Molecular beam epitaxy (MBE) is a process for growing thin, epitaxial films of a wide variety of materials, ranging from oxides to semiconductors to metals. It was first applied to the growth of compound semiconductors. That is still the most common usage, in large part because of the high technological value of such materials to the electronics industry. In this process beams of atoms or molecules in an ultra-high vacuum environment are incident upon a heated crystal that has previously been processed to produce a nearly atomically clean surface. The arriving constituent atoms form a crystalline layer in registry with the substrate, i.e., an epitaxial film. These films are remarkable because the composition can be rapidly changed, producing crystalline interfaces that are almost atomically abrupt. Thus, it has been possible to produce a large range of unique structures, including quantum well devices, superlattices, lasers, etc., all of which benefit from the precise control of composition during growth. Because of the cleanliness of the growth environment and because of the precise control over composition, MBE structures closely approximate the idealized models used in solid state theory.
This discussion is intended as an introduction to the concept and the experimental procedures used in MBE growth. The refinement of experimental procedures has been the key to the successful fabrication of electronically significant devices, which in turn has generated the widespread interest in the MBE as a research tool. MBE experiments have provided a wealth of new information bearing on the general mechanisms involved in epitaxial growth, since many of the phenomena initially observed during MBE have since been repeated using other crystal growth processes. We also summarize the general types of layered structures that have contributed to the rapid expansion of interest in MBE and its various offshoots. Finally we consider some of the problems that remain in the growth of heteroepitaxial structures, specifically, the problem of mismatch in lattice constant between layers and between layer and substrate. The discussion is phenomenological, not theoretical; MBE has been primarily an experimental approach based on simple concepts. 相似文献
A discussion on the use of Auger electron spectroscopy as a quantitative tool to determine the growth mode of metals on single crystal oxide surfaces is presented. In the case of Pd grown epitaxially on MgO(100), the three-dimensional character of the growth is easily seen at coverage above one monolayer. However, in the submonolayer regime, and mainly at low substrate temperatures, the AES results are ambiguous. The combination of AES with the more sensitive helium-atom diffraction method allows us to demonstrate that the growth is three-dimensional from the early stages, the particles becoming flatter when the substrate temperature decreases. We compare our results with other growth studies on different metal/oxide systems. At low temperature, the ideal growth modes are not always observed, the final morphology of the films being determined mainly by kinetic effects. Thus a pseudo-Stranski-Krastanov growth mode is often obtained with formation of 2D islands followed by 3D clustering from a critical submonolayer coverage. 相似文献
The selectivity in parallel and consecutive heterogeneous catalytic reactions over nonuniform surfaces has been analyzed within the framework of the surface electron gas model. Equations for the selectivity are derived in the case of slow adsorption (no rate-limiting steps). Conditions when the nonuniform character of real catalytic surfaces should be taken into account in modeling of the selectivity behavior are discussed. 相似文献
Summary The static response to a charge impurity near a metal surface is computed in the RPA approximation for the case of a finite
barrier. Particular attention is given to the problem of the correct infinite-volume limit and to the contribution of the
one-particle states with energy higher than the potential barrier, which has been found to be not negligible. An integral
equation for the self-consistent potential is derived and solved numerically. Results for Na and Zn are exhibited, showing
remarkable difference with respect to the infinite-barrier case.
Riassunto La risposta statica ad un'impurezza di carica in prossimità della superficie di un metallo è calcolata nell'approssimazione
delle fasi a caso (RPA) nel caso di una barriera finita. Sono trattati con particolare cura i problemi del corretto limite
di volume infinito e del contributo degli stati ad una particella con energia superiore alla barriera di potenziale, contributo
che risulta non trascurabile. Si deriva e risolve numericamente un'equazione integrale per il potenziale autoconsistente.
Sono presentati i risultati per Na e Zn, da cui si può rilevare una notevole differenza rispetto al caso della barriera infinita.
