Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions

Abstract

Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. Here we present evidence of such a built-in potential across polar LaAlO3 thin films grown on SrTiO3 substrates, a system well known for the electron gas that forms at the interface. By performing tunneling measurements between the electron gas and metallic electrodes on LaAlO3 we measure a built-in electric field across LaAlO3 of 80.1 meV/Å. Additionally, capacitance measurements reveal the presence of an induced dipole moment across the heterostructure. We forsee use of the ionic built-in potential as an additional tuning parameter in both existing and novel device architectures, especially as atomic control of oxide interfaces gains widespread momentum.As dictated by Maxwell's equations, the accumulation of screening charges at the boundary between dissimilar materials is one means 1 of ensuring a continuous electric displacement at the interface. 2 For instance, a layer of trapped screening charge forms the two dimensional electron gas that compensates a polarization mismatch at gallium nitride 3 and zinc oxide 4 based heterostructure interfaces. In insulating oxides, charge accumulation was observed at the interface between SrTiO 3 substrates with atomically precise surfaces and polar LaAlO 3 films. 5 LaAlO 3 thin films grown on singly terminated SrTiO 3 surfaces 6 comprise negatively charged AlO 2 and positively charged LaO end planes and are polar in the ionic limit. 1,7,8 When at least four unit cells (u.c.) of LaAlO 3 are deposited on TiO 2 terminated SrTiO 3 , an electron gas forms near the interface in SrTiO 3 . 5,9,10 It is often hypothesized that at a thickness of four u.c. the potential across LaAlO 3 exceeds the band gap of SrTiO 3 and electrons tunnel from the valence band of LaAlO 3 to the SrTiO 3 potential well, completely diminishing the potential across LaAlO 3 . 11 Thus within this picture, in the presence of an electron gas no field would be expected across the LaAlO 3 . However, if all the charge carriers do not lie precisely at the interface or have an extrinsic (oxygen vacancies 12 or cation doping 13 ) origin, the LaAlO 3 potential will not be fully screened and can thus be probed. 14 Alternatively, the precise band alignment between the LaAlO 3 and SrTiO 3 will also determine the strength of the residual fields in the LaAlO 3 . 15 Addressing this issue by determining the existence of an uncompensated built-in potential in LaAlO 3 is central to understanding the true nature of the polar LaAlO 3 /SrTiO 3 interface.We probe the potential landscape across the LaAlO 3 and the interface region in SrTiO 3 by employing a typical metal-insulator-metal capacitor geometry such that the LaAlO 3 thin films form the dielectric layer sandwiched between evaporated metallic electrodes an...

Towards electrical spin injection into LaAlO 3 –SrTiO 3

“…Insight into the transport mechanism at play in the device is already provided by the typical tunnelling resistance versus bias curve shown in figure 5d, showing a clear asymmetry between positive and negative bias, reminiscent of previous observations for Au and Pt reference electrodes in Singh-Bhalla et al [39]. This asymmetry mainly originates from the strong trapezoidal shape of the LAO barrier caused by the difference of the materials' workfunctions.…”

Nonvolatile Resistive Switching inPt/LaAlO3/SrTiO3Heterostructures

“…3(j), very short voltage pulses on the Pt electrode in our device can set the device because the LAO layer is extremely thin and the oxygen vacancies only need to migrate across the LAO/ STO interface to accomplish the forming process, realizing the LRS. Besides the applied bias, the internal field within the LAO layer is a function of several factors, including the film thickness, electrode type, and even surface absorbates [16,[51][52][53][54][55][56][57]. This built-in field may contribute to the asymmetry of the energy barrier for the migration of oxygen vacancies [ Fig.…”

Control of electronic conduction at an oxide heterointerface using surface polar adsorbates

“…The intense interest in the LaAlO 3 /SrTiO 3 interface [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] has led to several recent experimental observations that suggest a close relationship between the interface and the LaAlO 3 surface. These include the use of conducting atomic force microscopy probes to toggle a metal insulator transition [10][11][12] through the writing of surface charge 12,13 , and the use of capping layers of SrTiO 3 (ref.…”