We present an all-optical technique for measuring the quantum efficiency (QE) of luminescent materials; and, although generally applicable, we apply it as an example to silicon nanocrystals. The concept of internal quantum efficiency (IQE) for a light emission process; the fraction of electron-hole pairs that generate photons inside a material is quite a simple one, and yet in practice this is very difficult to measure. In fact, all existing methods rely on an estimation of IQE from the externalquantum efficiency (EQE); the ratio of photons emitted to those absorbed, typically with the assumption that IQE tends to unity at low temperature. This is an over-simplification and requires further assumptions about the dielectric materials that the photons traverse.

An alternative approach was proposed earlier [1], in which both the light and heat, generated by the excitation process, are measured simultaneouslyas functions of a third variable, e.g. excitation laser power. However, experimental limitations such as reabsorption of light in the sample and recovery of a non-local thermal signal mean that only the EQE is ever really determined, albeit accurately [2].

In this contribution, we describe an all-optical (pump-probe) technique in which we balance the light leaving a local (μm-scale) region of excitation (by measuring μ-PL) with the heat generated at the focussed laser spot (by measuring changes in intensity of a near-IR reflected probe). Critically, our optical arrangement (three confocal signals in one microscope objective) combined with an advanced DSP ‘lock-in’ technique, permits measurement of ΔR/R to within 1 part in 10⁶, corresponding to temperature changes on the mK scale. We present initial data on silicon nanocrystals, formed in glass cover-slips after ion implantation and thermal annealing, and discuss how the technique relates to a true measure of IQE with limitations and how it can be applied more generally to alternative material systems.

 References

 [1]     D J Dunstan, ‘On the measurement of absolute radiative and non-radiative recombination efficiencies in semiconductor lasers’, J. Phys. D: Appl. Phys. 25 (1992) 1825 – 1828

 [2]        K R Catchpole et al, ‘High external quantum efficiency of planar semiconductor strucutres’, Semicond. Sci. Technol. 19 (2004) 1232 – 1235