Resonant cavity infrared detectors (RCIDs) can reduce the noise in sensing a laser signal by strongly suppressing background photocurrent at wavelengths outside the narrow spectral band of interest. We recently reported an RCID with 100-nm-thick InAsSb/InAs absorber, GaAs/AlGaAs bottom mirror, and Ge/SiO2 top mirror. At T = 300 K, the external quantum efficiency reached 58% atλres ≈ 4.6 μm, with linewidth δλ = 27 nm. The characteristics at 125 K implied a specific detectivity of 5.5 × 1012 cm Hz½/W, which is more than 3× higher than for a state-of-the-art broadband HgCdTe device operating at that temperature. However, a prominent variation with mesa diameter of the deposited Ge spacer thickness made it difficult to predictably control λres for devices processed with a given diameter. This has been addressed by measuring the reflectivity spectrum following deposition of the spacer, so that thicknesses of the top mirror's SiO2 and Ge layers could be adjusted appropriately to attain a targeted resonance. This was especially beneficial in matching the λres for a small mesa, needed to minimize the capacitance in high-frequency measurements, to the emission wavelength of a given ewquantum cascade laser.

The infrared optical properties of thick GeSn films (>500 nm) having 10% Sn concentration and of SiGeSn layers, utilized for the growth of strain-relieved, direct-gap GeSn films by molecular beam epitaxy, are investigated. Two growth methods are used: a graded-growth structure and a stepped-growth structure that help us to illustrate the properties of the GeSn and SiGeSn layers. Interestingly, there can be strong absorption in SiGeSn films throughout the infrared. We observe an increase in infrared absorption with increasing Sn concentration up to 21% Sn and in films, where the Sn is held constant at 18%, with increasing Si concentration up to 30%. Cavity effects in the infrared transmission measurement of stepped-growth structures are observed and associated with reflections at growth interfaces. Si–Si bond formation is proposed to occur at high Si concentrations in SiGeSn films, and the bandgap in SiGeSn films appears to decrease with increasing Si and Sn concentrations.

Research has shown that free-space laser communication systems may experience fewer outages due to atmospheric impairments such as haze, fog, clouds, and turbulence by operating at a longer wavelength in the mid-wave or long-wave infrared, if disadvantages such as lower-performance transceiver components may be overcome. Here we report a resonant cavity infrared detector (RCID) with 4.6-µm resonance wavelength that enables 20-dB larger link budget than has been reported previously for ∼ 5 Gb/s operation. The device combines high responsivity, 1.97 A/W, with a low noise equivalent power (NEP) of 0.7 pW/ H z at room temperature, and a high bandwidth of 6.7 GHz at 3-dB. The relatively large surface-normal-incidence device with 30-µm diameter simplifies the coupling relative to intra-subband quantum cascade detectors. Although the RCID NEP is expected to increase with frequency to ∼ 1.5 pW/ H z , we estimate that the total equivalent noise power in a 2.5-GHz bandwidth is less than 200 nW. When combined with a relatively high power (∼100-mW) distributed-feedback quantum cascade laser, the difference of > 50 dB between modulated laser power and RCID noise significantly outpaces that of existing devices.

We report resonant cavity infrared detectors with a peak wavelength of 4.54-4.58 mu m that combine external quantum efficiency (EQE) exceeding 70% with spectral bandwidth 20-40 nm and <= 2% EQE at all non-resonance wavelengths between 4 and 5 mu m. A 300-nm-thick absorber assures that most of the radiation propagating in the cavity produces photocurrent rather than parasitic loss. The cavity is formed by heterogeneously bonding a midwave infrared (MWIR) nBn detector chip to a GaAs/AlGaAs distributed Bragg reflector, etching away the GaSb substrate, forming mesas with diameter approximate to 100 mu m, depositing a Ge spacer, and then depositing a single-period Ge-SiO2 top mirror. At all temperatures between 125 and 300 K, the responsivity at 150 mV bias exceeds 2.2 A/W and the EQE exceeds 61%. When the thermal background current for a realistic system scenario with f/4 optic that views a 300 K scene is derived from the observed EQE spectra, the resulting specific detectivity D* of 7.5 x 10(12) cmHz(1/2)/W at 125 K operating temperature is 4.5 times higher than for a state-of-the-art broadband MWIR HgCdTe device. Simulations of the cavity performance indicate that EQE > 90% may be feasible following minimization of parasitic optical loss and maximization of the photocarrier collection efficiency. Potential applications include free space optical communication, chemical sensing, on-chip spectroscopy, and hyperspectral imaging.

