Influence of SrTiO3 capping layer on the charge transport at the interfaces of SrTiO3/LaAlO3/SrTiO3 (100) heterostructure

The experimental realization of quasi-two-dimensional hole gas at the complex oxide interface has long been a grand challenge that may open up new possibilities for oxide-based electronics. ${\mathrm{SrTiO}}_{3}$ (STO)-capped ${\mathrm{LaAlO}}_{3}$ (LAO)/${\mathrm{SrTiO}}_{3}$ (100) is a potential candidate that has been recently demonstrated to show possible electron-hole bilayer conducting channels near the interfaces of the heterostructure. In this work we present a systematic investigation on the structural and magnetotransport properties of STO-capped and uncapped LAO/STO (100) samples that were grown by an oxide molecular beam epitaxy system. The STO/LAO/STO (100) samples exhibit metallic interfaces down to $T$ = 2 K even with a LAO thickness of merely 3 unit cells (3-uc). On the contrary for uncapped LAO(5-uc)/STO (100) samples the conducting interface becomes insulating at lower temperatures due to the surface adsorbate on top of the LAO as revealed from x-ray photoelectron spectroscopy measurements. The magnetotransport data at different back-gate voltages for STO-capped and uncapped LAO/STO (100) samples were analyzed and compared to each other using the two-band model. The presence of two types of charge carriers with opposite signs is uniquely found for STO/LAO/STO (100) samples which is in big contrast to the uncapped LAO/STO (100) samples with only electron-type carriers. Our results support the coexistence of electron-type and hole-type carriers in the STO/LAO/STO (100) heterostructure where their sheet densities and mobilities can be tuned significantly by electrical gating effect and the layer thickness of the heterostructure.