Branched Nanowire Growth

Metal nanoparticles can be used to seed the growth of nanowires of controlled diameter, length, and composition. We showed that dendritic (highly branched) nanowires could be grown by introducing a metal precursor during nanowire growth. Mn atoms deposited from the vapor phase self-assembled into catalysts for InAs nanowires, leading to generations of branches. This work demonstrated a new route to hierarchical nanowire structures.

Self-assembled Quantum Dots

Ferromagnetic MnAs quantum dots were grown epitaxially on InAs nanowires through on-nanowire self-assembly. Magnetic force microscopy was used to study the switching behavior of individual quantum dots and correlate with electron microscopy and diffraction studies of their crystal structure. The hybrid ferromagnetic/semiconductor QD/NW properties provide a promising basis for the development of nanowire spin-valves and magnetic memory devices.

Syntaxial Nanowire Growth

We discovered a novel syntaxial growth mechanism whereby two different nanowires- one metallic and one semiconducting- grow simultaneously from a common interface. The mechanism provides a useful approach to ideal nanoscale metal-semiconductor heterojunctions and represent the growth of electrical contacts ‘in situ’. The metallic phase is also magnetic, creating the opportunity to study spin-injection into a well-defined one-dimensional system.

Additional References

• J. L. Lensch-Falk, E. R. Hemesath, F. J. Lopez, and L. J. Lauhon, "Vapor-Solid-Solid Synthesis of Ge Nanowires from Vapor-Phase-Deposited Manganese Germanide Seeds," J. Am. Chem. Soc. 129, 10670 (2007).
• J. L. Lensch-Falk, E. R. Hemesath, D. E. Perea, and L. J. Lauhon, "Alternative catalysts for sub-eutectic growth of Si and Ge nanowires," J. Mater. Chem. (invited) 19, 849-857 (2009).

3D Atomic Composition Mapping

Intentional impurity atoms, or dopants, are used to control the electronic properties of semiconductors, but it is very challenging to determine the concentration and distribution of dopant atoms in extremely small structures such as nanowires. We pioneered the use of atom probe tomography to map the positions of dopant atoms in semiconductor nanowires (see also references below). This ability enables the quantitative correlation of electrical properties and the development of much needed models of nanowire doping (with Voorhees).

Single Atom Imaging

The vapor-liquid-solid method of nanowire growth is one of the most widely used, yet there is significant concern that metal nanoparticles used to grow the nanowires may lead to impurity incorporation. We have established both upper and lower bounds on the degree of gold catalyst contamination in silicon nanowires.

Polytypes in Nanowire Structures

Ordered arrays of stacking faults in semiconductors can lead to the formation of distinct polytypes- materials with the same chemical formula but distinct electronic structure. We have observed the formation of hexagonal inclusions within cubic silicon nanowires using correlated Raman spectroscopy and electron diffraction. Further investigations of the nanowire growth process may lead to the controlled growth of homojunction structures with novel electrical and optical properties.

Additional References

• D. E. Perea, J. E. Allen, S. J. May, B. W. Wessels, D. N. Seidman, & L. J. Lauhon, “Three-dimensional nanoscale composition mapping of semiconductor nanowires,” Nano Letters 6, 181 (2006). (Cover Article)
• J. Y. Chou, J. L. Lensch-Falk, E. R. Hemesath, and L. J. Lauhon, "Vanadium oxide nanowire phase and orientation analyzed by Raman spectroscopy," J. Appl. Phys. 105, 034310 (2009).
• D. E. Perea, E. Wijaya, J. L. Lensch-Falk, E. R. Hemesath, and L. J. Lauhon, "Tomographic analysis of dilute impurities in semiconductor nanostructures," Journal of Solid State Chemistry 181, 1645-1652 (2008).
• D. E. Perea, J. L. Lensch, S. J. May, B. W. Wessels, & L. J. Lauhon, “Composition analysis of single semiconductor nanowires using pulsed-laser atom probe tomography,” Applied Physics A, 85, 271-275 (2006).
• P. Adusumilli, L. J. Lauhon, D. N. Seidman, C. E. Murray, O. Avayu, and Y. Rosenwaks, “Tomographic study of atomic-scale redistribution of platinum during the silicidation of Ni0.95Pt0.05/Si(100) thin-films,” Appl. Phys. Lett. 94, 113103 (2009).
• Y.-C. Kim, P. Adusumilli, L. J. Lauhon, D. N. Seidman, S.-Y. Jung, H.-D. Lee, R. L. Alvis, R. M. Ulfig, J. D. Olson, “Three-dimensional atomic-scale mapping of Pd in Ni1-xPdxSi/Si(100) thin films,” Applied Physics Letters, 91, 113106 (2007).

