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Relaxed Si0.7Ge0.3 epitaxial layers that possess threading defect densities as low as 5×105 cm-2 have been grown on Si substrates by rapid thermal chemical vapor deposition. The layers were formed in a series of steps of constant Ge content, each step with either 6% or 3% more Ge than the previous one. At the low average grading rates investigated, the number of steps used to reach 30% Ge only slightly affected the density of individual threading dislocations, imaged by electron beam induced current (EBIC). However, the density of dark-line defects seen in EBIC, which are formed from pile-ups of threading dislocations, was strongly influenced only by the average grading rate. Both the dark line density and the dark spot density must be considered when assessing the surface of these relaxed layers. [ABSTRACT FROM AUTHOR]

Atomically sharp high-density tungsten nanotip arrays are fabricated by nanocasting using a patterned SiO2 template. The fabrication involves mainly top-down processes making it fully compatible with today’s Si technology. The obtained tungsten nanotip arrays show atomically sharp tips (radius of curvature smaller than 1 nm), high emitter density (∼5×1010 cm-2), perfect tip alignment, and excellent field emission properties with a turn-on field of 2.1 V/μm and a field enhancement factor of 3334. The unique fabrication process flow leads to the highly uniform height of the W nanotips, allowing for submicron cathode-anode spacing, which very much relaxes the requirement of vacuum level and makes it possible for low vacuum operation. [ABSTRACT FROM AUTHOR]

A 4-MW ion source was developed for the neutral beam injector (NBI) on Experimental Advanced Superconducting Tokamak (EAST). Breakdown nevertheless can happen during ion source conditioning and routine operations and is deleterious to the high-power ion source. To protect this ion source, a core snubber was designed to absorb the breakdown energy of the EAST-NBI ion source. A prototype core snubber was developed and tested using the ion-source test bed. The results show that with a core snubber, short-circuit currents at different high-power voltages were about one-tenth of the current without the snubber. The residual energy of the distributed capacitors had been absorbed successfully and the core snubber does protect the source from damage during breakdown. The results verified the successful development of a core snubber for the EAST-NBI. [ABSTRACT FROM AUTHOR]

This work presents a suspended graphene ribbon device for electrostatic discharge (ESD) applications. The device structure was proposed and fabricated after careful design considerations. Compared to the conventional ESD devices such as diodes, bipolar junction transistors, and metal-oxide-semiconductor field effect transistors, the proposed device structure is believed to render several advantages including zero leakage, low parasitic effects, fast response, and high critical current density. A process flow was developed for higher yield and reliability of the suspended graphene ribbons. Direct current (DC) and transmission-line pulse (TLP) measurements were carried out to investigate the switching behavior of the device, which is crucial for ESD operation. DC measurements with a different configuration were used to assess the mechanical shape evolution of the graphene ribbon upon biasing. Finite Element Simulations were conducted and agreed well with the experimental results. Furthermore, the current carrying capability of non-suspended graphene ribbons was tested using TLP. It was found that the critical current density of graphene is higher than that of copper wires widely used as interconnects in integrated circuits (ICs). [ABSTRACT FROM AUTHOR]

The diffusion processes and kinetic barriers of individual carbon adatoms and clusters on graphene surfaces are investigated to provide fundamental understanding of the physics governing epitaxial growth of multilayer graphene. It is found that individual carbon adatoms form bonds with the underlying graphene whereas the interaction between graphene and carbon clusters, consisting of 6 atoms or more, is very weak being van der Waals in nature. Therefore, small carbon clusters are quite mobile on the graphene surfaces and the diffusion barrier is negligibly small (~6meV). This suggests the feasibility of high-quality graphene epitaxial growth at very low growth temperatures with small carbon clusters (e.g., hexagons) as carbon source. We propose that the growth mode is totally different from 3-dimensional bulk materials with the surface mobility of carbon hexagons being the highest over graphene surfaces that gradually decreases with further increase in cluster size. [ABSTRACT FROM AUTHOR]

