Universality of electron mobility in LaAlO3/SrTiO3 and bulk SrTiO3

Abstract

Metallic LaAlO3/SrTiO3 (LAO/STO) interfaces attract enormous attention, but the relationship between the electron mobility and the sheet electron density, ns, is poorly understood. Here, we derive a simple expression for the three-dimensional electron density near the interface, n3D, as a function of ns and find that the mobility for LAO/STO-based interfaces depends on n3D in the same way as it does for bulk doped STO. It is known that undoped bulk STO is strongly compensated with N≃5×1018 cm−3 background donors and acceptors. In intentionally doped bulk STO with a concentration of electrons n3D<N, background impurities determine the electron scattering. Thus, when n3D<N, it is natural to see in LAO/STO the same mobility as in the bulk. On the other hand, in the bulk samples with n3D>N, the mobility collapses because scattering happens on n3D intentionally introduced donors. For LAO/STO, the polar catastrophe which provides electrons is not supposed to provide an equal number of random donors and thus the mobility should be larger. The fact that the mobility is still the same implies that for the LAO/STO, the polar catastrophe model should be revisited.

Controlling the electronic interface properties of AlO$_x$/SrTiO$_3$ heterostructures

“…The model yields two electron-like carrier populations, one with large density (n 2 ≈ 2 × 10 14 cm −2 ) and low mobility (µ 2 ≈ 140 cm 2 V −1 s −1 ) and one with much smaller density (n 1 ≈ 1 × 10 12 cm −2 )) and a significantly larger mobility (µ 1 ≈ 2000 cm 2 V −1 s −1 ). These data also reveal that a sizable fraction of the mobile charge carriers get frozen out at low temperatures, which is also observed in other STO based systems where doping is dominated by the formation of oxygen vacancies [8,15,16,31]. Interestingly, the measured sheet carrier densities are at all temperatures higher than in the AlO x /STO heterostructures of Sengupta et al [15] and the PLD-grown AlO x /STO heterostructures of Wolff et al [16], likely due to significantly higher oxygen vacancy concentrations in our samples.…”

Electron Mobility in γ−Al2O3/SrTiO3

“…Increasing the carrier density results in a pronounced increase in the mobility roughly described by µ~1 .5 until the mobility peaks at more than 100,000 cm 2 /Vs when the carrier density reaches (µ )~4 ⋅ 10 14 cm -2 . The positive correlation between the mobility and carrier density in GAO/STO is radically different from LAO/STO where the exponent is negative [19,20]. Interestingly, a positive exponent of ~1.5 is also observed in modulation doped electronic systems where the donors and electrons are spatially separated [42,43].…”

“…To date, the largest mobility of 140,000 cm 2 /Vs has been observed when STO is interfaced with γ-Al2O3 (GAO) [3], where oxygen vacancies account for the formation of the interface conductivity [3,17,18]. Contrary to the majority of other STO-based heterostructures [19,20], the highest mobilities in GAO/STO surprisingly occur at a high sheet carrier density exceeding 10 14 cm -2 , despite the abundance of oxygen vacancy donors in STO, which act as scattering sites [3,18,21]. A spatial separation of the electrons and donors within STO was recently proposed to be the origin of the high mobility in GAO/STO at low temperatures [21], but it remains to be settled unambiguously.…”

Electron mobility in oxide heterostructures

“…The figure also includes the scattering model presented in Section 3.3. Note that in order to plot the electron mobility data for bulk conducting STO together with the confined electron systems, the three-dimensional carrier density was translated into the equivalent two-dimensional density following the expression derived by Trier et al [65]. At first glance, it is clear from Figure 17 that some confined systems seem to follow an similar trend in µ(ns) as bulk conducting STO over two orders of carrier densities for 10 12 cm −2 < < 10 14 cm −2 .…”