Growth Of GaN Single Crystal By HVPE On Porous GaN Layers

Gallium nitride(Ga N)materials have wide applications in optoelectronics,power electronics etc.Because the research progress of bulk Ga N crystal is lagging behind that of Ga N-based devices,commercial Ga N-based devices are usually fabricated on foreign substrates.However,the poor quality of Ga N films induced by the heteroepitaxy severely limits the performance of the devices.Therefore,the fabrication of high-quality free-standing Ga N substrates is the key to solve this problem.Hydride vapor phase epitaxy(HVPE)has been considered as one of the promising candidates to commercially yield free-standing Ga N substrates due to its fast growth rate and low cost of equipment.However,since HVPE-Ga N is normally grown on foreign substrates,the separation of thick Ga N films from foreign substrates by using the thermal stress has always been a challenge to obtain the intact and high quality free-standing Ga N substrates.Currently,several studies have been reported to separate HVPE-Ga N from foreign substrates,such as mechanical lapping and laser lift-off,but those methods are generally complicated,and the cost is rather expensive.In this work,two different lift-off methods have been used to obtain 2-inch free-standing Ga N substrates.The main research content and achievement are as follows:1.A novel approach has been proposed to fabricate a nanoporous GaN sacrificial layer in order to ease the process of the separation of the thick Ga N films from the substrates.The spin-coating condition was optimized and we found that 4000 rpm spinning speed could offer the most uniform silica nanosphere layer.To understand the influence on the crystalline quality of subsequently thick Ga N films,we also prepared several nanoporous Ga N sacrificial layers in different thickness.The results show that excessively thick nanoporous layers were detrimental to high quality growth of thick Ga N films.On the other hand,it was difficult to form the pore structure if the nanoporous Ga N sacrificial layers were too thin.The optimized thickness of nanoporous Ga N sacrificial layers we found was within a range from 150 to 240 nm.After a regrowth of Ga N was taken on the Ga N template with the nanoporous Ga N sacrificial layer,an intact free-standing Ga N substrate was obtained by this method,which was free of any residual stress.The dislocation density is measured to be less than 107 cm-2 and the radius of curvature is approximately 7.4 m.2.Another lift-off method was investigated by inserting a TiN thin layer above the MOCVD-Ga N template.A 20-nm Ti thin film was prepared on Ga N template by DC magnetron sputter coating.Different annealing time and flow ratios between NH3 and H2 were used to optimize the formation of Ti N networks.It was found that longer annealing time(or higher ratio of H2)could enhance thermal agglomeration of Ti N leading to a larger size grids of network structure.In our work,the annealing time was set to 30 mins and the flow ratios between NH3 and H2 was 1:4 to growth Ti N networks.Porous Ga N layers with different V/III ratios were then grown on such Ti N networks.It was found that smaller ?/? ratio,such as 10,could be ascribe to larger pores of the porous Ga N layer,which contributes to a great reduction in dislocation density.3.The crystalline quality and impurity content of free-standing Ga N substrates obtained by those two methods have been compared by HRXRD,room temperature PL and SIMS.The results indicated that both of the crystalline quality and impurity content were improved by the lift-off method using Ti N network structure through in-situ nitridation of Ti films.