Raman Studies of GaNP Alloy
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Raman Studies of GaNP Alloy I. A. Buyanova1, W. M. Chen1, E. M. Goldys2, H. P. Xin3 and C. W. Tu3 Department of Physics and Measurement Technology, Linköping University, S-581 83 Linköping, SWEDEN 2 Division of Information and Communication Sciences, Macquarie University, NSW 2109 Sydney - Australia 3 Department of Electrical and Computer Engineering, University of California, La Jolla, CA 92093-0407, USA 1
ABSTRACT Raman scattering (RS) spectroscopy is employed to characterize the effect of nitrogen on structural properties of GaNxP1-x alloy with nitrogen composition up to 3 %. Two-mode behavior of the alloy is clearly shown. The frequency of the GaP-like LO phonons is found to decrease with N composition as –1.13 cm-1 x. This dependence is proposed to be largely due to the biaxial strain in the GaNP epilayers, as a result of lattice mismatch to the GaP substrate. The frequency of the GaN-like phonons is found to be more sensitive to nitrogen content, increasing with the rate of +2.6 cm-1x. The addition of nitrogen is also found to cause a dramatic quenching of the two-phonon Raman scattering and an appearance of the zone edge GaP-like vibrations. These effects are suggested to reflect local distortion in the GaNP lattice induced by nitrogen, as well as possible clustering of N atoms. INTRODUCTION Incorporation of nitrogen in GaP has been shown to induce strong modifications in the electronic properties of the resulting alloy such as a huge bowing in the band-gap energy [1, 2], splitting of the conduction band states into two subbands [3], and transformation from indirect to direct band-gap material [2 - 4]. The strong effect of nitrogen on the electronic properties of the GaNP alloy is caused, among other effects, by large differences in the lattice constant and stiffness between the parental GaN and GaP compounds [2]. These differences are expected to affect also structural properties of the material leading e.g. to local lattice distortion and nonuniform strain distribution in the alloy. However the effect of nitrogen incorporation on the structural quality of GaNP is basically unknown. In the present study we intend to examine this effect by using Raman Scattering (RS) spectroscopy and Scanning Electron Microscopy (SEM). EXPERIMENTAL DETAILS All investigated GaNP epilayers were grown by gas-source molecular-beam epitaxy (GS MBE). The thickness of the GaNP layers was varied between 0.25 and 0.75 µm. The nitrogen composition in the epilayer was determined from a simulation of a high-resolution X-ray rocking curve (XRC) data. The samples were grown on (100) GaP substrates starting from a 0.2 µm thick GaP buffer. The RS measurements were performed in a Renishow micro-Raman system based on a single grating monochromator equipped with a CCD camera. The RS spectra were excited either by the 325 nm line of a He-Cd laser or with the 514 nm line of an Ar+ laser (for resonant RS I5.4.1
measurements). A Gemini Leo microscope equipped with a MONOCL Oxford Instrument attachment was used for SEM measurements. The SEM image
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