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Temperature dependence of the Stark shifts of Er$^{3+}$ transitions in Er$_2$O$_3$ thin film on Si (001)

Optical transitions of an Er$_2$O$_3$ film on a Si substrate grown by using a metal-organic chemical vapor deposition technique were investigated in a wide temperature (300 K $\sim$ 5 K) and spectral (500 nm and 850 nm) ranges. Numerous sharp transitions corresponding to the Er$^{3+}$ ionic levels were observed, which show Stark shifts induced by the crystal field. With decreasing temperature from 300 K to 5 K, all transition peaks exhibit spectral red-shift. We believe that such red-shift behavior is due to the change of the crystal field together with the change of strain field induced by the film and the substrate in varying temperature. An interesting result is the total amount of red-shifts between 300 K and 5 K. We obtain that the higher transition energy peaks show bigger red-shifts. This is very consistent to all transition peaks. This is because the dipole moments between the transition levels are different that lead to different amounts of Stark shifts.