Kinetic Monte Carlo Simulation of Strained Heteroepitaxial Growth with Intermixing

An efficient method for the simulation of strained heteroepitaxial growth with intermixing using kinetic Monte Carlo is presented. The model used is based on a solid-on-solid bond counting formulation in which elastic effects are incorporated using a ball and spring model. While idealized, this model nevertheless captures many aspects of heteroepitaxial growth, including nucleation, surface diffusion, and long range effects due elastic interaction. The algorithm combines a fast evaluation of the elastic displacement field with an efficient implementation of a rejection-reduced kinetic Monte Carlo based on using upper bounds for the rates. The former is achieved by using a multigrid method for global updates of the displacement field and an expanding box method for local updates. The simulations show the importance of intermixing on the growth of a strained film. Further the method is used to simulate the growth of self-assembled stacked quantum dots

Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries

Background Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres. Methods This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries. Results In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia. Conclusion This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries

Growth and investigation of epitaxial semiconductor films

The work described in this thesis is concerned with the growth and examination of thin Si and Ge films. These were deposited by sublimation and evaporation techniques in UHV, using apparatus constructed by the author, The source of deposited material was heated by electron bombardment, while the substrates were heated either directly by the passage of an electric current or indirectly upon a Si bar, depending upon their electrical conductivity. The various substrates used were Si and the insulators sapphire, diamond and β-SiC. The grown films were studied by use of the optical microscope, the transmission electron microscope and the scanning electron microscope. The main object of the work was to investigate the structural perfection of these films, to deduce information concerning their growth mechanisms and to compare the observations with suitable theoretical models. The initial growth of Si films on unpreheated (111) Si substrates, held at temperatures around 900°C, was deduced to have been by nucleation (after an initial induction period), since irregular Si growth centres were produced. These eventually overlapped to give a channelled film containing few crystallographic defects. The dependence of the number density of Si growth centres upon deposition conditions was correlated with different theoretical relationships derived separately by Joyce et al (1967) and Logan (1969). It was deduced that the smallest stable nuclei contained 3 Si atoms, and that the activation energy for surface diffusion of Si was ~0.55 eV (Joyce et al) or ~1.1 eV (Logan). These observations and deductions were compared with analogous results obtained by Joyce et al, who grew Si films by silane pyrolysis. All the results were discussed in relation to initial growth processes, and various suggestions were put forward to account for certain unexpected features. Possible impurity effects were considered in detail and related to observed film growth characteristics. When (111) Si substrates were cleaned using heat treatment at 1250°C in UHV surface steps were generated, and these moved across the substrates in a [1̅1̅2] type direction. When Si deposition was initiated the direction of step motion reversed as growth took place. The steps were pinned in many places by impurity particles, which caused the production of deep pits in the grown films. Transmission electron energy loss analysis showed that the particles were SiC. The dependence of the Si film growth upon deposition conditions, the state of the substrate surface and the presence of contaminating gases was also investigated. The nature of both sublimation and growth step motion across substrate surfaces was analysed using theoretical models, and this showed that in the absence of pinning effects, the motion was generally that expected for a clean surface. Ge films were deposited on to (111) Si substrates at temperatures around 800°C. The thinnest films were composed of growth centres which were mostly triangular in shape, and this showed that initial growth had been by nucleation. A tentative comparison of experimental data with theoretical models indicated that the smallest stable Ge nucleus may have been composed of 4 atoms. TEM examination of the Ge films showed that they grew with the same crystallographic orientation as their Si substrates. However, the films contained many crystallographic defects, and some of the most prominent were continuous networks of misfit dislocations, which occurred over each entire Ge/Si interface. The spacings of the dislocations in such networks were non-ideal, and this was taken to indicate that considerable Ge/Si alloy formation had taken place. Deposition of Si on to heated (0001) sapphire substrates gave films which grew by initial nucleation with the epitaxial relationships (111)Si//(0001)Al2O3 and [11̅0]Si//[112̅0]Al2O3. The thinnest films were composed of equal proportions of two 60° rotational twins, and often contained large numbers of planar defects. However, films a substantial fraction of a micron in thickness contained misorientations of a more general nature, and with increasing thickness ultimately became polycrystalline. High resolution TEM studies provided no evidence of an interfacial misfit dislocation network, but instead indicated that a chemical reaction had taken place between the Si and the sapphire during deposition. Si films were also grown upon both (111) diamond and (111) βSiC substrates, and once again initial growth was by nucleation. Thicker films on diamond substrates exhibited significant orientation retention despite the very large crystallographic mismatch. Work was also carried out involving the high resolution TEM studies of closely spaced misfit edge dislocation networks at the Ge/(111)Si interface. It was generally possible to obtain either fringe or structural images depending upon the nature of the diffraction conditions, and this behaviour was examined in detail and discussed with special reference to the work of Thölén (1970). Use of the weak-beam technique (Cockayne et al, 1969) enabled the network structures to be investigated in detail. While it was observed that all the intrinsic dislocation nodes had a relatively uniform extension, the extrinsic nodes had mostly a much smaller extension and approximately half appeared to be completely contracted. Also, using 111̅ type Bragg reflections, the intensity of the TEM contrast exhibited by the different kinds of node stacking fault were markedly dissimilar, and depended upon the sense of the deviation from the exact Bragg position. These studies were extended to observations of dislocation networks produced by deformation in single crystal Si. Measurements of the dimensions of intrinsic nodes, which were all extended, yielded values for the intrinsic stacking fault energy in Si of ~58 or ~71 ergs cm-2, depending upon the type of measurement employed. Only half the extrinsic nodes were extended, and this behaviour was interpreted in terms of an energy barrier to node extension. Using 111̅ type reflections, the node contrast behaviour was analogous to that observed in the Ge/(111)Si system. This was correlated with the contrast behaviour of inclined stacking faults, and the image intensity appeared to depend upon the sign of (g.R)s. Using 224̅ type reflections, dissociated dislocation contrast was interpreted in terms of interactions between the strain fields of the bounding partial dislocations. In conclusion, suggestions were put forward concerning possible extensions of the present work, and these involved both film growth studies and investigations of TEM contrast behaviour

