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Self-sustained current oscillations in weakly coupled superlattices are studied by means of a self-consistent microscopic model of sequential tunneling, naturally including boundary conditions. Well-to-well hopping and recycling of charge monopole domain walls produce current spikes - high-frequency modulation - superimposed on the oscillation. For highly doped injecting contacts, the self-oscillations are due to the dynamics of monopoles. As the contact doping decreases, a lower-frequency oscillatory mode due to recycling and motion of charge dipoles is predicted. For low contact doping, this mode dominates and monopole oscillations disappear. At intermediate doping, both oscillation modes coexist as stable solutions and hysteresis between them is possible. ©1999 The American Physical Society.We thank Rosa Lo´pez and Professor Holger Grahn for helpful discussions. This work has been supported by the DGES ~Spain!, Grant Nos. PB97-0088 and PB96-0875, by the European Union TMR Contract Nos. ERB FMBX-CT97- 0157 and FMRX-CT98-0180, and by the Community of Madrid, Project No. 07N/0026/1998.Peer Reviewe

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