| 1. |
- Edwards, N.V., et al.
(författare)
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Optical characterization of wide bandgap semiconductors
- 2000
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Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 364:1, s. 98-106
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Tidskriftsartikel (refereegranskat)abstract
- Our work primarily concerns the characterization of wide-gap III-V nitride semiconductors, nondestructively and at variable temperature, with spectroscopic ellipsometry (SE) and reflectometry in the spectral range from 1.5 to 6 eV. In the case of GaN, there are three main concerns associated with such data: (a) the quantification of the dispersion of the index of refraction with energy, (b) the removal of surface overlayers in real-time, and (c) the determination of the variation of valence bands with biaxial stress and the quantification of residual stress in thin films. The SE and reflectance capabilities provide (1) broadband spectra from 1.5 to 6 eV, which yield information about (a) below the bandgap and (b) above it, and (2) high resolution spectra (less than 1 meV at 3.4 eV) in the vicinity of the gap (3.3-3.6 eV), which enables (c). Here we will discuss issues concerning the relation of (c) to GaN material and growth parameters, though similar data for other wide bandgap materials will be discussed where relevant. Specifically, optimal heterostructure design for potential valence band engineering applications will be discussed in the context of trends in residual stress as a function of film thickness, growth temperature and substrate orientation for GaN/AlN/6H-SiC heterostructures. Standard heterostructures are mostly compressive for samples less than about 0.7 µm thick, are tensile up to about 2 µm and then abruptly become less tensile with stress values near 1 kbar thereafter. Additionally, these trends can be circumvented for moderately thick (approximately 2 µm) GaN layers (normally>2 kbar, tensile) by the introduction of a `buried interface' approach, namely, a strain mediating layer (SML) above the standard high-temperature AlN buffer layer designed to yield a range of compressive stresses from 0 to 2 kbar. The strain characteristics but also the growth rates of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML. This is achieved by in situ techniques during crystal growth without degrading the optical and structural properties of the deposited layer, as confirmed by XRD, SEM, PL, and AFM data taken on the overlying GaN layers. These results are interpreted in terms of coefficient of thermal expansion data for the layers and data concerning the planarization of GaN layers and growth behavior in non-(0001) directions.
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| 2. |
- Meinicke, H., et al.
(författare)
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Tumour-associated changes in intestinal epithelial cells cause local accumulation of KLRG1(+) GATA3(+) regulatory T cells in mice
- 2017
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Ingår i: Immunology. - : Wiley. - 0019-2805. ; 152:1, s. 74-88
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Tidskriftsartikel (refereegranskat)abstract
- CD4(+) Foxp3(+) regulatory T (Treg) cells include differentiated populations of effector Treg cells characterized by the expression of specific transcription factors. Tumours, including intestinal malignancies, often present with local accumulation of Treg cells that can prevent tumour clearance, but how tumour progression leads to Treg cell accumulation is incompletely understood. Here using genetically modified mouse models we show that ablation of E-cadherin, a process associated with epithelial to mesenchymal transition and tumour progression, promotes the accumulation of intestinal Treg cells by the specific accumulation of the KLRG1(+) GATA3(+) Treg subset. Epithelial E-cadherin ablation activates the -catenin pathway, and we find that increasing -catenin signals in intestinal epithelial cells also boosts Treg cell frequencies through local accumulation of KLRG1(+) GATA3(+) Treg cells. Both E-cadherin ablation and increased -catenin signals resulted in epithelial cells with higher levels of interleukin-33, a cytokine that preferentially expands KLRG1(+) GATA3(+) Treg cells. Tumours often present reduced E-cadherin expression and increased -catenin signalling and interleukin-33 production. Accordingly, Treg cell accumulation in intestinal tumours from APC(min/+) mice was exclusively due to the increase in KLRG1(+) GATA3(+) Treg cells. Our data identify a novel axis through which epithelial cells control local Treg cell subsets, which may be activated during intestinal tumorigenesis.
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