Vacancy defect positron lifetimes in strontium titanate

Abstract

The results of positron annihilation lifetime spectroscopy measurements on undoped, electron irradiated, and Nb doped SrTiO3 single crystals are reported. Perfect lattice and vacancy defect positron lifetimes were calculated using two different first-principles schemes. The Sr vacancy defect related positron lifetime was obtained from measurements on Nb doped, electron irradiated, and vacuum annealed samples. Undoped crystals showed a defect lifetime component dominated by trapping to Ti vacancy related defects.Comment: 31 pages 5 figures 2 Table

Nonstoichiometry accommodation in SrTiO3thin films studied by positron annihilation and electron microscopy

“…10(a)] for all the films, and they are in good agreement with crystal SrTiO 3 measurements and the DFT lifetime values for V Sr and V Ti , respectively (Table I). 27,29 The uncertainties in fit components shown in Fig. 10 varied in accord with the intensity of trapping to V Sr and V Ti defects; the intensity of trapping to Sr vacancies was smallest for the 1.50 J cm −2 film as also seen in Fig.…”

“…4. The mean positron lifetimes measured in the films are all greater than the SrTiO 3 bulk lifetime of ∼155 ps, 27,32 indicating that positron trapping to open-volume defects occurs. The τ m increases systematically from ∼198 ps for the film grown using F o of 1.50 J cm −2 ( c ∼ 0) to 257 ps for the 2.00 J cm −2 film ( c ∼ 0.18 pm), which is expected to be Sr deficient.…”

“…The thin film is clearly differentiated from the substrate; the mean lifetime obtained for the substrate is ∼165 ps, consistent with values measured in bulk single crystals. 27 The values averaged over the implantation energy range for which the positrons are annihilating predominately from the films are shown in Fig. 4.…”

“…24,27,29 However, a lower limit for μ(V Sr ) can be estimated from the results of a recent VE-PALS study of MBE-grown La-doped SrTiO 3 thin films where positron trapping to vacancies was below the saturation trapping limit, allowing the positron trapping rate to V Sr defects to be estimated. 32 Cation vacancies are expected to act as acceptor defects, and previous studies had established a one to one relation between [La] ∼3 × 10 15 s −1 is obtained.…”

“…19,25,26 Positron lifetime values can by calculated using density functional theory (DFT). These have been performed for vacancy-related defects in SrTiO 3 , [27][28][29] and the values obtained for relevant defects are given in Table I. The two cation vacancy lifetimes have been calculated using the relaxed near-neighbor atom geometries given by first principles studies for the −2 and −4 charge states of the Sr and Ti vacancies, respectively, 25,29 the vacancy complex lifetime used unrelaxed atomic positions.…”