N. Lewis

Active site formation in the direct process for methylchlorosilanes

Abstract The direct process for methylchlorosilane production was studied in a laboratory-scale fluidized bed reactor. The silicon was found to have discrete sites at which silanes were produced. The density of sites was controlled by the nature of the native oxide layer over the silicon. The active sites displayed regular geometry, indicating an anisotropic silicon reaction which preferentially leaves (111) planes exposed as the reaction proceeded. Carbon and SiO2 layers 200 A thick prevented active site formation.

Epitaxial growth and critical current density of thin films of YBa2Cu3O7−x on LaAlO3 substrates

Thin films of the high‐temperature superconductor YBa2 Cu3 O7−x have been produced on (100) LaAlO3 substrates by coevaporation and furnace annealing. A 14‐μm‐wide and 400‐μm‐long constriction patterned on a 0.8‐μm‐thick film had a zero resistance transition temperature of 90 K, a transition width of 1.5 K, and a critical current density of 8×104 A cm−2 at 77 K. Although x‐ray diffraction shows a definite c‐axis alignment normal to the substrate plane, further analysis reveals that c‐axis alignment in the substrate plane is also present. The detailed microstructural picture is revealed by transmission electron microscopy: a continuous layer, about 0.2 μm thick adjacent to the substrate, with c axis normal to the substrate plane, and the remaining top portion of the film, with the c axis in the film plane. In spite of the bilayer structure, the film remains epitaxial (the axes of the superconductor are parallel to the 〈100〉 directions of the substrate).

Silicon and silicon dioxide thermal bonding for silicon‐on‐insulator applications

There has been a good deal of interest recently in the applicability of thermal bonding to silicon‐on‐insultator (SOI) technology. Thermal bonding (also called direct bonding) is accomplished by mating polished, properly hydrolyzed silicon and/or silicon dioxide surfaces, which are then annealed to promote diffusion bonding. In order to produce high‐quality SOI layers it must be demonstrated that the interface betweeen the wafers is void‐free over the entire surface of the wafer (4‐in. wafers in our study). We have found that the standard annealing step which has been used by other groups to form the wafer bond must be followed by a hyperbaric, high‐temperature annealing cycle in order to produce interfaces which are completely void‐free. In addition, we have found that mating the wafers in a controlled atmosphere is necessary to insure that voids do not remain after the thermal processing is complete. We shall present transmission electron micrographs which reveal the morphology of the bonded interface o...

Metal colloid morphology and catalytic activity : further proof of the intermediacy of colloids in the platinum-catalyzed hydrosilylation reaction

Abstract Transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) were used to analyze the platinum colloids formed during hydrosilylation reactions. Reaction solutions were analyzed for the addition of Et3SiH to either n-hexene or neohexene catalyzed by bis{1,3diviny1,1,1,3,3-tetramethyldisiloxane}Pt0 (Karstedts catalyst) and this analysis showed dramatic differences between the morphology of the Pt colloid formed in each case. The neohexene reaction, which gave 62% conversion to products in 1 h, contained polycrystalline, 2.5-nm-diameter Pt particles, whereas the n-hexene reaction only went to 10% conversion in 1 h and contained 1- to 1.5-nm Pt particles. The reaction of Et3SiH with styrene gave results similar to those with neohexene.

Heteroepitaxy of semiconductor‐on‐insulator structures: Si and Ge on CaF2/Si(111)

Si and Ge layers have been grown on CaF2/Si(111) by molecular‐beam epitaxy. Both Ge and Si grow as islands, and both the island size and the spacing between nucleation sites are considerably larger for Ge (∼300 nm) than for Si (∼100 nm). In addition, Ge and Si layers are found to be a mixture of type‐A (aligned with the underlying CaF2) and type‐B (rotated 180° about the surface normal with respect to the underlying CaF2) regions. The crystalline quality and surface morphology of the Ge layers are much better than those of the Si layers. This is thought to be due to the larger island size of the Ge deposits and to a greater ease of movement of the boundaries between type‐A and type‐B regions in Ge. Hall measurements show electron mobilities of up to 664 cm2/V s in Si layers and hole mobilities of 234 cm2/V s for Ge layers. Finally, the use of a GexSi1−x‐Si superlattice, when grown on a GexSi1−x buffer layer which is lattice matched to the CaF2 at the growth temperature, is shown to improve Si heteroepitaxy.

