Studies On Surface Physical Evolution And Crystal Quality Of GaN Thin Films Prepared By PEALD And MOCVD

GaN-based semiconductors have found a wide range of applications in fields such as optoelectronics and power devices due to their wide band gap,high saturated electron drift velocity and critical breakdown electric field.GaN thin films are mainly prepared by physical and chemical deposition on heterogeneous substrates.The most popular technique for the industrialization of GaN is MOC VD,in which chemical deposition involves the decomposition and synthesis of two gaseous sources,a metal organic source and ammonia,on the surface of the substrate.Due to the anisotropic nature of the growth,different epitaxy processes will occur on different substrates.There is both polar and non-polar growth in hexagonal GaN thin films,and polar growth is well established.This work focuses on non-polar growth of GaN,and attempts to analyze and optimize the surface processes using heterogeneous striped patterned substrates accompanied by epitaxial layer growth regulation to improve the crystal quality of a-plane GaN.Another technology in the field of chemical deposition,ALD,differs from traditional CVD(including MOCVD)in that the precursors for ALD are not fed into the cavity at the same time as in C VD,but are fed alternately and then adsorbed onto the substrate surface to react and form a film.Therefore,ALD technology can precisely control the film thickness in the nanometer range and can achieve low temperature growth,which has an important role in the realization of flexible twodimensional planar devices and the extension of Moore’s law.One commonality between ALD and MOCVD is that both are gaseous sources reacting on the surface in a process that is closely linked to the substrate or template used.This work focus on the use of monolayer MoS2 templates,which are better matched to the GaN lattice,and analyses the surface processes of GaN grown on these templates at different thicknesses in order to explore possible ways of implementing ALD at the material growth and heterojunction device levels.Based on the commonality of the growth of MOCVD and ALD and their different characteristics,these works present an in-depth study on the surface evolution mechanism of GaN deposited by PEALD on MoS2 templates,two-step growth to improve GaN crystal quality,and MOCVD growth of GaN combined with 3D-2D growth method on micro and nano graphic substrates.Specific findings of the study are as follows:(1)A clear 2D-3D(Stranski-Krastanov)growth mechanism was found when growing GaN films on Si/SiO2/monolayer MoS2 substrates using PEALD.In particular,the window conditions for the growth of PEALD-GaN on MoS2 templates and the physical evolution of the surface during the GaN growth process are investigated.The reasons for the hexagonal polycrystalline structure obtained by the growth of GaN in this 2D-3D mode are also analyzed in detail according to the surface evolution process.This study provides a basis for understanding the growth of GaN on mono-MoS2 layers using PEALD.(2)In improving the quality of GaN deposited by PEALD,based on the surface evolution process of GaN grown on MoS2 templates in(1)above,and also drawing on the conventional two-step growth method,a method(named in this study:twostep method)was proposed to grow GaN by using low-temperature growth in the initial 2D growth stage of GaN,followed by high-temperature 3D growth.High quality ultra-thin GaN films with[0001]preferred orientation,low impurity concentration(the oxygen content was measured as 3.6%)and balanced Ga/N stoichiometry ratio(Ga/N ratio is 0.95:1)were obtained.The mechanism of the twostep growth to improve the crystal quality is also explained by comparison with the growth of high-temperature PEALD-GaN directly on MoS2 templates.An achievable method is given for the preparation of future two-dimensional GaNbased planar devices that can be developed.(3)Fine string patterns of SiO2 with a period of 1:3 μm along the m-axis of GaN([1-100])direction,were prepared on r-plane sapphire substrates.By optimizing the micro-nano preparation process,especially the gas formulation during etching of SiO2,graphical results with straight edges and vertical sidewalls were obtained,and the specific effects of different gas formulations on the etching were also analyzed.A good process basis for the preparation of heterogeneous patterned substrates is provided for the subsequent lateral epitaxial growth of nonpolar a-plane GaN.(4)The 3D-2D growth method was used to grow non-polar a-plane GaN on the graphical substrate obtained in(3)above,and the conditions of the 3D and 2D layers were optimized to improve the quality of MOCVD-grown a-plane GaN.The XRC half-peak widths along the c-axis and m-axis of the a-surface GaN films were 367 arcsec and 384 arcsec,respectively,with anisotropy of 0.023 and layer misalignment density as low as 1.01×104 cm-1.This provides an available buffer layer preparation method for the subsequent preparation of non-polar devices.