Room-temperature Fabry-Perot resonances in suspended InGaAs/InP quantum-well nanopillars on a silicon substrate.

We present a new platform based on suspended III-V semiconductor nanopillars for direct integration of optoelectronic devices on a silicon substrate. Nanopillars grown in core-shell mode with InGaAs/InP quantum wells can support long-wavelength Fabry-Pérot resonances at room temperature with this novel configuration. Experimental results are demonstrated at a silicon-transparent wavelength of 1460 nm, facilitating integration with silicon platform.

Wurtzite-Phased InP Micropillars Grown on Silicon with Low Surface Recombination Velocity.

The direct growth of III-V nanostructures on silicon has shown great promise in the integration of optoelectronics with silicon-based technologies. Our previous work showed that scaling up nanostructures to microsize while maintaining high quality heterogeneous integration opens a pathway toward a complete photonic integrated circuit and high-efficiency cost-effective solar cells. In this paper, we present a thorough material study of novel metastable InP micropillars monolithically grown on silicon, focusing on two enabling aspects of this technology-the stress relaxation mechanism at the heterogeneous interface and the microstructure surface quality. Aberration-corrected transmission electron microscopy studies show that InP grows directly on silicon without any amorphous layer in between. A set of periodic dislocations was found at the heterointerface, relaxing the 8% lattice mismatch between InP and Si. Single crystalline InP therefore can grow on top of the fully relaxed template, yielding high-quality micropillars with diameters expanding beyond 1 µm. An interesting power-dependence trend of carrier recombination lifetimes was captured for these InP micropillars at room temperature, for the first time for micro/nanostructures. By simply combining internal quantum efficiency with carrier lifetime, we revealed the recombination dynamics of nonradiative and radiative portions separately. A very low surface recombination velocity of 1.1 × 10(3) cm/sec was obtained. In addition, we experimentally estimated the radiative recombination B coefficient of 2.0 × 10(-10) cm(3)/sec for pure wurtzite-phased InP. These values are comparable with those obtained from InP bulk. Exceeding the limits of conventional nanowires, our InP micropillars combine the strengths of both nanostructures and bulk materials and will provide an avenue in heterogeneous integration of III-V semiconductor materials onto silicon platforms.

Composition homogeneity in InGaAs/GaAs core-shell nanopillars monolithically grown on silicon.

Alloy composition homogeneity plays an important role in the device performance of III-V heterostructures. In this work, we study the spatial composition uniformity of n-In0.12Ga0.88As/i-In0.2Ga0.8As/p-GaAs core-shell nanopillars monolithically grown on silicon. Cross sections extracted along the axial and radial directions are examined with transmission electron microscopy and energy-dispersive X-ray spectroscopy. Interestingly, indium-deficient segments with width ∼5 nm are observed to develop along the radial ⟨112̅0⟩ directions in the InGaAs layers. We attribute this spontaneous ordering to capillarity effect and difference in group-III adatom diffusion lengths. The slight fluctuation in indium content (∼4%), however, does not induce any noticeable misfit defects in the pure wurtzite-phased crystal. In contrast, the heterostructure exhibits excellent alloy composition uniformity along the axial [0001] direction. Furthermore, abrupt transitions of gallium and indium are seen at the heterointerfaces. These remarkable properties give rise to extraordinary optical performances. Lasing is achieved in the core-shell nanopillars upon optical pump despite the observed alloy composition fluctuation in the radial directions. The results here reveal the potential of the InGaAs-based core-shell heterostructures as efficient optoelectronic devices and high-speed heterojunction transistors directly integrated on silicon.

Metastable growth of pure wurtzite InGaAs microstructures.

III-V compound semiconductors can exist in two major crystal phases, namely, zincblende (ZB) and wurtzite (WZ). While ZB is thermodynamically favorable in conventional III-V epitaxy, the pure WZ phase can be stable in nanowires with diameters smaller than certain critical values. However, thin nanowires are more vulnerable to surface recombination, and this can ultimately limit their performances as practical devices. In this work, we study a metastable growth mechanism that can yield purely WZ-phased InGaAs microstructures on silicon. InGaAs nucleates as sharp nanoneedles and expand along both axial and radial directions simultaneously in a core-shell fashion. While the base can scale from tens of nanometers to over a micron, the tip can remain sharp over the entire growth. The sharpness maintains a high local surface-to-volume ratio, favoring hexagonal lattice to grow axially. These unique features lead to the formation of microsized pure WZ InGaAs structures on silicon. To verify that the WZ microstructures are truly metastable, we demonstrate, for the first time, the in situ transformation from WZ to the energy-favorable ZB phase inside a transmission electron microscope. This unconventional core-shell growth mechanism can potentially be applied to other III-V materials systems, enabling the effective utilization of the extraordinary properties of the metastable wurtzite crystals.

Nanophotonic integrated circuits from nanoresonators grown on silicon.

