Non-linear behavior of Al-contacted pure amorphous boron (PureB) devices at low temperatures

Deposition of pure amorphous boron (PureB) layers on n-type Si results in p+n-like devices even in cases where B in-diffusion during the deposition is not expected. It is suspected that such behavior is due to the formation of an interfacial hole layer (IHL) between the PureB and Si. To further investigate physical mechanisms governing conduction of holes across the PureB/Si interface and through the IHL, electrical measurements were performed from room temperature down to cryogenic temperatures as low as 100 K. In this paper, current-voltage (I-V) measurements are made on structures where the PureB connects to p-type Si regions. One set of devices comprises ring-shaped structures designed for measuring the conductance through the IHL. In these structures, the PureB layer is deposited in rings that are contacted at the inner and outer perimeter with Al. Another set of samples includes devices where the PureB layer was deposited on p-type bulk Si. At room temperature, a close to linear change of current with voltage was seen irrespective of the PureB layer thickness and post-deposition processing. Lowering the operating temperature led to an increasingly non-linear I-V characteristics. Plausible explanations for the non-linear behavior are considered and discussed in the paper

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

In silicon technology, Schottky diodes mainly exhibit high current levels, and attempts are regularly made to reduce these by introducing 2D layers between the metal contact and the silicon. Defects in such interfacial layers, from weakly bonded structures to actual pinholes, can lead to high, localized metal-semiconductor Schottky currents. Using the example of diodes with an interfacial layer of pure boron (PureB) between an aluminum metallization layer and the Si, a signature for such ''nano-Schottky's'' is determined by evaluating the results of several different test-structure arrays and measurement techniques. An adapted bipolar-Type measurement is introduced as an additional method to determine whether any high current characteristics originate from a low Schottky barrier height over the entire diode surface or from a localized nano-Schottky structure

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

Limits on Thinning of Boron Layers With/Without Metal Contacting in PureB Si (Photo)Diodes

A little more than a monolayer-thick pure-boron (PureB) layer was deposited on silicon at 250 °C by chemical vapor deposition (CVD), forming junctions with low saturation current. They displayed the same efficient suppression of electron injection as PureB diodes fabricated with a few nm-thick PureB layer deposited at 400 °C. Assuming high concentrations of acceptor states at the B-to-Si interface, induced by a fixed negative charge in the range from \textsf {5}\times \textsf {10}^{\textbf {13}} cm ^{-\textbf {2}} to \textsf {5}\times \textsf {10}^{\textbf {14}} cm ^{-\textbf {2}} , would be consistent with the experiments and device simulations that exhibit an efficient suppression of electron injection. Metallization of the B-layers was studied, showing that in many situations, thinning of the layer to monolayer thickness will lead to a significant increase in the electron injection

Investigation of light-emission and avalanche-current mechanisms in PureB SPAD devices

The light emission from silicon PureB photodiodes was investigated in both forward- and avalanchemode operation and correlated to the presence of process-dependent defects that influence the reverse IV characteristics. As opposed to “defect-free” diodes with low dark currents and abrupt breakdown behavior, the diodes with defects had higher current levels and light-emitting spots appearing at voltages far below the breakdown voltage otherwise set by the implemented doping profiles. The role of such defect-related behavior for the application of the photodiodes as single-photon avalanche diodes (SPADs) and avalanche-mode light-emitting diodes (AMLEDs) is assessed in connection with the recent demonstration of these basic devices as both the light-emitting and light-detecting elements in optocoupler circuits integrated in CMOS for data transmission purposes