Charge Transfer and Built-in Electric Fields between a Crystalline Oxide and Silicon

We report charge transfer and built-in electric fields across the epitaxial ${\mathrm{SrNb}}_x{\mathrm{Ti}}_{1-x}{\mathrm{O}}_{3-\delta }/\mathrm{Si}(001)$ interface. Electrical transport measurements indicate the formation of a hole gas in the Si and the presence of built-in fields. Hard x-ray photoelectron measurements reveal pronounced asymmetries in core-level spectra that arise from these built-in fields. Theoretical analysis of core-level spectra enables built-in fields and the resulting band bending to be spatially mapped across the heterojunction. The demonstration of tunable charge transfer, built-in fields, and the spatial mapping of the latter, lays the groundwork for the development of electrically coupled, functional heterojunctions.

Figure 1

(a) $R_s$ for various $x$, showing anomalies (arrows) in the $x=0$, 0.084, and 0.20 heterojunctions. (b) $R_{xy}$ for the $x=0.20$ heterojunction, which exhibits a crossover in sign of the slope. Data are shown as symbols, while calculated fits to the data are shown as lines. (c) $n_h$ and ${\mu }_h$ of holes determined from fits of the $R_{xy}$ data for the $x=0.20$ heterojunction. (d) $R_{xy}$ for the $x=0.60$ heterojunction. (e) HAADF-STEM image of the $x=0.20$ heterojunction, showing an atomically abrupt interface.

Figure 2

(a)–(f) Ti $2p$, Nb $3d$, O $1s$, Sr $3d$, Si $2p$ spectra from $\mathrm{SNTO}/\mathrm{Si}$ $x=0.20$ (red) and $x=0$ (blue) heterojunctions. Spectra from a 1 at.% Nb-doped STO(001) single crystal and Si(100) substrate are also shown (green) for comparison. The Ti $2p$ spectra exhibit oxidation states of $0-4+$ as shown by fits in (b). Also, note the asymmetric features observed in the core-level spectra from the heterojunctions (arrows) that are not present in the spectra of bulk substrates.

Figure 3

(a) [(c)] Contour intensity plots of layer-resolved Si $2p$ and Ti $2p_{3/2}$ spectra vs distance from the interface resulting from fitting the spectra from the $x=0$ ($x=0.20$) $\mathrm{SNTO}/\mathrm{Si}$ heterojunction that takes into account built-in fields. Overlays of the sums of all spectra shown in (a) [(c)] to the experimental heterojunction spectra for the $x=0$ ($x=0.20$) heterojunction are on the right. (b) [(d)] Band edge profiles for the $x=0$ ($x=0.20$) $\mathrm{SNTO}/\mathrm{Si}$ heterojunction taken from the fits shown in (a) [(c)]. The conduction band-edge profiles are simply the valence band-edge profiles less the band gap for the appropriate material.