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We demonstrate the monolithic integration of plasmonic materials with semiconductor optoelectronic devices. Leveraging highly doped semiconductors as mid-IR plasmonic materials, we show significant performance improvement for both infrared detectors and emitters.

We numerically model and experimentally demonstrate sub-diffraction limited focusing of mid-infrared light using all-semiconductor hyperbolic metamaterial photonic funnels. Enhanced transmission through single funnels with aperture diameter /20 is demonstrated, in excellent agreement with our simulations.

Here we report a massively-parallel Fourier-optics convolutional processor accelerated 160x over spatial-light-modulators using digital-mirror-display technology as input and kernel showing an MNIST and CIFAR-10 accuracy of 96% and 54%, respectively.

InAlAs digital alloy avalanche photodiodes exhibit lower excess noise than those fabricated from conventional random alloy material. Experiment and Monte Carlo simulation both show that relative to the random alloy the ionization probability for electrons is slightly lower while that of holes is greatly suppressed. We propose that the suppression of carrier ionization probability in digital alloys happens because of the creation of minibands that localize carriers. The difference of suppression between conduction bands and valence bands comes from the difference of scattering path.

We demonstrate that plasmonic response can be tuned spectrally via the alloy composition of an epitaxial semimetallic film. Specifically, attenuated total reflectance studies show that the surface plasmon resonance of rare-earth monopnictide LaxLu1–xAs alloy films can be tuned across the mid-IR, while lattice-matching to technologically important substrates. The electrical properties are a strong function of the composition, which, in turn, manifests as tunability of the optical properties. The ability to produce tunable (semi)metals that can be epitaxially integrated with III–V emitters and absorbers is expected to enable a new generation of novel nanophotonic device architectures and functionality.

We report avalanche photodiodes (APDs) fabricated from the digital alloy Al0.8In0.2As0.23Sb0.77 (lattice-matched to GaSb). The APDs exhibit high avalanche multiplication and low excess noise.

We report avalanche photodiodes (APDs) fabricated from high-aluminum-content Al x In 1− x As y Sb1− y lattice matched to GaSb that is grown within the miscibility gap using a digital alloy approach. The material was initially characterized through a series of Al x In1− x As y Sb1− y ( x = 0.3, 0.4, 0.5, 0.6, 0.7) p-i-n structures. In order to achieve operation at telecommunications wavelengths, an Al x In1− x As y Sb 1− y separate absorption, charge, and multiplication APD has been developed. Very low excess noise, as characterized by k ∼0.01−0.05, has been achieved.

BGaAs layers with boron concentrations of 4.1%, 7.4%, and 12.1% are grown by molecular beam epitaxy on a GaP substrate and studied by optical absorption and photoreflectance (PR) spectroscopy with both temperature and hydrostatic pressure dependence. The direct optical transitions from the bands composing the valence band—namely heavy-hole, light-hole, and spin–orbit split-off—to the conduction band are clearly observed in the PR spectra. For the abovementioned optical transitions, their temperature dependencies are obtained in the range from 20 K to 300 K, and analyzed by Varshni and Bose–Einstein relations. Furthermore, the BGaAs alloys are also studied with hydrostatic pressure up to ∼18 kbar, revealing pressure coefficients of direct optical transitions. The obtained results are discussed within the framework of the band anticrossing model and chemical trends.

Background doping polarity is a key parameter in the design of numerous electrical and optoelectronic devices. It is especially critical for avalanche photodiodes (APDs). Recently, high-performance APDs have been demonstrated based on the AlInAsSb digital alloy materials system. A critical element of this work was the determination of the background doping polarity of the molecular beam epitaxial grown wafers. In this work, we determine the unintentional background doping polarity of Al0.7InAsSb using the double mesa capacitance-voltage technique. We fabricated two p-i-n Al0.7InAsSb structures: one with p-type background polarity and the other with n-type. The measurements indicate that devices with different background doping polarities show different capacitance relations to the mesa diameters; moreover, the relationship reverses at high voltage in a p-type background device. Subsequent simulations reveal that this reversal is caused by electrical field confinement after the depletion reaches the smaller top mesa. These findings are consistent with reports of reduced surface leakage current in double and triple mesa structures.

