Photoluminescence of chromium in gallium oxide and aluminum gallium oxide under pressure

Lauren Margaret Barmore

doi:10.7273/000005369

This work examines the effects of pressure and temperature on single crystals of Cr-doped gallium oxide (β-Ga2O3:Cr3+) and aluminum gallium oxide [(Al0.1Ga0.9)2O3, called AGO] by measuring the wavelength shift and intensity of the spectral R lines.R lines for both Ga2O3 and AGO showed a redshift with increasing temperature. At low temperatures, the R1 line intensity is dominant due to thermal depopulation of the R2 line. At temperatures greater than 50 K, the R2/R1 intensity ratio is strongly affected by nonradiative recombination. In Ga2O3, the nonradiative contribution is thermally activated, which results in strong emission at low temperatures, particularly for R1. AGO has smaller thermal activation barriers, possibly due to the increased disorder of the alloy which provides more nonradiative recombination pathways. Photoluminescence (PL) spectra of these materials were collected from samples in diamond anvil cells at pressures up to 9 GPa. The response of the Cr3+ R lines in β-Ga2O3 to hydrostatic pressure showed significant differences from ruby (Al2O3:Cr3+). The R1-R2 splitting, which arises due to the crystal field and spin-orbit coupling, is larger in β-Ga2O3 and increases further with applied pressure. A similar effect was observed in AGO. However, the R1 line in AGO shows a smaller shift versus pressure. X-ray diffraction (XRD) measurements of AGO indicate that its equation of state is similar to that of β-Ga2O3. Nonradiative recombination contributes significantly to the pressure-dependent intensity of the R lines in both β-Ga2O3:Cr3+ and AGO. β-Ga2O3:Cr3+ and AGO were examined under nonhydrostatic conditions by using mineral oil as a pressure-transmitting medium, which is nonhydrostatic above ~2.5 GPa. Similar to the case in ruby, the R1 line is much more sensitive to nonhydrostatic stress than R2. Spatially resolved PL of a β-Ga2O3:Cr3+ sample at 8 GPa in mineral oil showed significant variations in the R1 emission wavelength across the sample. A sample under hydrostatic pressure, in contrast, showed very homogenous emission wavelengths. These results suggest that the R1 line of β-Ga2O3:Cr3+ could serve as a sensitive indicator of nonhydrostatic stress, while the R2 line is insensitive to these perturbations. As part of this work, the hydrostatic limits of mineral oil and heptane were established, enabling their use as pressure-transmitting media. From synchrotron XRD experiments, a solid-to-solid phase transition was discovered in heptane with onset at 1.5 GPa. The phase transition resulted in discontinuous shifts in vibrational peaks in the IR spectra.

Photoluminescence of chromium in gallium oxide and aluminum gallium oxide under pressure

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