Low-Temperature Electrical Performance of PureB Photodiodes Revealing Al-Metallization-Related Degradation of Dark Currents

摘要

Pure boron (PureB) deposition as the anode region of Si photodiodes creates negative fixed charge at the boron/silicon interface, which is responsible for effective suppression of electron injection from the bulk, thus ensuring low saturation/dark current densities. This mechanism is shown here to remain effective when PureB diodes, fabricated at 700 degrees C, are operated at cryogenic temperatures down to 100 K. Although the PureB junctions were only a few nanometers deep, they displayed the same current-voltage (I-V) characteristics as conventional deep diffused p(+)-n junction diodes in the whole temperature range and also maintained ideality factors close to n = 1. Al-contacting was found to reveal process-related defects in the form of anomalous high current regions giving kinks in the I-V characteristics, often only visible at low temperatures. They were identified as minute Al-Si Schottky junctions with an effective barrier height of similar to 0.65 +/- 0.05 eV. In PureB single-photon avalanche diodes (SPADs), Al-Si perimeter defects appeared but did not affect the breakdown voltage characteristics set by implicit guard rings. Low series resistance required thin B-layers that promoted tunneling. In particular, for such thin layers, avoiding Al-related degradation puts stringent requirements on wafer cleaning and window etch procedures.