F+andF′Centers in Magnesium Oxide

Abstract

Single crystals of MgO have been additively colored by heating them in Mg vapor at temperatures from 1600 to 1800'C and pressures up to 4000 Torr. The F+ and F' optical absorption bands (due to anion vacancies containing one and two electrons, respectively) were observed to be extremely similar but distinguishable, and were observed at liquid-nitrogen temperature to peak at 4.96 and 5.03 eV, respectively.The oscillator strength for the F' center is estimated to be 0.70&0.05. A photoconversion process F+~F' was studied. The luminescence bands for these centers were observed at 3.1 and 2.4 eV, respectively. The density of F' centers 1' produced by the additive coloring process was observed to be related to the density of atoms in the vapor $«by the equation EF =1270K«e @'~~, where Q=1.53 eV. The temperature dependence of these two bands was measured and is discussed.

Luminescence characterization of ultrathin MgO films of high crystallinity prepared by pulsed laser deposition

“…Our MgO films exhibit the first strong excitation band at 6 eV. It is obvious that this excitation band cannot be due to the higher excited states of F + and F centres, while the first excitation bands in UV, with an oscillator strength as high as 0.7 for latter one [7], do not contribute strongly to the F + luminescence. On the other hand, since only the F + emission is detected under photoexcitation in this band, it is unlikely to be due to impurity absorption.…”

“…Obviously, the thin MgO films have an advantage over impurity, defect, and trap rich single crystals as suitable samples for revealing intrinsic dynamics of electronic excitations. It is interesting to note that the main overlapping absorption bands have been reported for the F + and F centre with maxima at 4.96 and 5.03 eV, respectively [7]. Our MgO films exhibit the first strong excitation band at 6 eV.…”

“…In the UV range, an emission band peaked at 5.3 eV is observed, arising from the recombination of electrons and holes at the vicinity of V Al centres, that consists of cation vacancies near Al 3+ ions (introduced as unintentional impurity) [6]. The broad band covering the range of 2-4 eV is a result of superposition of luminescence of Fand F + centres [7] with emission maxima at 2.4 and 3.15 eV, respectively. The band peaked at 2.85 eV is revealed in crystals with internal stress as reported in [16].…”

“…The fit of the fast emission component (see Fig. 3 captions for details) resulted in parameters (I max = 3.15 eV and FWHM 0.74 eV) close to those reported for the F + band under intra-centre excitation [7]. Additional support comes from the decay measurements of this emission, which can be approximated by single exponential with a characteristic time of 3.7 ns without any significant long components in the range of 200 ns (see inset of Fig.…”

“…The influence of isoelectronic Ca 2+ impurities in MgO and the resulting change in luminescence has been investigated in [5]. Effects induced in the optical spectra by heterovalent impurities-defect complexes (M 3+ + cation vacancy + hole, being known as V M -bands), which require charge compensation, have been analysed in [6]; the role of intrinsic colour centres (F, F + ) was studied in [7]. Recently it has been shown that surface morphology plays an important role in the formation of optical properties of pure and mixed MgO-CaO nanocrystals prepared by chemical vapour deposition [8,9].…”