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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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -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.
Highlights
O PERATION of silicon photodiodes at cryogenic temperatures is routinely exploited as a means of improving performance of detectors, such as charge-coupled devices (CCDs) [1], [2], single-photon avalanche diodes (SPADs) [3], and drift detectors [4], [5]
The two pure boron (PureB) diodes, PB6-guard rings (GRs) and PD20-GR, as well as the diffused diode PD0-GR, show practically the same I –V characteristics, with a significant difference only appearing in the series resistance at low temperatures
The temperature-dependent I –V characteristics of 700 ◦C PureB Si diodes, measured down to cryogenic temperatures, were found to be practically identical to that of diffused Si p+-n junctions with about the same apparent Gummel number of the p+-region, corresponding to an integral doping ∼5 × 1014 cm−2
Summary
O PERATION of silicon photodiodes at cryogenic temperatures is routinely exploited as a means of improving performance of detectors, such as charge-coupled devices (CCDs) [1], [2], single-photon avalanche diodes (SPADs) [3], and drift detectors [4], [5]. For SPADs, the dark count rate (DCR) is reduced, while the photon detection probability is increased, which has received much attention for potential use in high-sensitivity imagers and quantum computation [6], [7] The latter application has encouraged increased research into the operation of semiconductor devices at low temperatures. Devices incorporating these B-layers have been commercialized as PureB detectors for low-energy electrons [11], [12] and vacuum ultraviolet/near-ultraviolet (VUV/NUV) light [10], [13] The attraction for these applications lies in the extremely shallow junction depth and low dark currents, combined with exceptional optoelectronic stability and chemical robustness [14]–[16].
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