# Presentation: 504257

## 3-d Composition Mapping of Semiconductor Nanowires

- conductive substrate

- position-sensitive detector

- -Vpulse

- Semiconductor nanowires (NWs) are one-dimensional materials with unique electronic properties and are potential building blocks for new nanotechnologies.  The composition of single NWs has not been determined to better than 1%, but impurity atoms strongly influence NW properties at much lower concentrations.  For the first time, we have mapped the 3d composition of a NW with sub-nm resolution and single-atom sensitivity using atom-probe tomography (right). Single atoms were sequentially field-evaporated from the tip of a NW placed in a LEAP microscope.§  The original position of the atoms in the NW was reconstructed using software to produce 3d visualizations of nanowire composition.  Single impurity atoms and buried heterointerfaces were successfully imaged (next page).

- Lincoln J. Lauhon, MSE, Northwestern University, DMR-0449933*, DMR-0420532§

- *CAREER: Nanoscale Composition Control and Characterization of One-Dimensional Semiconductor Nanostructures
- §Acquisition of a Local-Electrode Atom-Probe (LEAP) Microscope (An Atom-Probe Tomograph) for Three-Dimensional Nanoscale Characterization of Materials, **P.I. David Seidman**, co-P.I. Lincoln Lauhon.

**L**ocal **E**lectrode **A**tom-**P**robe  (LEAP) Microscope

- local electrode

- nanowire array

**Notes:**
CAREER Research Objectives  The goal of the P.I.’s CAREER research program is to develop an integrated approach to the determination of structure-property relationships in nanoscale inorganic materials that will enable nanomaterials property engineering. Semiconductor nanowire synthesis will be closely integrated with advanced scanning probe microscopy techniques to provide quantitative insights into the effects of individual electronic and magnetic dopant atoms on the electronic, optical, and magnetic properties of one-dimensional nanowires.  One of the primary objectives of the proposed research is to map the composition of doped nanowires and nanowire heterostructures with single atom sensitivity and sub-nanometer resolution using atom-probe microscopy.  This will enable us to establish the limits of composition control for our approach to inorganic nanostructures and test the viability of impurity doping as a means of controlling the electrical and magnetic properties of 1-d semiconductors. 

This nugget shows that atom-probe tomography can be used to analyze semiconductor nanowires- a proof of principle demonstration. There are a number of reasons why this should be considered a breakthrough in the nanowire field in particular and the area of chemically synthesized semiconductor nanostructures in general.  They will be discussed on the notes page of the second slide.

Atom-probe tomography (APT) generates three-dimensional maps of sample composition by combining a field-ion microscope with a time-of-flight mass spectrometer, as shown in the first slide.  The resolution is better than 1 nm and the sensitivity is limited only by counting statistics.  APT is not new, but recent advances in instrumentation have made it possible to analyze volumes comparable to the active area of semiconductor devices, so it seems likely that this technique will soon become more widely known in both industry and academia.  Hard drive manufacturers currently make use of APT, and semiconductor chip makers are getting interested.  On the academic side, there has been an explosion in the amount of research on chemically synthesized nanostructures.

LEAP Analysis     The capability to map nanowire composition at the atomic scale originates from the sequential field evaporation of individual atoms from a nanowire tip. When a positive voltage is applied to a nanowire through the growth substrate, a very large electric field develops at the nanowire tip due to the high local radius of curvature. This local electric field reduces the potential energy barrier that bonds an atom to the surface, resulting in field evaporation of a positive ion.  In the LEAP microscope, sub-nanosecond voltage pulses are applied at 200 kHz to a conical electrode positioned above the nanowire, leading to sequential field evaporation of single ions at the pulsing frequency.  The mass-to-charge state ratio, m/n, of an evaporated ion is determined by measuring the time delay between the local electrode voltage pulse and the signal generated when the ion strikes a two-dimensional position sensitive detector; the calibrated time-of-flight between the nanowire tip and the detector is proportional to the square root of m/n.  The initial position of an atom on the nanowire tip is determined by reconstructing the flight path between the detector and the tip.  In this manner, a three-dimensional reconstruction of the nanowire composition may be generated atom-by-atom with sub-nm resolution. 

## 3-d Composition Mapping of Semiconductor Nanowires

- The positions of single gold atoms within an InAs nanowire are visualized below.  Only 2% of In and As atoms are shown to highlight the gold.  The gold atoms are trapped in the nanowire during the growth, which is directed by a liquid gold catalyst (at right).