Growth and investigation of epitaxial semiconductor films

The work described in this thesis is concerned with the growth and examination of thin Si and Ge films. These were deposited by sublimation and evaporation techniques in UHV, using apparatus constructed by the author, The source of deposited material was heated by electron bombardment, while the substrates were heated either directly by the passage of an electric current or indirectly upon a Si bar, depending upon their electrical conductivity. The various substrates used were Si and the insulators sapphire, diamond and β-SiC. The grown films were studied by use of the optical microscope, the transmission electron microscope and the scanning electron microscope. The main object of the work was to investigate the structural perfection of these films, to deduce information concerning their growth mechanisms and to compare the observations with suitable theoretical models. The initial growth of Si films on unpreheated (111) Si substrates, held at temperatures around 900°C, was deduced to have been by nucleation (after an initial induction period), since irregular Si growth centres were produced. These eventually overlapped to give a channelled film containing few crystallographic defects. The dependence of the number density of Si growth centres upon deposition conditions was correlated with different theoretical relationships derived separately by Joyce et al (1967) and Logan (1969). It was deduced that the smallest stable nuclei contained 3 Si atoms, and that the activation energy for surface diffusion of Si was ~0.55 eV (Joyce et al) or ~1.1 eV (Logan). These observations and deductions were compared with analogous results obtained by Joyce et al, who grew Si films by silane pyrolysis. All the results were discussed in relation to initial growth processes, and various suggestions were put forward to account for certain unexpected features. Possible impurity effects were considered in detail and related to observed film growth characteristics. When (111) Si substrates were cleaned using heat treatment at 1250°C in UHV surface steps were generated, and these moved across the substrates in a [1̅1̅2] type direction. When Si deposition was initiated the direction of step motion reversed as growth took place. The steps were pinned in many places by impurity particles, which caused the production of deep pits in the grown films. Transmission electron energy loss analysis showed that the particles were SiC. The dependence of the Si film growth upon deposition conditions, the state of the substrate surface and the presence of contaminating gases was also investigated. The nature of both sublimation and growth step motion across substrate surfaces was analysed using theoretical models, and this showed that in the absence of pinning effects, the motion was generally that expected for a clean surface. Ge films were deposited on to (111) Si substrates at temperatures around 800°C. The thinnest films were composed of growth centres which were mostly triangular in shape, and this showed that initial growth had been by nucleation. A tentative comparison of experimental data with theoretical models indicated that the smallest stable Ge nucleus may have been composed of 4 atoms. TEM examination of the Ge films showed that they grew with the same crystallographic orientation as their Si substrates. However, the films contained many crystallographic defects, and some of the most prominent were continuous networks of misfit dislocations, which occurred over each entire Ge/Si interface. The spacings of the dislocations in such networks were non-ideal, and this was taken to indicate that considerable Ge/Si alloy formation had taken place. Deposition of Si on to heated (0001) sapphire substrates gave films which grew by initial nucleation with the epitaxial relationships (111)Si//(0001)Al2O3 and [11̅0]Si//[112̅0]Al2O3. The thinnest films were composed of equal proportions of two 60° rotational twins, and often contained large numbers of planar defects. However, films a substantial fraction of a micron in thickness contained misorientations of a