Research On The Growth And Stress Relaxation Of ELOG GaN By Graphene Mask

Gallium nitride(GaN)is one of the most popular third-generation semiconductor materials at present,also known as wide band gap semiconductor materials.It has a wide range of applications and future development prospects in the fields of communications,new energy,energy-saving lighting,biomedicine and other fields,bringing us the technical advantages of high performance,low power consumption and low cost.Considering the lack and cost of GaN homogeneous substrate,GaN is mainly prepared on SiC,Sapphire,Si and other substrates by heteroepitaxial.However,the lattice mismatch and thermal mismatch between the heterogeneous substrate and GaN result in the high dislocation density and stress of GaN,which limits the performance of Gan-based devices.With the continuous pursuit of high quality GaN,the epitaxial laterally overgrown technique(ELOG)using periodic patterned masks to grow GaN has proved to be an effective way to reduce the dislocation density of GaN.The mask layer of the technology plays a key role in blocking the substrate dislocation.In recent years,the lack of hanging bonds on the surface of graphene makes it possible to use GaN as a mask material for lateral epitaxy.The graphene mask layer has many advantages in the epitaxial laterally overgrown GaN,mainly reflected in the following aspects.On the one hand,the graphene mask layer can block the dislocation from the substrate,its good thermal and conductive properties can bring performance advantages to GaN based devices.On the other hand,the weak van der Waals forces between graphene layers can alleviate the effects of heteroepitaxial mismatch and improve the crystal quality of GaN.On the contrary,the mismatch between the dielectric mask represented by SiO2 and GaN will lead to the introduction of additional stress.Therefore,it is of great significance to research the epitaxial laterally overgrown GaN by using two-dimensional material graphene mask.In this thesis,we mainly investigate the growth and stress relaxation mechanism of GaN by lateral epitaxy of two-dimensional material graphene mask,and explore the unique advantages of graphene mask.Multilayer graphene was grown on GaN/sapphire substrate by plasma enhanced chemical vapor deposition(PECVD),which was then prepared into patterned hexagonal mask layer by photography and etching process.Finally,GaN was grown by side epitaxial metal-organic chemical vapor deposition(MOCVD).The growth behavior and stress of GaN were characterized and analyzed,and the unique advantages and new mechanism of graphene mask were explored.The main content of this paper includes the following aspects:1.GaN growth behavior in hexagonal graphene masks with 70 μm mask width/10 μm window width and 20 μm mask width/4 μm window width filling factors(the ratio of mask width to window width)was investigated.The results showed that GaN does not grow in the lateral epitaxy mode on the graphene mask with 70 μm mask width/10 μm window width.The larger graphene mask width makes it possible for the graphene mask to decompose during the GaN lateral epitaxy process and thus cannot play the role of the pattern mask,and a large number of uncombined pits were left on the mask after GaN growth.By optimizing the filling factor of the mask,lateral epitaxial GaN about 2.8 μm thick was obtained on a hexagonal graphene mask with a mask width of 20 μm/window width of 4 μm.After the characterization of PL image,the dislocation density of GaN decreases to about 8.5×107 cm-2,which is an order of magnitude lower than that of GaN.On the other hand,the number of graphene layers also affects the lateral epitaxial growth of GaN,and relatively few layers of graphene may undergo rapid partial decomposition under the action of GaN substrate and fail to achieve the function of patterning mask.2.The growth behavior of ELOG GaN was compared between PECVD direct growth and wet transfer hexagonal graphene mask,and the effects of the two kinds of graphene mask on GaN growth were investigated.In the experiment,we found that the inhomogeneity and surface damage of graphene may be caused by human factors in the process of wet transfer,which may lead to the failure of graphene to play the role of mask,the dislocation density of GaN in the mask region cannot be significantly reduced.In contrast,the good homogeneity and integrity of the graphene grown directly by PECVD resulted in a good lateral epitaxy pattern for GaN.3.The stress relaxation effect of the graphene mask on the ELOG GaN was studied by systematic stress analysis.Raman,room-temperature PL and XRD were used to characterize the stress of GaN from the cross section,surface and lattice constants.Cross-section Raman showed that the compressive stress of GaN on the graphene mask was relatively relaxed.At the interface between the epitaxial GaN and the substrate GaN(the graphene insertion layer position),the maximum stress relaxation of 0.157 GPa was found for GaN.Subsequently,PL spectra showed that from one end of GaNs edge on the graphene mask to the other end of GaN’s edge on the graphene mask,the near-band emission peak(NBE)of GaN first showed a c blue shift and then a red shift,reflecting the stress relaxation of GaN during lateral epitaxial process.The maximum compressive stress exists in the window region GaN.The stress at both ends of GaN edge in the mask region is minimum,and the maximum stress relaxation of about 0.147GPa was obtained.Finally,XRD showed that the stress relaxation of GaN was reflected in lattice constants.Compared with the substrate GaN,the lattice constant of ELOG GaN on graphene was closer to that of free-standing GaN.This work provides new experience for the growth and stress relaxation of ELOG GaN by graphene mask.