Experimental investigation of long-wavelength optical lattice vibrations in quaternary AlxInyGa1−x−yN alloys and comparison with results from the pseudo-unit cell model
Introduction
The group-III nitride compounds have attracted considerable interest due to their wide applications in optoelectronic devices, which are active in the blue and ultraviolet spectral regions with high quantum efficiency [1], [2], [3], [4]. In particular, their binaries (AlN, GaN, and InN), ternaries, (InxGa1−xN, AlxGa1−xN, and AlxIn1−xN), and quaternaries AlxInyGa1−x−yN in wurtzite structures have direct band gaps and cover a broad spectral range from infrared to deep UV (0.7 eV for InN up to 6.2 eV for AlN) [5], [6]. It has been argued that group-III nitrides represent the ideal material system for optoelectronic applications, such as blue- and green-light-emitting diodes, laser diodes, and high-temperature and high-power electronic devices [7], [8]. However, the optical properties of the quaternary nitrides, mainly the phonons mode behavior, have not been widely investigated experimentally due to the lack of finding quaternary samples covering the whole composition range, besides this, quaternary nitrides are very complex materials and cannot be explained by the usual properties of wurtzite structure. Therefore, in this present work, we try to investigate the optical properties of the quaternary samples using Raman and FTIR spectroscopy measurements and compare the results with the theoretical model of lattice vibrations using a pseudo unit cell (PUC). On the other hand, the phonons mode results of quaternary samples remain incomplete experimentally to date. Also, there has been no agreement whether to consider the AlxInyGa1−x−yN with a one-mode [9], two-mode [10], [11], [12], or mix-mode behavior. Thus, this experimental study and the comparison with the theoretical study will provide some answers to this issue.
Section snippets
Experiment
The samples used in this study were alloys of AlxInyGa1−x−yN quaternary nitrides with different aluminum (Al) mole fraction x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. The samples were grown on c-plane (0 0 0 1) sapphire substrates with AlN buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique. The thickness of the AlInGaN epilayers is between 0.11 and 0.13 μm as measured by Filmetrics F20-VIS. Raman and FTIR spectroscopies were performed to
Pseudo-unit-cell model
The PUC model proposed by Chang and Mitra [13], [14] is an excellent theoretical model to investigate the phonon mode, phonon energy, and dielectric constant of mixed crystals. This model simplifies greatly the concept of the many-body and random problem of the mixed crystals, and also has the advantage that the Hamiltonian of the mixed system can be obtained directly. By using the PUC model, the electron–phonon interaction in ternary mixed crystals in the long-wavelength limit has been
Results and discussion
Raman spectroscopy analyses were performed at room temperature to investigate the optical phonon properties of the quaternary AlInGaN layers. Micro-Raman scattering experiments were carried out in the scattering configuration, with z parallel to the c axis. Under this configuration, the allowed zone-center optical phonon modes that can be detected of wurtzite structure layer will be A1(LO), E2(low), and E2(high).
Fig. 1 shows Raman spectral of the 0.11–0.13 μm thick AlInGaN
Conclusions
A comparison between the experimental measurements of Raman and FTIR with the theoretical calculations based on the PUC model for the zone-center optical phonons of quaternary nitride semiconductors of AlxInyGa1−x−yN with different aluminum (Al) mole fractions x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1 was performed. In this study, it was found that the optical phonon modes in AlxInyGa1−x−yN exhibit a two-mode behavior in the experimental work, which can also be
Acknowledgments
Financial support from Science Fund, Ministry of Science, Technology and Innovation (MOSTI) and Universiti Sains Malaysia are gratefully acknowledged.
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