Optical absorption and transport in semiconducting SrTiO3

Abstract

The optical absorption of reduced and doped SrTi03 has been measured between 0.39 and 7 p,m. Transport measurements were also carried out between room temperature and 10 K for various samples with different treatment. Both optical and transport properties were found to depend greatly on the methods of preparations of the samples. We found up to five absorption peaks in the visible and infrared regions, lying at 0.43, 0.52, 0.70, 1.2, and -4, 8 p, m. In all samples, the so called free-carrier absorption tail starts falling off at about 2.5 p,m and has a maximum between 4.5 and 5 p, m. The position of this peak cannot be explained by the free-carrier absorption theory of LO-mode scattering.The new absorption mechanisms are introduced in order to explain the results in this wavelength region. Of the five absorption peaks, the first, second, and fourth peaks are due to impurity or defect levels, whereas the third and fifth peaks are interpreted as being due either to interconduction-band or vacancy-level to conduction-band transitions. The conduction and scattering mechanisms are discussed.

Effects of hydrogen anneals on oxygen deficient SrTiO3−x single crystals

“…A peak at 2.9 eV has previously been observed in reduced, Fe-doped SrTiO 3 , but has not yet been assigned to a specific defect ͑complex͒. 4,6,21 The results show that hydrogen has a bleaching effect on these color centers. Such a procedure is apparently employed for some commercial, Verneuil-grown SrTiO 3 single crystals, which as-grown are dark in color and are made insulating and transparent by Ar/ H 2 anneals.…”

“…18 Iron on the Ti site can possess multiple valence states, which produce optical absorption peaks. 4,6,[19][20][21] Oxidizing anneals stabilize higher Fe valence states, while reducing anneals can stabilize Fe in lower valence states to compensate for oxygen vacancies, which can also form complexes with the Fe. A peak at 2.9 eV has previously been observed in reduced, Fe-doped SrTiO 3 , but has not yet been assigned to a specific defect ͑complex͒.…”

“…2,3 Electrically conducting SrTiO 3 crystals are often obtained by annealing in highly reducing, hydrogencontaining atmospheres. [4][5][6] The use of hydrogen as a reducing agent makes establishing the relationship between oxygen vacancies and the electrical properties complicated. In particular, interstitial hydrogen has been suggested to act as a shallow donor in SrTiO 3 and related materials.…”

Electrostatic modulation of the electronic properties of Nb-doped SrTiO3 superconducting films

“…Finally, the behavior of T c as a function of carrier concentration in doped STO is different from that of conventional low carrier superconductors for which T c increases monotonically with the carrier concentration. 17 In STO, T c first increases with the carrier concentration n then, for n larger than 1ϫ10 20 cm Ϫ3 , T c goes down, with superconductivity vanishing around 1ϫ10 21 cm Ϫ3 . It is generally accepted that this behavior can be explained by the plasmon and polar optic phonon model.…”

“…The ratio between the Nb doping and the active Nb donors is found to be less than previously reported ͑ϳ50%͒ for thin films 19 and single crystals. 20,21 Clearly, although the STO films described in the following are of high quality, a complex process which will require further investigation controls the fraction of active Nb donors. By monitoring the intensity of reflection high-energy electron diffraction during the deposition, stepflow type growth 22 was observed.…”