We report an nBn photodetector based on the AlInAsSb digital alloy materials system, which has the advantage of a near-zero valence band offset. These photodetectors have achieved 28% external quantum efficiency, dark current densities of 2.6 × 10−3 A/cm2 at 300 K and 1.8 × 10−9 A/cm2 at 100 K with −0.5 V bias, and detectivity of 1.7 × 1010 Jones at room temperature under 2 μm wavelength illumination.

As the focus of applied research in topological insulators (TI) evolves, the need to synthesize large-area TI films for practical device applications takes center stage. However, constructing scalable and adaptable processes for high-quality TI compounds remains a challenge. To this end, a versatile van der Waals epitaxy (vdWE) process for custom-feature Bismuth Telluro-Sulfide TI growth and fabrication is presented, achieved through selective-area fluorination and modification of surface free-energy on mica. The TI features grow epitaxially in large single-crystal trigonal domains, exhibiting armchair or zigzag crystalline edges highly oriented with the underlying mica lattice and only two preferred domain orientations mirrored at $180^\circ$. As-grown feature thickness dependence on lateral dimensions and denuded zones at boundaries are observed, as explained by a semi-empirical two-species surface migration model with robust estimates of growth parameters and elucidating the role of selective-area surface modification. Topological surface states contribute up to 60% of device conductance at room-temperature, indicating excellent electronic quality. High-yield microfabrication and the adaptable vdWE growth mechanism with readily alterable precursor and substrate combinations, lend the process versatility to realize crystalline TI synthesis in arbitrary shapes and arrays suitable for facile integration with processes ranging from rapid prototyping to scalable manufacturing., 26 pages including Supporting Information: 15 figures, 1 table

We report avalanche photodiodes fabricated from Al $_{x}$ In1− $_{x}$ As $_{y}$ Sb1− $_{y}$ wafers grown by molecular beam epitaxy using the digital alloy technique. A series of Al $_{x}$ In 1− $_{x}$ As $_{y}$ Sb1− $_{y}$ ( x = 0.3, 0.4, 0.5, 0.6, 0.7) p-i-n structures have been grown on GaSb substrate. Dark current, avalanche multiplication, excess noise, and external quantum efficiencies have been characterized. Very low excess noise, as characterized by k ∼ 0.01–0.05, has been achieved.

A novel ultrafast reflective grating-imaging technique has been developed to measure ambipolar carrier diffusion in GaAs/AlAs quantum wells and bulk GaAs. By integrating a transmission grating and an imaging system into the traditional pump–probe setup, this technique can acquire carrier diffusion properties conveniently and accurately. The fitted results of the diffusion coefficient and diffusion length in bulk GaAs agree well with the literature values obtained by other techniques. The diffusion coefficient and diffusion length of GaAs/AlAs quantum wells are found to increase with the well layer thickness, which suggests that interface roughness scattering dominates carrier diffusion in GaAs/AlAs quantum wells. With the advantages of simple operation, sensitive detection, rapid and nondestructive measurement, and extensive applicability, the ultrafast reflective grating-imaging technique has great potential in experimental study of carrier diffusion in various materials.

We report on the first demonstration of metal–insulator–semiconductor-type plasmonic lasers at the telecom wavelength (∼1.3 μm) using top-down fabricated semiconductor waveguides on single-crystalline metallic platforms formed using epitaxially grown Ag films. The critical role of the Ag film thickness in sustaining plasmonic lasing at the telecom wavelength is investigated systematically. Low-threshold (0.2 MW/cm2) and continuous-wave operation of plasmonic lasing at cryogenic temperatures can be achieved on a 150 nm Ag platform with minimum radiation leakage into the substrate. Plasmonic lasing occurs preferentially through higher-order surface-plasmon-polariton modes, which exhibit a higher mode confinement factor, lower propagation loss, and better field–gain coupling. We observed plasmonic lasing up to ∼200 K under pulsed excitations. The plasmonic lasers on large-area epitaxial Ag films open up a scalable platform for on-chip integrations of plasmonics and optoelectronics at the telecom wavelength.

