Field-Effect Transistor Figures of Merit for Vapor–Liquid–Solid-Grown Ge_1-xSn_x (x = 0.03–0.09) Nanowire Devices

Abstract

Ge1‑x Sn x alloys form a heterogeneous material system with high potential for applications in both optoelectronic and high-speed electronics devices. The attractiveness of Ge1‑x Sn x lies in the ability to tune the semiconductor band gap and electronic properties as a function of Sn concentration. Advances in Ge1‑x Sn x material synthesis have raised expectations recently, but there are considerable problems in terms of device demonstration. Although Ge1‑x Sn x thin films have been previously explored experimentally, in-depth studies of the electrical properties of Ge1‑x Sn x nanostructures are very limited, specifically those on nanowires grown via a bottom-up vapor–liquid–solid (VLS) process using metal catalysts. In this study, a detailed electrical investigation is presented of nominally undoped Ge1‑x Sn x bottom-up-grown nanowire devices with different Sn percentages (3–9 at. %). The entire device fabrication process is performed at relatively low temperatures, the maximum temperature being 440 °C. Device current modulation is performed through backgating from a substrate electrode, achieving impressive on–off current (I ON/I OFF) ratios of up to 104, showing their potential for electronic and sensor-based applications. Contact resistance (R C) extraction is essential for proper VLS-grown nanowire device electrical evaluation. Once the R C contribution is extracted and removed, parameter values such as mobility can change significantly, by up to 70% in this work. When benchmarked against other Ge1‑x Sn x electronic devices, the VLS-grown nanowire devices have potential in applications where a high I ON/I OFF ratio is important and where thermal budget and processing temperatures are required to be kept to minimum.

Facile synthesis of Ge1−xSnx nanowires

“…Firstly, SnI 4 was reduced by NaBH 4 at 300°C to form nanoscale molten spherical tin droplets. Then Ge(Ph) 3 Cl was reduced to form Ge 0 . Ge 0 attached on the surface of tin droplets where it could either diffuse around the outer surface of the droplet or penetrate and dissolve within it.…”

“…In summary, we have demonstrated a facile one-pot two-step synthesis of high aspect ratio Ge 1−x Sn x nanowires with an average diameter of 60±20 nm and average length of 5.5±2.0 μm. The nanowires were produced via self-catalyzed SLS growth in a process using low-cost commercially available precursors, namely Ge(Ph) 3 Cl, SnI 4 and NaBH 4 . Elemental analysis revealed highly homogeneous incorporation of Sn in the Ge matrix at up to 10 at%, as evidenced by results from both XRD and EDX spot analysis.…”

“…Tin nanoparticles were first formed by reducing SnI 4 with NaBH 4 at 300°C in TOA in a three-neck round bottom flask, followed by the swift loading Ge(Ph) 3 Cl and NaBH 4 consecutively, and then maintaining the temperature at 300°C for a few hours. The optimal molar ratio of SnI 4 to Ge(Ph) 3 Cl used was 1:30. This produced a black precipitate which was repeatedly washed in toluene and methanol prior to characterisation.…”

“…Ge 1−x Sn x has attracted much research interest as an exciting material with potential applications in nextgeneration rechargeable lithium-ion battery anodes and optoelectronic devices, due to their high carrier mobilities [1][2][3] and a direct band gap which can be tuned by varying the tin concentration [4][5][6][7][8][9][10][11][12]. Additionally, cubic Ge 1−x Sn x allows the lattice dimensions to be tuned over a wide range, which is beneficial when used as a buffer layer to reduce strain arising from lattice mismatch between III-V or II-VI compounds with silicon or germanium substrates [13][14][15].…”

“…Tin was homogeneously incorporated within the germanium matrix at up to 10 at%, with no tin segregation observed on either the surface or along the length of the nanowires. These nanowires were grown via the SLS mechanism through sequential reduction of SnI 4 and Ge(Ph) 3 Cl with NaBH 4 . This synthesis is simple and time-efficient due to the use of commercially available precursors and conventional laboratory glassware.…”