Резюме В случае конечного барьера в приближении случайных фаз вычисляется статический отклик на заряженную примесь вблизи металлической
поверхности. Особое внимание уделяется проблеме корректного предела бесконечного объема и вкладу одно-частичных состояний
с энергией выше потенциального барьера, который, как получено, не является пренебрежимо малым. Выводится интегральное уравнение
для самосогласованного потенциала и решается численно. Полученные результаы для Na и Zn обнаруживают значительное различие
с результатами для случая бесконечного барьера.
The G-protein-coupled receptor (GPCR) superfamily represents one of the largest classes of molecules involved in signal transduction
across the plasma membrane. Fluorescence-based approaches have provided valuable insights into GPCR functions such as receptor–receptor
and receptor–ligand interactions, real-time assessment of signal transduction, receptor dynamics on the plasma membrane, and
intracellular trafficking of receptors. This has largely been possible with the use of fluorescent probes such as the green
fluorescent protein (GFP) from the jellyfish Aequoria victoria and its variants. We discuss the potential of fluorescence-based approaches in providing novel information on the membrane
organization and dynamics of the G-protein-coupled serotonin1A receptor tagged to the enhanced yellow fluorescent protein (EYFP).
These authors contributed equally to the work. 相似文献
The current paper presents results of a quantum-chemical study of the surface structure of nanoparticles of both rutile and anatase crystallographic modifications. Different stages of the surface relaxation are discussed. Water adsorption is considered. The calculations were performed in the spd-basis by using semi-empirical quantum-chemical codes, both sequential and parallel. The results are mainly addressed to the study of the interface formed by titania nanoparticles and a set of carboxylated species, namely, benzoic, bi-isonicotinic acids as well as tris-(2,2′-dcbipyridine) Fe(II) complex placed on the surface of either rutile or anatase polymorphs.This revised version was published online in August 2005 with a corrected issue number. 相似文献
The dispersion behaviour of laser-synthesized silicon carbide nanoparticles (npSiC) in water is investigated by photon correlation spectroscopy (PCS). With regard to previous studies and due to an application in the processing of optical materials, this paper concerns low npSiC contents (from 0.05 to 10 wt.%). The role played by the particle surface state is be pointed out through the consideration of stochiometric (C/Si = 1), carbon-rich (C/Si > 1) and silicon-rich (C/Si < 1) nanopowders. Suspensions made from stoichiometric and silicon-rich nanopowders are easily dispersed and stable with time. The PCS measurements reveal in this case more than 95% of isolated nanoparticles, pointing out the key role of the oxidized layer covering the grain of silicon-rich samples. At the opposite, the carbon-rich powders are hardly dispersed in pure water, correlated with the presence of a relatively inert graphitic carbon layer at the grain surface. However, by addition of a commercial polymeric dispersant, all nanopowders induce high quality suspensions. In particular, the carbon-rich samples are easily dispersed, and possible dispersion mechanisms of npSiC in presence of a polymeric surfactant are discussed. The influence of the npSiC loading and the time evolution of the suspension are also presented. By considering stoichiometric, as well as carbon- and silicon-rich samples, this paper demonstrates the possibility to achieve high quality dispersions of SiC nanoparticles, whatever the chemical composition of the powder, as an easy step for optical material processing.This revised version was published online in August 2005 with a corrected issue number. 相似文献
The methodological aspects of PSEE spectroscopy of the surface of irradiated dielectrics have been considered. A generalized
method for processing the nonselective photostimulated electron emission (PSEE) spectra taking into account the effects of
radiation electrification and structural disordering is proposed and has been substantiated. The procedure of separation of
the emission contribution of discrete radiation centers providing, in the stationary approach, estimation of a number of parameters
and the concentration of emission-active defects of the surface layer of the material has been described. The potentialities
of the method have been demonstrated with the example of Be2SiO4 phenacite crystals and crystalline and glassy SiO2. Diamagnetic oxygen-deficient centers, body and surface E'-centers, as well as hole O
10
-centers on nonbridging oxygen atoms have been registered.
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 381–385, May–June, 2005. 相似文献