Spontaneous oscillations and chaos in silicon induced by excitonic impact ionization

Abstract

We have observed spontaneous photocurrent oscillations leading to chaos in boron doped Czochralski silicon under optical excitation at liquid helium temperatures. The silicon samples have been annealed at 450°C for more than 100 hours to contain thermal donor complexes in a concentration of about 10 16/cm 3. The oscillations start at a certain critical electric field, where the impact ionization of the thermal donor bound excitons by hot free carriers set in. The current-voltage characteristic shows a highly nonlinear behaviour with a negative differential resistance. Near the breakdown region, which is close to 150 V/cm at 2 K, the current oscillates between a few mA to some hundred mA in pulses of less than a microsecond at a frequency of around 10 kHz, critically dependent on the external parameters like the applied voltage, temperature, laser excitation intensity or the specific sample used. The physical mechanism causing these oscillations is suggested to be a critical balance between the impact ionization- and capture- rates of the thermal donor bound excitons.