148 research outputs found
Vacancy defects in epitaxial thin film CuGaSe2 and CuInSe2
Epitaxial thin film CuGaSe2 and CuInSe2 samples grown on GaAs substrates with varying [Cu]/[Ga,In] ratios were studied using positron annihilation Doppler-broadening spectroscopy and were compared to bulk crystals. We find both Cu monovacancies and Cu-Se divacancies in CuInSe2, whereas, in CuGaSe2, the only observed vacancy defect is the Cu-Se divacancy.Peer reviewe
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Infrared emission bands and thermal effects for 440-nm-emitting GaN-based laser diodes
Broad emission bands due to defects in (In,Ga,Al)N laser diodes operating at 440 nm are investigated using continuous-wave and pulsed currents. In addition to known yellow-green and short-wave infrared bands, defect emissions were observed even in the medium-wave infrared range. A separation from thermal radiation is possible. When using pulsed currents, a super-linearly increasing emission occurs at ∼1150 nm, which could be attributed to amplified spontaneous emission mainly due to the electroluminescence of deep defects in the optically active region. These results may be useful in interpreting the output power bottleneck of GaN-based lasers compared to mature GaAs-based lasers. © 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.514380
Spectroscopic signatures of a bandwidth-controlled Mott transition at the surface of 1T-TaSe
High-resolution angle-resolved photoemission (ARPES) data show that a
metal-insulator Mott transition occurs at the surface of the quasi-two
dimensional compound TaSe. The transition is driven by the narrowing of the
Ta band induced by a temperature-dependent modulation of the atomic
positions. A dynamical mean-field theory calculation of the spectral function
of the half-filled Hubbard model captures the main qualitative feature of the
data, namely the rapid transfer of spectral weight from the observed
quasiparticle peak at the Fermi surface to the Hubbard bands, as the
correlation gap opens up.Comment: 4 pages, 4 figures; one modified figure, added referenc
A complementary neutron and anomalous x-ray diffraction study
Distinguishing the scattering contributions of isoelectronic atomic species by
means of conventional x-ray- and/or electron diffraction techniques is a
difficult task. Such a problem occurs when determining the crystal structure
of compounds containing different types of atoms with equal number of
electrons. We propose a new structural model of Cu(InxGa1−x)3Se5 which is
valid for the entire compositional range of the CuIn3Se5–CuGa3Se5 solid
solution. Our model is based on neutron and anomalous x-ray diffraction
experiments. These complementary techniques allow the separation of scattering
contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly
identically in monoenergetic x-ray diffraction experiments. We have found that
CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification
exhibits a modified stannite structure (space group I4¯2m). Different
occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies
have been found at three different cationic positions of the structure
(Wyckoff sites 2a, 2b, and 4d) depending on the composition of the compound.
Significantly, Cu+ does not occupy the 2b site for the In-free compound, but
does for the In-containing case. Structural parameters, including lattice
constants, tetragonal distortions, and occupation factors are given for
samples covering the entire range of the CuIn3Se5–CuGa3Se5 solid solution. At
the light of the result, the denotation of Cu-poor 1:3:5 compounds as
chalcopyrite-related materials is only valid in reference to their
composition
Desempenho agronômico de canola em diferentes espaçamentos entre linhas e densidades de plantas
Wafer-Scale Epitaxial Modulation of Quantum Dot Density
Precise control of the properties of semiconductor quantum dots (QDs) is
vital for creating novel devices for quantum photonics and advanced
opto-electronics. Suitable low QD-density for single QD devices and experiments
are challenging to control during epitaxy and are typically found only in
limited regions of the wafer. Here, we demonstrate how conventional molecular
beam epitaxy (MBE) can be used to modulate the density of optically active QDs
in one- and two- dimensional patterns, while still retaining excellent quality.
We find that material thickness gradients during layer-by-layer growth result
in surface roughness modulations across the whole wafer. Growth on such
templates strongly influences the QD nucleation probability. We obtain density
modulations between 1 and 10 QDs/ and periods ranging from several
millimeters down to at least a few hundred microns. This novel method is
universal and expected to be applicable to a wide variety of different
semiconductor material systems. We apply the method to enable growth of
ultra-low noise QDs across an entire 3-inch semiconductor wafer
Wafer-scale epitaxial modulation of quantum dot density
Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-densities for single QD devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/µm2 and periods ranging from several millimeters down to at least a few hundred microns. This method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer
Miniband-related 1.4–1.8 μm luminescence of Ge/Si quantum dot superlattices
The luminescence properties of highly strained, Sb-doped Ge/Si multi-layer heterostructures with incorporated Ge quantum dots (QDs) are studied. Calculations of the electronic band structure and luminescence measurements prove the existence of an electron miniband within the columns of the QDs. Miniband formation results in a conversion of the indirect to a quasi-direct excitons takes place. The optical transitions between electron states within the miniband and hole states within QDs are responsible for an intense luminescence in the 1.4–1.8 µm range, which is maintained up to room temperature. At 300 K, a light emitting diode based on such Ge/Si QD superlattices demonstrates an external quantum efficiency of 0.04% at a wavelength of 1.55 µm
MicroRNA-21 targets tumor suppressor genes ANP32A and SMARCA4
MicroRNA-21 (miR-21) is a key regulator of oncogenic processes. It is significantly elevated in the majority of human tumors and functionally linked to cellular proliferation, survival and migration. In this study, we used two experimental-based strategies to search for novel miR-21 targets. On the one hand, we performed a proteomic approach using two-dimensional differential gel electrophoresis (2D-DIGE) to identify proteins suppressed upon enhanced miR-21 expression in LNCaP human prostate carcinoma cells. The tumor suppressor acidic nuclear phosphoprotein 32 family, member A (ANP32A) (alias pp32 or LANP) emerged as the most strongly downregulated protein. On the other hand, we applied a mathematical approach to select correlated gene sets that are negatively correlated with primary-miR-21 (pri-miR-21) expression in published transcriptome data from 114 B-cell lymphoma cases. Among these candidates, we found tumor suppressor SMARCA4 (alias BRG1) together with the already validated miR-21 target, PDCD4. ANP32A and SMARCA4, which are both involved in chromatin remodeling processes, were confirmed as direct miR-21 targets by immunoblot analysis and reporter gene assays. Furthermore, knock down of ANP32A mimicked the effect of enforced miR-21 expression by enhancing LNCaP cell viability, whereas overexpression of ANP32A in the presence of high miR-21 levels abrogated the miR-21-mediated effect. In A172 glioblastoma cells, enhanced ANP32A expression compensated for the effects of anti-miR-21 treatment on cell viability and apoptosis. In addition, miR-21 expression clearly increased the invasiveness of LNCaP cells, an effect also seen in part upon downregulation of ANP32A. In conclusion, these results suggest that downregulation of ANP32A contributes to the oncogenic function of miR-21
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