Browsing by Subject "Residual stresses"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • Thumbnail Image
    ItemOpen Access
    Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD
    (Springer New York LLC, 2017) Arslan, E.; Öztürk, M. K.; Tıraş, E.; Tıraş, T.; Özçelik, S.; Özbay, Ekmel
    High-resistive GaN (>108 Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1−xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.
  • Thumbnail Image
    ItemOpen Access
    Imaging flaws close to surface using focused surface acoustic waves
    (IEEE, 1986-11) Köymen, Hayrettin; Atalar, Abdullah; Çiloğlu, T.; Önder, Murat; Uzel, Ç.; Yavuz, H.
    The resolving power and detection ability of the focused surface acoustic wave (SAW) imaging modality is investigated in this paper. In this mode of imaging, conical bulk acoustic waves are used to generate and focus leaky surface acoustic waves on smooth surfaces of materials. Imaging systems built using this technique has diffraction limited focusing property. An imaging system using this focusing principle has been built, operating at 1.5 and 20 MHz. A slow mechanical scanning system controlled by a personal computer scans the surface of the object, and the data is acquired by the computer to generate a color or a black and white image on its graphic screen. The results of the initial experiments show that the inaging system is very sensitive to the grain structure and possible residual stresses on the surface of the object. It can resolve subsurface gratings of spacing less than a SAW wavelength very close to surface. The imaging system is inherently zero background, providing a high sensitivity not found in similar systems.