nBn Detectors Research Articles

Evidence for ionization damage in mid-wave infrared nBn detectors

The effects of 63 MeV proton and 60Co gamma irradiation on the operation of 4.3 μm cutoff nBn photodetectors are demonstrated separately, and both are shown to yield a total ionizing dose (TID) effect. The effect is shown here in an InAsSbBi nBn detector and has been observed in other bulk alloy nBn detectors, and is unusual as it is notably absent in superlattice nBn detectors. The non-antireflection coated detectors exhibit a pre-radiation quantum efficiency of 17% at 3.3 μm wavelength and a dark current density of 50 μA/cm2, or roughly 300× the Rule 07 expectation, at their ideal operating voltage of −0.4 V bias at 150 K. Step-wise proton irradiation and in situ measurement indicate that the dark current increases to about 400× Rule 07 at the highest proton dose level of 150 krad(Si) (9.10 × 1011 p+/cm2), while the quantum efficiency is degraded at a relatively faster rate than the majority of analogous detectors characterized by our lab. Both the photocurrent and dark current are also shown to exhibit a turn-on voltage magnitude reduction of 100 mV following either gamma or proton irradiation, a trend which is attributable to negative trapped charge at the barrier interface (TID effect). This theory is further supported by an observed capacitance density magnitude reduction with dose and affirmed with Silvaco TCAD simulations. Following both proton exposure and subsequent anneal and gamma exposure and subsequent anneal; dark current, photocurrent, and CV all approach their pre-radiation baseline values.

Barrier in the valence band in the nBn detector with an active layer from the type-II superlattice

Barrier in the valence band in the nBn detector with an active layer from the type-II superlattice

Effects of doping and minority carrier lifetime on mid-wave infrared InGaAs/InAsSb superlattice nBn detector performance

The effect of majority carrier concentration and minority carrier lifetime on the performance of mid-wave infrared ( λ cutoff = 5.5 μ m ) nBn detectors with variably doped InGaAs/InAsSb type-II superlattice absorbers is investigated. The detector layer structures are grown by molecular beam epitaxy such that their absorbing layers are either undoped, uniformly doped with a target density of 4 × 1015 cm−3, or doped with a graded profile, and variable-area mesa detector arrays are fabricated. Each material's temperature-dependent minority carrier lifetime is determined by time-resolved photoluminescence, and majority carrier concentration is extracted from capacitance–voltage measurements. Detector performance is evaluated with dark current and photocurrent measurements, from which quantum efficiency and shot-noise-limited noise-equivalent irradiance are calculated. The two doped detectors have lower dark current densities compared to their undoped counterpart due to the reduction in diffusion current as well as suppression of depletion current. Although both intentionally doped devices exhibit lower minority carrier lifetimes relative to the undoped device, the device with graded doping maintains a comparable quantum efficiency to the undoped device. Ultimately, the graded doping structure exhibits the highest sensitivity with a shot noise-limited noise-equivalent irradiance of 6.3 × 1010 photons/cm2 s in low-background light conditions, within a factor of 4× of an infrared detector pixel with Rule 07 dark current density and unity quantum efficiency. A detailed analysis of the dark current, quantum efficiency, and minority carrier lifetime provides insight into the material and device design factors that must be considered to realize a device with optimal sensitivity.

Design and modeling of high-performance mid-wave infrared InAsSb-based nBn photodetector using barrier band engineering approaches

We report a new nBn photodetector (nBn-PD) design based on the InAlSb/AlSb/InAlSb/InAsSb material systems for mid-wavelength infrared (MWIR) applications. In this structure, delta-doped compositionally graded barrier (δ-DCGB) layers are suggested, the advantage of which is creation of a near zero valence band offset in nBn photodetectors. The design of the δ-DCGB nBn-PD device includes a 3 µm absorber layer (n-InAs0.81Sb0.19), a unipolar barrier layer (AlSb), and 0.2 μm contact layer (n-InAs0.81Sb0.19) as well as a 0.116 µm linear grading region (InAlSb) from the contact to the barrier layer and also from the barrier to the absorber layer. The analysis includes various dark current contributions, such as the Shockley–Read–Hall (SRH), trap-assisted tunneling (TAT), Auger, and Radiative recombination mechanisms, to acquire more precise results. Consequently, we show that the method used in the nBn device design leads to diffusion-limited dark current so that the dark current density is 2.596 × 10−8 A/cm2 at 150 K and a bias voltage of − 0.2 V. The proposed nBn detector exhibits a 50% cutoff wavelength of more than 5 µm, the peak current responsivity is 1.6 A/W at a wavelength of 4.5 µm and a − 0.2 V bias with 0.05 W/cm2 backside illumination without anti-reflective coating. The maximum quantum efficiency at 4.5 µm is about 48.6%, and peak specific detectivity (D*) is of 3.37 × 1010 cm⋅Hz1/2/W. Next, to solve the reflection concern in this nBn devices, we use a BaF2 anti-reflection coating layer due to its high transmittance in the MWIR window. It leads to an increase of almost 100% in the optical response metrics, such as the current responsivity, quantum efficiency, and detectivity, compared to the optical response without an anti-reflection coating layer.Graphical

