Elad Koren

Measurement of Active Dopant Distribution and Diffusion in Individual Silicon Nanowires

We have measured the radial distribution and diffusion of active dopant atoms in individual silicon nanowires grown by the vapor-liquid-solid (VLS) method. Our method is based on successive surface etching of a portion of a contacted nanowire, followed by measurement of the potential difference between the etched and unetched areas using Kelvin probe force microscopy (KPFM). The radial dopant distribution is obtained by fitting the measured potentials with a three-dimensional solution of Poisson equation. We find that the radial active dopant distribution decreases by almost 2 orders of magnitude from the wire surface to its core even when there is no indication for tapering. In addition, the dopant profile is consistent with a very large diffusion coefficient of D approximately 1 x 10(-19) m(2) s(-1). This implies that phosphorus (P) diffusion during the VLS growth is remarkably high and subsequent thermal annealing must be used when a homogeneous dopant distribution is required.

Nonuniform doping distribution along silicon nanowires measured by Kelvin probe force microscopy and scanning photocurrent microscopy

We use Kelvin probe force microscopy and scanning photocurrent microscopy to measure the doping distribution along single phosphorous-doped silicon nanowire grown by the vapor-liquid-solid method. A nonlinear potential drop along biased silicon nanowires is detected both by measuring the surface potential directly via Kelvin probe force microscopy and by integrating the photocurrent measured by scanning photocurrent microscopy. These variations in the potential and field are further analyzed to extract the longitudinal dopant distribution along an individual silicon nanowire. The results show a very good agreement between the two methods to quantitatively detect potential, field, and doping variations within doped silicon nanowires.

Obtaining Uniform Dopant Distributions in VLS-Grown Si Nanowires

Semiconducting nanowires grown by the vapor-liquid-solid method commonly develop nonuniform doping profiles both along the growth axis and radially due to unintentional surface doping and diffusion of the dopants from the nanowire surface to core during synthesis. We demonstrate two approaches to mitigate nonuniform doping in phosphorus-doped Si nanowires grown by the vapor-liquid-solid process. First, the growth conditions can be modified to suppress active surface doping. Second, thermal annealing following growth can be used to produce more uniform doping profiles. Kelvin probe force microscopy and scanning photocurrent microscopy were used to measure the radial and the longitudinal active dopant distribution, respectively. Doping concentration variations were reduced by 2 orders of magnitude in both annealed nanowires and those for which surface doping was suppressed.

Direct measurement of individual deep traps in single silicon nanowires.

The potential of the metal nanocatalyst to contaminate vapor-liquid-solid (VLS) grown semiconductor nanowires has been a long-standing concern, since the most common catalyst material, Au, is known to induce deep gap states in several semiconductors. Here we use Kelvin probe force microscopy to image individual deep acceptor type trapping centers in single undoped Si nanowires grown with an Au catalyst. The switching between occupied and empty trap states is reversibly controlled by the back-gate potential in a nanowire transistor. The trap energy level, i.e., E(C) - E(T) = 0.65 ± 0.1 eV was extracted and the concentration was estimated to be ∼2 × 10(16) cm(-3). The energy and concentration are consistent with traps resulting from the unintentional incorporation of Au atoms during the VLS growth.

Coherent commensurate electronic states at the interface between misoriented graphene layers

Graphene and layered materials in general exhibit rich physics and application potential owing to their exceptional electronic properties, which arise from the intricate π-orbital coupling and the symmetry breaking in twisted bilayer systems. Here, we report room-temperature experiments to study electrical transport across a bilayer graphene interface with a well-defined rotation angle between the layers that is controllable in situ. This twisted interface is artificially created in mesoscopic pillars made of highly oriented pyrolytic graphite by mechanical actuation. The overall measured angular dependence of the conductivity is consistent with a phonon-assisted transport mechanism that preserves the electron momentum of conduction electrons passing the interface. The most intriguing observations are sharp conductivity peaks at interlayer rotation angles of 21.8° and 38.2°. These angles correspond to a commensurate crystalline superstructure leading to a coherent two-dimensional (2D) electronic interface state. Such states, predicted by theory, form the basis for a new class of 2D weakly coupled bilayer systems with hitherto unexplored properties and applications.

Direct measurement of nanowire Schottky junction depletion region

We have used Kelvin probe force microscopy to measure the surface potential of both doped and unintentionally doped (UID) Si nanowires Schottky junctions. The imaging of the Schottky junction together with 3D potential simulation and consideration of the convolution of the scanning tip enables us to determine the real surface potential. Highly doped n-type nanowires show smaller depletion regions compared to UID nanowires, and their potential profile was successfully modeled. For the UID nanowires, the measured potential profiles and, consequently, the depletion region indicate the presence of bulk deep traps with a concentration of ∼5 × 1017 cm−3.

Isolation of high quality graphene from Ru by solution phase intercalation

We introduce a method for isolating graphene grown on epitaxial Ru(0001)/α-Al2O3. The strong graphene/Ru(0001) coupling is weakened by electrochemically driven intercalation of hydrogen underpotentially deposited in aqueous KOH solution, which allows the penetration of water molecules at the graphene/Ru(0001) interface. Following these electrochemically driven processes, the graphene can be isolated by electrochemical hydrogen evolution and transferred to arbitrary supports. Raman and transport measurements demonstrate the high quality of the transferred graphene. Our results show that intercalation, typically carried out in vacuum, can be extended to solution environments for graphene processing under ambient conditions.