Very low excess noise Al0.75Ga0.25As0.56Sb0.44 avalanche photodiode.

AlxGa1-xAsySb1-y grown lattice-matched to InP has attracted significant research interest as a material for low noise, high sensitivity avalanche photodiodes (APDs) due to its very dissimilar electron and hole ionization coefficients, especially at low electric fields. All work reported to date has been on Al concentrations of x = 0.85 or higher. This work demonstrates that much lower excess noise (F = 2.4) at a very high multiplication of 90 can be obtained in thick Al0.75Ga0.25As0.56Sb0.44 grown on InP substrates. This is the lowest excess noise that has been reported in any III-V APD operating at room temperature. The impact ionization coefficients for both electrons and holes are determined over a wide electric field range (up to 650 kV/cm) from avalanche multiplication measurements undertaken on complementary p-i-n and n-i-p diode structures. While these ionization coefficients can fit the experimental multiplication over three orders of magnitude, the measured excess noise is significantly lower than that expected from the ß/α ratio and the conventional local McIntyre noise theory. These results are of importance not just for the design of APDs but other high field devices, such as transistors using this material.

Room-Temperature Midwavelength Infrared InAsSb Nanowire Photodetector Arrays with Al2O3 Passivation.

Developing uncooled photodetectors at midwavelength infrared (MWIR) is critical for various applications including remote sensing, heat seeking, spectroscopy, and more. In this study, we demonstrate room-temperature operation of nanowire-based photodetectors at MWIR composed of vertical selective-area InAsSb nanowire photoabsorber arrays on large bandgap InP substrate with nanoscale plasmonic gratings. We accomplish this by significantly suppressing the nonradiative recombination at the InAsSb nanowire surfaces by introducing ex situ conformal Al2O3 passivation shells. Transient simulations estimate an extremely low surface recombination velocity on the order of 103 cm/s. We further achieve room-temperature photoluminescence emission from InAsSb nanowires, spanning the entire MWIR regime from 3 to 5 µm. A dry-etching process is developed to expose only the top nanowire facets for metal contacts, with the sidewalls conformally covered by Al2O3 shells, allowing for a higher internal quantum efficiency. Based on these techniques, we fabricate nanowire photodetectors with an optimized pitch and diameter and demonstrate room-temperature spectral response with MWIR detection signatures up to 3.4 µm. The results of this work indicate that uncooled focal plane arrays at MWIR on low-cost InP substrates can be designed with nanostructured absorbers for highly compact and fully integrated detection platforms.

InGaAs-GaAs Nanowire Avalanche Photodiodes Toward Single-Photon Detection in Free-Running Mode.

Single-photon detection at near-infrared (NIR) wavelengths is critical for light detection and ranging (LiDAR) systems used in imaging technologies such as autonomous vehicle trackers and atmospheric remote sensing. Portable, high-performance LiDAR relies on silicon-based single-photon avalanche diodes (SPADs) because of their extremely low dark count rate (DCR) and afterpulsing probability, but their operation wavelengths are typically limited up to 905 nm. Although InGaAs-InP SPADs offer an alternative platform to extend the operation wavelengths to eye-safe ranges, their high DCR and afterpulsing severely limit their commercial applications. Here we propose a new separate absorption and multiplication avalanche photodiode (SAM-APD) platform composed of vertical InGaAs-GaAs nanowire arrays for single-photon detection. Among a total of 4400 nanowires constituting one photodiode, each avalanche event is confined in a single nanowire, which means that the avalanche volume and the number of filled traps can be drastically reduced in our approach. This leads to an extremely small afterpulsing probability compared with conventional InGaAs-based SPADs and enables operation in free-running mode. We show a DCR below 10 Hz, due to reduced fill factor, with photon count rates of 7.8 MHz and timing jitter less than 113 ps, which suggest that nanowire-based NIR focal plane arrays for single-photon detection can be designed without active quenching circuitry that severely restricts pixel density and portability in NIR commercial SPADs. Therefore, the proposed work based on vertical nanowires provides a new degree of freedom in designing avalanche photodetectors and could be a stepping stone for high-performance InGaAs SPADs.

High-efficiency ultrafast optical-to-electrical converters based on InAs nanowire-plasmonic arrays.

