Growth And Application Of Low Temperature AlN Thin Films Deposited By MOCVD

The growth of AlGaN/GaN MQWs is attracting more and more attention due to wide application in both optoelectronic and electronic devices such as ultraviolet light-emitting diodes and high electron mobility transistors. However, it is still a big challenge to grow high quality AlGaN/GaN MQWs because of the large lattice mismatch between AlGaN/GaN MQWs layer and GaN templates as well as their different coefficients of thermal expansion, which easily result in a great amount of structural defects and cracks and significant strain relaxation, tending to severely deteriorate device performance. Low temperature AIN(LT-AIN) interlayer has been commonly used as a strain engineering technique to achieve crack-free growth of thick AlGaN layers on GaN template. The elimination extent of defects and stress in overlayers through low temperature AIN (LT-AIN) interlayer is determined by the growth condition of LT-AIN interlayer on GaN. In this article, firstly, to obtain high quality thin films, we exploited LT-AIN interlayer beneath the MQWs layers, and carry out a study on the crystal quality of LT-AIN layers grown at different growth temperatures by metal-organic chemical vapor deposition (MOCVD), we also investigated the influence of growth temperatures of LT-AlN interlayer in the epitaxial crystal quality and optical electrical properties of AlGaN/GaN quantum well layer.The main research harvests are as follows:Morphology and microstructure evolution of LT-AlN layers at different growth temperatures by MOCVD were investigated. It is found that different growth temperatures of LT-AlN interlayer cause the different rms roughness and threading dislocations (TD) density of AlGaN/GaN MQWs. It can be noted that in the range of LT-AlN interlayer growth temperature studied here, both screw-type dislocation density and edge-type dislocation density rapidly decrease as the temperature rises at first, then increase with further rising temperature, and reach the minimum TDD obtained at640℃. However, atomic force microscopy analysis of LT-AlN layers shows the contrasting tendency. It is clear that the rms roughness of five samples first increases till the maximum at640℃and then decreases with growth temperature. The result of theoretical calculations of strain in LT-AlN layers shows the same tendency to the rms roughness. In our opinions, the growth temperatures of LT-AlN layer directly affect the mobility of Al atoms at the interface and the recombination rate, which is due to the different growth model of grain, and then directly influence the edge and screw type threading dislocation densities in the epitaxial layer, strain and surface morphology.Morphology and microstructure evolution of AlGaN/GaN multiple quantum-wells with low-temperature AlN interlayers were investigated. It is found that different growth temperature of LT-AIN interlayer cause the different rms roughness and threading dislocations (TD) density of AlGaN/GaN MQWs. The lowest TD density of MQWs is obtained when the growth temperature of LT-AIN interlayer is640℃, the highest mobility and the band edge emission peak intensity are also obtain at the same time. Atomic force microscopy analysis of LT-AIN layers shows the contrasting surface morphology, which is due to the different growth modes influenced by the temperature, and the different surface morphology directly affects the glide of misfit dislocations through primary slip system and TD blocking mechanism, resulting in different strain relaxation mechanisms of AlGaN/GaN MQWs.Using the high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) and other test methods, we focus on the effect of the LT-AIN interlayer grown at640℃on the epitaxial crystal quality of AlGaN/GaN quantum wells. The experimental data show that the interlayer grown at640℃can effectly block the threading dislocation from the GaN substrate reduce the dislocation density in the quantum well; and improve the epilayer surface roughness. The strain of AlGaN/GaN indued by the LT-AIN interlayer will lead to a blue shift of the photoluminescence peak.