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Visible-light-enhanced gating effect at the LaAlO3/SrTiO3 interface
来源：admin 发布时间：2016-05-17
The two-dimensional electron gas (2DEG) at the heterointerfaces between complex oxides has received attention in recent years because of its implementation for novel physics and prospective applications. The 2DEG confined to the LaAlO3/SrTiO3 (LAO/STO) interfaces is a representative system that has been extensively studied, and exotic properties including two-dimensional superconductivity, magnetism, enhanced Rashba spin–orbital coupling and strong electrical field effect have been observed. Among these, the field effect is particularly interesting. As already demonstrated, the transport behaviour can be tuned by a gate field across STO or LAO, undergoing a metal-to-insulator transition or a tunable superconducting transition. On the other hand, a dramatic modification of the interfacial conductivity can also be gained by depositing polar molecules or charges above the LAO layer. Obviously, gating effect has shown its potential in unravelling the emergent phenomena at complex oxide interfaces.
However, the electrical field effect for the complex oxide 2DEG is much more complicated than for the conventional semiconductor devices. In addition to electrons, there are many other factors such as ionic defects, trapped charges or ferroelectric instabilities in the system that can be severely affected by the applied electric field. As a consequence, significant hysteresis of interfacial conductivity can occur when cycling electrical bias through the STO crystal9 or scanning a biased tip across the LAO layer, the latter leads to conducting nanowires persisting for days. A few recent reports even show that the field effect actually exhibits two steps: a fast process is followed by an extremely slow process that usually lasts for thousands of seconds but owns a tuning ability comparable to or even stronger than the fast one. While the slow process yields additional freedom in controlling the physical properties of the 2DEG, its slow nature makes it hard to be exploited in any practical devices but causes an adverse influence to the reproducibility.
In this work, we report on a remarkable effect produced by combined electrical and optical stimuli for the 2DEGs at both amorphous and crystalline LAO/STO heterointerfaces (a-LAOSTO and c-LAO/STO, respectively). We found that an illumination of visible light drives the slow field-induced resistance growth into a great jump far beyond the scope of normal field effect, markedly enhancing the ability of the gate field to modulate charge carriers. The present work clearly demonstrates the mutual reinforcement of the effects of electrical gating and light illuminating on complex oxide interfaces.
However, the electrical field effect for the complex oxide 2DEG is much more complicated than for the conventional semiconductor devices. In addition to electrons, there are many other factors such as ionic defects, trapped charges or ferroelectric instabilities in the system that can be severely affected by the applied electric field. As a consequence, significant hysteresis of interfacial conductivity can occur when cycling electrical bias through the STO crystal9 or scanning a biased tip across the LAO layer, the latter leads to conducting nanowires persisting for days. A few recent reports even show that the field effect actually exhibits two steps: a fast process is followed by an extremely slow process that usually lasts for thousands of seconds but owns a tuning ability comparable to or even stronger than the fast one. While the slow process yields additional freedom in controlling the physical properties of the 2DEG, its slow nature makes it hard to be exploited in any practical devices but causes an adverse influence to the reproducibility.
In this work, we report on a remarkable effect produced by combined electrical and optical stimuli for the 2DEGs at both amorphous and crystalline LAO/STO heterointerfaces (a-LAOSTO and c-LAO/STO, respectively). We found that an illumination of visible light drives the slow field-induced resistance growth into a great jump far beyond the scope of normal field effect, markedly enhancing the ability of the gate field to modulate charge carriers. The present work clearly demonstrates the mutual reinforcement of the effects of electrical gating and light illuminating on complex oxide interfaces.
- A sketch of the experimental set-up. (b) Sheet resistance of a-LAO/STO, recorded in the presence/absence of a light of P=32 mW (λ=532 nm) while VG switches among −80, 0 and +80 V. (c) Enlarged view of the two-step feature of RS without light illumination. (d) Gate dependence of normalized sheet resistance, RS(VG,P)/RS(0,0), recorded at the time of 300 s after the application of VG. Arrow marks the RS corresponding to VG=−5 V. (e) Sheet resistance of c-LAO/STO, recorded in the presence/absence of a light of P=32 mW (λ=532 nm) as VG switches among −200, 0 and +200 V. All measurements were conducted at room temperature.
- Hall resistance, Rxy, of a-LAO/STO measured with an in-plane current of 10 μA under different gating/illuminating conditions. Without light illumination the data for VG=−100 V cannot be distinguished from those for VG=0, and therefore are not shown here. (b) Carrier density and sheet resistance as functions of light power, acquired under a fixed VG of −100 V. Solid lines are guides for the eye. Dashed line is the extrapolated nS–P relation. (c) Capacitance, Ca-LAO/STO, of a-LAO/STO as a function of gate voltage, measured under the a.c. amplitude of 0.5 V and frequency of 5 kHz. Labels in the figure denote light power (λ=532). (d) Carrier density change produced by capacitive effect, calculated by ΔnS=ε0εVG/d adopting the permittivity under a constant electrical field marked beside the curve and VG=100 V. Symbols are experimental values for |VG|=100 V extracted from literature, as indicated in the figure.
- Experiment set-up for the structural measurements of a-LAO/STO with simultaneously applied light illumination and gate field. (b) X-ray diffraction patterns of the 002 reflection of STO measured after a waiting time of 10 min upon the simultaneous application of light illumination (P=100 mW, λ=532 nm) and gate biases. The two shoulders developed on the low-angle side of the 002 reflection mark the lattice expansion in the near interface region of a-LAO/STO. Labels besides the curves indicate gate voltage. The total time required for each θ−2θ scanning is ~10 min. (c) A comparison of the lattice constants obtained with and without light illumination. The acceleration of the field-induced structural deformation by photoexcitation can be clearly seen. Solid lines are guides for the eye.
(a) The content of oxygen vacancy (marked by circled plus symbols) is considerably high at the LAO/STO interface due to the outward diffusion of oxygen ions from STO, resulting in electron doping (marked by circled minus symbols) and thus the 2DEG at the a-LAO/STO interface. The oxygen vacancy here may be mainly in the state with one deeply trapped electron,
, the most favourable state when vacancy content is high27, 28, 29, 30. (b) The inward migration of these interface oxygen vacancies under negative gate biases will induce an interface polarization phase25. Owing to the low mobility of s, however, it is difficult for the gate field alone to cause significant vacancy migration. As a result, a negative bias only slightly polarizes the interface region of STO, yielding a very weak tuning to sheet carriers. (c) Light illumination excites the trapped electron in , transitingthe latter into the 
state that is much more susceptible to external field. In this manner, it accelerates the electromigration of oxygen vacancies, thus the building up of the polarization phase that causes a strong extra tuning to sheet carriers.