Surface plasmon resonance (SPR) is a powerful tool to amplify coherent phonon signals in metal films. In a 40 nm Au film excited with a 400 nm pump, we observed an abnormally large electron–phonon coupling constant of about 8×1017 W/(m3K), almost 40× larger than those reported in noble metals, and even comparable to transition metals. We attribute this phenomenon to quantum confinement and interband excitation. With SPR, we also observed two coherent phonon modes in a GaAs/AlAs quantum well thin film. Our finding provides a new approach to generate high-frequency acoustic phonons in noble metals and to study their nonlinear nature of propagation.

Recently, using Al x In 1-y As y Sb 1-y grown as a digital alloy we have demonstrated APDs with noise as low as Si at telecommunication wavelengths (1300 nm to 1550 nm). In this paper, we a noise suppression mechanism in digital alloys. This is related to the fundamental issue of transport in ordered materials and provides the potential for "designer APDs" covering a broad range of spectral bands.

We report separate absorption, charge, and multiplication (SACM) avalanche photodiodes (APDs) in the Al x In 1-x As y Sb 1-y material system for 2-μm detection. Gain, dark current, and low excess noise are demonstrated.

Bismuth-antimony alloys (${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$) are topological insulators between 7 and 22% Sb in bulk crystals, with an unusually high conductivity suitable for spin-orbit torque applications. Reducing the thickness of epitaxial ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ films is expected to increase the maximum band gap through quantum confinement, which may improve isolation of topological surface-state transport. Like Bi(001) on Si(111), ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ has been predicted to form a black phosphoruslike allotrope with unique electronic properties in nanoscale films; however, the impact of Sb alloying on both the bulklike and nanoscale crystal structures on Si(111) is currently unknown. Here we demonstrate that the allotropic transition in ultrathin epitaxial ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ films on Si(111) is suppressed above 8--9% Sb, resulting in an unexpected (012) orientation within the topologically insulating regime. The metallic temperature-dependent conductivity associated with surface states in Bi(001) was not observed in the ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}(012)$ films, suggesting that the (012) orientation may significantly reduce surface-state transport. Growth on a Bi(001) buffer layer may prevent this orientation transition. Finally, we demonstrate that Sb alloying improves the continuity and quality of nanoscale ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}(012)$ films in the thickness regime expected for the black phosphorus allotrope, suggesting a promising route to large-area growth of puckered-layer two-dimensional ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$, which will be necessary to harness its unique electronic properties in practical applications.

In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime (λ/6000), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely "screens" the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.

Harmonic measurements of the longitudinal and transverse voltages in Bi-Sb/Co bilayers are presented. A large second harmonic voltage signal due to the ordinary Nernst effect is observed. In experiments where a magnetic field is rotated in the film plane, the ordinary Nernst effect shows the same angular dependence in the transverse voltage as the damping-like spin-orbit torque and in the longitudinal voltage as the unidirectional spin-Hall magneto-resistance respectively. Therefore, the ordinary Nernst effect can be a spurious signal in spin-orbit torque measurements, leading to an overestimation of the spin-Hall angle in topological insulators or semimetals., Comment: 5 pages, 4 figures

We have developed a transient thermoreflectance technique using picosecond pulsed and cw laser to study thermal conductivity and interface conductance in both thin-films and bulk materials. A real time-resolved system observes a thermal transport along the cross-plane direction of the sample during a single pulse excitation. The suggested TTR technique can measure thermal conductivity in up to a few hundred nm of thin films with a reasonable uncertainty by carefully selecting metal transducer thickness. In this paper, we examine thermal conductivity in several substrates including Si, GaAs, Sapphire, and Glass after depositing Au thin film as metal transducer and compare with reported values to validate our technique. For further study on our method, MoS2 thin-films with different thicknesses are prepared via exfoliating, and their thermal conductivity are measured as average value of 3.4 W/mK. Compared to TDTR technique, TTR is a simpler and inexpensive method to study thermophysical properties and can also measure in-plane thermal property using a grating imaging technique. TTR can be one of the available options for observing thermal transport phenomena in both horizontal and vertical directions with a simple and inexpensive preparation., Comment: 18 pages

