The Conduction And Magnetism Of Two-Dimensional Electron Gas At Oxide Heterointerfaces

Enormous technical advances make it possible to fabricate oxide in the atomic-scale and provide an opportunity to design the artificial nanostructure with novel physical properties.A high-mobility two-dimensional electron gas?2DEG?was discovered at the interface between two insulating perovskite oxides,such as LaAlO₃/SrTiO₃?LAO/STO?.When replacing the LAO with spinel y-Al₂O₃ and amorphous oxide,a 2DEG can still be fabricated.Moreover,the oxide 2DEG exhibits a large number of remarkable physical properties,such as superconductivity,ferromagnetism,spin-orbit interaction,quantum Hall effect,which attracts intensive studies and make it possible to get the new generation of all-oxide electronicsIn this thesis,high quality oxide thin films and high mobility 2DEG at oxide heterointerface has been successfully fabricated by pulsed laser deposition?PLD?technique.The transport property and magnetism of 2DEG have been studied and effectively tuned through changing the substrate,external doping and the application of gate voltage.The main results are shown as follows:1.The lack of spin polarization of 2DEG confined at the classic LAO/STO heterointerface limits its application on spintronics.Herein,a high-mobility spin-polarized 2DEG was fabricated in the LAO/STO heterointerface by inserting a single unit cell ferromagnetic insulating BaFeO₃?BFO?thin film.Compared to the LAO/STO,the 2DEG in LaAlO₃/BaFeO₃/SrTiO₃?LAO/BFO/STO?heterointerface not only has lower carrier density and higher mobility,but also exhibits an anomalous Hall effect?AHE?below 10K which is an irrefutable fingerprint of ferromagnetic order.The ferromagnetic nature of the single unit cell BFO buffer layer as well as the proximity-induced ferromagnetism of the STO-based 2DEG are confirmed by the superconducting quantum interference device?SQUID?magnetometer and element-specific X-ray magnetic circular dichroism?XMCD?measurements.In addition,the physical properties of the buried spin-polarized 2DEG,such as Rashba spin-orbit interaction,Kondo effect and AHE,are all found to be tunable by a back-gating electric-field2.The origin of the 2DEG confined at the oxide heterointerface is a research topic in oxide electronics.During the fabrication,the chemical redox reaction in the heterointerface can lead to the formation of 2DEG,such as the amorphous-LAO/STO?a-LAO/STO?heterostructure.Herein,the rutile single crystal TiO₂[TiO₂?R?]and anatase single crystal TiO₂[TiO₂?A?]were employed to replace the TiO₂-terminated STO substrate in the a-LAO/STO to tune the 2DEG by the variation of Ti-O angle Firstly,the a-LAO/TiO₂?A?and a-LAO/TiO₂?R?heterostructures and the interfacial 2DEGs were successfully fabricated.Moreover,the transport properties of 2DEGs in a-LAO/TiO₂?A?and a-LAO/TiO₂?R?are different from that of a-LAO/STO.The X-ray photoelectron spectrum and transport measurements indicate that the 2DEGs in these three heterostructures stem from the formation of oxygen vacancies.The different transport properties of the 2DEGs are due to the variations in activation energies for the diffusion of oxygen vacancies at substrate surface3.Based on the y-Al₂O₃/STO,the new spinel MAl₂O₄?M-Fe,Co,Ni?film was employed to replace the y-Al₂O₃ to fabricate the MAl₂O₄/STO heterostructure.Firstly,three MAl₂O₄ thin films exhibit ferromagnetic up to the room temperature.Moreover,the interfaces of the CoAl₂O₄/STO and NiAl₂O₄/STO are highly conducting,the mobility of their interfacial 2DEGs is about 3×10₄ cm^-2V^-1s^-1 which is comparable to y-Al₂O₃/STO.In contrast,the FeAl₂O₄/STO is insulating.Moreover,the 2DEGs of the CoAl₂O₄/STO and NiAl₂O₄/STO show AHE when the temperature is below 30K which results from the magnetic proximity effect induced by the top ferromagnetic spinel films.In addition,the ferromagnetism of ?-Al₂O₃/STO heterointerface which occurs below 5 K is related to oxygen vacancies at STO surface4.Doping which is a key materials' engineering tool in semiconductor technology can be employed in oxides.Herein,a new concept,fractional doping,on the basis of the lattice structure of ?-Al₂O₃ which consists of four neutral sublayers in 1 unit cell,was proposed.The nonmagnetic ZnO and magnetic Fe₃O₄ were employed to replace one sublayer in each ?-Al₂O₃ unit cell and adjust the position of the doping layer to tune the transport properties,respectively.Moreover,the AHE occurs in the fractionally doped heterointerface when the temperature is lower than 20 K.Through analyzing,the AHE of the ZnO doped ?-Al₂O₃/STO heterointerface which behaves similarly with that of y-Al₂O₃/STO stems from the oxygen vacancies in the STO surface.In contrast,the AHE in the Fe₃O₄ doped heterostructure results from the exchange effect between the magnetic Fe₃O₄ and the interfacial 2DEG.