Self-trapping and multiplication of electronic excitations inAl2O3and

Abstract

The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4-40 eV or electron pulses have been studied in ␣-Al 2 O 3 single crystals at 8 K. The intrinsic A ͑7.6 eV͒ and E emissions ͑3.77 eV͒ can be effectively excited in the region of long-wavelength ͑8.85-9.1 eV͒ and short-wavelength ͑9.1-9.3 eV͒ components of exciton absorption doublet, respectively. Fast ͑ϳ6 and ϳ20 ns͒ and slow ͑ϳ150 ns͒ 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 quadratically͒ with a rise of the excitation density by nanosecond electron pulses. In Al 2 O 3 :Sc, the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations ͑ϳ8.5 eV͒ as well as by the electron recombination with holes localized near Sc 3ϩ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of hϾ25 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 ␣-Al 2 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. ͓S0163-1829͑99͒01825-1͔

VUV spectroscopy and electronic excitations in nano-size alumina

“…Two types of self-trapped excitons (STEs) have been revealed in a-Al 2 O 3 with nanosecond emissions at 7.6 and 3.7 eV (Kirm et al, 1999). The shrinkage of the energy gap by w1.8 eV of transition alumina also influences the energetic positions of relaxed exited states.…”

“…The electronic properties of its stable a-phase have been thoroughly investigated (Valbis and Itoh, 1991;Kirm et al, 1999). The metastable polymorphs (b, g, d, Q, etc.…”

Fundamentos e análise de tintas refratárias de alta emissividade

“…Os materiais cerâmicos apresentam uma grande diferença de energia entre a banda de valência e a de condução, como se observa na α-Al 2 O 3 , que apresenta um intervalo de 9,4 eV entre as bandas [13]. Portanto, a transição eletrônica só ocorre para frequências de onda muito altas, pois estas são mais energéticas.…”

Low-temperature investigation of electronic excitations in wide-gap materials doped with RE3+ or Cr3+ ions

“…Используя s 2 -центры, а затем и некоторые редкоземельные и переходные ионы (например, Ce 3+ , Eu 3+ , Tb 3+ , Mn 2+ , Cr 3+ ), были изучены особенности собственных электронных возбуждений в широкощелевых галогенидах и оксидах металлов. Удалось также обнаружить и подробно исследовать основные механизмы размножения электронных возбуждений и фотонного умножения, когда поглощение одного фотона ведет к образованию двух электронно-дырочных (e-h) пар [5,6]; e-h пары и экситона [6,7]; прямому возбуждению горячим электроном проводимости центра люминесценции (аналог механизма Франка-Герца в газах) [8,9]; а также получению трех e-h пар при ионизации фотоном электрона из более глубокой, чем валентная (np 6 ), электронной оболочки ns 2 [10].…”