Fabrication and Characterisation of Epitaxial PMN-PT Nano-structures

Downscaling of the minimal feature size, or the gate length has been the key for the exponential im-provement of the performance of the logic CMOS integrated circuits over the last six decades. How-ever, since about 2003, some fundamental limits in the device physics were reached. Further scaling with increased clock speed at lower voltage was limited by excessive increase of power consumption and noise. This challenge motivated numerous efforts to search for next generation transistors. The piezoelectronic transistor (PET), based on piezoelectricity and piezoresistivity, is one of the proposed devices. In this work, we seek to contribute to the scaling of ferroelectrics lead magnesium niobate – lead titanate xPb(Mg1/3Nb2/3)O3-(1− x)PbTiO3 (PMN-PT) for PET applications. The first important issue is the deposition of high quality epitaxial PMN-PT films. We studied the two film compositions 60/40 and 67/33, which are at the morphotropic phase boundary (MPB) and slight-ly in the tetragonal phase. A process of growing epitaxial (001)-oriented PMN-PT thin films on SrRuO3-buffered SrTiO3 substrates by pulsed laser deposition was developed. The deposition condi-tions, such as substrate temperature, O2 pressure, target compositions were optimized for achieving smooth and phase pure perovskite thin films. The patterning process for damage-free nanostructures of PMN-PT was subsequently investigated. Pattern transfer was carried out with electron beam lithography (eBL) with the goal to reach sub-100 nm structures. We developed a novel lift-off technique consisting in using a bilayer mask incorporat-ing the electron-beam resist hydrogen silsesquioxane (HSQ) on top of amorphous aluminum oxide (AlOx). The bilayer mask forms undercut profile and is thermally stable at the high temperature depo-sition process. Epitaxial PMN-PT/SrRuO3 heterostructures were grown inside the mold of AlOx. The lift-off was performed in NaOH aqueous solution, leaving untouched the arrays of PMN-PT/SrRuO3 nanostructures. The film structures grown inside the openings follows the shape of the mold, produc-ing the obtuse angle at the edge of the nanostructure. The thickness of the nanostructures varied with the width of the openings, and is less than the thickness of plain films. We developed a model to pre-dict the thickness of the nanostructures. The minimum features fabricated with this method have a lateral size of 70 nm. The crystalline of the nanostructures were studied with Transmission electron microscopy (TEM). Piezoresponse force microscopy (PFM) revealed a clear polarization switching of the patterned, 12 to 15 nm high PMN-PT nanostructures with a piezoelectric response that was larger than ~10 pm/V and possibly reaching even ~150 pm/V, Furthermore, short-range (10 – 20 nm) ferroelectric domain pat-terns prominently appear after poling the nanostructured ferroelectrics, indicating the coexistence of tetragonal a- and c-domains, mixed with fewer rhombohedral domains. The presented nanofabrication method enables to step towards nanoscale engineering of ferroelectrics for the advancement of effi-cient electromechanical device applications.