Space charges and defect concentration profiles at complex oxide interfaces

Abstract

We discuss electronic and ionic defect concentration profiles at the conducting interface between the two wide-band-gap insulators LaAlO 3 and SrTiO 3 (STO). The profiles are deduced from a thermodynamic model considering a local space charge layer (SCL) originating from charge transfer to the interface region, thus combining electronic and ionic reconstruction mechanisms. We show that the electrical potential confining the two-dimensional electron gas (2DEG) at the interface modifies the equilibrium defect concentrations in the SCL. For the n-conducting interface, positively charged oxygen vacancies are depleted within the SCL, while negatively charged strontium vacancies accumulate. Charge compensation within the SCL is achieved by a mixed ionic-electronic interface reconstruction, while the competition between 2DEG and localized ionic defects is controlled by ambient pO 2. The concentration of strontium vacancies increases drastically in oxidizing conditions and exhibits a steep depth profile towards the interface. Accounting for the low cation diffusivity in STO, we also discuss kinetic limitations of cation defect formation and the effect of a partial equilibration of the cation sublattice. We discuss the resulting implications for low temperature transport.

Defect Control of Conventional and Anomalous Electron Transport at Complex Oxide Interfaces

“…The defect structure is inhomogeneous when traversing from regions close to the interface to regions farther away from the interface [13]. This will induce different mean scattering times τ i , leading to different mobilities, μ i ¼ eτ i =m à i , of the various charge carrier populations at low temperatures.…”

“…The understanding of these properties and, in particular, the interrelation between ionic defects and electronic or magnetic properties is extensively debated. To this end, it has been shown that gradual ionic defect distributions can cause space charges and inhomogeneous electronic carrier concentrations at oxide interfaces [11][12][13].…”

Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3

“…15 However, the temperature of 770 K in our experiments would be remarkably low for the formation of Sr vacancy defects, which are typically only considered at temperatures well above 1000 K due to kinetic limitations in the bulk. 15,20,21 However, due to the localization of the predicted effect at the surface, there would be no need for an ionic movement and hence the formation of V Sr at the surface might be possible even at lower temperatures.…”

“…This would freeze out some of the concentrations, leading to an effective charge separation at the surface. 15,20 The lower temperature limit at which the surface defect chemistry of the oxygen and strontium sublattices needs to be considered is already well investigated for acceptor-doped SrTiO 3 . However, it is much more complex for donor-doped SrTiO 3 , due to long equilibration times, secondary phase formation, 22 and the stronger influence of cations as electronic charge compensating species.…”

“…4 [14][15][16][17] A comparable scenario has been proposed also for electron gases formed at interfaces of SrTiO 3 . [18][19][20] Classical defect chemistry of donor-doped bulk SrTiO 3 is generally accepted for elevated temperatures above 1200 K. 21 At lower temperatures, however, it might not be possible to equilibrate either of the ionic sublattices in the entire sample due to sluggish ionic movement. This would freeze out some of the concentrations, leading to an effective charge separation at the surface.…”