Boron Delta-Doping in Si : B Stacks By RP-CVD Epitaxy
Authors: Ferrandon Alice , Deprat Fabien , Vives Jeremy , Frauenrath Marvin , Guyader Veronique , Lacomme Caroline , Madaci Ismail , Favre Luc , Berbezier Isabelle 
Published: December 2024

To reach the electrical performance of advanced CMOS technology, the source and drain contact resistance becomes a key figure of merit. By reducing the size of electrical devices, the minimum active carrier concentration required to obtain sufficiently low contact resistance and high-performance increases. However, incorporation of dopants by Reduced Pressure-Chemical Vapor Deposition (RP-CVD) is limited to <1020at/cm3 (substitutional atoms). For high B2H6 flow ratio (F[B2H6]/F[H2] of 7.1x10-06 at 750 °C, 10 torr), the material either forms defects, notably Boron clusters [1], or even turns polycrystalline which increase the resistivity [2]. To incorporate as many electrically active dopants as possible while maintaining a good crystalline quality, stacks of Boron saturated layers δ-(B) in between each Si:B thin film, known in the literature as delta-doping, are investigated to further reduce resistivity without degrading the crystalline quality [3].
In this presentation we show that the delta-doping method with the use of a stacked δ-(B)/Si:B structure, provides a factor two decrease of the resistivity limit and a ten times increase of electrically active B dopants as compared to a standard co-flow doping process. Such results are obtained for basic set of process parameters, with temperature and pressure set at 750 °C and 10 torr, respectively.
The influence of the number of δ-(B) layers and of the saturation times (per δ-(B) layer) were investigated from 2 to 23 and from 20 s to 600 s respectively. The resistivity reduces when increasing the saturation time and the number of saturation steps up to a plateau at saturation times around 400 s, whatever the number of δ-(B) layers (Fig. 1). In standard growth conditions (DCS flux, Pressure and Temperature), and at low B2H6 flow ratio ( a record resistivity of 8.9x10-04 Ω.cm was obtained which is a decade lower than the 9x10-03 Ω.cm obtained with the standard co-flow process. Secondary Ion Mass Spectrometry (SIMS) analyses and High-Resolution Transmission Electron Microscopy (HRTEM) images were performed on different stacks (see Fig. 2a and 2b). SIMS profiles showed that the total [B] was about 2x1021 at.cm-3 compared to electrically active [B] of 1.5x1020 at.cm-3, meaning that only 10% of incorporated Boron was electrically active.
Strain mapping analyses were performed on a stack with five δ-(B) layers. Despite the good crystalline quality and a low resistivity, the local strain mapping image revealed a dislocation associated to the high strain in the δ-(B) layers. In addition, the four hours required to process the epitaxial stack with the best resistivity result is prohibitive for industrial application.
To achieve lower contact resistance, high B2H6 flow ratios (>1x10-06) were investigated. The saturation time and number of δ-(B) layers were kept constant at 5s and 20, respectively, to obtain similar results but with shorter, more industry orientated process times (~ 5 min). The resistivity and the total [B] did not reduce further compared to previous results (at lower B2H6/H2 flow ratio). However, the SIMS and the TEM revealed a more homogeneous boron distribution over the entire depth of the layer (see Fig.2c).
The efficiency of the process, in terms of crystalline quality, was assessed for low and high B2H6/H2 flow ratios typically and >1x10-06 respectively using the soaking volume parameter (Vsoaking). It is defined as the product of B2H6 flow (in sccm) and flow time (in s) which gives the total volume of B2H6 introduced into the epitaxy chamber during a saturation step. Vsoaking initially evolves linearly up to 8x10-08 m-3 for low B2H6 flow and 2.5x10-08 m-3 for high B2H6 flow and then it saturates. At large Vsoaking, the crystalline quality degraded probably due to the strain in the δ-(B) which perturbates the crystal lattice. The results show that for the same amount of [B], the lower Vsoaking (1x10-7 m3), corresponding to the higher B2H6 flow process, lead to a higher crystalline quality.
In conclusion, in standard experimental conditions, the delta doping process leads to a decade lower resistivity while preserving excellent crystalline quality when using high B2H6 flow and low exposure times.
[1] Cristiano et al. / Appl. Phys. Lett., Vol. 83, No. 26, 29 December 2003
[2] J.M. Hartmann et al. / Journal of Crystal Growth 264 (2004) 36-47
[3] D. Kohen et al. / Journal of Crystal Growth 483 (2018) 285-290

Figure 1