Time-correlated single photon studies of hydrogenated amorphous silicon carbon

Abstract

A time correlated single photon counting system has been developed to probe very fast photoluminescence decay properties associated with excitonic recombination in hydrogenated amorphous silicon carbon. A detailed description of this system includes a unique unbiased distribution analysis technique capable of resolving up to five, nanosecond and subnanosecond component lifetimes. A resolution limit of 20 ps is demonstrated using synthetic data and down to 87 ps on a sample of known decay time. This is the first time that time correlated single photon counting has been apphed to probe the subnanosecond regime of the photoluminescence decay in hydrogenated amorphous silicon carbon thin films. Five samples of carbon to silicon atomic concentrations from 0.50 to 0.82 were deposited using an electron cyclotron resoucince plasma system with a liquid source of diethylsilane. Studies on absorption and emission spectra and luminescence decay acquired over the visible spectrum show that as the relative carbon concentration increases, the defect density, and bandtail broadening also increase as the photoluminescence peak emission decreases. The room temperature photoluminescence decay comprises of three and four distinct lifetime distributions which remain constant in time over the emission wavelengths studied and between samples; only the relative contribution of these individual components vary. Application of the bound exciton theory reveals that these four resolved distributions centered at 30 ps, 300 ps, 2 ns and 6-8 ns correspond to the recombination of the electrons with four possible hole states with respective wavefunction radii of 4.0A,2.9A,2.0A eind 1.6A. The relative probabihty of each transition depends on the density of the four hole states within each sample. The higher carbon concentration samples have shorter overall decay times, lower band gap energy, broader bcind tails and, hence a higher density of the most localized hole states. The percent contribution of the shorter lifetime components increases with carbon concentration and are attributed to increasing structural disorder. The sample of 0.50 Ccirbon concentration hcis the best overall photoluminescence properties, thus, establishing the capability of the electron cyclotron system to produce hydrogenated amorphous sihcon carbon thin films of good quality.