Photoemission study of $\mathbf{LaAl}{\mathbf{O}}_{\mathbf{3}}/\mathbf{SrTi}{\mathbf{O}}_{\mathbf{3}}$ and $\mathbf{LaAl}{\mathbf{O}}_{\mathbf{3}}/\mathbf{Nb}:\mathbf{SrTi}{\mathbf{O}}_{\mathbf{3}}$: Insulator-insulator versus insulator-semiconductor interfaces

By hard x-ray photoemission spectroscopy we have studied the electronic states of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$(100) and ${\mathrm{LaAlO}}_3/\mathrm{Nb}$ : ${\mathrm{SrTiO}}_3$(100) heterostructures, where the polar $\mathrm{LaAl}{\mathrm{O}}_3$ layer is grown on the insulating and conducting substrates, respectively. The analysis of the valence band has shown that the electron affinity is as small as $+0.4\mathrm{eV}$ in both the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures. The interface is “staggered type” (type II) in both of the heterostructures with the valence band maximum of $\mathrm{LaAl}{\mathrm{O}}_3$ above the valence band maximum of $\mathrm{SrTi}{\mathrm{O}}_3$. The valence band offset of 0.42 eV in the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructure has increased to 0.61 eV in the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ as the $\mathrm{LaAl}{\mathrm{O}}_3$ component shifts towards the lower binding energy side. The electronic states of $\mathrm{LaAl}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$ have been merged with each other in the lower energy part of the valence band of the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures.

Figure 1

From top to bottom, $\mathrm{Ti}2p_{3/2}$ core-level spectra of $\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ substrates (Nb 0.5 and 0.01 wt%), $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (Nb 0.5 and 0.01 wt%), and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (samples 1 and 2, thick curves). The sample $2 \mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructure was prepared by two-step postannealing, while the other heterostructures were prepared by single-step postannealing. The spectrum of the $\mathrm{LaAl}{\mathrm{O}}_3$/Nb 0.5 wt%:$\mathrm{SrTi}{\mathrm{O}}_3$ heterostructure and that of Nb 0.5 wt%:$\mathrm{SrTi}{\mathrm{O}}_3$ substrate are shifted by $\sim 0.07$ and $\sim 0.12\mathrm{eV}$ and overlaid on that of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ sample 2.

Figure 2

From top to bottom, Al $1s$ core-level spectra of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (Nb 0.5 and 0.01 wt%) and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (samples 1 and 2, thick curves). The spectrum of $\mathrm{LaAl}{\mathrm{O}}_3$/Nb 0.5 wt%:$\mathrm{SrTi}{\mathrm{O}}_3$ and that of $\mathrm{LaAl}{\mathrm{O}}_3$/Nb 0.01 wt%:$\mathrm{SrTi}{\mathrm{O}}_3$ are shifted by $\sim 0.17$ and $\sim 0.07\mathrm{eV}$ and overlaid on that of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ sample 2.

Figure 3

Energy difference between $\mathrm{Sr}3d_{3/2}$ and Al $2s$ core level of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (Nb 0.5 and 0.01 wt%) and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (samples 1 and 2) as a function of the electron-emission angle with respect to the sample surface.

Figure 4

From top to bottom, valence-band spectra of $\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ substrates (Nb 0.5 and 0.01 wt%), $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (Nb 0.5 and 0.01 wt%), and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (samples 1 and 2, thick curves).

Figure 5

Valence-band spectra of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (sample 2) (a) and $\mathrm{LaAl}{\mathrm{O}}_3$/Nb 0.01 wt%:$\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures (b) as well as the composed spectra, which are the sum of the spectrum of the bare Nb 0.01 wt%:$\mathrm{SrTi}{\mathrm{O}}_3$ substrate and that of $\mathrm{LaAl}{\mathrm{O}}_3$. The spectrum of $\mathrm{LaAl}{\mathrm{O}}_3$ is according to Ref. [33]. The decomposed $\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{LaAl}{\mathrm{O}}_3$ components (designated with “STO component” and “LAO component”, respectively) are shown in (c). The decomposed components are shown on an enlarged energy scale in (d). The lines are a guide to the eye.

Figure 6

Schematic band diagrams of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (a) and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ (b) heterostructures, where CBM, VBM, $E\mathrm{a}$ and Δ are the conduction-band minimum, valence band maximum, electron affinity, and valence band offset, respectively. We assume that the band profile of $\mathrm{SrTi}{\mathrm{O}}_3$ in $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures is not very steep on the length scale of photoemission escape depth $(\sim 6\mathrm{nm})$.