Suppression of vacancy defects in epitaxial La-doped SrTiO3 films

Abstract

Variable energy positron annihilation lifetime spectroscopy of high-mobility La-doped SrTiO3 grown by molecular beam epitaxy found that the films contained sufficiently low concentrations of Sr vacancies and vacancy cluster defects to allow the observation of positron annihilation events from the perfect lattice. This enabled the concentrations of charged cation vacancies to be estimated, and these were found to be at least an order of magnitude below the La-dopant concentrations.

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

“…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.…”

“…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. This suggests that the threshold concentration of cation vacancy defects above which saturation trapping is expected is of the order estimated earlier, ∼20-50 ppm.…”

Carrier-Controlled Ferromagnetism inSrTiO3

“…These results provide evidence for the absence of additional sources of carriers, such as oxygen vacancies, and of traps such as transition metal impurities or Sr vacancies (which would reduce the carrier concentrations relative to the La-dopant concentration), on the parts-per-million level. Furthermore, positron-annihilation studies of the La-doped films indicate Sr-vacancy concentrations of no more than approximately 1 Â 10 16 cm À3 in these films [20]. We note that the carrier densities in the uniformly doped samples investigated here are slightly less than the dopant concentration, because of the well-known surface depletion of SrTiO 3 [21].…”

Electric field-tunable BaxSr1−xTiO3 films with high figures of merit grown by molecular beam epitaxy

“…19 MBE-grown BST films have been reported, 20,21 but have thus far not exhibited improved properties relative to those deposited by other techniques. Here, we report on the dielectric properties of BST films grown by hybrid oxide MBE, a method which has previously been shown to allow for excellent stoichiometry control 22 and low defect densities 23,24 for SrTiO 3 films. Films are grown on Pt bottom electrodes to form parallel-plate capacitor structures, which are shown to combine high dielectric tunabilities with Q factors comparable to those of bulk BST.…”