Mapping the spatial distribution of charge carriers in LaAlO3/SrTiO3 heterostructures

Abstract

At the interface between complex insulating oxides, novel phases with interesting properties may occur, such as the metallic state reported in the LaAlO 3 /SrTiO 3 system [1].While this state has been predicted [2] and reported [3,4] to be confined at the interface, some works indicate a much broader spatial extension [5], thereby questioning its origin. Here we provide for the first time a direct determination of the carrier density profile of this system through resistance profile mappings collected in cross-section LaAlO 3 /SrTiO 3 samples with a conducting-tip atomic force microscope (CT-AFM). We find that, depending upon specific growth protocols, the spatial extension of the high-mobility electron gas can be varied from hundreds of microns into SrTiO 3 to a few nanometers next to the LaAlO 3 /SrTiO 3 interface.Our results emphasize the potential of CT-AFM as a novel tool to characterize complex oxide interfaces and provide us with a definitive and conclusive way to reconcile the body of experimental data in this system 2 Due to the richness of their intrinsic properties oxide materials can bring novel functionalities to electronics. Among all oxides, SrTiO 3 (STO) is particularly appealing because of its multifunctional character. It is a wide band gap semiconductor (3.2 eV) that becomes a high mobility metal [6] or even superconducting [7] upon oxygen vacancy doping.Stoichiometric STO is also at the fringe of ferroelectricity and its large dielectric permittivity, tunability and low microwave loss, makes it a good candidate for tunable microwave devicesThe potential of oxides is further enlarged by the fascinating properties of their interfaces, epitomized by the observation of ferromagnetism at the interface between two antiferromagnets [9] or of the quantum Hall effect in two-dimensional (2D) ZnO-based structures [10]. Recently, an electron gas with high electron mobility at low temperature (in the 10 4 cm 2 /Vs range) has been reported in heterostructures combining STO with LaAlO 3 (LAO), another band insulator [1]. Since its discovery by Ohtomo and Hwang [1], this unusual metallic state has been the object of many studies [2,3,4,5,11,12,13,14,15,16,17,18].Recent results include a two-dimensional superconducting behaviour at the LAO/STO interface below 200 mK [19], and indications of a ferromagnetic state below ~1K [20].The arguments invoked to explain the metallic state observed in such LAO/STO samples have been dual. A first interpretation relies on the transfer of a ½ e charge per unit cell at the interface, because of the polar discontinuity between positively charged LaO and neutral TiO 2 sub-planes [11]. In this picture a sheet carrier density n sheet ≈ 3.3 10 14 cm -2 is expected. However, in many cases the reported values of n sheet exceed this density by orders of magnitude [1,5,17]. This leads to another interpretation relying on the creation of oxygen vacancies in the STO substrate during the growth of the LAO film, which dopes the STO with electrons and makes it a high-mobility meta...

Nonvolatile Resistive Switching inPt/LaAlO3/SrTiO3Heterostructures

“…In contrast, a two-dimensional nature of the interface transport can be achieved in high-pressure-grown samples, with the electrons being confined a few nanometers near the interface. These transport results are consistent with previous reports on properly oxidized LAO/STO heterostructures [24][25][26]. It is noteworthy that we do not perform any postdeposition annealing in our sample synthesis; thus, as suggested in a recent study [27], oxygen vacancies exist in even the highpressure-grown samples and their contribution to the transport of LAO/STO heterostructures cannot be ignored.…”

“…Based on these transport data, we can conclude that oxygen vacancies play a key role in the conduction of low-pressure-grown samples by contributing high-density electrons near the interface [18]. The presence of oxygen vacancies causes the carrier distribution to extend deep into the STO substrate [25]. In contrast, a two-dimensional nature of the interface transport can be achieved in high-pressure-grown samples, with the electrons being confined a few nanometers near the interface.…”

“…This mechanism, along with the polar distortion and dipole screening in the LAO overlayers [16], can qualitatively explain the transport properties. In addition, factors like oxygen vacancies [17][18][19] and cation mixing [20][21][22] may influence the characteristics of the electron gas [23][24][25][26][27].…”

Nanometer-Scale Striped Surface Terminations on Fractured SrTiO3 Surfaces

“…Scanning probe techniques have been applied to a wide range of oxide materials [9][10][11][12][13][14] . However, and particularly with SrTiO 3 , these techniques have been utilized to investigate the surface morphology and resulting surface reconstructions following various sample preparation techniques that involve chemical and thermal processing [15][16][17][18][19] . Here we show by cross-sectional STM that fracturing SrTiO 3 (Fig.…”

Ferroelectrics at the nanoscale

“…Here, unique functional properties of interfaces themselves have been recently tailored and exploited. For example, the discovery of conducting interfaces between two non-conducting oxides (LaAlO 3 and SrTiO 3 ) [80] has stimulated a great deal of interest [81]. Specifically in relation to ferroelectrics, Bousquet et al [82] have recently discovered that tailored interfaces between SrTiO 3 and PbTiO 3 can induce a new form of improper ferroelectricity, whenever the interface density is great enough.…”