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  1. Abstract

    α-Sn and SnGe alloys are attracting attention as a new family of topological quantum materials. However, bulkα-Sn is thermodynamically stable only below 13C. Moreover, scalable integration ofα-Sn quantum materials and devices on silicon is hindered by their large lattice mismatch. Here, we grow compressively strainedα-Sn doped with 2-4 at.% germanium on a native oxide layer on a silicon substrate at 300–500C. Growth is found to occur by a reversedβ-Sn toα-Sn phase transformation without relying on epitaxy, with germanium-rich GeSn nanoclusters in the as-deposited material acting as seeds. The size ofα-Sn microdots reaches up to 200 nm, which is approximately ten times larger than the upper size limit forα-Sn formation reported previously. Furthermore, the compressive strain makes it a candidate 3D topological Dirac semimetal with possible applications in spintronics. This process can be further optimized to achieve optically tunable SnGe quantum material and device integration on silicon.

  2. The optical conductivity of single layer graphene (SLG) can be significantly and reversibly modified when the Fermi level is tuned by electrical gating. However, so far this interesting property has rarely been applied to free-space two-dimensional (2D) photonic devices because the surface-incident absolute absorption of SLG is limited to 1%–2%. No significant change in either reflectance or transmittance would be observed even if SLG is made transparent upon gating. To achieve significantly enhanced surface-incident optical absorption in SLG in a device structure that also allows gating, here we embed SLG in an optical slot-antenna-coupled cavity (SAC) framework, simultaneously enhancing SLG absorption by up to 20 times and potentially enabling electrical gating of SLG as a step towards tunable 2D photonic surfaces. This framework synergistically integrates near-field enhancement induced by ultrahigh refractive index semimetal slot-antenna with broadband resonances in visible and infrared regimes, ~ 3 times more effective than a vertical cavity structure alone. An example of this framework consists of self-assembled, close-packed Sn nanodots separated by ~ 10 nm nanogaps on a SLG/SiO2/Al stack, which dramatically increases SLG optical absorption to 10%-25% at λ = 600–1,900 nm. The enhanced SLG absorption spectrum can also be controlled by the insulator thickness.more »For example, SLG embedded in this framework with a 150 nm-thick SiO2 insulating layer displays a distinctive red color in contrast to its surrounding regions without SLG on the same sample under white light illumination. This opens a potential path towards gate-tunable spectral reflectors. Overall, this work initiates a new approach towards tunable 2D photonic surfaces.« less
  3. This paper addresses one of the key issues in the scientific community of Si photonics: thin-film quality and the light emission properties of band-engineered n+Germanium-on-Silicon (Ge-on-Si). Compared to the traditional delta doping approach, which was utilized in the first electrically-pumped Ge-on-Si lasers, we offer an n+Ge-on-Si thin film with better material quality and higher carrier injection efficiency grown by metal-organic chemical vapor deposition (MOCVD). The impacts of thermal cycle annealing and Si substrate offcut on the thin film quality were investigated, including surface roughness, strain, threading dislocation density, Si-Ge interdiffusion, and dopant diffusion. It was revealed that: 1) MOCVD overcomes the outdiffision issue of n-type dopants by having the dopant peaks at the bottom of the Ge films; 2) the characterization of the light emission properties of these MOCVD n+Ge-on-Si samples (1.0 × 1019cm−3doped) compared to delta-doped ultra-high vacuum chemical vapor deposition (UHVCVD) Ge, showing comparable photoluminescence (PL) spectral intensity at 1/4 of the doping level; 3) Detailed PL spectral analyses showed that population inversion from the direct gap transition has been achieved, and the injected electron density in the direct Γ valley is comparable to that of the delta-doped sample even though the n-type doping level is 75% less; and 4)more »Experimental evidences that Si-Ge interdiffusion has a much larger impact on PL intensity than threading dislocation density in the range of 108-109/cm3. These results indicate that MOCVD n+Ge is very promising to reduce the threshold of Ge gain media on Si notably.

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