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| 4. |
- Callaway, EM, et al.
(författare)
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A multimodal cell census and atlas of the mammalian primary motor cortex
- 2021
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 1476-4687 .- 0028-0836. ; 598:7879, s. 86-102
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Tidskriftsartikel (refereegranskat)abstract
- Here we report the generation of a multimodal cell census and atlas of the mammalian primary motor cortex as the initial product of the BRAIN Initiative Cell Census Network (BICCN). This was achieved by coordinated large-scale analyses of single-cell transcriptomes, chromatin accessibility, DNA methylomes, spatially resolved single-cell transcriptomes, morphological and electrophysiological properties and cellular resolution input–output mapping, integrated through cross-modal computational analysis. Our results advance the collective knowledge and understanding of brain cell-type organization1–5. First, our study reveals a unified molecular genetic landscape of cortical cell types that integrates their transcriptome, open chromatin and DNA methylation maps. Second, cross-species analysis achieves a consensus taxonomy of transcriptomic types and their hierarchical organization that is conserved from mouse to marmoset and human. Third, in situ single-cell transcriptomics provides a spatially resolved cell-type atlas of the motor cortex. Fourth, cross-modal analysis provides compelling evidence for the transcriptomic, epigenomic and gene regulatory basis of neuronal phenotypes such as their physiological and anatomical properties, demonstrating the biological validity and genomic underpinning of neuron types. We further present an extensive genetic toolset for targeting glutamatergic neuron types towards linking their molecular and developmental identity to their circuit function. Together, our results establish a unifying and mechanistic framework of neuronal cell-type organization that integrates multi-layered molecular genetic and spatial information with multi-faceted phenotypic properties.
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| 6. |
- Jacob, AP, et al.
(författare)
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Hydrogen passivation of self assembled InAs quantum dots
- 2002
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Ingår i: Journal of Applied Physics. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 92:11, s. 6794-6798
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Tidskriftsartikel (refereegranskat)abstract
- A systematic study on the hydrogen passivation of nonradiative centers in InAs quantum dots grown on GaAs substrates is presented. The samples used in this study were grown by molecular beam epitaxy. The structures contain an InxGa1-xAs insertion layer between the InAs quantum dots layer and the GaAs cap layer. The thickness and In concentration of the InxGa1-xAs are varied to achieve the emission wavelength at 1.3 mum. The samples after the H-2 plasma treatment show a significant increase of the photoluminescence intensity. The experimental results show that the quality of the InAs quantum dot structures does not degrade after the hydrogen (H-2) plasma treatments. The enhancement of the photoluminescence intensity from the InAs quantum dots is thought to be due to the passivation of nonradiative centers like defects in the structures. High resolution x-ray diffraction rocking curves are used to correlate photoluminescence results.
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| 7. |
- Jacob, AP, et al.
(författare)
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Room temperature luminescence from ZnSe1-xTex (x less than 1%) epilayers grown on (001) GaAs
- 2003
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Ingår i: Journal of Applied Physics. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 94:4, s. 2337-2340
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence experiments have been performed to systematically study the effect of thermal processing on ZnSe1-xTex (xless than1%) epilayers. Our results show that, a ZnSeTe epilayer under proper post growth thermal annealing can emit light in the visible range of 5500-7000 Angstrom at room temperature. Thus by systematically processing these samples, they could be used for II-VI laser diodes that can operate at room temperature. The results from hydrogen passivation study done on these samples are consistent with the previous reports that the broadband emission is related to an isoelectronic defect, i.e., excitons bound to the Te clusters.
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| 8. |
- Lu, W, et al.
(författare)
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Enhancement of room-temperature photoluminescence in InAs quantum dots
- 2003
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Ingår i: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 83:21, s. 4300-4302
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Tidskriftsartikel (refereegranskat)abstract
- We report pronounced enhancement of room-temperature photoluminescence up to 80-fold induced by proton implantation and the rapid thermal annealing process in a multilayer InAs/GaAs quantum-dot structure. This effect is studied by a combination of material methods and resulted from both proton passivation and carrier capture enhancement effects. The maximum photoluminescence peak shift is about 23 meV, resulting from the intermixing of quantum dots. Linear dependence behavior as observed for both the nonradiative recombination time and carrier relaxation time on the ion-implantation dose. Maximum enhancement of the photoluminescence is observed for a proton implantation dose of 1.0x10(14) cm(-2) followed by rapid thermal annealing at 700 degreesC. These effects will be useful for quantum dot optoelectronic devices.
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| 10. |
- Sundqvist, PA, et al.
