Improved Induced Diode Photodetectors by Increased Fixed Charge in PECVD Amorphous Silicon Nitride
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The predictable quantum efficient detector (PQED) is a cheaper, more practical alternative to the current radiometric primary standard, the cryogenic radiometer. The PQED is made of an induced diode, a rectifying junction based on a positively charged dielectric film which induces an n-type inversion layer on a p-type silicon substrate. Increasing the fixed charge Qf in the dielectric has been theoretically predicted to improve the quantum efficiency of the diode by decreasing the surface recombination velocity (SRV) at the dielectric-silicon interface, as well as improving the performance of the diode at high intensities. In this work, we purpose the replacement of silicon oxide (SiOx) with silicon nitride (SiNx) as the inversion-inducing dielectric layer as a means of increasing Qf.Amorphous PECVD SiNx has been shown to have a significantly higher Qf than thermally grown SiOx. A sixfold increase of Qf in SiNx by means of charge injection via bias soaking has also been demonstrated. The dielectric ideality of the nitride increases with the atomic nitrogen concentration x, and a high-x film has shown superior charge retention properties. The absorption in the high-x film is far less than 1% in the applicable wavelength ranges, but is larger in the low-x film. The impact of Qf on the SRV has been experimentally demonstrated, and may be fit to the extended SRH theory with the inclusion of a sub-surface damage term. Finally, it has been demonstrated that the aluminium contacts deposited for the purpose of charging the SiNx may be chemically removed while the injected charge remains. SiNx has thus been shown to be an interesting candidate for the PQED application.