Ultrahigh Responsivity-Bandwidth Product in a Compact InP Nanopillar Phototransistor Directly Grown on Silicon.

Highly sensitive and fast photodetectors can enable low power, high bandwidth on-chip optical interconnects for silicon integrated electronics. III-V compound semiconductor direct-bandgap materials with high absorption coefficients are particularly promising for photodetection in energy-efficient optical links because of the potential to scale down the absorber size, and the resulting capacitance and dark current, while maintaining high quantum efficiency. We demonstrate a compact bipolar junction phototransistor with a high current gain (53.6), bandwidth (7 GHz) and responsivity (9.5 A/W) using a single crystalline indium phosphide nanopillar directly grown on a silicon substrate. Transistor gain is obtained at sub-picowatt optical power and collector bias close to the CMOS line voltage. The quantum efficiency-bandwidth product of 105 GHz is the highest for photodetectors on silicon. The bipolar junction phototransistor combines the receiver front end circuit and absorber into a monolithic integrated device, eliminating the wire capacitance between the detector and first amplifier stage.

Illumination Angle Insensitive Single Indium Phosphide Tapered Nanopillar Solar Cell.

Low cost, high efficiency photovoltaic can help accelerate the adoption of solar energy. Using tapered indium phosphide nanopillars grown on a silicon substrate, we demonstrate a single nanopillar photovoltaic exhibiting illumination angle insensitive response. The photovoltaic employs a novel regrown core-shell p-i-n junction to improve device performance by eliminating shunt current paths, resulting in a high VOC of 0.534 V and a power conversion efficiency of 19.6%. Enhanced broadband light absorption is also demonstrated over a wide spectral range of 400-800 nm.

Valence band splitting in wurtzite InGaAs nanoneedles studied by photoluminescence excitation spectroscopy.

We use low-temperature microphotoluminescence and photoluminescence excitation spectroscopy to measure the valence band parameters of single wurtzite InGaAs nanoneedles. The effective indium composition is measured by means of polarization-dependent Raman spectroscopy. We find that the heavy-hole and light-hole splitting is ∼95 meV at 10 K and the Stokes shift is in the range of 35-55 meV. These findings provide important insight in the band structure of wurtzite InGaAs that could be used for future bandgap engineering.

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.

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.

GaAs-based nanoneedle light emitting diode and avalanche photodiode monolithically integrated on a silicon substrate.

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.

THz-bandwidth tunable slow light in semiconductor optical amplifiers.

We report tunable fractional delays of 250% for 700 fs pulses propagating in a 1.55 mum semiconductor optical amplifier at room temperature. This large fractional delay is attributed to a spectral hole created by the propagating pulses for pulses with duration shorter than the carrier heating relaxation time. Delay can be tuned electrically by adjusting the current with low amplitude variation across the tuning range.