Raman study of resonance effects in Ga 1-x Al x N solid solutions
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Internet Journal Nitride Semiconductor Research
Raman study of resonance effects in Ga solutions
1-xAl xN
solid
F. Demangeot1, J. Frandon1, M. A. Renucci1, H. Sands2, D. Batchelder2, S. Ruffenach-Clur3, Olivier Briot3 and Bernard Gil3 1Laboratoire
de Physique des Solides (ESA 5477 CNRS), Université Paul Sabatier, of Physics and Astronomy, University of Leeds, United Kingdom, 3Groupe d'Etude des Semiconducteurs, GES-CNRS, 2Department
(Received Monday, June 22, 1998; accepted Monday, November 30, 1998)
The photoluminescence and Raman spectra of several Ga1-xAlxN layers (0 ≤ x ≤ 0.86) grown on sapphire substrates by metal-organic vapor phase epitaxy have been recorded at room temperature, under an excitation at 244 nm. Using the photoluminescence spectra, the variation of the band gap of these alloys can be followed only up to x = 0.5. From resonant Raman scattering, it can be deduced that the band gap energy of the solid solution for x very close to 0.7 corresponds to the incident photon energy (5.08 eV). This result is confirmed by a detailed comparison of the present work with previous experimental data on the A1(LO) phonon peak position, obtained under visible excitation.
1
Introduction
Gallium aluminum nitride alloys are particularly suited to the technology of opto-electronic devices, particularly ultraviolet light emitters. Indeed, the width of their forbidden band can be tuned for many applications in this spectral range by varying the alloy composition. The basic physical properties of these solid solutions have been investigated only recently; concerning the dynamical properties of Ga1-xAlxN, they have been studied by Raman spectroscopy under excitation in the visible range [1] [2] [3]. The present measurements have been performed in view of studying the band gap in the Ga1-xAlxN alloys through resonant Raman scattering. Due to the strong electron-phonon Fröhlich interaction in polar crystals, a drastic enhancement of light scattering by polar LO phonons can be achieved in resonant conditions, i.e. when the energy of the incident (or scattered) photon is close to the band gap energy of the semiconductor. Generally, the resonant effect is evidenced on a given sample by varying the excitation energy. In the present work, this energy was kept constant and the intensity of the polar LO phonon was measured on several Ga1-xAlxN layers, thus varying the band gap energy with x. It should be noted that the LO phonon observable in backscattering geommetry along the c-axis of the wurtzite structure corresponds to the symmetry A1; according
to the selection rules, it is allowed only when the polarizations of the incident and scattered light are parallel. So, far from resonance, the contributions of the allowed and “ forbidden ” (resonant) scattering processes cannot be distinguished, while the latter predominates only in resonant conditions. 2
Samples and experiments
We present photoluminescence and Raman measurements on a series of alloy samples with the wurtzite structure, covering a wide compositional ra
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