Raman Characterization of Strained GaN y As 1-y and In x Ga 1-x N y As 1-y Epilayers
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Raman Characterization of Strained GaNyAs1-y and InxGa1-xNyAs1-y Epilayers Li-Lin Tay, David J. Lockwood, James A. Gupta and Zbig R. Wasilewski Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada K1A 0R6 ABSTRACT Pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y have been grown on GaAs(100) with y ranging from 0 to 0.05. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes are represented at y = 0.05 by the strong longitudinal optic (LO1) mode at 288.5 cm-1 and the weaker transverse optic (TO1) mode at 268.3 cm-1, while the GaN-like LO2 mode is observed at 474.8 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the GaN-like LO2 phonon band displays a sub-linear increase in wavenumber. Raman results for the unstrained quaternary alloy In0.06Ga0.94N0.02As0.98 are compared with those of GaN0.02As0.98. INTRODUCTION Recent advances in the growth of dilute nitride compounds have attracted, in particular, attention to GaNyAs1-y and InxGa1-xNyAs1-y due to their potential application as lasers, photodetectors and solar cells. Introducing dilute amounts of nitrogen into GaAs is complicated by the large difference between the GaAs and cubic GaN lattice constants. GaNAs epilayers grown coherently on GaAs exhibit tensile strain, which will, no doubt, have a significant influence on the properties of the alloy. In addition, the incorporation of nitrogen in GaAs causes a large bandgap bowing with a dramatic reduction of the fundamental bandgap [1]. Raman spectroscopy is a sensitive technique for investigating the nitrogen incorporation and any ordering effect in the crystalline lattice. This is because Raman frequencies of optical phonons are sensitive to atomic bonding arrangements [2]. In this paper, we present a Raman study of the optical phonon response to increasing the nitrogen concentration in molecular beam epitaxy (MBE) grown GaNyAs1-y samples, with 0 < y < 0.05. Raman scattering results from a lattice matched InxGa1-xNyAs1 epilayer will also be presented and compared with those of the equivalent GaNyAs1-y sample. EXPERIMENTAL DETAILS The GaNyAs1-y samples studied in this work were prepared in two growth runs, where graded nitrogen compositions were obtained by deliberately not using substrate rotation during growth, in order to exploit the natural, geometric flux gradients of the MBE system. Each of these two GaNyAs1-y strained epilayers nominally 35 nm thick was
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grown on a GaAs(100) substrate in a custom Thermo-VG Semicon V90 MBE system. Gallium was provided by a thermal effusion cell, with a valve cracker cell for As2 and active nitrogen was provided by an Applied-Epi Unibulb RF plasma cell using dynamic gas switching [3]. For each sample, the native surface oxid
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