Carrier recombination processes and divalent lanthanide spectroscopy in ${\text{YPO}}_4:{\text{Ce}}^{3+};L^{3+}$ $\left(L=\text{Sm},\text{Dy},\text{Tm}\right)$

We studied charge carrier trapping, detrapping, and recombination phenomena in ${\text{Ce}}^{3+}$ doped ${\text{YPO}}_4$, codoped with ${\text{Sm}}^{3+}$, ${\text{Dy}}^{3+}$, or ${\text{Tm}}^{3+}$. Ce ions trap the holes and Sm, Dy, and Tm trap electrons created during x-ray irradiation. By means of red to infrared stimulation, the trapped electrons can be back transferred to Ce leading to shorter wavelength ${\text{Ce}}^{3+}5d\text{−}4f$ luminescence. Excitation spectra for this recombination luminescence were recorded from 10 K to room temperature. It provides information on the excited state energies of divalent Sm, Dy, and Tm with respect to the lanthanide ground state energy and with respect to the mobility edge energy of ${\text{YPO}}_4$. From the temperature dependence, insight is obtained on the carrier recombination pathways. We will identify temperature independent tunneling recombination, recombination by thermal excitation to the conduction band, and phonon-assisted delocalization of electrons from impurity states within the conduction band.

Figure 1

(Color online) Scheme with the location of divalent and trivalent lanthanide electron donor and acceptor states in ${\text{YPO}}_4$.

Figure 2

(Color online) Luminescence excitation spectra of (a) 3.5 eV ${\text{Ce}}^{3+}$ emission and (b) 2.0 eV ${\text{Sm}}^{3+}$ emission in ${\text{YPO}}_4$ at 6 K. Characteristic features are indicated.

Figure 3

(Color online) OSL-excitation spectra of divalent Sm in ${\text{YPO}}_4:{\text{Ce}}^{3+};{\text{Sm}}^{3+}$ as function of temperature. Spectrum (a) is at 10 K and spectra (b) through (i) are from 160 to 300K with 20 K interval.

Figure 4

(Color online) OSL-excitation spectrum of divalent Dy in ${\text{YPO}}_4:{\text{Ce}}^{3+};{\text{Dy}}^{3+}$ as function of temperature. Spectra (a), (b), (c), and (d) are at 10 K, 40 K, 80 K, and 120 K, respectively. Between 120 and 300 K spectra are shown measured at 20 K intervals.

Figure 5

(Color online) OSL-excitation spectrum of divalent Tm in ${\text{YPO}}_4:{\text{Ce}}^{3+};{\text{Tm}}^{3+}$ as function of temperature. Spectra (a) and (b) are at 12 and 40 K. Between 40 and 300 K spectra are shown measured at 20 K intervals.

Figure 6

(Color online) Plausible recombination pathways during OSL (a) via the conduction band, (b) tunneling recombination and thermally excited recombination via the conduction band, and (c) tunneling recombination and thermally excited subband-gap barrier recombination.

Figure 7

(Color online) The divalent lanthanide states in ${\text{YPO}}_4$. Curve $4f\left(\text{CT}\right)$, $4f$ ground-state location derived from electron transfer data. ${}^{\ast }$ denotes $4f$ ground-state location derived from thermoluminescence glow curve analysis of ${\text{YPO}}_4:{\text{Ce}}^{3+};L^{3+}$. Curve $4f\left(\text{TL}\right)$, $4f$ ground-state location derived from thermoluminescence. Curve $5d\left(2+\right)$, the location of the lowest energy $4f^{n}5d$ states of the divalent lanthanides. $4f^{n+1}$ and $4f^{n}5d$ excited state levels of $L^{2+}$ are indicated by horizontal bars. The gray vertical bars indicate what states are reached during OSL.

Figure 8

(Color online) Arrhenius plots of OSL-excitation intensity. (a) At the maximum of the ${\text{Sm}}^{2+}5d_1$ [SmB] excitation band, (b) at the maximum of the ${\text{Dy}}^{2+}$ $\left[\text{LS}\right]5d_1$ band, and (c) at the maximum of the ${\text{Tm}}^{2+}$ $\left[\text{LS}\right]5d_1$ band. The slopes of line segments through part of the data are indicated in electron volt.