Matching Items (50)

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Towards high-efficiency thin-film solar cells: from theoretical analysis to experimental exploration

Description

GaAs single-junction solar cells have been studied extensively in recent years, and have reached over 28 % efficiency. Further improvement requires an optically thick but physically thin absorber to provide

GaAs single-junction solar cells have been studied extensively in recent years, and have reached over 28 % efficiency. Further improvement requires an optically thick but physically thin absorber to provide both large short-circuit current and high open-circuit voltage. By detailed simulation, it is concluded that ultra-thin GaAs cells with hundreds of nanometers thickness and reflective back scattering can potentially offer efficiencies greater than 30 %. The 300 nm GaAs solar cell with AlInP/Au reflective back scattering is carefully designed and demonstrates an efficiency of 19.1 %. The device performance is analyzed using the semi-analytical model with Phong distribution implemented to account for non-Lambertian scattering. A Phong exponent m of ~12, a non-radiative lifetime of 130 ns, and a specific series resistivity of 1.2 Ω·cm2 are determined.

Thin-film CdTe solar cells have also attracted lots of attention due to the continuous improvements in their device performance. To address the issue of the lower efficiency record compared to detailed-balance limit, the single-crystalline Cd(Zn)Te/MgCdTe double heterostructures (DH) grown on InSb (100) substrates by molecular beam epitaxy (MBE) are carefully studied. The Cd0.9946Zn0.0054Te alloy lattice-matched to InSb has been demonstrated with a carrier lifetime of 0.34 µs observed in a 3 µm thick Cd0.9946Zn0.0054Te/MgCdTe DH sample. The substantial improvement of lifetime is due to the reduction in misfit dislocation density. The recombination lifetime and interface recombination velocity (IRV) of CdTe/MgxCd1-xTe DHs are investigated. The IRV is found to be dependent on both the MgCdTe barrier height and width due to the thermionic emission and tunneling processes. A record-long carrier lifetime of 2.7 µs and a record-low IRV of close to zero have been confirmed experimentally.

The MgCdTe/Si tandem solar cell is proposed to address the issue of high manufacturing costs and poor performance of thin-film solar cells. The MBE grown MgxCd1-xTe/MgyCd1-yTe DHs have demonstrated the required bandgap energy of 1.7 eV, a carrier lifetime of 11 ns, and an effective IRV of (1.869 ± 0.007) × 103 cm/s. The large IRV is attributed to thermionic-emission induced interface recombination. These understandings can be applied to fabricating the high-efficiency low-cost MgCdTe/Si tandem solar cell.

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Date Created
  • 2015

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Growth and characterization of pyrite thin films for photovoltaic applications

Description

A series of pyrite thin films were synthesized using a novel sequential evaporation

technique to study the effects of substrate temperature on deposition rate and micro-structure of

the deposited material. Pyrite was

A series of pyrite thin films were synthesized using a novel sequential evaporation

technique to study the effects of substrate temperature on deposition rate and micro-structure of

the deposited material. Pyrite was deposited in a monolayer-by-monolayer fashion using

sequential evaporation of Fe under high vacuum, followed by sulfidation at high S pressures

(typically > 1 mTorr to 1 Torr). Thin films were synthesized using two different growth processes; a

one-step process in which a constant growth temperature is maintained throughout growth, and a

three-step process in which an initial low temperature seed layer is deposited, followed by a high

temperature layer, and then finished with a low temperature capping layer. Analysis methods to

analyze the properties of the films included Glancing Angle X-Ray Diffraction (GAXRD),

Rutherford Back-scattering Spectroscopy (RBS), Transmission Electron Microscopy (TEM),

Secondary Ion Mass Spectroscopy (SIMS), 2-point IV measurements, and Hall effect

measurements. Our results show that crystallinity of the pyrite thin film improves and grain size

increases with increasing substrate temperature. The sticking coefficient of Fe was found to

increase with increasing growth temperature, indicating that the Fe incorporation into the growing

film is a thermally activated process.

