Supercritical perpendicular collisionless shocks are characterized by a fraction of the incoming ions being reflected at the steep front. These reflected ions accumulate and form a foot, where the relative drift of the reflected ion beam versus the electrons can easily excite an electron cyclotron drift instability (ECDI). Here, we analyse the resulting wave emissions by two approaches. First, our linear dispersion analysis shows that several electron Bernstein harmonics can be unstable, their number being proportional to the drift, yet limited by the ion beam temperature. Second, the three local populations (incoming electrons/ions and reflected ions) of the foot region are introduced in full electromagnetic PIC periodic simulations in order to analyze the nonlinear regime of the ECDI. Main results are: (1) High gyro-harmonics develop over a short timescale less than the lower-hybrid period. (2) The spectral power shifts towards lower k-modes to accumulate on the first harmonic under the effects of two complementary processes: (i) trapping of the reflected ion beam, and (ii) resonance broadening. The latter acts to demagnetize the electrons in the dispersion relation. It initially applies to very high k-modes (kρe ≫ 1) and progressively, as time evolves, to lower k-modes, invalidating the existence of all gyro-harmonics, except the first one. (3) In the late stage a magnetic field component surprisingly develops in the energy spectrum that had so far been electrostatic. (4) The electrons are heated, which represents a source of preheating in the foot region of the shock front.