Study On The Growth Of AlGaN-based Materials And Its Related Ultraviolet Light-emitting Devices On SiC Substrates By MOCVD

Recent years,more and more research groups pay attention on the ?-group nitrides based ultraviolet?UV?light-emitting diodes?LEDs?.Comparing with the traditional UV source,such as mercury lamps,the UV LEDs have many advantages.They have a smaller volume,a longer working time and a lower energy consumption.Furthermore,it is easier for the UV LEDs for controlling their wavelength and integration.The UV LEDs has a variety of applications.They can be used in medical treatment,sterilization,water/air purification and protein analysis.Moreover,they are very useful for UV curing,security,white illumination and military industries,and so on.In a word,UV LEDs has a broad market and important application values.AlGaN alloys are direct-gap semiconductors.By adjusting the Al composition,the bandgap of AlGaN alloys can change from 3.42 eV to 6.2 eV,the corresponding wavelength range is 365 to 200 nm which accesses a large segment of the UV spectral range.AlGaN alloys have a perfect heat-resistance,radioresistance and without any toxicity.Therefore,AlGaN alloys are ideal materials for the preparation of UV LEDs.Up to now,the external quantum efficiency of AlGaN-based UV LEDs is lower than that of ?-group nitrides visible LEDs.This is because of the following reasons:?1?The poor crystalline quality of AlGaN alloys;?2?It is very difficult for realizing effective p-type doping in AlGaN;?3?There are obvious quantum confined Stark effect?QCSE?in the AlGaN quantum wells.To achieve high efficiency AlGaN-based UV LEDs,more research and exploration are needed.In this work,we used silicon carbon?SiC?as substrates for the growth of Al?Ga?N,AlGaN based multiple quantum wells?MQWs?,AlGaN-based distributed Bragg reflectors?DBRs?and LEDs because of its smaller lattice mismatches with AlN and GaN.The detailed research contents are following:1.We grew AlN epitaxial layers on SiC substrates by metal-organic chemical vapor deposition?MOCVD?.By optimizing the V/? ratio,we found the growth method of AlN transformed from three-dimensional method to two-dimensional method.And the surface morphology and crystal quality were improved as well.Then we adjusted the growth temperature under the condition of optimized V/? ratio.We found that the screw dislocation density was the lowest at 1050°C.2.We grew AlGaN on SiC substrates by MOCVD.There are large thermal expansion coefficient mismatch between SiC substrates and AlGaN.As a result,in-plane tensile stress would be introduced in AlGaN during the cooling down process and lots of cracks appear.In order to relieve this problem,we tried to accumulate compressive stress during the growth process.When the thickness and growth temperature were increased or buffer layers with Al content gradient were used,it is more easier to reduce the in-plane tensile stress.We also studied the influence of the growth parameters of buffer for the crystal quality and optical properties of AlGaN.3.We researched about the p-type doping of AlGaN.Firstly,we grew the p-Al_0.35Ga_0.65N.We found the samples had a poor surface morphology and remained a high resistance.Then we grew the p-AlGaN with Al content linearly changed from0.65 to 0.35 along the growth direction for the study of polarization-induced doping by MOCVD.The achieved the highest hole concentration of the p-AlGaN was 1.3×10¹77 cm^-3.4.The AlGaN-based multiple quantum wells?MQWs?were grown on SiC substrates.The influence of barrier width and well width for the properties of MQWs were studied.We found that a thinner well width was benefit for decrease the QCSE and improve the optical properties of MQWs.And a thicker barrier also had advantages for the optical properties of MQWs.5.We prepared AlGaN-based LEDs on SiC substrates.Based on the results of Al?Ga?N materials and AlGaN-based MQWs,we grew both vertically conducting LED and horizontal structure LED on SiC substrates.Comparing with the horizontal LED,the vertically conducting LED had a better performance.Under a injection current of 20 mA,the vertically conducting LED achieved electroluminescence with a wavelength of 324 nm.6.We designed and grew the AlGaN-based DBRs structures on SiC substrates for the further increase of device performance in the future.The 15 pairs of vertically conducting n-Al_0.4Ga_0.6N/n-Al_0.6Ga_0.4N DBRs nearly without crack were prepared successfully,as well as the 20 pairs of Al_0.4Ga_0.6N/Al_0.6Ga_0.4N DBRs without doping.