MOCVD Growth Of Semipolar AlInGaN And Characterization Of Optoelectronic Properties

Since the polarization electric field direction of semipolar(1122)plane AlInGaN quaternary material forms an angle of 58.4° with the growth direction,the quantum confined Stark effect(QCSE)can be effectively suppressed,thereby significantly improving the internal quantum efficiency.Meanwhile,this structural feature also makes the light emission polarization change from the strong TM mode to the TE mode with the increase in Al composition,which effectively improves the light extraction efficiency.The energy band gap and lattice constant of semipolar AlInGaN quaternary material can be adjusted independently by changing the composition of Al and In to achieve lattice matching between heterojunctions.Furthermore,high radiative recombination efficiency can be obtained in AlInGaN quantum wells even with high dislocation density due to the In-segregation effect.Therefore,semipolar AlInGaN quaternary alloys are regarded as ideal materials for the fabrication of high-efficiency ultraviolet light-emitting diodes(UV-LEDs).In this dissertation,semipolar(1122)plane AlInGaN quaternary epilayers were epitaxially grown on 2-inch(1010)m-plane sapphire substrates by metal organic chemical vapor deposition(MOCVD)technology,and surface morphology,crystal quality,and optoelectronic properties were characterized.The main research contents and results of this dissertation are as follows:1.The surface morphology and optical properties of semipolar(1122)plane AlGaN epilayers were improved by optimizing the growth parameters such as Ⅴ/Ⅲ ratio,temperature,and total flow in the reaction chamber.Semipolar(1122)plane AlGaN epilayers with Al composition of 27%and 53%were obtained,respectively.2.The effect of the hydrogen ratio in the reaction chamber on the properties of semipolar(1122)plane AlInGaN quaternary epilayers was investigated.An evident decrease in the number of hillock-like features and a slight increase in the size of hillock-like features were observed on the surfaces of the semipolar AlInGaN epilayers as the proportion of hydrogen in reaction gas was gradually increased.A root mean square(RMS)value as small as 2.7 nm and an indium composition of 3.2%were achieved by adding extra 15%hydrogen in reaction gas.3.The effect of growth pressure on the semipolar(1122)plane AlInGaN quaternary epilayers was investigated.An enhanced indium composition of 3.9%was obtained together with a 64%reduction in growth rate and dramatic degradation in surface morphology and crystalline quality by increasing the growth pressure from 40 to 100 Torr for the semipolar AlInGaN epilayer due to aggravated parasitic reactions.4.The semipolar(1122)plane AlInGaN quaternary epilayers were grown by the ammonia single pulsed flow growth technology.The In incorporation amount was guaranteed and the influence of parasitic reactions was suppressed by optimizing the ammonia pulsed flow time.The surface morphology of semipolar(1122)plane AlInGaN quaternary epilayers was effectively improved and the RMS value was further redused to 2 nm by using ammonia single pulsed flow growth technology.