Self-trapping and multiplication of electronic excitations in Al2O3 and Al2O3:Sc crystals

Abstract

The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4–40 eV or electron pulses have been studied in $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ single crystals at 8 K. The intrinsic A (7.6 eV) and E emissions (3.77 eV) can be effectively excited in the region of long-wavelength (8.85–9.1 eV) and short-wavelength (9.1–9.3 eV) components of exciton absorption doublet, respectively. Fast (∼6 and ∼20 ns) and slow (∼150 ns) components of the A emission correspond to the creation of singlet and triplet $p^{5}s$ excitons. The efficiency of the A emission in the region of band-to-band transitions is low. The intensity of A emission sharply increases (approximately quadratically) with a rise of the excitation density by nanosecond electron pulses. In ${\mathrm{Al}}_2{\mathrm{O}}_3:\mathrm{S}\mathrm{c},$ the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations (∼8.5 eV) as well as by the electron recombination with holes localized near ${\mathrm{Sc}}^{3+}$ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of $h\nu >25\mathrm{}\mathrm{eV}.$ One photon of 26–29 and 30–37 eV causes the ionization of the ${2p}^6$ or ${2s}^2$ shell of the oxygen ion and provides the creation of two or three electron-hole pairs, respectively. Long-term investigations of $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ crystals did not lead to the detection of immobile self-trapped holes or electrons. The A emission excited at the direct photocreation of excitons or at the recombination of free electrons and free holes is interpreted by us as the radiative decay of self-shrunk excitons. The theoretical model of Sumi allows the existence of such immobile self-shrunk excitons even if an electron and a hole do not separately undergo the self-trapping.