Assessment of surface recombination in mid-wave infrared InAsSb nBn detectors using transient microwave reflectance

Investigation of surface recombination is an important area for infrared detectors as the demand for smaller pixels increases. In this study, we use transient microwave reflectance to characterize the minority carrier lifetime of InAsSb nBn structures under three conditions: As-Grown, blanket Barrier-Etched, and SU-8 Passivated blanket barrier-etched. A qualitative comparison of these three samples shows that the minority carrier lifetime decreases for the Barrier-Etched sample compared to the As-Grown sample, indicating that the minority carrier lifetime is sensitive to changes in the sample surface, specifically the introduction of surface states. We compare these samples quantitatively using a polynomial fit (A−1 + Bn + Cn2). We find for the As-Grown sample A = 1.22 ± 0.45 µs, B = 2.5 ± 0.2 × 10−12 cm3/s, and C = 5.004 ± 4.996 × 10−31 cm6/s, for the Barrier-Etched sample A = 1.17 ± 0.40 µs, B = 9.9 ± 0.2 × 10−12 cm3/s, and C = 9.502 ± 9.498 × 10−30 cm6/s, and for the Passivated sample A = 1.34 ± 0.45 µs, B = 5.3 ± 0.5 × 10−12 cm3/s, and C = 5.500 ± 4.500 × 10−33 cm6/s.

All-epitaxial resonant cavity enhanced long-wave infrared detectors for focal plane arrays

We demonstrate a monolithic all-epitaxial resonant-cavity architecture for long-wave infrared photodetectors with substrate-side illumination. An nBn detector with an ultra-thin (t≈350 nm) absorber layer is integrated into a leaky resonant cavity, formed using semi-transparent highly doped (n++) epitaxial layers, and aligned to the anti-node of the cavity's standing wave. The devices are characterized electrically and optically and demonstrate an external quantum efficiency of ∼25% at T=180 K in an architecture compatible with focal plane array configurations.

Infrared nBn detectors monolithically integrated with metasurface-based optical concentrators

We demonstrate InAsSb-based infrared photodetectors monolithically integrated with metasurface lenses (metalenses) that act as optical concentrators. The metalenses are fabricated on the backside of a gallium antimonide substrate, and the photodetectors are fabricated on the frontside of the same substrate. The metalenses enhance the detector responsivity up to 10-fold and detectors retain the same detectivity at temperatures up to 40 K higher. Detectors integrated with metalenses exhibit detectivities greater than D*=1010 cmHz/W at λ=3.75 μm up to room temperature. Our metasurface-based optical concentrators are based on circular subwavelength nanoposts, which renders their performance polarization independent.

Electrical Characteristics of a Ga-free T2SL Mid-wave Infrared nBn Detector Based on an InAs/AlAsSb/InAsSb Barrier

Electrical Characteristics of a Ga-free T2SL Mid-wave Infrared nBn Detector Based on an InAs/AlAsSb/InAsSb Barrier

Comparative advantages of a type-II superlattice barrier over an AlGaSb barrier for enhanced performance of InAs/GaSb LWIR nBn photodetectors.

The barrier layer in InAs/GaSb LWIR nBn detector is usually composed of AlGaSb alloy, which has a non-negligible valence band offset and is sensitive to chemical solutions. In this work, we investigated a type-II superlattice (T2SL) barrier that is homogeneous with the T2SL absorber layer in order to resolve these drawbacks of the AlGaSb barrier. The lattice mismatch of the T2SL barrier was smaller than that of the AlGaSb barrier. At -70mV and 80 K, the dark current density and the noise equivalent temperature difference of the nBn devices with the T2SL barrier were 4.4×10-6A/cm2 and 33 mK, respectively.

Aдмиттанс МДП-структур на основе nBn-систем из эпитаксиального HgCdTe, разработанных для детектирования в спектральном диапазоне 3-5 μm

The admittance of test MIS devices based on nBn structures from Hg1-xCdxTe grown by molecular beam epitaxy (MBE) is investigated. The composition x in the absorbing layer was 0.29, and the composition in the barrier layer was 0.60. An equivalent circuit of a MIS device based on an nBn structure is proposed and the values of the elements of this circuit are found under various conditions. Comparison of the temperature dependence of the barrier resistance with the Rule07 model indicates the possibility of creating efficient nBn detectors based on MBE HgCdTe for the spectral range of 3–5 μm.

Admittance of metal–insulator–semiconductor devices based on HgCdTe nBn structures

Metal-insulator-semiconductor (MIS) structures were fabricated by depositing an Al2O3 dielectric on top of the contact layer of a mid-wave infrared nBn detector based on n-Hg1-xCdxTe grown by molecular beam epitaxy on GaAs (013) substrates. It is shown that when creating a backward electrode on the absorbing layer, the form of the capacitance–voltage characteristics depends not only on the properties of the contact layer, but also on the properties of the barrier and absorbing layers. An equivalent circuit of MIS structure based on nBn detector with mesa configuration is proposed. It is shown that measurements of the admittance of MIS structures with mesa configuration allow one to study the properties of the barrier layer in the nBn structure. The influence of infrared radiation on the temperature dependence of the barrier layer resistance is studied.

