Tuning the two-dimensional electron gas at the LaAlO${}_3/$SrTiO${}_3$(001) interface by metallic contacts

Density functional theory (DFT) calculations reveal that adding a metallic overlayer on LaAlO${}_3/$SrTiO${}_3$(001) alters significantly the electric field within the polar LaAlO${}_3$ film. For Al or Ti metal contacts the electric field is eliminated, leading to a suppression of the thickness-dependent insulator-to-metal transition observed in uncovered films. Independent of the LaAlO${}_3$ thickness, both the surface and the interface are metallic, with an enhanced carrier density at the interface relative to LaAlO${}_3/$SrTiO${}_3$(001) after the metallization transition. Monolayer thick contacts of Ti develop a finite magnetic moment and for a thin SrTiO${}_3$ substrate induce a spin-polarized two-dimensional electron gas at the $n$-type interface, due to confinement effects in the SrTiO${}_3$ slab. For transition (Fe, Co, Pt) and noble metal contacts (Cu, Ag, Au) a finite and even enhanced (Au) internal electric field develops within LaAlO${}_3$. Results for a representative series of metallic overlayers on LaAlO${}_3/$SrTiO${}_3$(001) (Na, Al; Ti, Fe, Co, Pt; Cu, Ag, Au) reveal broad variation of band alignment, size of Schottky barrier, carrier concentration and lattice polarization at the LaAlO${}_3/$SrTiO${}_3$(001) interface. The identified relationship to the size of work function of the metal on LaAlO${}_3$ provides guidelines on how the carrier density at the LaAlO${}_3/$SrTiO${}_3$ interface can be controlled by the choice of the metal contact.

Figure 1

Layer resolved density of states (LDOS) of (a) 1Ti/4LAO/STO(001) and (b) 1Ti/2LAO/STO(001) (with thin STO substrate). The potential build up in the uncovered LAO film (black line) is canceled when LAO is covered by a Ti overlayer (filled area). Additionally, there is a significant spin polarization both in the surface Ti as well as the interface TiO${}_2$ layers in 1Ti/$N$LAO/STO(001).

Figure 2

(a) Layer resolved density of states (LDOS) of 1Ti/2LAO/STO(001) within GGA with a 6.5-ML thick STO substrate. (b) Side view of the system with the electron density integrated in the interval $E_{\mathrm{F}}-0.65$ eV to $E_{\mathrm{F}}$, giving insight into the Ti $3d$ orbital occupation both at the surface and within SrTiO${}_3$.

Figure 3

Layer resolved density of states (LDOS) of 0.5, 2, and 3 ML Ti on 2LAO/STO(001) within GGA with a 6.5-ML thick STO substrate, showing similar band alignment as for 1Ti/2LAO/STO(001) (cf. Fig. 2) as well as a reduction of spin polarization of Ti in the contact layer with increasing thickness.

Figure 4

(a) Cation-anion buckling $\Delta z$ in $n$Ti/2LAO/STO(001), $n=0.5$–3 ML. Note that the strong lattice polarization within LAO for 3 and 4 ML LAO/STO(001) (orange line, empty symbols) is strongly suppressed once the metallic contact is added. (b) Layer resolved Ti $3d$ band occupation integrated between $E_{\mathrm{F}}-0.65$ eV and $E_{\mathrm{F}}$ for $n$Ti /2LAO/STO(001) with $n=$ 0.5 ML (blue circles), $n=$ 1 ML and thin (green stars)/thick STO substrate (black squares); $n=$ 2 ML (magenta diamonds) and $n=$ 3 ML (red triangles). (c) Positions of O $1s$ states with respect to $E_{\mathrm{F}}$. In 0.5 ML Ti there are two different oxygen sites in the top AlO${}_2$ layer with and without Ti on top that exhibit different properties.

Figure 5

Layer resolved DOS of $M/$2LAO/STO(001), $M=\mathrm{Na},\mathrm{Al}$, Fe, Co, Cu, Ag, Pt and Au monolayer thick contacts. Note that the internal electric field in the LAO film is canceled for Na and Al, strongly suppressed but increasing in the series from Fe to Pt and significantly enhanced compared to the uncovered film for an Au contact.

Figure 6

Majority spin band structures for Na, Al, Ti, Fe, Co, Cu, Ag, Pt, and Au contacts. The Ti $3d$ bands in the interface TiO${}_2$ layer are emphasized by circles with the $d_{xy}$ orbital being the lowest lying level at $\Gamma$. The highest occupation of the Ti $3d$ band at the LAO/STO(001) interface is observed for an Al contact and decreases in the series. Finally, for Au the Ti $3d$ band lies even above the Fermi level.

Figure 7

(a) Cation-anion buckling $\Delta z$ in 1 ML $M/$2LAO/STO(001). The strong lattice polarization within LAO for 3 and 4 ML LAO/STO(001) is strongly suppressed once the metallic contact is added. (b) Layer resolved Ti $3d$ band occupation integrated between $E_{\mathrm{F}}-0.65$ eV and $E_{\mathrm{F}}$ for 1 ML $M/$2LAO/STO(001). Note that charge in the AlO${}_2$ layer next to the interface is nearly vanishing in all cases, charge on the LaO and SrO layers is zero (not shown). (c) Positions of O$1s$ states with respect to $E_{\mathrm{F}}$.

Figure 8

Schematic band diagram of (a) LAO/STO(001) at the verge of an electronic reconstruction at the critical LAO thickness and (b) LAO/STO(001) covered by a metallic contact layer ($M$). Note that the potential build up that leads to an electronic reconstruction in the uncovered LAO/STO(001) system at the critical thickness is strongly reduced/eliminated in $M/$LAO/STO(001) except for the case of noble metal electrodes.