Through the use of multiple internal reflection and a modulation of the charge in the surface via the field effect, sufficient sensitivity is obtained to observe absorption of infrared radiation resulting from transitions involving the semiconductor surface states. This offers the possibility of obtaining the optical spectra and thus some rather direct information about the surface states. The principal other mechanism which contributes to the signal in this experiment is the free-carrier absorption arising from a modulation of the free-carrier density in the semiconductor space-charge region. The results of some observations on a silicon surface are discussed.

• Received 28 August 1961

DOI:https://doi.org/10.1103/PhysRev.125.1165