Temperature dependence of chemical-vapor deposition of pure boron layers from diborane

Abstract

Surface reaction mechanisms are investigated to determine the activation energies of pure boron (PureB) layer deposition at temperatures from 350 C to 850 C when using chemical-vapor deposition from diborane in a commercial Si/SiGe epitaxial reactor with either hydrogen or nitrogen as carrier gas. Three distinguishable regions are identified to be related to the dominance of specific chemical reaction mechanisms. Activation energies in H 2 are found to be 28 kcal/mol below 400 C and 6.5 kcal/mol from 400 C to 700 C. In N 2 , the value decreases to 2.1 kcal/mol for all temperatures below 700 C. The rate of hydrogen desorption is decisive for this behavior. Pure boron (PureB) layer depositions have in recent years been applied for creating the p þ -region of extremely shallow, less than 10-nm deep, silicon p þ n junction diodes for a number of leading-edge device applications. 1 Particularly impressive performance has been achieved for the application to bulk-Si photodiodes for detecting low penetration-depth beams. [2][3][4][5] Ideal diode characteristics have been achieved for deposition temperatures in the 400 C-700 C range. The option of depositing at temperatures below $500 C, which together with the fact that the deposition is conformal and highly selective to Si, makes PureB technology highly compatible with amorphous-/polysilicon-/ crystalline-silicon thin-film device processing. Moreover, these properties also make it an attractive process for creating junctions on silicon nanowires and advanced CMOS (complementary metal-oxide-semiconductor) transistors including source/drain in p-type FinFETs. 6,7 These applications require a sub-3-nm thick layer to avoid excess series resistance through the high-resistivity PureB layer.In the present work, the deposition is performed in a commercial Si/SiGe epitaxial reactor by exposing the Si surface to diborane (B 2 H 6 ). At 700 C, in the first few seconds of exposure, the boron atoms interact with the Si surface sites to quickly build up something like an atomic layer plane, and upon further deposition, the boron coverage readily exceeds one monolayer (1 ML). After this, the boron atoms will be deposited on a full PureB surface, which is a process that has a much slower, well-controlled deposition rate. In the past, it has been shown that less than 2-nm-thick layers can be deposited with good reliability and uniformity by a suitable adjustment of deposition parameters such as deposition time, temperature, partial pressures, and flow rates. 8 In this paper, an investigation is presented of the surface reaction mechanisms and activation energies of PureB layer deposition in the temperature range of 350 C to 850 C. At the lower temperatures, the carrier gas has a large influence on the ability to create the first full boron coverage of the Si. Nevertheless, by first creating a full PureB coverage at 700 C, which is smooth and uniform, and then proceeding with the low-temperature depositions, the boron-on-boron activation energies could be determined over the whole temp...

A Kinetic Model for Chemical-Vapor Deposition of Pure-Boron Layers from Diborane

“…The activation energies for reactions on H-terminated Si/PureB surface sites are larger than H-free sites (12). Thus these fluxes can be neglected as compared to those on H-free Si/PureB surface sites.…”

Low‐Temperature PureB CVD Technology for CMOS Compatible Photodetectors

“…In the high-temperature deposition, the temperaturerelated reactions [R1]-[R4] proceed, leading to the adsorption of boron atoms, which are deposited and/or suspended, as well as migration of the boron atoms along the surface. This leads to a smooth, uniform, closed boron layer [17]. Triggering the intermediate reactions also ensures very smooth layers with a roughness that can be around 2 angstroms, as measured by HRTEM imaging and atomic-force microscopy (AFM).…”

“…This pre-deposition prediction tool can be used to improve the set-up and control of the final deposition [17,18]. It is also very useful for transferring recipes from one reactor to the other.…”

“…The kinetics of boron CVD is investigated in references [17] and [18]. The lateral gas-phase diffusion length of boron atoms over silicon/boron surfaces during the CVD of pure boron layers is investigated in reference [20].…”

An analytical kinetic model for chemical-vapor deposition of pureB layers from diborane

“…31 If the density of the deposited PureB-layer is q B and the mass of the pure-boron atoms in the layer is M, the number of boron atoms in a unit volume of the PureB-layer is…”