High-pressure optical and vibrational properties of Ga2O3 nanocrystals

Abstract

In this project the optical and vibrational properties of monoclinic gallium oxide (β-Ga2O3) nanocrystals (NCs) are studied by Raman scattering spectroscopy under high-hydrostatic pressure conditions, from ambient pressure up to 21.6 GPa. Phonon pressure coefficients and Grüneisen parameters are obtained for different optical phonon modes of nanocrystalline β-Ga2O3. In the first part of the work, the investigated material is characterized by means of different techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman scattering. While XRD and SEM confirm the nanocrystalline nature of the investigated sample, from the Raman spectra we are able to properly identify the Raman-active modes of β-Ga2O3 at ambient pressure. By monitoring their peak position at different pressures, phonon pressure coefficients for several of the optical Raman-active modes of β-Ga2O3 have been successfully determined, with values significantly lower than those reported in previous works for bulk β-Ga2O3. This suggests that the compressibility of the NCs could be reduced with respect to the bulk material. In order to test the validity of the experimental data, density functional theory calculations of the structural properties of bulk β-Ga2O3 have also been performed as a function of pressure. From the ab initio calculations we obtain a bulk modulus of 160.7 ± 5.0 GPa for bulk β-Ga2O3, which is comparable, and even lower, than that measured in previous works for bulk material by means of synchrotron XRD as a function of pressure (~ 200 GPa). Our theoretical results thus confirm that the lower compressibility of the β-Ga2O3 NCs studied in this work may be a consequence of the nanocrystalline nature of the investigated material. The possible physical mechanisms giving rise to this observation are discussed in terms of similar results reported in the literature. It is concluded that more work dealing with the high-pressure structural and vibrational properties of β-Ga2O3 samples of different origin (i.e., bulk vs. NCs, and doped vs. undoped material) should be performed in order to fully understand the origin of the lower compressibility displayed by the nanocrystalline β-Ga2O3 sample studied in this work.