Photonic Nanobeam Cavities with Nanopockets for Efficient Integration of Fluorescent Nanoparticles.

Integrating fluorescent nanoparticles with high-Q, small mode volume cavities is indispensable for nanophotonics and quantum technologies. To date, nanoparticles have largely been coupled to evanescent fields of cavity modes, which limits the strength of the interaction. Here, we developed both a cavity design and a fabrication method that enable efficient coupling between a fluorescent nanoparticle and a cavity optical mode. The design consists of a fishbone-shaped, one-dimensional photonic crystal cavity with a nanopocket located at the electric field maximum of the fundamental optical mode. Furthermore, the presence of a nanoparticle inside the pocket reduces the mode volume substantially and induces subwavelength light confinement. Our approach opens exciting pathways to achieve tight light confinement around fluorescent nanoparticles for applications in energy, sensing, lasing, and quantum technologies.

Multiwavelength Single Nanowire InGaAs/InP Quantum Well Light-Emitting Diodes.

We report multiwavelength single InGaAs/InP quantum well nanowire light-emitting diodes grown by metal organic chemical vapor deposition using selective area epitaxy technique and reveal the complex origins of their electroluminescence properties. We observe that the single InGaAs/InP quantum well embedded in the nanowire consists of three components with different chemical compositions, axial quantum well, ring quantum well, and radial quantum well, leading to the electroluminescence emission with multiple wavelengths. The electroluminescence measurements show a strong dependence on current injection levels as well as temperatures and these are explained by interpreting the equivalent circuits in a minimized area of the device. It is also found that the electroluminescence properties are closely related to the distinctive triangular morphology with an inclined facet of the quantum well nanowire. Our study provides important new insights for further design, growth, and fabrication of high-performance quantum well-based nanowire light sources for a wide range of future optoelectronic and photonic applications.

Bottom-up, Chip-Scale Engineering of Low Threshold, Multi-Quantum-Well Microring Lasers.

Integrated, on-chip lasers are vital building blocks in future optoelectronic and nanophotonic circuitry. Specifically, III-V materials that are of technological relevance have attracted considerable attention. However, traditional microcavity laser fabrication techniques, including top-down etching and bottom-up catalytic growth, often result in undesirable cavity geometries with poor scalability and reproducibility. Here, we utilize the selective area epitaxy method to deterministically engineer thousands of microring lasers on a single chip. Specifically, we realize a catalyst-free, epitaxial growth of a technologically critical material, InAsP/InP, in a ring-like cavity with embedded multi-quantum-well heterostructures. We elucidate a detailed growth mechanism and leverage the capability to deterministically control the adatom diffusion lengths on selected crystal facets to reproducibly achieve ultrasmooth cavity sidewalls. The engineered devices exhibit a tunable emission wavelength in the telecommunication O-band and show low-threshold lasing with over 80% device efficacy across the chip. Our work marks a significant milestone toward the implementation of a fully integrated III-V materials platform for next-generation high-density integrated photonic and optoelectronic circuits.

Ultrathin Ga2 O3 Glass: A Large-Scale Passivation and Protection Material for Monolayer WS2.

Atomically thin transition metal dichalcogenide crystals (TMDCs) have extraordinary optical properties that make them attractive for future optoelectronic applications. Integration of TMDCs into practical all-dielectric heterostructures hinges on the ability to passivate and protect them against necessary fabrication steps on large scales. Despite its limited scalability, encapsulation of TMDCs in hexagonal boron nitride (hBN) currently has no viable alternative for achieving high performance of the final device. Here, it is shown that the novel, ultrathin Ga2 O3 glass is an ideal centimeter-scale coating material that enhances optical performance of the monolayers and protects them against further material deposition. In particular, Ga2 O3 capping of monolayer WS2 outperforms commercial-grade hBN in both scalability and optical performance at room temperature. These properties make Ga2 O3 highly suitable for large-scale passivation and protection of monolayer TMDCs in functional heterostructures.

Facet-dependent growth of InAsP quantum wells in InP nanowire and nanomembrane arrays.

Selective area epitaxy is a powerful growth technique that has been used to produce III-V semiconductor nanowire and nanomembrane arrays for photonic and electronic applications. The incorporation of a heterostructure such as quantum wells (QWs) brings new functionality and further broadens their applications. Using InP nanowires and nanomembranes as templates, we investigate the growth of InAsP QWs on these pure wurtzite nanostructures. InAsP QWs grow both axially and laterally on the nanowires and nanomembranes, forming a zinc blende phase axially and wurtzite phase on the sidewalls. On the non-polar {11[combining macron]00} sidewalls, the radial QW selectively grows on one sidewall which is located at the semi-polar 〈112[combining macron]〉 A side of the axial QW, causing the shape evolution of the nanowires from hexagonal to triangular cross section. For nanomembranes with {11[combining macron]00} sidewalls, the radial QW grows asymmetrically on the {11[combining macron]00} facet, destroying their symmetry. In comparison, nanomembranes with {112[combining macron]0} sidewalls are shown to be an ideal template for the growth of InAsP QWs, thanks to the uniform QW formation. These QWs emit strongly in the near IR region at room temperature and their emission can be tuned by changing their thickness or composition. These findings enrich our understanding of the QW growth, which provides new insights for heterostructure design in other III-V nanostructures.

On the origins of the green luminescence in the "zero-dimensional perovskite" Cs4PbBr6: conclusive results from cathodoluminescence imaging.

