MOCVD Growth And Characteristics Reseach Of N-polar GaN-based Nitride

Compared with metal polar materials,N-polar GaN-based materials have opposite polarization electric field and different atom arrangement order,which makes them have many excellent properties and great potential for wide applications.For example,the N-polar GaN/AlGaN-based HEMT possesses a unique band structure with a naturally formed back barrier which is powerful to enhance the electron confinement,and the N-polar GaN layer is easy to form good ohmic contact with the electrode materials.However,the surface atoms of N-polar materials have high chemical activity,which makes metal atoms with low mobility difficult to move freely on the surface,giving rise to the formation of a large number of hexagonal defects.In order to improve the crystalline quality and surface morphology of N-polar GaN-based materials,metal-organic chemical vapor deposition（MOCVD）technology was used to grow N-polar AlGaN epi-layers on c-plane sapphire substrate,and the material properties were studied in depth.The main research contents and results of this thesis are as follows:1.The relationship between the nucleation process and growth temperature of high temperature Al N（HT-Al N）layer was analyzed,and the N-polar AlGaN epi-layer with high crystalline quality and smooth surface morphology was obtained by setting the growth temperature of HT-Al N layer at 990℃.Si N_xinter-layer with high coverage rate was inserted between the optimized HT-Al N and epitaxial layer.It was found that the lateral side of the Si N_x nano-particles could provide nucleation sites and promote epitaxial lateral overgrowth.Therefore,the crystalline quality of epitaxial films.was improved successfully.2.N-polar AlGaN films were grown by NH₃ pulsed-flow technology with different duty ratio.It was found that metal-rich environment could be formed in the reaction chamber with the technology,and a large number of Al atoms rapidly accumulated near the hexagonal defects,resulting in the formation of the Al-GaN and Al-AlGaN clusters in a disordered phase.With the decrease in duty ratio from 100%to 16.7%,the AFM RMS decreases from 36.6 to 9.3 nm,indicating that the NH₃ pulsed-flow technology is useful to improve the surface morphology of AlGaN epi-layers.3.Based on the NH₃ pulsed-flow technology,an NH₃ purge term was added to develop the reformed pulsed-flow technology.The reformed pulsed-flow technology can promote the migration of Al atoms that have not migrated to suitable sites in time under metal-rich environment.When the NH₃ purge term was increased from 2 to 8 s,the XRC FWHM decreased gradually from 915 to 776 arcsec,and the AFM RMS value decreases gradually from 9.2 to 5.7 nm,indicating that the crystalline quality of N-polar AlGaN films have been successfully improved.In addition,the epitaxial films with high crystalline quality and smooth surface morphology have been obtained by synthesizing the optimum process parameters of the NH₃ pulsed-flow technology and the reformed pulsed-flow technology in sequence.