Growth And Characterization Of N-polar GaN-based Materials

N-polar GaN based materials have been widely applied in the fabrication of high-electron-mobility transistors(HEMTs),light emitting diodes(LEDs)and photoelectronic detectors,because of its advanced characteristics on heterojunction and ohmic contact.However,the growth of N-polar GaN-based materials with device quality remains a big challenge,due to high density of threading dislocations arising from the mismatches in lattice constant and thermal expansion coefficient between the sapphire substrates and N-polar GaN epi-layers.In this study,the N-polar GaN epi-layers were grown on(0001)-plane sapphire substrates in a low pressure(70 Torr)metal-organic chemical vapor deposition(MOCVD)system in order to investigate the effects of growth conditions on the characteristics.A reformed flow-rate-modulation technology was developed for the MOCVD growth of N-polar GaN epitaxial layer.The major research contents and the results achieved in this study are listed as below:Primary growth conditions of N-polar GaN epi-layers were investigated,such as the orientation of substrate,the nitridation temperature,the growth temperature and time of the GaN nucleation layer,the ?/? ratio,and the thickness of the N-polar GaN epi-layers.The results revealed that the hexagonal morphology of the N-polar GaN epi-layers can,be eliminated efficiently with by using the c-plane sapphire substrates with a vincinal degree of 2° towards m-plane.Besides,the optimized growth conditions for the N-polar GaN epi-layers were verified to be the nitridation temperature of 950 ?,the GaN nucleation layer grown at 470 ? for 10 min,a ?/? ratio of 512.Moreover,the results also demonstrated that the density of hexagonal cones decreased with the increase of the thickness of N-polar GaN epi-layers.Finally,the KOH solution etching method was proved to be effective for the judgement of the polarity of the grown GaN epi-layers.A reformed flow-rate-modulation technology was developed for the MOCVD growth of the N-polar GaN epitaxial layers on(0001)-plane sapphire substrates,based on the characterization results of the grown N-polar GaN epitaxial layers and the conventional flow-rate-modulation technology.To improve the crystalline quality of the N-polar GaN epitaxial layers,a GaN nucleation layer was grown at relatively low temperature with carefully-controlled pulsed supply of Ga source and was characterized with atomic force microscope(AFM).Furthermore,the electrical and optical properties of the grown N-polar GaN epitaxial layers were investigated extensively by means of Hall effect,photoluminescence(PL),and X-ray diffraction(XRD)measurements.The characterization results revealed that as compared with the N-polar GaN epitaxial layer grown over the conventional GaN nucleation layer which was deposited on the sapphire substrate with continuous supply of both N and Ga sources,the electrical and optical properties of the N-polar GaN epitaxial layer grown with the pulsed supply of Ga source for the GaN nucleation layer were significantly improved.N-polar AlGaN epi-layers with an Al composition of?10%were grown for the first time on(0001)-oriented c-plane sapphire substrates by MOCVD.Special attention was paid on the nitridation process in the epitaxial growth of N-polar AlGaN films.The optical microscope(OM)measurement results demonstrated that the size of hexagonal defects on the surface of N-polar AlGaN epi-layer decreased dramatically with our innovative nitridation process.Furthermore,the structural,electrical,and optical properties of N-polar AlGaN epi-layers were characterized extensively by means of X-ray rocking curves(XRCs),Hall effect,and PL spectroscopy.It was found that the defects-related blue-band(BB)emission was greatly suppressed and the threading dislocation(TD)density was significantly reduced by the optimized nitridation process.These characterization results reveal that the reformed nitridation process plays a vital role in the improvement in the crystalline quality as well as the electrical and optical properties of the N-polar AlGaN epi-layers.