Impact excitation and bottleneck effects in the time-resolved far-infrared photoresponse of high-purity InP

The time-resolved photoresponse of high-purity (μ_77K10 m² V^-1? sup -1---) n-type indium phosphide has been investigated at {λ}=118.8 {μ}m with use of a pulsed far-infrared laser. An effective conduction-band lifetime of 60 ns is found which increases slightly in a magnetic field. For sample dc bias voltages exceeding a few volts per centimeter, long tails develop in the photoresponse which last more than 10 {μ}s. The behavior of these tails is examined for various incident laser powers, magnetic fields, and sample bias voltages. It is shown that a four-level model involving impact excitation from impurity states is necessary to describe the photoconductive response. A critical requirement of the model is that the lifetimes of participating ``bottleneck'' states are 2 and 10 {μ}s. On the basis of known transition probabilities, these bottlenecks are therefore tentatively assigned to the 2p_- and 2s states.