Joint Effect Of Gate Bias And Light Illumination And Its Relaxation Behaviors Of Two-Dimensional Electron Gas At Complex Oxide Interface

As typical strongly correlated electronic materials,perovskite transition metal oxides and their heterostructures have shown many novel physical phenomena and broad application prospects.LaAlO₃?LAO?and Sr TiO₃?STO?are two typical insulating perovskite oxides.The two-dimensional electron gas?2DEG?at the LAO/STO interface has attracted great interest in recent years because of its numerous novel physical properties,such as 2D magnetism,2D superconductivity and enhanced Rashba spin–orbital coupling.Previous research works have shown that,light illumination can significantly enhance the gating effect of LAO/STO2DEG,where the variation of resistance is hundreds of times than that of the ordinary capacitance effect.This phenomenon has been called?light-enhanced gating effect??LEG?.Combined with a series of in situ characterizations,the researchers consider that the joint effect of gate bias and light illumination is derived from the lattice expansion and polarization at LAO/STO interface caused by the electro-migration of oxygen vacancies.However,there are many issues remain to be addressed,such as what will happen to a metallic 2DEG under the combined stimuli of gate field and light illumination and their dependence on temperature,what arethe dynamical behaviors of the electrons,ions and lattice during the LEG effect.These problems are very important for the design of new principle devices based on this effect.On the basis of previous works,we systematically studied the joint effect of gate bias and light illumination on the metallic LaAlO₃/SrTiO₃ interface at different temperatures.Subsequently,the dynamic changes of dielectric and structural properties at LAO/STO interface during the relaxation process of LEG effect were revealed and discussed.Finally,we foucs on the"exercise phenomenon"and"attenuation behavior"observed in the gating experiments,and the physical mechanisms were explained qualitatively.In the following are the main results we obtained:?1?We carried out a systemic investigation on the joint effect of gate bias and light illumination on metallic LAO/STO interface,which is insensitive to the individual light illumination.The resistance increased by more than 10 times under the combined stimuli of-100 V gate voltage and 10 mW light illumination,where the corresponding change produced by gate field alone is only few percents.The joint effect is strongly temperature dependent;it is very large at low and high temperatures,and relatively weak in the intermediate temperature range.By monitoring the sheet-resistance and Hall Effect during the LEG experiment,we demonstrated that the huge tuning of transport behavior is stemmed from the enhanced carrier depletion/injection by the LEG effect,while the carrier mobility remains nearly unaffected during the whole process.We also studied the LEG effect on the samples with different carrier densities,and found that the large tuning of resistance only appears in the low carrier density samples.It indicates that there is a critical carrier concentration for the observation of significant LEG effect,which is corresponding to the change density of the saturated polarization state at LAO/STO interface.?2?Using transient photovoltaic/photoconductivity technology,electrical transport measurements,X ray diffraction and Raman spectrum,we have determined the dynamic behavior of dielectric and structural properties at the interface during the relaxation process of LEG effect.The inconsistent relaxation of interface polarization and anionic migration/diffusion was revealed.When removing the light and electric field synchronously,the lattice polarization and lattice expansion at the interface will be disappeared in a short period of time?hundreds of seconds?,leading to about 95%restoration of the resistance to its initial state,resulting a rapid recovery of the transient photovoltaic signal enhanced by interface polarization.Unlike the fast relaxation of lattice polarization,the migration/diffusion of oxygen vacancies will lasts even tens of thousands of seconds,leading to a smooth decreasing of resistance and a slow recovery of the lifetime of transient photoconductivity signals.?3?Two new exotic phenomena during the gating experiments were demonstrated and well explained.Firstly,a strong training effect is induced by repeating cycling of LEG effect at room temperature,where the tuning ratio for the same gating filed was amplified by 8.3-fold after five cycles.By monitoring the discharging current in the LEG relaxtion process,we further consider this training effect is stem from the increasing of lattice polarization during repeating LEG tuning.The accumulation of oxygen vacancies,which can't fully recover to the interface in the short relaxation time,contributes to the increasing of lattice polarization and thus,the strong training effect.Additionally,at low temperatures,attenuation behaviors of the pure electric gating effect were observed,which will be suppressed by the light illumination.These behaviors are attributed to the charge carriers'exchange between the 2DEG and the localized states presented in proximity.Under light illumination,the appearance of interface polarization causes a strong depolarization field that could compensate the externally applied electric field,suppressing the charge carrier exchange and finally extinguishing the attenuation behaviors.