Scott J. Maddox

Ultrafast Dynamics of Surface Plasmons in InAs by Time-Resolved Infrared Nanospectroscopy

We report on time-resolved mid-infrared (mid-IR) near-field spectroscopy of the narrow bandgap semiconductor InAs. The dominant effect we observed pertains to the dynamics of photoexcited carriers and associated surface plasmons. A novel combination of pump-probe techniques and near-field nanospectroscopy accesses high momentum plasmons and demonstrates efficient, subpicosecond photomodulation of the surface plasmon dispersion with subsequent tens of picoseconds decay under ambient conditions. The photoinduced change of the probe intensity due to plasmons in InAs is found to exceed that of other mid-IR or near-IR media by 1-2 orders of magnitude. Remarkably, the required control pulse fluence is as low as 60 μJ/cm(2), much smaller than fluences of ∼ 1-10 mJ/cm(2) previously utilized in ultrafast control of near-IR plasmonics. These low excitation densities are easily attained with a standard 1.56 μm fiber laser. Thus, InAs--a common semiconductor with favorable plasmonic properties such as a low effective mass--has the potential to become an important building block of optically controlled plasmonic devices operating at infrared frequencies.

Enhanced low-noise gain from InAs avalanche photodiodes with reduced dark current and background doping

We reduced the room temperature dark current in an InAs avalanche photodiode by increasing the p-type contact doping, resulting in an increased energetic barrier to minority electron injection into the p-region, which is a significant source of dark current at room temperature. In addition, by improving the molecular beam epitaxy growth conditions, we reduced the background doping concentration and realized depletion widths as wide as 5 μm at reverse biases as low as 1.5 V. These improvements culminated in low-noise InAs avalanche photodiodes exhibiting a room temperature multiplication gain of ∼80, at a record low reverse bias of 12 V.

High-Gain InAs Avalanche Photodiodes

We report two InAs avalanche photodiode structures with very low background doping in the depletion region. Uniform electric fields and thick depletion regions have been achieved. Excess noise measurements are consistent with k~0 and gain as high as 70 at room temperature is observed. The measured gain-bandwidth product is >; 300 GHz. All measurements are consistent with Monte Carlo simulations.

Low-noise AlInAsSb avalanche photodiode

We report low-noise avalanche gain from photodiodes composed of a previously uncharacterized alloy, Al0.7In0.3As0.3Sb0.7, grown on GaSb. The bandgap energy and thus the cutoff wavelength are similar to silicon; however, since the bandgap of Al0.7In0.3As0.3Sb0.7 is direct, its absorption depth is 5 to 10 times shorter than indirect-bandgap silicon, potentially enabling significantly higher operating bandwidths. In addition, unlike other III-V avalanche photodiodes that operate in the visible or near infrared, the excess noise factor is comparable to or below that of silicon, with a k-value of approximately 0.015. Furthermore, the wide array of absorber regions compatible with GaSb substrates enable cutoff wavelengths ranging from 1 μm to 12 μm.

Growth and characterization of LuAs films and nanostructures

We report the growth and characterization of nearly lattice-matched LuAs/GaAs heterostructures. Electrical conductivity, optical transmission, and reflectivity measurements of epitaxial LuAs films indicate that LuAs is semimetallic, with a room-temperature resistivity of 90 μΩ cm. Cross-sectional transmission electron microscopy confirms that LuAs nucleates as self-assembled nanoparticles, which can be overgrown with high-quality GaAs. The growth and material properties are very similar to those of the more established ErAs/GaAs system; however, we observe important differences in the magnitude and wavelength of the peak optical transparency, making LuAs superior for certain device applications, particularly for thick epitaxially embedded Ohmic contacts that are transparent in the near-IR telecommunications window around 1.3 μm.

AlInAsSb separate absorption, charge, and multiplication avalanche photodiodes

We report Al<inf>x</inf>In<inf>1−x</inf>Asj, Sb<inf>1−y</inf>-based separate absorption, charge, and multiplication avalanche photodiodes (APDs) that operate in the short-wavelength infrared spectrum. These APDs exhibit low excess noise factor, corresponding to k = 0.01, and low dark current.

AlInAsSb/GaSb staircase avalanche photodiode

Over 30 years ago, Capasso and co-workers [IEEE Trans. Electron Devices 30, 381 (1982)] proposed the staircase avalanche photodetector (APD) as a solid-state analog of the photomultiplier tube. In this structure, electron multiplication occurs deterministically at steps in the conduction band profile, which function as the dynodes of a photomultiplier tube, leading to low excess multiplication noise. Unlike traditional APDs, the origin of staircase gain is band engineering rather than large applied electric fields. Unfortunately, the materials available at the time, principally AlxGa1−xAs/GaAs, did not offer sufficiently large conduction band offsets and energy separations between the direct and indirect valleys to realize the full potential of the staircase gain mechanism. Here, we report a true staircase APD operation using alloys of a rather underexplored material, AlxIn1−xAsySb1−y, lattice-matched to GaSb. Single step “staircase” devices exhibited a constant gain of ∼2×, over a broad range of applied ...

Growth and characterization of single crystal rocksalt LaAs using LuAs barrier layers

We demonstrate the growth of high‐quality, single crystal, rocksalt LaAs on III‐V substrates; employing thin well-behaved LuAs barriers layers at the III-V/LaAs interfaces to suppress nucleation of other LaAs phases, interfacial reactions between GaAs and LaAs, and polycrystalline LaAs growth. This method enables growth of single crystal epitaxial rocksalt LaAs with enhanced structural and electrical properties. Temperature-dependent resistivity and optical reflectivity measurements suggest that epitaxial LaAs is semimetallic, consistent with bandstructure calculations in literature. LaAs exhibits distinct electrical and optical properties, as compared with previously reported rare-earth arsenide materials, with a room-temperature resistivity of ∼459 μΩ-cm and an optical transmission window >50% between ∼3-5 μm.

Highly Strained Mid-Infrared Type-I Diode Lasers on GaSb

We describe how growth at low temperatures can enable increased active layer strain in GaSb-based type-I quantum-well diode lasers, with emphasis on extending the emission wavelength. Critical thickness and roughening limitations typically restrict the number of quantum wells that can be grown at a given wavelength, limiting device performance through gain saturation and related parasitic processes. Using growth at a reduced substrate temperature of 350 °C, compressive strains of up to 2.8% have been incorporated into GaInAsSb quantum wells with GaSb barriers; these structures exhibited peak room-temperature photoluminescence out to 3.96 μm. Using this growth method, low-threshold ridge waveguide lasers operating at 20°C and emitting at 3.4 μm in pulsed mode were demonstrated using 2.45% compressively strained GaInAsSb/GaSb quantum wells. These devices exhibited a characteristic temperature of threshold current of 50 K, one of the highest values reported for type-I quantum-well laser diodes operating in this wavelength range. This temperature stability is attributable to the increased valence band offset afforded by the high strain values, due to the simultaneously high quantum well indium and antimony mole fractions. Exploratory experiments using bismuth both as a surfactant during quantum well growth, as well as in dilute amounts incorporated into the crystal were also studied. Both methods appear to be promising avenues to surmount current strain-related limitations to laser performance and emission wavelength.

Nonlinear terahertz devices utilizing semiconducting plasmonic metamaterials

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density. Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials. We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies. Importantly, InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering, resulting in a reduced carrier mobility thereby damping the plasmonic response. We demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers, including flexible nonlinear absorbers achieved by transferring the disks to polyimide films. Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz (THz) optics and for passive protection of sensitive electromagnetic devices.