Optical Constants, Critical Points, and Phonon Modes of GaAsN Single Layers
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Optical Constants, Critical Points, and Phonon Modes of GaAsN Single Layers G. Leibiger, V. Gottschalch Faculty of Chemistry and Mineralogy, University Leipzig, Linnestrasse 3, 04103 Leipzig, Germany
A. Kasik, B. Rheinländer Faculty of Physics and Geoscience, University Leipzig, Linnestrasse 5, 04103 Leipzig, Germany
J. Šik, M. Schubert Center for Microelectronic and Optical Materials Research, and Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588
Abstract Spectroscopic ellipsometry (SE) is employed to study the optical properties of GaAs1-yNy [0% ≤ y ≤ 3.7%] single layers for photon energies from 0.75 eV to 4.5 eV and for wavenumbers from 100 cm-1 to 600 cm-1. We provide parametric model functions for the dielectric function spectra of GaAsN in both photon energy ranges. The model functions for photon energies from 0.75 eV to 4.5 eV excellently match dielectric function data obtained from a numerical wavelength-by-wavelength inversion of the experimental data. Criticalpoint analysis of the ellipsometric data is performed in the spectral regions of the fundamental band gap and the critical points E1 and E1+∆1. The band-gap energy is red shifted whereas the E1 and E1+∆1 transition energies are blue shifted with increasing y. For y ≤ 1.65% the observed blue shift of the E1 energy is well explained by the sum of the effects of biaxial (001) strain and alloying. The GaAsN layers show two-mode behaviour in the infrared spectral range (100 cm-1 to 600 cm-1). We detect the transverse GaAs- and GaN- sublattice modes at wavenumbers of about 267 cm-1 and 470 cm-1, respectively. The polar strength of the GaN TO mode increases linearly with y. This effect can be used to monitor the nitrogen composition in GaAsN layers. 1.Introduction The ternary alloy GaAsN has attracted increasing interest in the past few years. In contrast to the well known chemical trends of usual III-V compounds the incorporation of nitrogen into GaAs reduces drastically the band gap. As shown experimentally and theoretically, the bowing of the energy gap depends strongly on the N content. This is mainly due the large chemical and size differences of the As and N atoms. Alloying of indium with GaAsN, the quaternary compounds can be matched to the GaAs or the InP host lattices. GaAsN and InGaAsN are of high interest for optoelectronic devices operating at wavelengths of 1.3 µm and 1.55 µm, such as in optical-fibre communication networks. Considerable progress in development of Laser diodes, bipolar transistors, resonant-cavity enhanced photodiodes, and high-efficiency solar cells has been reported recently. However, knowledge of the material dielectric functions is needed for a precise device design. The refraction indices of InGaAsN for photon energies below the fundamental band gap were reported by Kitatani et al. [1] using spectroscopic ellipsometry (SE). Uesugi et al. [2] G6.35.1
measured the absorption indices of GaAsN also for photon energies below the fundamental band-gap. Grüning et al. [3] detected th
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