GROWTH AND CHARACTERIZATION OF EPITAXIAL Si/CoSi 2 AND Si/CoSi 2 /Si HETEROSTRUCTURES

Thin ( 2 films have been grown on (111) Siwafers in a UHV system using a variety of growth techniques including solid phase epitaxy (SPE), reactive deposition epitaxy (RDE), and molecular beam epitaxy (MBE). SEN and TEN studies reveal significant variations in the epitaxial silicide surface morphology as a function of the sillciqd formation method. Pinhole densities are generally greater than 10 7 cm -2 , although some reduction can be achieved by utilizing proper growth techniques. Si epilayers were deposited over the CoSi 2 films inthe temperature range from 550oC to 800oC, and the reesuulttinng structures have been characterized using SEM, cross—sectional TEN, and ion channeling measurements. These measurements show that the Si epitaxial quality increases with growth temperature, although the average Si surface roughness and the CoSi 2 pinhole density also increase as the growth temperature is raised.

Strain relief of large lattice mismatch heteroepitaxial films on silicon by tilting

Abstract In this paper, a general formula is calculated relating the misalignment of a heteroepitaxial overlayer with respect to the substrate to the defects at the interface. High resolution transmission electron microscopy studies are presented which show that the density of dislocations along the interface of GaAs grown in vicinal Si(100), with the Burgers vector perpendicular to the interface, is consistent with the amount of tilt observed in ion-channeling studies. Arguments are also presented which demonstrate that the tilt observed is an effective form of strain relief but that this mechanism can only operate when the heteroepitaxial growth is nucleated by island growth. One significant conclusion is that the lower bound observed on the threading dislocation density in GaAs on silicon is due to this mechanism of strain relief by tilting. These arguments should be valid for all large-lattice-mismatched heteroepitaxial systems in which the epitaxial layer has a larger lattice parameter than the substrate.

Novel pseudomorphic high electron mobility transistor structures with GaAs‐In0.3Ga0.7As thin strained superlattice active layers

The thin strained superlattice (TSSL) concept is introduced as a means for extending the practical range of application for pseudomorphic Inx Ga1−x As on GaAs. Growth and characterization results are presented for pseudomorphic high electron mobility transistor structures with GaAs‐In0.3 Ga0.7 As TSSL active layers grown by molecular beam epitaxy. The TSSLs are composed of three periods of GaAs(15 A)‐ In0.3 Ga0.7 As(h2 ), where h2 ranges from 30 to 52 A. Modulation doping of the TSSLs is provided by atomic planar‐doped Al0.3 Ga0.7 As overlayers with 45 A undoped spacers. 77 K Hall effect and transmission electron microscopy reveal that relatively thick TSSLs can be grown with high electronic and structural quality, comparable to much thinner In0.3 Ga0.7 As single quantum wells. Results are compared with a model for critical layer thickness and discussed in light of in situ reflection high‐energy electron diffraction measurements.

The effect of metal colloid morphology on catalytic activity: Further proof of the intermediacy of colloids in the rhodium-catalyzed hydrosilylation reaction

Abstract The relative activity of different morphological forms of Rh colloids were examined by transmission electron microscopy (TEM) or high resolution electron microscopy (HREM). The degree of agglomeration of the Rh colloid was affected by either its storage history (e.g. refrigerator or bench top) or by the use of a vinyl silicone stabilizer. The smaller particle size (higher surface area) yellow Rh colloid was more active than the larger particle size red Rh colloid. A black Rh colloid, which contained no stabilizer, was more active than both the yellow and red forms. The heterogeneous nature of both Pt and Rh catalyzed reactions was further confirmed by inhibition studies with mercury.

Film thickness dependence of critical current density and microstructure for epitaxial YBa2Cu3O7-x films

Films of nominal composition YBa2Cu3O7-x(YBCO) were made on (100) SrTiO3 substrates by coevaporation and furnace annealing. Film thickness was in the range 0.2-2.4 mu m. Chemical analysis by inductively coupled plasma emission spectroscopy reveals that the thinner films had an excess of Cu and Ba relative to Y, which led to Cu- and Ba-rich particles on the film surface. Transmission electron microscopy was used to define the epitaxial nature of the films: a continuous layer of about 0.4 mu m thickness adjacent to the substrate and with the c axis normal to the substrate plane, followed by material with the c-axis in the film plane. A simple model based on microstructural observations and the strongly anisotropic transport properties of YBCO is compared with the measurements of room-temperature resistivity and critical current density at 77 K as a function of film thickness. The critical current density is found to decrease as a function of increasing film thickness by a much larger factory than the simple model predicts.