R. Samlenski

Chemical vapour deposition and characterization of smooth {100}-faceted diamond films

Abstract Smooth {100}-faceted diamond films have been prepared by microwave-assisted chemical vapour deposition from CH4ue5f8H2 mixtures. Growth has been controlled by in-situ laser interferometry. Under appropriate growth conditions, the films show a fibre texture with the 〈100〉 fibre axis normal to the substrate. The surface is completely formed by well-aligned coplanar {100} facets. The structure and morphology of these films have been characterized by X-ray diffraction, ion channelling and angle-resolved optical scattering measurements. Fibre textures with an angular spread of the 〈100〉 directions as narrow as 1° have been observed. The coplanar {100} facets form an optically smooth surface, exhibiting pronounced specular reflection. The dependence of the texture formation and surface morphology on film thickness and deposition parameters, including substrate temperature and gas composition, has been studied. For lower than optimum growth temperatures the surface becomes rough as {111} facets develop in addition to the (still coplanar) {100} facets. For higher than optimum growth temperatures the size of the {100} facets increases and the texture axis tilts away from 〈100〉. By varying the gas composition, any texture axis between 〈100〉 and 〈110〉 can be established. Computer based growth simulations have been performed on the basis of evolutionary selection of specific crystallite orientations. It is shown that a growth parameter α = 3 1 2 V 100 V 111 , i.e. the ratio of the growth rates on {100} and {111} faces, determines the structure and morphology of the films. A novel two-step growth process based on the control of α permits the independent optimization of texture axis and surface morphology, offering the possibility to obtain smooth faceted diamond films for arbitrary film thickness.

Tetrahedral amorphous carbon films prepared by magnetron sputtering and dc ion plating

Highly tetrahedral, dense amorphous carbon (ta‐C) films have been deposited using rf sputtering of graphite by an unbalanced magnetron with intense dc Ar‐ion plating at low temperatures (<70u2009°C). The ratio of the argon ion flux to neutral carbon flux Φi/Φn is about 5. The film density and compressive stress are found to pass through a maximum of 2.7 g/cm3 and 16 GPa, respectively, at an ion plating energy of about 100 eV. Experiments with higher ion flux ratios of Φi/Φn=10 show that it is possible to deposit carbon films with densities up to 3.1 g/cm3 and sp3 contents up to 87%. Deposition of ta‐C in this experiment when the energetic species is Ar appears to require a minimum stress of 14 GPa to create significant sp3 bonding, which contrasts with the continuous increase in sp3 content with stress when the energetic species is C ions themselves. These results are used to discuss possible deposition mechanisms.

Characterisation and lattice location of nitrogen and boron in homoepitaxial CVD diamond

Abstract Homoepitaxial diamond films were prepared by microwave plasma assisted chemical vapor deposition (CVD) on {100} and {111} substrates from gas mixtures of 0.5 and 1.5 vol.% CH4 diluted in H2. One set of films was doped with boron by admixing trimethyleborate (TMB) with the reactant gas. The boron concentration ranged from 10 to 100 ppm in the gas phase. Another set of samples was doped with isotopic 15N by admixing 50 ppm 15N2 with the reactant gas. The films were characterised with respect to the incorporation and the lattice locations of the dopant atoms by nuclear reaction analysis (NRA) and ion channelling analysis. Both boron and 15N were found to incorporate preferentially in the {111} growth sectors. The B C ratios in the films were determined to be one order of magnitude less than the B C ratios in the gas phase, and the 15 N C ratios in the films were about four orders of magnitude less than the 15 N C ratios in the reactant gas. Both dopants were found to occupy preferentially substitutional sites in the host lattice. A substitutional fraction of 0.9–1.0 was determined for the boron dopant, a fraction of about 0.8 was derived for the nitrogen dopant.

Subplantation effect in magnetron sputtered superhard boron carbide thin films

Abstract Superhard amorphous boron carbide films with a film thickness of about 2 μm have been prepared by r.f.-magnetron sputtering of a boron carbide target in a pure argon discharge at a gas pressure of 1.6 × 10−3 mbar. The flux ratio of the argon ions to boron and carbon atoms has been kept constant at 3.5, while the energy of the argon ions is varied by applying a d.c.-substrate bias. The effect of argon ion implantation measured by Rutherford back scattering is discussed. When the argon ion energy is increased, the mechanical properties show extreme values at an argon ion energy of 74 eV, which can be explained quantitalively by knock-on subplantation. Stress up to 6.7 GPa and a micro-hardness up to 72 GPa are obtained. The hardness enhancement is correlated with the increase of stress. The influence of preferential sputtering of boron or carbon from the deposited B4C film can be neglected.

Deposition of ta-C:H films by r.f. plasma discharges

Abstract Diamond-like a-C:H films have been prepared by r.f. plasma deposition by increasing the plasma density using a magnetic field. A low-pressure r.f. discharge is used to minimize ion collisions in the plasma sheath. We find a maximum hardness of 50 GPa at a compressive stress of 10 GPa and an optical gap of 2.1 eV by varying pressure, magnetic field and r.f. power. The extreme hardness and high optical gap of these films is due to the rather high sp 3 content of 50% in the diamond-like hydrogenated films compared with a-C:H films normally produced by r.f. discharges. The higher sp 3 content can be explained by the subplantation model of Lifshitz et al. and Robertson. The films deviate from the fully constraint non-crystalline network (FCN) model of Angus et al.

