Optical and Structural Properties of AlGaN/GaN Quantum Wells Grown by Molecular Beam Epitaxy
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Centre de Recherche sur l'H6t6ro-Epitaxie et ses Applications - CNRS Rue B. Gr6gory, Sophia Antipolis, F-06560 Valbonne, France. II. Case Courtier 074. 34095 Montpellier Cedex 5, France. LPM - Universit6 d'Avignon - 33, rue Pasteur. 84000 Avignon, France.
** Groupe d'Etude des Semiconducteurs - CNRS - Universit6 Montpellier
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G11.7 (1999) ABSTRACT AIGaN/GaN quantum well (QWs) were grown on (0001) sapphire substrates by molecular beam epitaxy (MBE) using ammonia as nitrogen precursor. The Al composition in the barriers was varied between 8 and 27 % and the well thickness from 4 to 17 monolayers (MLs, 1ML = 2.59A). X-ray diffraction (XRD) experiments are used to investigate the strain state of both the well and the barriers. The QW transition energy are measured by low temperature photoluminescence (PL). A large quantum confined Stark effect is observed leading to QW luminescence much lower than the emission line of the GaN buffer layer for well width above a certain critical thickness. The built-in electric field responsible for such a phenomenon is deduced from fit of the PL data. Its magnitude is of several hundred kV/cm and increases linearly with the Al composition. INTRODUCTION The III-V nitride compounds have demonstrated their peculiar aptitude for producing light from the green to UV spectral range [I]. As for the case of other optoelectronic devices based on arsenide, phosphide or antimonide compounds, quantum wells (QWs) are currently used in the active region of nitride based light emitting diodes and laser diodes [1]. However, there is still only a few reports on the physical properties of nitride QWs compared to what is known about the prototypical system AlGaAs/GaAs. This is especially the case when considering the AIGaN/GaN QWs [2-8] though they appear very promising for extending the applications of nitrides to the far UV spectral region owing to the large direct band gap of AIN (6.2 eV/200 nm). It has been already shown that a large internal electric field of several hundred kV/cm takes place in the wurtzite AlGaN/GaN QWs [2-8]. Both piezoelectric and spontaneous polarization must be considered to account for the polarization field inside the quantum structure. Actually, spontaneous polarization has been theoretically considered because of the expected huge influence on the properties of AlGaN/GaN heterostructures [9,10]. In this paper, we report on AlGaN/GaN QWs grown by MBE on c-plane sapphire substrates. Their structural properties have been checked by XRD and their optical properties by low temperature PL. The Al composition of the AlGaN barriers was ranged between 8 and 27% and G 11.7 Mat. Res. Soc. Symp. Proc. Vol. 537 © 1999 Materials Research Society
the well thickness increases from 4 to 17 MLs. The internal electric field was deduced by fitting the PL data using a self-consistent envelope function calculation, including excitonic effects.
EXPERIMENTS The growth of GaN and AlGaN layers are carried out in a Riber 32 P MB
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