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 demonstrate a novel solid-state spectrometer employing a linear array of resonant cavity enhanced photodiodes (RCE-PDs) with a spatial chirp. By epitaxially grading the thicknesses of the distributed Bragg reflector mirrors, the chirp can cover a total bandwidth of ≥0.1 × λres where λres is the resonant wavelength. This new class of sensor is intended for analyzing IR absorption fingerprints and our group has already demonstrated conventional RCE-PDs between 2.2 – 7.8 μm. In theory the range between 1.55 and ~12 μm could be served using the same materials. This region covers important spectral fingerprints including chemical and pollutant gases, as well as threat agents including thiodiglycol and VX.
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.
Opto-electronic devices destined for space must be suitably radiation-hard, meaning that they must be resilient to the effects of high energy radiation in space. For high performance IR (infrared) space-based applications, the current material of choice is MCT (Mercury Cadmium Telluride). MCT is difficult and therefore expensive to fabricate and the constituent materials are becoming increasingly restricted by regulation. The new generation of barrier diode detectors based on III-V materials offer a promising alternative to MCT, providing comparable performance whilst offering devices that are compatible with volume manufacturing processes. As part of a DASA Space-to-Innovate Phase 1 funded project we have developed a novel radiation hard unipolar barrierbased ABaT™ III-V MWIR diode detector. The detector is being subjected to gamma and proton radiation testing to demonstrate its suitability for space environments. To compare the radiation performance of this diode, a number of other typical III-V detector diode structures have been fabricated and tested. In this paper we present the results of the project so far and future plans to develop this into detector arrays.
GaSb-based materials can be used to produce high performance photonic devices operating in the technologically important mid-infrared spectral range. Direct epitaxial growth of GaSb on silicon (Si) is an attractive method to reduce manufacturing costs and opens the possibility of new applications, such as lab-on-a-chip MIR photonic integrated circuits and monolithic integration of focal plane arrays (FPAs) with Si readout integrated circuits (ROICs). However, fundamental material dissimilarities, such as the large lattice mismatch, polar-nonpolar character of the III-V/Si interface and differences in thermal expansion coefficients lead to the formation of threading dislocations and antiphase domains, which effect the device performance. This work reports on the molecular beam epitaxial growth of high quality GaSb-based materials and devices onto Si. This was achieved using a novel growth procedure consisting of an efficient AlSb interfacial misfit array, a two-step GaSb growth temperature procedure and a series of dislocation filter superlattices, resulting in a low defect density, anti-phase domain free GaSb buffer layer on Si. A nBn barrier photodetector based on a type-II InAs/InAsSb superlattice was grown on top of the buffer layer. The device exhibited an extended 50 % cut-off wavelength at 5.40 μm at 200 K which moved to 5.9 μm at 300 K. A specific detectivity of 1.5 x1010 Jones was measured, corresponding in an external quantum efficiency of 25.6 % at 200 K.
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.
We present III-Sb resonant cavity-enhanced (RCE) photodetectors suitable for gas detection in the mid-wave infrared. AlAsSb/GaSb DBRs and absorbers of bulk InAsSb or a type-II InAsSb-InAs SLS were grown on GaSb, allowing for operation at 3.72 μm or 4.52 μm, with linewidth Δλ < 50 nm and Δλ < 70 nm, respectively. A barrier diode structure was used, and the absorber thickness was limited to 96 nm for InAsSb – or 192 nm for the SLS – in order to limit the dark currents. High quantum efficiency was obtained through the resonant optical field, while the remainder of the cavity was grown using wide-gap AlAsSb spacer layers not contributing to the dark current. By carefully compensation doping the AlAsSb layers, the 3.72 μm device was bandgap-engineered for a flat Fermi level in the thin absorber, and hence dark currents which scale with the absorber thickness. This can equate to a >20x reduction in noise compared with a conventional nBn detector with full thickness absorber. At 3.72 μm, performance above the BLIP limit imposed on broadband photodetectors was found by calculating for the specific detectivity.
