Research On The Epitaxial Growth And Characteristics Of N-polar AlGaN Films

Compared with conventional metal-polar III-nitrides,N-polar(0001)-oriented III-nitrides have lots of conspicuous advantages,such as low resistance ohmic contacts,two dimensional electron gas(2DEG)with high mobility at hetero-interface,and the polarization fields with the direction reverse from that of metal-polar III-nitrides.Therefore,N-polar(0001)-oriented III-nitrides are of great potentials for making advanced electronic and optoelectronic devices.However,N-polar AlGaN-based epitaxial materials have not been applied in the fabrication of electronic devices in large scale due to their unsatisfactory surface morphology and crystal quality as compared with metal-polar AlGaN-based materials.In addition,the domestic and overseas research works on N-polar AlGaN epi-layers are still at the initial stage.Therefore,it is meaningful to conduct systematic researches on the N-polar AlGaN materials.In this dissertation,the epitaxial growth of N-polar AlGaN materials on c-plane sapphire were performed by using metalorganic chemical vapor deposition(MOCVD).The Si-doped N-polar AlGaN epi-layers were also studied.Moreover,a technology featured with SiN_x insertion layer was developed for the growth of the N-polar GaN sub-micron hexagonal prisms.The major research contents and the results achieved in this dissertation are listed as follows:1.The N-polar AlGaN epi-layers were deposited on a high-temperature-grown AlN(HT-AlN)buffer which was directly grown on the c-plane sapphire substrates after the nitridation process.The effects of the growth conditions,such as nitridation time,growth temperature,and growth rate on the surface morphology,crystalline quality,and the electrical properties of the N-polar AlGaN epi-layers were studied in detail,respectively.The characterization results indicate that a prior nitridation treatment time of 3.5 min.for the sapphire substrate is most effective for the growth of the N-polar AlGaN epitaxial layers,resulting in the smoothest surface morphology,the highest crystalline quality,and the best electrical properties.In addition,with decreasing the growth temperature for the N-polar AlGaN layer,the Al composition of AlGaN layer decreased,and the hexagonal pyramids morphology gradually manifested.On the other hand,as the growth rate of the AlGaN layer decreased,the Al composition of AlGaN layer increased,whereas the size of hexagonal pyramids decreased.2.The Si-doped N-polar AlGaN epi-layers were successfully deposited on c-plane sapphire substrates.The effects of Si-doping levels on surface morphology,crystalline quality,and electrical characteristics of the N-polar AlGaN epi-layer samples were studied systematically.The research results show that the size of hexagonal pyramids significantly increased due to the incorporation of Si atoms into the epi-layers.Excess Si atoms that were doped into the N-polar AlGaN epi-layers existed in the form of Si_x or SiN_x clusters,which acted as a center of impurities,and thus a degraded crystalline quality.In addition,the electron concentrations of N-polar AlGaN epi-layers increased abruptly at first,and then became saturated.3.A SiNx insertion layer was grown on prior nitrided c-plane sapphire substrates using MOCVD technology,and the self-assembly N-polar GaN micro-hexagonal prisms were successfully fabricated subsequently.The SEM characterization results showed that the density of the hexagonal prisms diminished with the increase in growth time for SiN_x insertion layer,and the size of individual hexagonal prism also slightly decreased.