In:
ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2016-02, No. 30 ( 2016-09-01), p. 2031-2031
Abstract:
Ge has been expected as a high-performance scaled-CMOS platform due to its high carrier mobility. As for n MISFET 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 (ρ c ) [2]. In order to reduce parasitic resistances of the Ge- n MISFETs caused by the drawbacks described above, a carrier concentration of around 1x10 20 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 ρ c . Moreover, drive-current of GeOI- n MISFETs 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 SiH 4 , GeH 4 (10% in H 2 ), PH 3 (10% in He) and H 2 as a carrier gas to investigate the dopant activation rates. Growth temperature and mass-flow (F(gas)) ratio of GeH 4 to H 2 were fixed at 400 o C and F(GeH 4 )/F(H 2 ) = 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 ρ c 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 o C, 1 min. in N 2 ). GeOI- n MISFETs 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(PH 3 )/F(GeH 4 ) 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 7x10 19 cm -3 was observed. This value far exceeds the solid solubility of P in Ge of around 1x10 19 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 10 20 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 7x10 19 cm -3 . In contrast, Schottky-like contacts are observed for the P-implanted Ge having a carrier concentration of 2x10 19 cm -3 and the P-doped epitaxial Ge layer having a carrier concentration of 2x10 19 cm -3 . The ρ c extrapolated from TLM are 1.2x10 -6 Ωcm 2 for the epitaxial layer having carrier concentration of 7 x 10 19 cm -3 and 1.6 x 10 -5 Ωcm 2 for P-implanted one. Sheet resistances (R sh ) of the n + -Ge layers are plotted as a function of layer thickness as shown in Fig. 4. A low R sh 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 R sh 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- n MISFETs 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 4x10 17 cm -3 and TaN/Al 2 O 3 gate-stack was deposited after O 3 passivation [4]. An XTEM image of the GeOI- n MISFET is shown in Fig.6. Gate-length, L g , and GeOI thickness are 60 nm and 37 nm, respectively. I d -V d curves are shown in Fig. 7. I d of 300 μA/mm was obtained at V d =1V, V g -V th =1.5V. This is almost 2.7 times larger than the largest I d of Ge- n MISFETs ever reported [4, 5]. These results suggest that current drive of Ge- n MISFETs can be enhanced by reduction of parasitic resistances such as ρ c and R sh 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., JAP 104 , p.023523 (2008) [2] T. Nishimura., APEX 1 , 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
Type of Medium:
Online Resource
ISSN:
2151-2043
DOI:
10.1149/MA2016-02/30/2031
Language:
Unknown
Publisher:
The Electrochemical Society
Publication Date:
2016
detail.hit.zdb_id:
2438749-6
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