Experimental investigation of long-wavelength optical lattice vibrations in quaternary AlxInyGa1−x−yN alloys and comparison with results from the pseudo-unit cell model

doi:10.1016/j.physb.2011.01.033

Physica B: Condensed Matter

Volume 406, Issue 8, 1 April 2011, Pages 1379-1384

https://doi.org/10.1016/j.physb.2011.01.033 Get rights and content

Abstract

Raman and Fourier transform infrared (FTIR) spectroscopies have been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with aluminum (Al) mole fraction x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. Pseudo unit cell (PUC) model was applied to investigate the phonons frequency, mode number, static dielectric constant, and high frequency dielectric constant of the Al_xIn_yGa_1−x−yN mixed crystals. The theoretical results were compared with the experimental results obtained from the quaternary samples by using Raman and FTIR spectroscopies. The experimental results indicated that the Al_xIn_yGa_1−x−yN alloy had two-mode behavior, which includes A₁(LO), E₁(TO), and E₂(H). Thus, these results are in agreement with the theoretical results of PUC model, which also revealed a two-mode behavior for the quaternary nitride. We also obtained new values of E₁(TO) and E₂(H) for the quaternary nitride samples that have not yet been reported in the literature.

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, (In_xGa_1−xN, Al_xGa_1−xN, and Al_xIn_1−xN), and quaternaries Al_xIn_yGa_1−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 Al_xIn_yGa_1−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 Al_xIn_yGa_1−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 $z (x, unpolarized) \bar{z}$ 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 Al_xIn_yGa_1−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 Al_xIn_yGa_1−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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Experimental investigation of long-wavelength optical lattice vibrations in quaternary AlxInyGa1−x−yN alloys and comparison with results from the pseudo-unit cell model

Abstract

Introduction

Section snippets

Experiment

Pseudo-unit-cell model

Results and discussion

Conclusions

Acknowledgments

MRS Internet J. Nitride Semicond. Res.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Data in Science and Technology: Semiconductors

Phys. Status Solidi (b)

The Blue Laser Diode: GaN Based Light Emitters and Lasers

Fabrication and performance of GaN electronic devices

Mater. Sci. Eng.

Phys. Status Solidi (b)

Phys. Status Solidi (b)

Appl. Phys. Lett.

Phys. Rev. B

Phys. Rev.

Phys. Rev. B

J. Appl. Phys.

Experimental investigation of long-wavelength optical lattice vibrations in quaternary Al_xIn_yGa_1−x−yN alloys and comparison with results from the pseudo-unit cell model