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Date Created
  • 2014

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Luminescent cyclometalated platinum and palladium complexes with novel photophysical properties

Description

Organic light emitting diodes (OLEDs) is a rapidly emerging technology based on organic thin film semiconductors. Recently, there has been substantial investment in their use in displays. In less than

Organic light emitting diodes (OLEDs) is a rapidly emerging technology based on organic thin film semiconductors. Recently, there has been substantial investment in their use in displays. In less than a decade, OLEDs have grown from a promising academic curiosity into a multi-billion dollar global industry. At the heart of an OLED are emissive molecules that generate light in response to electrical stimulation. Ideal emitters are efficient, compatible with existing materials, long lived, and produce light predominantly at useful wavelengths. Developing an understanding of the photophysical processes that dictate the luminescent properties of emissive materials is vital to their continued development. Chapter 1 and Chapter 2 provide an introduction to the topics presented and the laboratory methods used to explore them. Chapter 3 discusses a series of tridentate platinum complexes. A synthetic method utilizing microwave irradiation was explored, as well as a study of the effects ligand structure had on the excited state properties. Results and techniques developed in this endeavor were used as a foundation for the work undertaken in later chapters. Chapter 4 introduces a series of tetradentate platinum complexes that share a phenoxy-pyridyl (popy) motif. The new molecular design improved efficiency through increased rigidity and modification of the excited state properties. This class of platinum complexes were markedly more efficient than those presented in Chapter 3, and devices employing a green emitting complex of the series achieved nearly 100% electron-to-photon conversion efficiency in an OLED device. Chapter 5 adapts the ligand structure developed in Chapter 4 to palladium. The resulting complexes exceed reported efficiencies of palladium complexes by an order of magnitude. This chapter also provides the first report of a palladium complex as an emitter in an OLED device. Chapter 6 discusses the continuation of development efforts to include carbazolyl components in the ligand. These complexes possess interesting luminescent properties including ultra-narrow emission and metal assisted delayed fluorescence (MADF) emission.

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  • 2014

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Photoluminescence enhancement of Ge quantum dots by exploiting the localized surface plasmon of epitaxial Ag islands

Description

This dissertation presents research findings regarding the exploitation of localized surface plasmon (LSP) of epitaxial Ag islands as a means to enhance the photoluminescence (PL) of Germanium (Ge) quantum dots

This dissertation presents research findings regarding the exploitation of localized surface plasmon (LSP) of epitaxial Ag islands as a means to enhance the photoluminescence (PL) of Germanium (Ge) quantum dots (QDs). The first step of this project was to investigate the growth of Ag islands on Si(100). Two distinct families of Ag islands have been observed. “Big islands” are clearly faceted and have basal dimensions in the few hundred nm to μm range with a variety of basal shapes. “Small islands” are not clearly faceted and have basal diameters in the 10s of nm range. Big islands form via a nucleation and growth mechanism, and small islands form via precipitation of Ag contained in a planar layer between the big islands that is thicker than the Stranski-Krastanov layer existing at room-temperature.

The pseudodielectric functions of epitaxial Ag islands on Si(100) substrates were investigated with spectroscopic ellipsometry. Comparing the experimental pseudodielectric functions obtained for Si with and without Ag islands clearly identifies a plasmon mode with its dipole moment perpendicular to the surface. This observation is confirmed using a simulation based on the thin island film (TIF) theory. Another mode parallel to the surface may be identified by comparing the experimental pseudodielectric functions with the simulated ones from TIF theory. Additional results suggest that the LSP energy of Ag islands can be tuned from the ultra-violet to the infrared range by an amorphous Si (α-Si) cap layer.

Heterostructures were grown that incorporated Ge QDs, an epitaxial Si cap layer and Ag islands grown atop the Si cap layer. Optimum growth conditions for distinct Ge dot ensembles and Si cap layers were obtained. The density of Ag islands grown on the Si cap layer depends on its thickness. Factors contributing to this effect may include the average strain and Ge concentration on the surface of the Si cap layer.

The effects of the Ag LSP on the PL of Ge coherent domes were investigated for both α-Si capped and bare Ag islands. For samples with low-doped substrates, the LSPs reduce the Ge dot-related PL when the Si cap layer is below some critical thickness and have no effect on the PL when the Si cap layer is above the critical thickness. For samples grown on highly-doped wafers, the LSP of bare Ag islands enhanced the PL of Ge QDs by ~ 40%.

