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Research Of GaN Growth On The Porous Substrates By HVPE

Posted on:2016-07-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y B DaiFull Text:PDF
GTID:1108330461984402Subject:Materials science
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GaN based Ⅲ/Ⅴ semiconductors are recognized as excellent materials for applications in microelectronic devices and optoelectronic devices. However, the development of high-performance GaN-based devices has been limited by the mismatch caused by hetero-epitaxial growth. It is widely recognized that the growth of bulk GaN single crystals with subsequent wafer slicing is a possible solution. The HVPE technology for obtaining high quality substrates of bulk GaN has been widely investigated. The self-separation of GaN crystal from substrate is very important. In this dissertation, the porous substrates were prepared by several methods to serve as the substrate of GaN crystal growth using HVPE. Specific as follows:1. The influence of atomic diffusion process on the growth model, the surface morphology and microstructure of GaN epitaxial layer was studied systematically. The relationship of Ⅴ/Ⅲ, stress and atomic mobility was studied. Then stress was connected with the growth kinetics by the atomic mobility. These results were helpful for the GaN growth on substrates with different cases of stress. High quality of GaN was grown on MOCVD-GaN/Al2O3 templates (MGA) and MOCVD-GaN/6H-SiC templates (MGS) through a hydride vapour phase epitaxy (HVPE) process in which the Ⅴ/Ⅲ ratio was controlled. It is found that a larger Ⅴ/Ⅲ ratio results in a higher atomic mobility. Stress changes the activation energy of surface which is related to the atomic mobility. Whereby the principle of offsetting the effect of stress on crystal growth by adjusting Ⅴ/Ⅲ was confirmed. The model of stress and surface kinetics extends the growth mechanism of GaN epitaxial, which is valuable for preparation of GaN crystal on the different novel types of substrate.2. A porous template was prepared under appropriate annealing conditions (HTAP, high temperature annealing porous). The HTAP template was used for the growth of free-standing GaN by HVPE. The porous structure in the HTAP is formed based on the the selective decomposition of GaN. The formation of the porous structure is related to the annealing temperature, annealing time, annealing atmosphere, doping and dislocation of the substrates. The self-separated GaN crystal grown on the porous template showed a smaller full width at half maximum (FWHM) for (002) and (102) reflections in the HRXRD measurement than that grown on the MGA template. The PL results indicated that the optical quality of the GaN crystal on the porous template was improved and the dislocation density decreased. The Raman results showed that the stress in the GaN crystal grown on the porous template is much smaller than that on the MGA template.3. An original high temperature annealing (HTA) technology assisted by SiO2 patterned masks, has been established to fabricate a porous substrate with layers of inverted pyramid structures. High quality of GaN has been prepared using this porous substrate by HVPE. The decomposition is selective for the dislocation at the appropriate annealing temperature, because the activation energy at a dislocation site is smaller than that at a perfect surface. So the decomposition is perfectly selective to obtain a large number of inverted pyramid porous structures. Porosity of porous GaN layer is related to the dislocation density. And there is some crystallographic orientation dependence which is believed to be the more chemically inert plane as a decomposition-stop plane. The formation of porous structure is sensitive to the lateral decomposition rate and vertical decomposition rate which is controlled by SiO2 patterned masks. The growth experiments demonstrated that such porous substrates were mechanically fragile because of the voids. The porous structures are suitable to be used as a flexible release layer for the growth of self-separated GaN crystals with low stress and defect density.4. A uniform porous structure was formatted between sapphire substrate and GaN crystals in the MOCVD-GaN epitaxial wafer after the two-step etching. Self-separated GaN crystals were successfully prepared using HVPE method on the porous substrate. The thermal phosphoric acid could reach the interface between the sapphire and the GaN crystal though the pipeline with tiny diameter, which was prepared in the internal MOCVD-GaN epitaxial films using the optimized preparation of electrochemical etching. The porous structure with a smooth surface was formed by the corrosion of the N face of GaN in the thermal phosphoric acid, because the N face of GaN is easier to be etched than the Ga face of GaN. Morphology of the porous structure is related to crystal quality, doping condition, electrolyte liquid, electrochemical potential and electrochemical device. It is proved that it is effective to improve the quality, reduce the residual stress and promote the separation using the porous substrates smooth surface and porous interface.
Keywords/Search Tags:HVPE, GaN, annealing, etching, porous substrate
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