SiGeSn/GeSn hetero- and multiple quantum well structures for optoelectronics on Si

Advanced information technology has to be able to cope with the enormous amounts and rates of data requirements. New architectures of computing systems, such as neuromorphic computing, will enable deep learning and massive parallel data handling. However, it will need also large amounts of data for training as well as fast transfer rates of data between logic and storage devices. Here, advanced chip and board designs, including silicon optical interposer may allow much higher density of signal traces between co-packaged chips. In particular co-packaged silicon photonic chips allow optical interconnections between systems-in-package. Thus silicon interposer can directly contain photonic devices based on group alloys. In a long term vision this technology might be enabled by GeSn lasers permitting to connect optically individual chips within the system-in-package.In the past years significant progress has been made to develop optically active devices based on Si. A direct band gap for GeSn alloys containing more than 8.5% of Sn was demonstrated and the optically pumped GeSn laser were reported [1,2]. In order to improve the device performance and achieve electrical operation at sufficiently low power still severe challenges have to be met. The GeSn active region has to be embedded in a heterostructure providing optical waveguiding and efficient carrier injection. The active region may contain quantum well structures to warrant low threshold currents and room temperature operation. Please click Additional Files below to see the full abstract

Cavity-enhanced single photon emission from a single impurity-bound exciton

Impurity-bound excitons in ZnSe quantum wells are bright single photon emitters--a crucial element in photonics-based quantum technology. But to achieve the efficiencies required for practical applications, these emitters must be integrated into optical cavities that enhance their radiative properties and far-field emission pattern. In this work, we demonstrate cavity-enhanced emission from a single impurity-bound exciton in a ZnSe quantum well. We utilize a bullseye cavity structure optimized to feature a small mode volume and a nearly Gaussian far-field transverse mode that can efficiently couple to an optical fiber. The fabricated device displays emission that is more than an order of magnitude brighter than bulk impurity-bound exciton emitters in the ZnSe quantum well, as-well-as clear anti-bunching, which verifies the single photon emission from the source. Time-resolved photoluminescence spectroscopy reveals a Purcell-enhanced radiative decay process with a Purcell factor of 1.43. This work paves the way towards high efficiency spin-photon interfaces using an impurity-doped II-VI semiconductor coupled to nanophotonics

Strain engineering for GeSn/SiGeSn multiple quantum well laser structures

Optically pumped GeSn laser have been realized, thus alloying of group IV elements germanium (Ge) and tin (Sn) has a large potential to be a solution for Si-photonics, since a direct bandgap for Sn incorporations above ~9 at.% is obtained [1]. The value of the bandgap can further be controlled by adding Si into the mix, which can be exploited for the formation of heterostructures for carrier confinement [2]. However, a sufficiently large difference in energy ΔE between the indirect L-valley and the direct Г-valley is required to achieve room temperature lasing. Recently lasing was reported at 180K in GeSn alloys with Sn concentrations as high as 22,3% [3]. Alternatively ΔE can be increased by adding tensile strain to the GeSn layers. Here we will discuss that an appropriate combination of Sn concentration and strain will be advantageous to tailor gain and temperature stability of the structures. Please click Additional Files below to see the full abstract

Electrical resistance of individual defects at a topological insulator surface

Three-dimensional topological insulators host surface states with linear dispersion, which manifest as a Dirac cone. Nanoscale transport measurements provide direct access to the transport properties of the Dirac cone in real space and allow the detailed investigation of charge carrier scattering. Here, we use scanning tunnelling potentiometry to analyse the resistance of different kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator thin film. The largest localized voltage drop we find to be located at domain boundaries in the topological insulator film, with a resistivity about four times higher than that of a step edge. Furthermore, we resolve resistivity dipoles located around nanoscale voids in the sample surface. The influence of such defects on the resistance of the topological surface state is analysed by means of a resistor network model. The effect resulting from the voids is found to be small compared to the other defects

Two-dimensional photonic crystal cavities in ZnSe quantum well structures

ZnSe and related materials like ZnMgSe and ZnCdSe are promising II-VI host materials for optically mediated quantum information technology such as single photon sources or spin qubits. Integrating these heterostructures into photonic crystal (PC) cavities enables further improvements, for example realizing Purcell-enhanced single photon sources with increased quantum efficiency. Here we report on the successful implementation of two-dimensional (2D) PC cavities in strained ZnSe quantum wells (QW) on top of a novel AlAs supporting layer. This approach overcomes typical obstacles associated with PC membrane fabrication in strained materials, such as cracks and strain relaxation in the corresponding devices. We demonstrate the attainment of the required mechanical stability in our PC devices, complete strain retainment and effective vertical optical confinement. Structural analysis of our PC cavities reveals excellent etching anisotropy. Additionally, elemental mapping in a scanning transmission electron microscope confirms the transformation of AlAs into AlOx by post-growth wet oxidation and reveals partial oxidation of ZnMgSe at the etched sidewalls in the PC. This knowledge is utilized to tailor FDTD simulations and to extract the ZnMgSe dispersion relation with small oxygen content. Optical characterization of the PC cavities with cross-polarized resonance scattering spectroscopy verifies the presence of cavity modes. The excellent agreement between simulation and measured cavity mode energies demonstrates wide tunability of the PC cavity and proves the pertinence of our model. This implementation of 2D PC cavities in the ZnSe material system establishes a solid foundation for future developments of ZnSe quantum devices

