Tunable Shortwave Infrared and Midwave Infrared Optoelectronics in Germanium/Germanium Tin Core/Shell Nanowires
Authors: Luo Lu , Assali Simone , Atalla Mahmoud , Koelling Sebastian , Moutanabbir Oussama 
Published: May 2020

Semiconductor nanowire(NW)-based optoelectronic devices can exhibit superior performances in comparison to their thin film counterparts, as a result of their improved electrical and optical properties. For example, the light-matter interaction in nanowires is enhanced due to the high surface-volume ratio and the light confinement effect resulting from the resonant cavity formed by the nanowire. The enhanced light absorption when using a nanowire geometry can be exploited to increase the responsivity of nanowire-based photodetectors. Light detection and emission at near-IR wavelengths (NIR, 0.8-1.5 μm) can be tailored via strain engineering using Ge-based NW devices.[1] Furthermore, by incorporating Sn in the Ge lattice a direct band gap can be achieved across the short-wave-IR (SWIR, 1.5-3.0 μm) and mid-IR (MIR, 3-8 μm) wavelength range.[2] Despite the equilibrium solubility of Sn in Ge being limited to ~1at.%, non-equilibrium growth methods recently developed in a chemical vapor deposition (CVD) reactor demonstrated a Sn content of 18 at.% with a room-temperature photoluminescence emission up to 4.0 μm [3]. Similarly, when moving to the nanoscale a Sn incorporation well above 10 at.% with a direct band gap emission was demonstrated using Ge/GeSn core/shell NWs.[4]
In this presentation, we will discuss the opto-electronic properties of Ge and Ge/GeSn core/shell NWs grown in a CVD reactor using a Ge or Si (111) substrate. The structural properties of the NWs were evaluated by combining Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) in order to understand the nanowire growth direction, crystalline properties as well as Sn distribution uniformity in the shell of the nanowires. Photoluminescence, absorption and Raman measurements will elucidate the effect of strain on the MIR direct bandgap emission/absorption of the GeSn shell.
Finally, wewill discuss the electrical characterization of individual Ge and GeSn NW photodetectors. Our devices are fabricated by Electro-Beam Photolithography (EBL) based on metal-semiconductor-metal (M-S-M) structure to address the dependence of the photocurrent under illumination at different SWIR-MIR wavelengths. The time response and transport properties of Ge and Ge/GeSn core/shell NW Field Effect Transistor (FET) in back-gated structure are compared with their counterparts of conventional thin film heterostructures.

References

Pilon, FT Armand, et al. "Lasing in strained germanium microbridges." Nature communications1 (2019): 1-8.

Assali, S., J. Nicolas, and O. Moutanabbir. "Enhanced Sn incorporation in GeSn epitaxial semiconductors via strain relaxation." Journal of Applied Physics2 (2019): 025304.

Assali, S., et al. "Atomically uniform Sn-rich GeSn semiconductors with 3.0–3.5 μ m room-temperature optical emission." Applied Physics Letters25 (2018): 251903.

Assali, S., et al. "Growth and optical properties of direct band gap Ge/Ge0. 87Sn0. 13 core/shell nanowire arrays." Nano letters3 (2017): 1538-1544.