Van Der Waals Eopitaxy Of Ultra-low Stress GaN Films On Graphene

Gallium nitride(GaN),a typical wide band gap semiconductor material with excellent photoelectric properties and stability,is very suitable for optoelectronic and microwave radio frequency devices in the fields of lighting and display,fifth generation(5G)communication,high frequency and high-power photoelectric equipment etc.At present,achieving batch production of stress-free GaN materials is the primary condition for the rapid development of GaN devices.Due to the inadequacy of native GaN substrate,most of GaN-based devices are mainly synthesized on sapphire,silicon carbide(SiC)or silicon(Si)substrates by heteroepitaxy.However,due to the existence of the lattice mismatch and thermal mismatch between the substrate and the epitaxial layer during heteroepitaxial,the large residual biaxial stress generated during the heteroepitaxy will result in microcracks or even fractures in the GaN material.The existence of residual stress limits the improvement of the quality of GaN crystal,which is not benefit to the subsequent preparation of devices.Graphene,a typical two-dimensional(2D)layered material with many fascinating properties,has been widely focused on.The growth of GaN films on graphene mitigate the lattice mismatch and improve the crystal quality of the films as well as alleviate biaxial stress generated by thermal mismatch,which ensure the epitaxial of high crystalline quality of stress-free GaN films.Therefore,the epitaxy of GaN on graphene has become an imported research field.In this thesis,we first grew graphene on sapphire with characteristic orientation via plasma enhanced chemical vapor deposition(PECVD).Then,the uniform GaN films growth on graphene was obtained via metal organic chemical vapor deposition(MOCVD)assisted with the sputtering AlN buffer layer.The ultra-low biaxial compressive stress(0.023 GPa)in GaN films was revealed.We confirmed a 30°crystallographic rotation angle between the epitaxial GaN and sapphire,and analyzed the effect of graphene on the structural orientation by characterizing the structural defects and crystal orientation of GaN on graphene.The intrinsic physical mechanism that the epitaxial graphene with characteristic orientation is crucial to release the stress in GaN was further revealed via analyzing this special orientation relationship.This mechanism was also verified by first-principles calculations.Due to the weak Van der Waals force between the graphene and the epitaxial layer at the interface,the ultra-low stress GaN film can be easily transferred to other substrates.The transferred GaN heterojunction was directly fabricated into a metal-insulator-semiconductor(MIS)prototype device from which typical electrical properties was obtained.Our work reveals the excellent stress-releasing effect of graphene and stress-releasing mechanism,which provides a new epitaxial strategy to guide the crystallographic epitaxy of GaN.