[Paper Review] Ge-Ge$_{0.92}$Sn$_{0.08}$ core-shell single nanowire infrared photodetector with superior characteristics for on-chip optical communication

Sudarshan Singh, Subhrajit Mukherjee+6 more|arXiv (Cornell University)|Feb 4, 2022

Photonic and Optical Devices37 references17 citations

TL;DR

This study presents a core-shell Ge-Ge₀.₉₂Sn₀.₀₈ single nanowire photodetector grown via chemical vapor deposition, achieving exceptional room-temperature responsivity (~70.8 A/W) and photoconductive gain (~57) at 1.55 µm, enabling high-performance on-chip infrared detection compatible with Si CMOS technology.

ABSTRACT

Recent development on Ge$_{1-x}$Sn$_x$ nanowires with high Sn content, beyond its solid solubility limit, make them attractive for all group-IV Si-integrated infrared photonics at nanoscale. Herein, we report a chemical vapour deposition-grown high Sn-content Ge-Ge$_{0.92}$Sn$_{0.08}$ core-shell based single nanowire photodetector operating at the optical communication wavelength of 1.55 $\mu$m. The atomic concentration of Sn in nanowires has been studied using X-ray photoelectron and Raman spectroscopy data. A metal-semiconductor-metal based single nanowire photodetector, fabricated via electron beam lithography process, exhibits significant room-temperature photoresponse even at zero bias. In addition to the high-crystalline quality and identical shell composition of the nanowire, the efficient collection of photogenerated carriers under an external electric field result in the superior responsivity and photoconductive gain as high as ~70.8 A/W and ~57, respectively at an applied bias of -1.0 V. The extra-ordinary performance of the fabricated photodetector demonstrates the potential of GeSn nanowires for future Si CMOS compatible on-chip optical communication device applications.

Motivation & Objective

• To develop a high-performance, CMOS-compatible infrared photodetector for on-chip optical communication using group-IV semiconductors.
• To overcome the challenges of lattice mismatch and defect formation in high-Sn-content GeSn alloys by employing a core-shell nanowire architecture.
• To achieve superior photodetection performance at 1.55 µm wavelength with high responsivity and gain at room temperature.
• To demonstrate the feasibility of using CVD-grown Ge-Ge₀.₉₂Sn₀.₀₈ core-shell nanowires for practical photodetector applications.

Proposed method

• Growth of Ge-Ge₀.₉₂Sn₀.₀₈ core-shell nanowires on Si(111) substrates via catalyst-assisted chemical vapor deposition at 330 °C for the Ge core and 310 °C for the GeSn shell.
• Use of X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine the Sn composition (~8.0 at.%) and strain state (~−2.6% uniaxial strain).
• Fabrication of a metal-semiconductor-metal (MSM) photodetector using electron beam lithography to pattern Cr/Au electrodes on isolated nanowires.
• Measurement of current-voltage (I-V) characteristics under dark and illuminated conditions to evaluate dark current, responsivity, and photoconductive gain.
• Analysis of photocurrent dependence on incident optical power using a power-law fit (Iph ∝ Pᵞ) to extract gain and responsivity.
• Use of noise current model 〈𝑖𝑛²〉 = [2𝑞𝑖𝑑 + 4𝑘𝐵𝑇/𝑅]Δ𝑓 to estimate signal-to-noise performance.

Experimental results

Research questions

• RQ1 Can a core-shell Ge-Ge₀.₉₂Sn₀.₀₈ nanowire structure effectively suppress defect formation and leakage current in high-Sn-content GeSn alloys?
• RQ2 What is the actual Sn composition and strain state in CVD-grown Ge-Ge₀.₉₂Sn₀.₀₈ core-shell nanowires, and how do they affect electronic properties?
• RQ3 Can a single nanowire MSM photodetector based on this material achieve high responsivity and gain at room temperature for 1.55 µm operation?
• RQ4 How does the device performance scale with applied bias and incident optical power?

Key findings

• The Ge-Ge₀.₉₂Sn₀.₀₈ core-shell nanowire structure exhibits a Sn composition of ~8.0 at.% as determined by XPS, consistent with Raman spectroscopy results of ~8.3 at.%.
• The nanowires show a uniaxial strain of −2.6%, indicating tensile stress in the Ge core that may assist in achieving a direct bandgap for improved absorption.
• The fabricated single nanowire MSM photodetector exhibits a dark current of ~10⁻⁹ A at zero bias, indicating low leakage.
• At zero bias and 1.55 µm illumination (~6.37 mW/cm²), the device shows a photocurrent response of ~280 pA with a rise time of ~8 s.
• The responsivity reaches ~70.8 A/W and the photoconductive gain reaches ~57 at an applied bias of −1.0 V, significantly outperforming most reported GeSn-based devices.
• The device maintains high detectivity (~8.4 × 10⁹ Jones) and exhibits a power-law dependence of photocurrent on incident power with exponent γ ≈ 0.98, indicating efficient carrier collection.