(Invited) Structural Characterization of SiC Crystals Grown By Solvent Laser Heated Floating Zone Method and Hot Wall Chemical Vapor Deposition for the Development of a Low Defect Density Bulk Growth Technique for SiC

Abstract

Structural perfection of silicon carbide (SiC) single crystal is critical in achieving high-performance power devices. This requirement is an ongoing challenge that is being addressed because of the high density of structural defects, particularly threading screw dislocations (TSDs) and basal plane dislocations (BPDs). While the deleterious impact of these defects has been reported, effort in reducing their densities is still ongoing. Researchers at NASA Glenn Research Center (GRC) have recently proposed a new bulk growth process based on axial fiber growth followed by lateral expansion for producing SiC boules with potentially as few as one TSD per wafer [1]. In this techniques, a long continuous single crystal SiC fiber containing a single TSD in the center is grown in the c-direction [0001] and then laterally enlarged (along <1-100> and <11-20> directions) into a boule by chemical vapor deposition (CVD) growth. To implement this novel growth technique, the growth process has been split into two parts that will be optimized separately: (1) Solvent laser heated floating zone (Solvent-LHFZ) method [2] to grow SiC at the end of a 4H-SiC ‘pseudo fiber’ seed using Fe as solvent. This technique combines the fiber growth ability of laser heated floating zone method with single crystal growth ability by traveling solvent method. (2) Lateral expansion of slivers or ‘pseudo-fibers’ of 6H-SiC and mixed 4H/6H polytype SiC by hot wall chemical vapor deposition (HWCVD) [3]. The seed slivers for both growth processes were cut from m-oriented SiC boule slices. Structural characterization of the grown crystals were carried out by synchrotron white beam x-ray topography (SWBXT), optical microscopy, SEM, HRTEM and Raman spectroscopy to map defect types, surface features and distribution [4]. The solvent-LHFZ grown region was single crystal and epitaxial with the seed but however characterized by inhomogeneous strains. The lateral grown regions were also found to be single crystal and epitaxial. The seed used contained stacking disorders and these are reproduced in the lateral growth sections. The Raman spectra confirms the existence of stacking faults with the density of faults higher in the seed than in the grown crystal. Bundles of dislocations are also observed propagating from the seed in m-axis lateral directions. Polytype phase transition and stacking faults were observed by high-resolution TEM (HRTEM), in agreement with SWBXT and Raman scattering. The implication of these results for implementing the novel growth process will be addressed.