Raman spectroscopy studies in InGaN/GaN wurtzite epitaxial films
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Raman spectroscopy studies in InGaN/GaN wurtzite epitaxial films. Maria R. Correiaa1, Sérgio Pereira1, Teresa Monteiro 1, Estela Pereira1 and Eduardo Alves2 1 Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal 2 I.T.N., Departamento de Física, E.N.10, 2686-935 Sacavém, Portugal ABSTRACT In this work we studied a set of nominally undoped epitaxial InxGa1-xN wurtzite films grown on (0001) sapphire substrates. In order to separate the contribution of the strain and indium content in the phonon mode frequency, indium mole fraction was determined using a strain insensitive method, Rutherford backscattering spectrometry (RBS). Strain was evaluated by comparing the lattice constants measured by X-ray diffraction (XRD) with the relaxed lattice parameters given by Vegard’s law. Samples with comparable indium content, but under different states of strain were used as reference. This allowed the behaviour of different Raman shift modes for both, strain and composition to be independently established. We also assess the potentiality of Raman spectroscopy for the evaluation of crystalline quality by comparing the results obtained with the ones provided by other well-established methods such as XRD and RBS. INTRODUCTION As technological and fundamental interest in InxGa1-xN properties continues to rise, it is increasingly important to establish the best techniques to assess relevant aspects of this material. Raman spectroscopy is a convenient non-destructive tool for the characterisation of semiconductors. As a rather sensitive, local, and fast technique it is a widely used method, namely in the study of optoelectronic properties of III-Nitrides heterostructures [1]. The optimisation of this technique, when applied to InxGa1-xN demands the establishment of the strain and indium content dependence for the Raman active modes. For this purpose the knowledge of the phonon frequencies and respective symmetry of the related binary compounds, GaN and InN, is essential. Raman modes for GaN are well known, however the measurement of phonon modes for relaxed InN has only recently been reported [2]. Both hexagonal GaN and InxGa1-xN have the wurtzite structure with two formula units per primitive unit cell. Group theory predicts the following Γ- point optical phonon modes for the wurtzite structure (point group C6v): A1(z)+E1(x, y) +2E2+2B1 [3], which are all Raman active except for the B1 modes. The A1(z) are polarised along z axis and E1(x, y) modes are polarised in the xy plane. A strong resonant enhancement of scattering by A1 (LO) GaN/(Al,InGa)N heterostuctures attributed to a Frölich mechanism has been observed [4,5,6,7]. This mechanism has been used to efficiently analyse thin heterostuctures. It is observed for incident photon energies close to the interband transition energy. This resonant effect allowed to distinguish between the two signals from GaN buffer and InxGa1-x N, by comparison of spectra obtained for the samples with low and high In concentration, respectively, the latter being close to resonant con
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