Alumni Dissertations and Theses

• SYSTEMATIC CONTROL OF SCHOTTKY BARRIER HEIGHT BY PARTISAN INTERLAYERS

Author:
Yang Li
Year of Dissertation:
2012
Program:
Physics
Raymond Tung
Abstract:

The relationship between the starting surface structure and the Schottky barrier height (SBH) in metal-silicon systems has been investigated to explore the possibility of modifying the interface dipole through the insertion of an inorganic interface layer. A systematic and comprehensible way to perform this modification has been introduced as a "partisan interlayer" method and was extensively studied for a variety of interlayer elements, along with several choices for the metal. Employing elements with a larger electronegativity than that of silicon, a monolayer of As, S, or Cl was deposited on Si surfaces and processed to form stable surface structures. The electron affinities of these surfaces were measured by Kelvin probe and found to increase significantly from the clean surface, consistent with the expected charge transfer from Si to the adsorbates and also in agreement with results of ab initio density functional theory calculations. Subsequent deposition of metal on these adsorbate terminated semiconductor (ATS) surfaces led to the fabrication of metastable interface structures with the SBH successfully and significantly modified in a predictable manner from the clean Si results. The chemical stability of these surfaces that weakens the interaction with the deposited metal, likely leads to the preservation of electric dipole from such partisan interlayers. The partisan interlayer method was found to work particularly well with As-terminated Si(111) surface, on which a interface behavior parameter exceeding 0.50 was found. This exceeded the S-parameter usually observed for covalent semiconductors such as Si, ~ 0.1, and highlighted a major reason for the adjustability of the SBH by the PI method. The SBH of all interfaces studied in this work was inhomogeneous. Making use of the theory of electronic transport through inhomogeneous SBH and temperature-dependent measurements, the extent of the SBH nonuniformity was routinely characterized from the Schottky diodes. The largest adjustment in the SBH was observed for Au on S-terminated Si(100), where the n-type junction became nearly perfectly ohmic. It was demonstrated, for the first time, that quantitative information on the distribution of the SBH and the lateral size of conduction patches ("hot spots") could still be obtained from ohmic junctions. The physical basis for these analyses and the special experimental conditions which enabled these analyses were carefully explained. The implications of these results for SBH control of MS systems in general and the understanding of the formation of SBH in general are also discussed.

• Strongly-correlated 2D Electron Systems in Si-MOSFETs

Author:
Shiqi Li
Year of Dissertation:
2015
Program:
Physics
Myriam Sarachik
Abstract:

Si-MOSFETs are basic building blocks of present-day integrated circuits. Above a threshold gate voltage, a layer of two-dimensional electrons is induced near the silicon-silicon dioxide interface of a Si-MOSFET. According to theory for noninteracting and weakly interacting electrons, no metallic state can exist in two dimensions in zero magnetic field in the limit of zero temperature. However, in strongly interacting electron systems the observation of a resistivity that changes from metallic to insulating temperature dependence has fueled a debate over whether this signals a quantum phase transition to a metallic phase in two dimensions. In this thesis I will present the results of two detailed experimental studies performed on high mobility Si-MOSFET samples. In the first study, we find the thermopower of this low-disorder, strongly interacting 2D electron system in silicon diverges at a finite disorder-independent density, providing evidence that this IS a transition to a new phase at low densities. For the second study, we conducted measurements on I-V characteristics as well as the AC voltage generated by the sample in the insulating phase. Nonlinear I-V characteristics observed in the insulating phase have been attributed to the presence of an additional conduction channel due to a sliding electron solid (Wigner crystal). We seek to provide evidence for the presence of a zero-field Wigner solid by detecting the noise generated by the sliding crystallites.

