(Invited) In-Situ Doped Epitaxial Growth of Highly Dopant-Activated n
+-Ge Layers for Reduction of Parasitic Resistance of Ge-nMISFETs
Authors: Moriyama Yoshihiko , Kamimuta Yuuichi , Ikeda Keiji , Sakai Akira , Tezuka Tsutomu 
Published: February 2020

Ge has been expected as a high-performance scaled-CMOS platform due to its high carrier mobility. As for nMISFET application, fabrication of shallow n
+/p junction having high electron concentration by ion-implantation method is problematic due to high diffusivity and low solubility limit of n-type dopants in Ge [1]. For a metal / n
+-Ge interface, moreover, Fermi-level is pinned to near the valence band edge, resulting in high specific contact resistance (ρc) [2]. In order to reduce parasitic resistances of the Ge-nMISFETs caused by the drawbacks described above, a carrier concentration of around 1x1020 cm-3 is needed in the n
+-S/D region. In this report, a shallow n
+-Ge layer was grown with in-situ P doped epitaxy using conventional low-pressure (LP) CVD for reduction of ρc. Moreover, drive-current of GeOI-nMISFETs was enlarged by utilizing the n
+-Ge:P layer to the S/D region.
P-doped n
+-Ge (Ge:P) layers were grown on p-Ge substrates with LP-CVD using SiH4, GeH4 (10% in H2), PH3 (10% in He) and H2 as a carrier gas to investigate the dopant activation rates. Growth temperature and mass-flow (F(gas)) ratio of GeH4 to H2 were fixed at 400 oC and F(GeH4)/F(H2) = 1.67 x 10-3, respectively. Growth pressure was varied from 5 to 80 Torr. Ti layers were deposited on the Ge:P layers to measure the ρc of Ti / Ge:P contact by a transmission line measurement (TLM). As a reference, Ti / Ge:P TLM patterns were also fabricated on a P ion-implanted (10 keV, 1x15cm-3) Ge layer on a p-Ge substrate after an activation anneal (600 oC, 1 min. in N2). GeOI-nMISFETs having the Ge:P S/D were fabricated to evaluate transistor characteristics.
Dopant and activated carrier concentration profiles of Ge:P samples as a parameter of total growth pressure are shown in Fig.1 (a) and (b), respectively. Here, F(PH3)/F(GeH4) is fixed to 3.0 x 10-3. Both dopant and carrier concentrations exhibit almost uniform profiles throughout the Ge:P layers. From Fig.1 (a) and (b), dopant and carrier concentration were plotted as a function of growth pressure as shown in Fig.2. This result exhibits a peak dopant activation rate of 0.7 at a growth pressure of 15 Torr. Here, an electron concentration of 7x1019 cm-3　was observed. This value far exceeds the solid solubility of P in Ge of around 1x1019 cm-3 at the growth temperature. Next, I/V curves observed in the TLM for the Ti / Ge:P and the Ti / P-implanted Ge layers having the same dopant concentrations of 1 x 1020 cm-3 are shown in Fig. 3. This result indicates that ohmic contacts are formed on the P-doped epitaxial Ge layer with a carrier concentration of 7x1019 cm-3. In contrast, Schottky-like contacts are observed for the P-implanted Ge having a carrier concentration of 2x1019 cm-3 and the P-doped epitaxial Ge layer having a carrier concentration of 2x1019 cm-3. The ρc extrapolated from TLM are 1.2x10-6 Ωcm2 for the epitaxial layer having carrier concentration of 7 x 1019 cm-3 and 1.6 x 10-5 Ωcm2 for P-implanted one. Sheet resistances (Rsh) of the n
+-Ge layers are plotted as a function of layer thickness as shown in Fig. 4. A low Rsh of 33 (Ω/sqr.) was shown for the 65-nm-thick P-doped Ge layer due to the high carrier concentration and the high dopant activation rate. The value of Rsh for the epi-layer agrees with the theoretically predicted value and is the lowest among the values ever reported for the Ti / Ge:P layer [3]. Finally, GeOI-nMISFETs with the Ti / Ge:P contacts were fabricated through gate-last process flow shown in Fig. 5. Here, electron concentration of the GeOI layer was 4x1017cm-3 and TaN/Al2O3 gate-stack was deposited after O3 passivation [4]. An XTEM image of the GeOI-nMISFET is shown in Fig.6. Gate-length, Lg, and GeOI thickness are 60 nm and 37 nm, respectively. Id-Vd curves are shown in Fig. 7. Id of 300 μA/mm was obtained at Vd=1V, Vg-Vth=1.5V. This is almost 2.7 times larger than the largest Id of Ge-nMISFETs ever reported [4, 5]. These results suggest that current drive of Ge-nMISFETs can be enhanced by reduction of parasitic resistances such as ρc and Rsh by utilizing the in-situ doped Ge:P layers to the S/D.
This work was granted by JSPS through FIRST Program initiated by the CSTP.

[1] M. Koike et al., JAP104, p.023523 (2008) [2] T. Nishimura., APEX1, p.051406 (2008) [3] B. Yang et al., Proc. of ISTDM, p.88 (2012) [4] Y. Kamimuta et al., Proc. of ISDRS, FP8-04 (2013) [5] C.T. Chung et al., Tech. Dig. IEDM, p.383 (2012)

Figure 1