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

Probing Transition Metal Dichalcogenides via Strain-Tuned and Polarization-Resolved Optical Spectroscopy

The strong light-matter interaction in the atomically thin transition metal dichalcogenides (TMDCs) has allowed the use of optical spectroscopy to investigate these materials in great depth. It has been shown that optoelectronic properties of ultrathin TMDCs are remarkably different from their bulk counterparts. Among them, this dissertation focuses on ultrathin MoTe2 (molybdenum ditelluride) and ReS2 (rhenium disulfide). We first introduce the fundamental properties of the two material systems, MoTe2 and ReS2, investigated in this dissertation. Specific experimental methods for optical spectroscopy of 2D materials, 2D sample preparation, and related optics calculations are presented. Absorption and photoluminescence measurements are applied to demonstrate that semiconducting MoTe2, an indirect band gap bulk material, acquires a direct band gap in the monolayer limit. Furthermore, strain-tuned optical spectroscopy on MoTe2 shows that tensile strain can significantly redshift its optical gap and partially suppress the intervalley exciton-phonon scattering. This suppression results in a narrowing of the near-band excitonic transitions. We also discuss the effect of strain on the transport properties of MoTe2 due to this reduction in scattering. We investigate monolayer ReS2 as a TMDC system exhibiting strong in-plane anisotropy. These properties are explored by polarization-resolved spectroscopy. We show how the accessible optical properties vary with optical polarization. We find that the near-band excitons in ultrathin ReS2, absorb and emit light along specific polarizations. We also show that purely non-contact, optical techniques can determine the crystallographic orientation of ReS2

Silicon-basedI nanostructures: Growth and Characterizations of Si2Te3 nanowires and nanoplates

Silicon-basedI nanostructures: Growth and Characterizations of Si2Te3 nanowires and nanoplate

Characteristics of nanocomposites and semiconductor heterostructure wafers using THz spectroscopy

All optical, THz-Time Domain Spectroscopic (THz-TDS) methods were employed towards determining the electrical characteristics of Single Walled Carbon Nanotubes, Ion Implanted Si nanoclusters and Si1-xGex HFO2, SiO2 on p-type Si wafers. For the nanoscale composite materials, Visible Pump/THz Probe spectroscopy measurements were performed after observing that the samples were not sensitive to the THz radiation alone. The results suggest that the photoexcited nanotubes exhibit localized transport due to Lorentz-type photo-induced localized states from 0.2 to 0.7THz. The THz transmission is modeled through the photoexcited layer with an effective dielectric constant described by a Drude + Lorentz model and given by Maxwell-Garnett theory. Comparisons are made with other prevalent theories that describe electronic transport. Similar experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in fused silica for which a similar behavior was observed. In addition, a change in reflection from Si1-xGex on Si, 200mm diameter semiconductor heterostructure wafers with 10% or 15% Ge content, was measured using THz-TDS methods. Drude model is utilized for the transmission/reflection measurements and from the reflection data the mobility of each wafer is estimated. Furthermore, the effect of high-K dielectric material (HfO2) on the electrical properties of p-type silicon wafers was characterized by utilizing non-contact, differential (pump-pump off) spectroscopic methods to differ between HfO2 and SiO2 on Si wafers. The measurements are analyzed in two distinct transmission models, where one is an exact representation of the layered structure for each wafer and the other assumed that the response observed from the differential THz transmission was solely due to effects from interfacial traps between the dielectric layer and the substrate. The latter gave a more accurate picture of the carrier dynamics. From these measurements the effect of interfacial defects on transmission and mobility are quantitatively discussed

Spin dynamics in van der Waals magnetic systems

The discovery of atomic monolayer magnetic materials has stimulated intense research activities in the two-dimensional (2D) van der Waals (vdW) materials community. The field is growing rapidly and there has been a large class of 2D vdW magnetic compounds with unique properties, which provides an ideal platform to study magnetism in the atomically thin limit. In parallel, based on tunneling magnetoresistance and magneto-optical effect in 2D vdW magnets and their heterostructures, emerging concepts of spintronic and optoelectronic applications such as spin tunnel field-effect transistors and spin-filtering devices are explored. While the magnetic ground state has been extensively investigated, reliable characterization and control of spin dynamics play a crucial role in designing ultrafast spintronic devices. Ferromagnetic resonance (FMR) allows direct measurements of magnetic excitations, which provides insight into the key parameters of magnetic properties such as exchange interaction, magnetic anisotropy, gyromagnetic ratio, spin–orbit coupling, damping rate, and domain structure. In this review article, we present an overview of the essential progress in probing spin dynamics of 2D vdW magnets using FMR techniques. Given the dynamic nature of this field, we focus mainly on broadband FMR, optical FMR, and spin-torque FMR, and their applications in studying prototypical 2D vdW magnets. We conclude with the recent advances in laboratory- and synchrotron-based FMR techniques and their opportunities to broaden the horizon of research pathways into atomically thin magnets

Recent Progress on Exciton Polaritons in Layered Transition‐Metal Dichalcogenides

Exciton polaritons (EPs) are half‐light, half‐matter quasiparticles formed due to the coupling between photons and excitons in semiconductors. Their uniqueness lies at the strong light–matter interactions and long‐distance transport, thus promising for many novel applications in photonics, information, and quantum technologies. Recently, EPs in group‐VI transition‐metal dichalcogenides (TMDs) have attracted a lot of research interest due to their room‐temperature stability, long‐distance propagation, and controllability through electric gating, valley‐selective optical pumping, and precise thickness control. In this progress report, recent studies of EPs in TMDs are reviewed, highlighting their key properties and functionalities, and then discussing the potential directions for future research

Transport and coupling of phonons, electrons, and magnons in complex materials

In nanoscale systems, in which the relevant length scales can be comparable to the mean free paths and wavelengths of the energy, charge and spin carriers, it is necessary to examine the microscopic transport of heat, spin and charge at the atomic scale and the quantization of the associated quasiparticles. The intricacies of the transport dynamics can be even more complicated in materials with atomic scale complexities, such as incommensurate crystals, magnetic materials, and quasi-one-dimensional systems. Meanwhile, the transport properties and coupling between these quasiparticles is important in determining the strength of various thermoelectric and spincaloritronic phenomena, as well as the reliability of nanoscale electronics. This work seeks to further the understanding of the complicated transport dynamics in complex structured materials at nanometer and micrometer length scales, and to address some of the fundamental questions about the interactions between energy, charge and spin carriers in the conducting polymer poly(3,4- ethylenedioxythiophene) (PEDOT), the incommensurate higher manganese silicide (HMS) thermoelectric material, and the magnetic insulator yttrium iron garnet (YIG). These questions are addressed through a number of combined experimental approaches through the use of thermal conductance and thermoelectric property measurements of suspended nanostructures, inelastic neutron scattering, Brillouin light scattering, and electron microscopy. According to in-plane thermal and thermoelectric transport measurements of PEDOT thin films, the electronic thermal conductivity of this conducting polymer is found to be significant and exceeds that predicted by the Wiedemann-Franz law for metals. Furthermore, thermoelectric transport measurements of suspended HMS nanoribbons show a reduction in the lattice thermal conductivity by approximately a factor of two compared to bulk HMS, which is qualitatively consistent with that predicted from a diffuson model for thermal conductivity derived from the phonon dispersion of HMS. Lastly, pressure dependent Brillouin light scattering spectroscopy is used to determine the influence of hydrostatic stress on the dispersions of magnons and phonons in YIG, in order to determine the magnon and phonon peak frequency shift associated with localized laser heating induced strain.Mechanical Engineerin