Engineering topics

Nelson, Christie

Publications and source records attributed to Nelson, Christie.

5 records

DOE OSTI2024

Local step-flow dynamics in thin film growth with desorption

Desorption of deposited species plays a role in determining the evolution of surface morphology during crystal growth when the desorption time constant is short compared with the time to diffuse to a defect site, step edge, or kink. However, experiments to directly test the predictions of these effects are lacking. Novel techniques such as in situ coherent x-ray scattering can provide significant new information. Herein we present x-ray photon correlation spectroscopy (XPCS) measurements during diindenoperylene (DIP) vapor deposition on thermally oxidized silicon surfaces. DIP forms a nearly complete two-dimensional first layer over the range of temperatures studied (40–120 ⁢°C), followed by mounded growth during subsequent deposition. Local step flow within mounds was observed, and we find that there was a terrace-length-dependent behavior of the step edge dynamics. This led to unstable growth with rapid roughening (β>0.5) and deviation from a symmetric error-function-like height profile. At high temperatures, the grooves between the mounds tend to close up leading to nearly flat polycrystalline films. In conclusion, numerical analysis based on a (1+1)-dimensional model suggests that terrace-length dependent desorption of deposited ad-molecules is an essential cause of the step dynamics, and it influences the morphology evolution.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Discovery of Charge Order in the Transition Metal Dichalcogenide Fe x NbS 2

The Fe intercalated transition metal dichalcogenide (TMD), Fe 1/3 NbS 2 , exhibits remarkable resistance switching properties and highly tunable spin ordering phases due to magnetic defects. We conduct synchrotron x-ray scattering measurements on both underintercalated ( x = 0.32 ) and overintercalated ( x = 0.35 ) samples. We discover a new charge order phase in the overintercalated sample, where the excess Fe atoms lead to a zigzag antiferromagnetic order. The agreement between the charge and magnetic ordering temperatures, as well as their intensity relationship, suggests a strong magnetoelastic coupling as the mechanism for the charge ordering. In conclusion, our results reveal the first example of a charge order phase among the intercalated TMD family and demonstrate the ability to stabilize charge modulation by introducing electronic correlations, where the charge order is absent in bulk 2H - NbS 2 compared to other pristine TMDs.

36 MATERIALS SCIENCE↗

DOE OSTI2022

High power density soft x-ray GaAs photodiodes with tailored spectral response

GaAs photodiode arrays have been designed for non-destructive monitoring of x-ray beam position in soft coherent beamline front ends in synchrotron light sources. A shallow p-on-n junction was employed to reduce the device photocurrent density to optimize the operation with beam power densities up to 20 W mm -2 , mainly coming from hard x-rays. With this approach, the photocurrent is primarily defined by the excess carriers generated by low-energy x-ray photons absorbed near the detector surface. The p–n junction structures were grown by molecular beam epitaxy and processed into 64-element linear photodiode arrays. The devices were characterized first in the visible range with a high-power Ar-ion laser and then tested in the soft and hard x-ray regions up to 10 keV at two beamlines of the National Synchrotron Light Source II. The responsivity was measured to be 0.16 A W -1 at 0.7 keV and 0.05 A W -1 at 6 keV in agreement with modeling. At higher x-ray energies the measured responsivity was lower than predicted in the framework of the carrier diffusion model; a possible explanation is discussed.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Polaronic Conductivity in Cr 2 Ge 2 Te 6 Single Crystals

We report that intrinsic 2D ferromagnetic semiconductors are an important class of materials for spin-charge conversion applications. Cr 2 Ge 2 Te 6 retains long-range magnetic order in the bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X-ray scattering, electronic, thermal transport and first-principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr 2 Ge 2 Te 6 are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr 2 Ge 2 Te 6 —based devices by electron–phonon- and spin–orbit coupling-based tailoring of polaron properties.

2D materials↗

DOE OSTI2021

Vacancy defect control of colossal thermopower in FeSb2

Abstract Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscopic study with electronic transport neutron, X-ray scattering, and first principle calculation, we identify atomic defects that control colossal thermopower magnitude and nanoprecipitate clusters with Sb vacancy ordering, which induce additional phonon scattering and substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of the unit cell P n n m → P m . The absence of Sb along [010] for high defect concentration forms conducting path due to Fe d orbital overlap. The connection between atomic defect anisotropy and colossal thermopower in FeSb 2 paves the way for the understanding and tailoring of giant thermopower in related materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