[en] He⁺ ions were implanted into a (111) epitaxial n-type silicon wafer at different dose rates (fluxes) ranging from 2.5 x 10¹² to 1.3 x 10¹³cm^-2s^-1 while keeping the incident energy and dose constant (1.6 MeV, 2 x 10¹⁶cm^-2). After implantation the samples were subjected to thermal annealing at 800^oC for 30 min. Cross section transmission electron microscopy (XTEM) was used to characterize the damage layer. Even in the as-implanted samples the TEM observations revealed the formation of a buried layer containing a dense array of small bubbles. After annealing, a large band of defects made up of bubbles and dislocations was observed in all samples. However, the characteristics of the damage layer found depended on the flux. For the lowest flux, only platelets and planar clusters of helium bubbles lying in the {001} planes were observed. Their nucleation is discussed in terms of the trap-mutation process. For higher fluxes a continuous band of bubbles with rows of prismatic punching related dislocation loops was observed. These dislocations can extend over several micrometers away from the buried layer and are emitted from clusters. For the highest flux these clusters were found to lie in the {100} or {110} planes. The plate-like structures are discussed in terms of the diluted system. The damage evolution with increasing dose rates is explained by taking into account the vacancy production. copyright 2001 American Institute of Physics

[en] Single crystals of 4H-SiC were implanted with helium ions at temperatures of 400 and 700 °C in a large range of fluences. The damage accumulation versus fluence was studied through the tensile elastic strain determined by using X-ray diffraction measurements. Results were analyzed via the multi-step damage accumulation model. At low dose (step 1) the strain can be described assuming a thermally activated process with low activation energy. Damage cross-sections, independent of implantation temperature, for interstitial-type defects were determined. With increasing dose, the contribution of other defects arises leading to an accelerated strain build-up namely the second step of the disordering process. However, in this regime, the strain cannot be fully described due to others operative mechanisms such as the formation of tiny bubbles under severe conditions of implantation. The formation of bubbles accelerates the development of the elastic strain. The values of damage cross-sections show that only small clusters contribute to the tensile elastic strain

[en] Microstructure in He implanted 4H-SiC under severe conditions followed by high-temperature annealing was studied by high-resolution transmission electron microscopy (HRTEM) using a Cs-corrected microscope. A huge density of stacking faults between large cavities was observed in the highly damaged zone. In order to get quantitative insights into the mechanisms operating in the stacking formation, HRTEM experiments and image simulations were completed by atomic structure calculations. Complex defects involving both Frank dislocations (FDs) and Shockley partial dislocations (PDs), which can either constitute scattering or compact fronts of dislocations, were revealed by HRTEM. In the latter case, the core of these complex defects extends over a few adjacent glide-set planes and the collective propagation of the PDs is associated with the formation of disordered ‘nanobands’ in the initial SiC microstructure. Multislice simulations of the experimental contrast of the FDs showed that they are formed by the condensation of a Si–C bi-layer. The gliding of the PDs allows accommodating the bending of the structure induced by the growth of FDs and is thermodynamically activated by polytypic transformation induced by the multiple shearing of basal planes, as calculated by using the axial next-nearest-neighbour Ising model. (paper)

[en] The phenomena of interaction and propagation of cracks under the contribution of hydrogen were studied in (001) silicon substrate in which an array of scattered over-pressurized He-plates was previously introduced at a given depth. Their propagation under subcritical regime was activated through diffusional supply of H atoms introduced by implantation/annealing. Interactions between the tips of non coplanar cracks take place in a nanometric scale; they can be of plastic-type leading to the formation of extended defects or of elastic-type resulting in deviations of crack-tip propagation. While the planar interactions facilitate the propagation of cracks, those of non coplanar-type stop them. The observations were carried out by transmission electron microscopy and the results were discussed and modelled by using concepts of elasticity and fracture mechanics.

[en] The effect of the crystalline orientation on the implantation-induced strain/stress and on the formation of He-plates was studied by combining high-resolution X-ray scattering and transmission electron microscopy. The highest strains are obtained in (001)-oriented implanted substrates regardless of the fluence and of the channeling effects. The anisotropic properties of the silicon that generate an anisotropic elastic response of the substrate were taken into account to explain these different values of strain. Upon specific thermal annealing, it is shown that the formation of He-plates occurs only in the (001) habit planes regardless of the orientation of the substrates, non-tilted and tilted (001)-, (110)-, and (111)-substrates. Moreover, the distribution of He-plates in the (001) variants was found to be strongly dependent on the angle of the habit plane with the surface and on the intensity of the implantation-induced strain/stress. The implantation-induced stress thus favors the formation of He-plates in specific planes (of low angle with the surface) forming different defect configurations. The nucleation and growth of He-plates are thus discussed with regard to the implantation-induced stress

