CRYSPE2-Crystal structure Preserving Electron - Beam Etching for Ge-Nanodevices

New materials like Ge-nanowires (Ge-NWs) and Germanium-on-insulator (GOI) enable innovative devices such as nanowire-transistors with small subthreshold leakage and GOI transistors with short cycle-times. As geometrical channel dimensions are crucial for the electrical properties, fine-tuning of critical dimensions by locally confined etching is the key to optimize Ge-based devices. Unfortunately, processing with a focused ion beam (FIB) leads to Ga implantation and destruction of the crystalline structure whether focused electron beam induced etching (FEBIE) is damage- and contamination-free. FEBIE of Si was demonstrated with XeF2 as etch gas but (i) suffers from spontaneous etching and (ii) is blocked by surface contaminants. Recently we have developed a process for controlled etching of Ge-substrates with the use of chlorine as etch gas. which is ideally suited for fabrication and fine-tuning of Ge-based devices for damage-free removal of FIB-damage. This project comprises 2 development objectives: 1. the etch process, 2. Ge-devices development. 1. "Development of the chlorine based etching process for Ge" has the goal to provide insights in surface mechanisms (adsorption, diffusion and desorption) of beam induced etching. Process optimization will provide improved etch rates, lower surface roughness, and an application for the 3-dimensional modification of Ge-nanowire structures. The etching process will enable damage-free removal of FIB-amorphized layers while maintaining full crystalline integrity of Ge-devices. 2. "Device fabrication and characterization" will utilize the previously developed FEBIE process for structuring and fine-tuning of nanodevices. The aim is the fabrication and electrical characterization of gated Ge- nanowire devices and of a GOI-MOSFET. Moreover, in-situ electrical analysis in the SEM during ongoing FEBIE etching and cryo-measurements down to 4 K will be performed. Using a chlorine-based etch gas mixture high-resolution nanostructuring of germanium can be performed in only those areas exposed to the electron beam. Yet, chlorine FEBIE fully maintains the crystalline structure of the sample but evades spontaneous etching. Project activities will align along 2 simultaneous activity lines - each conducted by one PhD student. 1. Etching of Ge-nanodevices with an focused electron beam using chlorine as etch gas : Etching of bulk Ge, Ge-NWs and GOI will be performed in a LEO 1530 electron microscope with chlorine gas injection. Vertical and horizontal thinning of Ge-NWs and MOSFET channels. 2. Analysis and Electrical Characterization of Ge-nanodevice: Processed surfaces will be investigated by TEM. Electrical properties of Ge-FETs will be investigated by C-V and I-V measurements. Electrical in-situ measurement during etching will be performed as well as structural (AFM), crystallographic and chemical (AES, SIMS) analysis. For the first time fine-tuning of device characteristics will be performed on Ge-NW and GOI devices as well as damage-free removal of amorphized surfaces for preparation of cross-sectional samples. Together with achieved new insights on the surface physics of adsorbed etchants and on their interaction with electrons aided by in-situ electrical monitoring during processing this project will reveal new insights beneficial both for industry and research.

In current computers and mobile phones both logic data processing in CPUs as well as data storage in flash memory relies on semiconductor transistors with features in the nanometer regime. Current fabrication technology can easily generate complicated 2D circuit patterns on planar substrates. A universal fabrication technology for real 3D nanostructures with arbitrary geometry is not yet available. Such 3D nanostructuring would enable vertical nanowire transistors for powerful 3D microelectronics and ultra-high-density 3D data storage as well as wearable electronics on non-flat surfaces, and 3D medical sensors.This project has advanced a mask-less, resistless nanofabrication technology that can build or modify 3D semiconductor nanodevices. A focused electron beam was scanned over the device region of interest and locally initiated a chemical reaction of the nanodevice surface with an added chemical gas. In short, this focused electron beam-induced processing (FEBIP) can be used to locally etch, deposit, or dope semiconductor materials, including 3D nanowires. This project has researched and perfected (i) the controlled chlorine-based etch process for semiconductors by FEBIP and (ii) the electrical trimming of semiconductor devices and the circuit integration of 3D nanomaterials using FEBIP. This project has also brought forth the following achievements: First, the electrical conductivity of individual nanodevices can be selectively modified by chlorine etching with the electron beam. With this custom tailoring of nanodevice cross-sections, this project has advanced a novel approach to modify 3D semiconductor nanodevices. Secondly, FEBIP with chlorine as a process gas cannot only be used for etching but also for nanoscale surface modification. This has even been achieved on the spherical surface of an Sinanowire and a Ge-nanowire. Surface analysis has indicated the presence of chlorine on the nanowire surface. This surface modification with chlorine has a tremendous impact and changes electrical conductivity without altering geometry. In this project, the fabrication of diode-like devices from a homogeneously p-doped semiconductor has been realized by surface modification using FEBIP with chlorine. This opens a new route for the fabrication of active transistors from plain nanowires.To facilitate a route for electrically connecting such nanowire devices, a method for the direct deposition of pure gold contacts was developed by this project. The decomposition of an organometallic gold precursor by FEBIP typically yields less-conductive carbon-rich material. By developing an in-situ purification procedure, it is possible to gain pure gold deposits.The results of this project will be highly relevant for future 3D nanotransistors and nanosensors. The chlorine termination of semiconductors is a first step towards the locally confined biofunctionalization of semiconductor biosensors. Pure gold nanostructures on planar and non-planar surfaces are also exciting components for nanoplasmonic devices.