14 research outputs found
Chemical and Electronic Structure of Surfaces and Interfaces in Compound Semiconductors
The interface formation between two different materials is important in applications for optoelectronic devices. Often, the success or performance of these devices is dependent on the formation of these heterojunctions. In this work, the surface and interfaces in such materials for optoelectronic devices are investigated by a suite of X-ray analytical techniques including X-ray photoelectron (XPS), X-ray excited Auger electron (XAES), and X-ray emission (XES) spectroscopies to provide novel insight.
For the group III-nitrides (e.g., AlxGa1-xN) used in many light emitting devices, a significant challenge exists to form an Ohmic contact. The electron affinities and band gaps of GaN and AlN are different, and thus it is difficult to find one contact scheme compatible for the entire AlxGa1-xN system. Contact schemes are empirically derived such that they result in optimal electrical properties, and thus this work focuses on providing a deeper understanding of the empirically derived contact-schemes. For the ndoped alloys, the presence of VN was identified at the V-AlxGa1-xN interface after contact formation. The amount of VN present varied for n-GaN and n-AlN, and was indicative of the VN dependency of the n-AlxGa1-xN composition. These findings provide detailed insight into the contact formation of (Al,Ga)N-based devices and the performance of V-based contacts.
Next generation thin film solar cells based on CdS/Cu(In,Ga)Se2 and CdTe/CdS heterojunctions, which are expected to replace the current Si-based technologies within a decade, are constantly driven to improve their device efficiencies. However, to optimize the entire device, the interfaces and layers within such a device must be understood. The interface formation between high-efficiency Cu(In,Ga)Se2 absorbers and CdS buffer layer was followed, and the findings suggest the presence of a S-containing interlayer between Cu(In,Ga)Se2 and CdS. For CdTe/CdS solar cells, post-absorber deposition processing (CdCl2 activation and back contact treatment) is necessary. The findings demonstrate that the CdCl2 activation drives the sulfur atoms from the CdS layer towards the back contact. While both of the processing steps influence the morphology of the back contact, the spectroscopic results suggest that the CdCl2 activation has a larger impact on the surface and interface composition involved in CdTe solar cells.
The surface and interface structure are complex in these optoelectronic devices, and they are expected to influence the electrical properties (and thus performance) of the final device. The goal of this dissertation is to provide new insight and physical explanations which could aid in future optimization and designs of heterojunctions
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Photonics Research and Development
During the period August 2005 through October 2009, the UNLV Research Foundation (UNLVRF), a non-profit affiliate of the University of Nevada, Las Vegas (UNLV), in collaboration with UNLVâs Colleges of Science and Engineering; Boston University (BU); Oak Ridge National Laboratory (ORNL); and Sunlight Direct, LLC, has managed and conducted a diverse and comprehensive research and development program focused on light-emitting diode (LED) technologies that provide significantly improved characteristics for lighting and display applications. This final technical report provides detailed information on the nature of the tasks, the results of the research, and the deliverables. It is estimated that about five percent of the energy used in the nation is for lighting homes, buildings and streets, accounting for some 25 percent of the average homeâs electric bill. However, the figure is significantly higher for the commercial sector. About 60 percent of the electricity for businesses is for lighting. Thus replacement of current lighting with solid-state lighting technology has the potential to significantly reduce this nationâs energy consumption â by some estimates, possibly as high as 20%. The primary objective of this multi-year R&D project has been to develop and advance lighting technologies to improve national energy conversion efficiencies; reduce heat load; and significantly lower the cost of conventional lighting technologies. The UNLVRF and its partners have specifically focused these talents on (1) improving LED technologies; (2) optimizing hybrid solar lighting, a technology which potentially offers the benefits of blending natural with artificial lighting systems, thus improving energy efficiency; and (3) building a comprehensive academic infrastructure within UNLV which concentrates on photonics R&D. Task researchers have reported impressive progress in (1) the development of quantum dot laser emitting diodes (QDLEDs) which will ultimately improve energy efficiency and lower costs for display and lighting applications (UNLV College of Engineering); (2) advancing green LED technology based on the Indium-Gallium-Nitride system (BU), thus improving conversion efficiencies; (3) employing unique state-of-the-art X-ray, electron and optical spectroscopies with microscopic techniques to learn more about the electronic structure of materials and contacts in LED devices (UNLV College of Science); (4) establishing a UNLV Display Lighting Laboratory staffed with a specialized team of academic researchers, students and industrial partners focused on identifying and implementing engineering solutions for lighting display-related problems; and (5) conducting research, development and demonstration for HSL essential to the resolution of technological barriers to commercialization
Zn-Se-Cd-S Interlayer Formation at the CdS/Cuâ‚‚ZnSnSeâ‚„ Thin-Film Solar Cell Interface
The chemical structure of the CdS/Cu2ZnSnSe4 (CZTSe) interface was studied by a combination of electron and X-ray spectroscopies with varying surface sensitivity. We find the CdS chemical bath deposition causes a "redistribution" of elements in the proximity of the CdS/CZTSe interface. In detail, our data suggest that Zn and Se from the Zn-terminated CZTSe absorber and Cd and S from the buffer layer form a Zn-Se-Cd-S interlayer. We find direct indications for the presence of Cd-S, Cd-Se, and Cd-Se-Zn bonds at the buffer/absorber interface. Thus, we propose the formation of a mixed Cd(S,Se)-(Cd,Zn)Se interlayer. We suggest the underlying chemical mechanism is an ion exchange mediated by the amine complexes present in the chemical bath
