  We report on terahertz (THz) radiation under electrical pumping from a
degenerate semiconductor possessing an electron accumulation layer. In InN, the
random grating formed by topographical defects provides high-efficiency
coupling of surface plasmon polaritons supported by the accumulation layer to
the THz emission. The principal emission band occupies the 2-6 THz spectral
range. We establish a link between the shape of emission spectra and the
structural factor of the random grating and show that the change of slope of
power dependencies is characteristic for temperature-dependent plasmonic
mechanisms. The super-linear rise of a THz emission intensity on applied
electric power provides advantage of such materials in emission yield.
