MFC from a Semiconductor in an STM 445


n-type and p-type semiconductors. The surface charge consists of ~400 minority carriers (Hagmann et al., 2015). Typically, dielectric relaxation takes place within 1 ps, which is 10−4 of the time between consecutive laser pulses; hence, the phenomena occurring in each pulse may be considered to be independent. The injected minority carriers move outward in a Coulomb explosion (Hagmann et al., 2015), as majority carriers move inward to complete neutralization in a distance of ~20 μm. We find that the probe must be within 200 μm of the tunneling junction to detect the MFC, but ideally this distance should be <20 μm. Initially, the MFC was measured with a high-speed


silicon PIN photodetector (DET 210; Thorlabs, Newton, New Jersey) in place of the tunneling junction. This was followed by measurements using the STM with a gold sample, to verify proper operation of the system, before beginning the measurements with the semiconductor. We were concerned that optical rectification could


take place at the contact between the probe and the semi- conductor, so an ohmic contact to the semiconductor was formed, using an Al/Ti bilayer metallization (Lin et al., 1994). The bilayer consisted of a 5-nm thickness of titanium covered by a 100-nm layer of aluminum, where each layer had a common width of 100 μm. However, this structure did not permit chemical cleaning of the semiconductor, which appears to be essential for the measurements. Our second approach, which was successful, was to clean the gallium nitride in HF and use a fine gold wire (8-mm long, 0.3-mm diameter) attached to the semiconductor with indium as a probe.


RESULTS AND DISCUSSION


Initial measurements of the MFC with the high-speed silicon PIN photodetector, as well as measurements with a gold sample in the STM, verified that the measurement system was operating properly. A series with 13 consecutive scans of the 18th harmonic (1.3366 GHz) with the photodetector showed an apparent linewidth (full-width at half-maximum) of 2.7Hz with the peak power having a mean value of −119 dBm. A series of 286 consecutive scans of the 100th harmonic (7.4254 GHz) with a gold sample in the STM showed an apparent linewidth of 2.6Hz with the peak power having a mean of −115 dBm. Using equation (1), this would be equivalent to a power of −92dBm at the fundamental frequency, which is consistent with earlier tests. The first measurements, using the semiconductor with


the Al/Ti bilayer metallization for an ohmic contact, gave no detectable microwave power, which we attribute to the inability to clean the semiconductor at the tunneling junction. Following this, the gallium nitride was cleaned with HF and a fine gold wire (8-mm long, 0.3-mm diameter) was attached to the semiconductor with indium to serve as the probe, to enable the following measurements of the MFC. Figures 4 to 7 show the average measured microwave power for 24 consecutive scans as a function of the frequency.


Figure 5. Mean power at the second harmonic of the microwave frequency comb for 24 consecutive scans with an n-type GaN sample in the scanning tunneling microscope.


Figure 4. Mean power at the first harmonic of the microwave frequency comb for 24 consecutive scans with an n-type GaN sample in the scanning tunneling microscope.


Figure 6. Expanded graph of the mean power at the first harmo- nic for 24 consecutive scans with an n-type GaN sample in the scanning tunneling microscope, with error bars.


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