Temperature dependence of terahertz emission from InMnAs

We have studied the terahertz radiation emitted by the dilute magnetic semiconductor InMnAs under femtosecond laser illumination, as a function of temperature. We observe a reversal of the polarity of the emitted terahertz field as the temperature is decreased, in both p-type and n-type materials. This effect is not observed in p-InAs. A competition between two oppositely directed currents, the photo-Dember current and the surface-field-induced current, can explain the observed polarity reversal. In contrast to nonmagnetic materials, these two currents are of comparable magnitude because the Mn impurity substantially reduces the carrier mobility in dilute magnetic semiconductors.