Research On Epitaxial Growth And Characterization Of Semi-polar AlGaN-based MQWs

In the past decade,there has been an outburst of the research on the AlGaN-based ultraviolet light emitting diodes(UV-LEDs)that have great potential in many application areas,such as sterilization,air and water purification and resin/ink hardening.However,up to date most of the AlGaN-based UV-LEDs are grown on(0001)c-plane sapphire substrates.Due to the spontaneous-and strain-induced piezoelectric fields existing inside the c-plane polar AlGaN-based UV-LEDs,there is a strong quantum confined Stark effect(QCSE),resulting in a remarkable decrease in light emission efficiency for these devices.In contrast,the semi-polar(11(?)2)plane AlGaN-based materials are promising candidates because of the significantly suppressed polarization electric fields along the growth direction and thereby to effectively improve the performance of the UV-LEDs.Therefore,it is meaningful to conduct systematic researches on the semi-polar AlGaN-based materials.In this thesis.the semi-polar(11(?)2)plane AlGaN films were successfully grown on non-poalr(10(?)0)-oriented m-plane sapphire substrates with an indium(In)-surfactant-assisted metal-organic chemical vapor deposition(MOCVD)technology.The properties of these epitaxial layers were also studied.Moreover,the growth and characteristics of the semi-polar(11(?)2)plane AlGaN multiple quantum wells(MQWs)and the Mg-doped semi-polar(11(?)2)plane AlGaN epi-layers were also investigated.The contents and major results achieved in this research were listed as follows:1.The semi-polar(11(?)2)plane AlGaN epi-layer samples were successfully grown on the non-polar(10(?)0)-oriented m-plane sapphire substrates with an In-surfactant-assisted MOCVD technology.The atomic force microscopy(AFM)measurement results demonstrate that the root-mean square(RMS)value for the semi-polar(11(?)2)plane AlGaN epi-layer samples has been decreased remarkably with increasing the TMIn mole flow rate,which can be attributed to the increase in surface adatom diffusion length induced by the indium-surfactant.Moreover,the native electron concentration of the unintentionally doped semi-polar(11(?)2)plane AlGaN epi-layers was decreased from 2.66×1017 to 2.97×1016 cm-3 due to the significant decrease in nitrogen vacancies(VN)with the indium-surfactant-assisted growth process.2.High quality semi-polar(11(?)2)plane AlxGa1-xN/AlyGa1-yN multiple quantum wells(MQWs)were successfully deposited on the non-polar(10(?)0)m-plane sapphire substrates with MOCVD technology and the effects of growth rate,In-surfactant and width of quantum well on the structural and optical properties of the semi-polar(11(?)2)plane AlGaN-based MQWs were investigated intensively.(1)The SEM characterization results showed that the surface morphology and the crystalline quality for the semi-polar(11(?)2)plane AlGaN MQWs could be improved remarkably by growing the MQWs with a relatively low growth rate.The photoluminescence(PL)spectra indicated that the emission peaks of the MQWs were located at approximately 279 nm.And the full width at half maximum(FWHM)value of the emission peak and the PL intensity of impurities were relatively small for the sample grown with a low growth rate.(2)The characterization results revealed that the surface morphology as well as the crystalline quality for the semi-polar(11(?)2)plane AlGaN MQWs could be improved remarkably by adopting In as surfactant during the MOCVD growth process.Furthermore,an enhanced internal quantum efficiency(IQE)could be achieved for the MQWs with the help of In-surfactant.(3)The PL spectra indicated that there was a clear blue shift for the peak position of the semi-polar(11(?)2)plane AlGaN-based MQWs with the well width decreased.Moreover,the built-in electric field for the semi-polar(11(?)2)plane AlGaN-based MQWs was calculated from the PL spectra to be 0.42 MV/cm,which indicated that the polarization field could be decrease significantly along the growth direction for the semi-polar(11(?)2)plane AlGaN materials.3.The Mg-?-doped semi-polar(11(?)2)plane Al0.2Ga0.8N films were successfully deposited on the non-polar(10(?)0)m-plane sapphire substrates with In-surfactant-assisted MOCVD technique and the effects of Cp2Mg mole flow rate on the characteristics of the AlGaN films were investigated.It was found that the surface morphology for the Mg-?-doped semi-polar(11(?)2)plane Al0.2Ga0.8N samples was monotonously degraded with increasing the Mg-doping level.Furthermore,a hole concentration as high as 4×1016 cm-3 was achieved at room temperature when the Cp2Mg mole flow rate was 0.22 ?mol/min.However,the hole concentration for Mg-6-doped semi-polar(11(?)2)plane Al0.2Ga0.8N samples decreased with further increasing the Cp2Mg mole flow rate due to the self-compensation effect.