Molecular beam epitaxy allows for the monolithic integration of wavelength-flexible epitaxial infrared plasmonic materials with quantum-engineered infrared optoelectronic active regions. We experimentally demonstrate a six-fold enhancement in photoluminescence from ultra-thin (total thickness of 1/32-th wavelength) long wavelength infrared (LWIR) superlattices grown on highly doped semiconductor designer metal virtual substrates when compared to the same superlattice grown on an undoped virtual substrate. Analytical and numerical models of the emission process via a Dyadic Greens function formalism are in agreement with experimental results and relate the observed enhancement of emission to a combination of Purcell enhancement due to surface plasmon modes as well as directionality enhancement due to cavity-substrate-emitter interaction. The results presented provide a potential path towards efficient, ultra-subwavelength LWIR emitter devices, as well as a monolithic epitaxial architecture offering the opportunity to investigate the ultimate limits of light-matter interaction in coupled plasmonic/optoelectronic materials.

Integration of embedded dielectric structures with crystalline III-V materials has generated significant interest, due to a host of important applications and material improvements that are central to high performance optoelectronic devices. The core challenge is the production of high-quality crystalline layers grown above embedded dielectric materials, requiring the growth processes of both lateral epitaxial overgrowth (LEO) and coalescence. In this review article, we provide a detailed and up-to-date description of the recent advances in both LEO and coalescence in III-V materials, from its extension to molecular beam epitaxial growth and high-quality coalescence in InP and GaAs to emerging applications that utilize encapsulated air voids to enhance optical devices. We also explore the epitaxial integration of other materials, particularly metals, with III-V semiconductors.

Atomic sheets of bismuth (Bi) have been expected to yield exotic optoelectronic properties, holding great promise for photodetector devices. However, existing Bi thin film photodetectors have limited performance in terms of photoresponsivity or response time, hindering its practical application. Herein, we report an experimental research progress on optoelectronic properties of epitaxial 2D Bi grown on Si(111) substrate. Our 2D Bi/Si(111) heterolayer exhibits inspiring photodetection performance, including a Vis-NIR broadband response with a responsivity up to 80 A W−1 and response time ∼3 μs, which is attributed to promoted generation and transportation of charge carriers in the heterojunction. 2D Bi/Si(111) here also demonstrates stable and reproducible photo switching behavior. This work paves an avenue to develop photodetectors based on heterointerface between group VA Xene and Si(111) with rapid switching behavior and adequate photoresponsivity.

Professor Arthur Gossard’s seminal contributions to fundamental physics often overshadow the immense impact he has had on advancing the performance and functionality of electronic and photonic devices. This paper attempts to, at least in part, capture this important aspect of Gossard’s continuing research contributions by reviewing three disparate examples, along with their device applications: epitaxial regrowth, digital alloy growth, and metal:semiconductor nanocomposites.

Conduction and valence band states for the highly mismatched alloy (HMA) Ge:C are projected onto Ge crystal states, Ge vacancy states, and Ge/C atomic orbitals, revealing that substitutional carbon not only creates a direct bandgap but also the new conduction band is optically active. Overlap integrals of the new Ge:C conduction band state with states from unperturbed Ge show that the new band cannot be attributed to any single Ge band but is a mixture of multiple Ge states. The Ge Γ conduction band valley state plays the largest single role, but L and X valley states collectively contribute a larger share than Γ due to the multiplicity of degenerate states. C sites structurally resemble uncharged vacancies in the Ge lattice, similar to Hjalmarson's model for other HMAs. C also perturbs the entire Ge band structure even at the deepest crystal core energy levels, particularly if staggered supercells are used to mimic a disordered alloy. Projection onto atomic sites shows a relatively weak localization compared with other HMAs, but it does show a strong anisotropy in probability distribution. L-valley conduction band states in Ge contribute to the conduction band minimum in Ge:C, but the optical transition strength in Ge:C remains within a factor of 2 of the direct gap transition in Ge.

