Dual microscopes illuminate electronic switching speeds

Designers of semiconductor devices are like downhill skiers---they thrive on speed. And achieving speed in the semiconductor business is all about the stuff you start with. While silicon is still the mainstay of the industry, circuit designers also would like to put materials like gallium nitride and silicon carbide into wider use. Such advanced semiconductor materials can operate at higher voltages and provide faster switching speeds, an important characteristic in determining how fast a semiconductor circuit can process information.

Reporting in the Sept. 22 issue of Applied Physics Letters, a National Institute of Standards and Technology (NIST) researcher and a Korean guest researcher describe a new method for scanning semiconductors for defects that may help accelerate the market for these newer materials. The duo combined an atomic force microscope with a scanning capacitance microscope and then added custom software and a simple on/off switch for the AFM's positioning laser.

The result is an instrument that can measure how fast a material generates electrical charges and then map those speeds in sections (at least for gallium nitride) that are only about 100 nanometers square. Current methods for measuring switching speed (carrier lifetime) produce only bulk averages.

According to NIST co-developer Joseph Kopanski, the system allows quick scanning of semiconductor wafers for defects that otherwise may not be found until an expensive device has already been built on the material. Most defects in semiconductors (i.e. sections with missing atoms) are presumed to slow down the speed that charges move through a material. Kopanski says further research using the new technique should determine if this assumption is correct. A patent application is pending on the technique.

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