Study On Some Key Technologies Of GaN Substrate

GaN has attracted extensive attentions in the area of Ⅲ-Ⅴ wide band gap semiconductor due to its supreme physicochemical properties.However, a few problems such as the lack of GaN single crystal substrate material and high densities of defects in normally heteroepitaxy GaN limit device applications. Now, there are two methods to prepare high quality GaN substrate material. One is GaN single crystal growth and the other is lifting off the substrate from the preparation of thick GaN film by Hydride vapor phase epitaxy (HVPE) or other heteroepitaxy technologies to get the freestanding GaN substrate. HVPE suits to grow large area thick GaN films because it has faster growth ratio and the required equipment is simple.In this paper, we study the related technologies on the preparation of GaN substrate material, including the preparation of GaN template with nanoholes to do the GaN self-lift-off, the chemical mechanical polishing (CMP) of GaN substrate materials and HVPE GaN epitaxial growth. The main content and results are listed as follow:1. We investigated the selective electrochemical etching of n-GaN. We demonstrate that the nanoholes made by electrochemical etching of n-GaN are influenced by two factors:the applied voltage and Si doping concentration of n-GaN. The size of holes and the density of holes increase with applied voltage increasing. And they also increase with the Si doping concentration increasing.2. We investigated the photo-electrochemical etching of n-GaN. We find that the ultraviolet light can enhancement the result of electrochemical etching of n-GaN, causing more holes and increasing the size of holes. When ultraviolet light illumination from the backside of the GaN sample, it can etching the UID-GaN in the interface between sapphire substrate and GaN epilayer.3. Based on the study of above, we conduct electrochemical etching and photo-electrochemical etching towards two different sample structures and obtain GaN template with nanoholes which shows many different morphology. The PL spectra of GaN template after etching show the enhancement of characteristic peaks and yellow-band luminescence. Raman spectra reveal a tensile stress relaxation in nano-void GaN template after etching. The XRD shows the etching decrease the crystal quality a little.4. We regrown 5μm GaN film on the above porous GaN template by MOCVD process. The etching sample has significant change during the epitaxial growth, forming cavernous structure and some samples even form hollow structure. By applying mechanical force, the GaN regrown epitaxial layer can be detach from the substrate. If we use HVPE as epitaxial growth methods, the process can be used to prepare large-area thick free-standing GaN substrate.5. We also investigated how to improve the processing method of chemical mechanical polishing (CMP) to obtain high smoothness and improve the thickness uniformity. The surface crystal quality of sapphire substrate, free-standing GaN and HVPE-GaN thick film are all improved.6. We investigated the growth of HVPE-GaN substrate by the multi-chip vertical HVPE epitaxial growth system our lab design. The thickness measurement of the produced HVPE-GaN substrate shows that the uniformity of layer thickness of 3-chip/6-chip growth is below 5%; the deviation between chips is about 4% when the GaN film is 20um and deviation between chips is about 4.29% when the GaN film is 35μm.