Since publication of the classic text on the electron microscope laboratory by Anderson, the proliferation of microscopes with field emission guns, imaging filters and hardware spherical aberration correctors (giving higher spatial and energy resolution) has resulted in the need to construct special laboratories. As resolutions iinprovel transmission electron microscopes (TEMs) and scanning transmission electron microscopes (STEMs) become more sensitive to ambient conditions. State-of-the-art electron microscopes require state-of-the-art environments, and this means careful design and implementation of microscope sites, from the microscope room to the building that surrounds it. Laboratories have been constructed to house high-sensitive instruments with resolutions ranging down to sub-Angstrom levels; we present the various design philosophies used for some of these laboratories and our experiences with them. Four facilities are described: the National Center for Electron Microscopy OAM Laboratory at LBNL; the FEGTEM Facility at the University of Sheffield; the Center for Integrative Molecular Biosciences at TSRI; and the Advanced Microscopy Laboratory at ORNL.

While electroluminescence has been demonstrated at terahertz frequencies from Si/SiGe quantum cascade emitters, to date no laser has been achieved due to poor vertical confinement of the optical mode. A method of increasing the vertical confinement of the optical mode for a Si/SiGe quantum cascade laser is demonstrated using silicon-on-silicide technology. Such technology is used with epitaxial growth to demonstrate a strain-symmetrised 600 period Si/SiGe quantum cascade interwell emission and the polarisation is used to demonstrate the optical confinement. Electroluminescence is demonstrated at ∼3 THz (∼100 μm) from an interwell quantum cascade emitter structure. Calculated model overlap and waveguide losses for ridge waveguides are comparable to values from GaAs quantum cascade lasers demonstrated at terahertz frequencies. The effects of high doping levels in Si/SiGe quantum cascade structures is also investigated with impurity emission demonstrated rather than intersubband emission for the highest doping levels used in the cascade active regions.

The way in which the Stranski-Krastanow epitaxial islanding transition can be controlled by strain due to elemental segregation within the initially-formed flat ‘wetting’ layer is examined in detail. Experimentally measured critical ‘wetting’ layer thicknesses for the InxGa1−xAs/GaAs system (x = 0.25 - 1) are demonstrated to show good agreement with values calculated using a segregation model. The strain energy associated with the segregated surface layer is determined for the complete range of deposited In concentrations using atomistic simulations. The segregation-mediated driving force for the Stranski-Krastanow transition is considered to be important also for all other epitaxial systems exhibiting the transition.

Boron is the most important p-type dopant in Si and it is essential that, especially for low energy implantation, both as-implanted B distributions and those produced by annealing should be characterized in very great detail to obtain the required process control for advanced device applications. While secondary ion mass spectrometry (SIMS) is ordinarily employed for this purpose, in the present studies implant concentration profiles have been determined by direct B imaging with approximately nanometer depth and lateral resolution using energy-filtered imaging in the transmission electron microscopy. The as-implanted B impurity profile is correlated with theoretical expectations: differences with respect to the results of SIMS measurements are discussed. Changes in the B distribution and clustering that occur after annealing of the implanted layers are also described.

The interest in the phenomenon of islanding in a range of semiconductor systems is in part due to the fundamental importance of the Stranski-Krastanow transition but also driven by potential device applications of self-organized quantum dot arrays. However, the mechanism underlying the island formation is still to a significant degree unclear. In the present work, we focus on the epitaxial InGaAs / GaAs(001) system, with layer deposition by molecular beam epitaxy. Atomic force microscopy is used to measure the surface topography of nominally 4nm thick InxGa1-xAs films. It is shown that the growth mode switches abruptly from flat layer to island growth if a critical Indium composition of x(In)≍0.25 is reached. The structure of such layers during early stages of growth is examined using energy-filtered transmission electron microscopy. Indium gradients in the islanded layers are measured and the driving force for the islanding transition itself is considered.

Boron is the most important p-type dopant in Si and it is essential that, especially for low energy implantation, both as-implanted B distributions and those produced by annealing should be characterized in very great detail to obtain the required process control for advanced device applications. While secondary ion mass spectrometry (SIMS) is ordinarily employed for this purpose, in the present studies implant concentration profiles have been determined by direct B imaging with approximately nanometer depth and lateral resolution using energy-filtered imaging in the transmission electron microscopy. The as-implanted B impurity profile is correlated with theoretical expectations: differences with respect to the results of SIMS measurements are discussed. Changes in the B distribution and clustering that occur after annealing of the implanted layers are also described.

SiGe/Si p-channel heteroepitaxial MOSFET test structures have been fabricated using solid-source molecular beam epitaxy. High-resolution transmission electron microscopy and energy-loss filtered imaging have been used to quantitatively determine the nanoscale Ge distributions across the SiGe alloy channel. The Ge profile at the edges of the alloy channel were found to be asymmetrical due to the effect of Ge segregation, with an exponential-like distribution directed toward the surface. The results agree well with the predictions of segregation theory and indicate that the concentration of Ge in the extended distribution lay in the range 10%-1% over a distance of several nanometers from the body of the channel. Secondary ion mass spectrometry measurements upon the same samples were insensitive to this short range extended Ge distribution.

Photocurrent spectroscopy of InAs/GaAs self-assembled quantum dots, studied as a function of applied electric field, is used to probe the nature of the confined electronic states. A field asymmetry of the quantum confined Stark effect is observed, consistent with the dots possessing a permanent dipole moment. The sign of this dipole indicates that for zero field the hole wavefunction lies above that of the electron, in disagreement with the predictions of all recent calculations. Comparison with a theoretical model demonstrates that the experimentally determined alignment of the electron and hole can only be explained if the dots contain a nonzero and non-uniform Ga content. The role of two different carrier escape mechanisms, tunneling and thermal excitation, is studied.

