The last decade has seen considerable interest in the use of germanium for high-speed low-power electronics. Despite the demonstration of high performance p-channel Ge transistors in planar and non-planar device technology, fabrication of n-channel Ge transistors faces a number of scientific and technological challenges which hinder the development of CMOS logic circuits based entirely on Ge.

Major challenge towards the establishment of Ge deep submicron n-channel MOSFETs technology is the control of the fast vacancy-mediated n-type dopant diffusion in Ge, which prevents the formation of shallow (x_j<20nm) and highly activated (active n-type dopant concentration ≥ 10²⁰cm^-3) n+/p junctions [1,2]. Additional issues in meeting this target are constituted by: (i) the significant simultaneous out-diffusion trend of n-type dopants and (ii) the Ge thermal etching during post-implantation annealing, which causes significant Ge substrate loss at temperatures above 500^oC, thus imposing the presence of a capping layer on Ge surface [3].

Up to now, a variety of approaches have been followed to suppress n-type dopant diffusion in Ge, the most representative being [1,2]: (a) co-implants with impurities such as carbon, fluorine or nitrogen (b) co-implants of two n-type dopants (e.g arsenic with antimony) (c) bulk interstitial injection by Ge self-implantation or GeOx clusters dissolution and (d) high temperature-short time non-conventional annealing schemes (Flash Lamp Annealing, Laser Annealing, Microwave Annealing).

More recently, the ability of surface capping layers to modify the near surface Ge vacancy concentration has been demonstrated [4]. This revealed “capping layer engineering” as an additional supporting technique to control n-type dopant diffusion in Ge.

During the last years, our work in the field has been devoted to the beneficial action of co-implanted nitrogen in suppressing low energy-implanted phosphorous in and out-diffusion in Ge [4-6]. The action of nitrogen is based on its individual anomalous diffusion [Figure(1)] and the simultaneous formation of complexes with phosphorous atoms [Figure(2)].

The aim of this work is to review the field comparing the above mentioned approaches, with particular emphasis on the action of nitrogen as phosphorous diffusion blocker. A series of experiments performed in low energy/high dose N₂/P co–implanted (100) Ge as function of the post-implantation annealing scheme and temperature, the surface capping layer stoichiometry and the level of simultaneous interstitial injection in the substrate will be presented and discussed. The experiments aspire to reveal the microscopic origin of nitrogen anomalous diffusion in Ge, the role of nitrogen in suppressing the damage caused in Ge substrate by non-conventional annealing schemes such as FLA [Figures (3)-(4)], as well as processing issues, limitations and strategies towards the formation of shallow and highly activated n⁺/p junctions in Ge based on N/P co-implants.

References:

[1] E. Simoen, M. Schaekers, J. Liu, J. Luo, C. Zhao, K. Barla and N. Collaert, Phys. Status Solidi A 213, No. 11, p. 2799–2808, (2016)

[2] C. Monmeyran, I.F. Crowe, R.M. Gwilliam, J. Michel, L.C. Kimerling, A.M. Agarwal, Internat. Mat. Rev., http://dx.doi.org/10.1080/09506608.2016.1261223, (2016)

[3] N.Ioannou, D.Skarlatos, C.Tsamis, C.A.Krontiras, S.N.Georga, A.Christofi, D.S. McPhail, Appl. Phys. Lett., 93, art.101910, (2008)

[4] D.Skarlatos, V.Ioannou-Sougleridis, M.Barozzi, G.Pepponi, D.Velessiotis, M.C. Skoulikidou, N. Z. Vouroutzis, K. Papagelis, P. Dimitrakis, C. Thomidis, B. Colombeau, ECS J. of Solid State Sci. and Technology., 6 (7), p.418, (2017)

[5] D.Skarlatos, M.Bersani, M.Barozzi, D.Giubertoni, N.Z.Vouroutzis and V.Ioannou-Sougleridis, ECS J. of Solid State Sci. and Technology., 1 (6), P315, (2012)

[6] C. Thomidis, M. Barozzi, M. Bersani, V. Ioannou-Sougleridis, N. Z. Vouroutsis, B. Colombeau, and D.Skarlatos, ECS Solid State Lett. 4, p.47, (2015).