(Invited) Germanium-Tin Based Nano-Electronic and Photonic Devices
Authors: Gong Xiao , Lei Dian , Wu Ying , Xu Shengqiang 
Published: December 2020

1. Introduction

Germanium-Tin (Ge1-x
Sn
x
) alloy has many attractive properties that make it promising for many applications in nano-electronic and photonic devices and systems. GeSn has both higher hole and electron mobilities than Ge and Si to possibly enable scaling of the supply voltage for future high performance field-effect transistors (FETs). In addition, the tunable bandgap of GeSn by changing Sn compositions helps to extend the photo detection range beyond what can be achieved by Ge, covering wavelength larger than 1.55 µm.
In this paper, we discuss the research and development of utilizing GeSn for various applications in nano-electronic and photonic devices. This includes multi-gate GeSn p-channel FETs (pFETs) on GeSn-on-insulator (GeSnOI) substrates, realization of ultra-low contact resistivity by incorporation Sn into Ge for the metal/GeSn S/D contact, and GeSn photodetectors for applications at 2 µm wavelength range.

2. GeSn pFinFETs with Sub-10 nm Fin Width on GeSnOI Substrates

Large-area GeSnOI substrates were fabricated using direct wafer bonding (DWB) and layer transfer techniques at a 200 mm wafer scale [1]. This GeSnOI substrate, together with the multi-gate architecture, enabled the realization of high performance Ge1-x
Sn
x
p-FETs with ultra-scaled channel length down to 50 nm [2]. An optimized try etch process was developed to fabricate GeSn FinFETs with fin width less than 10 nm. Excellent electrical characteristics were achieved with subthreshold swing of 63 mV/decade, intrinsic transconductance of 900 µS/µm at VDS
of -0.5 V, and high-field effective mobility of 275 cm2/V·s.

3. Metal/
GeSn p-type Contact with Ultra-low Contact Resistivity.

GeSn has a smaller hole Schottky barrier and a hole effective mass (mh
) as compared to Ge. These advantages make GeSn attractive for low-resistance p-type contacts. In-situ Ga doping was introduced during the epitaxial growth of GeSn using molecular-beam epitaxy (MBE) and more than 50% reduction of specific contact resistivity ρc
has been observed by incorporation of 5% Sn into Ge [3]. Further optimization of GeSn doping process by surface segregation or cold implantation reduces ρc
of Ti/p+-GeSn to sub-10-9 Ω-cm2 regime [4]. The ultra-low ρc
of Ti/p+-GeSn is retained after anneal at 420 oC for 1 hour, showing adequate thermal stability for BEOL process in current CMOS technology [5].

4. GeSn Photo-detectors for Applications at 2 µm Wavelength Range

High-speed photo detection was demonstrated beyond traditional telecommunication bands and reaching at 2 μm band, realized by GeSn/Ge multiple-quantum-well (MQW) photodiode on Si substrate [6]. GeSn/Ge MQW structure was employed to increase the critical thickness (hc
) of epitaxial GeSn on Ge virtual substrate. A decent responsivity of 15 mA/W was obtained at 2 μm. A low leakage current density of 44 mA/cm2 was achieved at reverse bias of 1 V, which is among the lowest reported values for all GeSn photodiodes. A 3-dB bandwidth (f
3-dB) larger than 10 GHz was experimentally demonstrated directly at 2 μm.

Acknowledgment

We would like to acknowledge the funding support from Singapore Ministry of Education Tier 2 grants (R-MOE2018-T2-1-137 and MOE2018-T2-2-154).
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[2] D. Lei et al., VLSI Symposia, 2018, 197-198.
[3] Y. Wu et al., J.
Appl. Phys., 122, 224503, 2017.
[4] Y. Wu et al., VLSI Symposia, 2018, 77-78.
[5] Y. Wu et al., IEEE Electron Device Lett., 40, 1575-1578, 2019.
[6] S. Xu et al., IEDM, 2018, 544-547.