A key issue on the development of EAST ion source was the junction design of insulator structure, which consists of three insulators and four supporting flanges of electrode grid. Because the ion source is installed on the vertical plane, the insulator structure has to withstand large bending and shear stress due to the gravity of whole ion source. Through a mechanical analysis, it was calculated that the maximum bending normal stress was 0.34 MPa and shear stress was 0.23 MPa on the insulator structure. Due to the advantages of simplicity and high strength, the adhesive bonding technology was applied to the junction of insulator structure. A tensile testing campaign of different junction designs between insulator and supporting flange was performed, and a junction design of stainless steel and fiber enhanced epoxy resin with epoxy adhesive was determined. The insulator structure based on the determined design can satisfy both the requirements of high-voltage holding and mechanical strength. [ABSTRACT FROM AUTHOR]

The authors studied the selective growth of Ge islands by molecular beam epitaxy on Si(001) covered with nanometer-scale patterned SiO2 mask generated using self-assembled diblock copolymer. Selective growth is made possible by Ge adatoms desorbing from the SiO2 surface as well as diffusing into the exposed Si area. For the Ge coverage of 2 nm, multiple islands are observed along the periphery of individual exposed Si areas. At 3.5 nm coverage, the coalescence of small islands with significant strain relaxation becomes evident. The ramifications of the multiple islands morphology and their coalescence on potential device applications are discussed. [ABSTRACT FROM AUTHOR]

We investigate the surface roughness and dislocation distribution of compositionally graded relaxed SiGe buffer layers by inserting two tensile-strained Si layers. The 20 nm thick strained Si layers, less than the critical thickness for dislocation formation, are inserted at 10 and 20% Ge content regions of the 1 μm thick graded SiGe layer with a final Ge content of 30%. The surface immediately after growing the second strained Si layer on SiGe with 20% Ge content is found to be flat with about 1.1 nm root-mean-square roughness. However, the crosshatched surface with the 7.8 nm roughness develops during subsequent SiGe growths, which is slightly less than the 10.3 nm value for SiGe without inserted Si layers. Another important issue of consideration is that inserting the strained Si layers leads to increased interaction among dislocations as shown by cross-sectional transmission electron microscopy. This study explores the possibility of using strained layers for achieving flat surfaces and illustrates the need for optimization when using this approach. [ABSTRACT FROM AUTHOR]

Field-effect transistors are fabricated from the Si/Si[sub 1-x]Ge[sub x] heterostructures. The density of the two-dimensional electron system (2DES) in the strained Si quantum well can be controllably tuned from 2.13×10[sup 11] to 4.24×10[sup 11] cm[sup -2]. The temperature dependence of the resistivity of the 2DES was measured and the apparent metal–insulator transition was observed. Its main features are similar to those reported in other semiconductor-based two-dimensional systems. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Porous Si is the semi-insulating state of Si, with low thermal expansion mismatch with bulk Si. As a result, it is an excellent material for crosstalk isolation in mixed-signal integrated circuits. We study the formation of isolated porous Si regions in p[sup -]-type and p[sup +]-type Si substrates with emphasis on the cross-sectional profile of the porous regions. Our study reveals that in addition to the primary undercut due to the isotropic nature of the anodization process, there exists a secondary undercut that is similar in shape to the bird’s beak commonly observed at the edge of field oxides in conventional Si complementary-metal-oxide-semiconductor process. The shape and the extent of the secondary undercut are dependent on the type of mask materials used during selective formation of porous Si as well as the substrate resistivity. The combined experimental and simulation studies pointed to two likely origins of secondary undercuts: the weak adhesion of some of the mask materials and current crowding in bulk Si substrates near the edge of the mask openings. Secondary undercuts result in the erosion of the precious Si chip surface area when a porous Si trench is used for rf crosstalk isolation, and should be minimized. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]