Optically Modulated Resistance Switching Polarities in BaTiO3 Thin Film

Optically Modulated Resistance Switching Polarities in BaTiO3 Thin Film Jing Wang, Bailey Keith Bedford, Changle Chen, Ludi Miao, Bingcheng Luo School of Science, Northwestern Polytechnical University, Xi’an, 710072, China Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Department of mathematical and Physics, Weinan Normal University, Weinan, 714000,China Abstract

Recently, resistive random access memory (RRAM) devices possessing many advantages such as simple structure, low cost, and a nonvolatile characteristic, ha ve been suggested as promising candidates for next generation no nvolatile memory applications [1][2][3] .
Furthermore, a number of studies have been focused on the functional tunable effect, the electroresistance effect induced by ferroelectric polarization [10] , the photoresistance effect induced by an ultraviolet laser [7] , giant tunneling electroresiatance in ferroelectric tunnel hunctions [11] and the resistive switching (RS) of polarities controlled by different deposition conditions [12] . Correspondingly, various microscopic mechanisms such as ferroelectric polarization [4,13] , conductive filaments mechanisms and Schottky barriers [14,15] at the interfaces have been proposed to explain the resistive switching effects. In spite of the excellent work in the field the ferroelectric resistive switching mechanisms are still controversial and in need of additional exploration. In the work of W. J. Ma et al. [16] , conductive filaments mechanisms are considered to explain the resistive switching. And in the work of Li et al. [12] , they observed two completely opposite RS polarities for the BTO films deposited in different oxygen pressures. They showed that ferroelectric polarization is the main cause of the RS effect when lower oxygen pressures is used during the film deposition while vacancy migration is the primary source of the RS effect when higher oxygen pressures is used.
In this work, we present a resistive switching behavior in a symmetric structure Pt/BTO/Pt, which combines the light induced resistance change and the polarities of 3 resistance switching. It is found that not only can the high and low resistance states of the film be switched by applying ultraviolet 365nm wavelength light but also that the polarities of resistance switching can be controlled under an opportune oxygen condition. The two mechanisms of RS, the ferroelectric polarization and oxygen vacanc ies, can coexist and compete with each other. Neutral oxygen vacancies can easily releases electrons to the conduction band under stimulation from certain wavelength of light [17] . So, we designed a film where the two mechanisms coexisted and then modulated the oxygen vacancy transition by light above the bandgap to demonstrate the change of the RS mechanisms.

Results and discussions
As illustrated in Figure 1 we estimate the direct bandgap of the thin BTO layer to be ~3.18eV. We used a modified square law based bandgap calculation using the (αhv) 2 vs hv plots, by extrapolating the linear portion of the absorption to the X-axis where the absorption coefficient becomes zero. This optical gap is consistent with the reported value of 3.20eV for bulk BTO [18] .
In order to confirm the stability and repeatability of the Pt/BTO/Pt device, we studied the ultraviolet (UV) periodic response. For this study, we periodically shone 365 nm UV light for 600 s. The UV response of BTO film is shown in Figure 2(a). It can be seen that current decreases rapidly when the sample is exposed to UV, followed by a gradual decrease in current until saturating at a value of 3.2 nA. When the light is turned off, the current increases gradually over 120 s to reach its original value of 0.09 nA. The behavior is observed to be periodic with the periodic illumination. To further understand the mechanism of the repetition state, we investigated the relaxation process during the UV response due to the switching of The transient photoresponse is dictated by the generation and recombination rates of photocarriers, typically in the ms range [19] . Oxygen vacancies serve as charge traps increasing the time constant to the seconds range. Due to the time of carrier drift being extremely short and the thermal excitation being slow the light on and light off processes are limited by the rate of photoexcitation and the rate of thermal excitation, respectively. The ratio 9.0 between the resistance for the light being on and off is consistent with previously reported values 6.1 [20] . It should be noted that both the oxygen vacancy mechanism and the ferroelectric polarization mechanism are present in all of the charge transport processes. Although the effect of the ferroelectric polarization is small it is still present when oxygen vacancies are playing an important role, and vice versa. There is a competition between oxygen vacancy mechanism and ferroelectric polarization but their inclusion would not change the physics picture presented.

Conclusions
In summary, we have fabricated a symmetric metal/ferroelectric/metal device to study the distinct RS effects on normal state and UV light illuminations. We find that the UV light can modulate the resistive state from a HRS to a LRS. More importantly, the polarity of the   Optical transmittance spectrum of the film is displayed in the inset.