Study of transport properties at LaAlO₃/SrTiO₃interface and its application in polar molecule sensor

Abstract

Two-dimensional electron gas (2DEG) at oxide interface is a very interesting system that has been extensively studied in recent years. In this system, a 2DEG with high mobility (~10³cm²V⁻¹s⁻¹ at 2 K temperature) is formed at the heterointerface between two band insulating materials LaAlO₃ (LAO) and SrTiO₃ (STO). This phenomenon has been explained by the model of electronic reconstruction due to the accumulation of polar discontinuity between the LaO and AlO₂layers. Based on the model of electronic reconstruction and reported fabrication of field-effect transistor (FET) by 2DEG in oxide, the electric transport properties of the 2DEG should be able to be modulated by an external electric field or some polar molecules. In my thesis work, the application of the 2DEG as a polar molecule sensor has been demonstrated. The oxide interfacial 2DEG was achieved by depositing LAO thin film on TiO2 terminated STO substrate by Laser-MBE. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were used to characterize the structure of the LAO/STO interface. The electric transport properties of the heterointerface, including mobility, charge density and conductivity, were measured at temperatures from 2.5 to 300K. When a droplet of water is added to the exposed LAO interface and fully covers the conducting channel, the I-V characteristic of the heterointerface changes to a typical FET type, but the rectify direction is different from the typical Schottky junction. An explanation to this phenomenon is proposed based on the fact that the polar of liquid molecules should affect the polarization field inside the LAO film; while this effect may change the conducting behavior of the heterointerface from metallic to semiconducting, and therefore, a Schottky junction may be formed between the 2DEG and the affect area. It is also interesting to find that an amorphous STO layer on top of LAO surface results in the same kind of current saturation as that affected by polar liquid, suggesting that the a-STO on LAO also changes the transport properties of the underlying 2DEG. These results not only promise the interest for understanding the oxide interfacial 2DEG, but also its potential applications in all-oxide device, thus may open new routes to complex oxide physics and ultimately for the design of devices in oxide electronics.