R. M. Tromp

The influence of the surface migration of gold on the growth of silicon nanowires

Interest in nanowires continues to grow, fuelled in part by applications in nanotechnology. The ability to engineer nanowire properties makes them especially promising in nanoelectronics. Most silicon nanowires are grown using the vapour–liquid–solid (VLS) mechanism, in which the nanowire grows from a gold/silicon catalyst droplet during silicon chemical vapour deposition. Despite over 40 years of study, many aspects of VLS growth are not well understood. For example, in the conventional picture the catalyst droplet does not change during growth, and the nanowire sidewalls consist of clean silicon facets. Here we demonstrate that these assumptions are false for silicon nanowires grown on Si(111) under conditions where all of the experimental parameters (surface structure, gas cleanliness, and background contaminants) are carefully controlled. We show that gold diffusion during growth determines the length, shape, and sidewall properties of the nanowires. Gold from the catalyst droplets wets the nanowire sidewalls, eventually consuming the droplets and terminating VLS growth. Gold diffusion from the smaller droplets to the larger ones (Ostwald ripening) leads to nanowire diameters that change during growth. These results show that the silicon nanowire growth is fundamentally limited by gold diffusion: smooth, arbitrarily long nanowires cannot be grown without eliminating gold migration.

Growth dynamics of pentacene thin films.

The recent demonstration of single-crystal organic optoelectronic devices has received widespread attention. But practical applications of such devices require the use of inexpensive organic films deposited on a wide variety of substrates. Unfortunately, the physical properties of these organic thin films do not compare favourably to those of single-crystal materials. Moreover, the basic physical principles governing organic thin-film growth and crystallization are not well understood. Here we report an in situ study of the evolution of pentacene thin films, utilizing the real-time imaging capabilities of photoelectron emission microscopy. By a combination of careful substrate preparation and surface energy control, we succeed in growing thin films with single-crystal grain sizes approaching 0.1 millimetre (a factor of 20–100 larger than previously achieved), which are large enough to fully contain a complete device. We find that organic thin-film growth closely mimics epitaxial growth of inorganic materials, and we expect that strategies and concepts developed for these inorganic systems will provide guidance for the further development and optimization of molecular thin-film devices.

Electronic and geometric structure of Si(111)-(7 × 7) and Si(001) surfaces

Abstract The atomic origins of the intrinsic surface states of the Si(111)-(7 × 7) and Si(001) surfaces have been identified using the recently developed method of current imaging tunneling spectroscopy (CITS). On Si(111)-(7 × 7) three filled and two empty surface states are found and directly identified with atomic features of the dimer-adatom-stacking fault model. On Si(001) one filled and one empty state are observed and identified with atomic features of a dimer model. The STM images of Si(001) are shown to be dominated by the surface electronic structure rather than geometric structure.

In situ ultrahigh vacuum transmission electron microscopy studies of hetero-epitaxial growth I. Si(001)/Ge

Abstract We use ultrahigh vacuum transmission electron microscopy (UHV-TEM) to study the growth of Ge on Si(001) in real time at different temperatures and for coverages ranging from the initial monolayers to the development and relaxation of 3D islands. During growth of the first monolayers the surface gradually changes from a disordered missing-dimer structure to a rather well ordered (2 × 8) reconstruction, an evolution clearly resolved by the TEM. As the coverage is increased 3D islands starts to form. The growth and relaxation of these islands are shown to depend significantly on the temperature, e.g. with different dislocations formed at high and low temperatures. We interpret this difference in terms of the brittle-ductile transition in Ge, below which dislocation glide is frozen out. An interesting observation is that islands grown at low temperatures are more fully relaxed than those grown at higher temperatures. At high enough temperature the islands are initially, up to a specific size, coherent with the substrate and further growth occurs in a remarkably oscillatory fashion with the introduction of each (60°-type) dislocation, where the core of the island, of about 2000 A in diameter, remains fully strained. However, in the low-temperature regime the islands grow relaxed from the outset with pure edge dislocations continuously being introduced in the moving edges. For temperatures less than 600°C the transition from 2D to 3D growth occurs via the formation of small and strained 3D islands, so-called “hut clusters”. We monitor the nucleation and characteristics of these clusters and discuss their possible role in the formation of relaxed 3D islands. The different growth mechanisms are discussed in terms of a simple model for the energetics of strain-relaxed islands, leading to a qualitative description of the temperature-dependent growth modes.

A new aberration-corrected, energy-filtered LEEM/PEEM instrument. I. Principles and design.

We describe a new design for an aberration-corrected low energy electron microscope (LEEM) and photo electron emission microscope (PEEM), equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control over the chromatic and spherical aberration coefficients C(c) and C(3), as well as the mirror focal length, to match and correct the aberrations of the objective lens. For LEEM (PEEM) the theoretical resolution is calculated to be approximately 1.5 nm (approximately 4 nm). Unlike previous designs, this instrument makes use of two magnetic prism arrays to guide the electron beam from the sample to the electron mirror, removing chromatic dispersion in front of the mirror by symmetry. The aberration correction optics was retrofitted to an uncorrected instrument with a base resolution of 4.1 nm in LEEM. Initial results in LEEM show an improvement in resolution to approximately 2 nm.

The adsorption of Ag on the Si(111) 7 × 7 surface at room temperature studied by medium energy ion scattering, leed and aes

Abstract We have studied the morphology and reactivity of ultrathin Ag films deposited on the Si(111) 7×7 surface at room temperature, employing the high depth resolution of Medium Energy Ion Scattering. In addition, LEED and AES measurements have been made. For coverages from zero to 5×1014 Ag atoms cm−2 we have observed the formation of a two-dimensional Ag layer, followed by the nucleation and growth of three-dimensional Ag islands for higher coverages (Stranski-Krastanov growth mode). Detailed analysis of the measured peak shapes in the energy spectra shows the presence of very high densities (≈ 1012 cm−2) of islands with height to width ratios less than 0.2. For the coverage range studied (zero to 1 × 1016 Ag atoms cm−2), the number of Si atoms, displaced from lattice sites does not increase above the value for the clean Si(111) 7×7 surface. This shows that no substantial mixing of Ag and Si occurs. The crystallographic relation between the Ag(111) crystallites and the Si substrate has been determined from LEED and Ion Blocking measurements, showing that part of the Ag crystallites have a preferential orientation, 180° rotated around the surface normal, with respect to that of the underlying substrate.

Growth temperature dependence of interfacial abruptness in Si/Ge heteroepitaxy studied by Raman spectroscopy and medium energy ion scattering

The influence of growth temperature on the interfacial abruptness of strained Ge layers, a few monolayers thick, embedded in Si has been studied using Raman spectroscopy to identify the presence of GeGe and GeSi bonds and medium energy ion scattering to characterize the spatial extent of the layers. Atomically sharp interfaces are observed for growth temperatures just above the crystalline to amorphous transition range, with pseudomorphic growth found for growth temperatures >∼250 °C. Asymmetric mixing of Ge into the Si capping layer occurs during growth at higher temperatures. Significantly less intermixing occurs on annealing after growth, pointing to the role of dynamical processes occurring at the growth front.

Are bare surfaces detrimental in epitaxial growth

For growth of epitaxial silicon‐germanium structures by hydride chemical vapor deposition (CVD), the growth front is hydrogen‐stabilized. Using medium energy ion scattering to examine the abruptness of an embedded Ge film in a Si(001) host, intermixing can be directly assessed. We have explored CVD films grown with varying hydrogen coverages, and find that adsorbed hydrogen serves a beneficial role in maintaining the abruptness of the interface. Embedded layers grown by molecular beam epitaxy are also more abrupt when the surface is stabilized, in this case by an adsorbed passivant such as Sb or As. Growth in the presence of a surface active agent (surfactant) results in greater control of constituents with no loss of epitaxial quality.

Surface Electronic Structure of Si(111)-(7 × 7) Resolved in Real Space

We have obtained the first energy-resolved real-space images of the filled and empty surface states of the Si(111)-(7 × 7) surface, with 3-A lateral resolution. This ability to resolve spatially these surface states with a scanning tunneling microscope depends upon a new method to acquire and separate geometric and electronic information. Our results not only are in good agreement with previous spectroscopic studies but also directly reveal the atomic location and geometric origin of the Si(111)- (7 × 7) surface states.

METALLIZATION INDUCED BAND BENDING OF SRTIO3(100) AND BA0.7SR0.3TIO3

We have investigated the interaction of Pt with single-crystal SrTiO3(001) and polycrystalline Ba0.7Sr0.3TiO3 thin films using photoemission spectroscopies. Significant band bending is caused by interface formation, determining the Schottky barrier height. We have depth profiled the band bending for Ba0.7Sr0.3TiO3 thin films, giving a direct measurement of the depletion depth and built-in potential.