Epitaxy of group IV Si-Ge-Sn alloys for advanced heterostructure light emitters

Over the last decades, silicon-based integrated circuits underpinned information technology. To keep up with the demand for faster and, becoming increasingly more relevant nowadays, energy-efficient electronics, smart solutions targeting power consumption are required. Integration of photonic components, e.g. for replacing part of copper interconnects, could strongly reduce on-chip dissipation. Prerequisite for efficient active optoelectronic devices, however not available in group IV elements, is a direct bandgap. Only recently though, a truly silicon-compatible solution was demonstrated by tin-based group IV GeSn alloys, which offer a direct bandgap for a cubic lattice and Sn concentrations above 9 at.%. Nevertheless, when moving from an experimental direct bandgap demonstration towards readily integrated light emitters, plenty of challenges have to be overcome. In this work, some of the remaining key aspects are investigated. Reduced-pressure chemical vapor deposition on 200 mm (Ge-buffered) Si wafers was used to form the investigated Si-Ge-Sn alloys. GeSn layers with subtitutionally incorporated Sn concentrations up to 14 at.%, considerably exceeding the solid solubility limit of 1 at.% Sn in Ge, were epitaxially grown to study growth kinetics. The necessary strain relieve in GeSn binaries was studied growing layers with thicknesses up to 1 µm, well above the critical thickness for strain relaxation. Influence of both, Sn incorporation and residual strain, on the optical properties was probed using temperature-dependent photoluminescence and reflection spectroscopy. Mid infrared light emission was found at wavelengths as long as 3.4 µm (0.37 eV) at room temperature. Overall, the investigated GeSn material system allows to cover a range up to about 2 µm (0.60 eV), making these binaries also interesting for a multitude of chemical and biological sensing applications. Efficient light sources further require the confinement of carriers in heterostructures. Therefore, also epitaxy of SiGeSn ternaries, which previously have been identified as optimal larger bandgap claddings, was scrutinized. The additional degree of compositional freedom was demonstrated by bandgap engineering, individually using strain relaxation, Si and Sn composition. Combining GeSn binaries and SiGeSn ternaries allowed formation of different diode structures. Light emitting diodes, both from GeSn homojunctions and multi quantum well heterojunctions, were epitaxially grown and studied for their emission characteristics. One drawback in these structures, however, is that they do not just yet feature a direct bandgap. Finally, several (so far undoped) direct bandgap GeSn/SiGeSn double heterostructures and multi quantum wells were investigated. The importance of defect engineering, that is separation of unavoidable misfit defects and active device regions, is stressed and fathomed for both designs. Excellent structural properties of the grown layers were proven by advanced characterization techniques, such as atom probe tomography or dark-field electron holography. Photoluminescence measurements were carried out to probe the optical quality of those structures, revealing strongly enhanced light emission from MQW structures, compared to bulk GeSn layers

Epitaxy of group IV Si-Ge-Sn alloys for advanced heterostructure light emitters

Over the last decades, silicon-based integrated circuits underpinned information technology. To keep up with the demand for faster and, becoming increasingly more relevant nowadays, energy-efficient electronics, smart solutions targeting power consumptionare required. Integration of photonic components, e.g. for replacing part of copper interconnects, could strongly reduce on-chip dissipation. Prerequisite for efficient active optoelectronic devices, however not available in group IV elements, is a direct bandgap. Only recently though, a truly silicon-compatible solution was demonstrated by tin-based group IV GeSn alloys, which offer a direct bandgap for acubic lattice and Sn concentrations above 9 at.%. Nevertheless, when moving froman experimental direct bandgap demonstration towards readily integrated light emitters, plenty of challenges have to be overcome. In this work, some of the remaining key aspects are investigated. $\textit{Reduced-pressure chemical vapor deposition}$ on 200mm (Ge-buffered) Si wafers was used to form the investigated Si-Ge-Sn alloys. GeSn layers with subtitutionally incorporated Sn concentrations up to 14 at.%, considerably exceeding the solid solubility limit of 1 at.% Sn in Ge, were epitaxially grown to study growth kinetics. The necessary strain relieve in GeSn binaries was studied growing layers with thicknesses up to 1 μm, well above the critical thickness for strain relaxation. Influence of both, Sn incorporation and residual strain, on the optical properties was probed using temperature-dependent photoluminescence and reflection spectroscopy. Mid infrared light emission was found at wavelengths as long as 3.4 μm (0.37 eV) at room temperature. Overall, the investigated GeSn material system allows to cover a range up to about 2 μm (0.60 eV), making these binaries also interesting for a multitude of chemical and biological sensing applications. [...