Universal distribution of optically excited carriers in tetrahedral amorphous semiconductors

The decay of optically excited electrons and holes in tetrahedrally coordinated amorphous semiconductors is a universal property of these amorphous solids [Phys. Rev. Lett. 84 (2000) 4180]. The experimental measurements employ a seldom-used electron spin resonance (ESR) detection technique that surpasses the sensitivity of previous measurements by several orders of magnitude. This technique employs second harmonic detection (SH-ESR) of the ESR. Measurements have been performed over a wide range of excitation intensities (nW/cm2 to W/cm2) on hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous germanium (a-Ge:H). Using the SH-ESR technique, the kinetics can be studied down to saturated carrier densities as low as approximately 1014 cm−3. In addition to the long-time decay curves, both the saturated densities of carriers after long-time irradiation and the most probable recombination lifetimes for the optically excited carriers in a-Si:H agree well with models for the universal diffusion and recombination of carriers at low temperatures. Similar results are obtained for samples of a-Ge:H with germanium dangling-bond densities ≲1017 cm−3. A comparison of the ESR lineshapes for the electrons and holes trapped in the conduction and valence band tails, respectively, with electrons and holes trapped in a divacancy in crystalline silicon shows that the asymmetries and localizations of the wavefunctions are similar in both cases.