Time-resolved luminescence study of recombination and defects in amorphous phosphorus

Abstract

Luminescence in semiconducting amorphous phosphorus is resolved into a band (HE) centred near 1.4 eV and a band (LE) centred near 1.15 eV by time-resolved luminescence measurements excited by 6 ns dye laser pulses. LE decays with a time constant around 150 ns. It is interpreted to be due to recombination of a singlet exciton state corresponding to the triplet found by ODMR. The singlet-triplet exchange splitting is 80 meV. HE shows fast initial decay within the first 800 ns followed by slow non-exponential decay, which is interpreted by recombination of pairs of separated carriers, recombining by radiative tunnelling with a distribution of lifetimes peaking around 3 ms. This interpretation is supported by the authors time-resolved photoconductivity measurements and the high quantum efficiency (0.85+or-0.3). HE shifts to lower energy with increasing delay time suggesting that charged defects are stable in a-P. The excitation spectra peak near 1.95 eV for both LE and HE, but light below 1.9 eV excites LE more efficiently than HE, as expected for excitons. Shape and Stokes shift of LE can be explained by strong electron-lattice coupling but the width of the HE bands is too large for this interpretation.