| Perovskite-type metal oxides have relatively simple lattice structures and extremely rich physical properties.The phenomena exist at surfaces and interfaces show different properties from the body make them more attractive.In 2004,A.Ohtomo and H.Y.Hwang found a quasi-two-dimensional electron gas(q-2DEG)with high mobility at the interface between two band insulators LaAlO3(LAO)and SrTiO3(STO).In the past decade,a lot of fundamental and application researches have been focused on this system,and remarkable progress has been made.After entering the 21st century,the properties of traditional semiconductor devices gradually reached the classic limit,and the constant pursuit of high-performance electronic devices made people turn to this kind of oxide semiconductor.Utilizing the pulsed laser deposition(PLD)system,LAO with thickness of more than four unit cells(uc)epitaxially grown on the TiO2-terminated STO forms q-2DEG.The thickness of the LaAlO3 layer can be precisely controlled by the in situ reflective high energy electron diffraction(RHEED).Combined with the electron beam evaporation technique,we fabricated high-quality metal/LAO/STO heterojunctions on the atomic-scale level.1.First,we systematically investigated the electrical transport properties of the LAO/STO interfaces after capping several different metal.The carrier density at the interface can be tuned by the metallic capping layer without changing the mobility.The carrier density variation is linearly dependent on the size of the metal work function,and it can be explained by the charge transfer mechanism between the interface and the metal layer through the LAO tunneling layer.Our results confirm the existance of the remaining built-in electric field in 5-uc-thick LAO layer with estimated value of 67.7 eV/A.Since the metallic overlayer is essential for devices,the present phenomena must be considered for future applications.2.Spin injection and relaxation of LAO/STO interface were studied.We annealed the LAO layer and deposited Co as ferromagnetic electrode on the LAO/STO heterojunction.Using the three-terminal method,the spin relaxation at the interface can be analyzed by the measured magnetoresistance.The magnetoresistance signal gradually weakens with the increasing temperature,but the spin relaxation time fitted at different temperatures is almost constant,about 30 ps.The spin diffusion length of LAO/STO interface is estimated to be about 30 nm at low temperature.3.The capping of Al enables the formation of q-2DEG at the interface of 1-3 uc LAO on STO which was originally insulating before capping.The properties of the q-2DEG induced by the A1 capping layer are essentially the same as q-2DEG without Al.Therefore,we can pattern q-2DEG by simply patterning the Al film on LAO(2 or 3 uc)/STO with one-step liftoff process.Our approach circumvents the difficulty of direct patterning oxide materials and provides a simple and robust patterning method for future device applications based on complex oxide interfaces.4.We deposited LaMnO3(LMO)as the buffer layer to block the carrier diffusion and studied the critical STO thickness of the formation of LAO/STO q-2DEG.We successfully realized the atomic epitaxial growth of 3 uc LMO/40 uc STO/10 uc LAO on the TiO2-terminated STO substrate.We found the critical thickness of STO to be 30 uc.On the 30 uc STO film,LAO layer with thickness above 10 uc can induce q-2DEG.The carrier density of the modulated LAO/STO interface is higher than that of the typical value.It is proved that the oxygen vacancy plays an important role in the formation of LAO/STO q-2DEG.5.By using magnetic exchange bias effect measurements with very high sensitivity,we present the direct evidence that the magnetic ground state of parent FeSe/STO films is antiferromagnetic.The post-annealing process suppresses the antiferromagnetic order of the monolayer and bilayer FeSe samples. |
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