  Weakly coupled semiconductor superlattices under dc voltage bias are
excitable systems with many degrees of freedom that may exhibit spontaneous
chaos at room temperature and act as fast physical random number generator
devices. Superlattices with identical periods exhibit current self-oscillations
due to the dynamics of charge dipole waves but chaotic oscillations exist on
narrow voltage intervals. They disappear easily due to variation in structural
growth parameters. Based on numerical simulations, we predict that inserting
two identical sufficiently separated wider wells increases superlattice
excitability by allowing wave nucleation at the modified wells and more complex
dynamics. This system exhibits hyperchaos and varieties of intermittent chaos
in extended dc voltage ranges. Unlike in ideal superlattices, our chaotic
attractors are robust and resilient against noises and against controlled
random disorder due to growth fluctuations.
