Effect of V/III Ratio on the Properties of GaN Layers Grown by Molecular Beam Epitaxy Using NH 3
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N. GRANDJEAN, M. LEROUX, J. MASSIES, M. MESRINE, AND P. LORENZINI Centre de Recherche sur 1'H&•ro-Epitaxie et ses Applications, Centre National de la Recherche Scientifique, Rue Bernard Gregory, Sophia Antipolis, 06560 Valbonne, France. ABSTRACT Ammonia as nitrogen precursor has been used to grow III-V nitrides by molecular beam epitaxy (MBE) on c-plane sapphire substrates. The efficiency of NH 3 has been evaluated allowing the determination of the actual V/III flux ratio used during the GaN growth. The effects of the V/III ratio variation on the GaN layer properties have been investigated by photoluminescence (PL), Hall measurements, atomic force microscopy (AFM), and secondary ion mass spectroscopy (SIMS). It is found that a high V/III ratio leads to the best material quality. Optimized GaN thick buffer layers have been used to grow GaN/AIGaN quantum well (QW) heterostructures. Their PL spectra exhibit well resolved emission peaks for QW thicknesses varying from 3 to 15 monolayers. From the variation of the QW energies as a function of well width, a piezoelectric field of 450 kV/cm is deduced. INTRODUCTION The family of III-V nitrides is now well recognized to be quite promising for short-
wavelength optical devices [I], but also for high power, high temperature, and high
frequency electronics. While the leading growth technique for these materials is the metalorganic chemical vapor deposition (MOCVD), MBE is also a viable technique as shown by the realization of LEDs [2,3] or modulation-doped field-effect transistors [4]. Two ways are presently followed to produce nitrogen radicals in MBE: either a N2 plasma or directly NH3 which is pyrolized at the growing surface. This paper reports on the MBE growth of III-V nitrides using NH3, and in particular on the effect of the V/MI flux ratio. It is shown that the optimization of the GaN growth allows the realization of high quality AlGaN/GaN quantum wells. EXPERIMENTS The growth of HI-V nitrides was carried out in a Riber 32P MBE system equipped with reflection high-energy electron diffraction (RHEED) and laser reflectivity facilities. Ga and Al are obtained from standard solid source effuision cells. N atomic species were provided by the decomposition of NH3 at the growing surface. The nitridation and the low temperature buffer layer procedures are described in ref 5. Alloy compositions and QW thicknesses are determined in situ from RHEED intensity oscillations [5].
69 Mat. Res. Soc. Symp. Proc. Vol. 512 0 1998 Materials Research Society
RESULTS The efficiency of NH 3, i.e. the nitrogen incorporation during growth, can be investigated through the variation of the growth rate as a function of the NH 3 incident flux (Fig. 1). When varying the NH 3 flux over a large range, keeping the Ga flux constant, the growth must be either N-limited or Ga-limited [6]. More precisely, the frontier between these two growth regimes corresponds in fact to the situation where the Ga flux is equal to the N atomic species available from the NH 3 decomposition. From the comparison of t
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