Extension of the Kelvin probe vibrating capacitor technique of measuring work function, via Illumination of the semiconductor surface, i.e., Surface Photovoltage Spectroscopy (SPS), has many potential applications in the field of surface analysis.
The combination of broad-band (white) and monochromatic radiation, together with measurement of the dark signal permits complete characterisation of the semiconductor work function via determination of the electron-affinity, surface potential and Local DensIty-of-States (LDOS). The work function is an extremely sensitive indicator of a wide range of surface processes, e.g., particle adsorption, stress, defect creation, phase-transitions, etc.
We Illustrate application of this technique in the study of the temperature dependent initial oxidation behaviour of p-type Si(111) 7×7 between 100 and 300 K. The SPV response of the clean surface at 100 K corresponds to the capture of photo-stimulated electrons by a band of surface states centered around 1.4, 1.7, 1.9 and 2.4 eV. This response completely disappears at the peak of the (dark) work function change (0.3L) corresponding to a near complete removal of dangling bond states. The temperature-dependent white-light SPS response permits determination of the band-bending throughout the adsorption process. We observe that at 100 K the band-bending substantially decreases during the initial adsorption phase (0.1 L), after this dose it remains constant. However at 300 K the band-bending decreases much later, i.e., >10 L, in conjunction with oxygen permeation through the surface layer.
In conclusion Surface Photovoltage Spectroscopy SPS is a simple and flexible method which can be used to follow the rather complex changes occurring at the semiconductor surface. It is a non-contact, nondestructive technique which allows simultaneous determination of both semiconductor band-bending and electron affinity.