Photoconductivity of Graphene in Proximity to $\mathrm{La}{\mathrm{AlO}}_3/\mathrm{Sr}{\mathrm{TiO}}_3$ Heterostructures: Phenomenon and Photosensor Applications

The proximal coupling between graphene and transition-metal-oxide heterostructures may integrate their unique features and further generate emergent states. Using the photoconductivity of graphene as an effective probe, we demonstrate the existence of a built-in polar field within the $\mathrm{La}{\mathrm{AlO}}_3$ layer of the $\mathrm{La}{\mathrm{AlO}}_3/\mathrm{Sr}{\mathrm{TiO}}_3$ heterostructures for both conducting and insulating $\mathrm{La}{\mathrm{AlO}}_3/\mathrm{Sr}{\mathrm{TiO}}_3$ interfaces. Such a polar field is a prerequisite for the validity of the electronic reconstruction mechanism for the interfacial conductivity. The built-in polar field is reflected by the hole doping in the graphene in proximity to the $\mathrm{La}{\mathrm{AlO}}_3/\mathrm{Sr}{\mathrm{TiO}}_3$ induced by pulsed deep-ultraviolet illumination regardless of the graphene’s carrier type. These photoresponse characteristics also render the $\mathrm{graphene}/\mathrm{La}{\mathrm{AlO}}_3/\mathrm{Sr}{\mathrm{TiO}}_3$ hybrid system a convenient deep-ultraviolet sensor. Moreover, we design an efficient broad-spectrum photodetector benefiting from the large in-plane electric field in graphene across the boundary between the $\text{graphene}/\mathrm{La}{\mathrm{AlO}}_3/\mathrm{Sr}{\mathrm{TiO}}_3$ and $\text{graphene}/\mathrm{Sr}{\mathrm{TiO}}_3$. Our findings may provide clues to the design of photosensors based on the hybrid structures of graphene and oxide heterostructures.

Figure 1

(a) Schematic of a graphene/LAO/STO device. (b) The graphene resistivity $\rho$ as a function of the illumination time $t$ for the graphene/5-uc-LAO/STO system under 6.7-eV UV illumination. The UV illumination is pulsed in regions I, II, and III. The dashed curve shows a domelike background. (c),(d) $\rho$ as a function of the gate voltage $V_g$, corresponding to the typical $\rho -V_g$ characteristics in regions I (c) and II (d) marked in (b), respectively. The dashed line indicates the resistivity at 0 V.

Figure 2

(a)–(c) The resistivity changes of graphene in response to pulsed UV illumination for regions I (a), II (b), and III (c) of Fig. 1, with graphene in the $p$-type (a), $n$-type (b), and deep $n$-type (c) regions, respectively. The colored stripes indicate the light-on states. (d),(e) Schematic band diagrams for the graphene/5-uc-LAO/STO system without (d) and with (e) UV illumination.

Figure 3

(a),(b) Photon-energy-dependent resistivity changes of graphene for the graphene/5-uc-LAO/STO system, with graphene in the $p$-type (a) and $n$-type (b) region, respectively. (c)–(f) The resistivity changes of graphene in the $n$-type region in response to 6.7-eV UV-light pulses for graphene/LAO (c), graphene/STO (d), and graphene/a-LAO/STO (e),(f). The a-LAO thicknesses are approximately 1.9 nm (e) and approximately 3.8 nm (f), respectively. The colored stripes indicate the light-on states.

Figure 4

(a)–(c) The resistivity changes of graphene in response to 6.7-eV UV-light pulses for the graphene/3-uc-LAO/STO system, with graphene in the $p$-type (a), $n$-type (b), and deep $n$-type (c) regions, respectively. The colored stripes indicate the light-on states. (d),(e) Schematic band diagrams for the graphene/3-uc-LAO/STO system without (d) and with (e) UV illumination.

Figure 5

(a) Schematic of the graphene in proximity to the STO substrate partly covered by a 5-uc-LAO layer. (b) The photocurrent of graphene with the 1.9-eV laser spot sweeping from the graphene/STO region to the graphene/LAO/STO region.