Subpicosecond hot carrier cooling in amorphous silicon

Abstract

Measurements of the cooling rate of hot carriers in amorphous silicon are made with a two-pump, one-probe technique. Pump photons at 2 eV create free carriers and pump photons at 1.42 eV heat the carriers up to 1.2 eV/pair. The experiment is simulated with a rate-equation model describing the energy transfer between a population of hot carriers and the lattice. An energy transfer rate proportional to the temperature difference is found to be consistent with the experimental data. An energy transfer rate independent of the temperature difference is inconsistent with the data. This contrasts with the situation in crystalline silicon and GaAs. The measured cooling time, 0.2 ps, is sufficient to explain the absence of avalanche effects in amorphous silicon at fields below 106 V/cm.

Ultrafast optical switching of photonic crystals

“…From this we conclude that a two-photon process is the dominant excitation mechanism in our experiments. The induced effects remain after the pump pulse, and decay at a time scale in between the decay time for free carriers in amorphous- [128] and single crystalline Si [135]. We therefore conclude that the changes are indeed caused my free carriers.…”

“…The decay times of about 18 ps are much shorter than carrier relaxation times in bulk Si, likely since our photonic crystals are made of polycrystalline silicon, whose lattice defects and grain boundaries act as efficient carrier recombination traps [110]. On the other hand, the decay time is longer than that of amorphous Si, in which recombination times as short as a few picoseconds have been observed [128]. The relatively fast decay time implies that switching could potentially be repeated at a rate above 25 GHz, which is relevant to possible future switching and modulation applications and will be discussed in Section 6.6.…”

Thermal quenching of defect photoluminescence and recombination rates of electron–hole pairs in a-Si:H

“…At low temperatures, in our model, the principal PL arises from radiative recombination of tail electrons with self-trapped holes, while the defect PL arises from radiative recombination of electrons at radiative defects with self-trapped holes. When electrons in the valence band and its tail states are excited into the conduction band by optical excitation, these high-energy electrons are cooled down at the bottom of the conduction band and also in its tail states through electron-phonon interaction [8][9][10]. However, the electron-phonon interaction is not almost involved in the radiative recombination responsible for the principal PL.…”

Time‐of‐Flight Analysis

“…However, it takes only picoseconds for the carriers to thermalize down to the mobility edge (for amorphous silicon, see Ref. [14]). Now, drift (and diffusion) at the mobility edge begins.…”