Temperature dependence of the formation of highly tetrahedral a-C:H

Abstract Deposition from a low pressure plasma beam source creates a highly tetrahedral form of hydrogenated amorphous carbon (ta-C:H)) which is analogous to the ta-C formed by deposition from a filtered cathodic arc or mass-selected ion beam. The properties of ta-C:H have been studied as a function of the substrate deposition temperatureTs and ion energy using electron energy loss spectroscopy, X-ray diffraction and atomic force microscopy. The density decreases suddenly for deposition temperatures above a threshold value, which is found to decrease with increasing ion energy. The films above the threshold are mainly sp2 bonded, with graphitic layering and high roughness. The variation of the film density with the ion energy andTs is consistent with deposition occurring by subplantation.

Characterization of homoepitaxial diamond films by nuclear methods

Isotopically marked 13C diamond films were deposited by microwave-plasma-assisted chemical vapour deposition homoepitaxially on {100} and {111} natural diamond substrates. The deposition was performed at 0.5 and 1.5 vol.% 13CH4 diluted in H2. In order to study the influence of nitrogen on diamond growth, N2 was admixed with the process gas for some samples. The thickness of the homoepitaxial films was determined by Rutherford backscattering, the crystalline quality by ion channelling measurements and hydrogen concentrations by nuclear reaction analysis. The defect density and growth rate of {111} films were found to increase at the higher methane concentration. The growth rate of {100} films also increased, but the defect density decreased at the higher methane concentration. The admixture of nitrogen with the process gas yielded about 25% higher growth rates and better crystalline qualities.

Lift-off technique of homoepitaxial CVD diamond films by deep implantation and selective etching

Abstract We report on the replication of diamond substrates and the production of thin free-standing monocrystalline diamond films by applying a “lift-off” technique as suggested by Parikh et al. A diamond substrate is first deeply implanted in order to create a damaged subsurface layer that is selectively etched after overgrowth. In this process incomplete delamination and chipping appeared to be a problem. In order to optimize the preparation conditions for a successful lifting, we implanted diamond substrates with O + ions of 4.5 MeV at various doses. The films were subsequently overgrown with microwave plasma-assisted CVD and etched in air at elevated temperatures. With implantation doses between 1 × 10 17 and 5 × 10 17 cm −2 the CVD layers could be completely removed. Using optimized conditions a multiple replication with consecutive application of the lift-off technique has been demonstrated. The films were characterized by optical microscopy, X-ray diffraction and Raman spectroscopy. For the best samples, which were successfully separated, the Raman spectra show a sharp diamond phonon line of 2.5 cm −1 half-width for both the substrate and the lifted layer. To get depth-resolved structural information, cross-sectional micro-Raman spectroscopy was applied on samples that were partially implanted and overgrown with 13 C. Polished side faces gave access to both the predamaged region and the undamaged reference area. The data indicate that the implantation damage is restricted to a thin subsurface layer with only a small influence on the structural quality of the overgrown film.

Ion channeling studies of hydrogen in homoepitaxially grown CVD diamond

Abstract Using ion channeling measurements in conjunction with the resonant 1 H( 15 N,αγ) 12 C nuclear reaction or with proton Rutherford backscattering spectrometry we studied the incorporation of hydrogen from the plasma into homoepitaxial (100) and (111) CVD diamond films. The measured hydrogen concentration and the structural defect density of the diamond lattice derived from proton channeling yields were observed to be correlated. Hydrogen lattice location studies for different incident ion channeling directions showed no dominant fraction of H occupying either one of the two theoretically predicted sites. These results suggest that in CVD films with quite high dislocation densities and relative H concentrations above 10 −3 , hydrogen is predominantly incorporated at structural defect sites uncorrelated with the lattice symmetry.

Liftoff-technique of single-crystal diamond plates : study of the lattice damage of the implanted substrates and the crystalline quality of the homoepitaxial films by ion channelling

Abstract Diamond substrates were deeply implanted by 4.5 MeV O+. The doses were in the range of 2.5 × 1016 up to 8.0 × 1017 at. cm−2. After annealing at 1200°C, homoepitaxial diamond films were deposited onto the substrates. The surface films were then removed by selective etching of the ion-damaged layer at 580°C in flowing air. This process was repeated with the remaining substrates. The lattice damage of the substrates and the crystalline quality of the homoepitaxial films were investigated by ion channelling at following process steps: (0) before implantation; (1) after implantation; (2) after annealing; (3) after deposition; (4) after the liftoff-step; and (5) after the second deposition step. The channelling analyses proved that it is possible to deposit high-quality diamond onto the ion-damaged substrates and to repeat the liftoff-technique without additional process steps such as, e.g., polishing the substrates after removing the epitaxial films.