher for compressive films. On the other hand, we found out that the elastic biaxial modulus of the amorphous and metallic films is systematically smaller than that of their crystalline c...

Hard graphitic-like amorphous carbon films with high stress and local microscopic density

In this work, we report unusual properties of amorphous carbon films prepared by ion beam-assisted deposition using different noble gases (neon, argon, and krypton). Independent of the noble gas ions used, the intrinsic compressive stress and plasmon energy increase sharply with the assisting ion beam energies up to 100 eV. Above this energy, the material properties depend on the mass of the ion. The highest values of stress (∼12 GPa) and plasmon energy associated with the C 1s core electron (29.5 eV) are of the same order of magnitude as those reported for highly tetrahedral amorphous carbon films. Structural results, however, indicate that the material is composed of a hard, highly stressed, and locally dense graphite-like network, i.e., a predominantly sp2-bonded material. It is suggested that the ion bombardment compacts the film structure by reducing the interplanar cluster distances, generating high compressive stress and high local density. The differences in the properties of the films introduced ...

Optoelectronic and structural properties of a-Ge1−xCx:H prepared by rf reactive cosputtering

Optoelectronic, structural, and mechanical properties of hydrogenated amorphous germanium carbon (a-Ge1−xCx:H) alloys are presented. The films were prepared by the rf cosputtering technique using graphite-germanium composite targets. Films with carbon contents in the 0<x<1 range were prepared under the same conditions used to obtain a-Ge:H films with good optoelectronic properties. The trends of the optical gap, infrared absorption, dark conductivity, and mechanical stress as a function of the carbon content suggest that the properties of films with low carbon concentration are mainly controlled by the incorporation of sp3 hybridized carbon. These films have good optoelectronic and structural properties. As the carbon content increases, the properties of the films are determined by the concentration of sp2 carbon sites.

THE SUBIMPLANTATION MODEL FOR DIAMOND-LIKE CARBON FILMS DEPOSITED BY METHANE GAS DECOMPOSITION

Abstract In this work, the formation of hard a-C:H films deposited on the cathode of an r.f. sputtering system through the decomposition of methane gas was explained using the subimplantation model. Even though in a r.f. plasma deposition the ions striking the films surface are not monoenergetic, the stress data match the theoretical model proposed by C.A. Davis. The stress versus bias plot shows a behavior similar to those already obtained for ta-C and ta-C:H films, which are prepared using monoenergetic ion beam.

Application of amorphous carbon based materials as antireflective coatings on crystalline silicon solar cells

We report on the investigation of the potential application of different forms of amorphous carbon (a-C and a-C:H) as an antireflective coating for crystalline silicon solar cells. Polymeric-like carbon (PLC) and hydrogenated diamond-like carbon films were deposited by plasma enhanced chemical vapor deposition. Tetrahedral amorphous carbon (ta-C) was deposited by the filtered cathodic vacuum arc technique. Those three different amorphous carbon structures were individually applied as single antireflective coatings on conventional (polished and texturized) p-n junction crystalline silicon solar cells. Due to their optical properties, good results were also obtained for double-layer antireflective coatings based on PLC or ta-C films combined with different materials. The results are compared with a conventional tin dioxide (SnO2) single-layer antireflective coating and zinc sulfide/magnesium fluoride (ZnS/MgF2) double-layer antireflective coatings. An increase of 23.7% in the short-circuit current density, ...

Bonding properties of rf-co-sputtering amorphous Ge–C films studied by X-ray photoelectron and Raman spectroscopies

The bonding properties of hydrogenated amorphous germanium–carbon (a-Ge1−xCx:H) alloy films, deposited by the rf-co-sputtering technique, were measured by Fourier transform infrared, micro-Raman and X-ray photoelectron spectroscopies. Films with carbon content in the 0 to 100 at.% range were prepared under the same deposition conditions used to prepare a-Ge:H films. The infrared spectra revealed that the carbon is bonded in both sp3 and sp2 configurations. XPS measurements show a chemical shift of the binding energy of the Ge 3d core electrons toward larger energies as the carbon content increases, while the line-width remains almost constant. On the other hand, the peak associated with the C 1s orbital displays a doublet related to the C–Ge and C–C bonds. The Raman spectroscopy data are analyzed over a wide frequency range of the Stokes scattering for different alloy compositions.

Thermal expansion coefficient of amorphous carbon nitride thin films deposited by glow discharge

Abstract The thermal expansion coefficient of a-CNx:H thin films was determined by the thermally induced bending technique. The films were deposited by glow discharge under methane and nitrogen atmosphere, and analyzed by FTIR and Raman spectroscopies, nanohardness, and stress measurements. Drastic changes of the film structure were observed as a result of the nitrogen incorporation, from 0 to 7%. The increase of nitrogen concentration reduces the deposition rate, stress, hardness, and the elastic constant of the films. It was also observed that the thermal expansion coefficient has a significant increase from approximately 2–9×10−6/K, which was associated with the increase in the sp2 concentration induced by the N incorporation, and with the increase in the concentration of CN bonds. In spite of that, stable and thick (∼2 μm) films were deposited at moderate deposition rate (0.3 nm/s), relatively high hardness (13 GPa), and low stress (0.6 GPa).

Optical properties of non-stoichiometric germanium nitride compounds (a-GeNx)

Abstract In this paper some optical properties of non-stoichiometric germanium-nitrogen compounds (a-GeN x ) are presented. It is shown that the inclusion of nitrogen atoms into the Ge network produces a band gap widening. The use of hydrogen to passivate the gap states of the material is also discussed.

Effects of stress on electron emission from nanostructured carbon materials

The electron field emission properties of highly graphite like (sp2 rich) amorphous carbon films have been investigated. These films were prepared by dual ion beam-assisted deposition technique, where the assisting energies were varied from 0 to 800 eV. Threshold fields as low as 8 V/μm is observed at an assisting energy of 400 eV, which is comparable to the best threshold fields observed in high sp3 carbon films. Surface nanostructures are found on these films during growth, but are thought not to be the primary reason for the observed low threshold fields. The combination of a highly graphite-like structure with a high intrinsic compressive stress and a high local (electronic) density, obtained from x-ray photoelectron spectroscopy, is identified as the source for the field enhancement. The controllable stress is thought to modify the band structures of the graphite-like sp2 rich component in the films, which results in high dielectric inhomogeneity. This analysis is in agreement with the concept of an ...

EXAFS study of noble gases implanted in highly stressed amorphous carbon films

Abstract In this work we report on the study of the local environment of Ar and Kr atoms implanted into an amorphous carbon (a-C) matrix. By intentionally changing the a-C deposition conditions we were able to trap noble gas (NG) atoms under different internal pressure (intrinsic stress) ranging from 1 up to 12 GPa. This enables us to investigate the effects of the internal pressure on the implanted NG atoms subjected to the highly strained environment of the carbon matrix. For this purpose, extended X-ray absorption fine spectroscopy (EXAFS) was performed on the K absorption edge of the implanted Ar (3.2 keV) and Kr (14.3 keV) atoms as a function of the intrinsic stress of the carbon matrix. The analysis of X-ray near edge spectroscopy (XANES) shows an evident increase of the white line for both gases as the stress increases. This result is indicative of clustering of the implanted NG atoms. The analysis of the EXAFS results for Ar atom indicates an increase of the first neighbor distance from 2.4 to about 2.8 A with increasing compressive stress. The latter figure is comparable to the Ar–Ar (Kr–Kr) inter-atomic distance of compressed solid Ar (Kr). This finding gives further evidence for the formation of small NG clusters in a highly strained a-C matrix.