  A full particle simulation study is carried out for studying
microinstabilities generated at the shock front of perpendicular collisionless
shocks. The structure and dynamics of shock waves are determined by Alfven Mach
number and plasma beta, while microinstabilities are controlled by the ratio of
the upstream bulk velocity to the electron thermal velocity and the
plasma-to-cyclotron frequency. Thus, growth rates of microinstabilities are
changed by the ion-to-electron mass ratio, even with the same Mach number and
plasma beta. The present two-dimensional simulations show that the electron
cyclotron drift instability is dominant for a lower mass ratio, and
electrostatic electron cyclotron harmonic waves are excited. For a higher mass
ratio, the modified two-stream instability is dominant and oblique
electromagnetic whistler waves are excited, which can affect the structure and
dynamics of collisionless shocks by modifying shock magnetic fields.
