Acid Etching Of Patterned Sapphire Substrate And Epitaxial Growth Of GaN And InGaN Crystal

High-quality GaN/InGaN crystal plays a key role in the realization of state-of-the-art RF,power switching,and optoelectronic devices.The growth of GaN thick-film/substrate by halide vapor phase epitaxy(HVPE)on flat c-plane sapphire substrate suffers from a series of problems to be solved,such as surface fragmentation,cracking,poor morphology,and high dislocation density,which arise from the large lattice mismatch and difference in thermal expansion coefficient between GaN and sapphire.On the other hand,molecular beam epitaxy(MBE)self-assembled growth of InGaN nanorods(NRs)on flat substrate faces frequently-occurring problems.In addition to the nonuniformity in the dimension and distribution of InGaN NRs,the undesired appreciable coalescence of neighboring NRs also hinders the subsequent active layer growth and device fabrication,as well as degrades the device performance by serious issues of current leakage and nonradiative carrier recombination caused by threading dislocation(TD)generated upon coalescence.It is well established that patterned sapphire substrate(PSS)can effectively facilitates the lateral epitaxy of HVPE-Ga N to improve its crystal quality and regulates the MBE self-assembled growth behavior of InGaN NRs by taking full advantage of the specific surface structural properties of high Miller index planes on PSS.However,there is a lack of intensive study revealing the advantage of PSS over flat c-plane sapphire substrate in the HVPE growth of GaN thick-film/substrate and relevant growth mechanism.On the other hand,there is also a lack of in-depth understanding regarding the MBE self-assembled growth mechanism of InGaN NRs on high Miller index planes of PSS,and thus the precise regulation of the self-assembled growth behavior of InGaN NRs is hard to achieve.As a result,this thesis systematically investigates the effects of etching time,volume ratio of H2SO4 to H3PO4,and etchant temperature on the crystallographic and morphological evolution of PSS,which contributes to the realization of precise regulation of microstructures exposed with high Miller index planes on PSS.After that,the capabilities of acid-etched PSS to facilitate the lateral epitaxy of GaN crystal to improve the crystal quality by HVPE as well as to regulate the MBE self-assembled growth behavior of InGaN NRs were studied and summarized,laying the groundwork towards the realization of the state-of-the-art GaN-based RF,power switching electronics,as well as InGaN NRs-based optoelectronic devices with enhanced performance.With cone and cylinder on the PSS as starting microstructures,the effect of etching time,volume ratio of H2SO4 to H3PO4,and etchant temperature on the crystallographic and morphological evolutions of PSS were systematically investigated.The correlation between surface structure and etching rate was established.The etching rate of crystallographic plane is proportional to the surface terrace density,as those high Miller index planes featuring higher terrace density thus exhibit higher etching rate than low Miller index c-plane.Moreover,the anisotropic etching capabilities of H2SO4 and H3PO4 were revealed and compared quantitatively.Larger volume fraction of H2SO4 in H2SO4 and H3PO4 mixture favors higher etching rate of low Miller index c-plane,by contrast,larger volume fraction of H3PO4 favors higher etching rate of high Miller index planes.With larger H2SO4 volume fraction in the mixture,the degree of coalescence between neighboring microstructures and their filling factor increase,along with the appearance of more diverse high Miller index planes and evolutional patterns.With decreasing temperature of H2SO4-based solution,the degree of coalescence and filling factor increase.The activation energy for etching reaction of various crystallographic planes was also calculated(87.0?108.3 k J/mol).Finally,well-controlled PSS patterns with various high Miller index planes were successfully fabricated with optimized parameters(e.g.,dimension,filling factor).Various patterns including truncated cone,faceted cone,triangular pyramid,hexagonal pyramid,and hexagram were successfully fabricated.Then,HVPE growth of GaN thin film,thick film,and substrate on optimized acidetched PSS has been studied.Compared with flat c-plane sapphire substrate,acid-etched faceted cone PSS favors greater surface morphology,lower dislocation density,and smaller residual stress of GaN crystal.Moreover,the effects of substrate temperature,HCl flow rate,and pattern morphology on the growth behaviors of GaN crystal on faceted cone PSS and flat c-plane sapphire substrate were also systematically investigated.Both higher substrate temperature and lower HCl flow rate favor the 2D growth of GaN single crystal filling the area between PSS patterns,and meanwhile inhibit the 3D deposition of polycrystalline GaN on the surface of patterns.Compared to triangular pyramid and hexagonal pyramid PSS,GaN crystal grown on the faceted cone PSS featuring highest aspect ratio and narrowest gap exhibits best surface morphology,lowest threading dislocation density,and smallest residue stress.With optimized growth parameters featuring cycles of low and high growth temperature,the crack-free GaN substrate with low TD density and small residual stress was achieved on faceted cone PSS,and the asgrown GaN substrate(thickness: 601 ?m)with the dislocation density of 1.89×106 cm-2 was achieved.The MBE self-assembled growth of InGaN NRs under varying Ga/In flux ratio was investigated on a series of high Miller index planes of faceted cylinder,triangular pyramid,and hexagonal pyramid PSSs.It is clearly demonstrated that lower Ga/In flux ratio favors the 3D growth of InGaN NRs and suppresses the 2D growth of compact layer,which is beneficial to the self-assembled growth of NRs with larger aspect ratio and higher density.The influence mechanism of high Miller index plane on the MBE self-assembled growth behavior of InGaN NRs was deeply revealed.High density of terraces on high Miller index plane and its high energy barrier of adatom diffusion contribute to the growth of InGaN NRs with high aspect ratio,large density,and good uniformity.Specifically,photoluminescence intensity of InGaN NRs on PSSs improved by up to 87% compared to those grown on flat c-plane sapphire substrate.Also the crystallographic orientation of as-grown InGaN NRs can be varied on the high Miller index planes of PSS,and the semipolar(11?02)InGaN NRs were fabricated on a high Miller index(4 5?1 38)plane of hexagonal pyramid PSS.In conclusion,the well-controlled fabrication of acid-etched PSS patterns with specific microstructure and high Miller index plane was achieved.Using acid-etched PSS,crack-free GaN substrate with low dislocation density and residual stress was successfully grown by HVPE.With the help of high Miller index planes on acid-etched PSS,the fabrication of well-aligned,uniformly,and densely distributed InGaN NRs with superior photoluminescence intensity were fabricated by MBE self-assembled growth.This work will contribute to the realization of high performance and more reliable GaN-based RF,power switching electronics,as well as InGaN NRs-based optoelectronic devices.