Study of LaAlO₃/SrTiO₃ 2DEG for sensor applications

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Study of LaAlO₃/SrTiO₃ 2DEG for sensor applications


Author: Chan, Ngai-yui
Title: Study of LaAlO₃/SrTiO₃ 2DEG for sensor applications
Degree: Ph.D.
Year: 2015
Subject: Electron gas.
Oxides -- Electric properties.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: 199 leaves : illustrations ; 30 cm
Language: English
InnoPac Record:
Abstract: Despite that both materials are band insulator, a conducting quasi-two dimensional electron gas (2DEG) at LaAlO₃/SrTiO₃ (LAO/STO) interface can be formed with a precise atomic stacking on the STO substrate. This discovery by Ohtomo and Hwang in 2004 initiated a great interest in this heterostructure and led to the further discovery of a plenty of exciting properties at the LAO/STO interface such as the switchable electronic properties, giant Seebeck coefficient, magnetic properties, superconductivity and photoconductivity. These significant results observed in this system indicate the importance of the interfacial effect and pave the way for potential applications in multifunctional oxide-based electronic devices. Nevertheless, a relatively weak area in this field of research is device application for such oxide interface 2DEG. The main difficulty to apply this 2DEG system in electronic devices is its relatively low response to external stimulus such as light and gas (since gas cannot react and be absorbed onto the heterostructure). In this thesis work, I have successfully invented a method to modify the surface of the LAO/STO heterostructure and significantly enhance the response to ultraviolet light and gases, therefore, made this fascinating 2DEG system more close to real application in sensing devices. Detailed results are: (1) The LAO/STO 2DEG heterostructure has been grown by laser molecular beam epitaxy. Growth conditions and substrate treatment process have been optimized to realize metallic conduction at the interface. The structural and transport properties of such heterostructure have been systematically characterized, and the results show that the 2DEG behavior at the LAO/STO interface is in the same quality as reported by other groups, indicating that it’s suitable for further device application. (2) Palladium (Pd) nanoparticles (NP) have been successfully grown on the LAO/STO structure. The Pd NP-decorated LAO/STO heterostructure exhibits a giant photo-response in the ultraviolet (UV) range. The conductance of the interface increases under the irradiation of UV light and a giant optical switching effect with a photoconductivity on/off ratio as high as 750% has been achieved at room temperature. This interesting phenomenon has been explained based on the Pd NPs' catalytic effect and surface/interface charge coupling, which modulates the charge distribution at the LAO/STO interface. (3) Besides the UV light sensing, the Pd NP-decorated LAO/STO heterostructure has also been found to be very sensitive to hydrogen gas down to ~2ppm in concentration, demonstrating the highly-sensitive and room-temperature workable gas sensing properties. Cross sensitivity tests indicate that the sensor shows response to ethanol, acetone and water gas vapor. The outstanding gas sensing properties are attributed to the Pd NPs catalytic effect to different gases, resulting in charge coupling between the gas molecules and Pd NPs through either a direct charge exchange or change of electron affinity. These results provide insight into the emerging properties at LAO/STO interfaces and demonstrate a novel conducting system with potential new physics and possible applications in the field for all-oxide devices, thus open a new route to complex oxide physics and ultimately for the design of devices in oxide electronics.

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