Investigation of Composition-dependent Optical Phonon Modes in Al x Ga 1-x N Epitaxial Layers Grown on Sapphire Substrat
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1167-O07-05
Investigation of Composition-Dependent Optical Phonon Modes in AlxGa1−−xN Epitaxial Layers Grown on Sapphire Substrates Jun-Rong Chen, Tien-Chang Lu, Hao-Chung Kuo, and Shing-Chung Wang Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 University Rd., Hsinchu, 300 Taiwan ABSTRACT We reported the systematical study of optical properties of hexagonal AlxGa1−xN 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 AlxGa1−xN 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 AlxGa1−xN 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. 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. 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 quantumwell structures, which have more advantages than GaAs-based material system [1]. Yu et al. studied the infrared reflectivity spectra of GaN and AlxGa1−xN with aluminum compositions of 0.087, 0.27, and 0.35 [2]. They found that the E2 mode, which arises from the disordered state of the alloys, can be observed in the refractivity spectrum of AlxGa1−xN. Holtz et al. reported optical studies on AlxGa1−xN alloy layers grown on (111)-oriented silicon substrates by combining Fourier transform infrared (FTIR) and Raman spectroscopy studies [3]. Besides, Hu et al. studied the optical properties of hexagonal AlxGa1−xN (x from 0.05 to 0.42) epitaxial films with Si doping concentration up to 1018 cm
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