Optimization And Characteristics Research In Crystalline Quality Of N-polar GaN-based Materials

Since the N-polarity group ? nitride material has different spontaneous polarization and piezoelectric polarization direction as compared with the metal-polarity group ? nitride material,the N-polarity group ? nitride material has the following incomparable advantages:enhanced devices can be realized by using AlGaN as the natural back barrier;low ohmic contact which is conducive to the realization of high-frequency and DC semiconductor devices;high surface chemical activity which exhibits excellent performance in the fields of high-frequency HEMT,high-sensitivity detectors and LEDs.However,the formation of hexagonal pyramid hillock-like defects on surface of N-polar GaN-based materials which is probably related to the very limited migration ability for group ? adatoms on the surface of N-polar materials,makes the crystalline quality of epitaxial films too poor to meet the requirements of devices fabrication.In this dissertation,the optimization of growth parameters of N-polar GaN epi-layer on nominally on-axis c-plane sapphire substrate was carried out in a metal organic chemical vapor deposition(MOCVD)reactor.Furthermore,the AlN seeding layer was deposited by utilizing the newly-developed flow-modulation technology,and the physical mechanism of polarity reversion from Al(Ga)-polar to N-polar of the subsequently grown AlGaN film was explored in detail.In addition,some researches have been conducted to investigate the influence of the substrate orientation on the properties of the N-polar AlGaN film by changing the miscut angle of c-plane sapphire substrate.The major research contents and the results achieved in this dissertation are listed as follows:1.Growth conditions such as pressure and ?/?ratio value that affect the growth rate were optimized,which led to significant improvements in the crystalline quality and surface morphology of the N-polar GaN epi-layer.Simultaneously,nitridation treatment time was optimized.The research results indicated that when the chamber pressure is 40 Torr,the defect size of the N-polar GaN epi-layer grown with a ?/? ratio value of 996 is significantly reduced,and the adjacent hexagonal pyramid defects are merged obviously on the steps.In addition,with increasing the nitridation treatment time,the defect size of the N-polar GaN epi-layer first increases and then decreases,whereas the carbon atoms is hardly unintentionally doped.The relatively good crystalline quality of N-polar GaN epi-layer was obtained with a nitridation treatment time of 2.5 min.2.The development and advantages of the flow modulation technology for the growth of N-polar epitaxial film were reviewed.The innovative flow-modulation technology was developed in this study for the deposition of AlN seeding layer,significantly improving the surface roughness of AlN seeding layer.Meanwhile,the polarity reversion was explained based on the atom migration mechanism.On the other hand,N-polar AlGaN films with different ?/? ratio values were grown on the AlN seeding layer deposited by using flow modulation technology,according to which the flow rate of NH₃ was fixed but the flow rates of TMA and TMG were changed.Due to the competition between the adduct formation pathway and the thermal decomposition pathway in the vapor deposition process,with the increase in?/?ratio,the crystalline quality of the films varied in a fluctuating mode.Furthermore,the defect size of N-polar AlGaN films decreased monotonously with the increase in?/?ratio,and the Alcomposition of AlGaN films can be increased to as high as 21%.3.The N-polar AlN/AlGaN films were grown on c-plane sapphire substrates with nominal miscut angles of 2°and 0.6°toward the m-axis.It was speculated that a suitable miscut sapphire substrate might enhance the migration ability of Al(Ga)atoms on the surface of the N-plane AlGaN film,effectively suppressesing the blue-band emission caused by defects.Meanwhile,step-flow mode growth was actually observed for N-polar AlGaN film,and the hexagonal pyramid hillock-like defects was almost completely disappeared with the improvement in surface roughness.