Project Outline:

The crystalline form of the element silicon builds the backbone of the digital world that changed our lives and human interaction in the last decades, and the continuous miniaturization of electronic devices on microchips led to ever-increasing computing speeds. This versatile material could also initiate breakthroughs in the field of quantum communication. Recent findings have shown that single photons, an essential ingredient of quantum cryptography, can be created by intentionally inducing defects into the silicon crystal lattice. Additionally, these single photons exhibit a wavelength in the telecommunication range. That is, the light particles can be, in general, efficiently transmitted through the worldwide glass fiber network. 

However, these quantum light sources are, to date, not sufficiently well understood. State-of-the-art approaches use carbon-ion implantation at high energies to create the necessary defects in the crystal lattice. Upon impact, a multitude of defect types is emerging in the crystal lattice, among them so-called “G-centers.” These “G-center” consist of a silicon atom that binds to two carbon atoms, and this configuration enables the emission of quantum light. Unfortunately, there is no way to predict at which depth in the crystal the defect is indeed created, i.e., directly underneath the surface or within a depth of 1500 lattice planes. However, this control over the vertical emitter position is crucial for this technology to succeed in the future since only precisely positioned quantum light emitters can be coupled to other photonic elements such as waveguides.

This project aims to enable precise vertical positioning through the growth of thin crystalline layers deposited in ultra-high-vacuum. These layers consist of silicon-germanium alloys or carbon-doped silicon. This epitaxial growth allows for the accurate fabrication of layer thicknesses, alloy concentrations, and doping concentrations with high precision. This project investigates if the defects can be preferentially created within the “softer” silicon-germanium alloy, as compared to the “harder” silicon crystal if the whole crystal is bombarded with the necessary carbon ions. Additionally, we investigate the possibility of creating defects in thin carbon-enriched silicon layers through the additional implantation with hydrogen. The required carbon for quantum light formation originates from the epitaxial layer, while the hydrogen ions lead to the mandatory defect formation in the crystal lattice. The resulting optical and quantum properties will be investigated by means of optical spectroscopy.

Project leader: Assist.-Prof. Dr. Moritz Brehm
Project duration: 01.02.2023-31.01.2026