Roman Drachev

Stacking faults created by the combined deflection of threading dislocations of Burgers vector c and c+a during the physical vapor transport growth of 4H–SiC

Observations have been made, using synchrotron white beam x-ray topography, of stacking faults in 4H–SiC with fault vectors of kind 1/6⟨202¯3⟩. A mechanism has been postulated for their formation which involves overgrowth by a macrostep of the surface outcrop of a c-axis threading screw dislocation, with two c/2-height surface spiral steps, which has several threading dislocations of Burgers vector c+a, with c-height spiral steps, which protrude onto the terrace in between the c/2-risers. Such overgrowth processes deflect the threading dislocations onto the basal plane, enabling them to exit the crystal and thereby providing a mechanism to lower their densities.

Characterization of 100 mm Diameter 4H-Silicon Carbide Crystals with Extremely Low Basal Plane Dislocation Density

Synchrotron White Beam X-ray Topography (SWBXT) studies are presented of basal plane dislocation (BPD) configurations and behavior in a new generation of 100mm diameter, 4H-SiC wafers with extremely low BPD densities (3-4 x 102 cm-2). The conversion of non-screw oriented, glissile BPDs into sessile threading edge dislocations (TEDs) is observed to provide pinning points for the operation of single ended Frank-Read sources. In some regions, once converted TEDs are observed to re-convert back into BPDs in a repetitive process which provides multiple BPD pinning points.

Basal plane dislocation multiplication via the Hopping Frank-Read source mechanism in 4H-SiC

Synchrotron white beam x-ray topography (SWBXT) observations are reported of single-ended Frank-Read sources in 4H-SiC. These result from inter-conversion between basal plane dislocations (BPDs) and threading edge dislocations (TEDs) brought about by step interactions on the growth interface resulting in a dislocation comprising several glissile BPD segments on parallel basal planes interconnected by relatively sessile TED segments. Under stress, the BPD segments become pinned by the TED segments producing single ended Frank-Read sources. Since the BPDs appear to “hop” between basal planes, this apparently dominant multiplication mechanism for BPDs in 4H-SiC is referred to as the “Hopping” Frank-Read source mechanism.

Basal Plane Dislocation Multiplication via the Hopping Frank-Read Source Mechanism and Observations of Prismatic Glide in 4H-SiC

In this paper, we report on the synchrotron white beam topographic (SWBXT) observation of “hopping” Frank-Read sources in 4H-SiC. A detailed mechanism for this process is presented which involves threading edge dislocations experiencing a double deflection process involving overgrowth by a macrostep (MP) followed by impingement of that macrostep against a step moving in the opposite direction. These processes enable the single-ended Frank-Read sources created by the pinning of the deflected basal plane dislocation segments at the less mobile threading edge dislocation segments to “hop” from one slip plane to other parallel slip planes. We also report on the nucleation of 1/3< >{ } prismatic dislocation half-loops at the hollow cores of micropipes and their glide under thermal shear stress.

Synchrotron X-Ray Topography Studies of the Propagation and Post-Growth Mutual Interaction of Threading Growth Dislocations with C-Component of Burgers Vector in PVT-Grown 4H-SiC

Synchrotron White Beam X-ray Topography (SWBXT) imaging of wafers cut parallel to the growth axis from 4H-SiC boules grown using Physical Vapor Transport has enabled visualization of the evolution of the defect microstructure. Here we present observations of the propagation and post-growth mutual interaction of threading growth dislocations with c-component of Burgers vector. Detailed contrast extinction studies reveal the presence of two types of such dislocations: pure c-axis screw dislocations and those with Burgers Vector n1c+n2a, where n1 is equal to 1 and n2 is equal to 1 or 2. In addition, observations of dislocation propagation show that some of the threading dislocations with c-component of Burgers adopt a curved, slightly helical morphology which can drive the dislocations from adjacent nucleation sites together enabling them to respond to the inter-dislocation forces and react. Since all of the dislocations exhibiting such helical configurations have significant screw component, and in view of the fact that such dislocations are typically not observed to glide, it is believed that such morphologies result in large part from the interaction of a non-equilibrium concentration of vacancies with the originally approximately straight dislocation cores during post-growth cooling. Such interactions can lead to complete or partial Burgers vector annihilation. Among the reactions observed are: (a) the reaction between opposite-sign threading screw dislocations with Burgers vectors c and –c wherein some segments annihilate leaving others in the form of trails of stranded loops comprising closed dislocation dipoles; (b) the reaction between threading dislocations with Burgers vectors of -c+a and c+a wherein the opposite c-components annihilate leaving behind the two a-components; (c) the similar reaction between threading dislocations with Burgers vectors of -c and c+a leaving behind the a-component.