• Order and asymmetry in jammed systems

Author:
Zhusong Li
Year of Dissertation:
2015
Program:
Physics
Mark Shattuck
Abstract:

Granular matter is composed of particles that are big enough that thermal effects may be neglected. We studied both granular flow and granular statics using numerical simulation. In granular flow, we simulated 2D granular flow in a hopper. A hopper is a container with an opening at the bottom. Simulated disks are placed in the hopper with the bottom closed and then released. We developed a new tangential force model to simulate hopper flow, that matches experiments and shows that the output flux is proportional to the bottom opening size to the 3/2 power. We also see clogging or jamming and estimate the jamming probability. We applied our force model to a 2D rotating drum simulation and studied the statistics of avalanches. In many systems, from earthquakes to plastic deformations the avalanche size probability F(I) is a power-law in avalanche size I. We find F(I) ~ I^(-1.29) for our system. We also find that the scaled average avalanche shape is parabolic as predicted by mean-field theoretical models. In many systems, we would like to measure the degree of crystallization. Q6 is common order metric used to detect hexagonal symmetry, and it works very well in mono-disperse systems. However, when we consider bi-disperse and poly-disperse systems, and other non-hexagonal lattices, Q6 is not as useful. We developed a new order metric, Voronoi entropy, based on Voronoi tessellation and information theory. The main idea of the Voronoi entropy is to detect and quantify unique Voronoi polyhedrons. Voronoi entropy can successfully find lattice order in bi-disperse crystal and other structures like simple cubic, body-centered-cubic, for which Q6 is not sensitive. We performed molecular dynamics (MD) simulations of binary Lennard-Jones systems to model the crystallization process during heating and cooling protocols in metallic glasses. We measured the minimum cooling rate Rc to crystallize a liquid and the minimum heating rate Rh to crystallize a glass formed prepared using a fast quench at rate Rp. We find: (1) Rh > Rc in all systems. (2) The asymmetry ratio Rh /Rc increases as Rc increases. (3) The critical heating rate Rh (Rp) has an intrinsic contribution Rh* and protocol-dependent contribution Rh − Rh* that increases with decreasing preparation cooling rates Rp. We show all of these findings are in agreement with classical nucleation theory.

• Spontaneous Time-Reversal Symmetry Breaking in Two Dimensional Electronic Systems

Author:
Wei Liu
Year of Dissertation:
2014
Program:
Physics
Alexander Punnoose
Abstract:

The discovery of high temperature superconductivity inspired a number of novel proposals, one of which, put forward by C.M.Varma, involves the breaking of time-reversal symmetry to explain the physics of the underdoped pseudogap phase. It was proposed that time-reversal symmetry is spontaneously broken as a result of strong repulsion between the Cu-O electrons to form loop-currents in the system. In this work, we developed a general theory to study the quantum phase transitions in the 2 dimensional strongly interacting electronic systems in which time-reversal symmetry is spontaneously broken in the ground state. We first applied the theory of magnetic groups to identify electronic current-loop patterns in two physically relevant systems: (i) 2-band model involving spinless electrons on a honeycomb lattice with next-nearest-neighbor interactions; (ii) 3-band $CuO_{2}$ model with and without lattice distortions. Next, by examining the correlation function within the standard ring and ladder Dyson series approximations, we identify the effective Hamiltonian with the relevant interactions responsible for creating low-energy fluctuations near the quantum critical point. The mean-field analysis of this effective Hamiltonian elucidated the fact that time-reversal symmetry breaking in a 2-band model is in the same universality class as the interband particle-hole pair condensation instability which occurs in the semi-conductors under large enough particle-hole attraction. Using Hubbard-Stratonovich transformation and functional integral method, we are able to investigate this instability and the static susceptibility in the condensed phase both in half-filling and doped case. Away from half-filling, because the condensates are metallic and couple to the gapless collective intraband particle-hole excitations, we find that the static susceptibility is generically negative as a result of this coupling, which implies that the condensates are unstable.