_*Single impurity atoms*_

- Perea *et al*, submitted to Nano Letters.

**Notes:**
This nugget shows that atom-probe tomography can be used to analyze semiconductor nanowires- a proof of principle demonstration. There are a number of reasons why this should be considered a breakthrough in the nanowire field in particular and the area of chemically synthesized semiconductor nanostructures in general.  

Most generally, given the amount of attention that has been devoted to nanocrystal and nanowire-based devices, it is perhaps surprising to note that the concentration and distribution of dopant atoms in these structures has never been determined quantitatively.  Impurity and/or dopant atoms, however, strongly influence the electrical properties of a semiconductor. Transmission electron microscopy is not generally sensitive to single dopant atoms, and secondary ion mass spectroscopy does not have the resolution to analyze individual nanostructures.  By imaging gold atoms in a nanowire using APT as shown in the second slide, we show that it is possible to determine the number and position of individual dopant or impurity atoms in nanowires.  One can now quantitatively link synthesis conditions to the nanowire composition, and the nanowire composition to the nanowire properties.   This can be a key enabler for the development of any device technology based on nanowires.

Second, and more specifically, many groups use the vapor-liquid-solid process to grow nanowires.  In VLS, a small metal particle is used to catalyze nanowire growth, and seen in the second slide.  People consistently ask whether any of the metal catalyst becomes incorporated in the nanowire itself, but no one has been able to answer this question definitively.  The answer has obvious consequences- gold, for example, forms a deep level in silicon and is a minority lifetime killer. We shown that this is possible to detect the catalyst in a VLS grown nanowire by analyzing Au-catalyzed InAs nanowires. We have not yet systematically analyzed nanowires to connect synthesis conditions to catalyst incorporation, but we will explore this issue in the technologically important example of silicon, which has shown some tendency to compensate n-type dopant atoms (unpublished prior results). 

Third, and more generally again, we have imaged a buried heterointerface as shown in the second slide.  Most semiconductor devices are based on functional interfaces, and the interface abruptness influences device operation.  We would therefore like to apply this method to image, for example, pn junctions and heterojunctions within semiconductor nanowires and connect the junction profiles with the nanowire electrical properties. 

Summary of Impact     
This is the first report of the mapping of individual impurity atoms in a chemically synthesized semiconductor nanostructure.  The demonstration of three-dimensional composition mapping of a semiconductor nanowire with sub-nm resolution establishes that LEAP microscopy can play an important role in the development of semiconductor nanostructure device technology by providing critical insight into the connection between synthesis schemes and nanoscale composition.  LEAP microscopy has the potential to be applied to other semiconductor nanostructures, including zero dimensional nanocrystals, by depositing samples on arrays of posts elevated from the substrate.  Furthermore, considering the concurrent development of nanoscale property measurements via scanned probes, LEAP microscopy promises to advance materials science by extending our understanding of structure-property relationships to the nanoscale limit. 

## 3-d Composition Mapping of Semiconductor Nanowires

- *CAREER: Nanoscale Composition Control and Characterization of One-Dimensional Semiconductor Nanostructures
- §Acquisition of a Local-Electrode Atom-Probe (LEAP) Microscope (An Atom-Probe Tomograph) for Three-Dimensional Nanoscale Characterization of Materials, **P.I. David Seidman**, co-P.I. Lincoln Lauhon.

- Lincoln J. Lauhon,MSE, Northwestern University, DMR-0449933*, DMR-0420532§

- Prof. Lauhon is the faculty director of the Research Experience for Science Teachers (REST) program at Northwestern.  He recently gave a presentation on nanoscale composition mapping to REST participants as part of a professional development workshop organized by the Nanoscale Science and Engineering Center for Learning and Teaching (NCLT). A video of the presentation will be available on the NCLT website _[http://www.nclt.us/](http://www.nclt.us/)__  pending publication of the research results. _

*Outreach*

*Education & Training*

- Daniel Perea, a first year grad student of Mexican descent, performed the atom-probe research shown here and has submitted a manuscript to Nano Letters.  In October, he will present his work at the annual conference of the Society for the Advancement of Chicanos and Native Americans in Science (SACNAS).  Danny was one of two students selected to represent Northwestern University at this conference.

- Danny Perea