Surface Microscopy of Atomic and Molecular Hydrogen from Field-Evaporating Semiconductors

We report on the microscopic behavior of residual hydrogen on nanometric field emitters. By using homogeneous or heterostructured semiconductor specimens analyzed in a laser-assisted atom probe, it is possible to study how the relative abundances of the ionic species H+, H-2(+), and H-3(+) depend on the microscopic electric field, estimated through post-ionization statistics. In the case of Ga-containing semiconductors, the relative abundances of H+, H-2(+), and H-3(+) follow a common trend, independent of the nonmetallic component of the matrix. The dependence of the total H flux on the electric field exhibits a more complex behavior, which depends also on the spatial direction of the variation of the field (in-depth or on-surface). The analysis of multiple detection events provides further insights into surface chemistry. Noticeably, H+-H-3(+) ion pairs are both correlated in number and separated by very small distances on the detector space, suggesting a possible reaction 2H(2) -> H-3(+) + H+ + 2e(-) taking place on the field emitter surface.