Dopant mapping in semiconductor nanostructures using atom probe tomography

Atom probe tomography (APT) was used to analyze the composition of semiconductor nanowires grown by the vapor-liquid-solid (VLS) mechanism. The feasibility of analyzing nanowires by APT was established with InAs nanowires using voltage pulsing in a Local Electrode Atom Probe microscope. The Au catalyst-nanowire interface was shown to be chemically abrupt, and the detection of single impurities was demonstrated. The results established that APT can play a unique and important role in the development of semiconductor nanowires and devices by providing quantitative insights into the effects of synthesis conditions on the nanoscale composition.;Pulsed laser APT was used to study the distribution of P in Ge nanowires, enabling direct comparisons with the concentrations of the precursors during synthesis by chemical vapor deposition. The detection of individual dopant atoms confirmed that dopants are incorporated via the VLS mechanism, and the incorporation rate could be controlled by the precursor flow rates. Differences in the precursor decomposition rates between the liquid catalyst and the solid nanowire surface gave rise to a heavily doped shell surrounding a lightly doped core.;Analysis of P-doped and B-doped Si nanowires indicated an enhanced concentration of dopants at the nanowire surface relative to the interior consistent with the spatial anisotropy observed in P-doped Ge nanowires. The detection of dopants in nominally undoped regions led to consideration and analysis of radial dopant diffusion during growth. Based on the significantly enhanced diffusion rates that would be needed to explain the data, the possibility of diffusion during APT analysis is suggested as an important component of future work. The abruptness of axial homojunctions was also studied. The equilibrium distribution coefficient of B between the liquid droplet and the solid Si nanowire, and the B composition within the liquid droplet during steady-state growth, are thereby determined for the first time. The relative dopant incorporation efficiencies of P and B in Si and P in Ge indicate that reaction kinetics at the liquid surface and/or gas-phase reactions play an important role in VLS dopant incorporation.;Atom probe tomography analysis was also extended to the distribution of Co and Mn in a Ge thin film superlattice. Visual inspection of the dopant distribution indicated dopant segregation. Radial distribution functions and frequency distribution analyses confirmed a departure from a random alloy and provided unambiguous evidence of phase separation in a regime that might be considered single-phase by other analytical techniques. The dopant rich regions may help propagate magnetic information through interactions between neighboring regions of high CoMn composition. The results suggest that APT may play a useful role in distinguishing between various hypotheses for the origins of magnetic ordering in doped semiconductors.