Towards Ultra-Low Specific Contact Resistance on P-Type and N-Type Narrow Bandgap GeSn Semiconductors
Authors: Abdi Salim , Atalla Mahmoud , Assali Simone , Kumar Aashish , Groell Léonor , Koelling Sebastian , Moutanabbir Oussama 
Published: May 2020

GeSn alloys are group IV semiconductors that have attracted remarkable interest owing to their ability of strain and bandgap engineering by controlling the Sn content, their compatibility with the Si CMOS platform and their tunable and direct band gap. These material properties make GeSn a promising candidate for many electronic and optoelectronic applications including among others tunnel field effect transistors, infrared (IR) photodectors, and IR emitters. However, in order to produce high quality GeSn devices to enable these applications, it is of paramount importance to develop ohmic metal contacts with very low specific contact resistivity on both n-type and p-type doped GeSn layers used in these devices. This could be achieved by realizing high doping levels and attaining low intrinsic barrier heights between the metal and the contacted GeSn layer.
The growth of metastable GeSn semiconductors is typically performed at temperatures well below 400 ºC to avoid Sn segregation and phase separation, which would compromise the opto-electronic properties of the material. [1] Therefore, major care is required when developing post-growth processes that are commonly required for device fabrication. For instance, ohmic contact formation using the conventional process of rapid thermal annealing of nickel contacts cannot be made and the GeSn cannot be cured after dopant implantation because of the very limited thermal budget. We investigated both the epitaxial growth of highly doped layers, as well as the passivation of undoped GeSn samples in order to circumvent these limitations and achieve the desired barrier heights for both p-type and n-type GeSn while varying the Sn content.
First, epitaxially grown GeSn layers with p- and n-type doping are demonstrated. For GeSn doping, the CVD growth of in-situ doped GeSn layers is done using B2H6 for p-type, and AsH3 for n-type. We show high active doping levels in the orders of 1019 and 1020 cm-3 for both p-GeSn and n-GeSn, respectively. These results were obtained from the capacitance voltage measurements (CV) using metal oxide semiconductor (MOS) back-to-back devices. In addition, secondary ion mass spectroscopy (SIMS) and atom probe tomography (APT) data support the high and uniform doping levels.
Secondly, since the strong Fermi level pinning (FLP) of Ge is a major problem in the development of n-type contacts due to the loss of metal work function modulation of the barrier height [2], it is expected that the GeSn materials system with low Sn levels exhibits the same behavior, thereby degrading the properties of direct metal/n-GeSn contacts. Therefore, we develop processes aiming to release the Fermi level pinning at metal/GeSn interface, thus restoring the metal work function control of the barrier height. Samples used in this study are CVD grown intrinsic GeSn relaxed samples with increasing Sn concentrations up to 11 at. %. These samples are unintentionally p-type with defect doping levels in the order of 1017 cm-3 as obtained by CV measurements on MOS devices. Our process starts by chemical passivation of these samples, then the deposition and patterning of four metals with different work functions close to the band gap is realized to obtain transfer length method (TLM) structures used for current-voltage (IV) measurements. We then extract the contact properties of these devices and estimate the change of the barrier height induced by the chemical passivation due to the depinning of the FL. This demonstrates the potential use of our process in the fabrication of future metal/n-GeSn contacts.

Acknowledgements

O.M. acknowledges support from NSERC Canada (Discovery, SPG, and CRD Grants), Canada Research Chairs, Canada Foundation for Innovation, Mitacs, PRIMA Québec, and Defence Canada (Innovation for Defence Excellence and Security, IDEaS).

References

[1] S. Assali et al., Enhanced Sn incorporation in GeSn epitaxial layers via strain relaxation, Journal of Applied Physics Vol. 125, 025304 (2019).
[2] T. Nishimura, K. Kita and A. Toriumi “Evidence for strong Fermi-level pinning due to metal-induced gap states at metal/germanium interface”, Appl. Phys. Lett. 91, 123123 (2007).