Charge compensation and mixed valency in ${\mathrm{LaAlO}}_3∕{\mathrm{SrTiO}}_3$ heterointerfaces studied by the FLAPW method

The electronic structures and properties of heterointerfaces in the perovskite oxide superlattices ${\mathrm{LaAlO}}_3∕{\mathrm{SrTiO}}_3\left[001\right]$ are presented using the first-principles all-electron full-potential linearized augmented plane-wave (FLAPW) method. Superlattices with three types of interfaces, (i) electron-doped, (ii) hole-doped, and (iii) both electron- and hole-doped, are studied and compared. For the electron-doped interface, the mixed valency of Ti along with the Jahn-Teller effect are found to explain the metallicity, in agreement with experiment, whereas for the hole-doped interface metallicity is found, in contrast to experiment. Oxygen vacancies introduce an additional $n$-type carrier to compensate the holes present at the interface, which now becomes insulating and agrees well with experiment. For a system with both types of interfaces, the metallicity found for the unrelaxed structure is changed to insulating after relaxation is included. For all cases, the relaxation results in a complicated buckled geometry, which is a good indication of the adjustment due to polarity discontinuity. Our findings support recent experimental results, namely, (i) the mixed-valence character of Ti at the electron-doped interface, and (ii) the existence and importance of oxygen vacancies at the hole-doped interface.

Figure 1

Schematic diagrams for tetragonal unit cells of superlattices with (a) $(\mathrm{LaO}{)}^{+}∕({\mathrm{TiO}}_2{)}^0$ interfaces, (b) $({\mathrm{AlO}}_2{)}^{-}∕(\mathrm{SrO}{)}^0$ interfaces, and (c) both of them.

Figure 2

The total and projected DOS near $E_F$ of superlattices with (a) $(\mathrm{LaO}{)}^{+}∕({\mathrm{TiO}}_2{)}^0$ and (b) $({\mathrm{AlO}}_2{)}^{-}∕(\mathrm{SrO}{)}^0$ relaxed interfaces.

Figure 3

Charge density in the unit cell of relaxed structural superlattices with (a) $(\mathrm{LaO}{)}^{+}∕({\mathrm{TiO}}_2{)}^0$ interfaces and (b) $({\mathrm{AlO}}_2{)}^{-}∕(\mathrm{SrO}{)}^0$ interfaces in an energy slice of $0.075\mathrm{eV}$ below $E_F$. The starting density is $1\times {10}^{-4}\mathrm{electrons}∕{\mathrm{a.u.}}^3$ and subsequent lines increase successively by a factor of $\sqrt{2}$.

Figure 4

Schematic diagrams for the atomic buckling in the [001] direction of superlattices with (a) $(\mathrm{LaO}{)}^{+}∕({\mathrm{TiO}}_2{)}^0$ interfaces, (b) $({\mathrm{AlO}}_2{)}^{-}∕(\mathrm{SrO}{)}^0$ interfaces, and (c) with both of them, and top views of (d) the $(\mathrm{LaO}{)}^{+}$ [or $(\mathrm{SrO}{)}^0$] layer and (e) the $({\mathrm{AlO}}_2{)}^{-}$ [or $({\mathrm{TiO}}_2{)}^0$] layer; thick lines denote buckled layers, shaded areas denote interfaces.

Figure 5

The total and projected DOS near $E_F$ of superlattices with $p$-type $({\mathrm{AlO}}_2{)}^{-}∕(\mathrm{SrO}{)}^0$ interfaces including (a) $25\%$ and (b) $50\%$ oxygen vacancies in the fixed structure. (OVS: oxygen vacant site; NOVS: nearest neighbor of oxygen vacant site; NNOVS: next-nearest neighbor of oxygen vacant site.)

Figure 6

The total and projected DOS near $E_F$ of the superlattice with both types of interfaces in the (a) fixed structure and (b) relaxed structure (IF: interface).