Research On Epitaxial GaN Films Based On Al₂O₃ And Ga₂O₃ Substrates

Due to the wide band-gap(3.39 eV)and high breakdown electric field,the third-generation compound semiconductor gallium nitride(GaN)has shown huge application potential on high-power and high-frequency devices,while the band-gap of the nitride alloy can be continuously changed with the change of composition.At T=300 K,it can change from 0.7eV to 6.2 eV,which covers the entire visible light spectrum and extends to the deep ultraviolet band.At present,GaN materials have played a pivotal role in the semiconductor industry system.The preparation of high-quality GaN thin films is an extremely important link in the GaN process.High-quality GaN thin films can lay a good foundation for the subsequent performance of GaN devices and even the realization of integrated circuit functions.However,at present,GaN bulk materials are not only expensive but also small in size.Therefore,it is currently the mainstream technology to grow single crystal GaN thin films by heteroepitaxial method on other substrates.Due to the inevitable lattice mismatch and thermal mismatch between the heterogeneous substrate and the epitaxial layer,there are still a large number of material defects on the as grown GaN film,which greatly limits the application of GaN in the field of electronics and optoelectronic devices.This problem still restricts the further development of the GaN material system,which is the bottleneck of the development of the GaN material system.This is mainly because some of the mechanistic issues of epitaxial growth of GaN films are still controversial,and the root cause of the corresponding problems in the growth process of GaN related materials is still unclear.In this paper,Al₂O₃ and Ga₂O₃ substrate were used to heteroepitaxial GaN film and study the mechanism of GaN growth.In addition,high-quality GaN thin films were prepared.The findings are as follows:1.Although the existing process of epitaxial GaN thin film based on Al₂O₃ substrate is relatively mature,the role of carrier gas in the growth process has not been thoroughly studied.Compared with the single carrier gas(H₂ or N₂)currently commonly used,we use N₂ in the nucleation layer stage and use H₂ to grow the high-temperature buffer layer,this method effectively improves the crystal quality of the GaN thin film.At the same time,it was found that when pure N₂ was used as the carrier gas,the roughening process was hardly experienced after the low-temperature nucleation stage,which would lead to the appearance of special morphologies such as conical and snowflake-shaped protrusions.2.Patterned substrate technology is considered as one of the most promising methods to effectively reduce the dislocation density.Obviously,the design of the size of the patterned substrate has become a key issue for this technology.Al GaN/GaN heterostructures were grown on nano-patterned substrates(NPSS),micro-patterned substrates(MPSS),and ordinary sapphire flat-chip substrates(CSS).Dislocations bend and annihilate during lateral growth,which is the main reason for dislocations to decrease.Compared with NPSS and CSS,the heterostructure grown on MPSS has the lowest dislocation density and the best crystal quality,and result in the highest carrier mobility.3.Ga₂O₃ substrate has become an important candidate material for epitaxial GaN films in recent years,and has attracted extensive attention.Compared to Si C or GaN,?-Ga₂O₃ has an E_g=4.8 eV,and a wider band-gap is very promising to achieve high breakdown voltage.Therefore,the performance of?-Ga₂O₃ based high-voltage rectifiers and enhanced metal oxide field effect transistors are expected to be better and to stand out in the next generation of power devices.At the same time,?-Ga₂O₃ as a substrate is compatible with the advantages of the two kinds of substrates currently used in GaN-based LEDs,namely the visible light transparency of sapphire and the conductivity of Si C.Based on the above two points,we have conducted a certain research based on Ga₂O₃ substrate epitaxial GaN film,the research progress is as follows:The annealing of the gallium oxide substrate at different temperatures in the H₂atmosphere proved that H₂ will etch the gallium oxide substrate directionally above600?.Therefore,when the Metal-Organic Chemical Vapour Deposition method is used to epitaxial GaN thin film on the gallium oxide substrate,it should be avoided using H₂ in the low temperature nucleation stage.GaN is not easy to form a film based on the Al N nucleation layer,and exists in the form of granular crystals.When the GaN nucleation layer was used,a GaN single crystal thin film was successfully prepared.At the same time,we found that the the GaN thin film prepared based on the epitaxial growth of gallium oxide is almost stress-free and easy to peel off.This property is currently the biggest advantage of gallium oxide compared to other substrates.