Vertical transport properties of weakly-coupled ac-driven gallium arsenide/aluminum arsenide superlattices

The vertical transport properties of ac-driven weakly-coupled superlattices have been investigated experimentally in this thesis. These include the frequency-locking phenomena, the ac-induced generation and annihilation of self-sustained current oscillations (SSCOs), and the appearance of current steps in dynamic voltage bands (DVBs) which exhibit the period-adding bifurcations.; In the study of the frequency-locking phenomena, it has been found that an ac-driven SSCO is frequency-locked into fac/n when the external ac frequency fac equals to nm f0, where f0 is the intrinsic frequency of the free SSCO and both n and m are integers with no common factors. In addition, a locking region for fac exists in the vicinity of nm f0. As long as f ac lies in this locking region, the frequency-locking into fac/n is observed. These phenomena are first explained qualitatively in terms of the limit cycle theory. Then a numerical simulation is performed using the discrete drift model. An excellent agreement between the experimental observations and the numerical results is obtained.; The ac-induced generation of self-sustained current oscillations is manifested by the expansion of dynamical voltage bands upon applying an external ac signal. The width of a DVB increases with the increase of the ac amplitude while the ac frequencies are not much higher than the intrinsic ones. Quantitatively, the ac-induced expansion of DVBs is successfully simulated using the discrete drift model. Qualitatively, a discussion of related phase portraits in the phase space offers a straightforward way to account for this DVB expansion. Furthermore, when an ac signal with frequency much higher than the intrinsic one is applied, the width of a DVB shrinks with increasing ac frequencies, indicating the annihilation of SSCOs. The annihilation of SSCOs at high ac frequencies is ascribed to the ac-induced localization of domain walls.; A series of current steps are observed in the dc current-voltage characteristics of DVBs under certain external ac driven conditions. On each current step the ac response is frequency locked to fac/n, where n is an integer larger than 1. When the applied ac frequency is equal to m X 3.6 KHz, where m is an integer and 3.6 KHz is close to the intrinsic frequency of free SSCOs within DVBs, these frequency-locked current steps are found to exhibit the so-called period-adding bifurcations represented by an arithmetical series of n = ml + 1, where l is an integer increasing from 1. The formation of the current steps in DVBs is explained based on the particular waveforms of the temporal current traces measured on current steps.; The results in this thesis not only demonstrate the richness of the nonlinear dynamics of weakly-coupled superlattices, but also reveal in details the crucial role of an external ac signal in the vertical transport properties of superlattices.