Irina G. Galben-Sandulache

The 3C-6H polytypic transition in SiC as revealed by diffuse x-ray scattering

The 3C-6H polytypic transition in SiC single crystals is studied by means of diffuse x-ray scattering. Based on numerical simulations of the diffuse scattering intensity distribution we unambiguously prove that the 3C-6H transition in SiC occurs through the glide of partial dislocations and not by the “layer displacement” mechanism (i.e., local diffusional rearrangement of the Si and C atoms). The technique is extremely sensitive and can be used as a nondestructive mean to obtain statistically relevant values of the transition level down to ∼0.05%.

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.

Optical Investigation of Electronic Properties in Bulk and Surface Region of Sublimation-Grown 3C-SiC Crystals

We applied a picosecond dynamic grating technique for studies of nonequilibrium carrier dynamics in a 0.8 mm thick bulk 3C-SiC crystal grown by the continuous feed physical vapor transport (CF-PVT) on 6H-SiC (0001) substrate. Investigation of carrier dynamics at surface or bulk excitation conditions was performed for excess carrier density in range from ~ 1017 cm-3 to ~ 1020 cm3 using for excitation weakly or strongly absorbed illumination. In DPBs free domains, the bipolar diffusion coefficient and carrier lifetime value at 300K were found gradually increasing with carrier density. The bipolar mobility vs. temperature dependence, μ. ~ T -k, provided a value k = 1.2 - 2 in range T < 100 K, thus indicating a negligible scattering by point and extended defects. These data indicated strong contribution of the carrier-density dependent but not defect-density governed scattering mechanisms, thus indicating high quality of the CF-PVT grown bulk cubic SiC. These studies were found in good correlation with the structural and photoluminescence characterization of the given crystal.

Quantitative analysis of diffuse X-ray scattering in partially transformed 3C-SiC single crystals

The X-ray scattering of partially transformed 3C-SiC single crystals is considered in detail. Extended diffuse scattering streaks, originating from stacking faults (SFs) lying in the {111} planes, are clearly observed in the wide-range reciprocal-space maps. The intensity distribution along the diffuse streaks is simulated with a model including the contributions of the diffuse scattering originating from the SFs [based on the pioneering theoretical description given by Kabra, Pandey & Lele (1986). J. Mater. Sci. 21, 1654–1666], the coherent scattering emanating from untransformed areas of the crystals and all θ-dependent terms that affect the scattered intensity (the layer structure factor, the irradiated volume and the polarization of the beam). The quantitative simulation of the diffuse streaks reveals that the transformation occurs through the glide of partial dislocations and allows one to derive the transformation level. It is shown that the 3C polytype is indeed unstable at high temperature. However, it is further shown that defect-free 3C-SiC single crystals remain stable at temperatures where 3C-SiC is known to be usually unstable (2173 K). The origin of this apparent stability is very likely of kinetic nature, i.e. the lack of crystalline defects inhibits the transformation.

On the Microstructure and the Polytype Transformation in 3C‐SiC Crystals Grown by LPE on (001) Substrates at Different Growth Conditions

This work focuses on the microstructure of SiC layers grown by liquid phase epitaxy at different temperatures using pure Si or Si with an additive as solvents. 3C‐SiC wafers with (001) orientation from HAST Corp. were used as seeds in all cases. The growth temperature was varied in the range of 1650° C to 1800° C and three different solvents were compared i.e. pure Si, Si+Al and Si+InP. The structural study revealed that when Al−Si melts were used, several short and long period polytypes, as well as Al inclusions were observed inside the layers. The growth at different temperatures did not affect the overall structural quality but only the type of the observed polytypes. When pure Si and Si+InP melts were used, no polytype transformations were observed. The increase of the temperature slightly improved the density of the defects. In terms of stacking fault density the lowest one (<8×103 cm−1) was achieved for the liquid phase epitaxy in the Si+InP melt.

Diffuse X-ray scattering from partially transformed 3C-SiC single crystals

The 3C–6H polytypic transition in 3C–SiC single crystals is studied by means of diffuse X-ray scattering (DXS) coupled with numerical simulations. It is shown that the presence of spatially correlated stacking faults (characteristic of this type of re-stacking transition) gives rise to extended diffuse scattering in the reciprocal space perpendicularly to the fault plane. 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.

Quality Investigation of 3C-SiC Crystals Grown by CF-PVT Technique

This work presents the crystalline quality investigation of 3C-SiC unseeded crystals grown from vapor phase. Samples were polished after different crystallographic planes from crystals grown with or without nitrogen flow. The structural and optical investigation showed that the central part of the samples exhibited a very good crystalline quality. The best samples proved to be the {100} growth sectors where the only defects found were stacking faults with a defect density under 103 cm-1. At the edges, i.e. between two adjacent growth sectors, structural investigation by transmission electron microscopy revealed stacking faults and hexagonal polytype inclusions. The nitrogen doping was found not to have an influence on the crystalline quality.

Heavily p-Type Doping of Bulk 6H-SiC and 3C-SiC Grown from Al-Si Melts

We report in this work, the solution growth of heavily p-type doped 3C-SiC and 6H-SiC. Description of the 3C and 6H-SiC crystals in terms of defects and resistivity are presented and discussed with respect to growth conditions such as temperature, Al content in the melt and seed polarity. Crystals and thick layers are investigated by means of TEM, NDIC microscopy and Raman.

TEM and LTPL Investigations of 3C-SiC Layers Grown by LPE on (100) and (111) 3C-SiC Seeds

In the present work the defects appearing in layers grown by liquid phase epitaxy on different substrates are compared. The used seeds were (i) 3C-SiC with (111) orientation, grown heteroepitaxially on (0001) 4H-SiC or 6H-SiC substrates by continuous feed physical vapour transport process and the vapour-liquid-solid mechanism, respectively, and (ii) 3C-SiC wafer with (100) orientation from HOYA. The structural and optical investigation showed that (i) on the (111) substrates, due to the appearance of silicon and 6H-SiC inclusions, a layer which consisted of a sequence of long period polytypes was formed. The dominant polytype formed was 21R-SiC, which after successive transformation to 39R- and 57R- SiC led to the formation of 6H-SiC on the top of the layer. (ii) On the (100) substrates, a 3C-SiC layer with comparatively uniform defect density was formed. The main defects were stacking faults and their density was reducing during the process.

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.