This paper reports on SiGeSn/GeSn multi-quantum-well microdisk lasers. The fabrication of the devices includes a selective under-etching step, which enhances the guiding of the whispering gallery modes inside the cavity. Lasing occurs under different electrical pumping conditions with a very low threshold current and for long, quasi-continuous wave pulses compared to previously reported GeSn-based microdisk lasers. Furthermore, the lasing threshold current is reduced by a factor of ten compared to similar double-heterostructure devices.
Group IV materials suffers from a lack of efficient light generation for the on-chip integration of active photonic component on silicon (Si). One of the solutions is to use new material like Germanium-tin alloys (GeSn) that can provide direct band gap alignment of the band structure. The use quantum well (QW) is known, in principle, to favor room temperature laser operation at reasonable thresholds over bulk material. While most of advances were performed with bulk materials, exploring adequate designs of GeSn/SiGeSn based QW including strain engineering should be helpful for futures developments of Si-based active photonic devices.
Here we demonstrate up to 290 K laser operation in GeSn/GeSn multi-QW microdisks cavities under optical pumping. The QW and barrier were performed by varying the Sn content. We used specific layer transfer technology and a Silicon Nitride (SiN) stressor layer was introduced to inject tensile strain in the active region such to enhance the directness of the transition. Interestingly this is the highest temperature of operation for GeSn quantum wells lasers. This progress opens the route towards room temperature electrically pumped laser operating.
Advancements in semiconductor materials, particularly within Group IV, are crucial to meet the demand for efficient and adaptable laser sources. Germanium-tin (GeSn) alloys have emerged as promising candidates, facilitating full monolithic integration into silicon photonics. Progress in optically pumped GeSn lasers is remarkable, but electrically injected ones face challenges due to low index contrast to effectively confine the optical mode. We propose an electrically pumped laser design based on GeSnOI (GeSn On Insulator) scheme. Modal analysis was performed at 2500 nm wavelength using finite element method, optimizing electromagnetic wave confinement, and mitigating direct electrical contact deposition on the active zone. Simulation results indicated that the most effective fabrication approach involves bonding with another silicon substrate using SiN dielectric layer as cladding, thus taking advantage of high optical index contrast. This advancement heralds the potential for room temperature operation of electrically pumped lasers.
Group IV materials suffers from a lack of efficient light generation for the on-chip integration of active photonic component on silicon (Si). One of the solutions is to use new material like Germanium-tin alloys (GeSn) that can provide direct band gap alignment of the band structure. The use quantum well (QW) is known, in principle, to favor room temperature laser operation at reasonable thresholds over bulk material. While most of advances were performed with bulk materials, exploring adequate designs of GeSn/SiGeSn based QW including strain engineering should be helpful for futures developments of Si-based active photonic devices.
Here we demonstrate up to 290 K laser operation in GeSn/GeSn multi-QW microdisks cavities under optical pumping. The QW and barrier were performed by varying the Sn content. We used specific layer transfer technology and a Silicon Nitride (SiN) stressor layer was introduced to inject tensile strain in the active region such to enhance the directness of the transition. Interestingly this is the highest temperature of operation for GeSn quantum wells lasers. This progress opens the route towards room temperature electrically pumped laser operating.
GeSn alloys are the most promising direct band gap semiconductors to demonstrate full CMOS-compatible laser integration with a manufacturing from Group-IV materials. Since the first demonstration of lasing with GeSn alloys up to 100 K, many researches were devoted to increase the laser operation up to room temperature. We will discuss the band sructure requirements and the practical issues that have to be addressed in order to reach robust gain with increasing temperature. We show that misfit defects managment and strain engineering are key ingredients. For that purpose we developped a GeSn-On-Insulator platform, that combine strain engineering , defective interfacial layer removal and laser resonator designs ad fabrication. Here we show that room temperature lasing, up to 300 K, can be obtained in microdisk resonators fabricated on a GeSnOI layer both with using high Sn-content in the gain medium, e. g. 17% or with applying tensile strain to a layer with lower Sn-content of 14%.
We present a suspended SiGeSn microring laser design that enables strain relaxation of the material layer stack, electrical pumping and adequate heat sinking. Using both strain and composition as two degrees of freedom to engineer the band structure, a direct bandgap is obtained in the gain material of a double heterostructure layer stack, and the L- to Γ-valley energy difference increased to 78 meV, by 66% compared to a non-underetched structure. The temperature dependent current threshold is modeled for the designed device and determined to be 18 kA/cm2 at 50 K. The fabrication process is outlined and first experimental electroluminescence results indicating the effectiveness of our approach are reported. At the time this proceedings paper is being submitted, electrically pumped lasing has also been achieved with a similar structure, with results that will be reported in a future publication.
