Semiconductors

An example of an in-situ work function topography of a clean Si(111) 7x7 single crystal is shown here. The work function is mostly flat with a depression in one area which is in the vicinity of the sample contact. Using optical pyrometry we can determine that this region of the sample was 50 ºC cooler and subsequent AES analysis indicated a modest build-up of carbon contamination (< 10%).
The Kelvin Probe has been used to study in detail the oxidation of Si surfaces which serves as a reminder of the sensitivity of the method. As the induced surface dipole at monolayer (ML) coverage is quite large- upwards of 1 V-, sub-mV work function accuracy infers resolutions of approximately 1/1000 ML in coverage. STM and AES can also be used to follow this reaction however STM topographies quickly become confused at relatively low coverage, e.g., > 0.1 ML and it is very difficult to obtain accurate chemical information below 0.3 ML. The KP data however are continuous throughout this region and offer additional information regarding atomic re-arrangements, density of surface states, etc.
We have employed similar measurements on polycrystalline metals such as Re and W in order to follow their high temperature (800 - 1400 oC) oxidation characteristics. The resulting high work function surface, i.e., >7 eV, will be used as positive ion sources.