Research On The Epitaxial Growth Of High-Quality GaN Films On SiC Substrates And Its Related Light Emitting Devices

The third generation of wide bandgap semiconductor materials represented by Ga N and Si C have been widely used in light emitting diodes?LEDs?,shortwavelength laser diodes?LDs?,ultraviolet detectors and microwave radio frequency devices due to their excellent photoelectric characteristics.At present,commercially available Ga N-based optoelectronic devices are mainly grown on Al₂O₃ substrates and Si substrates due to the lack of large-size Ga N substrates.However,the lattice mismatches for Al₂O₃ and Si between Ga N are 16 % and 20 %,respectively,the large lattice mismatch leads to a high threading dislocation?TD?density?109 cm-2?in Ga N films.Meanwhile,TDs can extend into the In Ga N/Ga N quantum well layers,resulting in a reduction of the luminous efficiency of the devices.The lattice mismatch between Si C and Ga N is only 3.4 %,and Si C has excellent thermal conductivity and electrical conductivity,which is appropriate for the fabrication of Ga N-based optoelectronic devices.In this paper,we use metal organic chemical vapor deposition?MOCVD?technology to conduct the controllable growth of high-quality Ga N films and its related light emitting devices on Si C substrates.The detailed research contents are as follows:Because the lattice mismatches for Si C and Ga N between Al N are 1 % and 2.4 % respectively.Thus,we choose Al N as the buffer layer,and the effects of thickness,growth temperature and V/III ratio of Al N buffer on the crystal quality,surface morphology,optical properties and stress characteristics of 1.5-?m-thick Ga N films have been emphatically studied.As a result,the performance of Ga N film is optimal when the thickness,growth temperature and V/III ratio of Al N buffer are 100 nm,1080 oC and 650,respectively.Due to the large difference?33.1 %?of thermal expansion coefficients between Ga N and Si C,substantial tensile stress would be introduced in as-grown Ga N films during the cooling down process,and the tensile stress in thin film increases with increasing the thickness of epitaxial layer.Finally,the increased stress would be released by the formation of cracks,which affects the properties of Ga N materials and its related devices.In order to reduce the tensile stress in Ga N films,we use the method of inserting a graded Al_xGa₁-x N?x=1-0?buffer between Al N and Ga N.In this paper,we investigated the influences of growth temperature,thickness and its growth rate of Al Ga N buffer on the crystal quality,surface morphology,optical properties and stress characteristics of 1.5-?m-thick Ga N films.In this case,the performance of Ga N film is optimal when Al Ga N growth temperature is 1100 oC,buffer thickness is 80 nm and its growth rate is 0.09 ?m/h.In addition,it is verified by Epi Curve in-situ monitoring system that the compressive stress is introduced during the growth of Al Ga N buffer,which effectively reduces the tensile stress in Ga N films.As a kind of porous nano-mask,Si Nx can not only block the dislocations,but also improve the stress state in Ga N films.We principally focused on the influences of deposition time and its insertion position of Si Nx interlayer on the growth mode,crystal quality,corrosion characteristics,surface morphology and stress characteristics of 4.5-?m-thick Ga N films.The performance of Ga N film is optimal when Si Nx growth time is 180 s and its deposition location is 0.2 ?m.Based on the above optimization,we obtained the optimal growth conditions of Si Nx interlayer.On this basis,we further optimized 1.5-?m-thick Ga N films prepared by series experiments of Al N buffer layer and graded Alx Ga1-x N buffer layer,the experiments results showed that the performance of Ga N films was significantly improved.At the same time,it lays a solid foundation for the fabrication of Ga N-based horizontal structure?HS?light emitting devices.Because the current congestion effect exists in HS devices,resulting in performance degradation after a long worktime.Si C substrate has high conductive and thermal conductivity characteristics,therefore it is very suitable for the preparation of vertical structure?VC?devices.Compared with HS,VC devices have the advantages of uniform current distribution,less heat generation,simple process and large active area.In order to obtain high-performance Ga N-based VC light emitting devices,the epitaxial growth of high-quality n-Ga N films on Si C substrates by using n-Al Ga N conductive buffer and Si Nx interlayer were performed.The effects of Si Nx growth conditions on the growth mode,crystallinity,corrosion characteristics,surface morphology,electrical properties and stress characteristics of n-Ga N films were primarily investigated.It has been found the properties of Ga N films were improved by increasing deposition time and reducing its deposition position of Si Nx interlayer.The fabrication of Ga N-based green LEDs was carried out by the use of high-quality Ga N templates.The designed wavelength of the devices was about 525 nm.Firstly,Ga N-based HS green LEDs were fabricated,among them,the preparation process,structural characteristics and photoelectric properties of the devices were introduced.Secondly,Ga N-based VC green LEDs were fabricated.In order to obtain crack-free LEDs devices with smooth surface morphology,we use the method of pre-introducing compressive stress during the growth process to counteract the tensile stress produced by the cooling process.In the experiments,the stress relaxation layers contain n-In Ga N/n-Ga N superlattice?SL?structure,n-In Ga N and n-Al Ga N/n-Ga N SL structure.The warpage condition of the above wafer can be determined by Epi Curve in-situ monitoring system.We have developed Ga N-based VC violet LDs on high-quality n-Ga N films,the lasing wavelength of the device was located at 410 nm.First of all,Ga N-based violet LDs without Al Ga N upper and lower confinement layers were prepared,among them,the preparation process,structural characteristics,thinning and cleavage process and the characteristics of optical pumping were primarily introduced.Furthermore,Ga N-based violet LDs with Al Ga N upper and lower confinement layers was fabricated,and the optical pumping of the LDs was realized.In the experiments,we use In Ga N instead of Ga N as the waveguide material,the propose is to introduce more compressive stress during the growth of the device,to avoid the formation of cracks.