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

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 °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 ~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.