Heteroepitaxial Growth And Electrical Transport Properties Of BiFeO₃ Ferroelectric Semiconductors

The heterostructures consisting of multiferroic BiFeO3(BFO) thin film have received noticeable attention because of their remarkable multiferroic and optical properties at room temperature. Especially, the resistance of these BFO based heterostructures can be reversibly switched between a high resistance state(HRS) and a low resistance state(LRS) by applying voltages, which indicates the potential application of BFO based heterostructrues in bistable memory devices. For practical application, the direct growth of the BFO on commonly used semiconductors, such as Si, GaAs, GaN has drawn much attention. Among these semiconductors, GaAs should play an important role as a typical Ш-V semiconductor because of its direct band gap and higher saturated electron mobility. If BFO and GaAs are integrated to form the BFO/GaAs integrated film, not only can the multi-fuctional integration of dielectric passive components and semiconductor active components be realized, but also we would exploit the coupling effect at the interface to develop some new devices. In previous study, researchers has epitaxial growth the BFO on GaAs successfully. But the electrical transport property it is still not fully understood. As the application of these BFO heterostructures is strongly dependent on the electrical transport property. Therefore, detailed knowledge of the charge carrier dynamics is crucial. In this work, the electrical transport property of BFO/NSTO/p-GaAs heterostructure was investigated.1.The BFO films were grown on the p-type GaAs(001) substrate by pulsed laser deposition(PLD) via a NSTO thin film as buffer layer and the structure property has been studied. The crystal structure of the samples was examined by X-ray diffraction(XRD) technique and the result indicating that the films have a mostly single(00l) orientation. The cross-sectional morphology of the heterostructure shows that the BFO film has grown columnarly on NSTO buffer layer with a clear interface. The amplitude and phase signals of vertical PFM were simultaneously obtained on the surface of the samples. A poly-domain state can be clearly seen under the PFM measurement implies a large polarization perpendicular to the BFO film.2.Different film thickness and buffer layer has been compared. The experiment has found that the dielectric properties is decreased with thickness of the thin film increased. When the thickness of the BFO film increased from 240 nm to 300 nm and 360 nm,respectively. The dielectric const decreased from 340 to 280 and 200. At the same time, there are certain constraints for the buffer layer thickness,when the thickness exceeds the 100 nm at this work, the performance of the heterojunction will decrease dramatically.3.The electric transport property has also been studied. The diode-like current characteristics with a rectifying ratio 200 and resistance phenomenon has been explained. Rectifying property was analyzed by the energy band structure of heterojunction. In fact the heterojunction can be treat as two back to back PN junction. According to the energy parameters, the band offset value of the conduction band and value band between BFO and NSTO is 0.7eV and 1.1eV. In a similar way, the value between NSTO and Ga As is 0.1eV and 1.68 eV. Since the barrier height for holes is 0.2eV larger than that for electrons, the hole current will be approximately a factor of 104 smaller than the electron current, suggesting that the current is dominated by the electrons in our heterostructure. In order to explain the resistive switching behavior. Combined with previous studies, a diode-like rectifying I-V characteristic without the switching behavior has been observed in STO/GaAs and other oxide/GaAs related heterostructures. As a consequence, we consider that the switching behavior in the BFO/NSTO/GaAs heterostructure could be attributed to the influence of BFO films. In our case, the observed resistive switching behavior in BFO/NSTO/GaAs heterostructure can be understood by the ferroelectric polarization modulation effect on the width of depletion region and then influence the height of potential barrier at the BFO/NSTO interface. As a result, the resistance switching behavior occurs. Finally, the leakage mechanism has been studied by the leakage mechanism theory. The result shows that heterojunction is dominated by Schottky emission mechanism.