Pump-probe scanning-tunneling-microscope light-emission spectroscopy of Sb₂Te₃

We have investigated the temporal behavior of Sb₂Te₃ after irradiation with picosecond (ps) pump-laser pulses using scanning-tunneling-microscope (STM) light emission synchronized with ps-probe-laser pulses delayed by times t after individual pump pulses. We determined the gap energy Δ E at the F point in the band diagram of Sb₂Te₃ from the STM light-emission spectra as a function of the delay time t. We found that Δ E increased monotonically with t from the original value of 1.62 eV for a delay time of 0 to 15.3 ps and it decreased toward that original value (1.62 eV) for t in the time span between 15.3 and 28 ps; we saw no change in Δ E for t ≥ 28 ps. By comparing this t-dependence of Δ E with the dielectric functions of the crystalline and amorphous phases of Sb₂Te₃, we have concluded that the phase transition from the crystalline phase toward the amorphous phase is induced by the pump pulses. The phonon energy of the A_2u mode is clearly seen in the STM light-emission spectra when Δ E is 1.62 eV, but not when Δ E is increased above 1.62 eV by pump-pulse irradiation. This feature appears to be consistent with the Raman signals, which are much stronger for the crystalline phase than for the amorphous phase.