Showing papers by "Theodore I. Kamins published in 2002"

Direct evaluation of composition profile, strain relaxation, and elastic energy of Ge:Si(001) self-assembled islands by anomalous x-ray scattering

Abstract: The growth of strained epitaxial self assembled nanocrystals is comprised of a variety of kinetic and thermodynamic factors that determine their morphology and size. Some of the significant factors to their stability are strain and interdiffusion. Here we directly measure the gradient of composition and strain in Ge nanocrystals grown on Si(001) using anomalous x-ray scattering. By combining our x-ray results, where we relate strain, interdiffusion, and shape with atomic force microscopy measurements, we have been able to determine the complete strain configuration of these islands. We show that the amount of elastic energy in pyramids and domes can be evaluated. The transition from pyramids to domes is accompanied by an increase of lattice parameter and enhancement of interdiffusion, both leading to a drastic decrease of the elastic energy stored per atom.

Twinning in TiSi2-island catalyzed Si nanowires grown by gas-source molecular-beam epitaxy

Abstract: Using TiSi2 islands as a catalyst, we have grown Si nanowires by gas-source molecular-beam epitaxy using Si2H6 as the gas source. The dominant TiSi2 islands are C49 phase with the orientation: Si[110]//C49-TiSi2[100] and Si(001)//C49-TiSi2(010). Twinning in the grown Si nanowires is observed by reflection high-energy electron diffraction and transmission electron microscopy. The twining also causes kinking, i.e., an abrupt change of growth direction of the Si nanowires. Lattice mismatch stress between the TiSi2 islands and the Si nanowires possibly leads to twinning and kinking of the Si nanowires.

Chemically vapor deposited Si nanowires nucleated by self-assembled Ti islands on patterned and unpatterned Si substrates

Abstract: When Ti is deposited on Si in the 600–700°C temperature range, the lattice mismatch between the Ti-containing deposit and the Si substrate causes TiSi x nanoislands to form. The nanoislands grow when annealed at temperatures above 800°C. When the nanoislands (either unannealed or annealed) are exposed to a Si-containing precursor gas, the Ti catalyzes the decomposition of the gas, allowing one-dimensional nanowires to grow. If oxide-patterned Si substrates are used, the Ti islands form selectively on the exposed Si and are preferentially positioned near the pattern edges. The subsequently grown Si nanowires are, therefore, positioned with respect to the larger lithographically formed pattern. Exposing the wires to an ion beam after deposition promotes the parallel alignment of nanowires.

Field effect transistor with channel extending through layers on a substrate

Abstract: A field effect transistor (FET) has a channel formed in a pore extending up from a conductive portion of a substrate through a stack of planar layers including a first insulating layer, a gate layer, and a second insulating layer. The pore can be upright or inclined relative to the layers. A nanoparticle used for a mask of a directional etching process ultimately defines the size of the pore and therefore the channel width. The substrate or a doped region of the substrate formed immediately beneath the channel can be a source/drain of the FET with the other drain/source being a doped region adjacent the top of the channel. The gate layer can form the gate or can contact a separate gate inside the pore.

Effect of phosphorus on island formation

Abstract: Adding a phosphorus-containing species during chemical vapor deposition of Ge islands on Si (0 0 1) modifies the island sizes and shapes, primarily by changing the surface energies and the relative surface energies of different surface facets. Three distinct island shapes occur, but the island types and their sequence of formation differ from those found with undoped Ge islands. The addition of phosphorus decreases the size of the multifaceted “domes”—the island shape that has a favored island size, providing an additional method for controlling the islands. The largest islands have a steep pyramidal structure not seen for undoped islands.

Heavy arsenic doping of silicon by molecular beam epitaxy

Abstract: As MOSFETs scale to deep-submicron dimensions, there has been an increasing demand for silicon epitaxial layers with abrupt doping profiles. For nanoscale devices, arsenic is an attractive n-type dopant because of its high solubility and low diffusivity, but suffers from surface segregation during epitaxy, making high- concentration incorporation with abrupt profiles difficult. In this paper we report results of arsenic incorporation in Si molecular beam epitaxy (MBE) using a unique combination of solid (As, Si) and gas (disilane) sources to achieve these goals.

Anomalous X-Ray Scattering On Self-Assembled Islands: Direct Evaluation Of Composition Profile, Strain Relaxation, And Elastic Energy

Abstract: The growth of Ge on Si(001) produces a wealth of morphologies of nanocrystals. After the deposition of a two-dimensional Ge film 3.5 monolayers thick, islands are formed, and several shape transitions can be observed depending on the growth temperature, rate and deposited thickness. In this work we combine atomic force microscopy and two different types of measurements of anomalous x-ray scattering to determine this elastic energy both in pyramid and dome shaped islands. By comparing pyramids and domes data, we have observed both an increase of lattice parameter and enhancement of interdiffusion for the domes. These results show that there is a drastic decrease of the elastic energy stored per atom upon this particular shape transition.

Effect Of Phosphorus On Ge/Si(001) Island Formation

Abstract: When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge in layers thicker than about four monolayers to form distinctive, three-dimensional islands. The shape of the islands is determined by the energies of the surface facets, facet edges, and interfaces. When phosphorus is added during the deposition, the surface energies change, modifying the island shapes and sizes, as well as the deposition process. When phosphine is introduced to the germane/hydrogen ambient during Ge deposition, the deposition rate decreases because of competitive adsorption. The steady-state deposition rate is not reached for thin layers. The deposited, doped layers contain three different island shapes, as do undoped layers; however, the island size for each shape is smaller for the doped layers than for the corresponding undoped layers. The intermediate-size islands are the most significant; the intermediate-size doped islands are of the same family as the undoped, multifaceted “dome” structures, but are considerably smaller. The largest doped islands appear to be related to the defective “superdomes” discussed for undoped islands. The distribution between the different island shapes depends on the phosphine partial pressure. At higher partial pressures, the smaller structures are absent. Phosphorus appears to act as a mild surfactant, suppressing small islands.

Ti-Island-Catalyzed Si Nanowire Growth by Gas-Source MBE: Morphology and Twinning

Abstract: Silicon nanowires catalyzed by Ti islands have been grown by molecular beam epitaxy (MBE) using Si2H6 as the gas source and characterized by in situ reflection high-energy electron diffraction (RHEED), scanning-electron microscopy (SEM) and transmission-electron microscopy (TEM). Approximately one monolayer of Ti was deposited on Si(001) wafers, which, during annealing, reacted with silicon and formed TiSi2 islands. After annealing, but before Si growth, the stoichiometric TiSi2 (C49) phase was observed with RHEED. The silicon nanowires are typically between 20 and 40 nanometers in diameter and several hundred nanometers long. The nanowires changed their growth direction several times during growth, resulting in complex RHEED patterns, which can be matched very well by simulated RHEED patterns calculated assuming that the nanowires change their direction by twinning along (111) planes. RHEED patterns of epitaxial silicon nanowires, first-order twinned nanowires (twinned relative to the substrate orientation), second-order twinned nanowires (twinned relative to the first-order twin), and TiSi2 were observed.