NANOPHOTONICS: Atom-probe microscope maps nanowires

A new type of microscope is being used by scientists at Northwestern ­University (Evanston, IL) to map the 3-D locations of millions of atoms in semiconductor nanowires. The resulting images, striking in their seeming concreteness, look a bit like vastly extended versions of the ping-pong-ball models of atomic structures that scientists once relied on. The instrument, called a local-electrode atom-probe (LEAP) microscope, was developed by Imago Scientific Instruments (Madison, WI), which commercially manufactures it.

Semiconductor nanowires may one day form the basis for smaller and faster photonic and electronic devices. Knowing the wires’ structure and make-up to atomic-scale accuracy is important in the effort to create practical components.

To acquire the 3-D data, a conical objective brought close to the nanowire applies subnanosecond voltage pulses at a 200-kHz frequency to the tip of the nanowire, ionizing atoms and evaporating them one at a time. The instrument determines the mass-to-charge ratio of each ion from the time the ion takes to reach a 2-D position-sensitive detector. If two separate atoms are evaporated off the specimen at once, their simultaneous detector strikes are ignored.

“The ions travel through a multichannel plate, generating an electron cascade that is used to determine the time of flight,” says Lincoln Lauhon, one of the researchers. “The ion then moves on to hit a delay-line detector, providing the position in two lateral dimensions. The time at evaporation provides the ion positions in the third (vertical) dimension.”

The researchers reconstructed the position of 1.3 × 106 atoms evaporated from an indium arsenide (InAs) nanowire, revealing the (0001) ­atomic planes, measuring the proportion of indium, arsenic, and a small concentration (100 parts per million) of gold atoms, and seeing the nanowire’s hexagonal facets.

Mapping interfaces

In results especially applicable to the photonics community, the researchers also mapped a nanowire with a gold(90%)-catalyst(10%)/InAs interface (see figure). An understanding of interfaces is crucial for getting at the detailed physics of a nanophotonic device and its fabrication. The interface was mapped in great detail, showing that the transition was abrupt, occurring within 0.5 nm.

“This technique can map insulators as well if the evaporation is triggered by fast laser pulses,” says Lauhon. “Imago also sells a LEAP microscope with pulsed-laser evaporation. We have tested this version on many samples at Imago, and are currently pursuing funding to acquire the pulsed-­laser upgrade.”

REFERENCE

1. Daniel E. Perea et al., Nano Letters 6(2) 181 (2006).