Self-assembled germanium islands grown on (001) silicon substrates by low-pressure chemical vapor deposition

The time evolution of self-assembled Ge islands, during low-pressure chemical vapor deposition (LPCVD) of Ge on Si at 650 Deg C using high growth rates, has been investigated by atomic force microscopy, transmission electron microscopy, and Rutherford backscattering spectrometry. We have found three different island structures The smallest islands are "lens-shaped" and characterized by a rather narrow size distribution, ~4nm high and ~20nm wide. Next to form are a distinct population of multifaceted "dome shaped" islands, up to 25nm high and 80-150 nm wide. Finally, the largest islands that form are square-based truncated pyramids with a very narrow size distribution ~50nm high and ~250nm wide. The pyramidal islands normally seen in the intermediate size range (~150nm) are not observed. The small lens-shaped islands appear to be defect free, while some of the multifaceted islands as well as all the large truncated pyramids contain misfit dislocations. The existence of multifaceted islands, in the size range where multifaceted "dome shaped" islands have previously been reported, is attributed to the high growth rate used. Furthermore, under the growth conditions used, the truncated-pyramid-shaped islands are characterized by a very narrow size distribution

Characteristics of fine and ultrafine aerosols in the London underground

\ua9 2022 The Authors. Underground railway systems are recognised spaces of increased personal pollution exposure. We studied the number-size distribution and physico-chemical characteristics of ultrafine (PM0.1), fine (PM0.1–2.5) and coarse (PM2.5–10) particles collected on a London underground platform. Particle number concentrations gradually increased throughout the day, with a maximum concentration between 18:00 h and 21:00 h (local time). There was a maximum decrease in mass for the PM2.5, PM2.5–10 and black carbon of 3.9, 4.5 and ~ 21-times, respectively, between operable (OpHrs) and non-operable (N-OpHrs) hours. Average PM10 (52 μg m−3) and PM2.5 (34 μg m−3) concentrations over the full data showed levels above the World Health Organization Air Quality Guidelines. Respiratory deposition doses of particle number and mass concentrations were calculated and found to be two- and four-times higher during OpHrs compared with N-OpHrs, reflecting events such as train arrival/departure during OpHrs. Organic compounds were composed of aromatic hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) which are known to be harmful to health. Specific ratios of PAHs were identified for underground transport that may reflect an interaction between PAHs and fine particles. Scanning transmission electron microscopy (STEM) chemical maps of fine and ultrafine fractions show they are composed of Fe and O in the form of magnetite and nanosized mixtures of metals including Cr, Al, Ni and Mn. These findings, and the low air change rate (0.17 to 0.46 h−1), highlight the need to improve the ventilation conditions

Characteristics of fine and ultrafine aerosols in the London underground.

Underground railway systems are recognised spaces of increased personal pollution exposure. We studied the number-size distribution and physico-chemical characteristics of ultrafine (PM0.1), fine (PM0.1-2.5) and coarse (PM2.5-10) particles collected on a London underground platform. Particle number concentrations gradually increased throughout the day, with a maximum concentration between 18:00 h and 21:00 h (local time). There was a maximum decrease in mass for the PM2.5, PM2.5-10 and black carbon of 3.9, 4.5 and ~ 21-times, respectively, between operable (OpHrs) and non-operable (N-OpHrs) hours. Average PM10 (52 μg m-3) and PM2.5 (34 μg m-3) concentrations over the full data showed levels above the World Health Organization Air Quality Guidelines. Respiratory deposition doses of particle number and mass concentrations were calculated and found to be two- and four-times higher during OpHrs compared with N-OpHrs, reflecting events such as train arrival/departure during OpHrs. Organic compounds were composed of aromatic hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) which are known to be harmful to health. Specific ratios of PAHs were identified for underground transport that may reflect an interaction between PAHs and fine particles. Scanning transmission electron microscopy (STEM) chemical maps of fine and ultrafine fractions show they are composed of Fe and O in the form of magnetite and nanosized mixtures of metals including Cr, Al, Ni and Mn. These findings, and the low air change rate (0.17 to 0.46 h-1), highlight the need to improve the ventilation conditions

Modeling for understanding of coronavirus disease-2019 (COVID-19) spread and design of an isolation room in a hospital

We have modeled the transmission of coronavirus 2019 in the isolation room of a patient suffering from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at the Royal Brompton Hospital in London. An adaptive mesh computational fluid dynamics model was used for simulation of three-dimensional spatial distribution of SARS-CoV-2 in the room. The modeling set-up is based on data collected in the room during the patient stay. Many numerical experiments have been carried out to provide an optimal design layout of the overall isolation room. Our focus has been on (1) the location of the air extractor and filtration rates, (2) the bed location of the patient, and (3) consideration of the health and safety of the staff working in the area

Modelling for understanding of COVID-19 spread and design of an isolation room in a hospital

