MOCVD Growth And Characterization Of Non-polar AlGaN-based Materials

AlGaN-based ultraviolet light emitting diodes?UV-LEDs?are of great potential for a wide range of applications,such as sterilization,water/air purification,and medical treatment.Currently,most of the AlGaN-based UV-LEDs have been fabricated based on polar c-plane AlGaN epi-layers.However,the light emission efficiency of the polar c-plane AlGaN-based UV-LEDs can be strongly reduced by the quantum confined Stark effect?QCSE?induced by the spontaneous and piezoelectric polarization fields with the order of magnitude up to several MV/cm.In contrast,there is no spontaneous polarization-induced built-in electric filed in a non-polar AlGaN epi-layer and the QCSE can be completely suppressed in non-polar AlGaN-based multiple quantum wells?MQWs?.Moreover,the acceptor activation energy in Mg-doped non-polar p-type AlGaN epi-layers can be reduced to approximately a half of that for Mg-doped polar p-type AlGaN epi-layers,which will be beneficial for realizing p-AlGaN with high Al content.Hence,high light emission efficiency can be expected for the non-polar AlGaN-based UV-LEDs.In this dissertation,the epitaxial growth of non-polar?11-20?-oriented a-plane AlGaN films on semi-polar?1-102?-oriented r-plane sapphire substrates was investigated experimentally using metalorganic chemical vapor deposition?MOCVD?.The properties of these epitaxial layers were also studied.The major research contents and the results achieved in this dissertation were listed as follows:1.Various kinds of intermediate layers were applied for the epitaxial growth of the non-polar?11-20?-oriented a-plane GaN and AlGaN films.1)The non-polar?11-20?-oriented a-plane GaN epi-layers were successfully grown on semi-polar?1-102?-oriented r-plane sapphire substrates with MOCVD by using the SiNx interlayer.It was found that the remarkable improvement in crystalline quality achieved could be attributed to the reduction in the pyramidal defects density as a result of the insertion of the SiNx interlayer for the non-polar?11-20?-oriented a-plane GaN epi-layers.Moreover,it was demonstrated that the SiN_x interlayer was powerful to suppress the crystalline quality anisotropy in the nonpolar?11-20?-oriented a-plane GaN epi-layer since there was only a slight difference of 3.3%?v.s.87.5%for the sample grown without the SiN_x interlayer?in full width at half maximum?FWHM?value of the X-ray rocking curve?XRC?between c-and m-direction for the sample grown with the SiNx interlayer.2)The non-polar?11-20?-oriented a-plane AlGaN epi-layers were successfully grown on semi-polar?1-102?-oriented r-plane sapphire substrates with MOCVD by using the MgN/AlGaN insertion layers.The characterization results demonstrated that the crystal quality,surface morphology,structural and optical properties of the non-polar?11-20?-oriented a-plane AlGaN epi-layers were significantly improved with the optimized MgN/AlGaN insertion layers.3)The non-polar?11-20?-oriented a-plane AlGaN epi-layers were successfully grown on semi-polar?1-102?-oriented r-plane sapphire substrates with MOCVD by using the MgN nucleation layer.It was revealed that the MgN nucleation layer?similar to nanometer-order patterned substrate?was able to compensate the in-plane strain induced by lattice mismatch,resulting in the improvement in the crystalline quality and reduction in the crystalline quality anisotropy for non-polar?11-20?-oriented a-plane AlGaN epi-layers.2.The non-polar?11-20?-oriented a-plane AlGaN epi-layers were successfully grown on semi-polar?1-102?-oriented r-plane sapphire substrates by using the innovative two-way pulsed-flow MOCVD technology.The root-mean-square?RMS?value was decreased from15.08 nm to 1.79 nm by using the innovative two-way pulsed-flow MOCVD technology.The results showed that when using the innovative two-way pulsed-flow method,solid particles or point defects generated by the so-called parasitic reaction between TMAl and NH₃ precursors could be effectively suppressed,leading to sufficient coalescence of large pyramidal defects and the formation of a smooth surface for the non-polar?11-20?-oriented a-plane AlGaN epi-layers.And the innovative two-way pulsed-flow method was effective to improve the surface morphology and reduce the defect density in the non-polar?11-20?-oriented a-plane AlGaN epi-layers.It was due to that there was more time for the Al adatoms to migrate to their appropriate lattice sites under an Al-rich growth condition,which was beneficial to epitaxial growth.In fact,an enhancement as large as 36.9%in the relative light transmittance was achieved within the wavelength range of 200-400 nm.3.The non-polar?11-20?-oriented a-plane AlGaN films were successfully grown on semi-polar?1-102?-oriented r-plane sapphire substrates with an indium-surfactant growth technology,and the effects of Trimethylindium mole flow rate on the structural and optical characteristics of the epi-layer samples were studied systematically.The characterization results showed that the indium-surfactant was useful to promote the incorporation of the metal atoms into correct crystal lattice sites and reduce the photoluminescence?PL?intensity of impurities.And the indium-surfactant was also effective to reduce the dislocation density and the structural anisotropy for the non-polar?11-20?-oriented a-plane AlGaN epi-layer,leading to the improvement in both optical property and crystalline quality for the non-polar?11-20?-oriented a-plane AlGaN epi-layer samples.