Lasers based on ion-bombarded Ge quantum dots on Si

Over the last years enormous efforts were made towards the fabrication of room-temperature silicon-based lasers that can be integrated into the existing silicon technology. These high-efficiency emitters are needed to overcome bottlenecks in the chip-to-chip and intra-chip data transfer rates caused by the presently used electrical interconnects. Here, I propose a detailed experimental study of a new form of epitaxial silicon-germanium (SiGe) quantum dots (QDs). SiGe QDs are objects a few nm in size that can nucleate on crystalline Si substrates upon deposition of a few atomic layers of Ge. Exposure of such QDs to Ge ion bombardment (GIB) with ion energies of about 2 keV leads to a partial amorphisation of the GIB-QDs. Interestingly, the light emission properties, especially at room-temperature, are substantially improved by this intentional partial transformation from the crystalline to an amorphous phase of parts of the QDs. Since the fabrication process of the GIB-QDs is fully compatible with the requirements of standard Si integration technology, applications such as photonic on-chip interconnects for higher data transfer rates and consequently faster computers become feasible and the resulting impact of the GIB-QDs is expected to be high. In the course of the project we want to gain detailed insights into structural and optical properties of the GIB-QDs in order to fully optimize their light emission properties. Hereby, several factors will have to be taken into account: How the PL efficiency of the GIB-QDs is influenced by the QD size, their shape and the energy of the implanted ions; How does the chemical composition within the QDs influences the amorphisation process and the resulting light emission properties of the GIB-QDs; What the limits are of thermal stability of the GIB-QDs, and how do they behave under thermal annealing; Can the light-emission properties from GIB-QDs be influenced by decorating the amorphous parts of the dots by foreign atoms such as hydrogen? What is the nature of the defects introduced e.g. by ion implantation? Can we identify and avoid detrimental defects? Can we achieve unambiguous signs of lasing from optically driven photonic cavities containing GIB-QDs? The proposed project aims to answer those questions about these novel nanostructures with the final aim to elucidate their suitability for high quality silicon-based light emitters for optical interconnects.

Only the fundamental understanding of the microstructural properties at the atomic scale allows for ongoing optimization and development of novel nanostructures. This is particularly true for the nanostructures that were investigated in this FWF-stand-alone project. The structures consist of two interlaced, but fundamentally different low-dimensional systems: Epitaxial quantum dots grown by molecular beam epitaxy and point defects that are intentionally introduced into the quantum dots through low-energy ion implantation. The resulting ion-bombarded quantum dots only consist of group-IV elements, the key components of all integrated technology that is the main driver for the digital transformation that continues to influence our lives nowadays. For reasons of limited bandwidth and ever-increasing power dissipation in current microprocessor chips, enormous research efforts have been undertaken to facilitate optical data transfer on or in-between single Si microprocessor chips. For making this concept of silicon-based photonics and photonic-integrated circuits feasible, one key ingredient is still missing, A silicon-based light source or silicon laser that can easily be implemented and is compatible with silicon integration technology. However, silicon is an inherently poor light emitter and lasing-operation using bulk silicon is not possible. In this project, we have demonstrated that the employed ion-bombarded germanium quantum dots have highly promising properties as light-emitters, even at room temperature, i.e. in an application-relevant temperature range. We explain this behavior by the specific defect-structure that is generated when germanium ions are implanted into else defect-free quantum dots. After the initial amorphization of the structure, recrystallization leads to the formation of a point-defect structure with low formation energy. Notably, this point defect is surrounded by lattice distortions containing about 45 atoms. These distortions turned out to be beneficial for the temperature-stability of the structural properties of the structures, i.e. the defect/dot complex can withstand high thermal budgets of larger than 600C for several hours without significant changes in the optical properties of the nanostructures. Furthermore, we evaluated the influence of quantum dot shape, chemical composition and energy of the implanted ions on the optical properties of the quantum dots. Thereby, the system turns out to be very robust against changes in these parameters, a factor that is beneficial for the future implementation into semiconductor devices. The optical properties of the optoelectronic devices containing ion-bombarded nanostructures were investigated and the first prototypes of light-emitting diodes were fabricated using silicon technology. Noteworthy, these diodes efficiently work even at temperatures beyond room temperature, i.e. at 100C, which is unique for silicon-based light sources. The success of the project emphasizes that ion-implanted silicon-germanium based quantum dots can provide an important step for the future implementation of silicon-based light emitters into CMOS-driven microelectronics with the goal to significantly enhance data transfer rates and speeds.