Back-reflection Topographs Research Articles

High-geometrical-resolution imaging of dislocations in SiC using monochromatic synchrotron topography

We demonstrate high-geometrical-resolution imaging of dislocations in 4H-SiC by monochromatic synchrotron topography (but still under the “integrated wave” condition). In back-reflection topographs, 1c screw dislocation images are “magnified” to appear as well-defined circular white spots, while basal plane dislocations with opposite edge Burgers vector components exhibit two distinct kinds of contrast features. All the dislocation images are precisely described by ray-tracing simulations. This imaging technique provides an accurate, comprehensive, and nondestructive characterization tool, which is needed by current SiC researchers is used for industrial applications. It also provides a simple picture for understanding the mechanisms underlying synchrotron diffraction imaging of defects.

Dislocations in GaAs p-i-n diodes grown by hydride vapour phase epitaxy

Hydride vapour phase epitaxy grown all-epitaxial p-i-n structures were studied by synchrotron X-ray topography. Three types of process induced dislocations were found: short threading dislocations, long straight interfacial dislocations and circular arc dislocations. The majority of the dislocations observed are short straight threading dislocations, the density of which is typically about 5000 cm−2. The dislocations at the p-i interface are long straight lines parallel to [110]. They are screw dislocations having their Burgers vector parallel to [110], calculated from the contrast analysis of the well resolved dislocation images. One sample also showed a dense misfit dislocation network at the n-side. However, no misfit dislocations were seen in the back-reflection topographs of the n-side of the other samples, which shows that it is possible to grow a misfit-dislocation free n-type GaAs layer onto the substrate side of a hydride vapour phase epitaxy grown GaAs surface after proper substrate removal.

The Characterization of Defects in Silicon Carbide Crystals by X-Ray Topography in the Back-Reflection Geometry

Synchrotron white-beam x-ray topographs taken in the back-reflection mode have proved a powerful tool in the study of defects in semiconductor-grade silicon carbide crystals. Capable of mapping the distribution of axial dislocations across a wafer's area (notably the devastating micropipe defect), it can also provide information on their natures. Under favorable conditions, various other types of defect may be observed in back-reflection topographs of SiC, among which are subgrain boundaries, inclusions, and basal plane dislocations. Observed defect images in backreflection topographs may be simulated using relatively simple computer algorithms based on ray tracing. It has been possible to use back-reflection topographs of SiC substrates with device structures deposited upon them to relate the incidence of defects to device failure.

The quality of 200 mm diameter epitaxial Si wafers for advanced CMOS technology monitored using synchrotron X-ray topography

The control and characterisation of wafer defect and strain distributions is of crucial importance for the development of advanced Ultra Large Scale Integration (ULSI) circuits. Within the IC manufacturing sector 0.35 μm linewidth-based advanced Complementary Metal Oxide Semiconductor (CMOS) logic has recently emerged at a high level of maturity, to be closely followed by an even more demanding 0.25 μm process. One very important issue is the need to ensure a uniform, high quality Si substrate, i.e. minimise defect densities and eliminate strain distributions in the starting wafer material. Synchrotron section and back-reflection topographic techniques were applied to 200 mm diameter p-Si wafers, upon which, boron and arsenic doped epitaxial silicon layers had been deposited. These wafers were supplied from manufacturers around the globe and revealed substantial differences in the overall quality of the epilayers and substrates. In all wafers the substrate quality varied significantly with position across the wafer, as measured by the presence of oxygen-related defects and dopant strain homogeneity. The strain field uniformity, induced by the growth of lightly doped Si epilayers, was also observed to vary qualitatively with location on a wafer. Back-reflection topographs verify that the quality of the epilayer-substrate interface improved as the thickness of the epilayer, or the gradient of dopant density across the interface, is reduced. Cellular strain-related structures, of the order of a few hundred μm in circumference, have been observed in the more stressed p on p + samples. Topographic results are in agreement with those obtained from X-ray diffraction measurements. Finally, an examination was carried out into the quality of commercially supplied 200 mm diameter Si wafers, revealing differences in the overall quality of the wafers.

Synchrotron x-ray topography analysis of GaAs layers grown on GaAssubstrates by liquid phase epitaxial lateral overgrowth

Synchrotron x-ray topography techniques in section and back-reflection geometries have been applied to silicon and tin doped GaAs layers grown by the liquid phase epitaxial lateral overgrowth (ELO) technique on (100) GaAs substrates. Back-reflection topographs show that the laterally grown parts of the ELO layers are nearly dislocation free in spite of a large density of defects in the substrate. Section topographs reveal novel and unique features which are attributed to the bending of the ELO layers induced by their interaction with the SiO2 mask.