Inclusion of Si atoms to the growth surface during the molecular beam epitaxy of Ge and Sn to form a SiGeSn alloy was identified as a reactive surface species and as a means to compensate strain, which allowed for the subsequent growth of GeSn alloys with high Sn content. The development of a SiGeSn virtual substrate having a 15% Sn concentration and lattice parameter larger than 5.72 Å is demonstrated, using atomic force microscopy, x-ray reciprocal space mapping, and transmission electron microscopy, as a method for the direct growth of thick (>500 nm) fully relaxed GeSn alloys with greater than 10% Sn. This buffer layer enables the monolithic integration of GeSn with silicon for optoelectronic applications, as the SiGeSn virtual substrate allows for selective chemical etching of GeSn, which is important for device fabrication.

We report a resonant cavity infrared detector (RCID) with an InAsSb/InAs superlattice absorber with a thickness of only ≈ 100 nm, a 33-period GaAs/Al0.92Ga0.08As distributed Bragg reflector bottom mirror, and a Ge/SiO2/Ge top mirror. At a low bias voltage of 150 mV, the external quantum efficiency (EQE) reaches 58% at the resonance wavelength λres ≈ 4.6 µm, with linewidth δλ = 19-27 nm. The thermal background current for a realistic system scenario with f/4 optic that views a 300 K scene is estimated by integrating the photocurrent generated by background spanning the entire mid-IR spectral band (3-5 µm). The resulting specific detectivity is a factor of 3 lower than for a state-of-the-art broadband HgCdTe device at 300 K, where dark current dominates the noise. However, at 125 K where the suppression of background noise becomes critical, the estimated specific detectivity D* of 5.5 × 1012 cm Hz½/W is more than 3× higher. This occurs despite a non-optimal absorber cut-off that causes the EQE to decrease rapidly with decreasing temperature, e.g., to 33% at 125 K. The present RCID's advantage over the broadband device depends critically on its low EQE at non-resonance wavelengths: ≤ 1% in the range 3.9-5.5 µm. Simulations using NRL MULTIBANDS indicate that impact ionization in the bottom contact and absorber layers dominates the dark current at near ambient temperatures. We expect future design modifications to substantially enhance D* throughout the investigated temperature range of 100-300 K.

For some applications, resonant cavity infrared detectors (RCIDs) offer advantages over traditional broadband photodetectors. The addition of a resonant cavity allows for higher external quantum efficiency (EQE), faster response time, and narrower spectral response for enhanced selectivity. Recently, the US Naval Research Laboratory demonstrated RCIDs with EQE of 34% and D∗ of 7 × 109 at room temperature, centered at 4.0 μm (46 nm FWHM). Princeton University has demonstrated that these RCIDs can detect gas-phase nitrous oxide (N2O) at room temperature with only a broadband light source and no other optical components. The results imply that a simple RCID-LED pair manufactured on a semiconductor wafer would provide a viable gas sensor. The manufacturing process could be completely automated, resulting in mass-producible optical gas sensors. Progress has been made for developing RCIDs at other wavelengths. Based on the achieved detection limit of 4% N2O at 4.0 μm, with 3 cm path length, leak detection of percentage-level concentrations of gases is definitely viable. The potential for operating at a more optimal wavelength to attain high-precision measurements at part-per-million (ppm) levels is still under investigation.

We achieved a detection limit of 400 ppmv N2O (5.5× 10 − 5 absorbance) at 19 °C and 4.0 µm, using emission from an interband cascade light-emitting device (ICLED) in combination with detection by a resonant cavity IR detector (RCID). These results demonstrate the feasibility of combining ICLEDs and RCIDs on a single mass-produced chip for optical gas sensing.

