Current self-oscillations, spikes, and crossover between charge monopole and dipole waves in semiconductor superlattices

Self-sustained current oscillations in weakly coupled superlattices are stu died by means of a self-consistent microscopic model of sequential tunnelin g, naturally including boundary conditions. Well-to-well hopping and recycl ing of charge monopole domain walls produce current spikes-high-frequency m odulation-superimposed on the oscillation. For highly doped injecting conta cts, the self-oscillations are due to the dynamics of monopoles. As the con tact doping decreases, a lower-frequency oscillatory mode due to recycling and motion of charge dipoles is predicted. For low contact doping, this mod e dominates and monopole oscillations disappear. At intermediate doping, bo th oscillation modes coexist as stable solutions and hysteresis between the m is possible. [S0163-1829(99)06031-2].