Preparation And Investigation Of Ultraviolet LED Based On SiC Substrate

(Al)GaN based materials are the ideal candidates for high-performance ultraviolet light emitting diode (UV-LED) because of their direct band gap which vary from 3.4 eV to 6.2 eV, good thermal and chemical stability. (Al)GaN based UV-LED due to its pollution free, narrow spectrum, low energy consumption, long service life and rapid response speed, has a broad application in solid-state lighting, UV curing, medical diagnosis, eliminate bacteria and antivirus and other fields. Because of the lack of affordable large size homoepitaxial substrates, (Al)GaN generally heteroepitaxy on foreign substrates. The low lattice mismatch to AlN(1%), high thermal conductivity and electrical conduction make SiC substrate admired as one of the most suitable foreign substrate for (Al)GaN epitaxial layer. However, because SiC absorbs UV light and huge thermal mismatch introduces a large number of cracks in the epitaxial film, it is difficult to prepare UV-LED with high light extraction efficiency. In this paper, we focus on the difficulties in preparation UV-LED on SiC substrate including epilayer growth and structure design. The main results are as follows:(1) The influence of the spacing between showerhead and susceptor (showerhead height) on the growth rate, crystalline quality and stress state of GaN films grown on SiC were investigated. It is found that, as the showerhead height increased from 7 mm to 13 mm, the growth rate decreased from 3.6 um/h to 1.8?m/h. The thermophoretic forces acting on the nano-scaled particles carry them in the direction opposite to the temperature gradient and towards the regions of lower temperature. For larger showerhead heights, the particles layers floating in the hotter middle of the chamber with the increasing of the adsorption and consumption of gallium, the predicted growth rate decreased dramatically. And the generation of tensile stress in GaN film increased due to the variation of initial island size.(2) The influence of ?/? ratio on properties of AlGaN was investigated. Using optimized ?/? ratio, it is found that the growth rate, Al-content and crystalline quality were improved. And on this basis, a crack-free 1.8?m AlGaN epilayer grown on SiC substrate was obtained by using the optimized SiNx interlayer. Finally, the growth mechanism of AlGaN on SiNx interlayer has been discussed.(3) Crack-free Al0.2Ga0.8N/GaN DBRs with high reflectivity were successfully grown on 6H-SiC substrate with a SiNx interlayer by MOCVD. Cross-sectional FE-SEM and HR-XRD results reveal good structural quality and very abrupt interfaces of the DBRs. The reflectivity of the DBR stack at 388nm was 92.8% and the stop-band bandwidth was 16 nm. Meanwhile, near-ultraviolet InGaN/AlGaN multiple quantum well (MQW) LEDs were grown on the AlGaN/GaN DBRs, the light output power for UV-LED with DBRs was approximately 56% higher than the UV-LED without DBRs.(4) Two types of deep-ultraviolet LED (DUV-LED) were successfully grown on 6H-SiC substrate based on crack-free Al0.5Ga0.5N epitaxial layer. The first involved using a thick p-GaN contact region to improve the forward voltage, but leads to absorption losses both in p-GaN region and SiC substrate. The second approach used a p-AlGaN superlattice to inject holes. This reduced absorption and extraction losses, but the resistance of the p-AlGaN lead to high electrical losses.(5) The polarity control was found to be a key factor determining the crystalline quality of AlN epitaxial film. A graded AlGaN interlayer (polarity control layer) was introduced between GaN and AlN layer, which effectively suppressed the decomposition of underlying GaN and controlled the crystal polarity of AlN film. High-quality AlN on GaN can be potentially achieved at a temperature as low as 1050?. On this basis, crack-free thick Alo.5Gao.5N layers were successfully fabricated by introducing a thin AlN interlayer on GaN epitaxial layer.(6) The p-GaN/Mg-doped Al0.3Ga0.7N/n+-GaN/n-SiC polarization-induced backward tunneling junction (PIBTJ) was grown on SiC substrate. The PIBTJ displays reliable and reproducible backward tunneling with a current density of 4.2 A/cm2 at the reverse bias of-1 V. The insertion of a graded AlN interlayer between PIBTJ and AlGaN MQWs layer can achieve a crack-free n-Al0.5Ga0.5N surface. And tunnel junctions can electrically couple a n-type material with a p-type material and eliminate the need for p-type contacts and improve the light absorption. The structure would be p-side down and inverted with respect to conventional structure. Finally, vertical conducting deep-ultraviolet light-emitters with a polarization-induced backward tunneling junction were grown by metal-organic chemical vapor deposition on n-type 6H-SiC substrate.