The Impact of Silicon CCD Photon Spread on Quantitative Analyses of Luminescence Images

Commercial and R&D photoluminescence imaging systems commonly employ indirect bandgap silicon charge-coupled device (CCD) imaging sensors. Silicon is a weak absorber of the near-infrared band-to-band emission of silicon, and significant lateral spreading of the luminescence signal can occur within the sensor. Uncorrected, this effect reduces image contrast, introduces artificial signal gradients, and limits the minimum feature size for which accurate quantitative measurements can be derived. Empirical quantification of the spreading effect defined in terms of the point-spread function (PSF) for the imaging apparatus allows for postprocessing deconvolution, which quantitatively improves image accuracy and contrast. Assessment of the impact of a photon spread indicates that signal smear under commonly occurring high contrast ratio scenarios is sufficient to warrant the application of deconvolution to improve the accuracy of quantitative data in calibrated luminescence images. With a carefully defined PSF, corrections to within ± 10% of the true signal ratio for small-area features can be achieved. Short-pass filtering provides partial correction of the photon spread with the advantage of reduced experimental complexity but, nonetheless, with limitations on the minimum feature size for which accurate signal ratios can be measured.