Epitaxial Growth Of AlGaN Based Deep Ultraviolet Light Emitting Diode By MOCVD

AlGaN based deep ultraviolet (UV) light emitting diode (LED) is a new kind of solid state UV light source. Compared with the conventional UV mercury lamb, UV-LEDs have many advantages, such as smaller size, lighter weight, lower power consumption, longer lifetime, more environmental friendly and can be continuously adjusted in emitting wavelength. Therefore, UV-LEDs gain extensive attention in the application areas related to UV light and begin to penetrate into some traditional applications of UV mercury lamb. However, the external quantum efficiency of AlGaN based UV-LEDs is far below the commercialized InGaN based blue LEDs, which is due to the problems such as high dislocation density in high Al content AlGaN, strong polarization effect in multi-quantum wells (MQWs), low hole injection efficiency and low light extraction from c-plane AlGaN. The low light emission power and poor quantum efficiency impede the application of UV-LEDs seriously.Aiming at the key technical issues in AlGaN based UV-LEDs, we begin with the epitaxy of high quality AIN and AlGaN layers by metal-organic chemical vapour deposition (MOCVD). Then some critical layers in the device structure such as AlGaN/AlGaN MQWs, and electron blocking layers (EBLs) have been optimized by combining theoretical simulation and experimental verification. Finally, we have obtained deep UV-LEDs with light emission power of1mW in several wavelengths. The detailed research contents in this dissertation are arranged as below.Firstly, high quality AIN templates have been obtained by using a combination of growth techniques of pulsed atomic layer epitaxy (PALE) and continuous growth. The growth rate of AIN with PALE method has been increased by adjusting the Ⅴ/Ⅲ ratio which also can improve the crystal quality. Combining the newly developed PALE growth with the conventional continuous growth method, we have effectively improved the crystal quality and surface morphology. Furthermore, we have adopted intermediate-temperature AIN interlayer to further reduce the dislocation density in AIN layers and have investigated the corresponding defect reduction mechanism.Secondly, for the first time the in-situ grown SiNx interlayers have been introduced in Si-doped Al0.4.sGa0.55N layers in order to reduce the dislocation density. We have found that the dislocation densities in Si-doped Al0.45Ga0.55N layers can be effectively reduced through optimizing the growth time of the SiNx interlayer. The mechanism for dislocation reduction in Al0.45Ga0.55N layers with SiNx interlayer has been explained with a newly established model.Thirdly,2.5μm thick crack-free Si-doped Al0.46Ga0.54N has been obtained. The effect of AIN/AlGaN superlattice with different periods on the quality of Si-doped Al0.49Ga0.51N has been investigated. Then, we designed two sets of high Al content AlGaN/AIN superlattices and succeeded in the fabrication of2.5μm crack free Si-doped Al0.46Ga0.54N with an electron concentration of3.09×1018cm-3.Fourthly, AlGaN/AlGaN MQWs with different well thickness and Si doping conditions have been studied systematically with simulation and experiment to find out their effects on light emitting property. We have obtained high efficiency290nm emitting MQWs with3nm well thickness and Si doing in both the wells and barriers.Fifthly, EBLs in UV-LEDs have been studied with APSYS software. We proposed a new EBL structure with Al content graded from0.9to0.4in the growth direction which proved to be simultaneously effective in electron blocking and hole injection. Large improvement in light output power and internal quantum efficiency have been achieved for the UV-LEDs with Al content graded EBL when compared with the reference sample with the conventional Al0.7Ga0.3N EBL in the simulation results.Finally, the effect of doping in the last quantum barriers has been systematically investigated with simulation and experiments. Mg-doped half of the last quantum barrier near to the EBL proved to be the most effective doping scheme in increasing the performance of UV-LEDs in experiment, which fits well with the simulation results.