Microscopic photoemissive probing of doped regions on semiconductor surfaces

The use of photoemission for microscopic probing of the material characteristics of a semiconductor surface is demonstrated. A focused UV laser beam is scanned across a patterned semiconductor, and differences in surface properties such as doping or oxide/metal deposits are observed as differences in emitted photocurrent. The experimental setup is described, and measurements on Si (100) surfaces are reported. Measurements of photoelectric yield versus laser power exhibited a linear dependence on power, indicating that single photon photoemission is the dominant effect observed in these experiments. A typical trace of the photocurrent versus time for a stationary laser spot on the weakly doped p-type silicon region is shown. On steady illumination, a photocurrent is measured which subsequently rises, reaches a maximum, and then decays to a nearly steady-state value. This phenomenon is believed to be the result of adsorbed gases, probably water vapor. Plots of photocurrent versus time obtained while scanning the beam across the p⁺-on-p and n⁺-on-p samples are also shown and discussed.