MFC from a Semiconductor in an STM 447
determination of the microwave attenuation, instead of only measuring the power that is delivered through the probe. All of the measurements with galliumnitride were made
Figure 8. Equivalent circuit for determining the power at harmo- nics of the microwave frequency comb with a grounded semiconductor sample in a scanning tunneling microscope.
under ultrahigh vacuum conditions, typically at a working pressure of 2×10−9 Torr. However, after the sample was cleaned with HF, it was exposed to the atmosphere for 60 min before the system was pumped. Thus, it is possible that vacuum may not be required for these measurements. Measurements of the attenuation of the MFC are
sensitive to the spreading resistance in a sub-nanometer spot at the tunneling junction, which may enable carrier profiling with sub-nanometer resolution. Roadmaps for the semi- conductor industry request that the spatial resolution for carrier profiling be finer than 10% of the dimension at each lithography node. However, this is not possible with present technology at the 7-nm node introduced in 2015, or the 5 and 2-nm nodes that are currently in research mode. SSRM is now the preferred method for high-resolution carrier profiling (Vandervorst et al., 2014), but it changes the lattice at the points where the diamond nanoprobes are inserted (Germanicus et al., 2015). Measurements with an MFC do not require probe insertion and also have the advantage of reduced signal to noise, because of the extremely narrow linewidth, and may determine the dielectric function of the semiconductor as well as the carrier concentration.
Figure 9. Calculated microwave power versus spreading resis- tance at the tunneling junction for the first and second harmonics at 74.254 and 148.508MHz.
calculated using equation (2) for the first and second harmonics at 74.254 and 148.508MHz, respectively. In these calculations, RS2 = 1kΩ, RL = 50Ω, CS = 6.4 pF, and C2 = 12 pF.
Measurements of the MFC with semiconductors require
(1) a forward-biased tunneling junction for greater DC tunneling current; (2) a probe making a surface contact
VIEW programming, enables fast and efficient acquisition, storage, retrieval, and analysis of the data so that every scan was captured and processed. The large data files show occasional sharp changes in the measured microwave power, which we attribute to the feedback mechanism stabilizing the DC tunneling current in the STM. Each time the tip-sample distance is automatically changed to stabilize the tunneling current, this also changes the tip-sample capacitance, which could cause the sharp changes in the measured microwave power. It may be possible to avoid this effect, without disabling the feedback, by switching the spectrum analyzer between the probe and the tip circuit. This would allowdirect
within 200 μm of the tunneling junction; (3) using only the first few harmonics, because the higher harmonics roll-off faster than with metal samples; and (4) a mode-locked laser having a photon energy less than the bandgap energy of the semiconductor, to prevent creating electron-hole pairswhich cause surge currents that interfere with the measurements. The NI PXIe-5668R high-performance VSA, with Lab-
CONCLUSION
Several significant observations can be made by examining Figure 9 and equation (4):
1. The quartic term for the frequency in the denominator of equation (4) causes a much faster roll-off in the microwave power at successive frequencies than that measured with metal samples, as shown in equation (1), and this difference is consistent with our measurements with semiconductors.
2. Figure 9 shows that a factor of 10 increase in the spreading resistance causes the microwave power to decrease by a factor of 100, which may be understood from the quadratic terms in the denominator of equation (4). Thus, measurements of the attenuation of theMFCare highly sensitive to the spreading resistance at the tunneling junction, which is inversely proportional to the carrier density in a sub-nanometer spot.
3. The spreading resistance at the contact of the surface probe with the semiconductor is not critical in these measure- ments, because it is in series with the spreading resistance at the tunneling junction, which is typically about 1MΩ. Thus, it is permissible to use iridium for attaching the probe to the sample, or even a simple pressure contact.
ACKNOWLEDGMENTS
The authors appreciate that National Instruments loaned them a PXI systemwith an NI PXIe-5668R high-performance
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