Epitaxial Growth And Characterization Of Non-polar AlGaN-based MQWS

Non-polar a-plane AlGaN-based films are potential materials for the fabrication of deep ultraviolet light emitting diodes(DUV-LEDs),and therefore it has an extraordinary scientific significance and practical value to perform growth and theoretical research on the non-polar(1120)a-plane AlGaN multiple quantum wells(MQWs).In this thesis,the non-polar a-plane AlGaN-based materials were grown with various pulsed-flow methods by using metalorganic chemical vapor deposition(MOCVD)technology.In particular,the non-polar a-plane AlGaN MQWs with an emission wavelength of 279.2 nm were successfully fabricated.The major works conducted in this research are listed as follows(1)Several non-polar a-plane AlGaN epi-layer samples were deposited on semi-polar(1012)r-plane sapphire substrates with various growth methods.Compared with conventional continuous-flow growth method,the parasitic gas phase reactions between trimethyl-aluminum(TMAl)and ammonia(NH3)could be suppressed effectively during the two-way pulsed-flow MOCVD growth process.Moreover,the surface morphology of the non-polar a-plane AlGaN epi-layer was improved evidently and the relative light transmittance was increased by 36.9%in the wavelength range of 200-400 nm(2)The growth technology for the non-polar a-plane Al0.34Ga0.66N films was reformed by modulating the NH3-flow duty-ratio in the pulsed-flow MOCVD growth process.It was found that the competitive adsorption for the growth of alloys was suppressed,and the incorporation of Al adatoms was increased.In fact,the typical pyramidal defects distributed on the non-polar a-plane AlGaN epi-layer superficies were reduced apparently.Furthermore,the free exciton(FE)-related emissions were also enhanced effectively(3)The non-polar a-plane AlGaN epi-layers with various Al compositions were grown with the pulsed-flow MOCVD growth technology,and a novel dual nitridation technology was developed.On the one hand,the Al composition of non-polar a-plane AlGaN epi-layer could be modulated accurately with the pulsed-flow growth technology which also makes the Ga and Al compositions of quantum wells and barriers uniform.On the other hand,smooth surface of non-polar a-plane AlGaN epi-layer with the root-mean-square(RMS)of 1.54 nm was obtained with appropriate dual nitridation processes for both sapphire substrate and low temperature-grown AlN nucleation layer,which offers the possibility to grow flat non-polar a-plane AlGaN MQWs with abrupt interfaces(4)Based on the growth methods developed in this study,an internal quantum efficiency(IQE)as high as 39%was achieved with the non-polar a-plane AlGaN-based MQWs with an emission wavelength of 279.2 nm.Meanwhile,there was no blue-shift in wavelength for the MQWs-related emission peak observed in the excitation power-modulated photoluminescence(EPM-PL)spectra even if the power density was greatly increased.This fact indicates that the band bending and quantum-confined Stark effect(QCSE)were fully eliminated in the non-polar a-plane AlGaN-based MQWs.