• Phase locking of solid-state laser arrays

Author:
LIPING LIU
Year of Dissertation:
2010
Program:
Physics
Ying-Chih Chen
Abstract:

This thesis reports a study of phase locking in solid-state laser arrays of a variety of configurations, including a 2x2 Nd:YVO4 continuous-wave laser array, a two-element passively Q-switched Nd,Cr:YAG laser array, a two-element continuous-wave ytterbium fiber laser array, and a two-element ytterbium fiber lasers passively Q-switched by stimulated Brillouin scattering. Phase locking is accomplished by coupling the lasing elements into a common Fourier-transform resonator, with the lasing elements placed at one focal plane of a converging lens and the output mirror placed at the other focal plane. The control of the relative phase among the elements is done by placing a spatial filter in front of the output mirror to introduce different modal losses. We have succeeded in achieving highly stable phase-locked operation in the in-phase mode in continuous-wave laser arrays. The fringe visibility of the phase-locked beam is nearly 1. As the coupling strength decreases, the transition from phase locked to unlocked mode is abrupt. Phase locking of nano-second pulsed laser arrays requires a tight control of the resonance frequencies and path lengths of the individual lasing elements to ensure the pulses generated by the individual elements to occur simultaneously. As the difference in path lengths increase or the coupling strengths decrease, the transition from the phase locked to the unlocked states is characterized by a gradual loss of coincidence of the pulses from the individual elements and a reduction in the fringe contrast in the combined laser beam. The current approach of phase locking has high efficiency and is applicable to two-dimensional laser arrays containing a large number of elements.

• Control of Exciton Photon Coupling in Nano-structures

Author:
Xiaoze Liu
Year of Dissertation:
2014
Program:
Physics
Vinod Menon
Abstract:

In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley polarization has also been discussed in this MoS2 microcavity for the possible applications in spin switches and logic gates. The direct dipole-dipole type excitonic interactions have also been studied in inorganic-organic nanocomposites, where ZnO nanowire is taken as the inorganic constituent and NTCDA thin films as the organic constituent. The excitonic interactions can be classified into weak coupling regime and strong coupling regime. Forster Resonant Energy Transfer (FRET), which is in the weak coupling regime, has been observed in this hybrid system. The optimized optical nonlinearity has also been determined in the hybrid system via Z-scan measurements. Exciton-plasmon polariton, another example of strongly coupled state which results from the interaction between excitons and plasmons when they are in resonance, has also been investigated in this thesis. Two rhodamine dyes spincoated on the silver thin films have separately been observed to be strongly coupled to the surface plasmon modes. With observed new polariton states, energy transfer mechanism has been discussed for nonlinear optical applications.

• Systematic Tuning of Silicon Schottky Barrier Height by Atomic Interlayers with Low Electronegativities

Author:
Wei Long
Year of Dissertation:
2012
Program:
Physics
Raymond Tung
Abstract:

The Schottky barrier height (SBH) is of great importance to the functionality of semiconductor devices, as it governs the carrier transport across the metal-semiconductor (MS) interface. The presence of the Fermi level (FL) pinning phenomena makes tuning the SBH a difficult goal to achieve. The technique of "partisan interlayer" (PI) was proposed recently to modify the SBH, where stable adsorbate-terminated semiconductor (ATS) surfaces were used to form SBs with subsequently applied metal. When elements with large electronegativities were used to form the ATS, the PI technique was effective in reducing the n-type SBH and increasing the p-type SBH, driven by the expected transfer of charge from the semiconductor to the adsorbates. In this thesis work, elements with electronegativities smaller than that of the semiconductor are used as surface termination. SBHs for Ag, Au and In on Si surfaces are found to increase for the n-type and decrease for the p-type interfaces, by as much as 0.25eV, when Ga, Mg and K are used to terminate the Si surfaces. The present results are thus in agreement with the expected charge transfers from elements with smaller electronegativities to silicon and illustrate the general validity of the PI technique. The chemical stability of these surfaces likely weakens the MS interaction and leads to the (partial) preservation of the surface dipole at the MS interface. However, large degrees of SBH inhomogeneity are observed for diodes on these surfaces, likely due to insufficient stability of these surfaces to completely withstand metal interaction. These results are discussed within the basic models of SBH formation and the implications of these results for SBH control of MS systems are also addressed.

