Resonance Spectroscopy of Electronic Levels in a Surface Accumulation Layer

Resonance transitions are observed between electronic levels in an accumulation layer on $n$-type (100)Si. Signals are studied at the 220-, 171-, and 118-\ensuremath{\mu}m lines of a watervapor (${\mathrm{H}}_{2}$O, ${\mathrm{D}}_{2}$O) molecular laser. Strong transitions from the lowest two-dimensional sub-band to the next higher band are observed. For 118-\ensuremath{\mu}m radiation the resonance occurs at about 0.6 \ifmmode\times\else\texttimes\fi{} ${10}^{12}$ electrons/${\mathrm{cm}}^{2}$.