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Sandhaya Koirala1 Seon Namgung Namgung1 Mahmoud Atalla1 Matthew C. Robbins1 Rui Ma1 Sang-Hyun Oh1 Steven Koester1

1, University of Minnesota, Minneapolis, Minnesota, United States

Sandhaya Koirala, Seon Namgung, Mahmoud Atalla, Matthew C. Robbins, Rui Ma, Sang-Hyun Oh, and Steven J. Koester

Monoclinic beta gallium oxide (β-Ga2O3) has recently emerged as an interesting material for next generation high power device applications, UV transparent electrodes, and solar blind photodetectors due to its ultra-wide bandgap of 4.6 eV and high breakdown field of ~ 8 MV/cm. Electronic and optoelectronic devices such as field-effect transistors [1] and photodetectors [2] have recently been demonstrated on β-Ga2O3. Recently, β-Ga2O3 has also been shown to be realizable in thin films enabling high-performance transistors to be fabricated [3], and offers a path to improving the thermal conductivity limitations of bulk β-Ga2O3. Bulk β-Ga2O3 is also known to have anisotropic electrical, and optical properties [4], but these properties have not been studied extensively in 2D thin films. In this work, we study the anisotropic optical properties of thin-film exfoliated β-Ga2O3 using polarization dependent Raman mapping, and describe the surprising result that multiple surface orientations of β-Ga2O3 are obtained through this process.
Samples were prepared by mechanical exfoliation from n-type Sn-doped (5.0×1018 cm-3) β-Ga2O3 wafers with (-201) surface orientation. Different thicknesses of films were characterized using optical contrast analysis, atomic force microscopy, and Raman spectroscopy. Interestingly we observed different surface orientations including (010) and (-201) from the (-201) host β-Ga2O3 crystal. This was confirmed optically from Raman spectroscopy by observing the polarization dependence of the allowed Ag and Bg phonon mode, i.e. in (010) only the Ag phonon modes are allowed, while on (-201) only the Bg mode is allowed, consistent with previous work on bulk crystals [5]. Polarization dependent Raman mapping of (010) oriented flakes shows strong optical anisotropies. The Raman mode Ag10 shows strong polarization dependence due to bending and stretching of Ga-O bonds. Similarly, the Ag3, Ag5, and Ag6 modes which are due to octahedral symmetry stretching and bending vibrational O-Ga-O modes also show polarization dependent intensity changes. Finally, Raman intensity mapping was performed to confirm the orientation-dependence and evaluate information on surface uniformity. Electrical measurements on the different orientation samples were found to be consistent with the Raman results. In conclusion, these results demonstrate the utility of using polarized Raman spectroscopy as an effective method for identifying the crystal orientation of exfoliated thin film β-Ga2O3. This work was supported by the University of Minnesota MRSEC under NSF Award DMR-1420013.
[1] W. S. Hwang, et al., Nat. Photonics 9, 247 (2015); [2] H. Zhou, et al., IEEE Elect. Dev. Lett. 38, 103 (2017); [3] G. C. Hu, et al., Optics Express, 23 13556 (2015); N. Ueda, et al., Appl. Phys. Lett. 71, 933 (1997); [5] C. Kranert, et al., Sci. Rep., 6 35964 (2016).

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