The interaction of free electrons with electromagnetic excitation is the fundamental mechanism responsible for ultra-strong confinement of light that, in turn, enables biosensing, near-field microscopy, optical cloaking, sub-wavelength focusing, and super-resolution imaging. These unique phenomena and functionalities critically rely on the negative permittivity of optical elements resulting from the free electrons. As result, progress in nanophotonics and nano-optics is often related to the development of new negative permittivity (plasmonic) media at the optical frequency of interest. Here we show that the essential mobility of free charge carriers in such conducting media dramatically alters the well-known optical response of free electron gases. We demonstrate that a ballistic resonance associated with the interplay of the time-periodic motion of the free electrons in the confines of a sub-wavelength scale nanostructure and the time periodic electromagnetic field leads to a dramatic enhancement of the electric polarization of the medium - to the point where a plasmonic response can be achieved in a composite material using only positive bulk permittivity components. This ballistic resonance opens the fields of plasmonics, nanophotonics, and metamaterials to many new constituent materials that until now were considered unsuitable for such applications, and extends the operational frequency range of existing materials to substantially shorter wavelengths. As a proof of concept, we experimentally demonstrate that ballistic resonance in all-semiconductor metamaterials results in strongly anisotropic (hyperbolic) response well above the plasma frequencies of the metamaterial components.

We report the first direct dry transfer of a single-crystalline thin film grown by molecular beam epitaxy. A double cantilever beam fracture technique was used to transfer epitaxial bismuth thin films grown on silicon (111) to silicon strips coated with epoxy. The transferred bismuth films retained electrical, optical, and structural properties comparable to the as-grown epitaxial films. Additionally, we isolated the bismuth thin films on freestanding flexible cured-epoxy post-transfer. The adhesion energy at the bismuth/silicon interface was measured to be ∼1 J/m2, comparable to that of exfoliated and wet transferred graphene. This low adhesion energy and ease of transfer is unexpected for an epitaxially grown film and may enable the study of bismuth’s unique electronic and spintronic properties on arbitrary substrates. Moreover, this method suggests a route to integrate other group-V epitaxial films (i.e., phosphorus) with arbitrary substrates, as well as potentially to isolate bismuthene, the atomic th...

Graphene has great potential for fabrication of ultrafast opto-electronics, in which relaxation and transport of photoexcited carriers determine device performance. Even though ultrafast carrier relaxation in graphene has been studied vigorously, transport properties of photoexcited carriers in graphene are largely unknown. In this work, we utilize an ultrafast grating imaging technique to measure lifetime (τr), diffusion coefficient (D), diffusion length (L) and mobility (μ) of photoexcited carriers in mono- and multi-layer graphene non-invasively. In monolayer graphene, D∼10,000 cm2/s and μ∼120,000 cm2/V have been observed, both of which decrease drastically in multilayer graphene, indicating that the remarkable transport properties in monolayer graphene originate from its unique Dirac-Cone energy structure. Mobilities of photoexcited carriers measured here are several times larger than the Hall and Field-Effect mobilities reported in literature (

Patterned arrays of graphene nanostructures, also referred as graphene metasurfaces, have proven to be capable of efficiently coupling with incident light by surface plasmon resonances. In this work, a new type of graphene metasurfaces with moire patterns using cost-effective and scalable moire nanosphere lithography (MNSL) is demonstrated. A large gradient in feature size (i.e., from sub-200 nm to 1.1 μm) of the graphene nanostructures exists in single metasurfaces. The in-plane quasi-periodic arrangement of the graphene nanostructures can be easily tuned to form a variety of moire patterns. The experimental measurement and numerical simulations show that the graphene moire metasurfaces support tunable and multiband optical responses due the size and shape dependences of surface plasmon resonance modes of graphene nanostructures. It is also demonstrated that the multiband optical responses of graphene moire metasurfaces can be tuned from mid-infrared (MIR) to terahertz (THz) regimes by choosing polystyrene spheres of different sizes for MNSL. These findings provide a cost-effective and scalable strategy to achieve ultrathin functional devices, including multiband light modulators, broadband biosensors, and multiband photodetectors, which feature tunable and multiband responses in wide range of wavelengths from MIR to THz.

