Pt/BiFeO3/Nb:SrTiO3异质结的光伏效应和光调控整流特性

Photovoltaic effect and photo-assisted diode behavior in Pt/BiFeO3/Nb-doped SrTiO3 heterojunction

铁电薄膜异质结的光伏效应因具有重要的应用前景而备受关注,而且其中多种光伏效应机制的共存带来了丰富而复杂的物理内涵.为了研究界面对光伏效应的重要作用,制备了基于BiFeO3铁电薄膜的具有"金属/铁电体/半导体"非对称电极结构的Pt/BiFeO3/Nb:SrTiO3异质结,并系统研究了其在不同波长(365和445 nm)激光照射下的光伏效应.在365 nm,74 mW/cm^2光照下,异质结的光伏开路电压高达0.55 V.而且,由于光激发和光吸收过程的不同,365 nm激光照射下该异质结的开路电压和短路电流比445 nm激光照射下的结果显著提高.随着温度降低,开路电压单调上升,而不同波长下的短路电流则表现出不同的变化规律.另外,随着光强的提高,异质结整流效应获得增强,通过分析,空间电荷限制电流传导机制对异质结输运有重要贡献,而光生载流子将通过填充缺陷影响输运特性.

The photovoltaic effect of ferroelectric BiFeO3(BFO)-based heterojunction has been one of hot subjects of theoretical and experimental studies due to its important application prospects,and the coexistence of varieties of photovoltaic effect mechanisms(bulk photovoltaic effect,domain wall effect,interfacial barrier effect,etc.)can bright rich and complicated physics nature.In order to investigate the important role that the interface plays in the photovoltaic effect,we prepare the Pt/BFO(60 nm)/Nb:SrTiO3(NSTO)heterojunction with an asymmetric metal/ferroelectric/semiconductor structure,and systematically investigate the photovoltaic effect under laser irradiation with different wavelengths(365 nm and 445 nm).The heterojunction exhibits much stronger open-circuit voltage(Voc,~0.55 V at 74 mW/cm2)and short-circuit current density(Jsc,~208μA/cm^2 at 74 mW/cm2)for the laser irradiation with 365 nm wavelength than those for the laser irradiation with445 nm wavelength,and the Voc and Jsc are both strengthened with the increase of light intensity.This is because the 365 nm light with the photon energy^3.4 eV can stimulate photon-induced carriers in both BFO(band gap^2.7 eV)and NSTO(band gap^3.2 eV)at both the Pt/BFO interface and the BFO/NSTO interface,while the 445 nm light with the photon energy^2.8 eV can only generate carriers in BFO.Thus the photovoltaic voltage is much bigger for the 365 nm light.Furthermore,the laser absorption process is much more efficient for the 365 nm light(79%absorbed in BFO and 21%absorbed in NSTO)than for the 445 nm light(21%absorbed in BFO).In addition,the temperature dependent Voc and Jsc are also investigated.It is found that for the 365 nm and 445 nm laser irradiation,the Voc increases with temperature decreasing,which is possibly due to the variations of the built-in potential,concentration of thermal charge carriers,and/or electron-phonon scatterings.The sharper variation of Voc above^200 K may suggest the more significant role of thermal charge carriers at high temperatures.Interestingly,the temperature dependent Jsc behaves differently for the 365 nm and 445 nm light.Under the 365 nm laser irradiation,the Jsc remains almost unchanged below170 K and increases sharply with temperature increasing above 170 K,which may be related to the dominant role of thermal excitation for the 365 nm light.While for the 445 nm light,the Jsc decreases with temperature increasing,which follows the variation trend of its Voc.What is more,the conduction mechanism of Pt/BFO/NSTO heterojunction under laser irradiation is also studied.It is found that the conduction for the445 nm light can be nicely described by the space-charge-limited bulk conduction(SCLC)model and the photon-generated carriers may fill the traps and thus leading the transition voltage to decrease.While for the365 nm light,the conduction is more complicated and cannot be described by the SCLC model.Our findings may be helpful in understanding the photovoltaic effect in transition-metal oxide based heterojunctions and designing photovoltaic devices.