Measurement of the GaAs/AlAs valence-band offset from a single quantum well near the Γ-X crossover

An iterative procedure which utilizes the type-I and type-II transitions of a GaAs/AlAs single quantum well is used to determine the GaAs/AlAs valence-band offset. The iteration is based on the fact that the confinement energy is not sensitive to the change of the barrier potential. It has the advantage that knowledge of the thickness of the quantum well is not necessary to determine the valence-band offset. Both the type-I and type-II transitions can be observed in the low-temperature photoluminescence spectrum if the GaAs is thin enough so that the lowest-energy level of the electron in the GaAs well is higher than the X-conduction-band minimum of the adjacent AlAs. We illustrate this procedure with a 35 Å GaAs/AlAs single quantum well, and the valence-band offset is found to be 36% of the Γ-gap difference

Photoluminescence measurements of tensile-strained GaAs/In0.07Al0.93As quantum wells

GaAs/In0.07Al0.93As tensile-strained quantum wells were grown on [001] GaAs substrates using molecular-beam epitaxy. The incorporation of tensile strain is made possible by preparing a 1-μm-thick In0.07Al0.93As relaxed buffer which is followed by the growth of quantum wells. The strain of the GaAs was measured using Raman spectroscopy and photoluminescence. The photoluminescence measurements from wells ranging in thickness from 25 to 100 Å reveal that the observed optical transition originates from the electron-light hole recombination for a 100 Å well and from the electron-heavy hole recombination if the well thickness is less than 40 Å. Therefore, a thick Al-rich InxAl1-x As relaxed buffer on the GaAs substrate can be used to engineer the relative energy position of the light and heavy holes for GaAs-based quantum wells

Mass spectrometry imaging identifies palmitoylcarnitine as an immunological mediator during Salmonella Typhimurium infection

Salmonella Typhimurium causes a self-limiting gastroenteritis that may lead to systemic disease. Bacteria invade the small intestine, crossing the intestinal epithelium from where they are transported to the mesenteric lymph nodes (MLNs) within migrating immune cells. MLNs are an important site at which the innate and adaptive immune responses converge but their architecture and function is severely disrupted during S. Typhimurium infection. To further understand host-pathogen interactions at this site, we used mass spectrometry imaging (MSI) to analyse MLN tissue from a murine model of S. Typhimurium infection. A molecule, identified as palmitoylcarnitine (PalC), was of particular interest due to its high abundance at loci of S. Typhimurium infection and MLN disruption. High levels of PalC localised to sites within the MLNs where B and T cells were absent and where the perimeter of CD169+ sub capsular sinus macrophages was disrupted. MLN cells cultured ex vivo and treated with PalC had reduced CD4+CD25+ T cells and an increased number of B220+CD19+ B cells. The reduction in CD4+CD25+ T cells was likely due to apoptosis driven by increased caspase-3/7 activity. These data indicate that PalC significantly alters the host response in the MLNs, acting as a decisive factor in infection outcome

DNA Methylation of the ABO Promoter Underlies Loss of ABO Allelic Expression in a Significant Proportion of Leukemic Patients

Background: Loss of A, B and H antigens from the red blood cells of patients with myeloid malignancies is a frequent occurrence. Previously, we have reported alterations in ABH antigens on the red blood cells of 55% of patients with myeloid malignancies. Methodology/Principal Findings: To determine the underlying molecular mechanisms of this loss, we assessed ABO allelic expression in 21 patients with ABH antigen loss previously identified by flow cytometric analysis as well as an additional 7 patients detected with ABH antigen changes by serology. When assessing ABO mRNA allelic expression, 6/12 (50%) patients with ABH antigen loss detected by flow cytometry and 5/7 (71%) of the patients with ABH antigen loss detected by serology had a corresponding ABO mRNA allelic loss of expression. We examined the ABO locus for copy number and DNA methylation alterations in 21 patients, 11 with loss of expression of one or both ABO alleles, and 10 patients with no detectable allelic loss of ABO mRNA expression. No loss of heterozygosity (LOH) at the ABO locus was observed in these patients. However in 8/11 (73%) patients with loss of ABO allelic expression, the ABO promoter was methylated compared with 2/10 (20%) of patients with no ABO allelic expression loss (P = 0.03). Conclusions/Significance: We have found that loss of ABH antigens in patients with hematological malignancies is associated with a corresponding loss of ABO allelic expression in a significant proportion of patients. Loss of ABO allelic expression was strongly associated with DNA methylation of the ABO promoter.Tina Bianco-Miotto, Damian J. Hussey, Tanya K. Day, Denise S. O'Keefe and Alexander Dobrovi

Effect of spacer layer thickness on tunneling characteristics in asymmetric AlAs/GaAs/AlAs double barrier structures

We present a first-time study of the effects of variable thickness undoped spacer layers of asymmetric AlAs/GaAs/AlAs double barrier structures on resonant tunneling current-voltage (I-V) characteristics. Although tunneling characteristics are only observed in the negative voltage sweep of the I-V curve, these structures do exhibit increased current density (jp) with increased spacer layer thickness (Lsp) until a threshold thickness of Lsp \u3e 10 nm is reached. Above Lsp = 10 nm, current density decreases. A modified asymmetric double barrier structure with reduced overall emitter-collector layer thickness was also examined. The latter provided insight into sample optimization as more prominent resonant tunneling characteristics were observed in comparison to the original structures. © 1995

Shape transition of InAs islands on InP (111)A

Atomic force microscopy (AFM) reveals that InAs islands grown on InP (111)A, as they grow in size, undergo a shape transition. Below a critical size of around 30 nm, round-shaped quantum dots form, while above this size they grow in the shape of triangles, reflecting the symmetry of the (111) substrates. The edges of triangular islands are aligned along the three equivalent (110) directions of the InP (111) surface. The triangular islands grow laterally much faster than vertically, indicating the aspect ratio decrease of the islands with increasing InAs coverage. Our results provide a better understanding of the self-organization behaviors of InAs on InP (111)A

Observation of ballistic transport in hot-electron vertical fet spectrometer using ultrathin planar-doped barrier launcher

Experimental evidence of ballistic transport by hot electron injection into a GaAs vertical FET channel using an ultrathin planar-doped barrier has been obtained for the first time. The spectroscopy exhibited a narrow energy spread of less than 50meV with an estimated 10% ballistic electrons. © 1991, The Institution of Electrical Engineers. All rights reserved

Measurement of the GaAs/AlAs valence-band offset from a single quantum well near the Γ-X crossover

An iterative procedure which utilizes the type-I and type-II transitions of a GaAs/AlAs single quantum well is used to determine the GaAs/AlAs valence-band offset. The iteration is based on the fact that the confinement energy is not sensitive to the change of the barrier potential. It has the advantage that knowledge of the thickness of the quantum well is not necessary to determine the valence-band offset. Both the type-I and type-II transitions can be observed in the low-temperature photoluminescence spectrum if the GaAs is thin enough so that the lowest-energy level of the electron in the GaAs well is higher than the X-conduction-band minimum of the adjacent AlAs. We illustrate this procedure with a 35 Å GaAs/AlAs single quantum well, and the valence-band offset is found to be 36% of the Γ-gap difference. © 1995 American Institute of Physics

Comparison of InGaAs(100) grown by chemical beam epitaxy and metal organic chemical vapor deposition

Secondary ion mass spectrometry is used to study the effects of substrate temperature on the composition and growth rate of InGaAs/InP(100) multilayers grown by chemical beam epitaxy, metal-organic chemical vapor deposition and solid source molecular beam epitaxy. The growth kinetics of the material grown by the different techniques are analyzed and compared