The growth of semiconductor heteroepitaxial layers is assuming ever greater importance due to the demands of modern electronic-device fabrication. Furthermore although low-strain heterosystems such as AlGaAs/GaAs remain the basis of many device structures, there is an increasing trend also to use more highly strained combinations such as InGaAs/GaAs and SiGe/Si. However the growth of the latter epitaxial systems must be approached with great care in order to achieve the optimum layer structural quality. Of course for any given alloy-layer composition, interfacial misfit defects in general will be introduced when the layer thickness exceeds a critical value, as originally described by Frank and van der Merwe, and Jesser and Matthews. (See the article by F.K. LeGoues in this issue for more general considerations of misfit-defect introduction.) In addition the morphology of such strained layers must be given very careful attention. It is the purpose of this article to examine our current understanding in this latter area.

When any epitaxial layer is grown, initially it might be expected that a flat surface would result under ideal growth conditions when internal defects have been eliminated. This would be expected to minimize the surface step density and hence the surface energy. However nature has a way of confounding our simplest expectations and while for homoepitaxial layers in the absence of kinetic effects this expectation virtually can be realized, the presence of strain in a heteroepitaxial system can severely affect the observed layer morphology.

The manner in which misfit strain can influence the morphology of heteroepitaxial layers is reviewed. Following a brief consideration of theoretical modelling, examples of experimental observations for two important materials systems, SiGe/Si and InGaAs/GaAs, are given. It is demonstrated that the formation of undulations of specific types is driven by partial elastic stress-relief and a lowering of the system free energy. Under these conditions, islands of deposit can be formed during initial growth and ripples can be produced upon continuous layers. Furthermore, the presence of surface morphological distortions and the accompanying strain fluctuations also can have a significant impact upon misfit dislocation introduction. Relationships between these fluctuations and dislocation source behaviour are described.

The MBE growth and related materials characterisation of InSb/InAlSb strained-layer structures is described. Band-gap considerations and critical thickness calculations are presented and indicate that this material system should offer considerable device potential. Detailed structural studies, performed using both transmission electron microscopy and X-ray diffraction, confirm the growth of high quality multiple quantum-wells, and 2K photoluminescence has shown corresponding energy upshifted transitions.

Ge has a diamond cubic crystal structure (α) while Sn exists in both the diamond cubic (α) and body centred tetragonal (β) allotropic forms. The solid solubilities between Ge and Sn are very small according to the equilibrium phase diagram. This paper describes an investigation into the manufacture of metastable diamond cubic Ge-Sn alloy layers using pulsed laser melting of evaporated thin films . Cross-sectional and plan-view transmission electron microscopy have been used to characterise the microstructures of the Ge-Sn alloy layers, with alloy compositions determined by energy-dispersive X-ray microanalysis. The metastable diamond cubic Ge-Sn alloy microstructures are discussed in detail as a function of processing conditions.

Advances in the study of high speed crystal growth from the melt are reviewed, with special emphasis on the fast melting and solidification of silicon achieved by use of Q-switched laser radiation pulses. Rapid melting of amorphous Si is confirmed to yield a liquid undercooled by several hundred Kelvins and, under suitable conditions, explosive crystal growth processes can occur. The latter involve the self-sustaining propagation of melt bands buried within the initially amorphous material. When the highest quench-rate conditions are established melting of even crystalline Si can yield a final amorphous solid phase. This breakdown in crystal growth is orientation dependent and can give regimes of crystal defect formation when amor-phization does not take place. The processes which characterize this limiting growth behaviour are discussed.

The use of Q-switched laser melting techniques to investigate new rapid solidification phenomena is described. It has been found that Si, Ge, GaP and GaAs can give rise to orientation dependent, kinetically-controlled defect generation processes during fast recrystallization from the melt. Indeed, these materials yield amorphous phases at sufficiently high solidification rates. Ultra-fast pulsed melting permits the study of the basic thermodynamic properties of amorphous solids. It is shown that amorphous Si melts to give a normal, low viscosity, undercooled liquid and that novel explosive crystal growth processes can occur in this low temperature regime.

Subnanosecond ultra-violet radiation pulses are used to produce relatively thick, large area amorphous layers onSi crystals by transient melting and solidification. The different behaviours of (001) and (111) Si orientations are highlighted. Observations of crystal growth phenomena during solidification at velocities lower than required for amorphization are correlated with theoretical predictions. Computer modelling of heat flow in amorphous silicon is refined.

Short (2.5nsec) pulses of ultra-violet radiation from a Q-switched laser system have been used to induce a range of defect transitions in the surface regions of (001) and (111) Si single crystal by transient melting and resolidification. Relatively large areas of Si have been uniformly processed and this has enabled the measurement of thresholds for both amorphization and extended defect production. The highest quench rates (isotherm velocities of ∼20m/sec) were achieved at the surface melting threshold near 0.2J/cm2 and both (001) and (111) Si could be amorphized with radiation energy densities close to this value. With increasing energy density the quench rate fell and (001) Si ceased to amorphize before (111) Si. Furthermore, over a range of high radiation fluxes the crystalline Si produced on (111) surfaces was highly defective and contained twins due to errors in liquid-phase epitaxy. The various observed structure transitions have been related to the predictions of crystal growth theory with account taken of melt undercooling effects. The amorphous Si produced during this work (up to almost 1000Å thickness) has been shown to be structurally similar to that produced by conventional methods. Both direct analysis and solid-phase regrowth experiments have demonstrated that its impurity content is negligible.