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Date Created
  • 2015

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Growth and characterization of novel thin films for microelectronic applications

Description

I studied the properties of novel Co2FeAl0.5Si0.5 (CFAS), ZnGeAs2, and FeS2 (pyrite) thin films for microelectronic applications ranging from spintronic to photovoltaic. CFAS is a half metal with theoretical spin

I studied the properties of novel Co2FeAl0.5Si0.5 (CFAS), ZnGeAs2, and FeS2 (pyrite) thin films for microelectronic applications ranging from spintronic to photovoltaic. CFAS is a half metal with theoretical spin polarization of 100%. I investigated its potential as a spin injector, for spintronic applications, by studying the critical steps involved in the injection of spin polarized electron populations from tunnel junctions containing CFAS electrodes. Epitaxial CFAS thin films with L21 structure and saturation magnetizations of over 1200 emu/cm3 were produced by optimization of the sputtering growth conditions. Point contact Andreev reflection measurements show that the spin polarization at the CFAS electrode surface exceeds 70%. Analyses of the electrical properties of tunnel junctions with a superconducting Pb counter-electrode indicate that transport through native Al oxide barriers is mostly from direct tunneling, while that through the native CFAS oxide barriers is not. ZnGeAs2 is a semiconductor comprised of only inexpensive and earth-abundant elements. The electronic structure and defect properties are similar in many ways to GaAs. Thus, in theory, efficient solar cells could be made with ZnGeAs2 if similar quality material to that of GaAs could be produced. To understand the thermochemistry and determine the rate limiting steps of ZnGeAs2 thin-film synthesis, the (a) thermal decomposition rate and (b) elemental composition and deposition rate of films were measured. It is concluded that the ZnGeAs2 thin film synthesis is a metastable process with an activation energy of 1.08±0.05 eV for the kinetically-limited decomposition rate and an evaporation coefficient of ~10-3. The thermochemical analysis presented here can be used to predict optimal conditions of ZnGeAs2 physical vapor deposition and thermal processing. Pyrite (FeS2) is another semiconductor that has tremendous potential for use in photovoltaic applications if high quality materials could be made. Here, I present the layer-by-layer growth of single-phase pyrite thin-films on heated substrates using sequential evaporation of Fe under high-vacuum followed by sulfidation at S pressures between 1 mTorr and 1 Torr. High-resolution transmission electron microscopy reveals high-quality, defect-free pyrite grains were produces by this method. It is demonstrated that epitaxial pyrite layer was produced on natural pyrite substrates with this method.

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Date Created
  • 2013

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ZnTe nanostructural synthesis for electronic and optoelectronic devices

Description

Zinc telluride (ZnTe) is an attractive II-VI compound semiconductor with a direct

bandgap of 2.26 eV that is used in many applications in optoelectronic devices. Compared

to the two dimensional (2D) thin-film

Zinc telluride (ZnTe) is an attractive II-VI compound semiconductor with a direct

bandgap of 2.26 eV that is used in many applications in optoelectronic devices. Compared

to the two dimensional (2D) thin-film semiconductors, one-dimensional (1D)

nanowires can have different electronic properties for potential novel applications.

In this work, we present the study of ZnTe nanowires (NWs) that are synthesized

through a simple vapor-liquid-solid (VLS) method. By controlling the presence or

the absence of Au catalysts and controlling the growth parameters such as growth

temperature, various growth morphologies of ZnTe, such as thin films and nanowires

can be obtained. The characterization of the ZnTe nanostructures and films was

performed using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy

(EDX), high- resolution transmission electron microscope (HRTEM), X-ray

diffraction (XRD), photoluminescence (PL), Raman spectroscopy and light scattering

measurement. After confirming the crystal purity of ZnTe, two-terminal diodes and

three-terminal transistors were fabricated with both nanowire and planar nano-sheet

configurations, in order to correlate the nanostructure geometry to device performance

including field effect mobility, Schottky barrier characteristics, and turn-on

characteristics. Additionally, optoelectronic properties such as photoconductive gain

and responsivity were compared against morphology. Finally, ZnTe was explored in

conjunction with ZnO in order to form type-II band alignment in a core-shell nanostructure.

Various characterization techniques including scanning electron microscopy,

energy-dispersive X-ray spectroscopy , x-ray diffraction, Raman spectroscopy, UV-vis

reflectance spectra and photoluminescence were used to investigate the modification

of ZnO/ZnTe core/shell structure properties. In PL spectra, the eliminated PL intensity

of ZnO wires is primarily attributed to the efficient charge transfer process

occurring between ZnO and ZnTe, due to the band alignment in the core/shell structure. Moreover, the result of UV-vis reflectance spectra corresponds to the band

gap energy of ZnO and ZnTe, respectively, which confirm that the sample consists of

ZnO/ZnTe core/shell structure of good quality.

