Monolithic Metal–Semiconductor–Metal Heterostructures Enabling Next-Generation Germanium Nanodevices

Abstract

Low-dimensional Ge is perceived as a promising building block for emerging optoelectronic devices. Here, we present a wafer-scale platform technology enabling monolithic Al-Ge-Al nanostructures fabricated by a thermally induced Al-Ge exchange reaction. Transmission electron microscopy confirmed the purity and crystallinity of the formed Al segments with an abrupt interface to the remaining Ge segment. In good agreement with the theoretical value of bulk Al-Ge Schottky junctions, a barrier height of 200 ± 20 meV was determined. Photoluminescence and μ-Raman measurements proved the optical quality of the Ge channel embedded in the monolithic Al-Ge-Al heterostructure. Together with the wafer-scale accessibility, the proposed fabrication scheme may give rise to the development of key components of a broad spectrum of emerging Ge-based devices requiring monolithic metal-semiconductor–metal heterostructures with high-quality interfaces.

A Top‐Down Platform Enabling Ge Based Reconfigurable Transistors

“…Both the STEM and HRTEM images reveal a sharp interface with finite (atomic level) roughness at the Al-Ge junction that is inclined with respect to the viewing direction and does not show any indications of chemical intermixing. Based on extensive TEM investigations on the Al-Ge exchange for vapor-liquid-solid grown Ge nanowires and top-down fabricated Ge nanosheets, [21] it was shown that the Al propagation is governed by Ge diffusion via surface channels on the Al to the extended contact pads. The Al replacing the Ge is thereby provided by effective Al self-diffusion.…”

“…Thus Al contamination in Ge is unlikely. [21] Figure 1d,e shows representative TEM images of a monolithic Al-Ge-Al heterostructure, with the interface being composed of a Ge {111} and an Al {200} facet, respectively. While both crystals are oriented in a [110] zone axis, a mutual in-plane rotation of 6.5° was measured, which might be associated to strain compensation and lattice relaxation to accommodate the lattice mismatch between Ge and Al.…”

“…While both crystals are oriented in a [110] zone axis, a mutual in-plane rotation of 6.5° was measured, which might be associated to strain compensation and lattice relaxation to accommodate the lattice mismatch between Ge and Al. [21] To underline the composition of the device stack, an EDX map of the fabricated device is shown in Figure S1 (Supporting Information). Further details regarding the evidence and characterization of Al-Ge-Al heterostructures, i.e.…”

“…Further details regarding the evidence and characterization of Al-Ge-Al heterostructures, i.e. energy dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD) investigations can be found in the work of Wind and Sistani et al [21] An example for a prospective wafer-scale fabrication process of Al-Ge-Al heterostructure nanosheet transistors is shown in Figure 1f. Thereto, the mesa structure is patterned using lithography and RIE.…”

“…Moreover, in contrast to common-metal-germanide/-silicide alloy formation, the Al-Ge exchange results in a highly conducting and pure metal with abrupt semiconductor/metal heterostructure contacts. [20,21]…”