Structural and electrical characterization of oxide heterointerfaces for 2-dimensional electron gas and water splitting photoanodes

Abstract

Since the heterointerface between different materials possesses various properties that is required in devices, It may be said that the interface is the device. In particular, conductivity manipulation at the heterointerface is one of the key technologies to manufacture semiconductor devices, as controlling the movement of electric charges to exhibit desired characteristics is required in electronic devices.<br/> Oxide composites are considered to be promising materials for many devices with various physical phenomena such as high-k dielectrics, high-temperature superconductivity and excellent ferroelectricity. As atomic scale deposition control has become possible with the recent technological development, novel properties which have not been found in bulk materials of natural systems have been discovered. Application studies on these novel properties have been actively carried out.<br/> In this theses, the possibility of controlling the lateral and vertical charge transfer by using the novel phenomena occurring at the heterojunction interface of the oxide composite is proposed. To accomplish this, three major studies were conducted.<br/> The first is the lateral carrier transport control of the two-dimensional electron gas in oxide heterointerface. The formation of two-dimensional electron gas at the heterointerface between SrTiO3 and LaAlO3 has been reported in 2004. The conductivity of the two-dimensional electron gas can be tuned depending on the degree of tilting of TiO6 octahedrons in the SrTiO3 substrate. The conductivity can be manipulated by inserting CaTiO3 which has tilted octahedrons. In addition, distortion of the TiO6 octahedrons are found in the study.<br/> The second is the vertical charge transfer control through band offset management. LaAlO3 is a material with a large dipole moment, which can influence the charge transfer at the heterojunction interface by utilizing the intrinsic electric field induced by the dipole moment. Schottky junctions are formed between Nb-doped SrTiO3 and WO3, and the Schottky barrier serves as a barrier for electrons to be transferred between materials. However, it has been shown that by inserting LaAlO3 interlayer at the interface, the Schottky barrier can be eliminated and Schottky junction can be effectively replaced with an Ohmic junction.<br/> The third is a charge transfer enhancement study using sequential type Ⅱ junctions in a photoelectrochemical water splitting device. Heterostructures having a type Ⅱ junction are often used in the photoanode material constituting photoelectrochemical water splitting cells. A photoelectrode having improved light absorption, photo-conversion efficiency, and electron-hole charge separation capability can be constructed by stacking three materials having a heterojunction structure composed of sequential type Ⅱ junctions.<br/> In this thesis, lateral and vertical charge transport control in the oxide heterojunction interface through the above studies are shown. This can be applied to electronic devices and solar water splitting cell. This thesis provide a basis for studying the properties of heterogeneous junctions.<br/>