A method for directly correlating site-specific cross-sectional and plan-view transmission electron microscopy of individual nanostructures.

A sample preparation method is described for enabling direct correlation of site-specific plan-view and cross-sectional transmission electron microscopy (TEM) analysis of individual nanostructures by employing a dual-beam focused-ion beam (FIB) microscope. This technique is demonstrated using Si nanowires dispersed on a TEM sample support (lacey carbon or Si-nitride). Individual nanowires are first imaged in the plan-view orientation to identify a region of interest; in this case, impurity atoms distributed at crystalline defects that require further investigation in the cross-sectional orientation.

Atomic structural analysis of nanowire defects and polytypes enabled through cross-sectional lattice imaging.

Correlated transmission electron microscopy imaging, electron diffraction, and Raman spectroscopy are used to investigate the structure of Si nanowires with planar defects. In addition to plan-view imaging, individual defective nanowires are imaged in axial cross-section at specific locations selected in plan-view imaging. This correlated characterization approach enables definitive identification of complex defect structures that give rise to diffraction patterns that have been misinterpreted in the literature.

Direct measurements of lateral variations of Schottky barrier height across "end-on" metal contacts to vertical Si nanowires by ballistic electron emission microscopy.

Ballistic electron emission microscopy measurements on individual "end-on" Au Schottky contacts to vertical Si nanowires (NWs) indicate that the local Schottky barrier height at the contact edge is 23 ± 3 meV lower than at the contact center. Finite-element electrostatic simulations suggest that this is due to a larger interface electric field at the contact edge resulting from (equilibrium) positive charge in Si/SiO(2) interface states near the Au/NW contact, induced by local band bending due to the high work function Au film.

Catalyst incorporation at defects during nanowire growth.

Scanning and transmission electron microscopy was used to correlate the structure of planar defects with the prevalence of Au catalyst atom incorporation in Si nanowires. Site-specific high-resolution imaging along orthogonal zone axes, enabled by advances in focused ion beam cross sectioning, reveals substantial incorporation of catalyst atoms at grain boundaries in <110> oriented nanowires. In contrast, (111) stacking faults that generate new polytypes in <112> oriented nanowires do not show preferential catalyst incorporation.

Relative influence of surface states and bulk impurities on the electrical properties of Ge nanowires.

We quantitatively examine the relative influence of bulk impurities and surface states on the electrical properties of Ge nanowires with and without phosphorus (P) doping. The unintentional impurity concentration in nominally undoped Ge nanowires is less than 2 x 10(17) cm(-3) as determined by atom probe tomography. Surprisingly, P doping of approximately 10(18) cm(-3) reduces the nanowire conductivity by 2 orders of magnitude.

Ordered stacking fault arrays in silicon nanowires.

Correlated Raman microscopy and transmission electron microscopy were used to study the ordering of {111} planar defects in individual silicon nanowires. Detailed electron diffraction and polarization-dependent Raman analysis of individual nanowires enabled assessments of the stacking fault distribution, which varied from random to periodic, with the latter giving rise to local domains of 2H and 9R polytypes rather than the 3C diamond cubic structure. Some controversies and inconsistencies concerning earlier reports of polytypes in Si nanowires were resolved.

Direct measurement of dopant distribution in an individual vapour-liquid-solid nanowire.

Semiconductor nanowires show promise for many device applications, but controlled doping with electronic and magnetic impurities remains an important challenge. Limitations on dopant incorporation have been identified in nanocrystals, raising concerns about the prospects for doping nanostructures. Progress has been hindered by the lack of a method to quantify the dopant distribution in single nanostructures.

Scanning photocurrent microscopy analysis of Si nanowire field-effect transistors fabricated by surface etching of the channel.

High-performance field-effect transistors were fabricated by etching the channel regions of surface-doped Si nanowires. On/off ratios of 10(6) and field effect mobilities up to 525 cm(2)/(V x s) represent significant improvements over transistors fabricated from uniformly doped n-Si nanowires. Analysis by scanning photocurrent microscopy (SPCM) confirmed that the devices function similarly to traditional metal-oxide semiconductor field-effect transistors; in accumulation, the device current is controlled by channel conductance modulation, while n(+)-n junctions determine subthreshold characteristics as the channel is depleted.

Syntaxial growth of Ge/Mn-germanide nanowire heterostructures.

We report the growth of free-standing one-dimensional Ge/Mn-germanide nanowire heterostructures by chemical vapor deposition and provide a detailed description of the growth mechanism. Self-assembled manganese-germanide particles seed the growth of Ge nanowires (GeNWs) and simultaneously elongate along a parallel axis, resulting in syntaxial growth of the two phases. The GeNW growth is limited by GeH 4 decomposition, whereas the germanide growth is limited by reaction of Mn at the growth interface.

High-resolution detection of Au catalyst atoms in Si nanowires.

The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature.