Jean Luc Demenet

PLASTIC BEHAVIOR OF 4H-SiC SINGLE CRYSTALS DEFORMED AT LOW STRAIN RATES

Department of MaterialsScience and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204, USA(Received April 18, 2000)(Accepted in revised form May 24, 2000)Keywords: Semiconductors; Mechanical properties; Dislocations; Brittle-to-ductile transitionIntroductionPlasticity of elemental and III-V compound semiconducting materials has been extensively studied ina wide range of stress and temperature; for a review, see, e.g., Refs. [1,2]. However, data on the yieldstress and other plastic properties of IV-IV compounds, e.g. different SiC polytypes, is lacking mainlybecause it was not possible to grow single crystals of these materials in large dimensions until recently.The first experiments on plasticity of 6H-SiC single crystals were performed by Fujitaet al. [3] on smallsamples cut from industrial Acheson-grown crystals. In the last few years, reasonably high quality bulksingle crystals of 4H-SiC and 6H-SiC are being grown by the modified Lely technique [4] and waferscut from these bulk ingots are now commercially available (Cree Research, Inc., Durham, NC, USA).Recently, Samant [5–7] performed compression experiments on the new high-quality 6H-SiC crystalsat moderate and high strain rates in order to measure the yield stress, t

Dislocation microstructures in Si plastically deformed at RT

Dislocation microstructures induced by plastic deformation at room temperature in Si have been investigated by TEM. Plastic deformation has been obtained by using two types of technique: deformation under a confining pressure of 5 GPa in an anisotropic multi-anvil apparatus and by surface scratching. The TEM observations show common features in the two deformation substructures which are characteristic of high stress-low temperature deformation. The deformation microstructures are built with dislocations with a/2110 Burgers vector in (111) planes which are undissociated. Such dislocations are mainly aligned along the screw orientation and 112 orientations at 30° from the Burgers vectors as well as along 132 orientations at 41° from the Burgers vector. The occurrence of those Peierls valleys confirms that different dislocation core configurations from those usually dealt with at higher temperatures have to be taken into account when dislocations are nucleated at very high stresses.

On a change in deformation mechanism in silicon at very high stress: new evidences

Abstract Dislocation configurations resulting from high stress low temperature deformation in silicon are found non-dissociated and elongated along different Peierls valleys as compared to usual results from the literature. Experiments on pre-deformed samples are reported and discussed in the context of a possible transition in the core structure at high stresses between dissociated glide and perfect shuffle configurations.

Kinetics of the 3C-6H polytypic transition in 3C-SiC single crystals: A diffuse X-ray scattering study

In this work, the kinetics of the 3C-6H polytypic transition in 3C-SiC single crystals are studied in details by means of diffuse x-ray scattering (DXS) coupled with numerical simulations and transmission electron microscopy and optical birefringence microscopy. Upon high-temperature annealing, spatially correlated stacking faults (SFs), lying in the {111} planes, are generated within the crystal and tend to form bands of partially transformed SiC. It is shown that the numerical simulation of the DXS curves allows to unambiguously deduce the transformation level within these bands, as well as the volume fraction corresponding to these bands. Increasing annealing time results (1) in the growth of the partially transformed regions by the glide of the partial dislocations bounding the SFs and (2) in the generation of new SFs within the crystal by means of a double-cross slip motion. The kinetics of each of these mechanisms are presented and discussed with respect to the annealing temperature, the initial SF density and crystalline quality.

Analysis of Dislocations Nucleated after Nano Indentation Tests at Room Temperature in 4H-SiC

4H-SiC intrinsic homoepitaxied single crystals have been nano indented at room temperature using a spherical indentor and the related deformation microstructures have been analyzed by Transmission Electron Microscopy. Dislocations are lying in the basal plane but have been found to be perfect, in contrast with observations made at higher temperature. Although such a change in deformation mechanism has been observed in other semiconductors such as Silicon and Indium Antimonide, it was unexpected in a very low stacking fault material such as SiC.

TEM Observations of 4H-SiC Deformed at Room Temperature and 150°C

4H-SiC single crystals have been deformed under 5 GPa at room temperature and 150°C using an anisotropic multi-anvil apparatus. Transmission Electron Microscopy observations show that the microstructure is composed of widely dissociated dislocations and perfect dislocations. The generation of such perfect dislocations could indicate that silicon carbide exhibits a change in deformation mechanism under very high stress, with a behavior similar to Si and III-V compounds. Introduction Due to the high strength of covalent bonds between atoms, semiconductors can be plastically deformed only at high temperature, say above about 0.5 Tm (Tm: melting temperature), when thermal activation is high enough to reduce the energy for breaking atomic bonds and generate dislocations. At low temperature, semiconductors deform by fracture and exhibit a brittle behavior. This well-known ductile-to-brittle behavior has been extensively studied in the case of silicon and, at a lower scale, for some other semiconductors. Recently, Zhang et al. performed fracture experiments on 4H-SiC using the four-point bend technique which confirm that the brittle-to-ductile transition could be associated to a change in dislocation core properties [1]. The brittle-to-ductile temperature, TBDT, deduced from these experiments is in fair agreement with the transition temperature, Tc, observed on the ln(τy) = f(1/T) plot, corresponding to the variation of the yield stress (τy) as a function of temperature obtained by compression tests at constant strain rate [2-4]. Above Tc, i.e. at low stress, deformation occurs by perfect, weakly dissociated, dislocations, whereas below Tc, i.e. at high stress, the deformation microstructure is composed by partial dislocations dragging large stacking faults [3,4]. Recent experiments on Si [5] and III-V compounds [6] have shown that under very high stress, perfect dislocations, possibly lying on the shuffle set, are generated, as predicted by theoretical calculations. Such level of very high stresses can be reached by accidental scratching during the manufacturing process of devices. In order to check if 4H-SiC exhibits a behavior similar to Si and III-V compounds, deformation experiments under very high stresses have been done and Transmission Electron Microscopy (TEM) has been used to characterize the resulting microstructure. Experimental Samples used in this study were cut from a 4H-SiC single crystal ingot grown at Cree Research Inc., using the technique of modified sublimation (seeded Lely), with a nitrogen content of about 2x10 18 cm -3 . They had a parallelepiped shape with final dimensions of typically 2x2x3.5 mm 3 after grinding and polishing at different diamond paste grades. The basal (0001) plane was oriented at Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 343-346 doi:10.4028/www.scientific.net/MSF.457-460.343

On the Stability of 3C-SiC Single Crystals at High Temperatures

The 3C-6H polytypic transition in 3C-SiC single crystals is studied by means of diffuse X-ray scattering (DXS) coupled with transmission electron microscopy (TEM). TEM reveals that the partially transformed SiC crystals contain regions of significantly transformed SiC (characterized by a high density of stacking faults) co-existing with regions of pure 3C-SiC. The simulation of the diffuse intensity allows to determine both the volume fraction of transformed material and the transformation level within these regions. It is further shown that the evolution with time and temperature of the transition implies the multiplication and glide of partial dislocations, the kinetics of which are quantified by means of DXS.

Dislocation Activity in 4H-SiC in the Brittle Domain

Results of deformation experiments on 4H-SiC single crystals below the usual brittle to ductile transition temperature are reported and discussed in comparison of previous literature data. Si-core and C-core partials are evidenced in the basal plane, and perfect dislocations are also observed on other crystallographic planes. These results could indicate that dislocation activity under high stress is more complex than expected.