Ce3+ energy levels relative to the band structure in CaS: evidence from photoionization and electron trapping

The energy positions relative to the host valence and conduction bands of the 5d excited states and 4f ground states of Ce3+ in CaS were determined from photoluminescence and photoconductivity measurements. The determination was based on the observation that the 5d electrons of Ce3+ are trapped after transport in the conduction band only when they were excited to the upper 5d-state (Eg), which, therefore, must lie above the minimum of the conduction band of CaS.