19December 2022in the uniformity of the epitaxial deposition on a large wafer, so that a Lidar laser die cut from one part of the wafer can be expected to perform and behave as one from any other part of the wafer. Once again, this property is readily characterized using a photoluminescence (PL) map across the wafer (figure 1b). If for some reason some of the dies from this wafer do not show optimal luminescence compared to others, the process engineer might be interested in understanding the nature of the defects causing this suboptimal luminescence efficiency and could use time-resolved PL to undertake these studies (figure 1c). Alternatively, and as part of a QA process the engineer might want to measurement the performance of the device under conditions similar to actual use by measuring the electroluminescence of the device before it is packaged (figure 1d). Fig. 1a Photoluminescence spectra for various semiconductor materials with different bandgaps. (b) Photoluminescence (PL) map of two inch Indium Phosphide water showing distribution of various parameters ­ an indication of homogeneity. (c) measurement of time-resolved PL at three points on an LED die. (d) Spatially resolved electroluminescence of an LEDExample 2: PhotovoltaicsDevelopment and fabrication of Photovoltaics show a similar need for multimodal characterization. For example, at the material stage it is important that the material bandgap be engineered to optimally absorb the solar spectrum. Once again, PL is a good technique for determining that property (figure 1a). The photovoltaic effect relies on the efficient movement of charge carriers either to the electrical load for use or to a battery for energy storage. Time-resolved PL is often used to characterize carrier dynamics (figure 1c) or Raman spectra to determine micro crystallinity (which in turn affects carrier dynamics (2a). Finally, in the QA process of the solar cell device one might be interested in measuring the overall device efficiency by measuring the spectral photocurrent response (figure 2b). (A)(B)(D)(C)(A)(B)
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