The Research On Preparation And Evaluation Of Nanopatterned Silicon Substrates For Gallium Nitride Epitaxial Films

As a typical third generation wide band gap semiconductor material,gallium nitride(GaN)presents a wide application prospects in the field of optoelectronic and electronic devices.Due to the advantages of low price,large-scale availability and good thermal and electrical conductivity,Si substrates are very promising for GaN epitaxial films.However,compared with the growth on common sapphire or silicon carbide(SiC)substrates,it is hard to achieve in-situ growth of high-quality GaN films on Si substrates due to the large lattice and thermal mismatch between the two materials.The application of patterned silicon substrates which consists of periodic pattern structure offers a solution to the problem.A well-known conventional technology of patterned silicon substrates preparation is the lithography method.Nevertheless,this technique has many disadvantages such as high cost,low yield and challenging to large-area fabrication,especially in the case where the silicon nanostructures are reduced to small sizes,particularly for sub-100 nm nano-scale.According to the above-mentioned,a simple method is proposed in this paper,in which the nanopatterned silicon(NPSi)substrates are prepared by dry etching using the anodic aluminum oxide(AAO)films as masks.Moreover,the as-obtained substrates are applied to the evaluation of GaN epitaxial films for the purpose of obtaining low-cost,high-through and large-scale preparation of NPSi substrates.The main contents of this dissertation include: the AAO film masks were initially prepared based on the direct anodization of sputtered Al films on silicon substrates;then the NPSi substrates were fabricated following the inductively coupled plasma(ICP)etching;subsequently,three-dimensional(3D)zinc oxide(ZnO)buffer layers grown on NPSi substrates were obtained by using atomic layer deposi tion(ALD)technique;finally,the GaN epitaxial films were manufactured by using laser pulse deposition(PLD)method.The main results of the dissertation are as follows:The effects of sputtering parameters on the roughness of Al films were investigated,the difficulties of direct anodization were analyzed,and the controllable preparation of AAO masks on Si substrates was realized.The surface roughness of Al thin films increases with increasing DC magnetron power,working pressure and substrate temperature.Eventually,the minimum mean surface roughness(Ra=1.99nm)for 600 nm Al thin films on Si substrates is achieved by using 60 W DC magnetron power,working pressure equal to 0.2 Pa and substrate temperature of 70 ?.The subsequent oxidation of silicon substrate is effectively avoided by using a self-designed electrolytic cell.The hole diameter and thickness of AAO film masks is effectively controlled by moderate modification of the pore-widening period and first anodization period,respectively.The effect of ICP etching parameters on the morphologies of NPSi substrates was investigated.In BCl3/Cl2/Ar mixed gas etching system,the nanopatterned structure is successfully transferred from the ultra-thin Si-based AAO film masks to Si substrates,following the two-step ICP etching.During ICP etching process,the ratio of the effects of isotropic chemical etching and anisotropic physical etching varies by changing the etching process parameters,which thus influences the etching rate and etching morphology of Si.Moreover,the pattern of a highly ordered porous array of the AAO film with aperture of 65 nm,wall thickness of 40 nm and depth of 330 nm is transferred to Si substrate with high quality,which is achieved by ICP etching with Cl2 concentration of 60%,while the first-etching time is 10 s,ICP power equales 900 W,RF power is 100 W and working pressure corresponds to 7 m Torr.Furthermore,the NPSi substrates including a uniform pore depth and different pore size are achieved by adjusting the pore-widening time in AAO film masks preparation,while the NPSi substrates with homogeneous pore size and various pore depth are obtained by controlling the etching time in the second etching step.Finally,the as-obtained NPSi substrates demonstrate excellent antireflection properties,which due to multiple reflection and refraction of the incident light on NPSi substrates.The optimal annealing parameters of 2D ALD-ZnO films and its photoluminescence properties caused by structural defects were analyzed.The deposition of 3D ALD-ZnO buffer layers on NPSi substrates was achieved and the influences of the size of NPSi substrates on the crystal quality of deposited ZnO buffer layers were revealed.In 2D deposition model,a thin ALD-ZnO film with higher crystallinity and better luminescence performance is obtained when the film annealed at 800 ? in oxygen.Additionally,the internal structure defects concentration of the film changes after annealing,which leads to the change of the number of peaks in UV emission and the change of the proportion of three visible peaks in visible emmision.According to the 2D deposition model,the 3D deposition of ZnO thin layer is achieved on NPSi substrate by adding a rea sonable “exposure period” for both precursors.Furthermore,a thin ZnO layer with ideal 3D morphology,optimal crystallinity(FWHM=0.351°)and luminescence performance is obtained when the pore-widening period corresponds to 45 min,which also proves that the Si nanopore diameter increase greatly benefits thin 3D ZnO layer growth when NPSi substrates morphology integrity is a requirement.The evaluation of the as-obtained NPSi substrates was completed by comparison of the crystal quality of GaN epitaxial films grown on NPSi substrate to the non-patterned Si substrate manufactured by using laser pulse deposition(PLD)method.The conventional XRD results indicates that GaN epitaxial thin films on NPSi substrates include higher crystallinity compared to that of planar Si substrates.HRXRD rocking curve results demonstrates that the 3.3×1010cm-2 dislocation reduction of GaN epitaxial thin films grown on NPSi substrates is associated to the bending of threading dislocations(TDs)during the overgrowth.Under identical epitaxial process conditions,the GaN thin film epitaxially grown on the NPSi substrate with a cubic structure(FWHM=482arcsec)achieves the crystal quality of the film on a sapphire substrate including a hexagonal structure,and the crystal quality of the as-obtained GaN epitaxial thin film is significantly improved.Micro-Raman results indicates that the stress caused by the crystal system and lattice mismatch between the substrate and the epitaxial layer is released significantly when the NPSi substrate is adopted.PL results demonstrates that the deep-level defects of the as-obtained GaN epitaxial thin film grown on a NPSi substrate are dramatically reduced,leading to the substantial improvement of the crystal quality and to the obvious enhancement of the luminescent performance.Therefore,the as-prepared NPSi substrates will have a better application prospect in the field of GaN epitaxial growth.