C. D. Moore

Thermal conductivity of nanoporous bismuth thin films

The thermal conductivity of nanoporous Bi thin films has been experimentally determined. Samples are fabricated by a liquid phase deposition, and their thermal conductivities are measured by a differential 3ω method. Nanoporous Bi thin films exhibit an order-of-magnitude reduction in thermal conductivity compared to that of solid films, most likely the result of a reduction in phonon mean free path. When porous Bi films are exposed to a hydrogen plasma, thermal conductivity measurements reveal no variation with extent of porosity, while electrical conductivity is much more sensitive to porosity, suggesting the possibility of independent control of these two intrinsic properties.

A SURFACTANT-MEDIATED RELAXED SI0.5GE0.5 GRADED LAYER WITH A VERY LOW THREADING DISLOCATION DENSITY AND SMOOTH SURFACE

A method to grow a relaxed Si0.5Ge0.5 graded layer with a very smooth surface and a very low threading dislocation density using solid-source molecular-beam epitaxy is reported. This method included the use of Sb as a surfactant for the growth of a 2 μm compositionally graded SiGe buffer with the Ge concentration linearly graded from 0% to 50% followed by a 0.3 μm constant Si0.5Ge0.5 layer. The substrate temperature was kept at 510 °C during the growth. Both Raman scattering and x-ray diffraction were used to determine the Ge mole fraction and the degree of strain relaxation. Both x-ray reflectivity and atomic force microscopy measurements show a surface root mean square roughness of only 20 A. The threading dislocation density was determined to be as low as 1.5×104 cm−2 as obtained by the Schimmel etch method. This study shows that the use of a Sb surfactant and low temperature growth is an effective method to fabricate high-quality graded buffer layers.

Effective compliant substrate for low-dislocation relaxed SiGe growth

An effective compliant substrate for Si1-xGex growth is presented. A silicon-on-insulator substrate was implanted with B and O forming 20 wt % borosilicate glass within the SiO2. The addition of the borosilicate glass to the buried oxide acted to reduce the viscosity at the growth temperature of Si1-xGex, promoting the in situ elastic deformation of the thin Si (∼20 nm) layer on the insulator. The sharing of the misfit between the Si and the Si1-xGex layers was observed and quantified by double-axis X-ray diffraction. In addition, the material quality was assessed using cross-sectional transmission electron microscopy, photoluminescence and etch pit density measurements. No misfit dislocations were observed in the partially relaxed 150-nm Si0.75Ge0.25 sample as-grown on a 20% borosilicate glass substrate. The threading dislocation density was estimated at 2×104 cm-2 for 500-nm Si0.75Ge0.25 grown on the 20% borosilicate glass substrate. This method may be used to prepare compliant substrates for the growth of low-dislocation relaxed SiGe layers.

Experimental study of a surfactant-assisted SiGe graded layer and a symmetrically strained Si/Ge superlattice for thermoelectric applications

Abstract A method to grow high-quality SiGe graded buffer layer is presented. The main concept of the method is to use Sb as a surfactant when growing SiGe graded layers. Compared with a Si 0.5 Ge 0.5 graded sample without Sb surfactant, the Sb-assisted Si 0.5 Ge 0.5 graded layer has much smoother surface and a significantly lower threading dislocation density. Thermal conductivity of a symmetrically strained Si/Ge superlattice grown on Sb-assisted Si 0.5 Ge 0.5 graded layer is also presented.

Thermal conductivity of Si/Ge superlattices

We report in this paper the thermal conductivity measurement of Si/Ge superlattices as a function of the temperature and the period thickness. The symmetrized Si/Ge superlattices are grown by MBE on Si substrates with a graded buffer layer. A comparative 3/spl omega/ method is used to measure the thermal conductivity of the buffer and the superlattices between 80K-300K. The thermal conductivity is carried out in conjunction with X-ray and TEM sample characterization. The measured thermal conductivity values are lower than that of their corresponding alloys and show a decreasing trend with increasing period thickness which are corroborated with the TEM characterization of the dislocation density.

Microstructure-property relations for porous bismuth films

Metallorganic decomposition of metal carboxylates is used to prepare porous thin films of bismuth. The general approach is to spin coat a precursor solution, bismuth 2-ethylhexanoate in a solvent of 2-methyl-1-propanol on the substrate of interest and then pyrolyze the resulting film under conditions which evolve the organic ligands but do not melt the bismuth. The controlled release of ligands creates the nanoporous morphology which is then preserved in the final film. Subsequent plasma etching and heat treatments are used to produce films of variable porosity. The film microstructure consists of nanoporous channels separating grains of bismuth. For films in the range of 30 % porosity, thermal conductivity values are on the order of 0.35 W/mK. The interrelationships among processing conditions, microstructure and thermal conductivity are discussed.

Substrate crystallinity and the performance of InP-based pseudomorphic high electron mobility transistors

We demonstrate a novel technique by which to measure the degree of crystalline perfection across semi-insulating InP substrates and show that the substrate perfection influences the crystallographic perfection and the electrical properties of subsequently deposited epitaxial layers. The substrate characterization technique is based on triple axis diffraction omega scans which are performed across the entire wafer area. Substrates with localized defective spots as well as overall substrate perfection are quantitatively measured. Pseudomorphic high electron mobility-type structures with strained In/sub 0.60/Ga/sub 0.40/As channels and nominally lattice-matched In/sub 0.52/Ga/sub 0.48/As buffer and supply layers were grown by molecular beam epitaxy and characterized by double and triple axis diffraction, Hall effect, and X-ray topography. Structures grown on high quality regions of the substrate exhibit sharper diffraction features and higher mobilities than those grown on low quality regions and we observe that there is a strong correlation between the level of diffuse scattering determined by triple axis diffraction and the channel mobility.