AlxIn1–xAsySb1–y digital alloys lattice-matched to GaSb were grown within the miscibility gap by molecular beam epitaxy, with aluminum fractions ranging from 0% to 80%. Photoluminescence spectra from AlxIn1–xAsySb1–y films and from AlxIn1–xAsySb1–y/GaSb type-II superlattices were used to determine the direct bandgap and the band offsets as a function of the aluminum fraction. Varying the aluminum content tuned the direct bandgap from 0.25 eV (0% aluminum) to 1.24 eV (75% aluminum), corresponding to photon wavelengths from 5000 to 1000 nm, with the transition from direct-gap to indirect-gap occurring at ∼1.18 eV (∼72% aluminum), or 1050 nm. This direct-gap tuning range of 0.93 eV is the largest reported for a III–V alloy lattice-matched to a commercially available substrate. The broadly tunable bandgap and type-I band alignments of this lattice-matched quaternary make it attractive for advanced mid-infrared and near-infrared detectors and sources.

Black phosphorus (BP) has attracted rapidly growing attention for high speed and low power nanoelectronics owing to its compelling combination of tunable bandgap (0.3 to 2 eV) and high carrier mobility (up to ∼1000 cm(2)/V·s) at room temperature. In this work, we report the first radio frequency (RF) flexible top-gated (TG) BP thin-film transistors on highly bendable polyimide substrate for GHz nanoelectronic applications. Enhanced p-type charge transport with low-field mobility ∼233 cm(2)/V·s and current density of ∼100 μA/μm at VDS = -2 V were obtained from flexible BP transistor at a channel length L = 0.5 μm. Importantly, with optimized dielectric coating for air-stability during microfabrication, flexible BP RF transistors afforded intrinsic maximum oscillation frequency fMAX ∼ 14.5 GHz and unity current gain cutoff frequency fT ∼ 17.5 GHz at a channel length of 0.5 μm. Notably, the experimental fT achieved here is at least 45% higher than prior results on rigid substrate, which is attributed to the improved air-stability of fabricated BP devices. In addition, the high-frequency performance was investigated through mechanical bending test up to ∼1.5% tensile strain, which is ultimately limited by the inorganic dielectric film rather than the 2D material. Comparison of BP RF devices to other 2D semiconductors clearly indicates that BP offers the highest saturation velocity, an important metric for high-speed and RF flexible nanosystems.

We investigate the impact of ErAs:GaAs and LuAs:GaAs superlattice structures with different LuAs/ErAs nanoparticle depositions and superlattice geometries on terahertz radiation properties of plasmonic photomixers operating at a 780-nm optical wavelength. Our analysis indicates the crucial impact of carrier drift velocity and carrier lifetime on the performance of plasmonic photomixers. While higher carrier drift velocities enable higher optical-to-terahertz conversion efficiencies by offering higher quantum efficiencies, shorter carrier lifetimes allow achieving higher optical-to-terahertz conversion efficiencies by mitigating the negative impact of destructive terahertz radiation from slow photocarriers and preventing the carrier screening effect.

