Tuning band alignment at a semiconductor-crystalline oxide heterojunction via electrostatic modulation of the interfacial dipole

We demonstrate that the interfacial dipole associated with bonding across the $\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{Si}$ heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into $\mathrm{SrTi}{\mathrm{O}}_3$. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.

Figure 1

(a) Annular dark field (ADF; left) and integrated differential phase contrast (iDPC; right) images of the 12 nm STO/Czochralski (CZ)-Si heterojunction. Scale bar is 1 nm. (b) ADF (left) and iDPC (right) images of the 12 nm STO/float-zone (FZ)-Si heterojunction. (c) and (d) Expanded views of the iDPC images for the 12 nm STO/CZ-Si and 12 nm STO/FZ-Si heterojunctions, respectively. (e) Polarization of the STO near the interface is manifested as O anions (pink) that are displaced downward relative to Sr (cyan) and Ti (green) cations.

Figure 2

(a) and (b) Scanning transmission electron microscopy (STEM) electron energy loss spectroscopy (EELS) mapping of the $\mathrm{Sr}{L}_{2,3}$ and Si $K$ edges for the 12 nm STO/Czochralski (CZ)-Si and 12 nm STO/float-zone (FZ)-Si heterojunctions, respectively. (c) and (d) STEM-EELS fine structure of the $\mathrm{Ti}{L}_{2,3}$ edge of the 12 nm STO/CZ-Si and 12 nm STO/FZ-Si heterojunctions, respectively.

Figure 3

(a) $R_s$ for the 12 nm STO/float-zone (FZ)-Si and 12 nm STO/Czochralski (CZ)-Si heterojunctions. (b) $R_{xy}$ for the 12 nm STO/FZ-Si (orange) and 12 nm STO/CZ-Si (purple) heterojunctions. Note the crossover in sign of $R_{xy}$ in the latter indicating the formation of a hole-gas. Raw data are shown as symbols, whereas fits to the data using a two-carrier model are shown as lines. (c) ${}^{18}\mathrm{O}^{-}$ time-of-flight secondary ion mass spectroscopy (ToF-SIMS) depth profiles in Si for the 12 nm STO/CZ-Si and 12 nm STO/FZ-Si heterojunctions showing higher density of ${}^{18}\mathrm{O}^{-}$ impurities in the former.

Figure 4

(a) Core-level O $1s, \mathrm{Sr}3d, \mathrm{Si}2p$, and $\mathrm{Ti}2p$ spectra for the 12-nm-thick STO/Czochralski (CZ)-Si (purple) and 12 nm STO/FZ-Si (orange) heterojunctions. Reference spectra from Si(001) and $\mathrm{Sr}{\mathrm{Nb}}_{0.01}{\mathrm{Ti}}_{0.99}{\mathrm{O}}_3$(001) single-crystal substrates are also shown. Arrows denote asymmetries observed in the 12 nm STO/CZ-Si heterojunction. (b) Fits to the asymmetric $\mathrm{Si}2p$ and $\mathrm{Ti}2p_{3/2}$ spectra of the 12 nm STO/CZ-Si heterojunction. (c) Band offsets across the 12 nm STO/CZ-Si and 12 nm STO/FZ-Si heterojunctions. Note the type-III (type-II) alignment in the 12 nm STO/CZ-Si (12 nm STO/FZ-Si) heterojunction.

Figure 5

(a) Structural model of the STO/Si interface; C-A shows the separation between the cation (C) and anion (A) atomic planes used as the measure of polarization in the STO film; L1–L4 label Si atomic planes near the interface where the As impurity is located. (b) Polarization of the STO film as a function of film thickness $N$ (in u.c.) and distance from the interface. (c) Valence band edge profiles across the interface (arrow) for various STO/Si heterojunctions. The $n$-type As dopant is situated in row L2. (d) Density of states (DOS) projected on each atomic plane of the polar undoped STO/Si heterojunction. (e) Charge of the STO film calculated as a sum of atomic charges of the Sr, Ti, and O. (f) The effect of As doping on the polarization in the 9-u.c.-thick STO film ($N=9$). Inset: Polarization in the planes 2–6 depending on the location of the As impurity.

Figure 6

(a) $R_s$ for the 4, 8, and 12 nm STO/Czochralski (CZ)-Si heterojunctions. (b) $R_{xy}$ for the 4- and 8-nm-thick STO/CZ-Si heterojunctions. (c) Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) depth profiles of ${}^{18}\mathrm{O}^{-}$ in Si for the 4, 8, and 12 nm STO/CZ-Si heterojunctions. The data for the 12 nm STO/FZ-Si heterojunction is also shown for comparison. (d) and (e) Fits to the $\mathrm{Si}2p$ and $\mathrm{Sr}3d$ spectra for the 4 and 8 nm STO/CZ-Si heterojunctions, respectively. (f) and (g) The corresponding band offsets for the 4 and 8 nm STO/CZ-Si heterojunctions, respectively.

Figure 7

(a) Valence band offset vs integrated area under ${}^{18}\mathrm{O}^{-}$ profile to a depth of 6 nm. (b) Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) data showing integrated intensity with depth used to determine the integrated area.

Figure 8

Panels (a) and (b) show $R_s$ and $R_{xy}$, respectively, for the 12 nm $\mathrm{STO}/n\text{−}\mathrm{Si}$ (${10}^{15}–{10}^{16}\mathrm{c}{\mathrm{m}}^{-3}$) heterojunction. (c) $\mathrm{Ti}2p, \mathrm{Si}2p$, and $\mathrm{Sr}3d$ core-level spectra for the 12 nm $\mathrm{STO}/n\text{−}\mathrm{Si}$ (${10}^{15}–{10}^{16}\mathrm{c}{\mathrm{m}}^{-3}$) heterojunction. Fits to the $\mathrm{Si}2p$ and $\mathrm{Sr}3d$ spectra are also shown. (d) $\mathrm{Ti}2p, \mathrm{Si}2p$, and $\mathrm{Sr}3d$ core-level spectra for the 12 nm $\mathrm{STO}/n\text{−}\mathrm{Si}$ (${10}^{17}–{10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$) heterojunction. Fits to the $\mathrm{Si}2p$ and $\mathrm{Sr}3d$ spectra are also shown. (e) and (f) The band offsets across the $\mathrm{STO}/n\text{−}\mathrm{Si}$ (${10}^{15}–{10}^{16}\mathrm{c}{\mathrm{m}}^{-3}$) and $\mathrm{STO}/n\text{−}\mathrm{Si}$ (${10}^{17}–{10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$) heterojunctions, respectively.