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The performance of semiconductor devices depends strongly upon the materials microstructure. Therefore the microstructural control to reduce the density of lattice defects is intrinsically important for fabrication of high performance devices. In this research, the microstructures have been analysed in detail and the mechanisms of microstructural changes in AlN thin films have been clarified for the establishment of the growth method of AlN on sapphire substrate. AlN templates for overgrowth of AlN thin layer were made by growing an AlN buffer layer grown on a (0001) sapphire substrate by the metal organic vapor phase epitaxy (MOVPE) growth process in a rather conventional way, but annealing treatments were added before and after the deposition of AlN buffer layer. The surface roughness of AlN was observed with an atomic force microscope (AFM) and XRC .The cross section of AlN template was observed by using conventional transmission electron microscopy (CTEM) at 200kV. High-angle annular dark-field (HAADF) scanning-TEM images were also observed with a scanning transmission electron microscope (STEM) at 300 kV. Thin foil specimens for TEM observation were made using a focused ion beam (FIB) mill with accelerating voltage of 15kV~3 kV. Prior to the deposition of a MT-AlN layer, the sapphire substrate was annealed at a high temperature TAn under the atmosphere of H2. For TAn<1250°C, the crystallinity improved but twisting domains appear above the temperature. Threading disloactions type c, type (a + c) and twisting domains with 200 - 500 nm in diameter along [2-1-10], with 108 cm-2 dislocation density was observed. However when the temperature was increased to 1350°C the threading dislocations were reduced and enhanced the adhesion between sapphire and AlN buffer layer. On the other hand, post deposition annealing at a high temperature between 1500oC to 1700oC for 2 hours under the atmosphere of N2+CO was carried out. Cross sectional TEM has revealed that after annealing at 1500oC, cone-shaped domains and threading dislocations remained. The morphology and the diffraction condition for the image contrast strongly suggest that the domains were inversion domains. Threading dislocations type-a and type-a+c dislocations were visible for g =01-10 under the two beam condition. However, after annealing at 1550oC, the inversion domains coalesced with each other to leave a single inversion domain boundary running in a zigzag laterally at the center of AlN buffer layer. The upper layer has Al- polarity where else the lower layer has N-polarity according to the HAADF and CBED images. The inversion domain boundary become smooth and flatter with the rising annealing temperature. It can be considered that domain boundaries can be eliminated by the lateral growth and mutual coalescence of domains even if boundaries penetrate vertically up to the top surface. The surface of MT-AlN buffer was finely rugged before the annealing, but became coarser and smoother with annealing. The changes in the surface morphology indicates the occurrence of grain coalescence. The density of TDs was reduced to 5×108 cm-2 after annealing at 1650oC roughly. In conclusion from this present research, it has been confirmed that pre deposition and post deposition annealing has been an effective treatment to control the microstructure and reduce dislocation density for advancement in the high quality fabrication of semiconductor devices.
Última atualização: 2016-07-24
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This book is partially my doctoral thesis to highlight the value of negotiation effort within requirements engineering process. The position of negotiation within the body of software engineering knowledge is presented and how the negotiation fits in is explained. The purpose and the strength of negotiation are elaborated to instil awareness among the software engineering community. This book also includes the justification of quality attributes feasibly achieved and measured at a very early stage of software development process. The metrics and measurement techniques are also presented. This book is written for practitioners who are looking for alternative ways of performing requirements engineering, for researchers who are exploring better ways of doing things and for academics who are looking for exciting things to share with their students.
malaysia language translation to english
Última atualização: 2014-08-27
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