Atomic-scale structural and electronic properties of ${\mathrm{SrTiO}}_3/\mathrm{GaAs}$ interfaces: A combined STEM-EELS and first-principles study

The electronic properties of epitaxial oxide thin films grown on compound semiconductors are largely determined by the interfacial atomic structure, as well as the thermodynamic conditions during synthesis. Ferroelectric polarization and Fermi-level pinning in ${\mathrm{SrTiO}}_3$ films have been attributed to the presence of oxygen vacancies at the oxide/semiconductor interface. Here, we present scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy analyses of GaAs films grown on ${\mathrm{SrTiO}}_3$ combined with first-principles calculations to determine the atomic and electronic structures of the ${\mathrm{SrTiO}}_3/\mathrm{GaAs}$ interfaces. An atomically abrupt SrO/As interface is observed and the interfacial SrO layer is found to be O-deficient. First-principles density functional theory (DFT) calculations show SrO/Ga and Sr/As interfaces are favorable under O-rich and O-poor conditions, respectively. The SrO/Ga interface is reconstructed via the formation of Ga-Ga dimers while the Sr/As interface is abrupt and consistent with the experiment. DFT calculations further reveal that intrinsic two-dimensional electron gas (2DEG) forms in both SrO/Ga and Sr/As interfaces, and the Fermi level is pinned to the localized 2DEG states. Interfacial O vacancies can enhance the 2DEG density while it is possible for Ga/As vacancies to unpin the Fermi level from the 2DEG states.

Figure 1

Ball-and-stick model of the ($2\times 2$) STO/GaAs interfaces in three-dimensional view. The top surface of GaAs is passivated by pseudohydrogen atoms and the bottom surface of STO is in contact with vacuum.

Figure 2

Atomic-resolution HAADF images of the STO/GaAs interface from two different views which are ${90}^{\circ }$ rotated: (a) the $\mathrm{STO}\left[010\right]\left|\right|\mathrm{GaAs}\left[110\right]$ epitaxy with full GaAs dumbbell structure at the interface and (c) the $\mathrm{STO}\left[100\right]\left|\right|\mathrm{GaAs}\left[1\overline{1}0\right]$ epitaxy with half GaAs dumbbell structure at the interface. The images are taken from dislocation-free areas and filtered using average background subtraction filter within a Gatan Digital Micrograph. Panels (b) and (d) show the intensity profiles of atomic lines centered on the Sr and Ti in STO averaged from (a) and (c), respectively.

Figure 3

(a) EEL spectra of a Ti $L_{3,2}$-edge taken from the STO bulk and the first to fourth ${\mathrm{TiO}}_2$ layers near the STO/GaAs interface. The dashed lines denote the positions of the four peaks in the bulk spectrum. (b) The O $K$-edge taken from the STO bulk and the first to fourth SrO layers near the STO/GaAs interface. The spectrum data are smoothed using a Gaussian function.

Figure 4

The computed ternary phase diagram of the formation energies of STO/GaAs interfaces with different stoichiometries under (a) Ga-rich (${\mu }_{\mathrm{Ga}}={\mu }_{\mathrm{Ga}}^{\mathrm{bulk}}$) and (b) As-rich (${\mu }_{\mathrm{As}}={\mu }_{\mathrm{As}}^{\mathrm{bulk}}$) conditions. The three axes are calculated as ${\mu }_{\mathrm{Sr}}^{*}={\mu }_{\mathrm{Sr}}-{\mu }_{\mathrm{Sr}}^{\mathrm{bulk}}, {\mu }_{\mathrm{Ti}}^{*}={\mu }_{\mathrm{Ti}}-{\mu }_{\mathrm{Ti}}^{\mathrm{bulk}}$, and ${\mu }_{\mathrm{O}}^{*}={\mu }_{\mathrm{O}}-\frac{1}{2}{\mu }_{{\mathrm{O}}_2}^{\mathrm{molecule}}$, where ${\mu }_{\mathrm{Sr}}^{\mathrm{bulk}}, {\mu }_{\mathrm{Ti}}^{\mathrm{bulk}}$, and ${\mu }_{{\mathrm{O}}_2}^{\mathrm{molecule}}$ are approximated by their DFT total energies. The allowed chemical potential area of stable STO without formation of other bulk materials is bounded by the solid lines corresponding to the constraints of ${\mathrm{TiO}}_2, {\mathrm{Ti}}_2{\mathrm{O}}_3$, and Ruddlesden-Popper structure ${\mathrm{Sr}}_6{\mathrm{Ti}}_5{\mathrm{O}}_{16}$. The region to the right of the red line denotes the formation of ${\mathrm{Ga}}_2{\mathrm{O}}_3$.

Figure 5

The DFT-optimized structures of SrO/Ga and Sr/As interfaces. (a) The ($2\times 2$) SrO/Ga interface viewed in $\mathrm{STO}\left[010\right]\left|\right|\mathrm{GaAs}\left[110\right]$ and $\mathrm{STO}\left[100\right]\left|\right|\mathrm{GaAs}\left[1\overline{1}0\right]$ directions. (b) A ($4\times 4$) view of the reconstructed GaAs (001) surface at the SrO/Ga interface with the top Ga atoms highlighted. The unit cell of $c(2\times 2)$ reconstruction is marked by a red square. (c) The ($2\times 2$) Sr/As interface viewed in $\mathrm{STO}\left[010\right]\left|\right|\mathrm{GaAs}\left[110\right]$ and $\mathrm{STO}\left[100\right]\left|\right|\mathrm{GaAs}\left[1\overline{1}0\right]$ directions. (d) A ($4\times 4$) view of the unreconstructed GaAs (001) surface at the Sr/As interface.

Figure 6

Projected DOS on each element from the first to fourth STO and GaAs unit cells for (a) a SrO/Ga interface and (b) a Sr/As interface. The yellow, purple, green, blue, and red curves represent DOS on Ga, As, Sr, Ti, and O, respectively. The Fermi level is shifted to zero, and the energy gaps near the Fermi level are marked by the shaded areas.

Figure 7

Charge density of Ti $d_{xy}$ and $d_{yz}+d_{xz}$ occupied states at the CBM of STO as a function the distance of ${\mathrm{TiO}}_2$ layers from the interface.

Figure 8

Band alignment diagrams of STO/GaAs heterostructures for (a) a SrO/Ga interface and (b) a Sr/As interface. The blue solid curve represents the profile of electrostatic potential of the heterostructure along the out-of-plane direction $V\left(z\right)$, and the purple dashed curve represents the macroscopic average of the electrostatic potential. The red lines indicate the averaged values of the potential in bulklike regions and the band alignment results. The conduction and valence band offsets are determined as $\mathrm{\Delta }{E}_{\mathrm{c}}=E_{\mathrm{c}}^{\mathrm{GaAs}}-E_{\mathrm{c}}^{\mathrm{STO}}$ and $\mathrm{\Delta }{E}_{\mathrm{v}}=E_{\mathrm{v}}^{\mathrm{GaAs}}-E_{\mathrm{v}}^{\mathrm{STO}}$. $\mathrm{\Delta }V$ stands for the difference of the averaged potential between GaAs and STO parts.

Figure 9

Projected DOS on each element in the first STO and GaAs unit cells for the defect-induced interfaces. The Fermi level is shifted to zero. SrO/${\mathrm{As}}_{0.5}$ is semiconducting with the band gap marked by the shaded red area.