ABSTRACT
The microstructure of the PbZr0.52Ti0.48O3 (PZT) films is known to influence the ferroelectric properties, but so far mainly the effect of the deposition conditions of the PZT has been investigated. To our knowledge, the influence of the underlying electrode layer and the mechanisms leading to changes in the PZT microstructure have not been explored. Using LaNiO3 (LNO) as the bottom electrode material, we investigated the evolution of the PZT microstructure and ferroelectric properties for changing LNO pulsed-laser deposition conditions. The explored deposition conditions were the O2 pressure, total pressure, and thickness of the electrode layer. Increasing both the O2 pressure and the thickness of the electrode layer changes the growth of PZT from a smooth, dense film to a rough, columnar film. We explain the origin of the change in PZT microstructure as the increased roughness of the electrode layer in relaxing the misfit strain. The strain relaxation mechanism is evidenced by the increase in the crystal phase with bulk LNO unit cell dimensions in comparison to the crystal phase with substrate-clamped unit cell dimensions. We explain the change from a dense to a columnar microstructure as a result of the change in the growth mode from Frank-van der Merwe to Stranski-Krastanov. The ferroelectric properties of the columnar films are improved compared to those of the smooth, dense films. The ability to tune the ferroelectric properties with the microstructure is primarily relevant for ferroelectric applications such as actuators and systems for energy harvesting and storage.
ABSTRACT
Lanthanum and lanthanum nitride thin films were deposited by magnetron sputtering onto silicon wafers covered by natural oxide. In situ and real-time synchrotron radiation experiments during deposition reveal that lanthanum crystallizes in the face-centred cubic bulk phase. Lanthanum nitride, however, does not form the expected NaCl structure but crystallizes in the theoretically predicted metastable wurtzite and zincblende phases, whereas post-growth nitridation results in zincblende LaN. During deposition of the initial 2-3â nm, amorphous or disordered films with very small crystallites form, while the surface becomes smoother. At larger thicknesses, the La and LaN crystallites are preferentially oriented with the close-packed lattice planes parallel to the substrate surface. For LaN, the onset of texture formation coincides with a sudden increase in roughness. For La, the smoothing process continues even during crystal formation, up to a thickness of about 6â nm. This different growth behaviour is probably related to the lower mobility of the nitride compared with the metal. It is likely that the characteristic void structure of nitride thin films, and the similarity between the crystal structures of wurtzite LaN and La2O3, evoke the different degradation behaviours of La/B and LaN/B multilayer mirrors for off-normal incidence at 6.xâ nm wavelength.
ABSTRACT
We studied the structure and optical properties of B(4)C/Mo/Y/Si multilayer systems. Using extended x-ray absorption fine structure measurements at the Y and Mo K-edge, the structure of the subnanometer thick Y layer and the underlying Mo layer were analyzed. It was found that even a 0.2 nm thick Y layer significantly reduced silicon diffusion toward Mo, thus reducing Mo silicide formation. Hard x-ray reflectometry showed that the difference in average interface roughness of the B(4)C/Mo/Y/Si multilayer structure compared to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures was negligible. Soft x-ray reflectometry showed optical improvement of B(4)C/Mo/Y/Si with respect to Mo/Si and B(4)C/Mo/B(4)C/Si multilayer structures.
ABSTRACT
The growth behavior of B(4)C interlayers deposited at the interfaces of Mo/Si multilayers was investigated using x-ray photoemission spectroscopy, x-ray reflectivity, and x-ray diffraction measurements. We report an asymmetry in the formation of B(4)C at the B(4)C-on-Mo interface compared to the B(4)C-on-Si interface. X-ray photoelectron spectroscopy (XPS) depth profiling shows that for B(4)C-on-Mo the formed stoichiometry is close to expectation (4:1 ratio), while for B(4)C-on-Si it is observed that carbon diffuses from the B(4)C interfaces into the multilayer, resulting in nonstochiometric growth (>4:1). As a result, there is a discrepancy in the optical response near 13.5 nm wavelength, where B(4)C-on-Mo behaves according to model simulations, while B(4)C-on-Si does not. The as-deposited off-stoichiometric B(4)C-on-Si interface also explains why these interfaces show poor barrier properties against temperature induced interdiffusion. We show that the stoichiometry of B(4)C at the Mo-Si interfaces is connected to the structure of the layers onto which B(4)C is grown. Because of enhanced diffusion into the amorphous Si surface, we suggest that deposited boron and carbon atoms form Si(X)B(Y) and Si(X)C(Y) compounds. The low formation enthalpy of Si(X)C(Y) ensures C depletion of any B(X)C(Y) interlayer. Only after a saturated interfacial layer is formed, does further deposition of boron and carbon atoms result in actual B(4)C formation. In contrast to the off-stoichiometric B(4)C growth on top of Si, B(4)C grown on top of Mo retains the correct stoichiometry because of the higher formation enthalpies for Mo(X)B(Y) and Mo(X)C(Y) formation and the limited diffusion depth into the (poly)-crystalline Mo surface.
