Si spontaneous emission during RTP and its impact on low-temperature pyrometry

Si fluorescence or spontaneous emission was discovered during the development of lower-temperature pyrometer. To reveal unambiguously the Si spontaneous emission, a high-power 980nm laser is used together with a high sensitivity IR spectrometer. Clear Si fluorescence spectra with peaks at ∼1140nm were obtained at different Si temperatures. The Si fluorescence peaks shift to longer wavelength, in agreement with Si bandgap narrowing with increasing temperatures. Wafers of different doping levels and types were studied for Si spontaneous emission. It is found that lightly doped (resisitivity ≪20 ohms-cm) Si has the highest level of Si spontaneous emission. On the other hand, heavily doped Si does not generate any Si spontaneous emission, mainly due to the higher recombination. Since the Si spontaneous emission has a broad spectrum, it spills into the RTP pyrometer spectral bandwidth and acts as s spurious pyrometer signal. Even though Si has very low efficiency for light emission due to its indirect bandgap, the fluorescence emitted light is still on the level of pyrometer signal equivalent to ∼200 to 250C.