Extremely thin single-crystal sheets have unique mechanical properties, which may influence the generation and behaviour of extended defects during heteroepitaxial growth. Using low-energy electron microscopy (LEEM) we investigate the earliest stages of inelastic strain relaxation in SiGe grown heteroepitaxially on Si-on-insulator (SOI) at a sensitivity not possible with other methods, employing a structure that forces dislocations, if they can form at all, to reside at the Si/oxide interface of SOI(0 0 1). LEEM confirms a lower dislocation line energy at the Si/amorphous oxide (SiO2) interface than at the crystalline SiGe/Si interface. The line energy is, however, nonzero, in contrast with earlier assumptions. The lower line energy makes the thermodynamic critical thickness for growth on SOI(0 0 1) lower than on bulk Si(0 0 1) for otherwise identical growth conditions. Nevertheless we can grow heteroepitaxial SiGe films on SOI(0 0 1) that are much thicker than even the thermodynamic critical thickness for growth on bulk Si(0 0 1), suggesting high kinetic barriers for dislocation formation or motion.

We report spatial arrangement of self-assembled Ge quantum dots on patterned structures on Si (001) without using complicated lithography. Starting from stripes and mesas fabricated by conventional lithography and plasma etching, we prepare sinusoidal Si stripes with narrow ridges and mesas with humped edges via high-temperature annealing. Deposited Ge self-assembles into coherent nanocrystals that align along the narrow ridges of the stripes and the sloped mesa edges. Enhanced strain relief at the ridges due to elastic relaxation and the high step densities on the shallow slopes at the edges are likely causes of nanocrystal alignment.

We demonstrate the use of low-energy electron microscopy (LEEM) as a tool for studying dis-location formation in low-Ge-content SiGe films on Si(001) and silicon-on-insulator. Compared to TEM, sample preparation for LEEM consists only of conventional surface cleaning. Yet, because of its sensitivity to local variations in surface strain on Si(001), LEEM can detect dislocations at the earliest stages of strain relaxation. In identically prepared SiGe films, the typical dislocation extends over the entire viewable region of several hundred microns in SiGe/Si, but is less than 100 microns in SiGe/SOI. In addition, dislocation cross-slip and threading segments are common in SiGe/SOI, but virtually non-existent in SiGe/Si. We have also observed dislocation formation in real-time during high temperature annealing. Preliminary results appear to demonstrate dislocation multiplication and blocking at a perpendicular glide plane. The applicability of LEEM to strain relaxation in other Si-based systems will be discussed.

We report a novel method of fabricating self-assembled carbon nanotube (CNT) on Si nanocrystals and the photocurrent from this network. Silicon-on-insulator (SOI) substrate with 10nm thin top silicon layer is annealed at elevate temperature in an ultra-high vacuum environment. The Si layer dewets and aggregates into Si nanocrystal islands with dimensions about 90 nm high, 100-150 nm wide, and 200nm apart. 1nm thin Fe film is deposited on the decomposed SOI as catalyst for CNT growth. The growth is done by chemical vapor deposition (CVD) at 900 °C with a flow of CH4 at 400sccm and H2 at 20sccm. The CVD grown CNTs show strong preferential growth on the top portion of the Si nanocrystals and form a suspended network connecting the nanocrystals. No photolithographic process is needed to create this self-assembled CNT network. We find that the reason that few CNT are found on the oxide surface is because of the influence of the island topography on the CH4 gas flow pattern, with feedstock unable to reach the oxide surface when the islands are close to each other. We demonstrate that, by shining a low power 650nm wavelength commercial red laser pointer on this network, it generates photocurrent on the level of 20nA photocurrent under 1 volt bias condition. Since a 100 mW 1.175 μm wavelength IR laser does not generate any distinguishable photocurrent in our measurement setup, we believe the photocurrent generated by 650 nm red laser mainly comes from the Si nanocrystals instead of the CNTs. We demonstrate that a dense, self-assembled CNT network can be formed on the decomposed Si nanocrystals and can be used as conducting media for electric measurement.

The strain driven self-assembly of faceted Ge nanocrystals during epitaxy on Si(001) to form quantum dots (QDs) is by now well known. We have also recently provided an understanding of the thermodynamic driving force for directed assembly of QDs on bulk Si (extendable to other QD systems) based on local chemical potential and curvature of the surface. Silicon-on-insulator (SOI) produces unique new phenomena. The essential thermodynamic instability of the very thin crystalline layer (called the template layer) resting on an oxide can cause this layer, under appropriate conditions, to dewet, agglomerate, and self-organize into an array of Si nanocrystals. Using low-energy electron microscopy (LEEM), we observe this process and, with the help of first-principles total-energy calculations, we provide a quantitative understanding of this pattern formation. The Si nanocrystal pattern formation can be controlled by lithographic patterning of the SOI prior to the dewetting process. The resulting patterns of electrically isolated Si nanocrystals can in turn be used as a template for growth of nanostructures, such as carbon nanotubes (CNTs). Finally we show that this growth may be controlled by the flow dynamics of the feed gas across the substrate.