(författare)
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Polarization of an exciton in a ZnO layer using a split gate potential
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 68:15
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we focus on structural and optical transitions of an exciton in a Zinc Oxide (ZnO) layer, which could be widely controlled by a split gate potential. We have solved the exciton problem by a self-consistent Schrodinger-Poisson technique, where the Hamiltonian includes the boundary conditions for the split gate structure. The gate voltage creates a paraboliclike potential, which at a typical threshold voltage separates or polarizes the exciton strongly. This sharp structural transition brings the exciton from being strongly correlated with a large overlap to a regime where the correlation is very small (with small overlap). The resulting structure for the exciton at negative gate voltages is a structure where the hole is located like a ring around a dotlike electron. For positive values of the gate voltage the situation is opposite. We have especially studied the ground-state binding energy and the optical transitions of the exciton. We found that the ground-state energy for ZnO could be tuned and the decrease of the ground-state energy can be as large as the double of the bulk exciton energy (60 meV for ZnO) with a gate voltage of -5 V. The ground-state energy is almost constant for small values of the gate voltage but at a typical threshold voltage (approximately -2 V) the energy suddenly changes and becomes linear with the gate voltage. We also analyze the lifetime for the exciton, which is shown to increase from nanoseconds to beyond milliseconds. This was shown to be an effect of the small overlap between the hole and the electron when the gate voltage increased above the threshold voltage. Stimulated by the long lifetime of the ground state of the exciton we also calculated the optical transition frequency and the corresponding oscillator strength for the transition between the ground state and the dominating excited (self-consistent) exciton states. The transition frequency was found to occur in the THz region and the oscillator strength in the range of 0.3-0.4 for gate voltages between -2 V and -5 V. In addition, we have also analytically described polarization and especially total charge densities for excitons in small linear electric fields.
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| 11. |
- Zhao, QX, et al.
(författare)
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Deep-level emissions influenced by O and Zn implantations in ZnO
- 2005
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Ingår i: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 87:21
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Tidskriftsartikel (refereegranskat)abstract
- A set of bulk ZnO samples implanted with O and Zn at various densities were investigated by photoluminescence. The implantation concentration of O and Zn is varied between 1x10(17)/cm(3) and 5x10(19)/cm(3). The samples were thermally treated in an oxygen gas environment after the implantation. The results clearly show the influence of O and Zn implantations on the deep-level emission. By comparing the photoluminescence spectra for the samples with different implantations, we can conclude that the V-Zn is responsible to the observed deep-level emission. In addition, a novel transition at the emission energy of 3.08 eV at 77 K appears in the O-implanted sample with 5x10(19)/cm(3) implantation concentration. The novel emission is tentatively identified as O-antisite O-Zn.
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| 12. |
- Zhao, QX, et al.
(författare)
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Effect of growth temperature and post-growth thermal annealing on carrier localization and deep level emissions in GaNAs/GaAs quantum well structures
- 2005
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Ingår i: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 86:12
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Tidskriftsartikel (refereegranskat)abstract
- We report results from investigation of the optical properties of GaNAs/GaAs quantum well structures. The structures were grown by molecular-beam epitaxy at different temperatures, and subsequently postgrowth thermal treatments at different temperature were performed. The results show that the carrier localization is smaller in a structure grown at a temperature of 580 degrees C in comparison with a structure grown at 450 degrees C. Both structures also show a broaden deep level emission band. Furthermore, the deep level emission band and the carrier localization effect can be removed by thermal annealing at 650 degrees C in the structure grown at 450 degrees C. The structure quality and radiative recombination efficiency are significantly improved after annealing. However, annealing under the same condition has a negligible effect on the structure grown at 580 degrees C. (C) 2005 American Institute of Physics.
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| 13. |
- Zhao, QX, et al.
(författare)
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Effects of monolayer AlAs insertion in modulation doped GaAs/AlxGa1-xAs quantum-well structures
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:16, s. 10984-10989
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Tidskriftsartikel (refereegranskat)abstract
- Symmetrically modulation doped GaAs/AlxGa1-xAs quantum-well structures, containing a monolayer thickness of single AlAs insertion in the well region, are studied by photoluminescence spectroscopy and electrical characterization. The aim of this study is to explore how the AlAs insertion influences the electronic properties of the structures and the mobility of the carrier confined in the well layer. We find that the electronic structure of the confined electrons is strongly influenced by the AlAs insertion in the modulation doped structures. The effective mass of the particles involved in the observed optical transition and the transition energy were deduced from magneto-optical measurements, while the mobility and carrier concentration were, obtained from the Hall measurements. The experimentally deduced transition energies are compared with the results from a simple self-consistent calculation.
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| 14. |
- Zhao, QX, et al.