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Date Created
  • 2017

Synthesis and in situ characterization of nanostructured and amorphous metallic films

Description

Nanocrystalline (nc) thin films exhibit a wide range of enhanced mechanical properties compared to their coarse-grained counterparts. Furthermore, the mechanical behavior and microstructure of nc films is intimately related.

Nanocrystalline (nc) thin films exhibit a wide range of enhanced mechanical properties compared to their coarse-grained counterparts. Furthermore, the mechanical behavior and microstructure of nc films is intimately related. Thus, precise control of the size, aspect ratio and spatial distribution of grains can enable the synthesis of thin films with exceptional mechanical properties. However, conventional bottom-up techniques for synthesizing thin films are incapable of achieving the microstructural control required to explicitly tune their properties. This dissertation focuses on developing a novel technique to synthesize metallic alloy thin films with precisely controlled microstructures and subsequently characterizing their mechanical properties using in situ transmission electron microscopy (TEM). Control over the grain size and distribution was achieved by controlling the recrystallization process of amorphous films by the use of thin crystalline seed layers. The novel technique was used to manipulate the microstructure of structural (TiAl) and functional (NiTi) thin films thereby exhibiting its capability and versatility. Following the synthesis of thin films with tailored microstructures, in situ TEM techniques were employed to probe their mechanical properties. Firstly, a novel technique was developed to measure local atomic level elastic strains in metallic glass thin films during in situ TEM straining. This technique was used to detect structural changes and anelastic deformation in metallic glass thin films. Finally, as the electron beam (e-beam) in TEMs is known to cause radiation damage to specimen, systematic experiments were carried out to quantify the effect of the e-beam on the stress-strain response of nc metals. Experiments conducted on Al and Au films revealed that the e-beam enhances dislocation activity leading to stress relaxation.

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  • 2017

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Study of doped magnetic systems

Description

Doping and alloying agents are commonly used to engineer the properties of magnetic materials. This study investigates the effects of doping manganese in thin films of Ni80Fe20 (permalloy) and Ni65Fe15Co20

Doping and alloying agents are commonly used to engineer the properties of magnetic materials. This study investigates the effects of doping manganese in thin films of Ni80Fe20 (permalloy) and Ni65Fe15Co20 magnetic systems for low power memory technologies, including those that operate at low temperature.

Elemental manganese is anti-ferromagnetic with a Neel temperature of 100 K. When used as a dopant in a magnetic material, it is found to often align its moment in an antiferromagnetic direction. Thus, the addition of manganese might be expected to reduce the overall saturation magnetization (MS) of the magnetic system. In this study, we show that the use of manganese dopants in Ni80Fe20 (permalloy) and Ni65Fe15Co20 thin films can reduce their saturation magnetization and still retain excellent switching properties.

Magnetic properties and transport properties were determined using Vibrating Sample Magnetometer. A 19% decrease in the MS of (Ni80Fe20)1-xMnx thin films and a 36% decrease for (Ni65Fe15Co20)1-xMnx thin films for dopant levels of x = 30%. The impact of depositing a ruthenium (Ru) under-layer for (Ni65Fe15Co20)1-xMnx system was also studied.

The structural (lattice parameters and phases), surface (roughness and topography) and electrical properties (resistivity and mean free path) of the Mn-doped Ni65Fe15Co20 films were determined with X-Ray Diffraction, Atomic Force Microscopy and Four-Point probe technique respectively.

The properties were analyzed and Ni65Fe15Co20 system with Ru- under-layer with 20 at. % Mn content was found to exhibit the following low-field switching properties at 10 K; MS~700 emu.cm-3, easy axis coercivity ~10 Oe and hard axis coercivity ~5 Oe, easy axis squareness ~0.9 and anisotropy field ~12 Oe, that are deemed useful for low-power memory applications that could be used at cryogenic temperatures.

To determine the transport properties thought these magnetic layers for use in superconductor/ferromagnetic memory structures, a study of the oxidation conditions of Al films was performed in order to produce a reliable aluminum oxide tunnel barrier on top of these films. The production of N-I-F-S (Normal metal-Insulator-Ferromagnet-Superconductor) tunnel junctions will allow for the investigation of the tunneling density of states as a function of ferromagnetic layer thickness, allowing for the determination of important transport parameters relevant to magnetic barrier Josephson junction devices.

