X-Ray Reciprocal space mapping studies of strained GaN/AlGaN quantum wells
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X-Ray Reciprocal space mapping studies of strained GaN/AlGaN quantum wells Olivier Briot, Sandra Ruffenach-Clur, Matthieu Moret and Roger Louis Aulombard Groupe d’Etudes des Semiconducteurs, UMR5650, CC074, Université Montpellier II, Place E.Bataillon 34095 Montpellier Cedex 5 France
ABSTRACT In this work, we report on the systematic X-ray reciprocal space mapping of a series of GaN/AlGaN samples, with different Al content and well thickness. For coherently grown samples, we present a calculation which allow us to precisely determine the strain state and Al content in the samples in one diffraction experiment. In our samples, both GaN and AlGaN are strained, and we discuss the effect of these strains on the band structure of GaN, which is probed by low temperature reflectivity and correlates perfectly our x-ray results. INTRODUCTION GaN/AlGaN quantum wells are currently the subject of considerable interest, both for their technological potential and theoretical interest. As a matter of fact, strong electric fields are present in these samples, due to both a spontaneous polarization and a piezoelectric polarization[1,2]. To analyze the optical data and the polarization effects, one must know the exact aluminum content of the barrier. This is usually tricky to determine from x-ray experiments, since the lattice parameters of the AlGaN alloy is affected both by the composition and strain effects induced by the layer onto it which it is grown. Moreover, since piezoelectric effects represent a large part of these effects, it is important to precisely determine and analyze the strain state of the structure.
EXPERIMENTAL The GaN/AlGaN heterostructures were grown by low pressure MOCVD using an AIXTRON 200/4 RF-S system. (0001) sapphire substrates were used and the structure of the samples is as follows : a 250 Å GaN nucleation layer is grown at low temperature (540°C), followed by a 1µm GaN buffer layer grown at 1140°C. The single quantum well is then grown, consisting in 300Å AlGaN barriers, with Al composition between 7 and 16.5 %, the GaN quantum well layer having a thickness between 3 and 25 monolayers. The reciprocal space mapping is realized on a Phillips Xpert diffractometer, using the CuKα1 radiation. Photoluminescence measurements were realized at 2K, using a HeCd 10 mW laser. Reflectivity was also recorded at 2K, using a white, low power, tungsten lamp. In order to get accurate angle measurements, we calibrated the ω angle using the sapphire (0006) Bragg peak.
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DISCUSSION The precise determination of the Aluminum composition can only be achieved provided that the AlGaN layer is coherently grown onto its substrate, i.e. if it is in an elastic state of strain, for which we can apply simple calculations. The method rely on the correlation of the a and c lattice parameters for the AlGaN layer. We will here restrict ourselves to the case of hexagonal epilayers grown along the (0001) direction, which corresponds to the vast majority of the nitrides samples. So, assuming that the AlGaN layer is elastically st
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