The Optics Laboratory
Group of Hans Hallen, Physics Department, North Carolina State University
The technique used is an all-optical method with high (<100 nm) spatial resolution. It utilizes NSOM to improve the spatial resolution, visible light (red or green) to create excess carriers, and infrared light (1.15 microns) to detect the carriers. For this wavelength of infrared light, free carrier absorption is the dominant carrier interaction. Thus, the infrared light signal is decreased by the presence of excess carriers. The time-dependence of the infrared signal variation is characteristic of the excess carrier lifetime.
Specifically, visible light excites electron-hole pairs between the valence band and conduction band of a semiconductor. IR light, with too low an energy to excite new electron-hole pairs, is used as a probe, as its absorption coefficient depends upon the carrier concentration:
The apparatus that accomplishes this is
An acousto-optic modulator is used to turn the visible light in and off -- a time constant cannot be measured without a time-dependent signal. The modulated light is coupled into the NSOM probe. Also coupled in (via a fiber-optic 'Y') is CW IR light. The Si sample absorbs all the visible light that isn't reflected. The infrared, now with a slight (1 part in 1000-10000) modulation superimposed on it, is analyzed with a lock-in amplifier and sent to a computer. The topography is also measured with the same probe for image comparison.
A timing diagram is shown in the center. The carrier population begins charging up when the visible light comes on, and discharges when the light is shut off. Shown is either a fast recombination rate or a very slow switching frequency, as the optical switching period far exceeds the recombination time, and saturation is reached. The IR signal is also shown at the top. The choice of visible switching frequency is quite important. It should have a period slightly less than the recombination time constant for the best signal.
More info is in the papers.
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