| In this thesis,aiming to obtain high quality AlGaN material for ultraviolet light emitting devices,AlGaN-based films and nanostructures were grown on different substrates by plasma-assited molecular beam epitaxy(MBE),and the crystal structure,surface morphology and optical properties of these materials were investigated.The main achievements are presented as following:1.High quality GaN material was obtained on GaN templates under optimized growth conditions.The growth model of GaN was analyzed by Reflection High Energy Electron Diffraction(RHEED).The effect of growth temperature and ?/?ratio on the surface of GaN film was studied.GaN material with smooth surface and low defect concentration of 5×108cm-2 was grown under slight Ga-rich regime at 710?,which is important for the growth of p-type GaN.2.Behaviors of Mg incorporation into GaN in molecular beam epitaxy were studied and p-type GaN material with high hole concentration of 2.8×1018cm-2 was achieved.In a certain regime,Mg could incorporate into GaN more effectively under higher temperature.However,that the temperature is too high will lead to more nitrogen vacancies and other related defects,and then the hole concentration decreases.Similarly,sufficient Ga atoms can improve the crystal quality and hole concentration of GaN film,but excessive Ga will suppress Mg incorporation.3.AlGaN films with A1 content of 26%were obtained on GaN templates by MBE.The influences of ?/? and growth on surface morphology and optical properties of AlGaN were investigated under different growth temperatures.It is founded that the Al content of AlGaN could be regulated effectively by changing Al flux.Furthermore,GaN sample grown under higher temperature and lager Ga flux ratio has better surface,stronger PL intensity and longer minority carrier lifetime under metal-rich regime.In addition,the photoluminescence(PL)properties of AlGaN indicate that there are two,type nonradiative recombination centres with different activation energy and the deeper one plays a leading role in restricting the minority carrier lifetime and luminous efficiency.4.Growth and p-type doping of AlGaN with high Al content of over 40%were achieved on AlN template on sapphires.The effect of different growth conditions on the growth and doping of AlGaN was investigated.It was observed that Al content,surface morphology and crystal quality will be improvd with increasing A1 flux ratio or growth temperature.The PL peak wavelength of 260 nm was obtained at room temperature.P-type AlGaN material was obtained at a relative low growth temperature and high Mg cell temperature with an extremely low activation.5.Deep ultraviolet emission was obtained from high A1 conetent AlGaN/AlN multi quantum well(MQW)structure by plasma-assisted MBE on AlN template on sapphire substrate.The emission wavelength was tuned by controlling the thickness of AlGaN quantum well,and the intensity was conditional on the quality of each epitaxial layer and hetero-interfaces in the AlGaN/AlN MQW.We have achieved deep ultraviolet emission at 260 nm from AlGaN(2.4 nm)/AlN(6.9 nm)MQW at room temperature.6.The selective-area growth of GaN-based nanocolumns(NCs)on silicon(Si)has been investigated experimentally.AlGaN material and hetero-structures were grown on GaN nanocolumns and then we extracted single NCs in a simple and feasible method.By the experiment on microstructure of GaN-based NCs,it was found that the nanocolumns have wurtzite structure and are oriented with[0001].We also observed that GaN-based nanocolumn has a core-shell shaped structure,which is attributed to different mobility of A1 and Ga on side wall during the growth process.The PL peak wavelength of AlGaN nanocolumns and AlGaN/AIN MQW nanocolumns was 305 nm and 290 nm at room temperature,respectively.In addition,a low-cost and facile scheme has been proposed and implemented to fabricate the selective-area growth nanocolumns,which contains the processes of nanosphere lithography and plasma-assisted MBE. |
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