Study on the Raman spectra of Al0.5In0.5P and AlxGa1-xN

• Main Authors: Ling-Yi Huang, 黃令宜
• Other Authors: Yang-Fang Chen
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/99233318143063550440

碩士 === 國立臺灣大學 === 物理學研究所 === 90 === In this thesis, we utilize micro-Raman spectroscopic system to study (i) the degree of ordering in Al0.5In0.5P alloys and (ii) the phonon mode behavior of AlxGa1-xN . The first part of this thesis is devoted to the study of the degree of ordering in AlInP by Raman scattering. It has been known that spontaneous long-range CuPt-like ordering results in (at k = 0) reduction of energy-gap, splitting of valence-band, and the anisotropy of the photoluminescence (PL) spectra. However, due to the nature of indirect band gap of Al0.5In0.5P alloys, the conventional (and non-destructive) determination of the order parameter from PL spectra can not be applied (; instead the destructive measurements were used). The anisotropic effects, which result from symmetric reduction, inspire us to suspect that the polarized Raman spectra of ordered alloys may differ from that of disordered alloys. In this thesis, we try to correlate this (or these) difference(s) with the degree of ordering in partially ordered Al0.5In0.5P. Raman scattering experiments were taken in -Z(X,X)Z and -Z(Y,Y)Z scattering configurations. We observed that, in -Z(Y,Y)Z configuration, the ratio R of the intensity of AlP-like LO phonon mode and the intensity of InP-like LO mode is quite different from that measured in -Z(X,X)Z scattering geometry, and it increases with increasing the degree of ordering. We thus introduce a new parameter related to the ratio R to denote the degree of ordering in Al0.5In0.5P alloys. Our results show that the newly defined parameter related to polarized Raman spectra is very easy to obtain and sensitive to the degree of ordering. We therefore point out that polarized Raman scattering provides an excellent tool for the determination of ordering in semiconductor alloys. In the second part of this thesis, we devote our attention to the study of the optical phonons in AlxGa1-xN alloys. We present the results of Raman measurements performed on AlxGa1-xN epitaxial layers grown by metal-organic chemical vapor deposition (MOCVD). Raman scattering experiments were taken in seven configurations, i.e., the -z(y',y')z, -z(x',y')z, -z(y,y)z, -z(x,y)z, -x'(z,z)x', -x'(y',z)x', and -x'(y',y')x' configurations to (i) demonstrate the hexagonal wurtzite structure in our samples and (ii) facilitate the observation of different phonon modes. The experimental (and fitting) results show that the energies (frequencies) of the A1(TO), E1(TO), A1(LO), and E1(LO) phonon modes smoothly increase with increasing aluminum content x, indicating a one-mode behavior. On the other hand, the E2 phonon mode exhibits different behavior. Its frequency increases (with increasing aluminum content x) much more slowly than the other phonon modes, and for x ≈ 0.4, a new phonon mode shows up which is shifted to higher frequencies by ~55 cm-1. This new line closes to the E2 phonon frequency of pure AlN. The intensity of E2 mode is strongly dependent on aluminum content x. All these features imply a two-mode behavior for the E2 phonon mode.