Experimental investigation of electronic properties of buried heterointerfaces of $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$

We have made very thin films of $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ on $\mathrm{Ti}{\mathrm{O}}_2$ terminated $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of ${10}^{16}{\mathrm{cm}}^{-2}$ and a mobility around ${10}^4{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ on the carrier density is found to be minimal.

Figure 1

(Color online) Resistance as a function of temperature for two 5 ML thick $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ samples, deposited at different oxygen background pressures: one sample deposited at ${10}^{-5}\text{Torr}$ ${\mathrm{O}}_2$ (red open circles) and one at ${10}^{-6}\text{Torr}$ ${\mathrm{O}}_2$ (blue open squares). Also given are some important parameters of both films at $4\mathrm{K}$.

Figure 2

(Color online) Transport measurements and analysis of films grown at low oxygen pressure $({10}^{-6}\text{Torr}$). (a) The mobility of the charge carriers, ${\mu }_{\mathrm{H}}$, as a function of temperature of the one (red triangles), three (orange squares), and five (black diamonds) monolayer thick films of $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$; the one monolayer sample has been oxidized $\left(6000\mathrm{L}\right)$. The sheet resistance, $R_{xx}$, is shown in the inset. (b) The sheet carrier density for the same samples calculated from the Hall resistance data ($R_{xy}$, see inset).

Figure 3

(Color online) Extra features show up in the resistance as a function of temperature plots during warmup (red open circles) than during cooldown (blue solid squares). These increases in resistivity have been observed in multiple samples and always occur at the same temperatures.

Figure 4

Normalized UPS $\left(\mathrm{He}\mathrm{I}21.2\mathrm{eV}\right)$ wide scans for samples with different $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ thicknesses on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$. A clear trend is visible as the spectrum changes from bare $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ (5% Nb doped $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ was used to avoid charging of the sample) to 3 ML of $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$. The thickness of the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ films was varied from $0.5\text{to}3\text{unit}$ cell layers based on RHEED oscillations. The dotted line is a guide to the eyes. These spectra were given an arbitrary offset for clarity.

Figure 5

(Color online) UPS near the Fermi level. Raw UPS $\mathrm{He}\mathrm{I}$ spectra of a one monolayer thick $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ layer on $\mathrm{Ti}{\mathrm{O}}_2$ terminated $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ prepared at $1\mathrm{Hz}$ (blue open squares, fitted with a solid blue line) and the same sample after exposure to $\sim 6000\mathrm{L}$ of oxygen (red open triangles, fitted with a dashed red line). Inset: 1 ML sample before oxidation upon subtraction of background indicated by the dotted green line in the main figure.

Figure 6

(Color online) Analysis of UPS data for different samples. (a) Comparison of the UPS signal near $E_F$ for different $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ film thicknesses by integration of the difference intensity (for example, from Fig. 5); the error bars indicate the statistical error; the connecting line is a guide to the eyes; the red open triangle is the value found for the $1\mathrm{Hz}$, 1 ML sample after exposure to $\sim 6000\mathrm{L}$ of oxygen. (b) Comparison of the integrated difference intensity as a function of the laser repetition rate (1 ML—0.5, 1, 2, $5\mathrm{Hz}$), where the error bars indicate the statistical error; the connecting line is a guide to the eyes; the open circle is the value found for a sample where the pressure quench described in the text is not applied; the solid triangle represents the value found for a sample made when a laser energy of $0.8\mathrm{J}{\mathrm{cm}}^{-2}∕\text{pulse}$ is used.