Naturally occurring materials with hyperbolic optical properties are attracting considerable interest due to their ability to confine light in small volumes and their resulting potential for applications in photonics. This paper uses a first-principles theoretical approach without adjustable parameters to investigate the hyperbolic optical properties of bulk calcite (CaCO3). This material exhibits natural hyperbolic behavior within its Reststrahlen bands at 1403–1552 and 864–887 cm−1. The calculated results are shown to be in good agreement with our reflectance data obtained from frequency- and polarization-dependent measurements. These results show that calcite is an attractive natural hyperbolic material; in addition, it has the advantage of low losses and is available commercially in a variety of crystal orientations.

Single crystal (−201) β-Ga 2 O 3 substrates doped with Si and Sn have been thermally annealed in N 2 and O 2 atmospheres. Structural and electrical properties evaluation was performed via a number of experimental methods in order to quantify the effects of the doping and annealing ambient on the properties of these samples. All samples annealed in O 2 exhibited significantly lower carrier concentration, as determined by capacitance–voltage measurements. Schottky barrier diodes exhibited excellent rectification when the Ga 2 O 3 was annealed in N 2 , and significantly lower forward current using O 2 -annealed Ga 2 O 3 substrates. Deep level transient spectroscopy revealed four deep trap levels with activation energies in the range of 0.40–1.07 eV. Electron spin resonance showed a decrease in shallow donor concentration, and cathodoluminescence spectroscopy revealed nearly two orders of magnitude lower emission intensity in O 2 -annealed Ga 2 O 3 samples. Raman spectroscopy revealed a carrier concentration dependent Raman mode around 254 cm −1 observed only when the final anneal of (−201) β-Ga 2 O 3 was not done in O 2 . Secondary ion mass spectroscopy measurements revealed diffusion of unintentional Fe towards the surface of the (−201) Ga 2 O 3 samples after annealing in O 2 . Depth resolved positron annihilation spectroscopy showed an increased density of vacancy defects in the bulk region of O 2 -annealed Ga 2 O 3 substrates.

Using a 2D metallic grating, we demonstrate the resonant enhancement of the external quantum efficiency of an nBn structure with a 0.5 mu m-thick-absorber. A total of two structures were studied with different barrier materials: ternary AlInSb and an InAs/AlInSb superlattice (SL). The device using an SL barrier had a diffusion-limited dark current of 6.7 mu A/cm(2) at 150 K, which was four times lower than the ternary-barrier device. The surface plasmon polariton (SPP) resonance wavelength for devices with six different grating periods varied as predicted by simulations. The quantum efficiency (QE) was enhanced by up to 56% by coupling to the SPP mode. A peak external QE of 39% was achieved at 4 mu m with a 1100 nm grating period.

A resonant-cavity detector with peak sensitivity at 4.0 μm reaches 34% external quantum efficiency at room temperature, despite having only five absorbing quantum wells. Multiple passes enhance the peak absorption by nearly x30. © 2019 The Author(s)

We report on an investigation of dark current contributions from common microscale crystallographic defects in type-II InAs/GaInSb superlattice infrared PIN type photodiode structures grown on (100) GaSb substrates and have identified three general classifications. Defects on several wafers of varying design were examined from multiple perspectives to correlate electrical activity with structural properties, to develop the identification and classification scheme. Active defects were first identified by current density vs voltage (J-V) measurements and electron beam induced current (EBIC) scans of individual diodes with micrometer resolution. The EBIC scans were then correlated with plan-view optical and atomic force microscopy images, both before and after anisotropic etch-pit analysis using a newly developed etchant. The atomic scale structure of active and inactive defects was then compared using cross-sectional transmission electron microscopy (TEM) on vertical slices of defects extracted using focused ion beam milling. Analysis of the TEM images yielded important clues as to the structure and root causes of benign and active defects, in which only significant disruptions at the epi-substrate interface appear to play a key role in producing microscale defects that efficiently promote dark current.

