Development of GaN Substrates and Homoepitaxy
Currently, III-nitride devices are almost grown on sapphire substrates owing to their reliability and cost. However, their performance is limited due to the heteroepitaxy which accompanies various issues, such as lattice mismatch, the difference of thermal expansion coefficients, high dislocation density, biaxial stresses, wafer bow, and so on. The main objectives of this dissertation are the development of GaN substrates and the study of homoepitaxial growth on GaN substrates. On the part of developing GaN substrates, the GaN thick films were grown by hydride vapor-phase epitaxy (HVPE) and separated from the sapphire substrates utilizing the laser lift-off (LLO) technique. Then, the GaN thick films were regrown on the Ga- or N-faces of freestanding GaN substrates to improve the crystal quality, substrate thickness, and wafer bow. Furthermore, the dry etching method was applied here to significantly reduce the wafer bow of freestanding GaN substrates. The bowing radius of a freestanding GaN substrate can be significantly increased from 1.5 to 17.8 m by ICP etching. It can further improve the fabrication efficiency and yield of the polish and chip processes. On the part of homoepitaxially growing III-nitride layers on GaN substrates by metal-organic chemical vapor deposition (MOCVD), the GaN substrates were first treated by the methods of dry etching and chemical-mechanical polish (CMP) to diminish the damage layer caused by mechanical polish (MP). And the undoped GaN thin films were homoepitaxially grown on GaN substrates by two-step growth. Overall, low dislocation density of 7×106cm-2; narrow (002) and (102) XRD FWHM of 92 and 54 arcsec can be obtained by homoepitaxially overgrowing GaN thin films on GaN substrates.
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