Ultrasensitive Force Detection

Silicon nanowires are single crystals with potentially ideal mechanical properties, but they are not useful without a means to stimulate and detect nanowire motion. Through a collaboration with Raffi Budakian’s group, we have established Michelson interferometry as a viable approach to detect motion in free-standing Si nanowires. The nanowires exhibit ultra-low mechanical dissipation and excellent displacement sensitivity.

Phase-change Materials

Vanadium oxide undergoes an insulator-to-metal transition near room temperature, leading to applications in thermochromic windows, gas sensors, and novel logic devices. We have provided new insights into the electrical switching behavior of vanadium oxide nanobeams by directly correlating the electrical properties with the spatial domain structure in real time. We confirmed the possibility of directly inducing the insulator-metal transition in the absence of a structural phase transformation, supporting the proposal to engineer a Mott field effect transistor from this material.

S. X. Zhang, J. Y. Chou, & L. J. Lauhon, “Direct correlation of structural domain formation with the metal insulator transition in a VO2 nanobeam,” submitted to Nano Letters.

Additional References

• J. M. Nichol, E. R. Hemesath, L. J. Lauhon, & R. Budakian, “Controlling the nonlinearity of Si nanowire resonators using active feedback” Appl. Phys. Lett. 95, 123116 (2009).

Potential Mapping in Nanowire Devices

Nanowire devices can offer higher performance than their planar counterparts, but a quantitative understanding of the operating principles is a necessary foundation for rational engineering and optimization.  We have used scanning photocurrent microscopy (SPCM) to image the operation of a high-performance nanowire field effect transistor under varying operating conditions.

Quantifying Minority Carrier Dynamics

The metal catalyst atoms used to grow semiconductor nanowires have the potential to seriously degrade the performance of minority carrier devices such as bipolar transistors and photovoltaics. By making the first measurements of minority carrier diffusion length in nanowires as a function of diameter and doping level, we demonstrated that the nanowire surfaces, and not metal impurity atoms, limit the diffusion length, a finding of significant importance for nanowire solar cells.

Scanning Photocurrent Microscopy

Nanowire devices are often assumed to operate according to simple one-dimensional models found in textbooks, but these assumptions are not always correct. We have developed a near-field implementation of scanning photocurrent microscopy (SPCM) to map the electronic structure of nanowire devices with high resolution. The influence of contact barriers as well as internal heterojunctions on device function can be systematically investigated.

Additional References

• E. Koren, J. E. Allen, E. R. Hemesath, L. J. Lauhon, & Y. Rosenwaks, “Non-Uniform doping distribution along silicon nanowires measured by Kelvin probe force microscopy and scanning photocurrent microscopy” Appl. Phys. Lett. 95, 092105 (2009).
• J. E. Allen, E. R. Hemesath, D. E. Perea, and L. J. Lauhon, “Non-Uniform Nanowire Doping Profiles Revealed by Scanning Photocurrent Microscopy,” Advanced Materials 21 (30) 3067-3072 (2009).
• Y. Gu, J. P. Romankiewicz, J. K. David, J. L. Lensch, & L. J. Lauhon, “Quantitative Measurement of the Electron and Hole Mobility-Lifetime Products in Semiconductor Nanowires,” Nano Letters 6, 948 (2006).
• Y. Gu & L. J. Lauhon, “Observation of space-charge limited current in CdS nanowires depleted by adsorbed oxygen,” Applied Physics Letters, 89, 143102 (2006).
• Y. Gu, J. P. Romankiewicz, & L. J. Lauhon, “Quantitative characterization of carrier transport in nanowire photodetectors,” Proceedings of SPIE 6479, 64790C-1 (2007). (Invited)
• Y. Gu, J. P. Romankiewicz, J. K. David, J. L. Lensch, E. -S. Kwak, T. W. Odom, & L. J. Lauhon, “Local photocurrent mapping as a probe of contact effects and charge carrier transport in semiconductor nanowire devices,” J. Vac. Sci. Tech. B 24, 2172 (2006).