Thermally assisted tunneling transport in La${}_{0.7}$Ca${}_{0.3}$MnO${}_3$/SrTiO${}_3$:Nb Schottky-like heterojunctions

We report on the electrical transport properties of all-oxide La${}_{0.7}$Ca${}_{0.3}$MnO${}_3$/SrTiO${}_3$:Nb heterojunctions with lateral size of just a few micrometers. The use of lithography techniques to pattern manganite pillars ensures perpendicular transport and allows exploration of the microscopic conduction mechanism through the interface. From the analysis of the current-voltage characteristics in the temperature range 20–280 K we find a Schottky-like behavior that can be described by a mechanism of thermally assisted tunneling if a temperature-dependent value of the dielectric permittivity of SrTiO${}_3$:Nb (NSTO) is considered. We determine the Schottky energy barrier at the interface, $q{V}_B=1.10\pm 0.02$ eV, which is found to be temperature independent, and a value of $\xi =17\pm 2$ meV for the energy of the Fermi level in NSTO with respect to the bottom of its conduction band.

Figure 1

(a) X-ray reflectivity and diffraction (inset) patterns of a 40-nm-thick LCMO layer. The presence of a large number of finite-thickness oscillations in the reflectivity data indicates that the samples are flat over long lateral distances. High-angle XRD evidences the epitaxial growth of the manganite layer with the (001) orientation on the STO substrate. (b) Low- and high- (inset) magnification HAADF images of the interface between the substrate and the manganite layer. The dashed box marks the region of the high-magnification image.

Figure 2

(a) Current density–voltage characteristics of a 7 × 14 μm${}^2$ LCMO/NSTO heterojunction, with a 60-nm-thick LCMO layer, at several temperatures (280, 260, 240, 220, 200, 180, 160, 140, 130, 120, 110, 100, 90, 80, 70, 60, 50, 40, 30, and 20 K from the left to the right). Isotherms at 200, 100, and 50 K are plotted as thicker lines for clarity. Inset is a schematic layout of the fabricated junctions showing the electrodes used (see text). (b) Semilogarithmic plot of the current density–voltage characteristics in the forward-bias region at several temperatures (280, 260, 240, 220, 200, 180, 160, 140, 130, 120, 110, 100, 90, 80, 70, 60, 50, 40, 30, and 20 K from the left to the right). Data at 200, 100, and 50 K are plotted as solid symbols for clarity. Solid lines are fits to Eqs. (1) and (2) in the text.

Figure 3

Temperature dependence of the ideality factor $n$ (a), the Schottky barrier height $V_B$ (b), and the saturation current density $J_s$ (c), as obtained from the fits shown in Fig. 2b.

Figure 4

Sketch of the potential profile for a LCMO/NSTO Schottky junction under a forward bias $V$. The arrows represent thermoionic emission (a) and thermally assisted tunneling (b) of electrons across the junction from the semiconductor NSTO to the metallic LCMO.

Figure 5

Open circles represent the energy $E_0$ = nkT determined as the inverse of the slope in ln$J-V$ plots (see text). The dotted line is the best fit to Eq. (4a) by using a constant value for the parameter $E_{00}$ defined by Eq. (4b). The solid line is the best fit to Eq. (4a) by allowing a temperature dependence of $E_{00}$ through a temperature-dependent value of the dielectric permittivity of NSTO. A weakly exponential decay of the dielectric permittivity is assumed, which is shown in the inset (solid line), to describe the temperature dependence of the values of the dielectric permittivity of NSTO (open squares) obtained from the fits to Eq. (4) of $E_0$ data shown in the main panel.

Figure 6

Plots of $\ln [J_S\left(T\right)\cosh (E_{00}/kT)/E_{00}^{1/2}T]-\xi /kT$ as a function of 1/$E_0$ for $\xi =0$ meV (solid squares) and for $\xi =17$ meV (open circles). Solid lines are linear fits to the data between 280 and 60 K for the case of $\xi =0$ meV, and to the data in the whole temperature range, between 280 and 20 K, for the case of $\xi =17$ meV.