Gokul Gopalakrishnan

Three-terminal field effect devices utilizing thin film vanadium oxide as the channel layer

Electrostatic control of the metal-insulator transition (MIT) in an oxide semiconductor could potentially impact the emerging field of oxide electronics. Vanadium dioxide (VO2) is of particular interest due to the fact that the MIT happens in the vicinity of room temperature and it is considered to exhibit the Mott transition. We present a detailed account of our experimental investigation into three-terminal field effect transistor-like devices using thin film VO2 as the channel layer. The gate is separated from the channel through an insulating gate oxide layer, enabling true probing of the field effect with minimal or no interference from large leakage currents flowing directly from the electrode. The influence of the fabrication of multiple components of the device, including the gate oxide deposition, on the VO2 film characteristics is discussed. Further, we discuss the effect of the gate voltage on the device response, point out some of the unusual characteristics including temporal dependence. A re...

Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions

200 nm diameter Au contacts were fabricated by e-beam lithography on sputtered thin film vanadium oxide grown on conducting substrates and current perpendicular to plane electron transport measurements were performed with a conducting tip atomic force microscope. Sharp jumps in electric current were observed in the I-V characteristics of the nano-VO2 junctions and were attributed to the manifestation of the metal-insulator transition. The critical field and dielectric constant were estimated from quantitative analysis of the current-voltage relationship and compared with reported values on micrometer and larger size scale devices. These results are of potential relevance to novel oxide electronics utilizing metal-insulator transitions.

Edge-induced flattening in the fabrication of ultrathin freestanding crystalline silicon sheets

Silicon nanomembranes are suspended single-crystal sheets of silicon, tens of nanometers thick, with areas in the thousands of square micrometers. Challenges in fabrication arise from buckling due to strains of over 10−3 in the silicon-on-insulator starting material. In equilibrium, the distortion is distributed across the entire membrane, minimizing the elastic energy with a large radius of curvature. We show that flat nanomembranes can be created using an elastically metastable configuration driven by the silicon-water surface energy. Membranes as thin as 6 nm are fabricated with vertical deviations below 10 nm in a central 100 μm × 100 μm area.

Fabrication of flat SiGe heterostructure nanomembrane windows via strain-relief patterning

The lattice mismatch between SiGe and Si in heteroepitaxial Si/SiGe/Si trilayers leads to buckling when confined nanomembrane windows formed from these heterostructures are released from silicon-on-insulator substrates. We demonstrate that large areas in which the curvature and curvature-induced strain are reduced by an order of magnitude can be produced by patterning the windows to concentrate buckling in narrow arms with low flexural rigidity supporting a flat central region. Synchrotron x-ray thermal diffuse scattering shows that the improved flatness of patterned windows permits fundamental studies with fidelity similar to what can be achieved with flat single-component Si nanomembranes.

Three-dimensional phonon population anisotropy in silicon nanomembranes

Nanoscale single-crystals possess modified phonon dispersions due to the truncation of the crystal. The introduction of surfaces alters the population of phonons relative to the bulk and introduces anisotropy arising from the breaking of translational symmetry. Such modifications exist throughout the Brillouin zone, even in structures with dimensions of several nanometers, posing a challenge to the characterization of vibrational properties and leading to uncertainty in predicting the thermal, optical, and electronic properties of nanomaterials. Synchrotron x-ray thermal diffuse scattering studies find that freestanding Si nanomembranes with thicknesses as large as 21 nm exhibit a higher scattering intensity per unit thickness than bulk silicon. In addition, the anisotropy arising from the finite thickness of these membranes produces particularly intense scattering along reciprocal-space directions normal to the membrane surface compared to corresponding in-plane directions. These results reveal the dimensions at which calculations of materials properties and device characteristics based on bulk phonon dispersions require consideration of the nanoscale size of the crystal.