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

Abstract

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.

Checks on temperature during on-wafer I-V characterization of Si diodes made with 2-D interfacial layers

“…The junction termination at the oxide perimeter can be a source of g-r leakage current that generally has n 2 and thus becomes more pronounced at low temperatures. In addition, weak spots in the B-layer can allow the Al-metallization to locally approach/contact the Si, forming nano-regions with Schottky-like high currents [8,9]. The latter effect also results in an increasing n value as the temperature decreases because the parasitic Schottky currents decrease less rapidly with temperature.…”

“…Both the BJT and 2-diode measurements are attenuated by the series resistance through the ~ 4-nm-thick bulk B-layer. This series resistance increases with decreasing temperature [9], leading to a decreased voltage window with the desired ideal I-V characteristics.…”

“…3b, becomes very high at this temperature because the temperature-dependent decrease of the electron current is lower than that of the hole current. The hole current is completely governed by the bandgap of the Si substrate, while the electron current is influenced by the much lower bandgap of the Al-metallization [9]. In Fig.…”

“…One of the studied diodes fabricated with a 5-min B-deposition is a good example of a low-saturation-current PureB diode, having an approximately 4-nm-thick B-layer. Another B-layer diode, has, after 1 s B-deposition time, only a fraction of a monolayer B coverage which results in about 10 times higher saturation current due to the proximity of the Al metal layer to the Si surface [8,9]. In addition, a 30 s P-deposition was applied, which is known to form a self-limiting monolayer on the Si surface [10] as well as a slight n-doping of the Si surface.…”

Identifying nano-Schottky diode currents in silicon diodes with 2D interfacial layers

“…Otherwise, the high excess currents were about a decade or so higher than the ideal p + n currents and the kinked behavior was evident. We believe that it is caused by high-injection effects at the nano-Schottky junction [12,14].…”