Ultra-High Boron Doping of Si and Ge for Nanoelectronics and Photonics
Authors: Hartmann Jean-Michel , Frauenrath Marvin , Richy Jérôme , Veillerot Marc 
Published: December 2020

In-situ boron doping of Si and Ge can be of use in various types of devices. Poly-Si:B gates can be integrated in p-type metal oxide semiconductor (pMOS) transistors. Monocrystalline Si:B layers can be used in p-i-n junctions for RF switches, attenuators, photo-detectors (PDs) or phase shifters. Ge:B layers can be integrated in the sources and drains of Ge pMOS transistors or as the p-type films in Ge p-i-n PDs. If the growth temperature is low enough, they can even be used in GeSn PDs or light emitting devices. We have explored here the boron-doping of Si and Ge with Si2H6 + B2H6 and Ge2H6 + B2H6. Such chemistries enable Si (Ge) epitaxy at temperatures lower than with conventional SiH4 (GeH4) precursors. Growth temperatures and pressures in our 200mm RP-CVD tool were 550°C, 20 Torr (Si) and 350°C, 100 Torr (Ge).

X-Ray Diffraction (XRD) was used to convert the tensile strain in Si:B or Ge:B into substitutional B concentrations, as shown in Fig. 1 and Fig.
2, with layer peaks moving away from the Si(001) substrate or the Ge Strain-Relaxed Buffers (SRB) as the diborane flow increased. The presence of marked thickness fringes testified to the high crystalline quality of the films. Secondary Ions Mass Spectrometry (SIMS) gave us the atomic B concentration. X-Ray Reflectivity on Si0.5Ge0.5 / Si:B stacks yielded Si:B Growth Rates (GR). Finally, differential weighting gave us the thickness of Ge:B layers grown on Ge SRBs and other GR values.

A huge Si:B GR increase, from 9.5 up to 40 nm min.-1, was evidenced when adding relatively small amounts of B2H6 to Si2H6, as shown in Fig. 3. This was due to the presence of B atoms on the surface which catalyzed H desorption, freeing sites for growth. Ultra-high substitutional and atomic boron contents were obtained in those Si:B layers (at most: 2.7x1020 and 1.1x1021 atoms cm-3), as shown in Fig. 4. Films became poly-cristalline for the highest F(B2H6)/F(Si2H6) Mass-Flow Ratios (MFRs) probed, without XRD peaks anymore and a boron ion concentration which was reduced compared to its peak value of 2x1020 cm-3. The good agreement between substitutional, atomic and ionic B concentrations for low to medium MFRs is to be underlined. By contrast, the growing discrepancy between XRD and SIMS for the highest diborane flows probed might partly be due to a SIMS artefact. Indeed, the Relative Sensistivity Factor used to convert the raw SIMS signals into B concentrations was for doped layers and not higher boron content binary alloys. The presence of B atoms in nanoclusters that did not contribute to XRD cannot be excluded, however.

A definite growth rate increase (from 6 up to 21 nm min.-1) and really high substitutional B concentrations (at most: 4.8x1020 atoms cm-3) were also obtained for Ge:B, as shown in Fig. 5 and Fig.
6. The good agreement between XRD and SIMS for medium F(B2H6)/F(Ge2H6) MFR is to be underlined. The growing discrepancy for high diborane flows might be due to the same SIMS artifact as before and to the presence of B atoms in nanoclusters.

x/(1-x) = m*F(B2H6)/F(Si2H6 or Ge2H6) relationships accounted for the semi-linear increase of the substitutional or atomic B concentration x with the diborane flow, with (i) msubst.
= 1.85 (matom.
= 7.45) in Si:B and (ii) msubst.
= 0.15 (matom.
= 0.68) in Ge:B. m values above 1 meant that B incorporation was easier than Si incorporation in Si:B. In contrast, B incorporation was more difficult than Ge incorporation in Ge:B alloys, as m was less than 1, then.

Current growth rate increases and B concentrations are definitely higher than with mainstream chemistries. For Si:B grown at 650°C, 20 Torr with SiH4 + B2H6, we had for instance GR increases from 9 up to 23 nm min.-1, with at most 9x1019 cm-3 B ions concentrations before layers became poly-crystalline (ECS Trans. 16 (10), 485 (2008)). For Ge:B grown at 400°C, 100 Torr with SiH4 + B2H6, GR increased from 10 up to 18 nm min.-1, with at most 1020 cm-3 B ions concentrations (Thin Solid Films 557, 4 (2014)). The advantages of switching over to higher order Si and Ge precursors and adopting lower growth temperatures are obvious.

Figure 1