We report on a multi-year development effort demonstrating resonant cavity enhanced photodiodes (RCE-PDs) with an all-epitaxial architecture, exploiting the GaSb matched material system and its increasingly popular family of IR absorbers. RCE-PDs redefine the relationship between signal and noise generation within IR detectors, breaking some of the performance limits constraining conventional IR detectors. High performance RCE-PDs are achieved with resonance wavelengths from the SWIR to the LWIR, using GaSb/AlAsSb DBR mirrors and InGaAsSb, InAs, InAsSb and T2SL absorbers. Finally, we introduce a graded thickness RCE-PD which can perform the function of a spectrometer on a chip.
We investigate the characteristics of the quaternary alloy InxGa1−xAsySb1−y as a viable alternative to extended InGaAs for sensing in short wavelength infrared. InxGa1−xAsySb1−yp-i-n photodetectors with 0 < x < 0.3 have been grown on GaSb substrates in the wavelength region between 1 - 3 µm. Absorption coefficient up to ∼ 104 cm−1 compares well with that of InGaAs, increasing for samples with narrower bandgaps. Capacitance measurements shed light on the intrinsic unintentional doping levels, which are up to an order of magnitude lower than in typical bulk GaSb, due to a reduction in native defects of the material. Current density initially decreases with addition of small fractions of In/As to GaSb, then proceeds to increase once again towards higher alloy fractions as the bandgap narrows.
Resonant cavity-enhanced photodiodes (RCE-PD) have previously been studied for the narrow spectral responses that can be achieved. A narrow response is useful for spectral sensing of a specific absorption line without interference from neighbouring absorption lines. We have designed and fabricated an array of photodiodes that each have a slightly shifted resonance wavelength; so combined photocurrent measurements from all the pixels can be used to not only monitor a single absorption line, but also monitor all absorption lines in a region of the infrared simultaneously. The RCE-PD array concept allows for many substances to be identified and measured, offering much more versatility than a measurement from a single-wavelength RCE-PD. The shift in resonance wavelength is created by inducing a thickness gradient in the epitaxial layers across the wafer. Using this technique, a fabricated 2-inch wafer showed a resonance shift between 1.9 μm and 2.5 μm. A 1D array of pixels was created with a shift in the resonance of 4nm per pixel. Light from a monochromator was used to test the ability of the array to determine the wavelength of the light. For four closely spaced wavelengths, an accuracy of ±2nm was seen.
Resonant cavity-enhanced photodetectors (RCE PDs) present a compelling alternative to broadband detection techniques in the field of gas detection and environmental sensing, due to the distinctive narrow-band absorption fingerprints of gases such as N2O (at 4.5 μm) or CO (4.6 μm). This characteristic aligns well with the operational mode of an RCE PD, whose VCSEL-like architecture results in a tuneable narrow-band spectral response with a significantly enhanced quantum efficiency. Additionally, unlike broadband detectors, RCE PDs are not subject to the broadband BLIP limit due to their high spectral selectivity, while the substantially reduced absorber volume offers commensurately reduced Auger and generation-recombination dark current densities. In this work, we present efforts to extend the operability of these structures beyond 4.0 μm wavelength by employing the type-II InAs/InAsSb superlattice as the absorber material. The tuneable bandgap of this structure allows to achieve and demonstrate a MWIR RCE PD with a highly thermally stable resonant response at ~ 4.45 μm, a Q factor of 85-95, full-width-at-half-maximum of ~ 50 nm and a peak quantum efficiency of 84% at 240 K - features which are promising for detection of gases such as CO and N2O. The broadband BLIP is also achieved at 180 K, a result which could potentially enable thermoelectrically cooled operation in the future. Finally, thanks to the inherent bandgap tunability of the InAs/InAsSb superlattice, extension of resonant response into the LWIR range is achievable with relatively straightforward changes to the already existing RCE PD structure.
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