There has been a growing interest in developing high-efficiency ultrafast optical-to-electrical converters for advanced imaging and sensing applications. Here, we propose a three-dimensional (3D) plasmonic platform based on InAs nanowire arrays with self-assembled gold gratings, which converts a telecom-wavelength (1550 nm) optical beam to sub-picosecond current pulses with quantum efficiency up to 18.3%, while operating in photovoltaic mode, i.e., at zero bias. Using a comprehensive 3D photoresponse model, we reveal that the incident photons form tightly confined fields near the gratings at nanowire tips, and thus a majority of the photogenerated carriers are efficiently routed to the metal within a few tens of nanometers distance, resulting in ultrafast current pulses. In addition, we show that the amplitude of current pulses is robust to the nanowire surface quality and can be effectively tuned by varying the doping levels in nanowires. This work paves a way to realizing a low-power, highly compact, and low-cost device scheme for ultrafast pulse generation.

Feasibility of achieving high detectivity at short- and mid-wavelength infrared using nanowire-plasmonic photodetectors with p-n heterojunctions.

Photodetection at short- and mid-wavelength infrared (SWIR and MWIR) enables various sensing systems used in heat seeking, night vision, and spectroscopy. As a result, uncooled photodetection at these wavelengths is in high demand. However, these SWIR and MWIR photodetectors often suffer from high dark current, causing them to require bulky cooling accessories for operation. In this study, we argue for the feasibility of improving the room-temperature detectivity by significantly suppressing dark current. To realize this, we propose using (1) a nanowire-based platform to reduce the photoabsorber volume, which in turn reduces trap state population and hence generation-recombination current, and (2) p-n heterojunctions to prevent minority carrier diffusion from the large bandgap substrate into the nanowire absorber. We prove these concepts by demonstrating a comprehensive three-dimensional photoresponse model to explore the level of detectivity offered by vertical InAs(Sb) nanowire photodetector arrays with self-assembled plasmonic gratings. The resultant electrical simulations show that the dark current can be reduced by three to four orders at room temperature, leading to a peak detectivity greater than 3.5 × 1010 cm Hz1/2 W-1 within the wavelength regime of 2.0-3.4 µm, making it comparable to the best commercial and research InAs p-i-n homojunction photodiodes. In addition, we show that the plasmonic resonance peaks can be easily tuned by simply varying the exposed nanowire height. Finally, we investigate the impact of nanowire material properties, such as carrier mobility and carrier lifetime, on the nanowire photodetector detectivity. This work provides a roadmap for the electrical design of nanowire optoelectronic devices and stimulates further experimental validation for uncooled photodetectors at SWIR and MWIR.

Uncooled Photodetector at Short-Wavelength Infrared Using InAs Nanowire Photoabsorbers on InP with p- n Heterojunctions.

In this work, we demonstrate an InAs nanowire photodetector at short-wavelength infrared (SWIR) composed of vertically oriented selective-area InAs nanowire photoabsorber arrays on InP substrates, forming InAs-InP heterojunctions. We measure a rectification ratio greater than 300 at room temperature, which indicates a desirable diode performance. The dark current density, normalized to the area of nanowire heterojunctions, is 130 mA/cm2 at a temperature of 300 K and a reverse bias of 0.5 V, making it comparable to the state-of-the-art bulk InAs p- i- n photodiodes. An analysis of the Arrhenius plot of the dark current at reverse bias yields an activation energy of 175 meV from 190 to 300 K, suggesting that the Shockley-Read-Hall (SRH) nonradiative current is the primary contributor to the dark current. By using three-dimensional electrical simulations, we determine that the SRH nonradiative current originates from the acceptor-like surface traps at the nanowire-passivation heterointerfaces. The spectral response at room temperature is also measured, with a clear photodetection signature observed at wavelengths up to 2.5 µm. This study provides an understanding of dark current for small band gap selective-area nanowires and paves the way to integrate these improved nanostructured photoabsorbers on large band gap substrates for high-performance photodetectors at SWIR.

Catalyst-free selective-area epitaxy of GaAs nanowires by metal-organic chemical vapor deposition using triethylgallium.

We demonstrate catalyst-free growth of GaAs nanowires by selective-area metal-organic chemical vapor deposition (MOCVD) on GaAs and silicon substrates using a triethylgallium (TEGa) precursor. Two-temperature growth of GaAs nanowires-nucleation at low temperature followed by nanowire elongation at high temperature-almost completely suppresses the radial overgrowth of nanowires on GaAs substrates while exhibiting a vertical growth yield of almost 100%. A 100% growth yield is also achieved on silicon substrates by terminating Si(111) surfaces by arsenic prior to the nanowire growth and optimizing the growth temperature. Compared with trimethylgallium (TMGa) which has been exclusively employed in the vapor-solid phase growth of GaAs nanowires by MOCVD, the proposed growth technique using TEGa is advantageous because of lower growth temperature and fully suppressed radial overgrowth. It is also known that GaAs grown by TEGa induce less impurity incorporation compared with TMGa, and therefore the proposed method could be a building block for GaAs nanowire-based high-performance optoelectronic and nanoelectronic devices on both III-V and silicon platforms.

