Publications

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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.

An InGaAs/GaAsSb Type-II superlattice is explored as an absorber material for extended short-wave infrared detection. A 10.5 nm period was grown with an InGaAs/GaAsSb thickness ratio of 2 with a target In composition of 46% and target Sb composition of 62%. Cutoff wavelengths near 2.8 μm were achieved with responsivity beyond 3 μm. Demonstrated dark current densities were as low as 1.4 mA/cm2 at 295K and 13 μA/cm2 at 235K at -1V bias. A significant barrier to hole extraction was identified in the detector design that severely limited the external quantum efficiency (EQE) of the detectors. A redesign of the detector that removes that barrier could make InGaAs/GaAsSb very competitive with current commercial HgCdTe and extended InGaAs technology.

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 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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