SiCC! Quantum light

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.

How can silicon, the heart of modern computer technology, be engineered to generate quantum light at telecom wavelengths crucial for data communication? The "SiCC! Quantum Light" project explored novel routes to fabricate quantum light sources based on color centers in semiconductors. The goals were to control the vertical position of the emitters and to improve the purity of the material surrounding the light sources. Because conventional methods for creating such color centers rely on ion implantation, achieving this level of control is inherently challenging. Here, we developed novel, self-organized, silicon-based color centers that form directly during the growth of crystalline structures and can be precisely confined within extremely thin layers. These color centers can also be integrated into bright, electrically-driven devices. At the heart of the research is a fully epitaxial fabrication process carried out at exceptionally low temperatures. Crystal growth is steered such that luminescent defect centers, i.e., atomic-scale light sources within the lattice, form spontaneously, yet in a highly controlled manner. Through atomic-level control of layers only a few atomic planes thick, the centers are positioned with precision, and their optical properties are homogenized. The result is robust, spectrally stable emission lines that are particularly valuable for future quantum-photonic applications. At the same time, the process is compatible with established silicon technology, which is a decisive advantage for subsequent scaling and for integration into existing chip manufacturing. Moreover, we demonstrate that the self-organized color centers were embedded into silicon light-emitting diodes (LEDs) operating at telecommunication-relevant wavelengths. Unlike conventional silicon LEDs, which are typically inefficient due to silicon's indirect bandgap, these devices leverage the defect centers as bright emission sites. They can be electrically driven and are, in principle, suitable for direct coupling to fiber-optic communication as well as for integration into silicon photonic circuits. This opens a pathway to cost-effective, compact light sources for data transmission, sensing, and quantum-optical applications. Several aspects are scientifically significant: Epitaxial self-organization enables unprecedented control over the position, density, and local environment of the color centers. The successful realization of LEDs further shows that these centers can be translated from optically excited laboratory demonstrations into practical, electrically-driven devices. In the long term, the project's results could pave the way for fully integrated silicon quantum photonics platforms, in which light sources, waveguides, and detectors operate together on a single chip, compatible with semiconductor industry standards. Beyond that, the engineered color centers provide a potentially mass-manufacturable basis for generating and controlling single photons at relevant wavelengths for quantum technologies. Taken together, these advances mark an important step toward faster, more secure, and more resource-efficient information processing with quantum light.