A three-dimensional insight into correlation between carrier lifetime and surface recombination velocity for nanowires.

The performance of nanowire-based devices is predominantly affected by nonradiative recombination on their surfaces, or sidewalls, due to large surface-to-volume ratios. A common approach to quantitatively characterize surface recombination is to implement time-resolved photoluminescence to correlate surface recombination velocity with measured minority carrier lifetime by a conventional analytical equation. However, after using numerical simulations based on a three-dimensional (3D) transient model, we assert that the correlation between minority carrier lifetime and surface recombination velocity is dependent on a more complex combination of factors, including nanowire geometry, energy-band alignment, and spatial carrier diffusion in 3D. To demonstrate this assertion, we use three cases-GaAs nanowires, InGaAs nanowires, and InGaAs inserts embedded in GaAs nanowires-and numerically calculate the carrier lifetimes by varying the surface recombination velocities. Using this information, we then investigate the intrinsic carrier dynamics within those 3D structures. We argue that the conventional analytical approach to determining surface recombination in nanowires is of limited applicability, and that a comprehensive computation in 3D can provide more accurate analysis. Our study provides a solid theoretical foundation to further understand surface characteristics and carrier dynamics for 3D nanostructured materials.

Telecom-Wavelength Bottom-up Nanobeam Lasers on Silicon-on-Insulator.

Semiconductor nanowire lasers are considered promising ultracompact and energy-efficient light sources in the field of nanophotonics. Although the integration of nanowire lasers onto silicon photonic platforms is an innovative path toward chip-scale optical communications and photonic integrated circuits, operating nanowire lasers at telecom-wavelengths remains challenging. Here, we report on InGaAs nanowire array lasers on a silicon-on-insulator platform operating up to 1440 nm at room temperature. Bottom-up photonic crystal nanobeam cavities are formed by growing nanowires as ordered arrays using selective-area epitaxy, and single-mode lasing by optical pumping is demonstrated. We also show that arrays of nanobeam lasers with individually tunable wavelengths can be integrated on a single chip by the simple adjustment of the lithographically defined growth pattern. These results exemplify a practical approach toward nanowire lasers for silicon photonics.

Diode Characteristics Approaching Bulk Limits in GaAs Nanowire Array Photodetectors.

We present the electrical properties of p-n junction photodetectors comprised of vertically oriented p-GaAs nanowire arrays on the n-GaAs substrate. We measure an ideality factor as low as n = 1.0 and a rectification ratio >108 across all devices, with some >109, comparable to the best GaAs thin film photodetectors. An analysis of the Arrhenius plot of the saturation current yields an activation energy of 690 meV-approximately half the bandgap of GaAs-indicating generation-recombination current from midgap states is the primary contributor to the leakage current at low bias. Using fully three-dimensional electrical simulations, we explain the lack of a recombination current dominated regime at low forward bias, as well as some of the issues related to analysis of the capacitance-voltage characteristics of nanowire devices. This work demonstrates that, through proper design and fabrication, nanowire-based devices can perform as well as their bulk device counterparts.

Monolithic InGaAs Nanowire Array Lasers on Silicon-on-Insulator Operating at Room Temperature.

Chip-scale integrated light sources are a crucial component in a broad range of photonics applications. III-V semiconductor nanowire emitters have gained attention as a fascinating approach due to their superior material properties, extremely compact size, and capability to grow directly on lattice-mismatched silicon substrates. Although there have been remarkable advances in nanowire-based emitters, their practical applications are still in the early stages due to the difficulties in integrating nanowire emitters with photonic integrated circuits. Here, we demonstrate for the first time optically pumped III-V nanowire array lasers monolithically integrated on silicon-on-insulator (SOI) platform. Selective-area growth of InGaAs/InGaP core/shell nanowires on an SOI substrate enables the nanowire array to form a photonic crystal nanobeam cavity with superior optical and structural properties, resulting in the laser to operate at room temperature. We also show that the nanowire array lasers are effectively coupled with SOI waveguides by employing nanoepitaxy on a prepatterned SOI platform. These results represent a new platform for ultracompact and energy-efficient optical links and unambiguously point the way toward practical and functional nanowire lasers.

