Controlled oxygen vacancies and space correlation with Cr 3+ in SrTiO3 For the development of semiconducting electronic devices based on perovskite compounds, it is essential to control point defects, such as oxygen vacancies (VO ) or transition-metal dopants. We present a structural study of Cr-related point defects in Cr-doped SrTiO 3. A special thermal reduction process introduces neutral VO as well as doubly charged VO ++ in the bulk. The amounts of both kinds of vacancies are directly controlled by the Cr concentration.
Current-induced bistable resistance effects on metal-insulator-metal (MIM) structures based on doped perovskite oxides, such as Cr-doped SrTiO3, have attracted substantial interest due to their potential technological use in non-volatile resistive memory devices. Although various models have been proposed for resistive switching in SrTiO3, a clear understanding of the role and the nature of intrinsic defects or dopants has not yet been achieved. In SrTiO3 an MIM transition is obtained at a small critical density of electron doping of 1018 e/cm3. Moreover, various cations cause substantial modifications of the electronic properties.
Ti4+
Cr3+
Ti3+
Empa Activities 09/10 Information, Reliability and Simulation Technology
We performed a structural study of a Cr-related point defect in Cr-doped SrTiO3. It is known that in SrTiO3 oxygen vacancies (VO) introduced by a reduction process can create a conducting state. However, by using Cr dopants, the insulating state is preserved via charge compensation by transforming the initial Cr 4+ to Cr 3+. The Cr 3+ ions act as a reservoir of electrons which can be transferred selectively to the conduction band by photo-excitation above 1.86 eV. In our investigation we found that, with our special thermal reduction process, two kinds of VO are introduced in the bulk: neutral VO and doubly charged VO++. The amounts of both kinds of VO are directly controlled by the Cr concentration, and this correlation accounts for the preservation of the insulating state of the reduced Cr-doped crystals. The capability of controlling point defects, such as VO or transition-metal dopants, is essential for the development of semiconducting electronic devices based on perovskite compounds. Based on EPR analysis of our Cr-doped samples we identified a linear complex of the form Cr 3+-O-Ti 3+-VO-Ti 3+ (Fig. 1), which is unique in its structure. It describes the spatial correlation between the neutral VO and the Cr ions in the bulk, and explains how this correlation accounts for the transport properties of SrTiO3.
Fabio La Mattina, in collaboration with J.G. Bednorz, IBM Zurich Research Laboratory (CH)
O
~R0 ~R0 R0+Δ V0 R0-Δ Δ
~R0 ~R0 Ti4+
ETi4+ ETi3+
Ti3+
Fig.1: Model of the Cr 3+-O-Ti 3+-Vo-Ti 3+ complex in reduced Cr-doped SrTiO3 with the Cr 3+ ion displaced by a distance D from the original 4+ position. The electric field generated by the two adjacent Ti 3+ and Ti 4+ may be responsible for the displacement.
Links: www.empa.ch/abt173
Contact: fabio.lamattina@empa.ch Reference: F. La Mattina, J.G. Bednorz, S. Alvarado, A. Shenghelaya, K.A. Müller, H. Keller, Phys. Rev. B 80, 075122 (2009)
63