Publications

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We demonstrate all-epitaxial structures capable of supporting short- and long-range surface plasmon polariton (SRSPP and LRSPP) modes in the long-wave infrared region of the electromagnetic spectrum. The SRSPP and LRSPP modes are bound to the interfaces of a buried heavily doped (n + +) semiconductor layer and surrounding quantum-engineered type-II superlattice (T2SL) materials. The surrounding T2SLs are designed to allow optical transitions across the frequency dispersion of the SPP modes. We map the SPP dispersion in our structure using grating-coupled angle- and polarization-dependent reflection and photoluminescence spectroscopy. The epitaxial structures are analytically described using a simplified three-layer system (T 2 SL / n + + / T 2 SL) and modeled using rigorous coupled wave analysis with excellent agreement to our experimental results. The presented structures offer the potential to serve as long-range interconnects or waveguides in all-epitaxial plasmonic/optoelectronic systems operating in the long-wave infrared.

We have fabricated and characterized AlInAsSb- and InPAsSb-absorber nBn infrared detectors with 200 K cutoff wavelengths from 2.55 to 3.25 μm. Minority-carrier lifetimes determined by microwave reflectance measurements were 0.2-1.0 μs in doped n-type absorber materials. Devices having 4 μm thick absorbers exhibited sharp cutoff at wavelengths of 2.9 μm or longer and softer cutoff at shorter wavelengths. Top-illuminated devices with n+ InAs window/contact layers had external quantum efficiencies of 40-50% without anti-reflection coating at 50 mV reverse bias and wavelengths slightly shorter than cutoff. Despite the shallow-etch mesa nBn design, perimeter currents contributed significantly to the 200 K dark current. Dark currents for InPAsSb devices were lower than AlInAsSb devices with similar cutoff wavelengths. For unoptimized InPAsSb devices with 2.55 μm cutoff, 200 K areal and perimeter dark current densities at -0.2 V bias in devices of various sizes were approximately 1x10-7 A/cm2 and 1.4x10-8 A/cm, respectively.

Infrared detectors using monolithically integrated doped semiconductor "designer metals"are proposed and experimentally demonstrated. We leverage the "designer metal"groundplanes to form resonant cavities with enhanced absorption tuned across the long-wave infrared (LWIR). Detectors are designed with two target absorption enhancement wavelengths: 8 and 10 μm. The core of our detectors are quantum-engineered LWIR type-II superlattice p-i-n detectors with total thicknesses of only 1.42 and 1.80 μm for the 8 and 10 μm absorption enhancement devices, respectively. Our 8 and 10 μm structures show peak external quantum efficiencies of 45 and 27%, which are 4.5× and 2.7× enhanced, respectively, compared to control structures. We demonstrate the clear advantages of this detector architecture, both in terms of ease of growth/fabrication and enhanced device performance. The proposed architecture is absorber- A nd device-structure agnostic, much thinner than state-of-the-art LWIR T2SLs, and offers the opportunity for the integration of low dark current LWIR detector architectures for significant enhancement of IR detectivity.

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Here, the design, fabrication, and characterization of an actively tunable long-wave infrared detector, made possible through direct integration of a graphene-enabled metasurface with a conventional type-II superlattice infrared detector, are reported. This structure allows for post-fabrication tuning of the detector spectral response through voltage-induced modification of the carrier density within graphene and, therefore, its plasmonic response. These changes modify the transmittance through the metasurface, which is fabricated monolithically atop the detector, allowing for spectral control of light reaching the detector. Importantly, this structure provides a fabrication-controlled alignment of the metasurface filter to the detector pixel and is entirely solid-state. Using single pixel devices, relative changes in the spectral response exceeding 8% have been realized. These proof-of-concept devices present a path toward solid-state hyperspectral imaging with independent pixel-to-pixel spectral control through a voltage-actuated dynamic response.

This report documents efforts to develop a wafer mapping measurement system for semiconductor characterization.

Characterization of vertical transport in semiconductor heterostructures is extremely difficult and often impractical. Measurements that are relatively straight forward in lateral transport using Hall methods, such as quantifying carrier density or mobility, have no analog in conventional vertical devices. Doppler charge velocimetry may provide an alternative approach to obtaining transport information. We hypothesize that we can drive vertical currents in structures like heterojunction bipolar transistors or nBn detectors, illuminate them with microwaves, and directly measure the carrier velocities through Doppler shifts imparted on the reflected microwave signal. Some challenges involve providing optical injection and working in the vertical geometry required to extract the desired information. While progress was made to this end, experiments have not yet proved successful. Implications for infrared material characterization are summarized at the end of this document.

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We report transport measurements of electrons on helium in a microchannel device where the channels are 200 nm deep and 3μm wide. The channels are fabricated above amorphous metallic Ta 40 W 40 Si 20 , which has surface roughness below 1 nm and minimal variations in work function across the surface due to the absence of polycrystalline grains. We are able to set the electron density in the channels using a ground plane. We estimate a mobility of 300cm2/Vs and electron densities as high as 2.56×109cm-2. We demonstrate control of the transport using a barrier which enables pinch-off at a central microchannel connecting two reservoirs. The conductance through the central microchannel is measured to be 10 nS for an electron density of 1.58×109cm-2. Our work extends transport measurements of surface electrons to thin helium films in microchannel devices above metallic substrates.

The oxidation mechanisms of exfoliated Gallium Selenide (GaSe) are strongly influenced by humidity. We have observed that the presence of water molecules leads to formation of Ga2O3, SeO2, and Se via sequence of intermediate reactions which include generation of aqueous solution of selenic acid. Raman spectra of GaSe flakes undergoing oxidation in a humidity-controlled environment reveal formation of selenic acid-related species causing Raman scattering signal in the regions around 830 cm-1 and around 1230 cm-1. This observation sheds light on the path of chemical reactions, going via an intermediate stage of formation of gallium hydroxide and selenium oxide-water complexes with further decompositions of these compounds to Ga2O3, SeO2, and amorphous selenium.

We present micro-scale time-resolved microwave resonator response (μ-TRMRR), a sensitive technique capable of measuring carrier lifetimes in micron-scale materials, something not typically achievable using common techniques like time-resolved photoluminescence or time-resolved microwave reflectance.

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Thermophotovoltaics (TPV) is the process by which photons radiated from a thermal emitter are converted into electrical power via a photovoltaic cell. Selective thermal emitters that can survive at temperatures at or above ∼1000°C have the potential to greatly improve the efficiency of TPV energy conversion by restricting the emission of photons with energies below the photovoltaic (PV) cell bandgap energy. In this work, we demonstrated TPV energy conversion using a high-temperature selective emitter, dielectric filter, and 0.6 eV In0.68 Ga0.32 As photovoltaic cell. We fabricated a passivated platinum and alumina frequency-selective surface by conventional stepper lithography. To our knowledge, this is the first demonstration of TPV energy conversion using a metamaterial emitter. The emitter was heated to >1000°C, and converted electrical power was measured. After accounting for geometry, we demonstrated a thermal-to-electrical power conversion efficiency of 24.1 0.9% at 1055°C. We separately modeled our system consisting of a selective emitter, dielectric filter, and PV cell and found agreement with our measured efficiency and power to within 1%. Our results indicate that high-efficiency TPV generators are possible and are candidates for remote power generation, combined heat and power, and heat-scavenging applications.

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