Nonlinear Transport Phenomena in AgCl and AgBr at High Electric Field

The mobility of electrons in zone-refined and impurity-doped AgCl and AgBr crystals at high electric fields has been investigated over the temperature range from $T=4^{\circ }$ K to 77°K in order to examine the energy-momentum relation and to clarify the scattering mechanisms of slow and fast electrons in ionic crystals. The highest-purity crystals were obtained from ingots zone-refined 300 times in halogen atmospheres. Several crystals containing known amounts of iron impurity were also investigated. A fast-pulse technique was used to observe both the transient photoconductivity and the Hall mobility of the photo-electrons. The electrode geometry of Redfield was used in the Hall-mobility measurements. At high electric fields, the photocurrent $Q(T,E)\mathrm{}$ was found to be proportional to the square root of the electric field $E$ in pure AgCl and AgBr crystals at low temperatures. The Hall mobility ${\mu }_H\mathrm{}(T,E)\mathrm{}$ of fast electrons in pure crystals at low temperatures was observed to be a decreasing function of the electric field, thus explaining the $Q(T,E)\mathrm{}$ data. Such nonlinear behavior is weaker in the impurity-doped crystals and gradually disappears even in pure crystals as the temperature rises, for the same range of the electric fields. These results are discussed in terms of the hot-electron theory. An energy-dissipation mechanism similar to the scattering due to the acoustical mode of lattice vibrations is suggested as effective for fast electrons in ionic crystals.