We report resonant-cavity infrared detectors with absorbers that consist of only five quantum wells, but exhibiting 34% external quantum efficiency at room temperature at the resonant wavelength of 4.0 μm. The FWHM linewidth is 46 nm, and the peak absorption is enhanced by nearly a factor of 30 over that for a single pass through the absorber. Although the Shockley-Read lifetime in the current material is much shorter than the state of the art, the dark current density is at the level of HgCdTe detectors as quantified by “Rule 07”. The Johnson-noise limited detectivity (D*) at 21°C is 7 × 109 cm Hz½/W. We expect that future improvements in the device design and material quality will lead to higher quantum efficiency, as well as a significant reduction of the dark current density consistent with the very thin absorber.

We report resonant-cavity infrared detectors with 34% external quantum efficiency at room temperature at the resonant wavelength of 4.0 μm, even though the absorber consists of only five quantum wells with a total thickness of 50 nm. The full width at half maximum (FWHM) linewidth is 46 nm, and the peak absorption is enhanced by nearly a factor of 30 over that for a single pass through the absorber. In spite of an unfavorable Shockley-Read lifetime in the current material, the dark current density is at the level of state-of-the-art HgCdTe detectors as quantified by "Rule 07." The Johnson-noise limited detectivity (D*) at 21°C is 7 × 109 cm Hz½/W. We expect that future improvements in the device design and material quality will lead to higher quantum efficiency, as well as a significant reduction of the dark current density consistent with the very thin absorber.

For diffusion limited nBn detectors, using an absorption layer much thinner than the optical attenuation length and minority carrier diffusion length can improve the dark current. As the absorber thickness decreases, the lower dark current increases the signal-to-noise ratio to provide greater sensitivity or higher temperature operation. However, if the quantum efficiency (QE) also decreases with absorber thickness, the advantage of reduced dark current is eliminated. Here we discuss the use of a metallic grating to couple the incident light into laterally-propagating surface plasmon polariton (SPP) modes, so as to increase the effective absorption length. We fabricate the gratings using a deposited Ge layer, which provides a uniform grating profile without increasing the dark current. Using this process in conjunction with a 0.5 μm-thick InAsSb absorber lattice-matched to GaSb, we demonstrate an external QE of 34% for T = 78–240 K. An nBn structure with an InAs0.8Sb0.2 absorber that is grown metamorphically on GaSb using a step-graded InGaSb buffer has a peak external QE of 39% at 100 K, which decreases to 32% by 240 K. Finally, we demonstrate that a grating with SPP resonance near the bandgap extends the absorption band, and can potentially reduce the dark current by a factor of 3–8 in addition to the 5× reduction due to the thinner absorber.

We demonstrate a strategy for increasing the operating temperatures of nBn midwave infrared (MWIR) focal plane arrays, based on the use of two-dimensional plasmonic gratings to enhance the quantum efficiency (QE) of structures with very thin absorbers. Reducing the absorber volume correspondingly reduces the dark current in a diffusion-limited photodiode, while light trapping mediated by the plasmonic grating increases the net absorbance to maintain high QE. The plasmonically enhanced nBn MWIR sensors with absorber thicknesses of only 0.5 μm exhibit peak internal QEs as high as 57%, which enables a 5-fold reduction in dark current. Numerical simulations indicate the potential for further improvement.

The narrow band gap and staggered band alignment of InAsSb alloys make it possible to engineer type-II superlattices (T2SLs) for mid-wave and long-wave (LW) infrared sensors operating in the 3–12 μm range. However, InAs/InAsSb T2SLs that are strain balanced to the underlying GaSb substrate have much lower absorption coefficients for LWIR operation because of the larger superlattice (SL) period, leading to reduced electron-hole overlap. The absorption coefficient of T2SLs can be greatly improved by growing on metamorphic buffers (MBs) with reduced lattice mismatch to the InAsSb layers, which allows the SL period to be reduced. For this study, MBs were capped with InAs/InAsSb T2SLs to assess the suitability of the materials for detector applications by X-ray diffraction and photoluminescence lifetime measurements. We show that the absorption of T2SLs can be significantly increased with no apparent degradation in the minority-carrier lifetime.