Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We ...employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a 1-10/(110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocation cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. This report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis.
HgTe-based colloidal quantum dots (CQDs) fabricated between 10 nm and 20 nm in size readily lead to infrared cutoff wavelengths between 3
μ
m and 12
μ
m, due to their quantum confinement. In ...previous work, infrared photodetection using these films has been demonstrated to detect radiation out to a wavelength of 12
μ
m, and imaging in the mid-wave infrared region. In this work, a complete focal plane array and imager was fabricated and its performance measured for detecting radiation out to 12
μ
m. The photoconductive and optical properties of these HgTe CQD films are described, along with recent advancements in CQD detector technology. Anticipated improvements in the CQD synthesis and film deposition chemistries and techniques can raise the specific detectivity of these CQD films, bringing them closer to room-temperature operation.
Low-cost silicon-based alternative substrates are an attractive choice for next-generation large-area high-resolution multicolor infrared (IR) detector arrays. However, the high density of ...dislocations formed during molecular-beam epitaxy growth of HgCdTe/CdTe/Si limits the performance of IR arrays, especially in the long-wavelength infrared (LWIR) region. Atomic hydrogen introduced by inductively coupled plasma (ICP) into HgCdTe is expected to passivate dislocations, bulk and surface defects, removing their contributions to dark current. Passivation using ICP hydrogenation can have different effects on HgCdTe photodiode performance, depending on which class of defects is being passivated. The infrared measurement analyzer (IRMA) was used to deconvolute the effects of hydrogenation on LWIR HgCdTe photodiodes through a reverse-modeling fit of the current–voltage (
I
–
V
) characteristic. This approach results in a fit with fewer false minima, low parameter error and bias, and high confidence in extracted device parameters. A description of this tool and its application to hydrogenated HgCdTe LWIR detectors is presented. Lower dark currents have been observed after hydrogenation of fully fabricated devices. Model-fits performed on a wide variety of LWIR HgCdTe photodiodes suggest that hydrogenation provides both surface and bulk quality improvements. These benefits of ICP hydrogenation have been retained over several months.
Dislocation cores have long dominated the electronic and optical behaviors of semiconductor devices and detailed atomic characterization is required to further explore their effects. Miniaturization ...of semiconductor devices to nanometre scale also puts emphasis on a material's mechanical properties to withstand failure due to processing or operational stresses. Sessile junctions of dislocations provide barriers to propagation of mobile dislocations and may lead to work‐hardening. The sessile Lomer–Cottrell and Hirth lock dislocations, two stable lowest elastic energy stair‐rods, are studied in this paper. More specifically, using atomic resolution high‐angle annular dark‐field imaging and atomic‐column‐resolved X‐ray spectrum imaging in an aberration‐corrected scanning transmission electron microscope, dislocation core structures are examined in zinc‐blende CdTe. A procedure is outlined for atomic scale analysis of dislocation junctions which allows determination of their identity with specially tailored Burgers circuits and also formation mechanisms of the polar core structures based on Thompson's tetrahedron adapted to reactions of polar dislocations as they appear in CdTe and other zinc‐blende solids. Strain fields associated with the dislocations calculated via geometric phase analysis are found to be diffuse and free of `hot spots' that reflect compact structures and low elastic energy of the pure‐edge stair‐rods.
CdTe is a material well-suited to solar cell applications due to its 1.5 eV direct bandgap and high optical absorption. To meet energy demands, CdTe solar cells must be produced at a low-cost and ...with high throughput which often demands the use of non-ideal polycrystalline CdTe. As a result of careful process control, current thin-film poly-CdTe cells have been shown to be somewhat defect tolerant with proven industry success. Yet despite this success poly-CdTe cells are still far from their predicted Shockley-Queisser theoretical limits. The next generation cells must demonstrate higher open-circuit voltages, fill factors, and longer minority carrier lifetimes. Playing a major role in doping, defect migration, carrier recombination, and current transport are 2D extended defects both within grains and between grains as grain boundaries (GBs). A further understanding of these defects is needed which exhibit either high symmetry such as the CSL structures or those mixed or random GBs with low symmetry. Their corresponding formation and electronic behavior will be needed to develop methods to mitigate their effects and instead promote higher doping with less minority carrier recombination. Predictions and guidance on electronic and thermodynamic properties can be obtained from model atomic structures within the framework of ab-initio density-functional theory. Bulk point defect formation energies were determined for comparison to calculations of point defects along GB structures. Model atomic structures of GBs can also be created rapidly and over a wide parameter space using the Grain Boundary Genie code developed for this project. Commonly observed low-angle and special coincident grain boundaries structures were created and a subset relaxed to determine their local strain environment and interfacial energy with for comparison to STEM observations. Additionally, a series of random angle or ‘mixed’ grain boundaries were created and investigated corresponding to possible interfaces between grains that cannot be observed in STEM.
Concerns about acquisition of antibiotic resistance have led to increasing demand for new antimicrobial therapies. OligoG CF-5/20 is an alginate oligosaccharide previously shown to have antimicrobial ...and antibiotic potentiating activity. We investigated the structural modification of the bacterial cell wall by OligoG CF-5/20 and its effect on membrane permeability. Binding of OligoG CF-5/20 to the bacterial cell surface was demonstrated in Gram-negative bacteria. Permeability assays revealed that OligoG CF-5/20 had virtually no membrane-perturbing effects. Lipopolysaccharide (LPS) surface charge and aggregation were unaltered in the presence of OligoG CF-5/20. Small angle neutron scattering and circular dichroism spectroscopy showed no substantial change to the structure of LPS in the presence of OligoG CF-5/20, however, isothermal titration calorimetry demonstrated a weak calcium-mediated interaction. Metabolomic analysis confirmed no change in cellular metabolic response to a range of osmolytes when treated with OligoG CF-5/20. This data shows that, although weak interactions occur between LPS and OligoG CF-5/20 in the presence of calcium, the antimicrobial effects of OligoG CF-5/20 are not related to the induction of structural alterations in the LPS or cell permeability. These results suggest a novel mechanism of action that may avoid the common route in acquisition of resistance via LPS structural modification.
High throughput experiments using a molecular recognition probe reveal a simple relationship between solvent functional group concentration and selective solvation. The 1 : 1 association constant for ...the H-bonding interaction between tri-
n
-butylphosphine oxide and 4-phenylazophenol was measured in 1088 different alkane-ether mixtures using a UV-Vis plate reader. Although the stability of the complex decreased with increasing concentration of the more polar ether cosolvent as expected, the results show that it is the functional group composition rather than the constitution of the solvent molecules or the properties of the bulk liquid that determines the solvation thermodynamics. Thus the solvent properties of a simple ether can be reproduced by an appropriate mixture of a polyether and an alkane that has the same net concentration of ether oxygen functional groups. The results suggest that solvation may be understood at the molecular level simply by considering the polarities and the concentrations of the functional groups present in the solvent, because these are the parameters that affect local solvation interactions with the solutes.
High throughput experiments using a molecular recognition probe reveal a simple relationship between solvent functional group concentration and selective solvation.
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