Materials Science Forum Vols. 483-485

Abstract: 6H-SiC single crystalline substrates were implanted at room temperature with 2 MeV Al2+ ions to fluences from 2×1014 Al2+ cm-2 to 7×1014 Al2+ cm-2 and with different current densities (from 6.6 to 33×1010 Al2+ cm-2 s-1). The depth profile of the damage induced by the Al2+ ions was determined by Backscattering Spectrometry in channeling geometry (BS/C) with a 3.5 MeV He2+ beam. The BS/C spectra were evaluated using the RBX code. The samples were subsequently annealed at 1100°C in N2 for one hour, in order to analyze their structural recovery by BS/C and the amount of the remaining defects by means of Electron Paramagnetic Resonance (EPR). The results from the BS/C spectra corresponding to the as-implanted samples indicate that the damage depends strongly on the total fluence but also, although to a lesser extent, on the beam current density. The BS/C measurements reveal that all the samples, except the one implanted with the highest fluence, recover completely their original crystalline structure after the annealing. Furthermore the angular anisotropy of the EPR spectra indicates that the implanted region recovered a good crystallinity, although some residual defects were observed.

Abstract: Thin 3C-SiC films epitaxially grown on Si-substrate are substantially improved by the FLASIC process, which involves irradiation with flash lamps with pulse duration of 20ms. The disadvantages of the standard FLASIC process are the undulations introduced in the SiC film due to melting of the Si-substrate and the Si mass transport near the SiC/Si interface during the flash. An improved structure was realised in order to minimize the undulations of the SiC, improving also the quality of the film. This structure involves the deposition of a silicon overlayer (SOL) on the initial SiC layer, followed by an additional SiC capping layer acting as a source for SiC transfer by liquid phase epitaxy to the lower SiC layer. Significant mass SiC transport from the upper to the lower SiC layer through the SOL occurs during the flash. The new structure is characterized as inverse - FLASiC. The structural characteristics of the new structure were studied by transmission electron microscopy and atomic force microscopy.

Abstract: 4H-SiC samples were bent in compression mode at temperature ranging from 400°C to 700°C. The introduced-defects were identified by Weak Beam (WB) and High Resolution Transmission Electron Microscopy (HRTEM) techniques. They consist of double stacking faults bound by 30° Si(g) partial dislocations whose glide locally transforms the material in its cubic phase. The velocity of partial dislocations was measured after chemical etching of the sample surface. The formation and the expansion of the double stacking faults are discussed.

Abstract: The pressure dependences of the second-order elastic constants ij C and the velocity of sound in 3C-SiC and 2H-SiC crystals are calculated in the framework of the Keating model with the additional assumption that the central α and the noncentral β force constants are linear functions of external hydrostatic pressure. Grüneisen parameters for the different acoustic modes of 3C-SiC have been also calculated. The third –order elastic constants ijk C for 3C-SiC are determined from the dependences of ij C on the pressure.

Abstract: A structural characterisation of the first [01-15] grown 6H SiC crystals is presented. They show a different micro domain structure outside the facetted region as compared to conventionally [0001] grown crystals. It is imposed by the reduced rotational symmetry for this direction which favours the activation of a low number of glide systems.

Abstract: X-Ray Diffraction Topography (XRDT) and Optical Microscopy (OM) are adopted to study extended structural defects in 6H-SiC bulky crystals. Topographs are taken by means of White Beam Synchrotron Radiation Source (WB-SRS-XRDT) and by means of monochromatic radiation (MoKα1) with conventional source (Lang method). All studied samples are characterised by the presence of linear defects, dislocations and microchannels, uniformly distributed in the crystal. Such defects draw a net of independent systems of parallel lines, with different orientation and different contrast widths. Micro-channels are parallel to the c axis, whereas dislocations are perpendicular or nearly parallel to the c axis. The last are unit screw dislocations. It has been concluded that the growth mechanism is driven by screw dislocations and that channels results from the coalescence of parallel dislocations.

Abstract: We report defects study in 4H-SiC bulk crystals grown by sublimation method on micropipe filled seed crystals oriented (0001) on-axis. The seed crystals of 1~3 inch in diameter were prepared from the large 4H-SiC bulk crystals. Before the sublimation growth, micropipes of the seed crystals were filled with epilayers grown by micropipe filling technique of CVD method. We confirmed about 95% of micropipes perfectly disappeared in the grown crystal. The mechanism of the micropipe extinction was also defined by defect analysis.

Abstract: The aim of the present work is to grow 3C-SiC on (0001) 6H-SiC seeds using the Physical Vapour Transport (PVT) method and to study the electrical and structural properties of the grown material. Photoluminescence (PL)-mappings reveal that the overgrown layer consists predominantly of the 3C-SiC polytype and capacitance-voltage (C-V) measurements result in a net nitrogen donor concentration of 1x1016cm-3. Transmission Electron Microscopy (TEM) observations also confirm that the overgrown layer is of the 3C-SiC polytype having the cubic [111] crystallographic direction parallel to the c-axis of the 6H-SiC substrate. In some cases, twin crystals of 3C-SiC are formed immediately after the interface and, in a few cases, small 6H-SiC inclusions are observed in the cubic film having the same orientation as the substrate. The film near the substrate/overgrown interface shows a high density of defects such as dislocations and stacking faults (SF’s), which propagate into the overgrown layer. Finally although there is a rapid decrease of the defect density within the first 60 µm from the interface, the SF density remains almost constant within the last 100 µm below the surface.

Abstract: We investigated the structure of the in-grown stacking faults (SFs) in the 4H-SiC epilayers. The in-grown SFs exhibited the photoluminescence (PL) peaks representing phonon replicas with bandgap of 2.710 eV. The in-grown SFs were confirmed to be triangular-shaped by PL mapping and KOH etch pit observation. High-resolution TEM image showed that the in-grown SFs have an identical stacking sequence that differ from single or double Shockley SF. In addition, the density of the in-grown SF depended on growth conditions.

Abstract: The effects of hydrogen and proton irradiation on stacking fault formation in 4H-SiC are investigated by an optical pump-probe method of imaging spectroscopy. We report optically stimulated nucleation and expansion of stacking faults (SFs) in 0.6 keV 2H+ implanted n-/n+ and p+/n-/n+ structures. The activation enthalpy for recombination enhanced dislocation glide (REDG) in hydrogenated samples (~0.25 eV) is found to be similar to that in a virgin material. Our results indicate that SFs mainly nucleate at the internal n-/n+ interface, beyond reach of hydrogen, thus justifying minor SF passivation effect. No REDG could be initiated optically in 2.5 MeV proton irradiated samples due to radiation defects providing alternative recombination channels to bypass the REDG mechanism. The radiation damage was verified by DLTS, revealing several new levels below EC in the range 0.4-0.80 eV, and by PL, showing the onset of D-center related luminescence band and concurrent increase of the nonradiative recombination rate.