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-μm 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-μm radiation the resonance occurs at about 0.6 × ${10}^{12}$ electrons/${\mathrm{cm}}^2$.

• Received 25 March 1974

DOI:https://doi.org/10.1103/PhysRevLett.32.1251