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Register a new userUltra-low threshold cw and pulsed lasing in tensile strained GeSn alloys
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Added by
Moustafa El Kurdi Dr.
Publication date
2020
fields
Physics
and research's language is
English
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GeSn alloys are the most promising semiconductors for light emitters entirely based on group IV elements. Alloys containing more than 8 at.% Sn have fundamental direct band-gaps, similar to conventional III-V semiconductors and thus can be employed for light emitting devices. Here, we report on GeSn microdisk lasers encapsulated with a SiNx stressor layer to produce tensile strain. A 300nm GeSn layer with 5.4 at.% Sn, which is an indirect band-gap semiconductor as-grown with a compressive strain of -0.32 %, is transformed via tensile strain engineering into a truly direct band-gap semiconductor. In this approach the low Sn concentration enables improved defect engineering and the tensile strain delivers a low density of states at the valence band edge, which is the light hole band. Continuous wave (cw) as well as pulsed lasing are observed at very low optical pump powers. Lasers with emission wavelength of 2.5 um have thresholds as low as 0.8kWcm^-2 for ns-pulsed excitation, and 1.1kWcm^-2 under cw excitation. These thresholds are more than two orders of magnitude lower than those previously reported for bulk GeSn lasers, approaching these values obtained for III-V lasers on Si. The present results demonstrate the feasabiliy and are the guideline for monolithically integrated Si-based laser sources on Si photonics platform.
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By independently engineering strain and composition, this work demonstrates and investigates direct band gap emission in the mid-infrared range from GeSn layers grown on silicon. We extend the room-temperature emission wavelength above ~4.0 {mu}m upon post-growth strain relaxation in layers with uniform Sn content of 17 at.%. The fundamental mechanisms governing the optical emission are discussed based on temperature-dependent photoluminescence, absorption measurements, and theoretical simulations. Regardless of strain and composition, these analyses confirm that single-peak emission is always observed in the probed temperature range of 4-300 K, ruling out defect- and impurity-related emission. Moreover, carrier losses into thermally-activated non-radiative recombination channels are found to be greatly minimized as a result of strain relaxation. Absorption measurements validate the direct band gap absorption in strained and relaxed samples at energies closely matching photoluminescence data. These results highlight the strong potential of GeSn semiconductors as versatile building blocks for scalable, compact, and silicon-compatible mid-infrared photonics and quantum opto-electronics.
Ge-Sn alloys with a sufficiently high concentration of Sn is a direct bandgap group IV material. Recently, ion implantation followed by pulsed laser melting has been shown to be a promising method to realize this material due to its high reproducibility and precursor-free process. A Ge-Sn alloy with ~9 at.% Sn was shown to be feasible by this technique. However, the compressive strain, inherently occurring in heterogeneous epitaxy of the film, evidently delays the material from the direct bandgap transition. In this report, an attempt to synthesize a highly-relaxed Ge-Sn alloy will be presented. The idea is to produce a significantly thicker film with a higher implant energy and doses. X-ray reciprocal space mapping confirms that the material is largely-relaxed. The peak Sn concentration of the highest dose sample is 6 at.% as determined by Rutherford backscattering spectrometry. Cross-sectional transmission electron microscopy shows unconventional defects in the film as the mechanism for the strain relaxation. Finally, a photoluminescence (PL) study of the strain-relaxed alloys shows photon emission at a wavelength of 2045 nm, suggesting an active incorporation of Sn concentration of ~6 at.%. The results of this study pave way to produce high quality relaxed GeSn alloy using an industrially scalable method.
Strain engineering in Sn-rich group IV semiconductors is a key enabling factor to exploit the direct band gap at mid-infrared wavelengths. Here, we investigate the effect of strain on the growth of GeSn alloys in a Ge/GeSn core/shell nanowire geometry. Incorporation of Sn content in the 10-20 at.% range is achieved with Ge core diameters ranging from 50nm to 100nm. While the smaller cores lead to the formation of a regular and homogeneous GeSn shell, larger cores lead to the formation of multi-faceted sidewalls and broadened segregation domains, inducing the nucleation of defects. This behavior is rationalized in terms of the different residual strain, as obtained by realistic finite element method simulations. The extended analysis of the strain relaxation as a function of core and shell sizes, in comparison with the conventional planar geometry, provides a deeper understanding of the role of strain in the epitaxy of metastable GeSn semiconductors.
THz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The contribution from ultrafast demagnetization/remagnetization is extremely weak, in the comparison. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures. The THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta) based FM/NM spintronic heterostructures have been studied for an optimized performance for THz generation when pumped by sub-50 fs amplified laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an efficient THz source. The optimized FM/NM spintronic heterostructure on a quartz substrate, having alpha-phase Ta as the nonmagnetic layer, show the highest damage threshold as compared to those with Pt, irrespective of their generation efficiency. The damage threshold of the Fe/Ta heterostructure on quartz substrate is ~85 GW/cm2.
The generation and control of exotic phenomena in organic electroluminescent microcavities, such as polariton lasing and non-linear optical effects, operating in strong and ultra-strong coupling regimes, is still a great challenge. The main obstacles originate from the small number of molecular classes investigated as well as from the absence of an efficient strategy aiming at the maximization of polariton states population. Here we report on bright polariton organic light emitting diodes made of a coumarin fluorescent dye emitting layer, working in the ultra-strong coupling regime up to a coupling strength of 33%. Owing to a high radiative decay emission, a large Stokes shift and a fine cavity-exciton tuning, the radiative pumping mechanism of polariton states has been fully optimized, leading a large portion (25%) of the emissive electrically pumped excitons to be converted in polariton emission. The resulting polariton OLEDs showed electro-optical performances up to 0.2% of external quantum efficiency and 700 cd/m2 of luminance, corresponding to the highest values reported so far for this class of devices. Our work gives clear indications for an effective exploitation of organic polariton dynamics towards the development of novel quantum optoelectronic devices.
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