Self-Assembled Monolayers Impact Cobalt Interfacial Structure in Nanoelectronic Junctions
Controlling and understanding the
interface formation between organic
molecular layers and ferromagnetic materials is a crucial aspect in
the implementation of organic spintronics. Here, the formation of
thiol-containing molecular monolayers on template-stripped cobalt
and oxidized cobalt surfaces is achieved. The successful attachment
and quality of the aliphatic molecular structure and cobalt surface
was followed with X-ray spectroscopic measurements. The self-assembly
of octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA) are contrasted,
finding the self-assembly of the bifunctional molecule profoundly
different than the thiol-alone species. In particular, the MHA-cobalt
surface exhibits a very different interface following self-assembly
of the MHA species. Two different models of the interface formation
are proposed based on the results. The data suggest the MHA/ethanol
removes the cobalt oxide during the self-assembly as the prevailing
model. The impact of this Co/molecule interface on electron transport
through Co/molecule/Si molecular junctions is also discussed. These
results provide insight into <i>ex situ</i> modification
and functionalization of ferromagnetic interfaces impacting an important
aspect of spin-based devices
Site- and Symmetry-Resolved Resonant X-ray Emission Study of a Highly Ordered PTCDA Thin Film
Electrical and Physical Characterization of Bilayer Carboxylic Acid-Functionalized Molecular Layers
We have used flip chip lamination (FCL) to form monolayer
and bilayer
molecular junctions of carboxylic acid-containing molecules with Cu
atom incorporation. Carboxylic acid-terminated monolayers are self-assembled
onto ultrasmooth Au by using thiol chemistry and grafted onto n-type
Si. Prior to junction formation, monolayers are physically characterized
by using polarized infrared absorption spectroscopy, X-ray photoelectron
spectroscopy, and near-edge X-ray absorption fine structure spectroscopy,
confirming the molecular quality and functional group termination.
FCL was used to form monolayer junctions onto H-terminated Si or bilayer
junctions of carboxylic acid monolayers on Au and Si. From the electrical
measurements, we find that the current through the junction is attenuated
as the effective molecular length within the junction increases, indicating
that molecules are electrically active within the junction. We find
that the electronic transport through the bilayer junction saturates
at very thick effective distances possibly because of another electron-transport
mechanism that is not nonresonant tunneling as a result of trapped
defects or sequential tunneling. In addition, bilayer junctions are
fabricated with and without Cu atoms, and we find that the electron
transport is not distinguishably different when Cu atoms are within
the bilayer
Correction to Redox-Active Molecular Nanowire Flash Memory for High-Endurance and High-Density Nonvolatile Memory Applications
Redox-Active Molecular Nanowire Flash Memory for High-Endurance and High-Density Nonvolatile Memory Applications
In
this work, high-performance top-gated nanowire molecular flash
memory has been fabricated with redox-active molecules. Different
molecules with one and two redox centers have been tested. The flash
memory has clean solid/molecule and dielectric interfaces, due to
the pristine molecular self-assembly and the nanowire device self-alignment
fabrication process. The memory cells exhibit discrete charged states
at small gate voltages. Such multi-bit memory in one cell is favorable
for high-density storage. These memory devices exhibit fast speed,
low power, long memory retention, and exceptionally good endurance
(>10<sup>9</sup> cycles). The excellent characteristics are derived
from the intrinsic charge-storage properties of the protected redox-active
molecules. Such multi-bit molecular flash memory is very attractive
for high-endurance and high-density on-chip memory applications in
future portable electronics
Attachment of a Diruthenium Compound to Au and SiO<sub>2</sub>/Si Surfaces by “Click” Chemistry
Fabrication of electrodes with functionalized
properties is of
interest in many electronic applications with the surface impacting
the electrical and electronic properties of devices. We report the
formation of molecular monolayers containing a redox-active dirutheniumÂ(II,III)
compound to gold and silicon surfaces via “click” chemistry.
The use of Cu-catalyzed azide–alkyne cycloaddition enables
modular design of molecular surfaces and interfaces and allows for
a variety of substrates to be functionalized. Attachment of the diruthenium
compound is monitored by using infrared and photoelectron spectroscopies.
The highest occupied molecular (or system) orbital of the “clicked-on”
diruthenium is clearly seen in the photoemission measurements and
is mainly attributed to the presence of the Ru atoms. The “click”
attachment is robust and provides a route to investigate the evolution
of the electronic structure and properties of novel molecules attached
to a variety of electrodes. The ability to attach this redox-active
Ru molecule onto SiO<sub>2</sub> and Au surfaces is important for
the development of functional molecular devices such as charge-based
memory devices
Interface Engineering To Control Magnetic Field Effects of Organic-Based Devices by Using a Molecular Self-Assembled Monolayer
Organic semiconductors hold immense promise for the development of a wide range of innovative devices with their excellent electronic and manufacturing characteristics. Of particular interest are nonmagnetic organic semiconductors that show unusual magnetic field effects (MFEs) at small subtesla field strength that can result in substantial changes in their optoelectronic and electronic properties. These unique phenomena provide a tremendous opportunity to significantly impact the functionality of organic-based devices and may enable disruptive electronic and spintronic technologies. Here, we present an approach to vary the MFEs on the electrical resistance of organic-based systems in a simple yet reliable fashion. We experimentally modify the interfacial characteristics by adding a self-assembled monolayer between the metal electrode and the organic semiconductor, thus enabling the tuning of competing MFE mechanisms coexisting in organic semiconductors. This approach offers a robust method for tuning the magnitude and sign of magnetoÂresistance in organic semiconductors without compromising the ease of processing