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

The time-resolved photoresponse of high-purity (${\mathit{\mu }}_{77\mathrm{K}\mathrm{\approxeq }10}$ ${\mathrm{m}}^2$ ${\mathrm{V}}^{\mathrm{-}1}$? sup -1—) n-type indium phosphide has been investigated at λ=118.8 μm 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 μs. 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 μs. On the basis of known transition probabilities, these bottlenecks are therefore tentatively assigned to the 2$p_{\mathrm{-}}$ and 2s states.