Albert V. Davydov

In situ atomic-scale imaging of electrochemical lithiation in silicon

In lithium-ion batteries, the electrochemical reaction between the electrodes and lithium is a critical process that controls the capacity, cyclability and reliability of the battery. Despite intensive study, the atomistic mechanism of the electrochemical reactions occurring in these solid-state electrodes remains unclear. Here, we show that in situ transmission electron microscopy can be used to study the dynamic lithiation process of single-crystal silicon with atomic resolution. We observe a sharp interface (~1 nm thick) between the crystalline silicon and an amorphous Li(x)Si alloy. The lithiation kinetics are controlled by the migration of the interface, which occurs through a ledge mechanism involving the lateral movement of ledges on the close-packed {111} atomic planes. Such ledge flow processes produce the amorphous Li(x)Si alloy through layer-by-layer peeling of the {111} atomic facets, resulting in the orientation-dependent mobility of the interfaces.

Sub-bandgap defect states in polycrystalline hafnium oxide and their suppression by admixture of silicon

The crystallinity of atomic layer deposition hafnium oxide was found to be thickness dependent, with the thinnest films being amorphous and thick films being at least partially crystalline. Hafnium oxide films fabricated by metalorganic chemical vapor deposition are mostly monoclinic. Formation of hafnium silicate by admixture of 20% Si prevents crystallization. Electronic defects are reflected by an absorption feature 0.2–0.3 eV below the optical bandgap. These defects arise in polycrystalline, but not in amorphous, hafnium-based oxides.

Fabrication of GaN-based nanoscale device structures utilizing focused ion beam induced Pt deposition

In this work we have demonstrated nanoscale GaN device structures made from individual GaN nanowires and electrical contacts utilizing focused ion beam (FIB) induced Pt deposition. These GaN nanowires were grown by direct reaction of Ga vapor with NH3 and had diameters ranging from 100nmto250nm and lengths up to 200μm. As-grown nanowires were dispersed on SiO2 coated p++ Si substrate. A 30keV Ga+ ion beam was used to dissociate (trimethyl)methylcyclopentadienyl-platinum precursor for depositing Pt contacts to GaN nanowires. FIB-deposited Pt contacts to GaN nanowires showed nonlinear I-V characteristics, which turned linear after annealing at 500°C for 30s in argon. Resistivity of a GaN nanowire measured using a four terminal contact geometry fabricated by depositing Pt with a FIB was in the range of 5×10−3Ωcm. Temperature dependent resistivity measurement of the GaN nanowire revealed semiconducting behavior with a weak temperature dependence of the resistivity. In this study both Ohmic and Schottky contac...

GaN Nanowires Grown by Molecular Beam Epitaxy

The unique properties of GaN nanowires grown by molecular beam epitaxy are reviewed. These properties include the absence of residual strain, exclusion of most extended defects, long photoluminescence lifetime, low surface recombination velocity, and high mechanical quality factor. The high purity of the nanowires grown by this method allows for controllable n-type doping. P-type doping presents more challenges but has been demonstrated in active light-emitting diode devices. The present understanding of nucleation and growth of these materials is also reviewed.

Diameter dependent transport properties of gallium nitride nanowire field effect transistors

The authors report transport property measurements of individual GaN nanowire field effect transistors and the correlation of the electron mobilities with the existence of grain boundaries in these nanowires. Room temperature field effect electron mobilities as high as 319cm2V−1s−1 were obtained for the 200nm diameter nanowires. Mobilities calculated from these reliable nanowire field effect transistors indicated that the surface scattering plays a dominant role in smaller diameter nanowires, whereas for intermediate diameter devices transport is dominated by grain boundary scattering. Reduction of the mobility with decreasing diameter of nanowires can be explained using “continuous surface” model.