Monolithically Integrated InGaAs Nanowires on 3D Structured Silicon-on-Insulator as a New Platform for Full Optical Links.

Monolithically integrated III-V semiconductors on a silicon-on-insulator (SOI) platform can be used as a building block for energy-efficient on-chip optical links. Epitaxial growth of III-V semiconductors on silicon, however, has been challenged by the large mismatches in lattice constants and thermal expansion coefficients between epitaxial layers and silicon substrates. Here, we demonstrate for the first time the monolithic integration of InGaAs nanowires on the SOI platform and its feasibility for photonics and optoelectronic applications. InGaAs nanowires are grown not only on a planar SOI layer but also on a 3D structured SOI layer by catalyst-free metal-organic chemical vapor deposition. The precise positioning of nanowires on 3D structures, including waveguides and gratings, reveals the versatility and practicality of the proposed platform. Photoluminescence measurements exhibit that the composition of ternary InGaAs nanowires grown on the SOI layer has wide tunability covering all telecommunication wavelengths from 1.2 to 1.8 µm. We also show that the emission from an optically pumped single nanowire is effectively coupled and transmitted through an SOI waveguide, explicitly showing that this work lays the foundation for a new platform toward energy-efficient optical links.

High Quantum Efficiency Nanopillar Photodiodes Overcoming the Diffraction Limit of Light.

InAs1-xSbx nanowires have recently attracted interest for infrared sensing applications due to the small bandgap and high thermal conductivity. However, previous reports on nanowire-based infrared sensors required low operating temperatures in order to mitigate the high dark current and have shown poor sensitivities resulting from reduced light coupling efficiency beyond the diffraction limit. Here, InAsSb nanopillar photodiodes with high quantum efficiency are achieved by partially coating the nanopillar with metal that excites localized surface plasmon resonances, leading to quantum efficiencies of ∼29% at 2390 nm. These high quantum efficiency nanopillar photodiodes, with 180 nm diameters and 1000 nm heights, allow operation at temperatures as high as 220 K and exhibit a detection wavelength up to 3000 nm, well beyond the diffraction limit. The InAsSb nanopillars are grown on low cost GaAs (111)B substrates using an InAs buffer layer, making our device architecture a promising path toward low-cost infrared focal plane arrays with high operating temperature.

Comparative study of photoluminescence from In0.3Ga0.7As/GaAs surface and buried quantum dots.

The optical properties of In0.3Ga0.7As/GaAs surface quantum dots (SQDs) and buried QDs (BQDs) are investigated by photoluminescence (PL) measurements. The integrated PL intensity, linewidth, and lifetime of SQDs are significantly different from the BQDs both at room temperature and at low temperature. The differences in PL response, measured at both steady state and in transient, are attributed to carrier transfer between the surface states and the SQDs.

High-Quality InAsSb Nanowires Grown by Catalyst-Free Selective-Area Metal-Organic Chemical Vapor Deposition.

We report on the first demonstration of InAs1-xSbx nanowires grown by catalyst-free selective-area metal-organic chemical vapor deposition (SA-MOCVD). Antimony composition as high as 15 % is achieved, with strong photoluminescence at all compositions. The quality of the material is assessed by comparing the photoluminescence (PL) peak full-width at half-max (fwhm) of the nanowires to that of epitaxially grown InAsSb thin films on InAs. We find that the fwhm of the nanowires is only a few meV broader than epitaxial films, and a similar trend of relatively constant fwhm for increasing antimony composition is observed. Furthermore, the PL peak energy shows a strong dependence on temperature, suggesting wave-vector conserving transitions are responsible for the observed PL in spite of lattice mismatched growth on InAs substrate. This study shows that high-quality InAsSb nanowires can be grown by SA-MOCVD on lattice mismatched substrate, resulting in material suitable for infrared detectors and high-performance nanoelectronic devices.

Axial diffusion barriers in near-infrared nanopillar LEDs.

