Study On Growth Mechanism Of High Quality AlN By Molecular Beam Epitaxy And Physical Properties Of Related Materials

AlGaN is a very important semiconductor material for applications ofmicroelectronic and optoelectronic devices, including high electron mobilitytransistors (HEMTs), UV light emitting diodes (LED), UV laser diode (LD) and UVphotodetectors (PD). AlGaN-based material growth and stress control are twoimportant and difficult issues in III-Nitride research, and have attracted muchattention. The high quality AlN growth is of significance for realizing high qualityAlGaN. In this dissertation, the growth mechanism of AlN grown on sapphiresubstrate by plasma-assisted molecular beam epitaxy (PA-MBE) was studiedsystematically. Besides, the influences of SiN_x on stress state of AlGaN/GaNheterostructure were also investigated.Firstly, the effects of nitridation temperature and time on AlN crystallinequality were observed. It is found that nitridation at high temperature for alonger time can reduce dislocation density. The X-ray photoelectronspectroscopy results indicate that the formation of AlN is more structurallycomplete using this process condition. Therefore, it can act as a better initialtemplate to improve the crystalline quality of the epi-layer grown above.The influences of thickness of nucleation layer on AlN epi-layercrystalline quality were investigated next. A critical nucleation thickness forrealizing high quality AlN films has been observed. When the nucleationthickness is above a certain value, the edge-type threading dislocations (TDs)increase with nucleation thickness, whereas the screw-type TDs show anopposite trend. It is considered that, the thicker nucleation layer with largernuclei can reduce screw-type TDs, while the thinner nucleation layer withlower nuclei density can prevent the formation of edge-type TDs.Afterwards, V/III ratio for main layer growth was carefully optimized.The effects of growth mode under Al-rich condition with different V/III ratioon threading dislocations (TDs) and surface morphology were observed and discussed. Using optimized V/III ratio, high quality AlN with a thickness of 4μm was realized. Then an interlayer structure was adopted. By optimizing thethickness of interlayer, the TD density and surface roughness can be reducedsimultaneously. High quality 1-μm-thick AlN was obtained with an optimizedinterlayer thickness of 50 nm. By transmission electron microscopy (TEM),the screw and edge-type TD densities penetrating to surface density aredetermined as low as 5×10⁷ and 6.5×10⁹ cm^-2, respectively. A very smoothsurface with an rms roughness of 0.4 nm was also realized.Besides, to study the stress control effects of SiN_x films on AlGaN/GaNheterostructure, SiN_x deposited by plasma enhanced chemical vapordeposition (PECVD) in different conditions on two dimensional electron gas(2DEG) characteristics were investigated. The DC as well as gate-lag anddrain-lag measurements on HEMT devices with and without SiN_x were alsoperformed. SiN_x deposited using high frequency source can enhance the 2DEGand reduce current collapse by inducing extra tensile strain in AlGaN layer aswell as surface state passivition. Oppositely, SiN_x deposited using lowfrequency source weakens 2DEG greatly due to an extra compressive strainand ion bombardment damage, and corresponding HEMT devices failedpermanently.