Materials Science Forum Vols. 679-680

Abstract: Extensive study of various process parameters to influence on the growth of 4H-SiC crystal has been carried out using the transformation of the 6H-SiC seed by a PVT method. The axial temperature gradients were increased throughout increasing the crucible length along growth direction in order to enhance the growth rate and transformed crystal yield. The N2/Ar gas ratio used during the crystal growth related with carrier concentration/mobility of grown crystal. In the initial stage of growth, foreign polytypes such as 6H/15R were observed on 6H-SiC seed crystal but 4H crystals were entirely grown after the process optimization. While the typical absorption spectrum of SiC seed crystal indicated that the SiC polytype was the 6H-SiC with fundamental absorption energy of about 3.02eV, absorption spectrum of grown SiC crystal exhibited 4H-SiC with fundamental absorption energy of about 3.26eV. The entirely transformed SiC region exhibited lower micropipe density than 6H/4H transition region.

Abstract: The single crystal ingots by using a sublimation technique were grown on 6H-SiC dual-seed crystals with opposite face polarities and then SiC crystal wafers sliced from the SiC ingot were systematically investigated to find out the polarity dependence of the crystal quality. The growth rate of the SiC crystal grown in this study was about 0.2mm/hr. N-type 2’’ SiC crystals exhibiting the 4H- and 6H-SiC polytype were successfully fabricated on C-face and Si-face, respectively. The incorporation of nitrogen donors in the SiC crystals grown on the C-face seed crystal was exhibited to be higher than in SiC crystals grown on a Si-face crystal. When the SiC crystal ingot proceeded to grow, the SiC crystal region grown on the C-face seed crystal was enlarged compared to the SiC crystal region on the Si-face seed crystal.

Abstract: Three dimensional kinetic Monte Carlo simulations on super-lattices are applied to study the evolution of stacking faults during epitaxial growths. We show that, in the case of misoriented close packed substrates, these defects can either extend throughout the entire epilayer (i.e. extended from the substrate up to the surface) or close in dislocation loops, in dependence of the deposition conditions. We explain this behavior in terms of a surface kinetic competition between these defects and the surrounding crystal: if the local growth rate of the defect is larger compared with that of the perfect crystal the defect will expands, otherwise it will closes. This mechanisms allows to explain several experimental results on homo and hetero epitaxies.

Abstract: Homoepitaxial growth on 4° off-axis substrates with different off-cut directions, i.e. [11-20] and [1-100], was investigated using a commercial CVD reactor. The characteristics of the growth process on substrates with different off-cut directions were determined with respect to applicable C/Si ratio, growth rate and n- and p-type doping range. Stable step flow growth was achieved over a broad range of C/Si ratio at growth rates ~ 15 µm/h in both cases. The n-type doping level of epilayers can be controlled at least in the range from 5  1014 cm-3 to 3  1017 cm-3 on both types of substrates. Highly p-type epilayers with p = 2  1019 cm-3 can also be grown on [1-100] off-cut substrates. Hence, the growth process for standard substrates was successfully transferred to [1-100] off-cut substrates resulting in epilayers with similar doping levels. The dislocation content of the grown epilayers was investigated by means of defect selective etching (DSE) in molten KOH. For both off-cut directions of the substrates, similar densities of threading edge dislocations (TED), threading screw dislocations (TSD) and basal plane dislocations (BPD) were found in the epilayers. Epilayers with very low BPD density can be grown on both kinds of substrates. The remaining BPDs in epilayers are inclined along the off-cut direction of the substrate. The surface morphology and roughness was investigated by atomic force microscopy (AFM). The epilayers grown on [1-100] off-cut substrates are smoother than those on standard substrates.

Abstract: The epitaxial growth at 100 µm/h on on-axis 4H-SiC substrates is demonstrated in this study. Chloride-based CVD, which has been shown to be a reliable process to grow SiC epitaxial layers at rates above 100 µm/h on off-cut substrates, was combined with silane in-situ etching. A proper tuning of C/Si and Cl/Si ratios and the combination of different chlorinated precursors resulted in the homoepitaxial growth of 4H-SiC on Si-face substrates at high rates. Methyltrichlorosilane, added with silane, ethylene and hydrogen chloride were employed as precursors to perform epitaxial growths resulting in very low background doping concentration and high quality material, which could be employed for power devices structure on basal-plane-dislocation-free epitaxial layers.

Abstract: The effectiveness of an in-situ growth interrupt in nitrogen doped 8° off-cut epilayers was investigated using ultraviolet photoluminescence imaging. Low-doped n-type epilayers (<1016 cm-3) exhibited an abrupt increase in BPD to TED conversion at the growth interrupt and achieved 96-99% conversion overall (< 10 BPDs/cm-2), while high-doped epilayers had minimal conversion at the interrupt (< 1%) and overall (< 30%). This large discrepancy suggests nitrogen prohibits or alters the conversion mechanism at the growth interrupt. Therefore, a novel SEM technique was developed to "freeze-in" the interface morphology and help elucidate the conversion mechanism. Preliminary results suggest that preferential etching at the point of BPD intersection with the surface is greatly reduced in highly doped layers, which inhibits the conversion mechanism.

Abstract: Spatially resolved micro-photoluminescence has been used to study the Single Shockley faults surface density and properties on 4H-SiC epitaxial layers. The improvement of quality of epitaxial layers due to the chemical vapor deposition process has been studied by measuring the reduction of mean density of Single Shockley faults. The change of faults density has been correlated to the different precursor gas used for the growth. In fact trichlorosilane has been used instead of silane. The change of precursor led to two different advantages: the reduction of basal plane dislocation surface density and the capability to increase the growth rate of the process. Both these features allow reducing the density of Single Shockley faults.

Abstract: This paper deals with the formation and propagation of twin boundaries (TBs) inside 3C-SiC layers grown heteroepitaxially on -SiC substrate. The equivalent probability of nucleating 60° rotated 3C islands on such substrate lead to the systematic formation of TB upon coalescence of these islands. Elimination of these defects should occur by bending of the propagation direction. Bending through incoherent TBs is usually encountered during both VLS and CVD growth and it generates crystalline defects due to high built-in energy. One would prefer coherent TBs, formed by two-by-two annihilation of neighbouring TBs, which do not form new defect except microtwin inclusion at the interface. Such TB annihilation seems to be a specificity of growth by VLS mechanism. The mechanism of such bending is discussed

Abstract: A chloride-based chemical-vapor-deposition (CVD) process has been successfully used to grow very high quality 3C-SiC epitaxial layers on on-axis α-SiC substrates. An accurate process parameters study was performed testing the effect of temperature, in situ surface preparation, precursor ratios, nitrogen addition, and substrate polytype and polarity. The 3C layers deposited showed to be largely single-domain material of very high purity and of excellent electrical characteristics. A growth rate of up to 10 µm/h and a low background doping enable deposition of epitaxial layers suitable for MOSFET devices.

Abstract: In this work we analyze the static behavior of cantilevers elaborated on the basis of 3C SiC thin films grown by chemical vapor deposition on (100) and (111) oriented silicon substrates. A direct microscope observation of cantilever bending indicates the opposite sign of stress gradient (respectively negative and positive) for both film orientations. The correlation of this observation with the commonly admitted nature of intrinsic stress for each orientation (respectively compressive and tensile) leads us to an unexpected conclusion: instead of relaxing, the absolute value of the intrinsic stress increases from the interface to the layer surface. We propose an analytical model that could explain this phenomenon.