This experimental study reveals intriguing thermoelectric effects and devices in epitaxial bismuthene, two-dimensional (2D) bismuth with thickness ⩽30 nm, on Si (111). Bismuthene exhibits interesting anisotropic Seebeck coefficients varying 2-5 times along different crystal orientations, implying the existence of a puckered atomic structure like black phosphorus. An absolute value of Seebeck coefficient up to 237 μV K-1 sets a record for elemental Bi ever measured to the best of our knowledge. Electrical conductivity of bismuthene can reach up to 4.6 × 104 S m-1, which is sensitive to thickness and magnetic field. Along with a desired low thermal conductivity ∼1.97 W m-1 K that is 20% of its bulk form, the first experimental zT value at room temperature for bismuthene was measured ∼10-2, which is much higher than many other VA Xenes and comparable to its bulk compounds. Above results suggest a mixed buckled and puckered Bi atomic structure for epitaxial 2D bismuth on Si (111). Our work paves the way to explore potential applications, such as heat flux sensor, energy converting devices and so on for bismuthene.

Raman and coherent phonon spectroscopies were used to investigate the thickness-dependent phononic properties of ultrathin single-crystal Bi films prepared by molecular beam epitaxy on Si(111) substrates. Both the A1g and Eg Raman peaks disappeared in the Raman spectra of a 4 nm Bi film, indicating a complete transition from the low-symmetry A7 structure to the high-symmetry A17 structure. Coherent phonon signals of the A1g mode also showed a strong dependence on the film thickness, where thin samples (≤15 nm) exhibited lower phonon frequency and shorter phonon lifetimes than the thick samples (≥30 nm). This difference is attributed to a shallower energy potential barrier caused by both a permanent phase transition, which is determined by the film thickness, and a temporary structural transition by photo-excited carriers. Our results not only provide evidence of a phase transition from the A7 to the A17 structure with the decreasing Bi film thickness but also reveal the influence of this phase transition on phonon dynamics. Understanding these material performance traits will facilitate modern application of Bi thin films in electronic devices.

We report the room temperature photoluminescence and electroluminescence properties of boron incorporated into highly strained InGaAs, forming BGaInAs, grown on GaAs substrates. X-ray diffraction was used to determine the alloy composition and strain of BGaInAs quantum wells on GaAs. As expected, the addition of boron reduced the quantum well compressive strain, preventing strain-relaxation and enabling extension of the peak emission wavelength of InGaAs quantum wells to 1.3 μm on GaAs. We also report both the longest wavelength emission observed from BGaInAs (1.4 μm) and electrically injected photoemission from a dilute-boride active region. We observed a blueshift in electroluminescence, due to unintentional in situ annealing of the active region, which we mitigated to demonstrate a path to realize true 1.3 μm emitters in the presence of in situ annealing.

We differentiate the effect of strain induced by lattice-mismatched growth from strain induced by mechanical deformation on cubic nonradiative Auger recombination in narrow-gap GaInAsSb/GaSb quantum well (QW) heterostructures. The typical reduction in the Auger coefficient observed with lattice-mismatched growth appears to be due to the concomitant compositional change rather than the addition of strain, with implications for mid-IR semiconductor laser design. We induced a range of internal compressive strain in five samples from −0.90% to −2.07% by varying the composition during the growth and mechanically induced a similar range of internal strain in analogous quantum well membrane samples. We performed time-resolved photoluminescence and differential reflectivity measurements to extract the carrier recombination dynamics, taken at 300 K with carrier densities from 2.7 × 10 18 cm−3 to 1.4 × 10 19 cm−3. We observed no change with strain in the cubic Auger coefficient of samples that were strained mechanically, but we did observe a trend with strain in samples that were strained by the QW alloy composition. Measured Auger coefficients ranged from 3.0 × 1 0 − 29 cm6 s−1 to 3.0 × 1 0 − 28 cm6 s−1.

Coherent acoustic phonon dynamics in single-crystalline bismuth nanofilms transferred to a glass substrate were investigated with ultrafast pump–probe spectroscopy. Coherent phonon signals were substantially enhanced by more than four times when compared with as-grown films on Si (111) substrates. Furthermore, more than 10% reduction of the acoustic phonon velocity was observed when the film thickness decreases to 22 nm, which is attributed to the modified phonon dispersion in extremely thin films from quantum confinement effects.

