The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO3/SrTiO3 interface has been a topic of debate. Metallic interfaces with similar electronic properties produced by amorphous oxide overlayers on SrTiO3 [Y. Chen et al., Nano Lett. 11, 3774 (2011); S. W. Lee et al., Nano Lett. 12, 4775 (2012)] have called in question the original polarization catastrophe model [N. Nakagawa et al., Nature Mater. 5, 204 (2006)]. We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO3 substrates with crystalline and amorphous overlayers of LaAlO3 of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (∼1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restored the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case, but the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO3 overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO3/SrTiO3 heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO3/SrTiO3 heterostructures. Furthermore, it was found that the crystallinity of the LaAlO3 layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO3 overlayers. PACS numbers: 73.20.-r 73.21.Ac 73.40.-c 71.23.CqThe two-dimensional electron gas (2DEG) appearing at the interface between the band insulators LaAlO 3 (LAO) and SrTiO 3 (STO) has attracted much attention since its discovery by Ohtomo and Hwang [1]. It has stimulated a substantial body of experimental and theoretical work [2-25], but, its origin is still controversial [26]. Three different mechanisms have been proposed. First is interface electronic reconstruction to avoid the polarization catastrophe induced by the discontinuity at the interface between polar LAO and nonpolar STO [2][3][4]. Second is doping by thermal interdiffusion of Ti/Al or La/Sr atoms at the interface [13]. A third possible mechanism is creation of oxygen vacancies in STO substrates during the deposition process [9][10][11]27,28]. Oxygen vacancies are known to introduce a shallow intragap donor level close to the conduction band of STO [29], and their action may be specific to this one substrate. The thermal interdiffusion mechanism was discounted in recent work [25], which studied the effect of a mixed interface layer. It i...
We report optical, electrical and magnetotransport properties of oxygen deficient SrTiO(3) (SrTiO(3-x)) thin films fabricated by pulsed laser deposition technique. The oxygen vacancies (O(vac)) in the thin film are expected to be uniform. By comparing its electrical properties to those of bulk SrTiO(3-x), it was found that O(vac) in bulk SrTiO(3-x) is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the SrTiO(3-x) film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be reexcited by Joule heating, electric and intriguingly magnetic field.
Since the discovery of two-dimensional electron gas (2DEG) at the oxide interface of LaAlOinterface. The minimal thickness of the polar layer t C that is required for electronic reconstruction is t C = 0 P E/eP, where P is the dielectric constant of the polar material, E is the energy gap separating the valance band of the polar layer and the conduction band of the nonpolar material, and P is the electric polarization of polar layers [10]. Taking P = 24, E as STO bandgap of 3.2 eV, and P = 0.526 C m -2 for the LAO/STO (001) interface, t C is calculated to be 4 unit cells (uc) Here we show that this can be accomplished by replacing LAO with LSAT - When grown on STO, the lattice mismatch for LSAT/STO is only 1.0%, which is only one third of the value of LAO/STO (3.0%). Furthermore, STO and LSAT both undergo a similar cubic-to-tetragonal transition below 100 K [25,26], whereby maintaining the structural coherency. Fig. 1(c).Given the nonpolar nature of SrTiO 3 (001), a polar-discontinuity-induced 2DEG is expected at the LSAT/STO (001) interface.The thickness-dependent transport data at 2 K for the LSAT/STO (001), (110), and (111) interfaces are summarized in Figs. 2(a)-2(c), respectively. In Fig. 2(a), the (001) interface becomes conducting when covered by a LSAT layer with thickness t ≥ 5 uc. Furthermore, the low-temperature sheet conductance of LSAT/STO increasers with t, reaching its highest value at t ≈ 12 uc. This conductance improvement is not caused by any increases of carrier density, but it is brought about by a great enhancement of carrier mobility S , which reaches its peak of 35,000 cm 2 V -1 s -1 at t = 12 uc.And this high carrier mobility is about 30 times larger than that of LAO/STO interfaces prepared under similar conditions [5,21,22]. Also, we note that clear Shubnikov-de Haas conductance oscillations can be observed at 2 K for (001) interfaces with high carrier mobility (Fig. S3 in Supplementary Materials [30]). In addition, the transport data clearly show that there are two critical thicknesses for the LSAT/STO (001) interface: one is at 5 uc where the 2DEG is established and the other is around 12 uc where the mobility is greatest.On the other hand, this high mobility 2DEG is also observed at the annealed (110)-and (111)-orientated LSAT/STO interfaces, which is similar to the LAO/STO interface with different orientations [22,31]. But unlike LSAT/STO (001) interface, both the (110) and (111) (110) and (111) LSAT/STO interfaces show much more robust metallicity. For example, our data show that the high-mobility 2DEG can be maintained in the LSAT/STO (110) and (111) interfaces with a 50-uc-thick LSAT layer, while the LAO/STO (110) and (111) interfaces show low-temperature insulating behavior when LAO thickness is beyond just 10 uc [22].Therefore, two major differences between LSAT/STO and LAO/STO interfaces can be found in Fig. 2. One is the much higher carrier mobility and more robust metallicity at the LSAT/STO interface, and the other one is the observation of two critical th...
The ability to change states using voltage in ferroelectric tunnel junctions (FTJs) offers a route for lowering the switching energy of memories. Enhanced tunneling electroresistance in FTJ can be achieved by asymmetric electrodes or introducing metal-insulator transition interlayers. However, a fundamental understanding of the role of each interface in a FTJ is lacking and compatibility with integrated circuits has not been explored adequately. Here, we report an incisive study of FTJ performance with varying asymmetry of the electrode/ferroelectric interfaces. Surprisingly high TER (∼400%) can be achieved at BaTiO3 layer thicknesses down to two unit cells (∼0.8 nm). Further our results prove that band offsets at each interface in the FTJs control the TER ratio. It is found that the off state resistance (R(Off)) increases much more rapidly with the number of interfaces compared to the on state resistance (ROn). These results are promising for future low energy memories.
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