Polarized Raman spectra in β-Ga2O3 single crystals
Introduction
Monoclinic β-Ga2O3 has a wide bandgap-energy of 4.8–4.9 eV, and has attracted considerable attention because of its potential use in UV transparent electrodes [1], photodetectors [2], [3], and field-effect transistors (FETs) [4]. The biggest advantage of this material is the availability of low-cost and large-size single-crystalline substrates grown by the melt-growth method [5]. Vertical current injection has been demonstrated in nitride-based blue light-emitting-diodes fabricated on the substrates [6]. Single-crystalline β-Ga2O3 has superior material properties for high-power and high-voltage FETs owing to its high breakdown electric field of 8 MV/cm [4], [7]. Therefore, β-Ga2O3 is considered as a promising material for future development of low-cost and environment-friendly devices.
In general, phonon modes have received considerable attention because information on them is important in considering the carrier transport, and the radiative and nonradiative optical processes. Despite its superior material properties, reports on its lattice vibration modes are still limited [8], [9], [10]. The only report [8] made on the polarized Raman spectroscopy is on a single crystal grown by the floating zone (FZ) growth method. By comparing the results with the valence force field calculation, they have obtained complete description of the modes by combining local vibrations such as stretching, bending, twisting, rocking, etc. A phase transformation from the monoclinic β-phase to the corundum α-phase has been studied by the X-ray diffraction and Raman spectroscopy under a pressurized condition [9]. First principle calculations have predicted the phonon frequencies and dielectric and thermodynamic properties [9], [10].
In this article, polarized Raman spectra are shown to discuss the lattice vibration modes in device-grade β-phase Ga2O3 substrates. The results are compared with the previous experimental and theoretical results to show the uniformity of crystalline orientation and surface flatness of the present substrates.
Section snippets
Experiment
A (100) Si-doped bluish substrate was grown using the edge-defined film-fed growth (EFG) method. The Hall-effect measurement yielded the carrier concentration n=4.9×1018 cm−3, the Hall electron mobility μΗ=93 cm2/(V s), and the resistivity ρ=1.4×10−2 Ω cm at room temperature. A (010) Mg-doped colorless substrate grown by the FZ growth method [5] exhibited a semi-insulating behavior, and ρ of 6×1011 Ω cm was obtained with Mg concentration of 4×1018−2×1019 cm−3. A (001) unintentionally doped (undoped)
Results and discussion
Unpolarized Raman spectra of the substrates are summarized in Fig. 2. As expected from the polarization selection rules, the Ag modes were selectively observed for the (010) Mg-doped substrate. On the contrary, both Ag and Bg mode peaks appeared in the spectra for the (100) Si-doped and (001) undoped substrates. By referring the valence force field calculation [8], the Raman peaks can be divided into three parts: libration and translation of the GaIO4 chains for the Ag(1)−Ag(3), Bg(1), and Bg
Conclusion
We have succeeded in measuring the polarized Raman spectra, in which the polarization selection rules were perfectly reproduced. To the best of our knowledge, this is the first experimental observation of the complete set of polarized Raman spectra of β-Ga2O3. The results are ensured by the high uniformity of crystalline orientation and surface flatness of the present substrates.
Acknowledgments
This work was supported in part by Grants-in-Aid for Scientific Research Nos. 25390071, 25420341, and 25706020 from MEXT, Japan. T. O. is grateful to Prof. M. Sato, Dr. H. Nagai, and Ms. C. Mochizuki for their help in the experiments.
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