Tapering Control of Si Nanowires Grown from SiCl4 at Reduced Pressure

Device applications of tapered Si nanowire (SiNW) arrays require reliable technological approaches for fabricating nanowires with controlled shape and orientation. In this study, we systematically explore effects of growth conditions on tapering of Si nanowires grown by chemical vapor deposition (CVD) at reduced pressure from SiCl(4) precursor. Tapering of SiNWs is governed by the interplay between the catalyzed vapor-liquid-solid (VLS) and uncatalyzed vapor-solid (VS) growth mechanisms. We found that the uncatalyzed Si deposition on NW sidewalls, defined by a radial growth rate, can be enhanced by lowering SiCl(4)/H(2) molar ratio, applying higher gas flow rate, or reducing growth pressure. Distinct dependences of the axial and radial growth rates on the process conditions were employed to produce SiNWs with a tapering degree (i.e., a ratio of the radial/axial growth rates) varying by almost 2 orders of magnitude. The results are explained by an interplay between the thermodynamic and kinetic effects on the axial (VLS) and radial (VS) growth rates. Established correlation between the SiCl(4)/H(2) molar ratio and vertical alignment of nanowires was used to develop a two-stage growth procedure for producing tapered SiNW arrays with a predominantly vertical orientation.

Effect of Growth Orientation and Diameter on the Elasticity of GaN Nanowires. A Combined in Situ TEM and Atomistic Modeling Investigation

We characterized the elastic properties of GaN nanowires grown along different crystallographic orientations. In situ transmission electron microscopy tensile tests were conducted using a MEMS-based nanoscale testing system. Complementary atomistic simulations were performed using density functional theory and molecular dynamics. Our work establishes that elasticity size dependence is limited to nanowires with diameters smaller than 20 nm. For larger diameters, the elastic modulus converges to the bulk values of 300 GPa for c-axis and 267 GPa for a- and m-axis.

Vertical 2D/3D Semiconductor Heterostructures Based on Epitaxial Molybdenum Disulfide and Gallium Nitride

When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 μm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm(2) and current spreading in the MoS2 monolayer of approximately 1 μm in diameter.

Effect of AlN buffer layer properties on the morphology and polarity of GaN nanowires grown by molecular beam epitaxy

Low-temperature AlN buffer layers grown via plasma-assisted molecular beam epitaxy on Si (111) were found to significantly affect the subsequent growth morphology of GaN nanowires. The AlN buffer layers exhibited nanowire-like columnar protrusions, with their size, shape, and tilt determined by the AlN V/III flux ratio. GaN nanowires were frequently observed to adopt the structural characteristics of the underlying AlN columns, including the size and the degree of tilt. Piezoresponse force microscopy and polarity-sensitive etching indicate that the AlN films and the protruding columns have a mixed crystallographic polarity. Convergent beam electron diffraction indicates that GaN nanowires are Ga-polar, suggesting that Al-polar columns are nanowire nucleation sites for Ga-polar nanowires. GaN nanowires of low density could be grown on AlN buffers that were predominantly N-polar with isolated Al-polar columns, indicating a high growth rate for Ga-polar nanowires and suppressed growth of N-polar nanowires under typical growth conditions. AlN buffer layers grown under slightly N-rich conditions (V/III flux ratio = 1.0 to 1.3) were found to provide a favorable growth surface for low-density, coalescence-free nanowires.

Refractive index study of AlxGa1−xN films grown on sapphire substrates

A prism coupling method was used to measure the ordinary (no) and extraordinary (ne) refractive indices of AlxGa1−xN films, grown by hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD) on sapphire, at several discrete wavelengths from 442 nm to 1064 nm. In addition, spectroscopic transmittance and reflectance, correlated with the prism coupling results, were used to measure no as a continuous function of wavelength between the band gap of each sample (255 nm to 364 nm, depending on Al fraction) and 2500 nm. The Al mole fractions (x), determined by energy dispersive x-ray spectroscopy (EDS), were x=0.144, 0.234, 0.279, 0.363, 0.593, and 0.657 for the HVPE-grown samples, and x=0.000, 0.419, 0.507, 0.618, 0.660, and 0.666 for the MOCVD-grown samples. The maximum standard uncertainty in the EDS-determined value of x was ±0.02. The maximum standard uncertainty in the refractive indices measured by prism coupling was ±0.005 and a one-Sellmeier-term equation was adequate to fit ...