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

The processes of intrinsic and extrinsic luminescence excitation by synchro tron radiation of 4-40 eV or electron pulses have been studied in alpha-Al2 O3, 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 e V) and short-wavelength (9.1-9.3 eV) components of exciton absorption doubl et, respectively. Fast (similar to 6 and similar to 20 ns) and slow (simila r to 150 ns) components of the A emission correspond to the creation of sin glet and triplet p(5)s excitons. The efficiency of the A emission in the re gion of band-to-band transitions is low. The intensity of A emission sharpl y increases (approximately quadratically) with a rise of the excitation den sity by nanosecond electron pulses, in Al2O3:Sc, the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations (similar to 8. 5 eV) as well as by the electron recombination with holes localized near SC 3+ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply incre ases at the energy of exciting photons of hv >25 eV. One photon of 26-29 an d 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, respecti vely. Long-term investigations of alpha-Al2O3 crystals did not lead to the detection of immobile self-trapped holes or electrons. The A emission excit ed 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 sel f-shrunk excitons. The theoretical model of Sumi allows the existence of su ch immobile self-shrunk excitons even if an electron and a hole do not sepa rately undergo the self-trapping. [S0163-1829(99)01825-1].