Scanning-probe Single-electron Capacitance Spectroscopy

The overall goal of the following experiment is to observe and to spatially resolve the charging and discharging of single electrons in nanoscale conducting systems located beneath non-conductive surfaces. This is achieved by loading the sample onto a cryogenic scanning probe microscope to achieve low temperatures and low noise levels, enabling observation of single electron behavior. As a second step, use the microscope in scanning tunneling microscopy mode to bring the tip to approximately one nanometer away from the top surface of the sample, which positions the tip in a suitable location for performing the capacitance measurements.

Next, use the microscope in capacitance mode utilizing the extremely sensitive charge detection circuitry to detect the image charge induced on the tip by electron motion at the subsurface system. This allows determination of the electronic structure of the subsurface quantum system. Results are obtained that show individual electrons tunneling onto and off of nanoscale subsurface systems.

Peaks and capacitance versus voltage curves mark the addition energies of electrons. In the quantum system, Semiconductor devices are getting smaller and smaller. The smallest possible device is a single do in atom or an impurity atom.

Many proposed devices involve small numbers of interacting dots. Our method can resolve the basic electronic structure of these minute systems. This method can provide insight into the electronic structure of subsurface, docents, and semiconducting samples at its heart.

This is a capacitance method, which can be extended to variety of low temperature local measurements such as surface dielectric properties and work function mapping. These experiments are done on a cryogenic capable scanning probe microscope with its associated electronics. In addition to the coaxial wires for bias, voltage, and tunneling current, ensure that at least two additional coaxial wires and a ground wire extend from the electronics rack to the tip area of the microscope.

These will be used to carry signals for the cryogenic amplifier. Next, begin assembly of the cryogenic amplifier circuit based on the high electron mobility transistor hemp. Use a scribe to cleave an approximately one centimeter by one centimeter chip from a gallium arsenide wafer.

Then use deposition to form several gold pads of approximately one millimeter by one millimeter on the surface. Now, prepare a sharp tip from a noble metal wire here. Diagonal cutters are used to snip an 80 20 platinum iridium wire using cryogenic compatible epoxy.

Attach a gold wire to each of the gold pads on the gallium arsonite chip. Additional wires have been added on this chip. They can be easily removed if they are not needed at this point, take precautions to avoid introducing stray charges.

When working with the hemp epoxy, the biasing resistor, the tip and the hemp onto the gallium arsenide melting chip. Once the epoxy has cured properly, use a wire bonder loaded with gold wire to bond the source drain and gate elements of the hemp to separate gold pads of the chip bond. Temporary wires connecting the gate and source or drain pads to ensure the gate does not become charged with respect to the source drain channel.

To attach the mounting chip to the microscope first ground the coaxial wires on the microscope to which the wires from the chip will be soldered. Then affix the mounting chip atop the scanning pizzo tube. Use indium solder to connect the gold wires on the chip to the appropriate coaxial wires.

After testing, the integrity of the hemp mounts the sample. This sample is mounted on baka style ramps that allow it to walk in and out in response to voltages applied to the supporting piezo tubes. With the microscope and STM mode, move the sample into range to ensure the sample and tip can approach each other successfully.

After a successful test, walk the sample far out of range to protect the tip during microscope handling. To prepare for lower temperature operation, transfer the microscope from the laboratory benchtop to the cryostat. The cryostat should be capable of achieving the desired base temperature of the microscope 4.2 kelvin or below.

After pumping the microscope to a vacuum of a few micro tour, lower an inch or two of the microscope into the cryostat and wait for the temperature to equilibrate. This can take up to tens of minutes. Repeat lowering an inch or two at a time until the microscope is in place.

The complete immersion process can take almost a day. The microscope should then be left to thermally equilibrate. Finally, isolate the cryostat and microscope assembly from vibrations.

A bungee cord suspension system attached to the cryostat is used in this experiment. Use the suspension system to lift the assembly a few inches off the ground and maintain it at that height. Monitor the height to know if the cryostat sinks and needs to be resus suspended.

After performing STM scans, start capacitance mode measurements by disabling the feedback loop in the STM controller with the tip retracted. A few tens of nanometers from its STM position offset the lateral position of the tip to an area of the sample, which has not recently been scanned. To switch the wiring configuration to capacitance mode, first, protect the hemps by grounding all coaxial wires.

Terminating the wires with T connectors allows the wires to remain grounded while other connections are being made. Next, connect the coaxial wires to relevant voltage sources and resistors, the locke and amplifier and the function generator. Set all voltage sources to zero and turn them on.

Unground the coaxial wires being careful to unground the gate wire. Last to protect the hemp, increase the voltage sources to the desired levels. Adjust the hemp and lock an amplifier for optimal performance.

Then wait for the hemp to stabilize. At this point, it is possible to perform scanning, charge accumulation imaging and capacitance voltage spectroscopy. This is an example of a charge accumulation image.

The sample was silicone doped with boron acceptors with an aerial density of 1.7 times 10 to the 15th per meter squared in a delta dope layer 15 nanometers below the surface at 4.2 kelvin. As indicated by the scale, brighter colors indicate increased charging. The bright spots are interpreted as marking the location of individual subsurface boron atoms.

The blue dot indicates a particular bright spot where point C versus V spectroscopy was performed. The largest peak in the C versus V data is interpreted to be from charge entering the doin directly below the tip Nearby peaks are due to nearby dots. Their centers are shifted in amplitudes decreased with respect to the main peak.

Due to increased distance of the DO pins. The peaks are broadened along the voltage axis by effects accounted for in the model that has been developed as indicated by the agreement of the model curve with the data. The C versus V spectroscopy data shown here is for gallium arsenide delta dope with a layer of silicone donors of aerial density, 1.25 times 10 to the 16th per meter squared, located 60 nanometers below the surface at 300 millikelvin.

It also shows a series of charging peaks, most of which are consistent with groups of many electrons entering and leaving the do opens a single electron peak is indicated with the red arrow. The data on the right are from repeated measurements of the peak indicated by the red arrow on the plot on the left. When the data are averaged, a fit is made and shown here in green.

This fit curve is consistent with the expected shape for a single electron peak under the experimental conditions. After watching this video, you should have a good understanding of the hands-on aspects of performing scanning single electron capacitance measurements While attempting this procedure. It's important to remember to avoid destroying the sensitive hemp by taking precautionary steps to prevent static buildup between the gate and the source drain channel.