Study On The Epitaxial Growth Technology Of Non-polar AlGaN-Based Multiple Quantum Well

Deep ultraviolet light-emitting diode?DUV-LED?fabricated based on AlGaN materials is one of the ideal candidates to replace traditional mercury lamp with high effciency and free of pollution.In this thesis,high quality non-polar a-plane AlGaN-based materials and multiple quantum wells?MQWs?with an emission wavelength of 280 nm were successfully grown on r-plane sapphire substrate by using metalorganic chemical vapor deposition?MOCVD?technology.This will pave the way to fabricate non-polar AlGaN-based DUV-LED with high efficiency.The major works conducted in this research are listed as follows:1.With the introduction of two-way pulsed-flow growth technology,the parasitic gas phase reactions between trimethyl-aluminum?TMAl?and ammonia?NH₃?could be suppressed effectively during the epitaxial growth of AlGaN-based materials.The surface morphology and relative light transmittance for the non-poalr AlGaN epi-layer was improved evidently owing to the supressed parasitic gas phase reactions.The surface roughness expressed in root mean square?RMS?value for the non-polar Al_0.68Ga_0.32N epi-layer grown with two-way pulsed-flow growth method was decreased to 1.79 nm which is much lower than that the RMS value of15.08 nm for the sample grown with conventional continuous-flow growth method.A TMAl flow duty-ratio modulation method was developed based on the two-way pulsed-flow growth technology.This method turned out to be most effective to modulate the Al composition in the non-polar AlGaN epi-layers as compared with any other conventional growth technologies.2.The epitaxial lateral overgrowth of AlGaN-based materials was successfully performed by only using the inserted interlayer grown without any masks.The crystalline quality was improved remarkably and the anisotropy in crystalline quality was suppressed effectively owing to the successful compensation of the strains in the epi-layers by using this method.Furthermore,the density of the featured pyramidal defects distributed on the surface of non-polar AlGaN-based materials was reduced evidently.3.The effect of dual nitridation processes for both r-plane sapphire substrate and low temperature-grown AlN nucleation layer?LT-AlN NL?on the crystalline quality of the non-polar a-plane AlGaN epi-layer was studied intensively.It was found that the AlN grains with proper density and height could be formed with appropriate nitridation process for sapphire substrate which played a critical role in the growth process of the subsequently-grown epi-layers.By optimizing the nitridation process for sapphire substrate and LT-AlN NL,the RMS value for the non-polar Al_0.53Ga_0.46N epi-layer was further decreased to 1.54 nm.This RMS value was small enough to grow MQWs structure.4.An internal quantum efficiency?IQE?as high as 39%was achieved with the non-polar AlGaN-based MQWs with an emission wavelength of 279.2 nm grown on r-plane sapphire substrate by using the newly-developed TMAl flow duty-ratio modulation method and the optimized dual nitridation processes.No blue-shift in wavelength for the MQWs-related emission peak was observed in the photoluminescence spectra even if the excitation power density was significantly increased from 6.8×10³ to 2.1×10⁵ W/cm².This fact indicates that the non-polar AlGaN-based MQWs grown in this study are indeed free of quantum-confined Stark effect?QCSE?.