The growth of GaAs/GaAsP axial heterostructures is demonstrated and implemented as diffusion current barriers in nanopillar light-emitting diodes at near-infrared wavelengths. The nanopillar light-emitting diodes utilize an n-GaAs/i-InGaAs/p-GaAs axial heterostructure for current injection. Axial GaAsP segments are inserted into the n- and p-GaAs portions of the nanopillars surrounding the InGaAs emitter region, acting as diffusion barriers to provide enhanced carrier confinement. Detailed characterization of growth of the GaAsP inserts and electronic band-offset measurements are used to effectively implement the GaAsP inserts as diffusion barriers. The implementation of these barriers in nanopillar light-emitting diodes provides a 5-fold increase in output intensity, making this a promising approach to high-efficiency pillar-based emitters in the near-infrared wavelength range.

Improved quantum dot stacking for intermediate band solar cells using strain compensation.

We use thin tensile-strained AlAs layers to manage compressive strain in stacked layers of InAs/AlAsSb quantum dots (QDs). The AlAs layers allow us to reduce residual strain in the QD stacks, suppressing strain-related defects. AlAs layers 2.4 monolayers thick are sufficient to balance the strain in the structures studied, in agreement with theory. Strain balancing improves material quality and helps increase QD uniformity by preventing strain accumulation and ensuring that each layer of InAs experiences the same strain. Stacks of 30 layers of strain-balanced QDs exhibit carrier lifetimes as long as 9.7 ns. QD uniformity is further enhanced by vertical ABAB… ordering of the dots in successive layers. Strain compensated InAs/AlAsSb QD stacks show great promise for intermediate band solar cell applications.

Direct-bandgap epitaxial core-multishell nanopillar photovoltaics featuring subwavelength optical concentrators.

Semiconductor nanomaterials have recently fueled numerous photonic scientific fields. Arrays of nanopillars (NPs) have been examined by the photovoltaic (PV) community as highly efficient solar absorbers, with potential material/cost reductions compared to planar architectures. Despite modeled predictions, experimental efficiencies are limited by surface recombination and poor light management, once integrated in a practical PV device. In this Letter, we correlate optoelectronic modeling with experimental results for direct-bandgap arrays of core-multishell GaAs NPs grown by selective area, catalyst-free epitaxy and capped by epitaxial window layers, with efficiencies of 7.43%. Electrically, improved open-circuit voltages are yet partly affected by residual surface state density after epitaxial passivation. Optically, dome-shaped indium-tin-oxide (ITO) top electrode functions as a two-dimensional (2-D) periodic array of subwavelength lenses that focus the local density of optical states within the NP active volume. These devices provide a path to high-efficiency NP-based PVs by synergistically controlling the heteroepitaxy and light management of the final structure.

Thin 3D multiplication regions in plasmonically enhanced nanopillar avalanche detectors.

We demonstrate a nanopillar (NP) device structure for implementing plasmonically enhanced avalanche photodetector arrays with thin avalanche volumes (∼ 310 nm × 150 nm × 150 nm). A localized 3D electric field due to a core-shell PN junction in a NP acts as a multiplication region, while efficient light absorption takes place via surface plasmon polariton Bloch wave (SPP-BW) modes due to a self-aligned metal nanohole lattice. Avalanche gains of ∼216 at 730 nm at -12 V are obtained. We show through capacitance-voltage characterization, temperature-dependent breakdown measurements, and detailed device modeling that the avalanche region is on the order of the ionization path length, such that dead-space effects become significant. This work presents a clear path toward engineering dead space effects in thin 3D-confined multiplication regions for high performance avalanche detectors for applications in telecommunications, sensing and single photon detection.

Three-dimensional core-shell hybrid solar cells via controlled in situ materials engineering.

Three-dimensional core-shell organic-inorganic hybrid solar cells with tunable properties are demonstrated via electropolymerization. Air-stable poly(3,4-ethylenedioxythiophene) (PEDOT) shells with controlled thicknesses are rapidly coated onto periodic GaAs nanopillar arrays conformally, preserving the vertical 3D structure. The properties of the organic layer can be readily tuned in situ, allowing for (1) the lowering of the highest occupied molecular orbital level (|ΔE| ∼ 0.28 eV), leading to the increase of open-circuit voltage (V(OC)), and (2) an improvement in PEDOT conductivity that results in enhanced short-circuit current densities (J(SC)). The incorporation of various anionic dopants in the polymer during the coating process also enables the tailoring of the polymer/semiconductor interface transport properties. Systematic tuning of the device properties results in a J(SC) of 13.6 mA cm(-2), V(OC) of 0.63 V, peak external quantum efficiency of 58.5%, leading to a power conversion efficiencies of 4.11%.