Microstructural Evolution and Coherent Islands

1992 ◽  
Vol 280 ◽  
Author(s):  
Jeff Drucker
Keyword(s):  
Growth Rate ◽  
Surface Diffusion ◽  
Microstructural Evolution ◽  
Rigid Substrate ◽  
Island Growth ◽  
Equilibrium Solubility

ABSTRACTMicrostructural evolution in systems containing strained islands (coherent, incoherent or both) is investigated. The growth rate of an individual island coarsening in an ensemble of strained islands is obtained by including elastic effects on surface diffusion of adatoms to and the equilibrium solubility of strained islands. For strained islands growing on a quasi-rigid substrate, coherent islands grow more slowly than incoherent islands of the same radius. Consequently, the island growth rate accelerates at the coherent to incoherent transition. The model agrees with recent experimental observations in Ge/Si(100) heteroepitaxy.

The growth, ecology and population structure of giant tortoises on Aldabra

1971 ◽  
Vol 260 (836) ◽  
pp. 327-372 ◽  
Keyword(s):  
Growth Rate ◽  
Carapace Length ◽  
Size Range ◽  
Habitat Preferences ◽  
Local Variation ◽  
Island Population ◽  
Island Growth ◽  
Age Classes ◽  
Mean Values ◽  
Differential Movement

The giant tortoise of Aldabra, Geochelone gigantea , shows quite marked changes in proportions with age, although during growth the relations between the length of the carapace and various measurements of the plastron and scutes involve not only strong but also weak allometry. Certain scutes show a predisposition to split during growth. Accidental damage to the carapace is frequent. Males reach over 100 cm in carapace length but females are smaller, up to 80 cm. There is no segregation between the sexes in any single measurement investigated, except among the very largest animals. A general appraisal of carapace and tail shape is sufficient to sex only animals above 60 cm in carapace length. The number of annuli on each scute corresponds to the number of years of age at least up to the formation of the tenth to fifteenth annulus. A general assessment of the pattern of growth is made by plotting body measurements against number of annuli. Growth curves of individual tortoises are reconstructed by relating measurements of successively formed annuli to age. Growth rate is recorded by plotting the difference between successive pairs of annulus measurements against age. The growth rate of ageable tortoises varies between local populations on South Island and between populations of South and Middle Island. Growth rate declines with age, reaching asymptotes at mean values of between 20 and 30 years. Some individuals exhibit sudden increases in growth rate after several years of very slow growth. There is a well-marked daily cycle of activity, feeding being limited to the early morning and late evening. Agonistic behaviour is virtually absent. Breeding is seasonal and the males select partners from within a limited size range of tortoises. Most mating attempts are unsuccessful. On Aldabra, tortoises occur in a wide variety of habitats, in each of which they depend on a different plant species or vegetational association for food. On coastal plains the chief source of food is Sporobolus virginicus . A variety of small herbs is consumed on the barren stretches of coastal champignon. Distribution in these areas is profoundly affected by the availability of shade. Further inland, the tortoises browse heavily on Guettarda speciosa in woods dom inated by this tree. They take advantage of seasonal successions in the vegetation associated with freshwater pools, feeding on each community as it develops. Most of the woody plants near the pools are ignored. On the platin, browsing is selective and the regeneration of some trees is held in check. A very im portant food source here is the `tortoise turf’ (a sward in which Panicum sp. is often dominant) developed under conditions of heavy grazing and susceptible to erosion by wind and the tortoises themselves. On Middle Island, where the population is small, the tortoises exert very little effect on the vegetation. Associations with other animals are mostly casual, but along the south coast dunes Coenobita rugosus is dependent on tortoise faeces for food. Fossilized tortoise bones have been discovered at many points on Aldabra, deposited in brown limestone. They probably date from before the interstadial of about 30 000 years ago. Some adult tortoises range over 7 km or more, across a variety of habitats, but many individuals appear to be sedentary. The population of South Island is enormous of the order of 100 000 animals with a density of about 30 hm -2 on the platin. Higher densities are reached in Guellarda woodland. Local variation in numbers, size range and age structure depend on habitat preferences, differential movement of age classes and regional differences in growth rate. Attempts at assessing age class distribution are affected particularly by undersampling of the younger age classes, and the difficulty of counting the worn growth rings in animals with more than about 14. In the census sample, which may itself be an imperfect sample of the whole South Island population, at least 35 % of the animals are below 20 years of age and only about 20 % can have reached sexual maturity. More than 50 age classes may be present, but this and similar deductions are still speculative.

“Enhanced Layer Coverage of Thin Films by Oblique Angle Deposition”

2004 ◽  
Vol 859 ◽  
Author(s):  
Tansel Karabacak ◽  
Gwo-Ching Wang ◽  
Toh-Ming Lu
Keyword(s):  
Thin Films ◽  
Surface Diffusion ◽  
Kinetic Monte Carlo ◽  
Monte Carlo Algorithm ◽  
Oblique Angle Deposition ◽  
Oblique Angle ◽  
Island Growth ◽  
Average Growth ◽  
Substrate Rotation ◽  
Angle Deposition

ABSTRACTThe characteristics of nucleation and island growth in oblique angle deposition with substrate rotation have recently attracted interest due to the formation of novel 3D nanostructures by a physical self-assembly process. In this study, we present the results of a solid-on-solid growth simulation by a kinetic Monte Carlo algorithm that explores the layer coverage evolution of thin films during oblique angle deposition. The simulations accounted for oblique incidence flux, shadowing effect, surface diffusion, and substrate rotation. The layer coverage, the ratio of average island volume to average island size, and root-mean-square (RMS) roughness values are reported for the initial stages of island growth from submonolayer thicknesses up to a few monolayers. RMS roughness was also investigated for later stages of the growth. Our results show that, for small deposition angles and with limited or no surface diffusion included, the average growth rate of islands is faster in lateral directions that results in enhanced layer coverages and smoother films. This is due to that the sides of the islands can be exposed to the incident flux more effectively at small deposition angles. On the other hand, normal incidence and high oblique angle depositions give poorer layer coverages and much rougher films due to the slower growth rates in lateral directions.

