Here, we use a novel growth scheme to overcome this roadblock and directly grow on-chip InGaAs nanopillar lasers, demonstrating the potency of bottom-up nano-optoelectronic integration. Unique helically-propagating cavity modes are employed to strongly confine light within subwavelength nanopillars despite low refractive index contrast between InGaAs and silicon. These modes thereby provide an avenue for engineering on-chip nanophotonic devices such as lasers. Nanopillar lasers are as-grown on silicon, offer tiny footprints and scalability, and are thereby particularly suited to high-density optoelectronics. They may ultimately form the basis of the missing monolithic light sources needed to bridge the existing gap between photonic and electronic circuits. 2Since the first laser demonstrated that stimulated emission processes in an optical medium can implement a powerful, coherent light source 1 , the field of photonics has witnessed an explosion of applications in telecommunications, lighting, displays, medicine, and optical physics amongst others. Integration of photonic and electronic devices to leverage the advantages of both has subsequently attracted great interest. In particular, integration of optical interconnects onto silicon (Si) chips has become critical as ongoing miniaturization of Si logic elements has incurred a bottleneck in inter-and intra-chip communications 2,3 . Efforts towards creating on-chip light sources for optical interconnects have included engineering silicon and germanium for optical gain 4-6 and stimulated Raman scattering 7-9 . Concurrently, III-V lasers have been heterogeneously bonded onto silicon substrates [10][11][12] . However, numerous challenges face these approaches. Wafer bonding have low yields because of a stringent surface flatness requirement down to the atomic scale, while group IV emitters must overcome an indirect band gap that offers exceedingly inefficient radiation. Monolithic growth of high-performance III-V lasers on silicon thereby remains a "holy grail" for cost-effective, massively scalable, and streamlined fabrication of on-chip light sources.The fundamental roadblock facing monolithic integration up to now has been a large mismatch of lattice constants and thermal expansion coefficients between III-V materials and The nanopillar-based laser is schematically depicted in Figure 1A. shows the hexagonal cross-section of the nanopillar, which results from its unique single crystal wurtzite structure 15 . As we will later show, the as-grown nanopillar structure provides a natural optical cavity supporting unique resonances that have not been observed before to the best of our knowledge. As such, nanopillars do not require additional top-down processing to form on-chip optical cavities. Instead, they provide a viable bottom-up approach towards integrating light sources and resonators onto a silicon chip.Importantly, nanopillars possess several critical advantages for optoelectronic integration onto silicon. They grow at a low temperature of 400 °C, which is dra...

We report the use of highly ordered, dense, and regular arrays of in-plane GaAs nanowires as building blocks to produce antiphase-domain-free GaAs thin films on exact (001) silicon. High quality GaAs nanowires were grown on V-grooved Si (001) substrates using the selective aspect ratio trapping concept. The 4.1% lattice mismatch has been accommodated by the initial GaAs, a few nanometer-thick with high density stacking faults. The bulk of the GaAs wires exhibited smooth facets and a low defect density. An unusual defect trapping mechanism by a “tiara”-like structure formed by Si undercuts was discovered. As a result, we were able to grow large-area antiphase-domain-free GaAs thin films out of the nanowires without using SiO2 sidewalls for defect termination. Analysis from XRD ω-rocking curves yielded full-width-at-half-maximum values of 238 and 154 arc sec from 900 to 2000 nm GaAs thin films, respectively, indicating high crystalline quality. The growth scheme in this work offers a promising path towards integrated III-V electronic, photonic, or photovoltaic devices on large scale silicon platform.

Monolithic integration of III-V compound semiconductor devices with silicon CMOS integrated circuits has been hindered by large lattice mismatches and incompatible processing due to high III-V epitaxy temperatures. We report the first GaAs-based avalanche photodiodes (APDs) and light emitting diodes, directly grown on silicon at a very low, CMOS-compatible temperature and fabricated using conventional microfabrication techniques. The APDs exhibit an extraordinarily large multiplication factor at low voltage resulting from the unique needle shape and growth mode.

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The heterogeneous integration of III-V optoelectronic devices with Si electronic circuits is highly desirable because it will enable many otherwise unattainable capabilities. However, direct growth of III-V thin film on silicon substrates has been very challenging because of large mismatches in lattice constants and thermal coefficients. Furthermore, the high epitaxial growth temperature is detrimental to transistor performance. Here, we present a detailed studies on a novel growth mode which yields a catalyst-free (Al,In)GaAs nanopillar laser on a silicon substrate by metal-organic chemical vapor deposition at the low temperature of 400 °C. We study the growth and misfit stress relaxation mechanism by cutting through the center of the InGaAs/GaAs nanopillars using focused ion beam and inspecting with high-resolution transmission electron microscopy. The bulk material of the nanopillar is in pure wurtzite crystal phase, despite the 6% lattice mismatch with the substrate, with all stacking disorders well confined in the bottom-most transition region and terminated horizontally. Furthermore, InGaAs was found to be in direct contact with silicon, in agreement with the observed crystal orientation alignment and good electrical conduction across the interface. This is in sharp contrast to many III-V nanowires on silicon which are observed to stem from thin SiN(x), SiO(2), or SiO(2)/Si openings. In addition, GaAs was found to grow perfectly as a shell layer on In(0.2)Ga(0.8)As with an extraordinary thickness, which is 15 times greater than the theoretical thin-film critical thickness for a 1.5% lattice mismatch. This is attributed to the core-shell radial geometry allowing the outer layers to expand and release the strain due to lattice mismatch. The findings in this study redefine the rules for lattice-mismatched growth on heterogeneous substrates and device structure design.

Nanoscale self-assembly offers a pathway to realize heterogeneous integration of III-V materials on silicon. However, for III-V nanowires directly grown on silicon, dislocation-free single-crystal quality could only be attained below certain critical dimensions. We recently reported a new approach that overcomes this size constraint, demonstrating the growth of single-crystal InGaAs/GaAs and InP nanoneedles with the base diameters exceeding 1 μm. Here, we report distinct optical characteristics of InP nanoneedles which are varied from mostly zincblende, zincblende/wurtzite-mixed, to pure wurtzite crystalline phase. We achieved, for the first time, pure single-crystal wurtzite-phase InP nanoneedles grown on silicon with bandgaps of 80 meV larger than that of zincblende-phase InP. Being able to attain excellent material quality while scaling up in size promises outstanding device performance of these nanoneedles. At room temperature, a high internal quantum efficiency of 25% and optically pumped lasing are demonstrated for single nanoneedle as-grown on silicon substrate. Recombination dynamics proves the excellent surface quality of the InP nanoneedles, which paves the way toward achieving multijunction photovoltaic cells, long-wavelength heterostructure lasers, and advanced photonic integrated circuits.

Hydrogen production via water splitting is considered to be one of the most promising technologies in the future hydrogen economy, where the critical challenge in this technology is exploring high-efficient and cost-effective electrocatalysts. Currently, extensive works from both experimental and theoretical investigations have shown that two-dimensional (2D) layered materials can be highly energetic electrocatalysts for electrically driven hydrogen production. Herein, recent progress in 2D layered materials such as electrocatalysts, including graphene, graphitic carbon nitride, transitional-metal dichalcogenides, and MXenes for the hydrogen evolution reaction (HER) will be systematically discussed and summarized. This review provides a broad overview on a wide range of strategies to design and fabricate 2D layered materials as electrocatalysts with high-catalytic performance for HER and aims to give potential avenues for the design of catalysts for commercial applications. In addition, the key scientific issues to address the 2D layered materials as HER catalysts are highlighted, and the perspective on the future development is given at the end.

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