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Date Created
  • 2018

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Heterojunction and nanostructured photovoltaic device: theory and experiment

Description

A primary motivation of research in photovoltaic technology is to obtain higher efficiency photovoltaic devices at reduced cost of production so that solar electricity can be cost competitive. The majority

A primary motivation of research in photovoltaic technology is to obtain higher efficiency photovoltaic devices at reduced cost of production so that solar electricity can be cost competitive. The majority of photovoltaic technologies are based on p-n junction, with efficiency potential being much lower than the thermodynamic limits of individual technologies and thereby providing substantial scope for further improvements in efficiency. The thesis explores photovoltaic devices using new physical processes that rely on thin layers and are capable of attaining the thermodynamic limit of photovoltaic technology. Silicon heterostructure is one of the candidate technologies in which thin films induce a minority carrier collecting junction in silicon and the devices can achieve efficiency close to the thermodynamic limits of silicon technology. The thesis proposes and experimentally establishes a new theory explaining the operation of silicon heterostructure solar cells. The theory will assist in identifying the optimum properties of thin film materials for silicon heterostructure and help in design and characterization of the devices, along with aiding in developing new devices based on this technology. The efficiency potential of silicon heterostructure is constrained by the thermodynamic limit (31%) of single junction solar cell and is considerably lower than the limit of photovoltaic conversion (~ 80 %). A further improvement in photovoltaic conversion efficiency is possible by implementing a multiple quasi-fermi level system (MQFL). A MQFL allows the absorption of sub band gap photons with current being extracted at a higher band-gap, thereby allowing to overcome the efficiency limit of single junction devices. A MQFL can be realized either by thin epitaxial layers of alternating higher and lower band gap material with nearly lattice matched (quantum well) or highly lattice mismatched (quantum dot) structure. The thesis identifies the material combination for quantum well structure and calculates the absorption coefficient of a MQFl based on quantum well. GaAsSb (barrier)/InAs(dot) was identified as a candidate material for MQFL using quantum dot. The thesis explains the growth mechanism of GaAsSb and the optimization of GaAsSb and GaAs heterointerface.

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Date Created
  • 2011

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An in situ Surface Stress Study of Electrochemical Phenomena: Electrodeposition and Molecular Adsorption

Description

Over the last decade copper electrodeposition has become the dominant process by which microelectronic interconnects are made. Replacing ultra-high vacuum evaporative film growth, the technology known as the Cu damascene

Over the last decade copper electrodeposition has become the dominant process by which microelectronic interconnects are made. Replacing ultra-high vacuum evaporative film growth, the technology known as the Cu damascene process has been widely implemented in the microelectronics industry since the early 2000s. The transition from vacuum film growth to electrodeposition was enabled by solution chemistries that provide "bottom-up" or superfilling capability of vias and trenches. While the process has been and is used widely, the actual mechanisms responsible for superfilling remain relatively unknown. This dissertation presents and discusses the background and results of experimental investigations that have been done using in situ electrochemical surface stress monitoring techniques to study the evolution of stress on Cu{111} thin film electrodes. Because of its extreme sensitivity to the structure on both the electrode and solution sides of the interface, surface stress monitoring as analytical technique is well suited for the study of electrodeposition. These ultra-high resolution stress measurements reveal the dynamic response of copper electrodes to a number of electrochemical and chemical experimental variables. In the case of constant current pulsed deposition and stripping, the surface stress evolution depends not only on the magnitude of the current pulse, but also shows a marked response to plating bath composition. The plating bath chemistries used in this work include (1) additive free, (2) deposition suppressing solutions that include polyethylene glycol (PEG) and sodium chloride (NaCl) as well as (3) full additive solution combinations which contain PEG, NaCl, and a one of two deposition accelerating species (bis-(sodiumsulfopropyl)disulfide (SPS) or mercaptopropane sulfonic acid (MPS)). The development of thin film stress is further investigated through a series of solution exchange experiments that correlate the magnitude of electrode exchange current density and the stress state of the film. Remarkably, stress changes as large as ~8.5 N/m are observed during solution exchanges at the open circuit potential. Overall, this research demonstrates that solution chemistry can have a large impact on thin film stress evolution, even for very small deposition thicknesses (e.g. <10 ML) or in the absence of net addition or removal of material from the electrode.

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Date Created
  • 2011