Characterization Of BST Thin Films Prepared By Pulsed Laser Deposition

Barium strontium titanate (BST) have recently been widely investigated as promising candidates for the applications in tunable microwave devices, dynamic random access memories (DRAMs) and Uncooled Focal Plane Arrays (UFPA), due to their high dielectric constant, low dielectric loss, low leakage current, smooth film surface, and the highly nonlinear dependence of the dielectric constant with the applied electric field, namely, electric tunabilty, etc. With the development of technology and the requirement of VLSI industry, Barium strontium titanate Ferroelectric thin film should be widely used as for its high performance in both electric and optical properties. However, the film fabrication technology didn't reach the requirement of VLSI industry, which limits the application of BST film. For that reason, research in BST film preparation and the improvement of BST film properties is needed.To solve these problems, this paper has concentrated on several aspects and the main results and conclusions can be summarized as following:1. Using Pulsed Laser Deposition technology, BST film samples have been successfully prepared on different substrates. By changing the deposition condition and film preparation process, for example, the annealing temperature, choosing of substrates, etc, the influence in film properties including physicality and electric properties are studied.2. XRD spectra shows that the intensity of BST peaks are increased as the depositon temperature changed from 700℃to 750℃, so are the film quality and the crystallization. Films deposited on LNO and Pt substrate are both well crystallized and show different orientation. The grain size of each film is 70nm and 50nm. Results show that with the using of LNO buffer layer, which is of almost the same crystal structure with BST, film's grain size is increased and the choosing of different substrate strongly influence the the orientation and crystallization of BST thin films.3. The microscopic surface morphology for BST thin films deposited on Pt and LNO bottom electrodes was studied by atomic force microscopy (AFM). The results show that the two kinds of BST films are well-crystallized, pinhole-free, and crack-free. BST film deposited on LNO has larger and more uniform grain size (60-70nm) and larger than that grown on Pt, whose grain size is 40-50nm. Additionally, The surface roughness of BST/Pt/Ti/Si(100) (RMS=10.218nm) is apparently larger than that of BST/LNO/Si(100)(RMS=4.301nm). In whole, films growing on LNO electrode have smoother surface, larger grain size and better crystallization. Consequently,The nucleation and growth dynamics of the BST film show obvious bottom electrode dependence.4. XPS spectra shows that the Ba, Sr, Ti,O element in BST film fabricated by PLD method represent in a typical chemical state of pervoskite structure. And the result given by AES spectra shows that C element pollution, which is caused by air contact.5. We tested the electric properties of the BST samples, including the C-F curves, dielectric loss properties, C-V curves, hysteresis loops, etc. Results show that the dielectric properties depend on frequency, especially on high frequency area. Differences in buffer layer also show strongly influence.6. In our study, the asymmetry of films' dielectric property is studied carefully. Results show that the asymmetry is the result of film property, not the testing method. And the difference in top electrode and bottom electrode will increase the asymmetry. In order to improve the asymmetry, using same electrode (Pt electrode), improved the asymmetry property remarkably, as the rate of asymmetry decreased from 50.38% to 17.86%.7. BST thin films show great potential in microwave applications. In this paper, the dielectric tunability is also studied. Results show that LNO buffered sample show better tunability, at 10 kHz and 100 kHz, the tunability is 45.3% and 39.5%. However, tunability of sample deposited on Pt sub is about 30% during different testing frequency. The result of tunability is consistent with the result of C-V testing. Micro structure and dielectric property directly influence the performance of tunability. In order to fabricate high tunability BST films, film preparation process should be improved.8. We also tested the ferroelectric properties of these film samples. Results show that with an applied voltage of 8 V, the remanent polarization (2Pr) and the coercive electric field (Ec) changed from 0.409μC/cm2 and 22.04kV/cm to 0.393μC/cm2 and 12.09kV/cm, as the bottom electrode changes from LNO to Pt, respectively. Film deposited on Pt electrode exhibits larger and clearer hysteresis loops, and the hysteresis loops of film deposited on LNO buffer layer apparently show a shift along the electric field axis, which is consistent with the C-V measurement. This suggests that the domain structure is different in these films, the grain size and using of buffer layer may cause the result.