Electrostatic gating field and light illumination are two widely used stimuli for semiconductor devices. Via capacitive effect, a gate field modifies the carrier density of the devices, while illumination generates extra carriers by exciting trapped electrons. Here we report an unusual illumination-enhanced gating effect in a two-dimensional electron gas at the LaAlO3/SrTiO3 interface, which has been the focus of emergent phenomena exploration. We find that light illumination decreases, rather than increases, the carrier density of the gas when the interface is negatively gated through the SrTiO3 layer, and the density drop can be 20 times as large as that caused by the conventional capacitive effect. This effect is further found to stem from an illumination-accelerated interface polarization, an originally extremely slow process. This unusual effect provides a promising controlling of the correlated oxide electronics in which a much larger gating capacity is demanding due to their intrinsic larger carrier density.
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 applications1,2,3,4,5,6,7,8,9,10,11,12,13. 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 superconductivity3, magnetism5, enhanced Rashba spin–orbital coupling6 and strong electrical field effect4,7,8,9,10,11,12,13 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 transition9 or a tunable superconducting transition3,13. On the other hand, a dramatic modification of the interfacial conductivity can also be gained by depositing polar molecules or charges above the LAO layer7,8. 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 days10,11. A few recent reports9,14 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 one14. 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.
Illumination-accelerated gating effect
Details for sample fabrication and resistive measurements are described in the Methods. Figure 1 shows the typical resistive responses of our devices to electrical and optical stimuli. As schemed in Fig. 1a, a gate voltage, VG, between −100 V and 100 V was applied to the back gate of STO while the a-LAO/STO interface was grounded and the sheet resistance, RS, was recorded in the presence/absence of a light illumination. In all cases, the leakage current (<7 nA) was much lower than the in-plane current applied for resistive measurements, 1 μA (Supplementary Fig. 1). As shown in Fig. 1b,c, without illumination, the application of a VG=−80 V yields two distinct processes marked respectively by a slight jump and a followed steady increase of RS. The first minor jump is the normal gating effect, stemming from the field-induced charge density change in the backgate–interface capacitor. The latter process is extremely slow, lasting for >2,000 s without saturation and produces an RS increase much larger than the first jump. This process can be well described by the Curie–von Schweidler law RS∝(t−t0)α, which implies a wide distribution of the energy barriers that impede the carrier depletion (see Supplementary Fig. 2)15.
Remarkably, such a field effect is significantly modified by light illumination. Aided by a light of 32 mW (λ=532 nm), as shown by the red curve in Fig. 1b, gate field drives RS into a sudden jump to a steady state of 200-fold resistance, that is, the slow process has been markedly accelerated by light illumination, and this change is reversible for the repeated on–off operations of the gate field. As summarized in Fig. 1d, a light of 32 mW pushes the RS(VG=−100 V,P)/RS(0,0) ratio from ~1.2 to ~202, amplifying the gating effect by ~170-fold. Moreover, even a VG as low as −5 V can cause a 17-fold RS growth (marked by an arrow). This bias is only one-tenth of that usually required to get comparable effect using a backgate without light9,16. The gating effect of positive VG was also enhanced by illumination but it is relatively weak (see Fig. 1b and Supplementary Figs 3 and 4). Similar illumination-enhanced gating effect is also observed in c-LAO/STO (Fig. 1f), suggesting that it is a quite universal phenomenon, independent of the characteristics of the electronic transport of the interface (it is semiconducting for a-LAO/STO and metallic for c-LAO/STO; refer to Supplementary Fig. 5) and the crystal structure of the LAO overlayer (crystalline or amorphous).
Carrier density tuning beyond capacitive effect
To gain a further understanding of this illumination effect, we examined the sheet carrier density, nS, by Hall measurement. From the linear Rxy–H relation in Fig. 2a, the initial nS can be deduced, and it is ~7 × 1012 cm−2, where Rxy is the Hall resistance. There are no detectable changes in the Rxy–H dependence measured immediately after the application of a |VG|=100 V, indicating that the change in carrier density is tiny. It is consistent with the result deduced from the capacitance data in Fig. 2c, for P=0, which is only ~4% of the initial nS, where Ca-LAO/STO is the capacitance of the backgate–interface capacitor, e is the electron charge and S≈5 mm2 is the interface area. In contrast, in a light of P=6 mW (the highest intensity available for our Hall-effect measurement system), a VG=−100 V reduces the nS from ~7.0 × 1012 to ~1.3 × 1012 cm−2 (Fig. 2b) and the mobility from ~25.8 to ~1.2 cm2 V−1 s−1 (deduced from the data in Fig. 2b). This extraordinarily large ΔnS is confirmed by the sudden Ca-LAO/STO drop shown in Fig. 2c for VG<−20 V and P=32 mW, which suggests the exhaustion of sheet carriers. A large ΔnS (~1.1 × 1013 cm−2 for a VG of −200 V) is also detected in illuminated c-LAO/STO (Supplementary Fig. 6). However, the illumination enhancement is almost absent when the interface is positively gated. In this case, as shown by the data of VG=0 (black) and 100 V (magenta) in Fig. 2a, ΔnS increases slightly and can be ascribed to the illumination-generated extra photocarriers.
