We are developing a new method for the carrier profiling of semiconductors that shows promise for nm-resolution which is required at the new sub-10 nm lithography nodes. A modelocked ultrafast laser focused on the tunneling junction of a scanning tunneling microscope (STM) generates a regular sequence of pulses of minority carriers in the semiconductor. Each pulse of carriers has a width equal to the laser pulse width (e.g. 15 fs). In the frequency domain, this is a microwave frequency comb (MFC) with hundreds of measurable harmonics at integer multiples of the laser pulse repetition frequency (e.g. 74 MHz). After the minority carriers diverge rapidly into the semiconductor as a Coulomb explosion, the pulses become broader and decay, so that the MFC has less power with a spectrum limited to the first few harmonics. The frequency-dependent attenuation of the MFC is determined by the resistivity of the semiconductor at the tunneling junction so SFCM is closely related to Scanning Spreading Resistance Microscopy (SSRM). Harmonics of the MFC are measured with high speed, and high accuracy because the signal-to-noise ratio is approximately 25 dB due to their extremely narrow (sub-Hz) linewidth. Now we superimpose a low-frequency signal (e.g. 10 Hz) on either the applied bias or the voltage that is applied to the piezoelectric actuators of the STM to cause sidebands at each harmonic of the MFC which are less affected by the artifacts.
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Method of carrier profiling utilizing dielectric relaxation
A mode-locked laser injects pulses of minority carriers into a semiconductor sample. A microwave frequency comb is then generated by the currents formed in the movement of majority carriers native to the semiconductor and the injected minority carriers. These carriers move to cause dielectric relaxation in the sample, which can be used to determine carrier density within the sample. Measurements require close proximity of transmitter and receiver contacts with the sample and may profile a semi-conductor with a resolution of approximately 0.2 nm.
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- Award ID(s):
- 1648811
- NSF-PAR ID:
- 10066871
- Date Published:
- Journal Name:
- U.S. patentSearch claims & abstracts
- ISSN:
- 1069-3645
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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