MBE growth of compound semiconductors: Part II. Applications of the stochastic model

1992 ◽  
Vol 7 (5) ◽  
pp. 1235-1242 ◽  
Author(s):  
R. Venkatasubramanian
Keyword(s):  
Growth Rate ◽  
Surface Roughness ◽  
Phase Separation ◽  
Stochastic Model ◽  
Diffusion Process ◽  
Surface Diffusion ◽  
Growth Front ◽  
Mbe Growth ◽  
Temperature Range ◽  
Kinetics Of

In this part of the work (Part II), two typical applications of the stochastic model to the MBE growth kinetic studies are presented. The applications are the MBE growth kinetics of a hypothetical compound semiconductor, ab, and diamond cubic alloy, ax. In this study, the effect of the surface diffusion process on the MBE growth kinetics is analyzed. In the case of the compound, ab, the results of the present stochastic model are compared with that of a Monte Carlo simulation study in the temperature range of 600–850 K. The results of the two studies agree qualitatively. Higher substrate temperatures result in higher growth rate and growth front smoothness due to higher surface diffusion. Beyond 800 K, the growth rate and the growth front smoothness become independent of temperature because of the saturation of the interlayer diffusion process. In the case of the alloy studies, the kinetics of a hypothetical diamond cubic alloy in which the thermodynamics favors phase separation, is studied in the temperature range of 573–898 K. Below 648 K, due to negligible surface diffusion, there is no clustering of the alloy, but the surface roughness is very large. In the intermediate temperature range of 573–798 K, with increasing temperature, the surface diffusion increases, resulting in more clustering and less surface roughness. Above 798 K, due to very high surface diffusion, complete phase separation of the alloy and a smooth surface result.

Surface diffusion of Fe and island growth of FeSi2 on Si(111) surfaces

1996 ◽  
Vol 287 (1-2) ◽  
pp. 93-100 ◽  
Author(s):  
A. Wohllebe ◽  
B. Holländer ◽  
S. Mesters ◽  
C. Dieker ◽  
G. Crecelius ◽  
...  
Keyword(s):  
Surface Diffusion ◽  
Island Growth

Theoretical Analysis of Non-Catalytic Growth of Nanorods on a Substrate

2006 ◽  
Vol 963 ◽  
Author(s):  
S. Joon Kwon ◽  
Jae-Gwan Park
Keyword(s):  
Growth Rate ◽  
Aspect Ratio ◽  
Theoretical Analysis ◽  
Surface Diffusion ◽  
Diffusion Time ◽  
Probable Mechanism ◽  
Growth Time ◽  
Catalytic Growth ◽  
One Dimensional ◽  
The Mean

ABSTRACTA theoretical analysis explaining non-catalytic growth of one-dimensional (1D) nanorods on a substrate is presented. The nuclei undergo cluster migration which continues until the mean free time of the adatoms is larger than surface diffusion time during several consecutive nuclei growth steps. The most probable mechanism is the migration of six adatoms into one fixed adatom. After the cluster migration, the nuclei grow in an isotropic manner, until the nucleus reaches the size limit. The 1D growth of nanorods on the nuclei begins when the reactant dose is smaller than a certain value. The growth rate of the height is greater than that of the radius. This difference in the growth rate causes the aspect ratio to increase with growth time. The presented analysis explains well the experimental results of the non-catalytic growth of nanorods.

The Effect of Phosphine Pressure on the Band Gap of Ga0.5In0.5P

1994 ◽  
Vol 340 ◽  
Author(s):  
Sarah R. Kurtz ◽  
D. J. Arent ◽  
K. A. Bertness ◽  
J. M. Olson
Keyword(s):  
Growth Rate ◽  
Surface Structure ◽  
Band Gap ◽  
Surface Diffusion ◽  
Parameter Space ◽  
Growth Temperature ◽  
Growth Rates ◽  
Diffusion Length ◽  
Group V ◽  
Group Iii

ABSTRACTThe band gap of Ga0.51n0.5P is studied as a function of phosphine pressure, B-type substrate misorientation, growth rate, and growth temperature, with emphasis placed on the effect of the phosphine pressure. Over most of the parameter space explored (high temperatures, large substrate misorientations, and low growth rates), the band gap increases with decreasing phosphine. This increase is proposed to be caused by lower phosphorus coverage of the surface, resulting in a different surface structure that doesn't promote ordering. The implications of this effect on the observed variations of band gap with growth temperature, substrate misorientation, and growth rate are discussed. For regions of parameter space in which the ordering appears to be kinetically limited by surface diffusion, the band gap increases slightly with phosphine pressure, consistent with observations that increased group-V pressure decreases the group-III surface diffusion length.

Monotectic Al–Bi–Sn alloys directionally solidified: Effects of Bi content, growth rate and cooling rate on the microstructural evolution and hardness

2015 ◽  
Vol 653 ◽  
pp. 243-254 ◽  
Author(s):  
Thiago A. Costa ◽  
Emmanuelle S. Freitas ◽  
Marcelino Dias ◽  
Crystopher Brito ◽  
Noé Cheung ◽  
...  
Keyword(s):  
Growth Rate ◽  
Cooling Rate ◽  
Microstructural Evolution ◽  
Directionally Solidified
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