Direct band gap achievements in germanium by alloying with tin or by tensile strain engineering has enabled, in recent years, several demonstrations of laser emission in the 2-5µm wavelength range. This fast and promising emergence of CMOS-compatible laser technology in the Mid-IR faces, however, major issues, e.g. high threshold power densities, which limit the integration of GeSn as a gain media on a silicon chip for cost-efficient sensing and/or short-range Datacom devices. We show that combining tensile strain and Sn alloying enables one to effectively engineer the material band structure and its optical gain properties. We also evidence the importance of defects management on GeSn lasing characteristics, beyond the band-structure engineering. We discuss the potential of GeSnOI technology to address the above-mentionned aspects, which enabled to drastically reduce the lasing thresholds in microdisk laser cavities and reach continuous-wave operation in GeSn.
GeSn is discussed as solution to realize the dream of a group IV light source integrated on a Si chip. Sn added into a Ge lattice decreases the conduction band energies leading to a direct bandgap semiconductor band structure. However, the compressive strain increases the direct band energy imposing a large Sn content in the GeSn bulk. In spite of many difficulties regarding the growth of epitaxial GeSn alloys on Si, several hundred nm thick GeSn layers with various Sn concentrations up to 15% could be realized and used as gain material for lasers. Nowadays research concentrates on increasing the Sn content towards 20 at% as well as structural layout. The challenge here is the decreasing quality at high Sn contents and the isolation of the active layer from the mists formed at the interface with Ge/Si which increase the laser threshold. In this direction we discuss the influence on lasing and threshold of MQW SiGeSn/GeSn heterostructures with different quantum well thicknesses. Other solution proposed is the change of intrinsic strain type from compressive into tensile by introducing Si3N4 stressors and also GeSn on Insulator technology. These methods are well known in CMOS technology and can be applied to very low Sn content GeSn alloys. The discussion on the best way to reach room temperature laser is addressed both theoretical and experimental.
Energy-efficient integrated circuits for on-chip or chip-to-chip data transfer via photons could be tackled by monolithically grown group IV photonic devices. The major goal here is the realization of fully integrated group IV room temperature electrically driven lasers. An approach beyond the already demonstrated optically-pumped lasers would be the introduction of GeSn/(Si)Ge(Sn) heterostructures and exploitation of quantum mechanical effects by reducing the dimensionality, which affects the density of states. In this contribution we present epitaxial growth, processing and characterization of GeSn/(Si)Ge(Sn) heterostructures, ranging from GeSn/Ge multi quantum wells (MQWs) to GeSn quantum dots (QDs) embedded in a Ge matrix. Light emitting diodes (LEDs) were fabricated based on the MQW structure and structurally analyzed via TEM, XRD and RBS. Moreover, EL measurements were performed to investigate quantum confinement effects in the wells. The GeSn QDs were formed via Sn diffusion /segregation upon thermal annealing of GeSn single quantum wells (SQW) embedded in Ge layers. The evaluation of the experimental results is supported by band structure calculations of GeSn/(Si)Ge(Sn) heterostructures to investigate their applicability for photonic devices.
Nils von den Driesch, Daniela Stange, Stephan Wirths, Denis Rainko, Gregor Mussler, Toma Stoica, Zoran Ikonic, Jean-Michel Hartmann, Detlev Grützmacher, Siegfried Mantl, Dan Buca
The experimental demonstration of fundamental direct bandgap, group IV GeSn alloys has constituted an important step towards realization of the last missing ingredient for electronic-photonic integrated circuits, i.e. the efficient group IV laser source. In this contribution, we present electroluminescence studies of reduced-pressure CVD grown, direct bandgap GeSn light emitting diodes (LEDs) with Sn contents up to 11 at.%. Besides homojunction GeSn LEDs, complex heterojunction structures, such as GeSn/Ge multi quantum wells (MQWs) have been studied. Structural and compositional investigations confirm high crystalline quality, abrupt interfaces and tailored strain of the grown structures. While also being suitable for light absorption applications, all devices show light emission in a narrow short-wave infrared (SWIR) range. Temperature dependent electroluminescence (EL) clearly indicates a fundamentally direct bandgap in the 11 at.% Sn sample, with room temperature emission at around 0.55 eV (2.25 µm). We have, however, identified some limitations of the GeSn/Ge MQW approach regarding emission efficiency, which can be overcome by introducing SiGeSn ternary alloys as quantum confinement barriers.
