Effect of high pressure on radiative recombination in hydrogenated amorphous silicon

Abstract

The first studies of photoluminescence (PL) at high pressure in hydrogenated amorphous silicon are reported. A cryogenic diamond-anvil press is employed in the range 0-75 kbars, 10-110 K. Whereas the PL intensity is strongly quenched by compression, the peak position is only weakly shifted to lower energy. This interesting dichotomy indicates that compression affects radiative and nonradiative processes differently. The quenching data suggest an increase in the intragap defect density by ∼${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$, after pressurizing to 42 kbars and lowering to 1 atm. This is interpreted as evidence for compression-induced structural changes, associated with voids, which augment the nonradiative defect population. Although the pressure coefficient, -2.0±0.5 meV/kbar for the PL peak is similar to that of the absorption edge in crystalline (and amorphous) Si, the possibility of structural changes discourages a direct connection between the two. Useful "Gruneisen parameter" expressions, which are derived for the Stokes shift and PL linewidth, indicate marginal compatibility between the Stokes-shift hypothesis and the pressure data. No strong effect of pressure on the PL thermal behavior is observed. With increasing pressure the radiative decay rate exhibits nonmonotonic behavior, first increasing to a maximum at 23 kbars and then decreasing. This complex dependence, which could not be explained within a simple (donor-acceptor-type) radiative tunneling picture, may reflect details of the recombination process. A model is suggested involving small-polaron hopping between inequivalent sites.