Materials Science Forum Vols. 527-529

Abstract: Defects are introduced into (11-20) highly N-doped 4H-SiC by one surface scratch followed by annealing at 550°C or 700°C with or without an additional compressive stress. The defects are planar and always consist of double stacking faults dragged by a pair of partial dislocations. In a pair, the partial dislocations have the same line direction, Burgers vector and core composition. All the identified gliding dislocations have a silicon core. An analysis of their expansion during annealing proves that C(g) partial segments can be created but that C(g) partial dislocations are immobile.

Abstract: Using light emission imaging (LEI), we have determined that not all planar defects in 4H-SiC PiN diodes expand in response to bias. Accordingly, plan-view transmission electron microscopy (TEM) observations of these diodes indicate that these static planar defects are different in structure from the mobile stacking faults (SFs) that have been previously observed in 4H-SiC PiN diodes. Bright and dark field TEM observations reveal that such planar defects are bounded by partial dislocations, and that the SFs associated with these partials display both Frank and Shockley character. That is, the Burgers vector of such partial dislocations is 1/12<4-403>. For sessile Frank partial dislocations, glide is severely constrained by the need to inject either atoms or vacancies into the expanding faulted layer. Furthermore, these overlapping SFs are seen to be fundamentally different from other planar defects found in 4H-SiC.

Abstract: With a simple processing sequence using only patterned aluminum (Al), dislocations and stacking faults were examined in thick n- epitaxial layers including a grown p+ layer on top. The thicknesses of the n- layers were 100 and 150 μm, values that are typical for fabricating 10 kV PiN diodes. High temperature sintering of the metal film was avoided making this a potentially nondestructive scheme for evaluating SiC epitaxy. Faulting of basal plane dislocations (BPDs) and the resulting forward voltage, Vf, increase were examined at current densities up to 20 A/cm2. A simple guard ring structure defined in the Al pattern was successfully used to confine the current through the epitaxial layers to the area inside of the ring. Kelvin contacts compensated for voltage drops at the Al/SiC interfaces. As a result, current-voltage characteristics and electroluminescent imaging were obtained across a known area of the PiN layer at currents densities ranging from 20 A/cm2 to 7 × 10-3 A/cm2.

Abstract: 4H-SiC pin diodes were fabricated on epitaxial layers grown by Sandwich Sublimation Method (SSM). I-V and photoemission measurements were conducted on these devices. These measurements show hot spots responsible for a soft breakdown and evidence triangular shape defects previously observed in 4H-SiC pin diodes made on CVD epitaxial layers. These results agree with the morphology studies which indicate that the SSM-grown layers have a higher number of structural defects than their counterparts.

Abstract: We investigate the possibility of controlling formation of stacking faults (SFs) at the interface region by implanting the 4H-SiC substrate with low-energy antimony ions (75 keV Sb+) prior to conventional CVD growth of the homoepitaxial layers. This approach is based on the solidsolution hardening concept, according to which interaction of impurity atoms with dislocations makes the motion of the latter more difficult. Photoluminescence imaging spectroscopy is employed to investigate incorporation of Sb+ implants at the buried interface and also to assess its impact on structural degradation. Spectral results are analyzed considering both the onset of n-type doping and irradiation damage. The latter factor was estimated separately from supplementary measurements of high-energy (2.5 MeV H+) proton-irradiated 4H-SiC epilayers. We compare results of optically stimulated SF formation in virgin and Sb implanted regions and provide a comprehensive picture of the defect evolution, including microscopic details of the imminent nucleation sites.

Abstract: Plan-view transmission electron microscopy (TEM) was applied to investigate the origin of surface defects in 4H-SiC homoepitaxial films. Their origin existed at the substrate/epi-film interface. Hence, almost the entire thickness of the epi-film was removed by plasma etching leaving a very thin film. Then, the etched epi-crystal was thinned from the substrate side so that we could observe the crystallographic imperfections at the substrate/epi-film interface in plan-view TEM. Morphological features of the epi-film surface remained unchanged after the plasma etching process. Hence, one-to-one correspondence between surface defects and crystallographic imperfections was confirmed by comparing optical micrographs and TEM images. Crystallographic imperfections associated with “carrot defects” were observed. They were composed of stacking faults on the (0001) plane and partial dislocations bounding them. These imperfections originated from foreign particles at the interface. From X-ray energy-dispersive spectrometry (XEDS), it was confirmed that particles contained zirconium (Zr). Selected area diffraction patterns showed that the particles were crystalline.

Abstract: The structure of the “star” defect in 4H-SiC substrates and its effects on the extended defect structures in the epilayers were studied by molten KOH etching and transmission x-ray topography. Star defects consist of a center region with high densities of threading dislocations (both edge and screw types) and six arms of dislocation arrays extending along <11-20> directions. In addition, multiple linear dislocation arrays extending perpendicular to the off-cut direction were observed in the epilayers. Dislocation arrays extending along <11-20> directions are consistent with the slip bands generated by the prismatic slip: a/3<11-20>{1-100}. Bands of linear dislocation arrays extending perpendicular to the off-cut direction correspond to the threading edge dislocations nucleated during epitaxial growth.

Abstract: The detailed properties of the dislocations of SiC crystals were analyzed using ultrahigh-quality substrates manufactured by RAF (repeated a-face) growth method by means of bulk X-ray topography. From this analysis, we could reveal the detailed features of one type of basal plane dislocations and two types of threading dislocations. The basal plane dislocations were screw type with Burgers vector were parallel to <11-20> direction. One of the threading dislocations was mixed type close to screw dislocation parallel to the growth direction with Burgers vector of 1c+na (n=0, 1, 2, …). Another was the edge type parallel to the c-axis, which was lying between two basal plane dislocations. Moreover, these dislocations were found to be connecting with each other, constituting large network structures.

Abstract: Among the types of dislocation seen in homo-epilayers of SiC grown upon 4H-SiC wafers with an 8° surface offcut are basal plane dislocations propagated into the epilayer at an 8° inclination, and threading edge dislocations. These types may be imaged by monochromatic synchrotron x-ray topography in the grazing-incidence reflection geometry using the 11 2 8 reflection. Equations needed to apply the ray-tracing method of computer simulating x-ray topographic defect images in this experimental geometry were derived and used to simulate images of the threading edge dislocations. Simulations of the threading edge dislocations showed 4 μm wide white ovals with narrow arcs of dark contrast at their ends, inclined relative to the g-vector of the topograph according to the sign of their Burgers vector. These resembled the experimental topographs, inasmuch as was possible at the maximum resolution of x-ray topographs.

Abstract: We have developed non-destructive in-house observation techniques for dislocations and stacking faults (SFs) in 4H-SiC epilayers. Low temperature photoluminescence (PL) mapping was carried out at 100K using He-Cd laser (325 nm) as an exciation source. PL mapping at ~420 nm was used to investigate basal plane dislocations (BPDs), Shockley stacking faults (SSFs) and boundary, while PL mapping at ~470 nm and 100K obtained in-grown SF images. In addition, using a high-resolution laboratory X-ray topography system with a four-crystal collimator, we succeeded in recording BPDs propagating along [11-20]. From the measurement results, new evaluation techniques for dislocations and SFs other than KOH etching and Synchotron radiation topography were demonstrated on Si- and C-face 4H-SiC epilayers.