Improvement of Crystalline Quality of Group Iii Nitrides on Sapphire Using Low Temperature Interlayers

In organometallic vapor phase epitaxial growth of group III nitrides on sapphire, insertion of a low temperature interlayer is found to improve crystalline quality of AlxGa1-xN layer with x from 0 to 1. Here the effects of the low temperature deposited GaN or AlN interlayers on the structural quality of group III nitrides is discussed. INTRODUCTION From the late 60’s to the early 70’s Maruska and Tietjen [1], Pankove et al. [2] and Monemar [3] demonstrated the significant potential of GaN as a solid source blue light emitter. However, the difficulty in growing high crystalline quality GaN free of cracks, and in fabricating p-type GaN, have long prevented the practical application of GaN. The use of low-temperaturedeposited buffer layer (LT-BL) [4,5] and the realization of p-type GaN [6,7] triggered the vast expansion of nitride research worldwide [8]. Today, bright blue light emitting diodes (LEDs), green LEDs and white LEDs based on nitrides have been commercialized, and purple, deep purple and even UV laser diodes have been fabricated. Microwave field effect transistor (FET) and solar-blind UV detectors are also available. These results are remarkable considering the 16% lattice mismatch between GaN and sapphire, which were accommodated by the LT-BL approach. Nevertheless, GaN grown on sapphire using a LT-BL still contains high threading dislocations (TDs) densities on the order of 10-10cm that originate at the interface between GaN and the LT-BL and/or sapphire substrate [9,10]. TDs affect performances of several devices such as LEDs or FETs because some of the TDs act as non-radiative recombinaiton centers [11] and/or scattering centers in the transport of electrons [12]. Recently, we found that insertion of low-temperature-deposited GaN or AlN interlayers (LTGaN IL or LT-AlN IL) between high-temperature-grown GaN (HT-GaN) layers reduces TD densities of HT-GaN [13-15]. We also found that the low temperature interlayer is effective not only for the improvement of the crystalline quality of the HT-GaN layer, but also for that of HTAlGaN and HT-AlN. In this study, X-ray diffraction (XRD), transmission electron microscopy (TEM) as well as microscopic observations were performed to investigate the effects of LT-ILs on the structural properties of the topmost GaN, AlGaN and AlN layers. EXPERIMENTS Nominally undoped GaN, AlGaN and AlN layers were grown on (0001) sapphire substrates in a horizontal reactor [16]. TMGa, TMAl and ammonia were used as Ga, Al and nitrogen source gases, respectively, at around 140 Torr growth pressure. Fig.1 schematically shows the structure of the sample used in this study. In this paper, the LTlayer between sapphire substrate and HT-GaN is called buffer layer (BL), while the LT-layer between HT-nitride is called interlayer (IL). Just after thermal annealing of the sapphire (0001) substrate in a hydrogen flow at 1,150°C, the graphite susceptor was cooled down to 500°C in order to deposit either a LT-AlN BL or a LTGaN BL on the sapphire substrate. The thickness was ~20nm. HT-GaN was grown at 1,050°C on the LT-BL. The thickness of the first HT-GaN layer was fixed at ~1 μm. The deposition conditions for LT-AlN IL or the LT-GaN IL was just the same as that for the first LT-BL. HTnitrides were grown on top of the LT-IL. For comparison, HT-AlGaN and HT-AlN were also grown on LT-BL or on HT-GaN not covered with a LT-IL.