edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 4 6 the tool supplier, or they must be characterized individually. A more convenient way is to see the SPEM system as a whole and determine a wavelength dependent system transformation factor Csys(λ). A thorough determination of Csys(λ) is the key enabler to increase the trustworthiness of SPEM results. THE REFERENCE LIGHT SOURCE A meticulous calibration of a SPEM setup requires a light source with a known spectrum. This light source is used to generate a reference spectrumthat canbe used for calibrating the SPEM systemwith a substitution method. The light source chosen must feature a continuous spectrum with spectral components in the infrared regime (for InGaAs detectors). A couple of commercially available light sources offer an enclosed spectral distribution. However,most likely, theseout of thebox solutionswill not fit into the SPEM measurement setup. Furthermore, as it was shown inReference 10, the aging of the reference light source influences the outcome of the calibration significantly. Thus, it is crucial to have a calibrated spectrometer available, which allows for regular checking of the emitted spectra of the reference light source. Amore convenient solution for a reference light source is offered by typical thermionic emission-based lamps. These lamps emit a continuous spectrum including light in the infrared (IR) region. For example, the broadband infrared tungsten bulb HEP3965 is a suitable candidate that offers a small form factor and an adjustable emission spectrum by controlling the power consumption. However, the characterization of the emitted spectrum under given boundary conditions needs to be performed. Finding a suitable and certified spectral calibration system can be tricky but often photovoltaic labs have these systems available. Finally, a pinhole is required to simulate a point shaped light source. Depending on the objectives that need to be calibrated, 1 to 10 µmpinholes had proven to be suitable. Having a point-shaped light source is essential for SPEM measurements as pointed out later. To be flexible enough during the SPEM system calibration and to not oversaturate the SPEM detector, it is suggested to measure the spectra of the reference light source for various power supplies. Voltages of 1.3 up to 3 V are a good reference. Last but not least, it is suggested that the lamp shouldbe initially operated for several hours to address the burn-in effect and for each measurement the lamp must be fully warmed to be stabilized. As a last step to finalize the reference light source setup, it is necessary to calculate a fitting function for the measured spectra. This is crucial as most likely the data points of the spectral measurement tool will not map to the ones from a typical SPEM set up. DISPERSION CHARACTERISTIC AND SPATIAL DISPLACEMENT When the SPEM measurement is performed with the helpof a prism, a point-shaped light source is transformed into a line area as shown in Fig. 3. The XY coordinate of a point-shaped emission is defined as the origin, and the resulting spectral coordinates are noted in relation to this origin. However, the assumption of a point-shaped emission is a theoretical construct. A spatially extended emission is of greater practical relevance. In this case, the coordinates of the maximum intensity of the emission must be noted and defined as the origin. It must be noted that the spatial extent of the photon emission is an irrevocable boundary condition of real SPEMmeasurements, which always leads to a deviation from the theoretical model and thus to inaccuracies in the spectrumextraction. To minimize the spatial extent of the photon emission, a lower magnification can be chosen if the associated Fig. 3 Schematic shows how a prism is used in a photon emission microscope to analyze an emission signal spectrally. A local XY point source information (left) is spread into a spectral λ-y information (right).[11]
RkJQdWJsaXNoZXIy MTMyMzg5NA==