Infrared reflectance of optical phonon modes in AlGaN epitaxial layers grown on sapphire substrates Jun-Rong Chen, Tien-Chang Lu*, Gen-Sheng Huang, Tsung-Shine Ko, Hao-Chung Kuo, Shing-Chung Wang Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Ta Hsueh Rd., Hsinchu, 30010 Taiwan ABSTRACT We reported the systematical study of optical properties of hexagonal Al x Ga 1−x N epitaxial films grown on c-sapphire substrate using metal-organic chemical vapor deposition. By performing Fourier transform infrared spectroscopy measurements, the high-frequency dielectric constants and phonon frequencies can be obtained by theoretically fitting the experimental infrared reflectance spectra using a four-phase layered model. The high-frequency dielectric constant of Al x Ga 1−x N varies between 4.98 and 4.52 for ε ∞,⊥ (polarization perpendicular to the optical axis) and between 4.95 and 4.50 for ε ∞,// (polarization parallel to the optical axis) respectively when the aluminum composition changes from 0.15 to 0.24. Furthermore, from experimental infrared reflectance spectra of Al x Ga 1−x N films, a specific absorption dip at 785 cm −1 was observed when the aluminum composition is larger than 0.24. The dip intensity increases and the dip frequency shifts from 785 to 812 cm −1 as aluminum composition increases from 0.24 to 0.58. According to the reciprocal space map of x-ray diffraction measurements, the emergence of this dip could be resulted from the effects of strain relaxation in AlGaN epitaxial layers due to the large lattice mismatch between GaN and AlGaN epitaxial film. Keywords: AlGaN, Sapphire, Infrared reflectance, FTIR 1. INTRODUCTION Hexagonal GaN and AlN semiconductors, and AlGaN alloys, have attracted considerable attention due to their successful applications in the fabrication of high-performance electronic and optoelectronic devices, such as heterostructure field-effect transistors, 1 ultra-violet light-emitting diodes, 2 and laser diodes. 3-5 By varying the alloy composition, different electrical and optical properties can be obtained in a wide spectral range from 3.4 to 6.3 eV. In order to further engineer these alloys and related optoelectronic devices, it is necessary to work on the fundamental properties of these materials. The infrared optical response of these alloys is important for the determination of crystal quality and phonon properties. Recently, Sun et al. proposed the idea of developing terahertz quantum cascade lasers with GaN/AlGaN quantum-well structures, which have more advantages than GaAs-based material system. 6 Since the carrier dynamics and temperature-dependent performance of quantum cascade lasers are closely related to the phonon energies, the study of composition-dependent phonon energies will be also conducive to the design and develop of nitride-based quantum cascade lasers. Generally, GaN, AlN, and Al x Ga 1−x N alloys can be deposited on sapphire, silicon carbide, silicon, or GaAs substrates by using different growth methods. 7, 8 Except for GaAs substrates, GaN and its related compounds grown on these substrates have the hexagonal wurtzite structure (α-phase). Therefore, they are natural optically anisotropic uniaxial crystals. 9 In the hexagonal Al x Ga 1−x N alloys with space group 4 6v C , both the transverse-optical (TO) and longitudinal-optical (LO) modes spit into the axial (A 1 ) and planar (E 1 ) optical modes due to the anisotropy in the macroscopic electric field induced by polar phonons. The A 1 and E 1 modes are both infrared and Raman active according to the selection rules. Yu et al. studied the infrared reflectivity spectra of GaN and Al x Ga 1−x N with aluminum compositions of 0.087, 0.27, and 0.35. 10 They found that the E 2 mode, which arises from the disordered state of the alloys, can be observed in the refractivity spectrum of Al x Ga 1−x N. Holtz et al. reported optical studies on Al x Ga 1−x N alloy layers grown on (111)-oriented silicon substrates by combining Fourier transform infrared (FTIR) and Raman spectroscopy studies. 11 Besides, Hu et al. studied the optical properties of hexagonal Al x Ga 1−x N (x from 0.05 to 0.42) epitaxial films with Si doping concentration up to 10 18 cm −3 grown on c-plane sapphire substrates using infrared *timtclu@mail.nctu.edu.tw; phone +886-3-571-2121 ext. 31234; fax +886-3-571-6631 Gallium Nitride Materials and Devices III, edited by Hadis Morkoç, Cole W. Litton, Jen-Inn Chyi, Yasushi Nanishi, Euijoon Yoon, Proc. of SPIE Vol. 6894, 68941U, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.760296 Proc. of SPIE Vol. 6894 68941U-1