Hot Carrier Enhancement of Dember Photorefractive Space-Charge Fields in Zincblende Semiconductors

We use a novel, nondegenerate, polarization-sensitive, transient-grating technique1 to monitor the picosecond dynamics of the photorefractive effect in undoped CdTe and InP:Fe at 960 nm. The technique circumvents the limited temporal resolution of the two-beam coupling geometry by using a time-delayed third probe pulse (with a duration of <5 psec) to read the gratings written in the semiconductor. The technique also exploits the crystal symmetry of zincblende semiconductors by using an optically induced anisotropy in the crystal index of refraction2 to separate the photorefractive gratings from the stronger, co-existing instantaneous bound-electronic and freecarrier gratings. In both semiconductors, the photorefractive effect is associated with the Dember field between mobile electron-hole pairs, in contrast to the more conventional photorefractive space-charge field connected with the separation of a mobile carriers species from a stationary, but oppositely charged, mid-gap state. In the undoped CdTe sample, which possesses no optically-active mid-gap levels, the electron-hole pairs are produced by two-photon absorption of 1.3 eV photons across the 1.44 eV band-gap of the semiconductor. The resultant ~1 eV excess carrier energy, which allows hot carrier transport to dominate the initial formation of the space-charge field, causes up to an order of magnitude enhancement in the photorefractive effect on picosecond timescales. After the carriers have cooled and the initial overshoot in the space-charge field has decayed, the photorefractive effect is observed to decay as the Dember field is destroyed by ambipolar diffusion of the electron-hole pairs across the grating period. In InP:Fe on the other hand, the electron-hole pairs are produced predominantly by direct single-photon band-to-band absorption into the band-tail of the semiconductor (band-gap ~1.35 eV), since the iron dopant only dominates the linear absorption at longer wavelengths. This means that the carriers are generated with little excess energy. Consequently, no hot carrier enhancement of the photorefractive effect was observed, and once formed, the Dember space-charge field decayed directly by ambipolar diffusion.