| We use moveable and fixed capacitive probes to experimentally study the spatially-dependent electric field associated with a spontaneous periodic and driven chaotic current instability in voltage-biased p-type ultrapure Ge in the post-breakdown regime at 4.2 K. For dc voltage bias, the data show that each period of the periodic current instability is associated with the nucleation and subsequent movement of a high field space-charge domain along the conduction direction. When the sample is biased below the threshold for spontaneous instability, a solitary domain is nucleated in response to a brief upward pulse in the applied field. The onset of the instability for increasing bias is characterized by non-periodic spikes which are due to the movement of solitary domains across the sample. The transition from infrequent spikes to a periodic oscillation is well-described by the predictions for type III intermittency. When the oscillation is driven chaotic, for increasing drive amplitude we observe first the onset of temporal chaos and later the loss of spatial coherence due to the nucleation and destruction of high field domains in the bulk of the sample. |
