Impurity‐Associated Colour Centres in Mg‐ and Ca‐Doped Al₂O₃ Single Crystals

Abstract

I n Mg-and Ca-doped cc-Al,O, crystals irradiated by X-rays a t 80 K new paramagnetic colour centres are observed having an absorption band a t about 255 nm and an emission band a t 310 nm. The hyperfine structure of the EPR spectrum indicates that the unpaired electron spin interacts mainly with two AlZ7 nuclei. The optical and EPR spectra are consistent with the model of a centre involving one electron trapped a t an anion vacancy adjoining the substitutional divalent ion ( F~Q or Fc,). The luminescence band a t 290 nm observed in the TSL peak a t 225 K is suggested to result from the recombination of the holes with the FM~(Fc,) centres.

Ultraviolet nanophosphors

“…F + -centers with Mg 2+ substituting for Al 3+ on neighboring sites (designated as F + -Mg centers in this paper) also give rise to even shorter wavelength emission that arises from recombination of holes with F + -Mg centers. F + -Mg centers have a high cross section for hole recombination, higher in fact than that of isolated F + -centers [8]. Recombination of ionized oxygen vacancy centers therefore decreases the density of F + -Mg and F + -centers during electron irradiation.…”

“…For example the F + center in MgO has an oscillator strength of 0.8 [6] and the F + center in Al 2 O 3 has a decay time of 50 ns at a wavelength of 325 nm [7], indicating a similar high radiative efficiency. In Mg-doped sapphire, the presence of Mg on a site neighboring an F + center shifts the emission wavelength to a shorter value [8]. Since divalent cations can stabilize and shorten the emission wavelength of ionized F-centers, mixed oxides of MgO and Al 2 O 3 are intriguing candidates for UV generation.…”

Photoluminescence study of photochromically and radiochromically transformed Al2O3:C,Mg crystals used for fluorescent nuclear track detectors

“…1 shows PL spectra of an Al 2 O 3 :C,Mg crystal after different procedures of annealing, bleaching with pulsed laser light and irradiation. The excitation/emission bands in the spectra are assigned as follows: 255/325 and 240/325 are single oxygen vacancies charge compensated by Mg impurities -F þ (Mg) centres (Kulis et al, 1981), the 300/500 band is attributed to F 2 centres (Evans et al, 1994), 435/510 is attributed to the aggregate F 2 2þ (2Mg) centres, 350/385 is attributed to F 2 þ (Mg) centres and the 620/750, 335/750 and 260/750 are attributed to F 2 þ (2Mg) centres (Akselrod et al, 2003, Sanyal andAkselrod, 2005). This assignment was done based on literature data and our experiments with photochromic and thermal transformations of colour centres in Al 2 O 3 :C,Mg.…”

Visible Luminescence of Colour Centres in Sapphire

“…I n crystals treated a t oxygen partial pressures of the order of lo5 P a holes are localized on oxygen ions adjacent to the Mg2+ ions giving rise to the [Mg]O centres [8]. Thermal treatment in the atmosphere of CO gas results in the formation of F+-type centres [20].…”