Pulsed MOCVD Growth And Characterization Of GaN/AlN Superalttice

GaN/AlN superlattice（GaN/AlN-SL） can be widely used in photoelectric detectors,lasers and infrared devices. But there are still some difficulties in the mass production ofhigh quality GaN/AlN superlattice, i.e. growth of the SL with low defect and sharpinterface at low cost. The molecular beam epitaxy（MBE） technique used inGaN/AlN-SL growth is advantageous to form monoatomic smooth SL interface, but it ishigh in cost and low in growth rate and not suitable for mass production. Metal organicchemical vapor deposition（MOCVD） is a technique widely used in commercialproduction with reasonable cost, but the interface and the surface of MOCVD grown SLare usually not as good as those grown by MBE. So it is necessary to optimize theMOCVD growth parameters of GaN/AlN-SL.In this dissertation, the MOCVD growth of GaN/AlN-SL is studied to improve theSL surface and interface quality, and it is also investigated the potential advantage ofusing GaN/AlN-SL, or quasi-AlGaN barrier, instead of an alloy-AlGaN barrier layer inAlGaN/GaN heterostructures to form high performance high-Al componentAlGaN/GaN heterostructures. The major work and results are listed as follows.Firstly, according to plused MOCVD method, a feature of our self-built MOCVDequipment, it is proposed a new way to grow the GaN/AlN-SL. The ammonia is neverinterrupted in growth but controlled to flow at different rates for GaN layer and AlNlayer, and only a single pulse of MO source（TEGa/TMAl） is injected into the growthchamber at the beginning of depositing each GaN/AlN layer. In this way, V/III ratiocould be separately optimized for AlN layer and GaN layer to improve the crystalquality. Meanwhile, the interrupt following the MO source pulse in the deposition ofeach layer provides time for adjusting ammonia flow rate and switching different MOsource, and most importantly for the surface with metal atoms just deposited to benitrified to improve the two-dimensional smoothness. As a result, it is obtained the highquality GaN/AlN SL with smooth surface and ultra-thin GaN/AlN period thickness.Secondly, the optimized temperature for the grow of GaN/AlN SL is obtainedthrough series of SL growth experiments. It is found that at the growth temperaturehigher than the optimized one decomposition of GaN layers occurs resulting in seriousdeterioration of the crystal quality of GaN/AlN SL. While at the lower growthtemperature surface depression can be observed which also leads to poor superlattice quality.Thirdly, thin-GaN/AlN-SL is used as barrier layer on top of GaN template to formquasi-AlGaN/GaN heterostructures. The equivalent Al component of quasi-AlGaN is ashigh as45%, and the comparison of the quasi-Al_0.45Ga_0.55N/GaN heterostructure withconventional alloy-Al_0.45Ga_0.55N/GaN heterostructure shows that quasi-AlGaN/GaNheterostructure has a2D electron gas（2DEG） with much higher sheet density andtherefore a much higher depletion voltage. It is also found that utilizing GaN/AlN-SL asquasi-AlGaN barrier layer can increase the critical thickness for strain relaxation ofhigh-Al component AlGaN/GaN heterostructures.Finally, the one-dimensional self-consistant Schr dinger-Poisson model is used tostudy the quasi-AlGaN/GaN heterostruture. The2DEG distribution and energy bandprofile are obtained for the quasi-AlGaN/GaN heterostructures with different equivalentAl components, different SL period numbers, and different GaN/AlN period thickness.It is found that for the same Al component and thickness of AlGaN barrier, the2DEGsheet carrier density of quasi-AlGaN/GaN heterostructure is much higher thanalloy-AlGaN/GaN heterostructure even when no strain relaxation is considered.Altogether, GaN/AlN-SL used as barrier layer in GaN heterostructures not onlyincreases the2DEG density, but also increases the critical thickness of high Alcomponent AlGaN barrier layer.