202 research outputs found
Band anticrossing effects in highly mismatched semiconductor alloys
The first five chapters of this thesis focus on studies of band anticrossing (BAC) effects in highly electronegativity- mismatched semiconductor alloys. The concept of bandgap bowing has been used to describe the deviation of the alloy bandgap from a linear interpolation. Bowing parameters as large as 2.5 eV (for ZnSTe) and close to zero (for AlGaAs and ZnSSe) have been observed experimentally. Recent advances in thin film deposition techniques have allowed the growth of semiconductor alloys composed of significantly different constituents with ever- improving crystalline quality (e.g., GaAs{sub 1-x}N{sub x} and GaP{sub 1-x}N{sub x} with x {approx}< 0.05). These alloys exhibit many novel and interesting properties including, in particular, a giant bandgap bowing (bowing parameters > 14 eV). A band anticrossing model has been developed to explain these properties. The model shows that the predominant bowing mechanism in these systems is driven by the anticrossing interaction between the localized level associated with the minority component and the band states of the host. In this thesis I discuss my studies of the BAC effects in these highly mismatched semiconductors. It will be shown that the results of the physically intuitive BAC model can be derived from the Hamiltonian of the many-impurity Anderson model. The band restructuring caused by the BAC interaction is responsible for a series of experimental observations such as a large bandgap reduction, an enhancement of the electron effective mass, and a decrease in the pressure coefficient of the fundamental gap energy. Results of further experimental investigations of the optical properties of quantum wells based on these materials will be also presented. It will be shown that the BAC interaction occurs not only between localized states and conduction band states at the Brillouin zone center, but also exists over all of k-space. Finally, taking ZnSTe and ZnSeTe as examples, I show that BAC also occurs between localized states and the valence band states. Soft x-ray fluorescence experiments provide direct evidence of the BAC interaction in these systems. In the final chapter of the thesis, I describe and summarize my studies of optical properties of wurtzite InN and related alloys. Early studies performed on InN films grown by sputtering techniques suggested a direct bandgap of {approx}1.9 eV for this semiconductor. Very recently, high-quality InN films with much higher mobility have become available by using the molecular beam epitaxy growth method. Optical experiments carried out on these samples reveal a narrow bandgap for InN of 0.77 eV, much lower than the previously accepted value. Optical properties of InGaN and InAlN ternaries on the In rich side have also been characterized and are found to be consistent with the narrow bandgap of InN. The bandgap bowing parameters in these alloys were determined. In the context of these findings, the bandgap energies of InGaN and InAlN were found to cover a wide spectral range from the infrared for InN to the ultraviolet for GaN and deep ultraviolet for AlN. The significance of this work is rooted in many important applications of nitride semiconductors in optoelectronics and solar energy conversion devices
Thermodynamics of Strained Vanadium Dioxide Single Crystals
Vanadium dioxide undergoes a metal-insulator transition, in which the strain condition plays an important role. To investigate the strain contribution, a phenomenological thermodynamic potential for the vanadium dioxide single crystal was constructed. The transformations under the uniaxial stress, wire, and thin film boundary conditions were analyzed, and the corresponding phase diagrams were constructed. The calculated phase diagrams agree well with existing experimental data, and show that the transformation temperature (and Curie temperature) strongly depends on the strain condition
Ultrafast Charge Transfer in Atomically Thin MoS2/WS2 Heterostructures
Van der Waals heterostructures have recently emerged as a new class of
materials, where quantum coupling between stacked atomically thin
two-dimensional (2D) layers, including graphene, hexagonal-boron nitride, and
transition metal dichalcogenides (MX2), give rise to fascinating new phenomena.
MX2 heterostructures are particularly exciting for novel optoelectronic and
photovoltaic applications, because 2D MX2 monolayers can have an optical
bandgap in the near-infrared to visible spectral range and exhibit extremely
strong light-matter interactions. Theory predicts that many stacked MX2
heterostructures form type-II semiconductor heterojunctions that facilitate
efficient electron-hole separation for light detection and harvesting. Here we
report the first experimental observation of ultrafast charge transfer in
photo-excited MoS2/WS2 heterostructures using both photoluminescence mapping
and femtosecond (fs) pump-probe spectroscopy. We show that hole transfer from
the MoS2 layer to the WS2 layer takes place within 50 fs after optical
excitation, a remarkable rate for van der Waals coupled 2D layers. Such
ultrafast charge transfer in van der Waals heterostructures can enable novel 2D
devices for optoelectronics and light harvesting
Pressurizing Field-Effect Transistors of Few-Layer MoS2 in a Diamond Anvil Cell
Hydrostatic pressure applied using diamond anvil cells (DAC) has been widely
explored to modulate physical properties of materials by tuning their lattice
degree of freedom. Independently, electrical field is able to tune the
electronic degree of freedom of functional materials via, for example, the
field-effect transistor (FET) configuration. Combining these two orthogonal
approaches would allow discovery of new physical properties and phases going
beyond the known phase space. Such experiments are, however, technically
challenging and have not been demonstrated. Herein, we report a feasible
strategy to prepare and measure FETs in a DAC by lithographically patterning
the nanodevices onto the diamond culet. Multiple-terminal FETs were fabricated
in the DAC using few-layer MoS2 and BN as the channel semiconductor and
dielectric layer, respectively. It is found that the mobility, conductance,
carrier concentration, and contact conductance of MoS2 can all be significantly
enhanced with pressure. We expect that the approach could enable unprecedented
ways to explore new phases and properties of materials under coupled
mechano-electrostatic modulation.Comment: 15 pages, 5 figure
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