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

Abstract

A little more than a monolayer-thick pureboron (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 5 × 10 13 cm −2 to 5 × 10 14 cm −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.

Nanometer-thin pure boron CVD layers as material barrier to Au or Cu metallization of Si

“…The two largest diodes have more than a decade higher current and display a kink at about 0.45 V after which the current reduces to PureB-like levels. Such a kink was previously observed for Almetallization of large area PureB diodes with thin B-layers and was related to a few pinhole-sized defects where the metal could get close to or even contact the Si [21]. For the defected Au-coated sample, it is clear that the Au has found structurally weak spots or pinholes in the B-layer through which it has come into contact with the Si, etched visible pits, and, under the influence of the dissolved Si, migrated large distances along the surface of the otherwise closed B-layer.…”

“…The lower the J e , the lower the dark current. The relationship between J e and the concentration of fixed negative charge, N I , at the B-to-Si interface, was studied experimentally and via simulations in [21]. It was concluded that for a B-deposition temperature of 450°C, there would be no actual B-doping of the bulk Si, but the experimental PureB diodes without metal contacting had an electron saturation current density, J se , at room temperature (RT) of about 20 pA/cm 2 to 30 pA/cm 2 .…”

“…As an example, to illustrate the expected current trends, we applied the same simulation parameters for bulk B and Si as used in [21] to make a 1-dimensional simulation of the PureB diode characteristics when contacted by metals with different work functions. In Fig.…”

“…In this paper, it was also demonstrated that in simulations of experimental results, the effect of the interface and surrounding (charged) layers can often be reproduced by assuming the diode to have an appropriate SBH and surface recombination velocity. Alternatively, for PureB diodes, the model defining an interfacial fixed negative charge layer has been successful in describing a vast amount of experimental data [21,26]. The bulk B-parameters of these very thin B-layers are not well known, but because the layers are so thin, the exact B-parameters may only have a small impact on the diode I-V behavior as compared to the external influence of parameters like N I and / M .…”

Towards electrostatic doping approaches in ultra-thin body semiconductor materials and devices

“…Alternatively, an inversion layer at the interface can also be formed by employing a non-insulating layer such as pure boron (PureB). In these "PureB" diodes, the potential responsible for holding the holes at the interface, has been proposed to originate from a high concentration of fixed negative charge created by the B-to-Si bonds [171]. Due to the overall attractive characteristics of this type of junction, it is now widely applied in high performance photodiodes [172].…”