According to the capacitor model, the depleted carrier density by a negative VG is ΔnS=ε0ε|VG|/d, where ε is the relative dielectric constant of STO and d is the thickness of STO. Adopting the ε values in ref. 17, the tuned carrier density by the capacitive effect can be calculated. As shown in Fig. 2d, the illumination-enhanced gating effect is much stronger than the simple gating effect that is always well described by the conventional capacitive effect. This result strongly suggests that additional mechanisms are at work under light illumination.
To explore the origin of this unusual illumination-enhanced asymmetric gating effect, we examined the dependence of the field effect on light wavelength, λ. As shown in Fig. 3a, the tuned value of RS drops rapidly as λ increases from 532 to 980 nm, suggesting that the photoexcitation of trapped electrons play a key role in the observed effect, although, counter-intuitively, the photoexcitation process significantly decreases, rather than increases, nS. Furthermore, a strong-to-weak crossover of the illumination effect occurs at λ~850 nm (Fig. 3b, VG=−20 V), corresponding to a photon energy of ~1.4 eV. This value coincides well with the reported deep oxygen vacancy states with one trapped electron in STO18,19.
Gating-induced and illuminating-enhanced lattice polarizations
As reported, oxygen vacancies (VO) tend to pile up close to the STO surface20,21,22,23 and drift slowly under electrical field24. A recent study has shown that the electromigration of oxygen vacancies can lead to a polarity-asymmetric interface polarization25, which was built up in >20 h under a strong field. This slow buildup of the polar phase is reminiscent of the slow gating process observed when only the electrical field is applied (Fig. 1b). It is therefore possible that the illumination-enhanced gating effect is triggered by the acceleration of the establishment process of this interface phase. Direct evidence comes from Fig. 4, where a field-induced structural deformation of a-LAO/STO is indicated by X-ray diffraction. Figure 4a is the experimental set-up for structural measurements. Figure 4b is the θ−2θ diffraction patterns, and Fig. 4c is the deduced out-of-plane lattice constant of STO, as a function of electric biases. When illuminated, as shown by Fig. 4b, a low-angle shoulder of the 002 reflection of STO emerges and develops above VG≈−300 V, indicating an out-of-plane lattice expansion. However, no structural changes are observed up to the gate bias of −700 V without illumination. This result indicates that the light illumination indeed helps the gate field in inducing a structural deformation. As revealed by the previous work25, the lattice expansion of STO is a signature of interface polarization25. The structural distortion could not be a thermal effect as it continues to remain once it has appeared even after the sample is shaded from light, and the illumination alone produces no effect on structure (see Supplementary Fig. 7). Notably, the threshold VG for structural deformation is much higher than that for remarkable resistance tuning. It may be a consequence of uneven gating due to preferential carrier exhaustion around electrode, which confines gate field to the close proximity of the electrode. The uneven tuning can be sensed by sheet resistance since the latter is susceptible to local environment but not by X-ray diffraction unless VG is so high that the strongly gated area has well outward extension. We also performed the X-ray diffraction measurements for a Ti (30 nm)/STO/Ti (200 nm) capacitor structure and observed a similar lattice expansion (Supplementary Fig. 7). Without uneven gating, here the interface phase appears under a VG below −200 V. Corresponding to the emergence of interface phase, forbidden shifts in Raman spectra were detected, implying an inversion symmetry breaking (Supplementary Fig. 8). These results strongly suggest that light illumination has greatly accelerated the formation of the interface polarization phase.
On the basis of the above analyses, we can present a scenario for the illumination-enhanced gating effect. As schemed in Fig. 5a, the oxygen vacancy concentration at the LAO/STO interface is considerably high due to the outward oxygen ion diffusion from the STO substrate during the deposition of the LAO overlayer, and the resulted electron doping leads to the 2DEG at the a-LAO/STO interface21,22,23 and might also contribute to the conduction of the c-LAO/STO interface. Without illumination, negative bias only slightly polarizes the interface region of STO (Fig. 5b), yielding the slow RS growth following the first sudden RS jump (Fig. 1c). Light illumination accelerates interface polarization by enhancing the electromigration of oxygen vacancies, probably by exciting the trapped electrons in deep oxygen vacancy states (Fig. 5c). This polarization yields an extra tuning to nS and a weakening of the interfacial confining well of 2DEG. These two effects markedly reduce nS, amplifying the gating effect.