Graphitization of the Seeding Surface during the Heating Stage of SiC PVT Bulk Growth

A transient numerical simulation of the temperature field distri bu ion in a conventional resistively heated SiC PVT growth reactor revealed that the uni ntended seeding substrate sublimation typically exists during the furnace heat up stage. Conseque ntly, this would lead to the seeding surface graphitization due to nonstoichiometry of SiC evaporat ion. Suppression of the seeding surface graphitization requires significant reduction of the furnace heat up time. An optimal elevation of the argon partial pressure and seeding substrate tem p rature during the heat up stage would also promote repression of the seed graphitization. Introduction The elimination of silicon and carbon second phase inclusions (bulk defects) that may serve as new micropipe generation centers [1] is essential for high quality SiC bulk growth. While the silicon second phase formation at the growing surface of SiC was previously disc ussed and has been experimentally proven [2], the mechanisms of carbon second phase gener ation is not completely understood. One of the possible mechanisms being proposed here may result f rom unintended seeding substrate sublimation (thermal etching), which typically exists during the heating stage of SiC PVT bulk growth. Model description Transient numerical simulation of the temperature field distributi on during the initial (furnace heat up) stage and steady state phase of growth run i n a conventional resistively heated PVT reactor was performed using GAMBIT-1.3.1/FIDAP-8.6 software pa ckage. The numerical model incorporated the axi-symmetric geometry of the calculati on domain, time dependence of the heating power rise at the growth beginning and the temperature de pendent physical properties of the seeding substrate, SiC source material, argon/Si xCy vapor mixture and the materials used in the furnace design. Conduction in the solid elements of the calculation domai n and radiation along with conduction in the fluid (gaseous) regions represent the heat transfer mechanisms incorporated in the model, whereas the convective component of heat transfer was omitted [3, 4]. Results and Discussion According to the simulation results, the heating up rate of the sourc e material region significantly differs from that of the seeding substrate (see Fig. 1). Such a situation is primarily attributed to the large difference in the thermal conductivities of SiC source material λP and the seeding substrate λC (λC/λP>100 at 2500 K) [5, 6]. This difference determines the typical temperature rise dynamics of the source material and the grow in crystal shown in Fig. 2. As is clear from the plot, the furnace heat up stage continues ~3 hr afte r the heating power switch on. At the beginning of this stage 0 < t < tB the major part of the source material region is not heated up yet. In fact, the temperature inside the source material volume (point 3 in Fig.1) is significantly Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 99-102 doi:10.4028/www.scientific.net/MSF.433-436.99

Deflection of Threading Dislocations with Burgers vector c/c+a Observed in 4H-SiC PVT –Grown Substrates with Associated Stacking Faults

A review is presented of Synchrotron White Beam X-ray Topography (SWBXT) studies of stacking faults observed in PVT-Grown 4H-SiC crystals. A detailed analysis of various interesting phenomena were performed and one such observation is the deflection of threading dislocations with Burgers vector c/c+a onto the basal plane and associated stacking faults. Based on the model involving macrostep overgrowth of surface outcrops of threading dislocations, SWBXT image contrast studies of these stacking faults on different reflections and comparison with calculated phase shits for postulated fault vectors, has revealed faults to be of basically four types: (a) Frank faults; (b) Shockley faults; (c) Combined Shockley + Frank faults with fault vector s+c/2; (d) Combined Shockley + Frank faults with fault vector s+c/4.

Fundamental Limitations of SiC PVT Growth Reactors with Cylindrical Heaters

The ability to set and accurately control the desired growth conditions is crucial in order to attain high quality bulk growth of Silicon Carbide (SiC), especially when the ingot size is large (> 2” in diameter by > 2” long). However, these two aspects of SiC PVT (Physical Vapor Transport) growth technology are severely limited in “conventional” SiC PVT growth reactors with single cylindrical heaters. To overcome such shortcomings, an “alternative” furnace design with two plane resistive heaters is proposed. In order to verify benefits of this design, numerical modeling and comparative procedures have been employed. Detailed comparative analysis revealed two fundamental disadvantages of the conventional furnace design, attributed to (a) – significantly higher in magnitude and spatially nonuniform distribution of the thermal stress that consequently deteriorates structural quality of the growing SiC boule, and (b) – inability to grow long (> 2”) monocrystalline ingots of SiC. Furthermore, the potential of the alternative furnace design to overcome fundamental limitations of the conventional design is also analyzed, with particular attention being paid to the processes of source material recrystallization.

Point and planar defect formation in SiC during PVT growth

The spontaneous nucleation of “negative” crystals from the solute of vacancies in SiC does not appear to be dominant due to the low super-saturation of vacancies. However, clustering of the vacancies is possible due to the energy gain in the system caused by coalescence of any two vacancies. The major reasons for point and planar defect formation in SiC are the liquid phase of free silicon and non-stoichiometry of the vapor.