Preparation And Electrical Properties Of ZnO/BaTiO₃ Hetero-structure Grown By Chemical Solution Deposition (CSD )

The Metal-Ferroelectric-Semiconductor (MFS) structures can be applied in the fields of metal-ferroelectric-semiconductor field effect transistor (MFSFEF), non-destructive read-out ferroelectric random access memory (NDRO FeRAM), infrared and uvtraviolet detector and electro-optic devices and so on, thus their fabrication technique and growth control have been extensively studied. Aiming at the growth and properties of ZnO, BaTiO₃ (BTO) and ZnO-BTO thin films, these works had touched the following aspects via chemical solution deposition (CSD):Growth behaviors of ZnO thin film were affected by the preparation technology. Sufficient oxygen was helpful to the crystallization of ZnO thin films. Highly c-axial orientation of the ZnO films deposited on amorphous glass were achieved under a range of the temperatures of 550℃⁶50℃, and the root-mean-square(RMS) of these thin films would be increased with the annealing temperature in some degree. Meanwhile, the growth mechanisms of ZnO films were found to be different for different kinds of solvents and pre-heating temperatures. On the one hand, it was easy to control the orientation of (002)ZnO in the PVA-Zn network by using PVA as the solvent. On the other hand, the orientation of ZnO thin film depended on the surface structure of the templates by using 2-methoxyethanol (MOE) as the solvent.Growth behaviors of BTO thin film were investigated on different substrates. On amorphous glass substrate, the films were polycrystalline with random crystal orientation. On Pt(111)/Ti/SiO2/Si substrate, it was hard to deposit in the experimental conditions. But the enhanced quality and preferred (001) crystal orientation of BTO thin films were obtained using LaNiO₃ as a buffer layer, indicating that the growth of BTO thin films depended on the surface structure of the templates. Furthermore, the coating thicknesses from 1 layer to 7 layers just affected the RMS and grains of the BTO thin films instead of the orientation. Under optimized process condition of ZnO and BTO thin films, the ZnO/LaNiO₃ and ZnO/BTO/LaNiO₃ heterostructure were successfully fabricated. The orientation relationships were found to be (002)ZnO/LaNiO₃ and (002)ZnO/(001)BTO/LaNiO₃ thin films, providing the highest degree of c-axial orientation of ZnO and BTO thin films. Both of them revealed good surface quality, with the RMS of 10.5 nm and 12 nm, respectively.The Uv-vis spectra of ZnO thin film showed that the ZnO reference layer presented a high transparency (above 80 %) in visible light wavelength range and band gap was about 3.24 eV, which would strongly affected the interface band structure of ZnO/BTO heterostructure. The capacitance-voltage (C-V) characteristic of BTO thin films measured at 1 MHz and 100 kHz exhibited an asymmetric bufferfly shaped hysteresis loop, suggesting the existence of many mobile electrons or charges in the film-electrode interface resulting in bias voltage. Compared to BTO/LNO thin films, the leakage current of ZnO/BTO/LNO heterostructure was greatly improved after the deposition of ZnO thin films on BTO/LNO. And C-V curve of ZnO/BTO/LNO heterostructure exhibited a counterclockwise hysteresis loop for MFS capacitor confirming the ferroelectric nature. The memory windows increased and then reduced as the frequency changed from 100 kHz¹ MHz, which might relate to electron injection. Therefore, these results suggest that BTO-ZnO (MFS) heterostructures by CSD can be used for memory device working at room temperature, in which the ZnO plays the role of the semiconductor.