Effect of light illumination on the electromigration of oxygen vacancies in STO can be identified from the transient leakage current recorded under a constant d.c. bias. As well established, prior to resistance degradation, a broad current peak will appear when the VOs in the near region of the anode reach cathode25,26. As shown by the Supplementary Fig. 9, for the STO biased by a VG of −600 V, the current peaks at ~580, ~115 and ~16 min for the light power of 0, 40 and 100 mW, that is, illumination indeed accelerates the migration of oxygen vacancies.
As revealed by ref. 25, the polarization will disappear in several seconds after removing the external field. This is consistent with our observation that RS quickly drops back when the gate bias is removed (Fig. 1b). No additional tuning is observed under positive gate fields since there are no structural changes (Fig. 4b). In conclusion, our present work has revealed a unique control of the 2DEG confined at the LAO/STO interface with complementary electric and light stimuli. The principle proven here could be extended to a wide variety of complex oxide systems with ferroelectric instabilities, pioneering a new avenue for the resistive tuning of oxide interfaces.
The samples a-LAO/STO were prepared by depositing an amorphous LAO layer, ~12 nm in thickness, on TiO2-termined (001)-STO substrates (3 × 5 × 0.5 mm3) using the pulsed laser (248 nm) ablation technique. In the deposition process, the substrate was kept at ambient temperature and the oxygen pressure at 10−3 mbar. The fluence of the laser pulses was 1.5 J cm−2 and the repetition rate was 1 Hz. The target–substrate separation was 4.5 cm. A shadow mask was employed to get the Hall-bar-shaped samples. For comparison, sample c-LAO/STO with a crystalline LAO overlayer (4 unit cells in thickness) was also prepared at a temperature of 800 °C and the oxygen pressure of 10−5 mbar. The fluence of the laser pulses was 0.7 J cm−2 and the repetition rate was 1 Hz. After deposition, the sample was in situ annealed in 200 mbar of O2 at 600 °C for 1 h and then cooled to room temperature in the same oxygen pressure. The detailed procedures for sample preparation can be found in ref. 21 for amorphous overlay and in ref. 7 for crystalline overlayer.
Ultrasonic Al wire bonding (20 μm in diameter) was used for electrode connection. Four-probe technique was adopted for resistance measurements. The four welding spots were well aligned and the separation between the neighbouring spots was ~0.4 mm. The formula of RS≈(L/W)R was adopted for the convention of four-probe resistance to sheet resistance, where L and W are, respectively, the long and wide dimensions of the measured plane. Transverse electrical field was applied to STO through an Ag electrode underneath STO and the LAO/STO interface was grounded. The direction from substrate to interface was defined as positive. The applied current for resistance measurements was 1 μA. Lasers with the wavelengths between 532 and 980 nm were used in the present experiments. The spot size of the light was ~0.4 mm in diameter, focusing on the space between two inner Al wires. Under the gate voltage of −100 V, the leakage current was ~0.7 nA without illumination and at most ~7 nA under light illumination (refer to Supplementary Fig. 1). The crystal structure of the gated c-LAO/STO was measured by a Bruker diffractometer (D8 Discover, Cu Kα radiation), using the X-ray parallelized and monochromatized by an asymmetric Ge 2202-Bounce monochromator. Capacitance was measured by the Precision Impedance Analyzer (Agilent 4294 A), adopting the a.c. amplitude of 0.5 V and the frequencies of 100 Hz and 5 kHz. The data were recorded after an interval of 60 s after the application of VG and the whole measurement from −40 V to 40 V takes 180 s. All data, except for the RS–T relations, were acquired at ambient temperature.
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This work has been supported by the National Basic Research of China (2012CB925002, 2011CB921801) and the National Natural Science Foundation of China (11374348 and 11134007). Y.W.X. and H.Y.H. acknowledge the support from the Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. J.R.S. thanks Professor J.W. Cai for his help in preparing the sample for structural analysis.
The authors declare no competing financial interests.
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Lei, Y., Li, Y., Chen, Y. et al. Visible-light-enhanced gating effect at the LaAlO3/SrTiO3 interface. Nat Commun 5, 5554 (2014). https://doi.org/10.1038/ncomms6554
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