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Laser Spectroscopy Overcomes Measurement Challenge

Photonics.com
Sep 2013
PHILADELPHIA, Sept. 12, 2013 — A "remarkably simple" laser-based approach overcomes the challenge of measuring key aspects of electron behavior while designing ever-smaller components -- which could allow cellphones, laptops and tablets to get increasingly thinner and more energy efficient.

“One of the most important features of the interface [between two semiconductor materials] is the height of the energy step required for the electron to climb over, known as band offset. Current methods for measuring this step height in planar devices are not practical for nanoscale devices, however, so we set off to find a better way to make this measurement,” said Guannan Chen, a graduate student in Drexel University’s Materials Science and Engineering department and lead author of the group’s recent Nano Letters paper (doi: 10.1021/nl401737u).


Key mechanisms of the measurement process are illustrated in this electron microscope image of the nanowire device.
Courtesy of Drexel University.


Measuring the band offset faced by electrons jumping from one material to another guides the redesign and prototyping of nanoscale components to make them as efficient and effective as possible -- but it hasn't always been easy. So Chen and a team of international collaborators created the tool using laser-induced current in a nanowire device and its wavelength dependence to derive the band offset. They extrapolated the band-offset data by continuously changing the wavelength of the laser and measuring the photocurrent responses.

“Using the interface within a coaxial core-shell semiconductor nanowire as a model system, we made direct measurements of the band offset for the first time in nanowire electronics,” Chen said. “This is a significant cornerstone to freely design new nanowire devices such as solar cells, LEDs and high-speed electronics for wireless communications. This work can also extend to broader material systems which can be tailored for specific application.”

With a better understanding of the material and electron behavior, the team will continue to pursue novel nanoscale optoelectronic devices such as new-concept transistors, electron-transfer devices and photovoltaic devices.

“This remarkably simple approach to obtaining a key characteristic in individual nanowires is an exciting advance,” said Drexel engineering professor Dr. Jonathan E. Spanier, the lead investigator. “We anticipate it will be a valuable method as we develop nanoscale electronic devices having completely new and important functionalities.”

The study, which was funded primarily by the National Science Foundation, also included researchers from Lehigh University; the National Research Council – Institute for Microelectronics and Microsystems (IMM-CNR) and the University of Salento, both in Italy; Weizmann Institute of Science and Negev Nuclear Research Center, both in Israel; and the University of Alabama.


GLOSSARY
electron
A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.
laser spectroscopy
That part of the science involved in the study of the theory and interpretation of spectra that uses the unique characteristics of the laser as an integral part in the development of information for analysis. Raman spectroscopy and emission spectroscopy are two areas where lasers are used.
photocurrent
The current that flows through a photosensitive device, such as a photodiode, as the result of exposure to radiant power.
transistor
An electronic device consisting of a semiconductor material, generally germanium or silicon, and used for rectification, amplification and switching. Its mode of operation utilizes transmission across the junction of the donor electrons and holes.
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