Growth And Properties Of Freestanding GaN Crystal By HVPE

GaN is a very important direct wide bandgap semiconductor material and is widely used in optical electronic devices and power electronic devices. Becauce of the lack of GaN crystal substrate, most of GaN is grown on foreign substrate. The lattice mismatch and thermal mismatch between GaN and the substrate induces high dislocation density in GaN epilayer, which greatly reduces the performance and lifetime of GaN based devices. Therefore, GaN crystal substrate is in urgent need. Hydride vapor phase epitaxy (HVPE) method is considered to be the most promising growth method of GaN crystal. However, most of GaN grown by HVPE is on foreign substrate, so separation between GaN and foreign substrate become a big chanllenge to obtain GaN crystal substrate. At present, the most common methods to separate GaN and foreign substrate are mechanical lapping and laser lift-off. In addition, the GaN crystal can self-separate from the foreign substrate with special structure after growth.In this work, the HVPE growth process of GaN crystal was optimized. Freestanding GaN was obtained by using low temperature buffer layer on sapphire substrate and C face SiC substrate. The main research achievements are as follows:1. The influence of ?/? on the crystal quality and photoelectric properties of GaN was studied. According to our rearch, with the increase of ?/?, the dislocation density decreased, the residual stress increased, the carrier concentration decreased, the electron mobility increased, the optical property became better. When the ?/? increased from 10 to 50, the dislocation density decreased one order of magnitude, the residual stress increased three times. With the increase of ?/?, the diffusion length of Ga atom became longer. So the growth mode transform from 3D mode to 2D mode. As a result, dislocation density decreased and the residual stress increased. The surface EBSD of GaN showed that, the quality of EBSD Kikuchi patterns became better and the angle of the deviation from the ideal situation for crystal orientation became larger. SIMS was used to identify the impurities in GaN, according to the results, with the increase of ?/?, the concentration of Si increased and the concentration of O decreased, the concentration of C decreased at first and then increased and the concentration of H was basically unchanged. With the increase of ?/?, the carrier concentration of GaN decreased from 2.118×1018cm-3 to 7.414×1017cm-3, the electron mobility increased from 65 cm2/V sec to 240 cm2/V sec, the band edge emission of PL became stronger and the yellow luminescence band became weaker.2. A method of freestanding GaN crystal growth on sapphire substrate using GaN low temperature buffer layer was proposed. The effect of thickness and annealing temperature of the low temperature buffer layer on the crystal quality of GaN was studied in order to determine the best buffer layer process and acquire high quality freestanding GaN. When the thickness of buffer layer was 800nm, and the annealing temperature was 1090?, the quality of HT-GaN was the best, the FWHMs of GaN (002) and (102) were 250arcsec and 215arcsec, the residual stress was small and the optical property was the best. According to the EBSD result, the BS value was relatively high which means the internal stress was small. The HT-GaN was separated from the substrate with a thickness of more than 300?m when the best growth condition was used.3. High quality freestanding GaN crystal was directly grown on C face SiC by using GaN low temperature buffer layer. The comparison of HT-GaN grown under different low temperature buffer layer process and ?/? was made. When the HT-GaN was grown without buffer layer, it was polycrystal and could not separate from the substrate. When the HT-GaN was grown on the buffer layer without annealing process, it was polycrystal but could separate from the substrate. According to the investigation, the low temperature buffer layer and annealing process are necessary for acquiring high quality GaN and reducing residual stress. Through the comparison of HT-GaN grown on low temperature with different thickness and the ?/? during the HT-GaN growth, when the buffer layer was grown for 30 min, annealed for 10min and the ?/? was 90, the crystal quality was the best. GaN was self-separated from SiC after growth and SiC substrate could be used repeatedly. The HT-GaN grown with the above condition was characterized. The GaN crystal was Ga face GaN. The crystal quality was good, the FWHM of (002) was 261arcsec and the FWHM of (102) was 272arcsec. The HT-GaN was stress free. The EBSD mapping made at the cross section of GaN proved that with the increase of thickness the crystal quality became better and the internal stress decreased. The PL test indicated good optical property of freestanding GaN.4. According to the realationship between strain energy of dislocation and free energy for annealing pits formation, a new method to characterize the dislocation in GaN-high temperature annealing method was carried out. The strain energy of dislocation reduces the free energy for annealing pits formation, as a result, the pits appear at the dislocation termination. As the strain energy of screw and mixed dislocation is larger than edage dislocation, the screw and mixed dislocation is corresponding to large annealing pits and edage dislocation is corresponding to small dislocation. So we can characterize dislocations according to the difference of annealing pits. The morphology of annealing pits in the GaN surface at different annealing temperature was investigated, and the best annealing condition was conformed as annealing 10min at 1100?. According to the AFM results, there were two kind of annealing pits:big annealing pits and small annealing pits and the shape of big annealing pits was either inverted hexagonal pyramid shape or inverted trapezoidal shape. The face of the big pits was {2023}.Comparasion between high temperature annealing method, KOH-NaOH etching method and CL method was made, and the annealing method proved to be effective to identify dislocation type and evaluate dislocation density. The TEM results showed that the annealing pits were terminal of dislocations and the large pit corresponding to mixed and screw dislocations while small pits corresponding to edge dislocations.