This project will use ultra-sensitive charge detectors coupled to a scanning needle, similar to an Atomic Force Microscope, to make nanoscale-resolution maps of the electrical potential of a surface of solid or soft materials. A novel sensor, based on high-speed version of the most sensitive charge detector, single-electron transistor (SET), will be used. This sensor employs arrays of SETs and radio-frequency reflectometry to give information on fast charging processes, occurring in a microsecond time frame. The capabilities of this technique enable experiments that cannot be performed using traditional measurements of surface potentials. For instance, in existing measurements the scanning probe disturbs the surface that is being measured, making it difficult to measure charges, such as electrons, that are only weakly confined to an area of the surface. In contrast, the measurements made in this project will be much less disruptive to the surface, making it possible to observe phenomena such as Wigner localization of electrons. The project also involves an outreach program that will target middle school students in South Bend Schools through classroom activities involving faculty and graduate students, and field trips to bring students to the Notre Dame labs. These activities will contact 50 - 100 students per year. Middle school aged children are an excellent group for outreach since they are advanced enough to understand science, but are still making choices about their areas of interests. South Bend public schools have a diverse student population with a large number of students from groups underrepresented in the areas of science and technology.
This project will use ultra-sensitive electrometers coupled to a scanned probe system to map the potential of material surfaces. In surface potential measurements, two methods dominate: Kelvin Probe Force Microscopy (KPFM) and Electrostatic Force Microscopy (EFM). Kelvin probe is the more popular technique because it produces a quantitative measurement of the contact potential difference (CPD) while EFM can measure only qualitative changes in CPD. This project design and fabricate scanning single-electron transistor probes (S-SETs) and use them to characterize the surface charge and potential of semiconductor surfaces. The S-SET probes will use radio-frequency reflectometry to enable measurements of the CPD with high charge and potential sensitivity, as well as high spatial and temporal resolution. Arrays of SETs will be used to provide improved immunity to random background charge fluctuations, and to provide improved matching to the RF circuit, eliminating the need for a complicated resonant network. In addition, the use of an RF SET will enable a Kelvin force measurement, not yet reported in the literature, that will be less invasive than those made by conventional KPFM. With the scanned probe system it will be possible to characterize a variety of surfaces including semiconductors and insulators. In particular, the system will be used to investigate Wigner localization of electrons within semiconductor quantum dots.
