Graphene growth through a recrystallization process in plasma enhanced chemical vapor deposition.

Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (I D/I G < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq.

Relation between growth rate and structure of graphene grown in a 4″ showerhead chemical vapor deposition reactor.

The chemical vapor deposition (CVD) growth of graphene on copper is controlled by a complex interplay of substrate preparation, substrate temperature, pressure and flow of reactive gases. A large variety of recipes have been suggested in literature, often quite specific to the reactor, which is being used. Here, we report on a relation between growth rate and quality of graphene grown in a scalable 4″ CVD reactor. The growth rate is varied by substrate pre-treatment, chamber pressure, and methane to hydrogen (CH4:H2) ratio, respectively. We found that at lower growth rates graphene grains become hexagonal rather than randomly shaped, which leads to a reduced defect density and a sheet resistance down to 268 Ω/sq.

Material and doping transitions in single GaAs-based nanowires probed by Kelvin probe force microscopy.

We demonstrate the potential of Kelvin probe force microscopy for simultaneously probing the topography and the work function of individual nanowires. Our technique allows us to visualize both the material and the doping contrast in single GaAs-based nanowires without the need to electrically contact the nanowires. In a GaAs/GaP heterostructure nanowire, a core-shell structure is found. This is attributed to a thermally activated radial overgrowth of GaAs, while in the GaP region the vertical nanowire growth dominates. In partially p-doped GaAs nanowires the doping transitions can be localized and the width of the depletion layer is estimated.

Preparation by ion milling and TEM investigation of embedded needle-shaped crystals of H-Nb2O5.

A method for preparing needle-shaped and platelike crystals for electron microscopical investigation was elaborated. Crystals of H-Nb2O5 were embedded in a synthetic resin and disks were cut off perpendicular to the desired direction of observation. The thickness of the sample was reduced by planar grinding and then by using a dimple grinder and furthermore by ion milling with argon ions. With the precision ion milling system small crystal areas were selected and subsequently irradiated. The TEM investigations showed that the desired crystallographic orientation was reached and that the crystal structure has been preserved. The contrast of highly resolved images was reduced by an amorphous surface layer which was not removable.