(författare)
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Effects of nitrogen incorporation on the properties of GaInNAs/GaAs quantum well structures
- 2005
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Ingår i: Journal of Applied Physics. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 97:7
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Tidskriftsartikel (refereegranskat)abstract
- We report results from theoretical and experimental investigations of GaInNAs/GaAs quantum well structures. Optical transition energies for samples with different In and N concentrations were determined by photoluminescence measurements. The results show that the reduction of the ground-state transition energy by the introduction of N decreases with increasing In concentration. The experimental data are compared with calculations using the effective-mass approximation. Modifications of the band-gap energy due to N incorporation were accounted for using the two-level repulsion model. Proper effective-mass and band offset values, based on recent experimental work, were used. Calculated and measured transition energies show good agreement. The critical thickness, lattice constant, strain, and optical transition energies are discussed for GaInNAs/GaAs quantum well structures tuned for emission at 1.3 and 1.55 mu m, in particular. Such a simple model, within the effective-mass approximation, is a very useful guide for device design. (C) 2005 American Institute of Physics.
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| 15. |
- Zhao, QX, et al.
(författare)
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Nonradiative centers in InAs dots grown on GaAs substrates for 1.3 mu m emission
- 2003
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Ingår i: Physics Letters A. - : Elsevier Science B.V., Amsterdam.. - 0375-9601 .- 1873-2429. ; 315:02-Jan, s. 150-155
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Tidskriftsartikel (refereegranskat)abstract
- Nonradiative centers in InAs dots grown on GaAs substrates are investigated in this study. The emission from InAs dots close to 1.3 mum is monitored under different excitation densities and different excitation energy. The used samples were also treated by hydrogen plasma in order to suppress the nonradiative centers. The purpose of this work is to study how nonradiative centers influence the efficiency of InAs dots emission and whether the nonradiative centers can be reduced. Our results clearly illustrate that there indeed exist nonradiative centers, both at the interface between the InAs dots and surrounding layers and in the GaAs layers, which can be suppressed by H-treatments. A technique to estimate relative amount of nonradiative centers is also discussed.
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| 16. |
- Zhao, QX, et al.
(författare)
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Optical recombination of ZnO nanowires grown on sapphire and Si substrates
- 2003
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Ingår i: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 83:1, s. 165-167
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Tidskriftsartikel (refereegranskat)abstract
- ZnO nanowires have been grown on sapphire and Si substrates using catalytic growth. A strong near-band-gap ultraviolet emission is observed at room temperature. By carefully studying the temperature dependence of ZnO wire emission, we found that the room-temperature UV emission contains two different transitions; one is related to the ZnO free exciton and the other is related to the free-to-bound transition. The bound state has a binding energy of about 124 meV. The results from optical measurements show that a high quality of ZnO nanowires grown on sapphire and Si substrates has been achieved.
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| 17. |
- Zhao, QX, et al.
(författare)
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Radiative recombination of localized excitons and mobility edge excitons in GaInNAs/GaAs quantum wells with strong carrier localization
- 2005
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Ingår i: Physics Letters A. - : Elsevier Science B.V., Amsterdam.. - 0375-9601 .- 1873-2429. ; 341:04-Jan, s. 297-302
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Tidskriftsartikel (refereegranskat)abstract
- The radiative recombination in InxGa1-xN0.01As0.99/GaAs quantum well structures exhibiting strong carrier localization was investigated by optical spectroscopy. For In-concentration from 0 to 30%, the results indicate that the degree of carrier localization decreases with increasing In-concentration. At temperatures below 100 K, the mobility edge excitons as well as localized excitons are identified and their transitions energies strongly depend on the excitation intensity. At elevated temperatures the localized excitons become quenched. The temperature dependence of the photoluminescence emission energy shows different behaviors at different excitation intensities.
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| 18. |
- Zhao, QX, et al.
(författare)
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Strong enhancement of the photoluminescence-efficiency from InAs quantum dots
- 2003
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Ingår i: Journal of Applied Physics. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 93:3, s. 1533-1536
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Tidskriftsartikel (refereegranskat)abstract
- InAs quantum dots (QDs) have been investigated using optical spectroscopy, in order to understand the experimental observation of strong enhancement of their photoluminescence efficiency. When a tunneling barrier is introduced between the, InAs layer and the GaAs cap layer, the intensity of the InAs QD emission increases by more than an order of magnitude at the excitation density of 60 W/cm(2). The enhancement of the optical recombination efficiency is due to the suppression of the nonradiative transitions in the wetting layer. The strong enhancement of the InAs emission can lead to an increase in the optical gain of the InAs laser structure
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