Amplitude-mode electrostatic force microscopy in UHV: Quantification of nanocrystal charge storage

Charge storage in Si and Ge dots (8-60 nm) has been studied via electrostatic force microscopy (EFM) in UHV using a nontraditional amplitude-mode (AM) inversion method to detect cantilever frequency shift and surface potential. Specifics of the AM technique are discussed relating to resonance curve inversion, nonlinear tip-surface coupling, and force vs force gradient "nulling" to extract surface potentials. Single dots were charged via contact electrification with the atomic force microscopy (AFM) tip, followed by EFM, to quantitatively determine how the dot charging level depends on injection bias and dot size. Correlating surface potential with dot charge is addressed by calculating the dipole-dipole, dipole-charge, and capacitive forces acting on the AFM tip in a rigorous manner. For this, the tip-substrate capacitive interaction was studied in detail to account for tip geometry and screening effects due to the dot dielectric volume. In every instance, the change in surface potential measured above a charged dot was seen to be carrier-type dependent and vary linearly with injection bias, where the charging efficiency (e(-)/V applied bias) was determined by only the dot-substrate contact area. Charging efficiencies for small dots (8 nm) were similar to 10 e(-)/V and larger dots (50-60 nm) showed similar to 200-400 e(-)/V, agreeing well with the predicted single e(-) charging behavior of the tip-dot-substrate double tunnel junction (orthodox theory). These experiments demonstrate that AM-EFM can quantitatively measure charge near the level of single electrons-so long as tip shape and screening effects (perturbation of the tip-substrate electric field by the nanostructure being interrogated) are understood and incorporated into models for static charge determination.