18,084 research outputs found
Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy
The control of optically driven high-frequency strain waves in nanostructured
systems is an essential ingredient for the further development of
nanophononics. However, broadly applicable experimental means to quantitatively
map such structural distortion on their intrinsic ultrafast time and nanometer
length scales are still lacking. Here, we introduce ultrafast convergent beam
electron diffraction (U-CBED) with a nanoscale probe beam for the quantitative
retrieval of the time-dependent local distortion tensor. We demonstrate its
capabilities by investigating the ultrafast acoustic deformations close to the
edge of a single-crystalline graphite membrane. Tracking the structural
distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an
acoustic membrane breathing mode with spatially modulated amplitude, governed
by the optical near field structure at the membrane edge. Furthermore, an
in-plane polarized acoustic shock wave is launched at the membrane edge, which
triggers secondary acoustic shear waves with a pronounced spatio-temporal
dependency. The experimental findings are compared to numerical acoustic wave
simulations in the continuous medium limit, highlighting the importance of
microscopic dissipation mechanisms and ballistic transport channels
Spatial mapping of band bending in semiconductor devices using in-situ quantum sensors
Band bending is a central concept in solid-state physics that arises from
local variations in charge distribution especially near semiconductor
interfaces and surfaces. Its precision measurement is vital in a variety of
contexts from the optimisation of field effect transistors to the engineering
of qubit devices with enhanced stability and coherence. Existing methods are
surface sensitive and are unable to probe band bending at depth from surface or
bulk charges related to crystal defects. Here we propose an in-situ method for
probing band bending in a semiconductor device by imaging an array of
atomic-sized quantum sensing defects to report on the local electric field. We
implement the concept using the nitrogen-vacancy centre in diamond, and map the
electric field at different depths under various surface terminations. We then
fabricate a two-terminal device based on the conductive two-dimensional hole
gas formed at a hydrogen-terminated diamond surface, and observe an unexpected
spatial modulation of the electric field attributed to a complex interplay
between charge injection and photo-ionisation effects. Our method opens the way
to three-dimensional mapping of band bending in diamond and other
semiconductors hosting suitable quantum sensors, combined with simultaneous
imaging of charge transport in complex operating devices.Comment: This is a pre-print of an article published in Nature Electronics.
The final authenticated version is available online at
https://dx.doi.org/10.1038/s41928-018-0130-
Surface versus bulk characterization of the electronic inhomogeneity in a VO_{2} film
We investigated the inhomogeneous electronic properties at the surface and
interior of VO_{2} thin films that exhibit a strong first-order metal-insulator
transition (MIT). Using the crystal structural change that accompanies a VO_{2}
MIT, we used bulk-sensitive X-ray diffraction (XRD) measurements to estimate
the fraction of metallic volume p^{XRD} in our VO_{2} film. The temperature
dependence of the p was very closely correlated with the dc
conductivity near the MIT temperature, and fit the percolation theory
predictions quite well: (p - p_{c})^{t} with t = 2.00.1
and p_{c} = 0.160.01. This agreement demonstrates that in our VO
thin film, the MIT should occur during the percolation process. We also used
surface-sensitive scanning tunneling spectroscopy (STS) to investigate the
microscopic evolution of the MIT near the surface. Similar to the XRD results,
STS maps revealed a systematic decrease in the metallic phase as temperature
decreased. However, this rate of change was much slower than the rate observed
with XRD, indicating that the electronic inhomogeneity near the surface differs
greatly from that inside the film. We investigated several possible origins of
this discrepancy, and postulated that the variety in the strain states near the
surface plays an important role in the broad MIT observed using STS. We also
explored the possible involvement of such strain effects in other correlated
electron oxide systems with strong electron-lattice interactions.Comment: 27 pages and 7 figure
Development of suspended thermoreflectance technique and its application in thermal property measurement of semiconductor materials
Doctor of PhilosophyDepartment of Mechanical and Nuclear EngineeringGurpreet SinghThis dissertation details the development of a new scientific tool for the thermal characterization of freestanding micro/nano-scale materials, with specific application to thin films. The tool consists of a custom-designed and calibrated opto-electric system with superior spatial and temporal resolutions in thermal measurement. The tool, termed as Suspended ThermoReflectance (STR), can successfully perform thermal mappings at the submicron level and is able to produce unconstrained thermal conductivity unlike other optical measurement techniques where independent conductivity measurement is not possible due to their reliance on heat capacity. STR works by changing the temperature of a material and collecting the associated change in light reflection from multiple points on the sample surface. The reflection is a function of the material being tested, the wavelength of the probe light and the composition of the specimen for transparent and quasi-transparent materials. Coupling the change in reflection, along the sample’s length, with the knowledge of heat conduction allows for the determination of the thermal properties of interest. A thermal analytical model is developed and incorporated with optical equations to characterize the conductivity of thin films. The analytical model is compared with a finite element model to check its applicability in the STR experiment and data analysis. Ultimately, thermal conductivity of 2 µm and 3 µm thick Si samples were determined using STR at a temperature range of 20K – 350K and compared to literature as a validation of the technique.
The system was automated using a novel LabView-based program. This program allowed the user to control the equipment including electronics, optics and optical cryostat. Moreover, data acquisition and real-time monitoring of the system are also accomplished through this computer application.
A description of the development, fabrication and characterization of the freestanding thin films is detailed in this dissertation. For the most part, the thin films were fabricated using standard microfabrication techniques. However, different dry and wet etching techniques were compared for minimum surface roughness to reduce light scattering. The best etching technique was used to trim the Si films for the desired thicknesses. Besides, vapor HF was used to avoid stiction-failure during the release of suspended films
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