MOCVD Epitaxial Growth Of GaN Based Blue LEDs On SiC Substrates

GaN based semiconductors have many excellent optical and electrical properties, andare very suitable for the preparation of luminescent devices. In early1993, GaN basedblue LED has been successfully developed, generalized and applied. However, research onhigh performance GaN based LEDs has always been popular, the reason is thatcommercial GaN based blue LEDs are based on the sapphire substrates resulting in someinherent defects, such as the heat dissipation problem and efficiency droop effect at highinjection current. If GaN materials are chosen as substrates for homoepitaxy, the aboveproblems can be resolved, but high quality, large size GaN single crystal which is suitablefor the fabrication of the GaN based devices is still not available in the domestic.In order to overcome the difficulties, we decide to adopt SiC single crystal the latticeof which matched GaN well as the substrates to develop GaN based blue LEDs. For SiCsubstrates have good heat conduction ability, and can solve the heat dissipation problemeffectively, they are more suitable for the preparation of high power luminescent devices.At the same time, the characteristic of easily cleaving reduces technology difficulty ofdevice fabrication. Most importantly, the lattice mismatch between SiC and GaN is3%,while the lattice mismatch between SiC and AlN is only1%which usually acts as thebuffer layer material. Adopting SiC substrates, the density of misfit dislocation can begreatly reduced, and therefore SiC substrates are ideal for preparation of high quality GaNepilayers.In the front half part of the research, growth parameters of epitaxial layers areoptimised to meet the growth of LEDs. Subsequent research is based on the previouslyoptimised growth parameters results to grow LED structure. Uniformity of emissionwavelengths is optimized to improve the utilization rate of the epitaxial wafer. Theresearch will include the following contents:(1) As the buffer layer, AlN has a good wettability with SiC. In the initial growthstage, the growth mode turns into two-dimensional planar growth. Through depositing200nm AlN buffer layer under1100℃, we can obtain GaN layer with high crystalline quality.At the same time, it introduces a sufficient amount of compressive stress avoidingcracking of thin film during the cooling process. The Al composition graded AlGaN buffer layers are inserted between AlN and GaN layer to overcome the nucleation caused by thelarge lattice mismatch at the adjacent interface. Consequently it reduces the misfitdislocations, and compressive stress generated by lattice mismatch between differentcomponents persists.(2) In active region, quantum wells are designed to be the barrier/cap/well preventingthe damage to the well layer under high temperature during the growth of barrier layer.And the growth conditions of InGaN well layer are optimized, the effects of In/Ga inputratio and growth temperature on In component of InGaN well layer are studiedrespectively. When the Ga input flow is certain, increase of In/Ga input ratio contributes tothe increase of In component of InGaN materials. Growth under low temperaturecontributes to the incorporation of In, while the crystalline quality of InGaN material ishigher at high temperatures. Regulation of the emitting wavelength of quantum well canbe realized by changing the growth temperature of well layer. Every1℃the growthtemperature of well layer increases, a2nm blue shift of emitting wavelength is observed.(3) If warping exists during the quantum well structures, it will certainly lead to apoor heat contact between wafer and susceptor. Thus simple adjustment to the temperaturezone parameters will not improve the uniformity of emitting wavelength obviously. In thisexperiment, the convex warping during the growth of quantum well is effectivelycompensated by adopting SiC substrates with initial curvature of290km-1, then theuniformity of emitting wavelength improves greatly.