There exists significant interest in the demonstration and development of alternative mid-infrared emitters, with future applications for thermal scene projection, low-cost infrared sensing, and possible long-wavelength quantum communication applications. Type-II In(Ga)Sb quantum dots grown in InAs matrices have the potential to serve as a viable material system for wavelength-flexible, mid-infrared sources. Here, we dramatically expand the range of potential applications of these mid-infrared quantum emitters through the demonstration of surface-emitting electrically pumped mid-infrared light-emitting diodes with active regions utilizing type-II In(Ga)Sb quantum dots. Two device structures were studied, the first iteration being a single In(Ga)Sb insertion layer within a simple PIN structure and the second being a design engineered for improved room temperature emission with the addition of lattice matched AlAsSb cladding at the anode to block electrons and five layers of In(Ga)Sb dots to increase the effective volume of active material. Samples were grown by molecular beam epitaxy and the electrical and optical properties for each design were characterized as a function of temperature.

Semiconductors such as InAs with high dopant concentrations have a variety of applications, including as components of mid-infrared optoelectronic devices. Unfortunately, growth of these materials by molecular beam epitaxy is challenging, requiring high growth rates and low growth temperatures. We show that the use of a bismuth surfactant improves silicon incorporation into InAs while simultaneously reducing the optical scattering rate, increasing the carrier mobility, reducing surface roughness, and enabling growth at higher substrate temperatures and slower growth rates. We explain our findings using microscopic theories of dopant segregation and defect formation in III-V materials.

Photoreflectance (PR) and photoluminescence (PL) has been measured on the set of four samples containing 12 periods of a GaSb and an AlInAsSb digital alloy with various average Al fractions, both 12nm thick. Using 8kp with envelope function approximation, PL and PR spectra are qualitatively explained. States in conduction bands are classified in three general groups, which are used during further systematic description of various superlattice cases.

Low noise avalanche photodiode has been realized by InAlAs digital alloy, wherein minigaps in band structure takes role of modulating the ionization coefficient ratio k to be as low as 0.01.

Digital alloy In 0.52 Al 0.48 As avalanche photodiodes exhibit lower excess noise than those fabricated from random alloys. This paper compares the temperature dependence, from 203 K to 323K, of the impact ionization characteristics of In 0.52 Al 0.48 As and Al 0.74 Ga 0.26 As digital and random alloys. These results provide insight into the low excess noise exhibited by some digital alloy materials, and these materials can even obtain lower excess noise at low temperature.

We describe the molecular beam epitaxial growth, characterization, and device performance of conventional, staircase, and photoconductive avalanche photodetectors grown with AlInAsSb digital alloys. In particular, this is the first low-noise III-V avalanche photodiode alloy family and offers the band engineering flexibility necessary to achieve staircase avalanche photodiode operation.

We describe the molecular beam epitaxial growth of BGaAs on GaP, towards the integration of direct bandgap light emitting diodes coherently on silicon.

We report the design and operation of a nonlinear intersubband polaritonic metasurface for mid-infrared second harmonic generation. The metasurface is made of plasmonic nanoresonators filled with a multiple-quantum-well semiconductor heterostructure. Unlike the previously reported nonlinear intersubband polaritonic metasurfaces that employ full-metal backplanes below the etched metal–semiconductor nanoresonators, the metasurface reported here employs an incomplete backplane that is complementary to the pattern of the top metallization of the etched semiconductor heterostructure nanoresonators. The new approach produces high-electric-field localization and enhancement in the nanoresonators, while requiring simplified fabrication and allowing the metasurface to operate in both transmission and reflection regimes.

Avalanche photodiodes (APDs) can provide higher sensitivity than p-i-n photodiodes owing to their internal gain. However, the gain, which originates from impact ionization is also a source of noise. Recently, using AlxIn1-xAsySb1-y grown as a digital alloy we have demonstrated APDs with noise as low as Si at telecommunication wavelengths (1300 nm to 1550 nm). In this paper, we report on additional AlInGaAsSb digital alloys and demonstrate noise suppression relative to random alloys. This is related to the fundamental issue of transport in ordered materials and provides the potential for “designer APDs” covering a broad range of spectral bands.