SPATIAL MEASUREMENTS OF MOVING SPACE-CHARGE DOMAINS IN P-TYPE ULTRAPURE GERMANIUM

We use a moveable capacitive probe to study experimentally the spatially dependent voltage, electric field, and charge density associated with a periodic current instability in voltage-biased ultrapure p-type Ge at 4.2 K. The data show that each period of oscillation is associated with the movement of a high-field space-charge domain along the conduction direction. The domains are 2 mm wide, approximately 3 V/cm in amplitude, and move at a velocity of approximately 1200 cm/s. The onset of the instability for increasing bias field is characterized by nonperiodic current spikes, which are due to the nucleation and movement of solitary domains across the sample. We have also succeeded in nucleating a solitary domain when the sample is biased below the threshold for spontaneous current instability by applying a brief upward pulse in the applied field.