There is great interest in the use of highly-efficient all-inorganic halide perovskites CsnPbBr2+n for optoelectronic applications. There however remains considerable debate as to the origins of the green luminescence in the zero-dimensional phase of the perovskite Cs4PbBr6, with theories suggesting it originates either from defects in the Cs4PbBr6 lattice or CsPbBr3 impurities/inclusions. The confusion has arisen due to the two phases being miscible and typically co-existing. Moreover, low impurity levels of CsPbBr3 in Cs4PbBr6 are difficult to detect by XRD measurements, yet have much stronger photoluminescence than bulk CsPbBr3 that exhibits quenching, further contributing to the confusion as to the origins of the green photoluminescence. With the rise of significant debate and misconceptions, we provide conclusive evidence that the green emission from Cs4PbBr6 is indeed due to nanocrystalline CsPbBr3 impurities. This is demonstrated by undertaking cathodoluminescence and EDX measurements on samples prepared mechanochemically by ball-milling. Cathodoluminescence imaging clearly shows the presence of small crystals embedded in/or between larger crystallites of Cs4PbBr6 and they emit around 520 nm. EDX shows that the smaller crystal inclusions have a Pb : Br ratio that is approximately 2 times higher, confirming the CsPbBr3 phase, which has the expected size-dependent shift to shorter wavelengths (about 528 to 515 nm). These studies make significant inroads into understanding these lead halide perovskites for their use in a variety of optoelectronic and photovoltaic applications.

Introduction of TiO2 in CuI for Its Improved Performance as a p-Type Transparent Conductor.

The challenges of making high-performance, low-temperature processed, p-type transparent conductors (TCs) have been the main bottleneck for the development of flexible transparent electronics. Though a few p-type transparent conducting oxides (TCOs) have shown promising results, they need high processing temperature to achieve the required conductivity which makes them unsuitable for organic and flexible electronic applications. Copper iodide is a wide band gap p-type semiconductor that can be heavily doped at low temperature (<100 °C) to achieve conductivity comparable or higher than many of the well-established p-type TCOs. However, as-processed CuI loses its transparency and conductivity with time in an ambient condition which makes them unsuitable for long-term applications. Herein, we propose CuI-TiO2 composite thin films as a replacement of pure CuI. We show that the introduction of TiO2 in CuI makes it more stable in ambient conditions while also improving its conductivity and transparency. A detailed comparative analysis between CuI and CuI-TiO2 composite thin films has been performed to understand the reasons for improved conductivity, transparency, and stability of CuI-TiO2 samples in comparison to pure CuI samples. The enhanced conductivity in CuI-TiO2 stems from the highly conductive space-charge layer formation at the CuI-TiO2 interface, whereas the improved transparency is due to reduced CuI grain growth mobility in the presence of TiO2. The improved stability of CuI-TiO2 in comparison to pure CuI is a result of inhibited recrystallization and grain growth, reduced loss of iodine, and limited oxidation of the CuI phase in the presence of TiO2. For optimized fraction of TiO2, an average transparency of ∼78% (in 450-800 nm region) and a resistivity of 14 mΩ·cm are achieved, while maintaining a relatively high mobility of ∼3.5 cm2 V-1 s-1 with hole concentration reaching as high as 1.3 × 1020 cm-3. Most importantly, this work opens up the possibility to design a new range of p-type transparent conducting materials using the CuI/insulator composite system such as CuI/SiO2, CuI/Al2O3, CuI/SiNx, and so forth.

Shape Engineering of InP Nanostructures by Selective Area Epitaxy.

Greater demand for III-V nanostructures with more sophisticated geometries other than nanowires is expected because of the recent intensive investigation of nanowire networks that show great potential in all-optical logic gates, nanoelectronics, and quantum computing. Here, we demonstrate highly uniform arrays of InP nanostructures with tunable shapes, such as membrane-, prism-, and ring-like shapes, which can be simultaneously grown by selective area epitaxy. Our in-depth investigation of shape evolution confirms that the shape is essentially determined by pattern confinement and the minimization of total surface energy. After growth optimization, all of the different InP nanostructures grown under the same growth conditions show perfect wurtzite structure regardless of the geometry and strong and homogeneous photon emission. This work expands the research field in terms of producing nanostructures with the desired shapes beyond the limits of nanowires to satisfy various requirements for nanoelectronics, optoelectronics, and quantum device applications.

Radial Growth Evolution of InGaAs/InP Multi-Quantum-Well Nanowires Grown by Selective-Area Metal Organic Vapor-Phase Epitaxy.

III-V semiconductor multi-quantum-well nanowires (MQW NWs) via selective-area epitaxy (SAE) is of great importance for the development of nanoscale light-emitting devices for applications such as optical communication, silicon photonics, and quantum computing. To achieve highly efficient light-emitting devices, not only the high-quality materials but also a deep understanding of their growth mechanisms and material properties (structural, optical, and electrical) are extremely critical. In particular, the three-dimensional growth mechanism of MQWs embedded in a NW structure by SAE is expected to be different from that of those grown in a planar structure or with a catalyst and has not yet been thoroughly investigated. In this work, we reveal a distinctive radial growth evolution of InGaAs/InP MQW NWs grown by the SAE metal organic vapor-phase epitaxy (MOVPE) technique. We observe the formation of zinc blende (ZB) QW discs induced by the axial InGaAs QW growth on the wurtzite (WZ) base-InP NW and propose it as the key factor driving the overall structure of radial growth. The role of the ZB-to-WZ change in the driving of the overall growth evolution is supported by a growth formalism, taking into account the formation-energy difference between different facets. Despite a polytypic crystal structure with mixed ZB and WZ phases across the MQW region, the NWs exhibit high uniformity and desirable QW spatial layout with bright room-temperature photoluminescence at an optical communication wavelength of ∼1.3 µm, which is promising for the future development of high-efficiency light-emitting devices.

Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells.

Perovskite material with a bandgap of 1.7-1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation.