T. Clement

Vapor-liquid-solid growth of germanium nanostructures on silicon

The pressure and temperature dependencies for vapor-liquid-solid (VLS) growth of Ge nanostructures on Si using chemical vapor deposition are reported. Gold nanodots self-assembled by evaporation on clean hydrogen-terminated and heated Si substrates are used to seed the liquid eutectic VLS growth. Digermane pressures are varied from 4×10−5 to 1×10−2Torr and substrate temperatures from 400 to 600°C for heteroepitaxial growth on Si(111). Two types of nanostructures are identified, nanowires and nanopillars, with a transition from nanopillar growth to nanowire growth occurring with increasing pressure. Nanowires are characterized by rapid vertical growth, long-aspect-ratio structures, and linear dependence of the growth rate on pressure. At lower pressures a transition to nanopillars is observed; these exhibit both vertical and lateral growth with typical aspect ratios of 1:2. For Si(111) substrates nanowires grow epitaxially with their growth axis along the ⟨111⟩ direction. High-resolution transmission elect...

Discrete magnetic microfluidics

We present a method to move and control drops of water on superhydrophobic surfaces using magnetic fields. Small water drops (volume of 5–35μl) that contain fractions of paramagnetic particles as low as 0.1% in weight can be moved at relatively high speed (7cm∕s) by displacing a permanent magnet placed below the surface. Coalescence of two drops has been demonstrated by moving a drop that contains paramagnetic particles towards an aqueous drop that was previously pinned to a surface defect. This approach to microfluidics has the advantages of faster and more flexible control over drop movement.

In situ studies of semiconductor nanowire growth using optical reflectometry

The authors report the use of in situ optical reflectometry to determine the incubation time for the onset of growth, mean growth rate, and average length of Si nanowires during chemical vapor deposition vapor-liquid-solid synthesis. Results for the constructive and destructive interferences of 635nm linearly polarized laser light scattering from growing nanowire layers are compared to simulations. This real time optical reflectance approach is shown to quantitatively determine nanowire growth rates as well as reveal a pressure dependence for the time to nucleate nanowire growth.

Nanopillar growth mode by vapor-liquid-solid epitaxy

We report epitaxial growth of Ge nanopillars (NPs) on Si(100) by vapor-liquid-solid (VLS) growth from digermane. This growth morphology is characterized by short, low-aspect-ratio pillars and is markedly different from the long, narrow nanowires (NWs) previously reported for VLS growth. The NP growth mode occurs at low digermane pressures. It is attributed to surface-diffusion-induced lateral growth in combination with an insufficient Ge concentration gradient in the AuGe eutectic to catalyze NW growth. High resolution electron microscopy confirms that the NPs are epitaxial with the Si (100) substrate and are fully relaxed and strain free.

Ultrafast carrier dynamics in semiconductor nanowires

Ultrafast wavelength-tunable optical measurements on semiconductor nanowires allow us to independently probe the dynamics of electrons, holes, and defect states. These investigations reveal the influence of two-dimensional confinement on carrier dynamics in these nanosystems.

Ultrafast Carrier Dynamics in Semiconductor Nanowires

Time-resolved measurements of carrier dynamics in Ge and GaN nanowires reveal that carrier relaxation in these systems is governed by surface states and defects. This has significant implications for nanowire-based devices in photonics and thermoelectrics.

Synthesis of silicon and germanium nanowires.

The vapor-liquid-solid growth process for synthesis of group-IV semiconducting nanowires using silane, germane, disilane and digermane precursor gases has been investigated. The nanowire growth process combines in situ gold seed formation by vapor deposition on atomically clean silicon (111) surfaces, in situ growth from the gaseous precursor(s), and real-time monitoring of nanowire growth as a function of temperature and pressure by a novel optical reflectometry technique. A significant dependence on precursor pressure and growth temperature for the synthesis of silicon and germanium nanowires is observed, depending on the stability of the specific precursor used. Also, the presence of a nucleation time for the onset of nanowire growth has been found using our new in situ optical reflectometry technique.