Tuning polarization states and interface properties of ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructure by metal capping layers

How to tune two-dimensional electron gas at interface of heterostructures is becoming an important question for both fundamental physics and electronic applications. Here, using density functional theory calculations, we find that the polarization state of ${\mathrm{BaTiO}}_3$ in metal capped ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures changes dramatically, depending on the termination of ${\mathrm{BaTiO}}_3$ and the different metal layers ($M=\text{Al}$, Fe, Pt Au). Most interestingly, for Pt on the BaO-terminated ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$, interface conductivity can be tuned. With a paraelectric state in ${\mathrm{BaTiO}}_3, \text{Pt}\_\mathrm{BaTiO}{}_3/{\mathrm{SrTiO}}_3$ remains insulating at interface, while when ${\mathrm{BaTiO}}_3$ is ferroelectric, a hole- or electron-conducting ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ interface can be realized, depending on its polarization direction in ${\mathrm{BaTiO}}_3$. This conducting interface and the top Pt layer screen the depolarization field, and thus stabilize the ferroelectricity in ${\mathrm{BaTiO}}_3$. Our result provides important clues for the reversibly tunable conductivity at oxide interfaces.

Figure 1

Structural guidance for $\text{Pt}\_f\text{BTO/STO}\_\text{BaO}, p\text{BTO/STO}\_\text{BaO}$, and $o\text{BTO/Pt}\_\text{BaO}$. (a) $\text{Pt}\_f\text{BTO/STO}\_\text{BaO}$ is a structure with Pt capping layer, BaO termination, and a $\mathrm{ferro}(+)$ state of BaO. In a $\mathrm{ferro}(+)$ state of BaO, the O-Ba-O and O-Ti-O chains buckle toward BTO/STO interface. (b) $p\text{BTO/STO}\_\text{BaO}$ is a structure with BaO termination and a para state of BTO. Para indicates no polarization in BTO. (c) $o\text{BTO/Pt}\_\text{BaO}$ is a structure with BTO layers on Pt substrate, BaO termination, and a $\mathrm{ferro}(-)$ state of BTO. The $\mathrm{ferro}(-)$ here indicates a ferroelectric state of BTO with O-Ba-O and O-Ti-O chains buckle toward BTO/$M$ interface.

Figure 2

The different structural and electronic properties of $p\text{BTO/STO}\_{\mathrm{TiO}}_2$ and $p\text{BTO/STO}\_\text{BaO}$. (a) The displacements between cations and oxygen atoms in each sublayer of $p\text{BTO/STO}\_{\mathrm{TiO}}_2$ and $p\text{BTO/STO}\_\text{BaO}$. A positive value indicates a cation moves towards the BTO/STO interface, assuming the oxygen atom is fixed. (b),(c) The layer-projected density of stats (LDOS) of surface BTO and interface STO layers for $p\text{BTO/STO}\_{\mathrm{TiO}}_2$ and $p\text{BTO/STO}\_\text{BaO}$, respectively.

Figure 3

(a) Work functions of Al, Fe, BTO, Au, and Pt. The vacuum level is aligned to the same value. (b)–(e) Structural guidance for Al/BTO_BaO, Fe/BTO_BaO, Au/BTO_BaO, and Pt/BTO_BaO, respectively, and their interface charge density differences with an isosurface value of 0.0016 $e^{-}/{\AA }^3$. The charge density difference is calculated by subtracting the sum of the charge densities of the BTO and metal capping layers from the total charge density of the whole interface slab. The blue and red area represents charge depletion and accumulation, respectively. For clarity, only the interface BTO and metal capping layers are shown here.

Figure 4

Profiles of $xy$-plane averaged and macroscopically averaged electrostatic potentials along [001] direction. The cyan (green) line indicates the $xy$-plane averaged (macroscopically averaged) potential for para state of BTO while the orange (red) line is for $\mathrm{ferro}(+)$ state of BTO. (a) Comparison between para and $\mathrm{ferro}(+)$ BTO/STO with ${\mathrm{TiO}}_2$ termination of BTO. (b) Comparison of Pt_BTO/STO between para and $\mathrm{ferro}(+)$ BTO with BaO termination of BTO.

Figure 5

Layer-projected density of states (LDOS) for $p\text{BTO/STO}\_{\mathrm{TiO}}_2, f\text{BTO/STO}\_{\mathrm{TiO}}_2, \text{Pt}\_p\text{BTO/STO}\_\text{BaO}$, and $\text{Pt}\_f\text{BTO/STO}\_\text{BaO}$. The purple lines are LDOS of STO, red lines of BTO, and orange lines of the Pt capping layer.

Figure 6

$xy$-plane averaged interface charge density. (a) $xy$-plane averaged interface charge density for $f\text{BTO/STO}\_{\mathrm{TiO}}_2$ along [001] direction. (b) $xy$-plane averaged interface charge density for $\text{Pt}\_f\text{BTO/STO}\_\text{BaO}$ along [001] direction.

Figure 7

Electronic properties of $\text{Pt}\_o\text{BTO/STO}\_\text{BaO}$. (a) Profiles of $xy$-plane averaged and macroscopically averaged electrostatic potentials along [001] direction for $\text{Pt}\_o\text{BTO/STO}\_\text{BaO}$. (b) Layer-projected density of states for $\text{Pt}\_o\text{BTO/STO}\_\text{BaO}$. (c) Structural guidance for $\text{Pt}\_o\text{BTO/STO}\_\text{BaO}$ and its corresponding interface charge density distribution with an isosurface value of $1.0\times {10}^{-4}{e}^{-}/{\AA }^3$. For clarity, only the interface BTO and STO layers are shown.