more general nature, and with increasing thickness ultimately became polycrystalline. High resolution TEM studies provided no evidence of an interfacial misfit dislocation network, but instead indicated that a chemical reaction had taken place between the Si and the sapphire during deposition. Si films were also grown upon both (111) diamond and (111) βSiC substrates, and once again initial growth was by nucleation. Thicker films on diamond substrates exhibited significant orientation retention despite the very large crystallographic mismatch. Work was also carried out involving the high resolution TEM studies of closely spaced misfit edge dislocation networks at the Ge/(111)Si interface. It was generally possible to obtain either fringe or structural images depending upon the nature of the diffraction conditions, and this behaviour was examined in detail and discussed with special reference to the work of Thölén (1970). Use of the weak-beam technique (Cockayne et al, 1969) enabled the network structures to be investigated in detail. While it was observed that all the intrinsic dislocation nodes had a relatively uniform extension, the extrinsic nodes had mostly a much smaller extension and approximately half appeared to be completely contracted. Also, using 111̅ type Bragg reflections, the intensity of the TEM contrast exhibited by the different kinds of node stacking fault were markedly dissimilar, and depended upon the sense of the deviation from the exact Bragg position. These studies were extended to observations of dislocation networks produced by deformation in single crystal Si. Measurements of the dimensions of intrinsic nodes, which were all extended, yielded values for the intrinsic stacking fault energy in Si of ~58 or ~71 ergs cm-2, depending upon the type of measurement employed. Only half the extrinsic nodes were extended, and this behaviour was interpreted in terms of an energy barrier to node extension. Using 111̅ type reflections, the node contrast behaviour was analogous to that observed in the Ge/(111)Si system. This was correlated with the contrast behaviour of inclined stacking faults, and the image intensity appeared to depend upon the sign of (g.R)s. Using 224̅ type reflections, dissociated dislocation contrast was interpreted in terms of interactions between the strain fields of the bounding partial dislocations. In conclusion, suggestions were put forward concerning possible extensions of the present work, and these involved both film growth studies and investigations of TEM contrast behaviour.</p

Bi incorporation in GaAs(100)-2×1 and 4×3 reconstructions investigated by RHEED and STM

Bi acts as a surfactant in molecular beam epitaxy (MBE) growth on GaAs(100). Incorporation is achieved by disequilibrium at growth temperatures below ∼450 °C. Bi can however affect the static reconstruction up to 600 °C. Two reconstructions are considered in this work: dynamic (2×1) and static c(8×3)/(4×3), which are shown to be the dominant reconstructions for GaAsBi MBE. Bi storage in these two reconstructions provides an explanation of RHEED transitions that cause unintentional Bi incorporation in the GaAs capping layer. Finally dynamic observations of the (2×1) reconstruction are used to explain growth dynamics, atomic ordering and clustering observed in GaAsBi epilayers which have a direct influence on photoluminescence linewidth broadening in mixed anion III–V alloys

Advanced TEM Analysis of Strain-Balanced Si/SiGe Resonant Tunnelling Diode Structures

We present a TEM analysis of a series of Si/SiGe resonant tunnelling diode structures which contain Si1-xGex quantum wells (x of 0.4 and 1.0) grown on a range of (001) Si1-yGey virtual substrates (y of 0.15, 0.2 and 0.3). It is found that the dislocation density in the graded region of the virtual substrate increases with the y parameter, as expected. The subsequent effect of this, however, is to increase the amplitude of RMS roughness of the substrate surface, which ranges from 2.0 to 6.7 rim. Quantum wells with x = 0.4 are found to be highly planar; however, at x = 1.0 the wells undulate significantly due to misfit strain