• Proton Pumping in Cytochrome c Oxidase

Author:
Jianxun Lu
Year of Dissertation:
2015
Program:
Physics
Marilyn Gunner
Abstract:

Cytochrome c oxidase (CcO) is a large trans-membrane protein, which is the final enzyme in the respiratory electron transport chain in mitochondria or aerobic bacte- ria. It implements proton pumping through the mitochondrial membrane against the electrochemical gradient, by utilizing the chemical energy released by reducing O2 to water. The active site of the chemical reaction is called the Binuclear Center (BNC) that is made up of heme a3, CuB, a Tyrosine residue and their ligands. The protein is reduced four times by electron from cytochromes c to reduce O2 and to generate four different BNC redox states step by step. In each reduction step a proton is delivered to the BNC and another proton is pumped across the protein to increase the trans-membrane proton gradient. In CcO, the pumped proton is firstly located in the proton loading site (PLS), and then is released out of the protein. In these processes, a high conserved Glutamate residue, plays an essential role on the proton translocation either to the BNC or the PLS. In this thesis, Multi-Conformational Continuum Electrostatics (MCCE) and Molecular Dynamics (MD) are combined to study the proton affinity (pKa) of the high conserved Glutamate residue and the identity of the PLS. This Glutamate residue is located in a hydrophobic cavity in the protein, and the simulations show that the hydration of the cavity is controlled by the protonation state of the propionic acid of heme a3, a group on the proton outlet pathway. The changes in hydration and elec- trostatic interactions lower the proton affinity by at least 5 kcal/mol. The identity of the residues in the PLS is another open question in CcO research, and various groups above the BNC have been considered as candidates. We designed a new model for the simulation via separating the catalytic cycle into smaller substates and monitoring the charge of all residues in the protein. The results demonstrates the PLS is a cluster rather than a single residue, and the proton affinity of the heme a3 propionic acids primarily determines the number of protons loaded into the PLS.

• Exploring Non-Equilibrium Dynamics in Time Dependent Density Functional Theory

Author:
Kai Luo
Year of Dissertation:
2015
Program:
Physics
Neepa Maitra
Abstract:

Time-dependent density functional theory(TDDFT) is a method of choice for calulations of excitation spectra and response properties in materials science and quantum chemistry. The many-body problem is mapped into a set of one-body Schr\"odinger equations, called the Kohn-Sham(KS) equations. In principle, the one-body potential can be chosen such that the density of the interacting system is exactly reproduced by the KS system. However, one component of the one-body potential has to be approximated and is typically adiabatic". Though in linear response regime adiabatic approximations give quite good spectra, it is important to explore their performances in non-equilibrium dynamics. \noindent In this thesis, I will present the results of the explorations on non-equilibrium dynamics in TDDFT. For the first study, a decomposition of exact exchange-correlation potential into kinetic and interaction components is derived. We compare the components with that of adiabatic" counterparts in non-perturbative dynamics and find that the interaction component is less poorly approximated adiabatically than the kinetic component. A salient feature is that step structures generically appear, of relevance in the second study. We prove that the step structures only appear in the non-linear response regime. We find an exact condition which is typically violated by the approximations in use today. Spuriously time-dependent spectra in TDDFT can be explained and we find that the more the condition is violated the worse the dynamics is. In last, we envision that orbital functionals are able to incorporate the memory effects and compensate the deficiencies of the adiabatic" approximations.

• Hybridized criticality and elementary excitations in LiHoF4

Author:
Haifu Ma
Year of Dissertation:
2015
Program:
Physics
Abstract:

In this dissertation, I study the magnetic properties of LiHoF4. Quantum criticality in rare earth ferromagnet LiHoF4 is complicated by the presence of strong crystal field and hyperfine interactions resulting, e.g., in incomplete mode softening reported by Ronnow etal. We construct a systematic framework for treating elementary excitations in this material across the phase diagram. These excitations interpolate between purely electronic, nuclear and lattice modes and exhibit two-types of quantum critical softening, both complete (as anticipated by elementary treatments, see e.g. Sachdev) but also incomplete, in close correspondence with nuclear scattering results.