High-speed silicon modulators based on the plasma effect in reverse-biased p(i)n junction phase shifters have been extensively investigated. The main challenge for such modulators is to maximize their modulation efficiency without compromising high-speed performance and insertion losses. Here, we propose a highly efficient silicon modulator based on a Mach-Zehnder Interferometer in which the doping profile of a vertical pin junction is precisely controlled by means of in-situ doping during silicon epitaxial growth. The precise level of control afforded by this fabrication procedure allows separately optimizing doping concentrations in the immediate vicinity of the junction and in surrounding electrical transport layers at the nanometric scale, enabling high performance levels. Free carrier absorption losses are minimized by implementing high carrier densities only in the waveguide regions where they benefit the most, i.e., in the immediate vicinity of the junction. Since these devices rely entirely on single crystal silicon, performance degradation caused by poor transport and high optical losses in poly- or amorphous silicon (as utilized in similar vertical phase shifter geometries such as semiconductor-insulator-semiconductor capacitive phase shifters) is avoided. Furthermore, unlike conventional plasma effect silicon phase shifters, the bandwidth of the proposed phase shifters is largely independent of the applied reverse voltage and the phase shift versus applied voltage is linearized, making them more suitable for complex modulation formats. The efficiency of the single ended phase shifters is expected to reach a VπL of 0.56 V•cm and absorption losses of α=4.5 dB/mm, a good performance metric for depletion-type modulators. Lumped element Mach-Zehnder Modulators as well as travelling-wave modulators with phase matching based on meandered waveguides have been designed and their RF characteristics simulated and optimized with Ansoft HFSS. First experiments have validated the growth of the epitaxial stack and complete devices are currently being fabricated.
The development of the selective laser sintering (SLS) technique used for rapid building of 3D models from metal powders is presented. The aim of research efforts in laser sintering is to create strong and dense models with no need post-machining. The feasibility of SLS is demonstrated by the mechanical properties of models obtained, which nearly the equivalent to the products formed by sintering in conventional way. However, the range of the materials available to SLS technique is limited to weldable metals and alloys. The main benefit of this laser application is the exact control of process parameters. A Nd:YAG laser working in CW regime was used. It was obtained sintered powder parts with a fine accuracy up to 0.4 mm. The shrinkage degree of height was lower than 10 percent. The microstructure and mechanical properties of SLS processed materials are similar with those of conventionally processed material. The strength and the bulk hardness of the sintered material were tested.
The hot cracking problem encountered when 1 mm thick stainless steel sheets are seam welded using a long pulse Nd:YAG laser is studied. Cracks are observed only in regions where the primary austenite was formed in the cooling phase, depending on it amount in the weld. The initiators of cracks were found between the heat affected zone and base metal as a band of segregate. High repetition rates decrease the crack formation at constant average power and welding sped. The primary austenite dendrites are strongly influenced by cooling rate. Melt thickness increase with pulse energy when pulse duration and repetition rate are kept constant. The optimum focus position was approximately 0.5 mm below the level of the workpiece front face and produced good quality seam welds.
The processes involved in the melting and solidification of powders have been studied, in order to obtain 3D pieces. The experiments have been performed with a CW Nd:YAG laser, but the Q-switched regime has been also used. Measurements about compacting rate for different parameters (scanning speed, thickness of layer) and a comparison between the two regimes have been made.
Obtaining sharp and sometimes special patterns on hard materials surfaces (texturing) is rather difficult by conventional procedures, which made the laser an attractive choice in this matter. Absorption of Nd:YAG's 1.06 micrometers radiation in most steels and ceramics, which are the common materials for textured surfaces, is good such that average power lasers can be used. In this paper we are concerned with the principal phenomena involved in the texturing process and with the sought of a simple mathematical model that will allow to choose right working regime, and make estimations about the system performances (speed, power consumption, etc.). Experimental data were obtained by using both a Electro-Optically Q-switched CW laser with an average power of up to 100 W and a pulsed free running laser with an average power of up to 100 W and pulse energy of up to 10 J. The experimental set-up includes an X-Y stage and two gold deflectors used to cover the entire surface.
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