[en] The growth of Ti₃SiC₂ thin films were studied onto α-SiC substrates differently oriented by thermal annealing of TiAl layers deposited by magnetron sputtering. For any substrate’s orientation, transmission electron microscopy coupled with x-ray diffraction showed the coherent epitaxial growth of Ti₃SiC₂ films along basal planes of SiC. Specifically for the (1120) 4H-SiC, Ti₃SiC₂ basal planes are found to be orthogonal to the surface. The continuous or textured nature of Ti₃SiC₂ films does not depend of the SiC stacking sequence and is explained by a step-flow mechanism of growth mode. The ohmic character of the contact was confirmed by current-voltage measurements.

[en] Damage formation in implanted 4H-SiC was studied as a function of dose and temperature of implantation. At RT the maximal strain as well as the surface swelling linearly increases suggesting a point defects swelling. With increasing temperature the slope decreases due to irradiation-induced dynamic recovery with activation energy of 0.13±0.02eV. From 300°C the amorphisation is avoided and the strain build-up can be fitted according to a direct impact model. At 300°C the as-induced strain profile consists of three different zones of damage with depth, resulting from the damage accumulation in the near surface region, the formation of Xe-vacancy complexes in the ion distribution and beyond a zone of end-of-range strain associated with interstitial accumulation. (paper)

[en] In this work, ion implantations with in situ transmission electron microscopy observations followed by different rates of temperature ramp were performed in (001)-Si to follow the evolution of He-plates under the influence of hydrogen. The JANNUS and MIAMI facilities were used to study the first stages of growth as well as the interactions between co-planar plates. Results showed that under a limited amount of H, the growth of He-plates resulting from a subcritical stress-corrosion mechanism can be fully described by the kinetic model of Johnson-Mehl-Avrami-Kolmogorov with effective activation energy of 0.9 eV. Elastic calculations showed that the sudden and non-isotropic coalescence of close He-plates occurs when the out-of-plane tensile stress between them is close to the yield strength of silicon. After hydrogen absorption, surface minimization of final structure occurs.

[en] The AlN_xO_y system offers the possibility to obtain a wide range of responses, by tailoring the properties between Al, AlN and Al₂O₃, opening a significant number of possible applications. The aim of this work is to correlate the optical properties of AlN_xO_y thin films with their composition and structural features, taking as reference the binary systems AlN_x and AlO_y. In the AlN_x system, the increase in the nitrogen content induced a wide variation in the optical properties, ranging from the typical profile of a polycrystalline Al-type film towards nearly constant reflectance values as low as 5%, as well as a smooth increase in samples transparency as the ratio N/Al approached unit. In the case of the AlO_y system, the reflectance also decreased as the oxygen content increased; however, the transition to transparent films (Al₂O₃-like) was more abrupt. The ternary system AlN_xO_y, revealed optical responses that ranged from a typical profile of a polycrystalline Al-type film towards low and constant reflectance values in a wide range of x and y coefficients, ending up as semi-transparent when Al₂O₃-like films were formed. The unusual low optical reflectance of some films reveals some potential applications in solar power systems and sensors.

[en] The crystalline-to-amorphous transformation induced by lithium ion implantation at low temperature has been investigated. The resulting damage structure and its thermal evolution have been studied by a combination of Rutherford backscattering spectroscopy channelling (RBS/C) and cross sectional transmission electron microscopy (XTEM). Lithium low-fluence implantation at liquid nitrogen temperature is shown to produce a three layers structure: an amorphous layer surrounded by two highly damaged layers. A thermal treatment at 400 °C leads to the formation of a sharp amorphous/crystalline interfacial transition and defect annihilation of the front heavily damaged layer. After 600 °C annealing, complete recrystallization takes place and no extended defects are left. Anomalous recrystallization rate is observed with different motion velocities of the a/c interfaces and is ascribed to lithium acting as a surfactant. Moreover, the sharp buried amorphous layer is shown to be an efficient sink for interstitials impeding interstitial supersaturation and {311} defect formation in case of subsequent neon implantation. This study shows that lithium implantation at liquid nitrogen temperature can be suitable to form a sharp buried amorphous layer with a well-defined crystalline front layer, thus having potential applications for defects engineering in the improvement of post-implantation layers quality and for shallow junction formation.