Investigation of GaN epilayers growth mechanisms using in-situ reflectance in MOCVD
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Investigation of GaN epilayers growth mechanisms using in-situ reflectance in MOCVD Matthieu Moret, Olivier Briot, Sandra Ruffenach-Clur and Roger-Louis Aulombard GES, Université Montpellier II, CC074, Pl. E Bataillon, 34095 Montpellier Cedex 5, France. ABSTRACT GaN is a wide gap semiconductor which is now used to produce blue and green light emitting diodes, blue laser diodes and which has numerous other potential applications, like high frequency HEMT transistors, UV sensors, etc. A complicated two step process, using a low temperature buffer layer, subsequently annealed and followed by the deposition of the monocrystalline semiconductor was developed, and due to the excellent results obtained following this method, a rush towards applications resulted. There is now a need to investigate in more detail the growth mechanisms, and the influence of the growth parameters, in order to ensure a better reproducibility of the results. In this paper, we report an investigation of the growth mechanisms and the influence of the growth parameters using in-situ reflectance experiments. The reflectance measurements allow us to follow the growth rates, and the changes in the surface morphology (transitions between islands growth and 2D growth). Additional exsitu measurements (AFM ) were performed at different stages of the growth process to ensure additional information. As a result, we demonstrate that the recrystallization of the low temperature buffer layer is a critical step, which is drastically influenced by the composition of the annealing atmosphere (amount of ammonia present in the gas phase), while the deposition temperature and buffer thickness have a moderate effect. We will discuss here the growth mechanisms which may be involved to explain such a behavior. INTRODUCTION The nitride LEDs and LDs commercialized by Nichia Chemical Co. are evidence of the device potential for III-Nitrides. A lot of effort has gone into producing other devices like HEMT transistors [1] or UV photodetectors [2], but the complicated two step process used to grow high crystalline quality epilayers and the high density of dislocations act against a full success [3]. Today it seems very important to better understand the mechanisms involved in the growth in order to ensure a better reproducibility and moreover, better results in the rush towards devices. The two step growth process was first introduced by Yoshida et al. [4] for MBE growth, and applied to MOCVD growth four years later by Amano and Akasaki [5]. This process consists in the growth of a low temperature buffer layer before the growth of the high temperature epilayer, in order to limit the problem of the large lattice mismatch and expansion coefficients difference between the substrate and the epilayer. This buffer layer is annealed at high temperature to recrystallize. Then, we grow the main layer at high temperature. In each of these steps, the temperature, the V/III ratio, the pressure and the ambient (hydrogen, nitrogen or an ammonia/hydrogen mixture) are all parameters to be o
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