Study On Preparation Method And Mechanism Of Graphene-assisted Flexible Nitride Materials

GaN and AlN,as the third-generation semiconductors,are widely used in microwave radio frequency,power electronics and optoelectronics.In the 5G era,there will be more abundant application scenarios,such as 5G base stations,satellite communications,military radar and so on.By virtue of its unique physical and chemical properties,the combination of two-dimensional materials with group III nitrides in quasi-van der Waals epitaxy and two-dimensional electrodes has injected new development momentum into the application of nitrides.Among them,the epitaxial growth of nitride with graphene has become a major research focus in recent years.In this paper,the preparation and growth mechanism of flexible GaN thin films are studied in depth.the main work and achievements are as follows:（1）Graphene/LT-AlN/HT-AlN buffer layer is proposed to grow high-quality single crystal GaN thin films,and large-area flexible GaN thin films are obtained by transfer method.The growth mechanism of AlN was analyzed by Raman strain of graphene and SEM images.The2D peak of the Raman spectrum of the transferred graphene is shifted from 2684.10 cm^-1 to2711.38 cm^-1 after the growth of LT-AlN/HT-AlN,indicating that graphene is subjected to compressive stress during the growth process.It is observed that AlN at the grain boundary of graphene presents a variety of crystal orientations,and the combination is poor,while AlN on the unit cell of graphene presents a single crystal orientation and combines well.The results show that large-area single crystal of graphene can improve the quality of AlN.The LT-AlN/HT-AlN/GaN structure was transferred to Si substrate and flexible substrate by thermal stripping tape.the FWHM of rocking curve of（002）and（102）crystal plane of GaN thin film was 202 arcsec and 432 arcsec,respectively.The roughness of GaN thin film was0.188 nm,and the stripped GaN thin film area was 2×1.5cm²,indicating that this method can realize large area and low-cost transfer.It was found that graphene was not stripped along with LT-AlN/HT-AlN/GaN structure,but remained on the original substrate,indicating that the original substrate and graphene can be reused to prepare strippable GaN.（2）A method of sputtered AlN on sapphire substrate and then transferring graphene was proposed to prepare higher quality single crystal GaN films.the growth mechanism of GaN was analyzed by first-principles simulation and high-performance ultraviolet LED devices were developed.According to the first-principles calculation,the sputtered AlN/Graphene composite buffer layer has stronger electrostatic potential,and it is predicted that the composite buffer layer has obvious effect on improving the quality of GaN.Comparing the Raman test results of GaN on graphene buffer layer,sputtered AlN buffer layer and sputtered AlN/Graphene composite buffer layer,it is found that graphene is helpful to the growth of stress-free GaN.In sputtered AlN/Graphene composite buffer layer,the quality of GaN films on single-layer graphene and double-layer graphene is compared.The results show that GaN films on single-layer graphene have higher quality,and more folds on double-layer graphene produce more dislocations in GaN films.The flexible GaN films with Raman E₂^H peak at568 cm^-1,（002）plane rocking curve FWHM of 186 arcsec,（102）plane rocking curve FWHM of 326 arcsec and RMS roughness of 0.269nm were obtained.～400nm UV LED devices were fabricated based on GaN thin films grown on sputtered AlN/Graphene composite buffer layer.After LED full-structure epitaxy and device standard die-casting process,a high-performance LED with a turn-on voltage of 2.8V and a leakage current of2n A@-2V is obtained,which proves that graphene will not have a negative impact on the electrical performance of LED.The LED devices based on GaN epitaxial layer grown on the composite buffer layer verify the performance of GaN materials,which provides a new idea for the application of graphene and other two-dimensional materials in optoelectronic devices.（3）Two growth modes of nitride on graphene are proposed.The research on the growth mechanism of nitride on Graphene/LT-AlN/HT-AlN buffer layer can be divided into two interfaces:Graphene/LT-AlN and HT-AlN/GaN.Growth of LT-AlN on Graphene/LT-AlN interface is mainly influenced by van der waals force of graphene.GaN thin film growth on LT-AlN/HT-AlN belongs to the layered growth mode of heteroepitaxial growth.At last,the interface Graphene/AlN interface was stripped off,and the obtained structure was LT-AlN/HT-AlN/GaN.Growth mechanism of GaN thin films on AlN/Graphene composite buffer layer by sputtered is mainly Graphene/GaN interface.At this interface,the growth of GaN is not only affected by van der waals force,but also the sputtered AlN on the lower layer of graphene brings higher electrostatic potential to the surface of graphene,which improves the nucleation probability of GaN on graphene.graphene can accelerate the lateral epitaxy of GaN,promote the generation of short-range stacking faults and block the extension of threading dislocations,thus reducing the dislocation density in the epitaxial layer.The interface of GaN grown on this buffer layer is Graphene/GaN interface,GaN thin films with high quality can be obtained.In conclusion,high quality GaN thin films were obtained and LED devices are fabricated on this basis,which verifies the material properties.At the same time,two growth modes of nitride on graphene are analyzed theoretically based on the experimental data,which provides a new way for the application of GaN materials in flexible devices.