There are several methods of epitaxial growth in thin films which have been used mainly for producing electron devices : vapor phase epitaxy (VPE), liquid phase epitaxy (LPE), and solid phase epitaxy (SPE). Molecular beam epitaxy (MBE) is a rather new technique which has been developed as one of the VPE methods. MBE is currently being used to produce thin film devices such as heterojunction lasers, superlattices devices for negative resistance, tunnel diodes, and filed effect transistors. Moreover, MBE has been shown to be a powerful means to study fundamental processes in crystal growth on very clean surfaces in ultra-high vacua (UHV). The fundamental aspects of MBE are reviewed with emphasis on growth kinetics and surface interaction of impinged particles, such as adsorption-desorption, life time of impinged particles, surface migration, and reaction between molecules on surfaces. The characterization and assessment of thin films synthesized by MBE are also discussed.
The MBE apparatus has progressed rapidly in the last two years, leading to the appearance of high performance device based on the MBE method. The recently developed MBE apparatus is described and, also, the history and the futuretrends of such systems.
Homogeneous and asymmetric membranes were fabricated from the mixture of polytrimethylvinylsilane (PTMVS) and polydimethylsiloxane (PSIO). The ratio of the PSIO content to that of the PTMVS was varied. A scanning electron microscope was used to observe the surface and the cross section of the membranes. X-ray diffraction of the membrane and the permeability of N2, O2 and CO2 through the membrane were measured to characterize the PTMVS-PSIO blend in the membranes. Electron microphotographs of the asymmetric membranes showed that the thickness of the dense surface layer decreases, and the pore diameter of the porous substructure becomes larger as the PSIO content increases. X-ray diffraction reveals that the PTMVS and PTMVS-PSIO blend membranes have amorphous structures, and that their interplanar distances become shorter with the increase of PSIO content. This indicates that PSIO molecules are inserted into the structure of PTMVS and are mixed homogeneously at the molecular level. The permeability coefficients of the blend membranes become higher as the PSIO content increases. Moreover, the separation factors are intermediate between those of the pure PTMVS and PSIO membranes.
GaAs-AlAs superlattices were grown by molecular beam epitaxy with various widths of the GaAs quantum well. A non-destructive X-ray diffraction technique was shown to be a practical method to measure the GaAs quantum well width Lz with an accuracy of 10%. Photoluminescence measurements showed the main carrier recombination process to be from the n=1 electron quantum level to the n=1 heavy hole level. Optical absorption spectra exhibited the sharply peaked structure assocated with excitons at room temperature. MQW laser diodes with GaAs-Asx Ga1-xAs superlattice active layers had lower threshold current density than in conventional DH laser diodes. This illustrated the good optical quality of the superlattices.
The effect was investigated of growth temperature on the photoluminescence efficiency of MBE-grown n-GaAs/N-AlGaAs single-heterostructure (SH) layers doped with Si. At growth temperatures higher than 700°C, high optical quality Si-doped N-AlxGa1-xAs layers in the range 0≤x<0. 3 were obtained with photoluminescence intensity comparable to that of LPE material. The lateral uniformity of the n-GaAs layer in one of the SH wafers was evaluated by measuring the light output of fabricated Zn-diffused LED's. The variation of light output of the 1039 tested diodes was less than 3.5%, a significantly smaller variation than that obtained from the same geometry LED's made by LPE. These results suggest that MBE is a more promising epitaxial growth technique than conventional techniques for application to integrated optics.
Experiments on the surface fluorination of graphite electrodes were made in a low pressure glow discharge. Three gases were used in the discharge : SF6, CF4, and CF2Cl2. High voltage AC and DC were directly applied between the graphite and an aluminum counter electrode. After the discharge, measurements of the contact angle of the graphite surface were made by the drop method. The drop contact angle relates to the degree of surface fluorination. Only CF4 caused a significant increase of the drop contact angle related to the above listed gases. When the graphite surfaces were observed by a scanning electron microscope, thin solid products were found. These products were shown to be fluorographite by ESCA analysis. However, the mass of the graphite electrode decreased with increase of discharge time, an unexpected result. It seemed that the graphite fluorination reactions in the discharge were carried out through parallel reaction paths in a fluorographite (solid) and a fluorocarbon (gas) producing process.
Surface problems encountered during plasma confinement experiments in tokamak devices are described in the following context : causes of impurity introduction into the plasma, control of impurity release, fuel recycling, testing of first wall material, and a problem related to additional heating. On the basis of these studies, remaining and future problems are also discussed.