We have modelled the transmission of coronavirus 2019 (COVID-19) in the isolation room of a patient suffering from severe acute respiratory syndrome coronavirus 2 (SARS CoV 2) at the Royal Brompton Hospital (RBH) in London. An adaptive mesh computational fluid dynamics (CFD) model was used for simulations of 3D spatial distribution of SARS CoV 2 in the room. The modelling set-up is based on data collected in the room during the patient stay. A number of numerical experiments have been carried out to provide an optimal design layout of the overall isolation room. Our focus has been on (1) the location of the air extractor and filtration rates; (2) the bed location of the patient; and (3) consideration of the health and safety of the staff working area

Structural and compositional evolution of self-assembled germanium islands on silicon (001) during high growth rate LPCVD

Understanding the process of self-organization of Ge nanostructures on Si with controlled size distribution is a key requirement for their application to devices. In this study, we investigate the temporal evolution of self-assembled islands during the low pressure chemical vapour deposition (LPCVD) of Ge on Si at 650°C using high growth rates (6-9 nm/min). The islands were characterized by atomic force microscopy, transmission electron microscopy, Rutherford backscattering spectrometry and micro-Raman spectroscopy. We found that the first nanostructures to assemble were small islands, with a narrow size distribution, typical of the 'lens-shaped' structures reported in previous studies. Next to form were a population of larger 'lens-shaped' islands with a similar surface density to that of the small islands, but with broad height and width distributions. These islands differ from the pyramid-shaped islands previously reported for a similar size range. On further Ge deposition, the population evolves into one of large square-based truncated pyramids with a very narrow size distribution. Such pyramidal structures were previously reported at smaller sizes. Furthermore, we see no evidence of the multifaceted domes previously reported in this size range. The small 'lens-shaped' islands appear to be strained, whilst some of the intermediate-sized islands and all the large truncated pyramids contain misfit strain relaxation induced defects. Additionally, in the both the intermediate size 'lens-shaped' islands and in the large size truncated pyramidal islands, there is evidence of Si-Ge strain-induced alloying, more significant in the first than in the latter. Our observation of 'lens shaped' islands and truncated pyramids at larger sizes than are normally observed, suggests a kinetically driven process that delays the evolution of energetically favourable island structures until larger island sizes are reached

Influence of H2 preconditioning on the nucleation and growth of self-assembled germanium islands on silicon (001)

Understanding the effects of growth conditions on the process of self-organisation of Ge nanostructures on Si is a key requirement for their practical applications. In this study we investigate the effect of preconditioning with a high-temperature hydrogenation step on the nucleation and subsequent temporal evolution of Ge self-assembled islands on Si (001). Two sets of structures, with and without H2 preconditioning, were grown by low pressure chemical vapour deposition (LPCVD) at 650°C. Their structural and compositional evolution was characterised by Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM) and micro-Raman (μRaman) spectroscopy. In the absence of preconditioning, we observe the known evolution of self-assembled Ge nanostructures on Si (001), from small islands with a narrow size distribution, to a bimodal size distribution, through to large islands. Surface coverage and island size increase steadily as a function of deposition time. On the H2 preconditioned surface, however, both nucleation rates and surface coverage are greatly increased during the early stages of self-assembly. After the first five seconds, the density of the islands is twice that on the unconditioned surface, and the mean island size is also larger, but the subsequent evolution is much slower than in the case of the unconditioned surface. This retardation correlates with a relatively high measured stress within the islands. Our results demonstrate that standard processes used during growth, like H2 preconditioning, can yield dramatic changes in the uniformity and distribution of Ge nanostructures self-assembled on Si. © 2004 Materials Research Society

Raman study of the strain and H2 preconditioning effect on self-assembled Ge island on Si (001)

An investigation of the microscopic mechanisms of Ge self-assembling island growth is of great importance for future optoelectronic applications of quantum dot nanostructures. In this study, two sets of self-assembled germanium islands on Si (001) substrate, with and without preconditioning using a high-temperature hydrogenation step on their nucleation and subsequent temporal evolution, were grown by low-pressure chemical vapor deposition (LPCVD). The average germanium concentration, mean diameter of Ge crystalline regions and the strain inside the germanium quantum dots are characterized with high resolution micro-Raman spectroscopy (?RS). Both the intensity and peak position of the Si–Si vibration mode at about 520.07 cm?1 in the Raman spectra have been used as a reference to separate the germanium Raman signal from the overlapping localized Si–Si optical phonon at ?300 cm?1. In the absence of preconditioning, both the island size and germanium composition increase steadily as a function of deposition time. However, on the H2 preconditioned surface, the nucleation and growth rates are greatly increased during the first stages and slow down significantly after deposition for 10 s. Our results indicate that the compressive strain inside the islands acts as a barrier for Ge adatoms to diffuse from the wetting layer into the islands. For the growth times used in this study, for both sets of samples with and without H2 preconditioning, the normalized rate of increase of the Ge concentration (%? [Ge]/? t) decreases by ?0.13/s for a 1% compressive strain increase. The H2 preconditioning can initially increase the density of island nucleation sites, but cannot accelerate the Ge island growth. It tends to lower %? [Ge]/? t by 0.015/s instead. The decreased strain due to surface roughing is the principal reason why the Ge islands grow so rapidly at the beginning on the H2 preconditioned samples