Surface morphology control of strained InAs/GaAs(331)A films: From nanowires to island-pit pairs

We have studied the effect of molecular beam epitaxy growth conditions on the surface morphology of strained InAs/GaAs(331)A films. Our results reveal that InAs nanowires aligned along the [1 (1) over bar0] direction are formed under As-rich conditions, which is explained by the effect of anisotropic buffer layer surface roughing. Under In-rich conditions, however, the surface morphology of the InAs layers is characterized by a feature of island-pit pairs. In this case, cooperative nucleation of islands and pits can lower the activation barrier for domain growth. These results suggest that the surface morphology of strained InAs layers is highly controllable. (C) 2005 American Institute of Physics.

Corrugated surfaces formed on GaAs(331)A substrates: the template for laterally ordered InGaAs nanowires

Morphology evolution of high-index GaAs(331)A surfaces during molecular beam epitaxy (MBE) growth has been investigated in order to achieve regularly distributed step-array templates and fabricate spatially ordered low-dimensional nano-structures. Atomic force microscope (AFM) measurements have shown that the step height and terrace width of GaAs layers increase monotonically with increasing substrate temperature. By using the step arrays formed on GaAs(331)A surfaces as the templates, we have fabricated highly ordered InGaAs nanowires. The improved homogeneity and the increased density of the InGaAs nanowires are attributed to the modulated strain field caused by vertical multi-stacking, as well as the effect of corrugated surface of the template. Photoluminescence (PL) tests confirmed remarkable polarization anisotropy.

Self-organized InAs quantum wires on GaAs (331)A substrates

Self-organized InAs quantum wires (QWRs) were fabricated on the step edges of the GaAs (331)A surface by molecular beam epitaxy. The lateral size of InAs QWRs was saturated by the terrace width (i.e., 90 nm) while the size along the step lines increased with the increasing thicknesses of the InAs layers, up to 1100 nm. The height of InAs QWRs varied from 7.9 nm to 13 nm. The evolution of the morphology of InAs QWRs was attributed to the diffusion anisotropy of In adatoms.

Corrugated surfaces formed on GaAs (331)a substrates: The template for laterally ordered InGaAs nanowires

Morphology evolution of high-index (331)A surfaces during molecular beam epitaxy (MBE) growth have been investigated in order to uncover their unique physic properties and fabricate spatially ordered low dimensional nanostructures. Atomic Force Microscope (AFM) measurements have shown that the step height and terrace width of GaAs layers increase monotonically with increasing substrate temperature in conventional MBE. However, this situation is reversed in atomic hydrogen-assisted MBE, indicating that step bunching is partly suppressed. We attribute this to the reduced surface migration length of Ga adatoms with atomic hydrogen. By using the step arrays formed on GaAs (331)A surfaces as the templates, we fabricated laterally ordered InGaAs self-aligned nanowires.

Surface morphology evolution of strained InAs/GaAs(331)a films

Surface morphology evolution of strained InAs/GaAs(331)A films was systematically investigated in this paper. Under As-rich conditions, InAs elongated islands aligned along [1 (1) over bar0] are formed at a substrate temperature of 510 degrees C. We explained it as a result of the anisotropic diffusion of adatoms. Under In-rich conditions, striking change has occurred with respect to the surface morphology of the InAs layers. Instead of anisotropic InAs elongated islands, unique island-pit pairs randomly distributed on the whole surface were observed. Using cooperative nucleation mechanisms proposed by Jesson et al. [Phys. Rev. Lett. 77, 1330 (1996)], we interpret the resulting surface morphology evolution.

Morphology Evolution of (331)A High-Index Surfaces During Atomic Hydrogen Assisted Molecular Beam Epitaxy (MBE)

Step like morphology of (331)A high-index surfaces during atomic hydrogen assisted molecular beam epitaxy (MBE) growth has been investigated. Atomic Force Microscope (AFM) measurements show that in conventional MBE, the step heights and terrace widths of GaAs layers increase monotonically with increasing substrate temperatures. The terrace widths and step densities increase with increasing the GaAs layer thickness and then saturates. And, in atomic hydrogen assisted MBE, the terrace width reduces and density increases when depositing the same amount of GaAs. It attributes this to the reduced surface migration length of Ga adatoms with atomic hydrogen. Laterally ordered InAs self-aligned nano-wires were grown on GaAs (331)A surfaces and its optical polarization properties were revealed by photoluminescence measurements.

GaAs(331)A面InAs自组织纳米结构形貌演化

采用分子束外延方法在CaAs(331)A高指数衬底上制备自对齐InAs纳米线(QWRs)或者三维(3D)岛状结构.InAs纳米线(QWRs)选择性生长在CaAs层的台阶边缘.通过原子力显微镜(AfM)仔细研究了InAs纳米微结构的表面形貌,发现不同的生长条件,包括:衬底温度、生长速率、和InAs层厚度等,对InAa表面形貌有很大的影响.如,低温更容易导致线状纳米微结构的形成,而高温更利于3D岛状结构形成.表面形貌的转变归结于表面能同应变能之间的竞争.

Response to comment on “The response of vegetation on the Andean flank in western Amazonia to Pleistocene climate change” – M.L. Cárdenas et al., Science 331, 1055 (25 February 2011)”.

Puyasena et al. question our interpretation of climate-driven vegetation change on the Andean flank in western Amazonia during the middle Pleistocene and suggest that the use of Podocarpus spp. as a proxy of past climate change should be reassessed. We defend our assertion that vegetation change at the Erazo study site was predominantly driven by climate change due to concomitant changes recorded by multiple taxa in the fossil record.

Concerning CP violation in 331 models

We consider the implementation of CP violation in the context of 331 models. In particular we treat a model where only three scaler triplets are needed in order to give all fermions a mass while keeping neutrino massless. In this case all CP violation is provided by the scalar sector.

Efeitos de Z' nos modelos left-right, B-L e 331

Pós-graduação em Física - IFT

The young stellar cluster [DBS2003] 157 associated with the H ii region GAL 331.31-00.34

We report a study of the stellar content of the near-infrared (NIR) cluster [DBS2003] 157 embedded in the extended H ii region GAL 331.31-00.34, which is associated with the IRAS source 16085-5138. JHK photometry was carried out in order to identify potential ionizing candidates, and the follow-up NIR spectroscopy allowed the spectral classification of some sources, including two O-type stars. A combination of NIR photometry and spectroscopy data was used to obtain the distance of these two stars, with the method of spectroscopic parallax: IRS 298 (O6 V, 3.35 +/- 0.61 kpc) and IRS 339 (O9 V, 3.24 +/- 0.56 kpc). Adopting the average distance of 3.29 +/- 0.58 kpc and comparing the Lyman continuum luminosity of these stars with that required to account for the radio continuum flux of the H ii region, we conclude that these two stars are the ionizing sources of GAL 331.31-00.34. Young stellar objects (YSOs) were searched by using our NIR photometry and mid-infrared (MIR) data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) survey. The analysis of NIR and MIR colourcolour diagrams resulted in 47 YSO candidates. The GLIMPSE counterpart of IRAS 16085-5138, which presents IRAS colour indices compatible with an ultracompact H ii region, has been identified. The analysis of its spectral energy distribution between 2 and m revealed that this source shows a spectral index a= 3.6 between 2 and m, which is typical of a YSO immersed in a protostellar envelope. Lower limits to the bolometric luminosity and the mass of the embedded protostar have been estimated as L= 7.7 x 10(3) L? and M= 10 M?, respectively, which correspond to a B0B1 V zero-age main sequence star.