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A kinematic (geometrical) diffraction simulation model has been developed to provide understanding of direct dislocation images on synchrotron white-beam X-ray topographs, and has been successfully applied to illustrate the contrast formation mechanisms involved in images of micropipe-related superscrew dislocations in silicon carbide crystals. The coincidence of the simulations with the contrast features of the superscrew dislocation images, recorded using a series of synchrotron topography techniques, shows that this model is capable of revealing the detailed diffraction behavior of the highly distorted region around the dislocation core and determining the quantitative characteristics of the dislocations. The simulation technique is thus demonstrated to be a simple but efficient method for interpretation of synchrotron topographs, and may be applied to explain the topographic contrast characters of general crystal defects.

Synchrotron white beam x-ray topography (SWBXT) and Nomarski optical microscopy (NOM) have been used to characterize 4H-SiC epilayers and to study the character of triangular inclusions therein. 4H-SiC substrates misoriented by a range of angles from (0001), as well as (1 100) and (1120) oriented substrates were used. For epilayers grown on substrates misoriented by 3.5° from (0001) toward , the triangular inclusions were identified as consisting of two 3C-SiC structural configurations which are related to each other by a 180° rotation about the [111] axis. The epitaxial relationships between the 3C inclusions and the 4H-SiC epilayers (or substrates) were also determined. No evidence was found for the nucleation of 3C-SiC inclusions at superscrew dislocations (along the [0001] axis) in the 4H-SiC substrates. Increasing the off-axis angle of the substrates from 3.5 to 6.5° was found to greatly suppress the formation of the triangular inclusions. In the case of substrates misoriented by 8.0° from (0001) toward , the triangular inclusions were virtually eliminated. The crystalline quality of 4H-SiC epilayers grown on the substrates misoriented by 8.0° from (0001) was very good. For the (1100) and (1120) samples, there is no indication of 3C-SiC inclusions in the epilayers. Possible formation mechanisms and the morphology of 3C-SiC inclusions are discussed.

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.

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.

Molecular beam epitaxy (MBE) regrowth on a corrugated surface is improved dramatically using a novel technique by which not only a clean surface can be obtained but also the shape of grating can be precisely controlled. A GaAlAs/GaAs multi-quantum well gain-coupled distributed feedback (DFB) laser with an absorptive grating is fabricated all by MBE for the first time. DFB mode oscillation within a range of at least 80/spl deg/C is achieved. Stable single longitudinal mode oscillation is maintained up to 20 mW. >

Taking into consideration of gain coupling, characteristics such as single-mode selectivity, chirping and light output efficiency of photonic integrated circuits (PIC) integrating distributed feedback (DFB) semiconductor lasers and electroabsorption modulators are analyzed for the first time. Devices with conventional index-coupled (IC), quarter-wave-shifted (QWS) and gain-coupled (GC) DFB laser sections are calculated and compared. The chirping of GC-PIC is nearly the same as that of IC-PIC and QWS-PIC, but the single-mode selectivity of GC-PIC is much better. With a [HR-AR] coating , which is advantageous for high-power operation, GC-PIC shows a much higher probability of single-mode operation than IC-PIC, while QWS-PIC does not function at all.

Ferroelectric domains and strains in a barium titanate (BaTiO3) single crystal, grown by the top-seeded solution growth technique, have been investigated by synchrotron white-beam X-ray topography (SWBXT) and optical microscopy. Conventional radiation X-ray topography studies in this system had previously been hampered by the presence of internal strain fields which made it difficult to observe the overall domain structures. SWBXT was found to be more suitable for this kind of study, with the overall domain configuration in the (100) crystal plate being readily observed. Also the condition of ΔP·g = 0 is almost obeyed for the 90° a–a walls which are predominant in a thin crystal. The 90° a–c wall satisfied the invisibility criterion, leaving only residual strain images. Some curved images of 90° a–a walls were observed, which could be interpreted as resulting from the interference between the 90° a–a domain walls and the internal strain field.