Hollow-core screw dislocations in 6H-SiC single crystals: A test of Frank’s theory

Hollow-core screw dislocations, also known as “micropipes”, along the [0001] axis in 6H-SiC single crystals, have been studied by synchrotron white beam x-ray topography (SWBXT), scanning electron microscopy (SEM), and Nomarski optical microscopy (NOM). Using SWBXT, the magnitude of the Burgers vector of screw dislocations has been determined by measuring the following four parameters: (1) the diameter of dislocation images in back-reflection topographs; (2) the width of bimodal dislocation images in transmission topographs; (3) the magnitude of the tilt of lattice planes on both sides of dislocation core in projection topographs; and (4) the magnitude of the tilt of lattice planes in section topographs. The four methods show good agreement. SEM results reveal that micropipes emerge as holes on the as-grown surface, with their diameters ranging from about 0.1 to a few micrometers. Correlation between topographic images and SEM micrographs shows that micropipes are hollow-core screw dislocations with Burgers vector magnitudes from 2c to 7c (c is the lattice parameter along the [0001] axis). There is no empirical evidence that 1c dislocations have hollow cores. The Burgers vector magnitude of screw disloca-tions, b, and the diameter of associated micropipes, D, were fitted to Frank’s prediction for hollow-core screw dislocations: D = μb2/4π2γ, where μ is shear modulus, and γ is specific surface energy. Statistical analysis of the relationship between D and b2 shows that it is approximately linear, and the constant γ/μ ranges from 1.1 × 10−3 to 1.6 × l0−3 nm.

Quantitative analysis of screw dislocations in 6H−SiC single crystals

Screw dislocations along the [0001] axis in 6H−SiC single crystals have been studied extensively by Synchrotron White-Beam X-ray Topography (SWBXT), Scanning Electron Microscopy (SEM), and Nomarski Optical Microscopy (NOM). Using SWBXT, the magnitude of the Burgers vector of screw dislocations has been determined by measuring the following four parameters: 1) the diameter of dislocation images in back-reflection topographs; 2) the width of bimodal dislocation images in transmission topographs; 3) the magnitude of the tilt of lattice planes on both sides of dislocation core in projection topographs; and 4) the magnitude of the tilt of lattice planes in section topographs. The four methods show good agreement. SEM results reveal that micropipes in the form of hollow tubes run through the crystal emerging as holes on the as-grown surface, with their diameters ranging from about 0.1 to a few micrometers. Correlation between topographic images and SEM micrographs shows that micropipes are screw dislocations with Burgers vector magnitudes from 2c to 7c (c is the lattice constant along the [0001] axis). There isno were fitted to Frank’s prediction for hollow-core screw dislocations:D =μb2/4π 2 γ, where μ is the shear modulus, and γ is the specific surface energy. Statistical analysis of the relationship betweenD and b2 shows that it is approximately linear, and the constant, γ/μ, obtained from the slope, ranges from 1.1×10−3 to 1.6×10−3 nm.

Determination of Burgers Vector of Screw Dislocations in 6H-SiC Single Crystals by Synchrotron White Beam X-Ray Topography

AbstractIndividual screw dislocations along the [0001] axis in 6H-SiC single crystals have been characterized by means of Synchrotron White Beam X-ray Topography (SWBXT). The magnitude of the Burgers vector was determined from: (1) the diameter of circular diffraction-contrast images of dislocations in back-reflection topographs, (2) the width of bi-modal images associated with screw dislocations in transmission topographs, (3) the magnitude of the tilt of the lattice planes on both sides of dislocation core in projection topographs, and (4) also the magnitude of the tilt of the lattice planes in section topographs. All of the four methods showed reasonable consistency. The sense of the Burgers vector can also be deduced from the abovementioned tilt of the lattice planes. Results revealed that in 6H-SiC a variety of screw dislocations can be found with Burgers vector magnitude ranging from 1c to 7c (c is the lattice constant along [0001] axis). This work demonstrates that SWBXT can be used as a quantitative technique for detailed analyses of line defect configurations.

X-Ray Topographic Studies of Defects in PVT 6H-SiC Substrates and Epitaxial 6H-SiC Thin Films

ABSTRACTSynchrotron white beam X-ray topography has been used to characterize defect structures in (0001) 6H-SiC substrates grown by the sublimation physical vapor transport (PVT) technique as well as in 6H-SiC epitaxial thin films grown on these substrates. Defects revealed in 6H-SiC substrates include super screw dislocations and basal plane dislocations. It has been found that back-reflection topographs are particularly suitable for imaging such super screw dislocations as well as basal plane dislocations whenever transmission topography is not applicable. Epitaxial 6H-SiC thin films grown on such (0001) substrates (tilted a few degrees towards the a-axis) were also examined by using surface sensitive grazing Bragg-Laue topography. It has been shown that super screw dislocations were replicated in the epitaxial thin films but no basal plane dislocations were revealed in the thin films. Results from various topographic techniques are discussed.