Flexible Thin-Film InGaAs Photodiode Focal Plane Array

Natural imaging systems such as the retina and the compound eye employ a conformal architecture that provides an aberration-free image with wide field of view (FOV) and very low <i>f</i>/number. However, most artificial imagers such as conventional cameras are limited to a planar architecture demanded by the use of brittle semiconductor focal plane arrays (FPAs). High-resolution image formation on this flat field requires multiple bulky optical elements. Here we demonstrate a general approach to fabricating complex circuits and in particular FPAs on flexible and/or conformable substrates that can be shaped to overcome these fundamental limitations. An 8 × 100, lightweight, thin-film In_0.53Ga_0.47As p<i>-</i>i<i>-</i>n photodiode FPA with sensitivity to wavelengths as long as λ = 1650 nm is fabricated on a thin flexible plastic foil following transfer by adhesive-free bonding of the epitaxial layers that are subsequently lifted off from the parent InP substrate. The array is shaped into either a convex cylindrically curved imager to achieve a 2π FOV or, when formed into a concave shape, to provide high-resolution and compact spectral decomposition over a wide wavelength range. The array exhibits ∼99% fabrication yield with ∼100% peak external quantum efficiency at λ = 1300 nm. The unique features of this flexible thin-film FPA provide a new paradigm for realizing advanced electronic and imaging applications

Thin-Film Architectures with High Spectral Selectivity for Thermophotovoltaic Cells

Thermophotovoltaic (TPV) systems are a promising technology for distributed conversion of high-temperature heat to electricity. To achieve high conversion efficiency, the transport of sub-bandgap radiation between the thermal emitter and PV cell should be suppressed. This can be achieved by recycling sub-bandgap radiation back to the emitter using a spectrally selective cell. However, conventional TPV cells exhibit limited sub-bandgap reflectance. Here we demonstrate thin-film In_0.53Ga_0.47As-based structures with high spectral selectivity, including record-high average sub-bandgap reflectance (96%). Selectivity is enabled by short optical paths through a high-quality material fabricated using epitaxial lift-off, high-reflectance back surfaces, and optimized interference. In addition, we use a parallel-plate TPV model to evaluate the impact of specific structural features on performance and to optimize the cell architecture. We show that a dielectric spacer between InGaAs and the Au back surface is an important feature that enables a predicted TPV efficiency above 50% (with a power output of 2.1 W/cm²), significantly higher than current TPV devices. This work provides guidelines for the design of high-efficiency, low-cost TPV generators