Harnessing light with photonic circuits promises to catalyse powerful new technologies much like electronic circuits have in the past. Analogous to Moore's law, complexity and functionality of photonic integrated circuits depend on device size and performance scale. Semiconductor nanostructures offer an attractive approach to miniaturize photonics. However, shrinking photonics has come at great cost to performance, and assembling such devices into functional photonic circuits has remained an unfulfilled feat. Here we demonstrate an on-chip optical link constructed from InGaAs nanoresonators grown directly on a silicon substrate. Using nanoresonators, we show a complete toolkit of circuit elements including light emitters, photodetectors and a photovoltaic power supply. Devices operate with gigahertz bandwidths while consuming subpicojoule energy per bit, vastly eclipsing performance of prior nanostructure-based optoelectronics. Additionally, electrically driven stimulated emission from an as-grown nanostructure is presented for the first time. These results reveal a roadmap towards future ultradense nanophotonic integrated circuits.

High brightness InP micropillars grown on silicon with Fermi level splitting larger than 1 eV.

The growth of III-V nanowires on silicon is a promising approach for low-cost, large-scale III-V photovoltaics. However, performances of III-V nanowire solar cells have not yet been as good as their bulk counterparts, as nanostructured light absorbers are fundamentally challenged by enhanced minority carriers surface recombination rates. The resulting nonradiative losses lead to significant reductions in the external spontaneous emission quantum yield, which, in turn, manifest as penalties in the open-circuit voltage. In this work, calibrated photoluminescence measurements are utilized to construct equivalent voltage-current characteristics relating illumination intensities to Fermi level splitting ΔF inside InP microillars. Under 1 sun, we show that splitting can exceed ΔF ∼ 0.90 eV in undoped pillars. This value can be increased to values of ΔF ∼ 0.95 eV by cleaning pillar surfaces in acidic etchants. Pillars with nanotextured surfaces can yield splitting of ΔF ∼ 0.90 eV, even though they exhibit high densities of stacking faults. Finally, by introducing n-dopants, ΔF of 1.07 eV can be achieved due to a wider bandgap energy in n-doped wurzite InP, the higher brightness of doped materials, and the extraordinarily low surface recombination velocity of InP. This is the highest reported value for InP materials grown on a silicon substrate. These results provide further evidence that InP micropillars on silicon could be a promising material for low-cost, large-scale solar cells with high efficiency.

Single crystalline InGaAs nanopillar grown on polysilicon with dimensions beyond the substrate grain size limit.

Monolithic integration of III-V optoelectronic devices with materials for various functionalities inexpensively is always desirable. Polysilicon (poly-Si) is an ideal platform because it is dopable and semiconducting, and can be deposited and patterned easily on a wide range of low cost substrates. However, the lack of crystalline coherency in poly-Si poses an immense challenge for high-quality epitaxial growth. In this work, we demonstrate, for the first time, direct growth of micrometer-sized InGaAs/GaAs nanopillars on polysilicon. Transmission electron microscopy shows that the micrometer-sized pillars are single-crystalline with pure wurzite-phase, far exceeding the substrate crystal grain size ~100 nm. The high quality growth is enabled by the unique tapering geometry at the base of the nanostructure, which reduces the effective InGaAs/Si contact area to <40 nm in diameter. The small footprint not only reduces stress due to lattice mismatch but also prevents the nanopillar from nucleating on multiple Si crystal grains. This relaxes the grain size requirement for poly-Si, potentially reducing the cost for poly-Si deposition. Lasing is achieved in the as-grown pillars under optical pumping, attesting their excellent crystalline and optical quality. These promising results open up a pathway for low-cost synergy of optoelectronics with other technologies such as CMOS integrated circuits, sensing, nanofluidics, thin film transistor display, photovoltaics, and so forth.

Unconventional growth mechanism for monolithic integration of III-V on silicon.

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.

Nanolasers grown on silicon-based MOSFETs.

We report novel indium gallium arsenide (InGaAs) nanopillar lasers that are monolithically grown on (100)-silicon-based functional metal-oxide-semiconductor field effect transistors (MOSFETs) at low temperature (410 °C). The MOSFETs maintain their performance after the nanopillar growth, providing a direct demonstration of complementary metal-oxide-semiconudctor (CMOS) compatibility. Room-temperature operation of optically pumped lasers is also achieved. To our knowledge, this is the first time that monolithically integrated lasers and transistors have been shown to work on the same silicon chip, serving as a proof-of-concept that such integration can be extended to more complicated CMOS integrated circuits.

Planar high-numerical-aperture low-loss focusing reflectors and lenses using subwavelength high contrast gratings.

We propose planar, high numerical aperture (NA), low loss, focusing reflectors and lenses using subwavelength high contrast gratings (HCGs). By designing the reflectance and the phase of non-periodic HCGs, both focusing reflectors and lenses can be constructed. Numerical aperture values as high as 0.81 and 0.96 are achieved for a reflector and lens with very low losses of 0.3 and 0.2 dB, respectively. The design algorithm is also shown to be readily extended to a 2D lens. Furthermore, HCG optics can simultaneously focus the reflected and transmitted waves, with important technological implications. HCG focusing optics are defined by one-step photolithography and thus can be readily integrated with many devices including VCSELs, saturable absorbers, telescopes, CCDs and solar cells.