Inserting an infrared detector architecture into an optical cavity between two high-reflectivity mirrors allows incident light to reflect and pass through the detector multiple times, thereby enhancing absorption within the active region. This allows for a 40-100x thinner optical absorbing region compared to conventional infrared detector structures which reduces the detector dark current and noise and enhances SNR. We report the design, growth, fabrication and characterization of resonant cavity enhanced MWIR photodiodes on GaSb substrates. Devices on GaSb use AlAsSb/GaSb mirrors, AlAsSb spacer layers, and a narrow 96 nm InAsSb absorber. Dark current and detectivity behavior better than equivalent broadband nBn detectors in the literature have been observed. 34nm linewidth detector response is observed. Resonant cavity-enhanced photodiodes with resonant wavelengths of 3.6μm and 3.72μm are demonstrated with dark currents equal to or lower than Rule 07 over the operating temperature range of the device. D* in excess of 1×1010 cm Hz1/2W-1 at 300K and 8×1010 cm Hz1/2W-1 at 250K have been achieved. Amethyst Research has produced packaged resonant-cavity detectors. The 3.6 μm resonant-cavity enhanced photodiode was packaged within an Amethyst Research designed pre-amplifier package with an integrated TEC for detector cooling.
InAs/InAs1-xSbx type-II strained-layer superlattices (SLS) are a structure with potential infrared detection applications, owing to its tunable bandgap and suppressed Auger recombination. A series of medium-wavelength infrared (MWIR) InAs/InAs0.815Sb0.185 SLS structures, grown as undoped absorption epilayers on GaAs, were fabricated using molecular beam epitaxy in order to study the dependence of the ground state transitions on temperature and superlattice period thickness. Photoluminescence peaks at 4 K were obtained with the use of a helium-cooled micro-PL system and an InSb detector, and temperature-dependent absorption spectra were measured in the range 77 K – 300 K on a Fourier Transform Infrared (FTIR) spectrometer, equipped with a 1370 K blackbody source and a DTGS detector. An nBn device sample with the absorber structure identical to one of the undoped samples was also grown and processed with the goal of measuring temperature-dependent spectral response. A model for superlattice band alignment was also devised, incorporating the Bir-Pikus transformation results for uniaxial and biaxial strain, and the Einstein oscillator model for bandgap temperature dependence. Absorption coefficients of several 1000 cm-1 throughout the entire MWIR range are found for all samples, and temperature dependence of the bandgaps is extracted and compared to the model. This and photoluminescence data also demonstrate bandgap shifts consistent with the different superlattice periods of the three samples.
Laser-based stand-off sensing of threat agents (e.g. explosives, toxic industrial chemicals or chemical warfare agents), by detection of distinct infrared spectral absorption signature of these materials, has made significant advances recently. This is due in part to the availability of infrared and terahertz laser sources with significantly improved power and tunability. However, there is a pressing need for a versatile, high performance infrared sensor that can complement and enhance the recent advances achieved in laser technology. This work presents new, high performance infrared detectors based on III-V barrier diodes. Unipolar barrier diodes, such as the nBn, have been very successful in the MWIR using InAs(Sb)-based materials, and in the MWIR and LWIR using type-II InAsSb/InAs superlattice-based materials. This work addresses the extension of the barrier diode architecture into the SWIR region, using GaSb-based and InAs-based materials. The program has resulted in detectors with unmatched performance in the 2-3 μm spectral range. Temperature dependent characterization has shown dark currents to be diffusion limited and equal to, or within a factor of 5, of the Rule 07 expression for Auger-limited HgCdTe detectors. Furthermore, D* values are superior to those of existing detectors in the 2-3 μm band. Of particular significance to spectroscopic sensing systems is the ability to have near-background limited performance at operation temperatures compatible with robust and reliable solid state thermoelectric coolers.
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