High-performance midwavelength infrared detectors based on InAsSb nBn design**Project supported by the National Key Technologies Research and Development Program of China (Grant No. 2018YFA0209104) and the Major Program of the National Natural Science Foundation of China (Grant No. 61790581).

we report nBn photodetectors based on InAs0.91Sb0.09 with a 100% cut-off wavelength of 4.75 μm at 300 K. The band of an nBn detector is similar to that of a standard pin detector, but there is special wide bandgap AlAs0.08Sb0.92 barrier layer in the nBn detector, in which the depletion region of nBn detector exists. The nBn design has many advantages, such as low dark current and high quantum efficiency, because the nBn design can suppress the generation–recombination (GR) current that is the main composition of standard pin detector dark current. The constant slope of the Arrhenius plot of J0–1/T indicates the absence of the generation–recombination dark current. We fabricate an nBn detector with a quantum efficiency (QE) maximum of ∼ 60% under −0.2-V bias voltage. The InAsSb nBn detectors may be a competitive candidate for midwavelength infrared detector.

Temperature dependence of quantum efficiency enhancement using plasmonic gratings on nBn detectors with thin absorbers

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 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. We discuss the use of a metallic grating to couple the incident light into laterally propagating surface plasmon polariton (SPP) modes and increase the effective absorption length. We fabricate the gratings using a deposited Ge layer, which provides a uniform 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 to 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 another factor of 3 to 8 times in addition to the 5 × reduction due to the thinner absorber.

High performance nBn detectors based on InGaAsSb bulk materials for short wavelength infrared detection

We report a low dark current, high quantum efficiency nBn photodetectors based on In0.28Ga0.72As0.25Sb0.75 bulk materials with a cut-off wavelength of 3μm at room temperature. Generation-Recombination current was suppressed using an nBn design to shift deplete region from In0.28Ga0.72As0.25Sb0.75 active region into a wide band gap AlGaSb barrier region. The Arrhenius plots of R0A-1/T show that there is no Generation-Recombination current detected in nBn device, whereas pin devices have a Generation-Recombination dominant dark current at temperatures ranging from 160K to 220K. Optical characterizations at 300K show the nBn device using InGaAsSb as an absorption material has a relative high quantum efficiency. The nBn device exhibits a peak specific detectivity of 4.8×1010 Jones under 200mV reverse bias voltage at 300K. The low dark current, high quantum efficiency and band gap tunability are expected to make InGaAsSb bulk material a competitive candidate for short wavelength infrared regime.

Resonant cavity-enhanced photodetector incorporating a type-II superlattice to extend MWIR sensitivity.

Mid-infrared resonant cavity-enhanced photodetectors (RCE PD) present a promising technology for targeted gas detection. We demonstrate an RCE PD incorporating an InAs/InAsSb superlattice as the detecting element, extending the resonant wavelength beyond 4 µm. AlAsSb/GaSb mirrors and a unipolar barrier active region paralleling an nBn structure are also used, and performance is compared to a conventional broadband nBn detector incorporating the same superlattice. The RCE PD exhibited a Q-factor of ∼90 and an extremely stable resonance wavelength. Peak responsivity was 3.0 A W-1 at 240 K, equalling 84% quantum efficiency, a 5.5 times increase over the reference nBn at the same wavelength. Dark current density was 3.3×10-2 A cm-2 at 240 K, falling to 2.7×10-4 A cm-2 at 180 K. The broadband BLIP limit is approached at 180 K with specific detectivity of 2.1×1011 cm Hz1/2 W-1, which presents the potential of achieving BLIP-limited operation in the thermoelectric cooling regime.

Wavelength dependence of intrinsic detection efficiency of NbN superconducting nanowire single-photon detector**Project supported by the National Key R&D Program of China (Grant No. 2017YFA0304000), the National Natural Science Foundation of China (Grant Nos. 61671438 and 61827823), the Science and Technology Commission of Shanghai Municipality, China (Grant No. 16JC1400402), Program of Shanghai Academic/Technology Research Leader, China (Grant No. 18XD1404600), and the

Superconducting nanowire single-photon detectors (SNSPDs) have attracted considerable attention owing to their excellent detection performance; however, the underlying physics of the detection process is still unclear. In this study, we investigate the wavelength dependence of the intrinsic detection efficiency (IDE) for NbN SNSPDs. We fabricate various NbN SNSPDs with linewidths ranging from 30 nm to 140 nm. Then, for each detector, the IDE curves as a function of bias current for different incident photon wavelengths of 510–1700 nm are obtained. From the IDE curves, the relations between photon energy and bias current at a certain IDE are extracted. The results exhibit clear nonlinear energy–current relations for the NbN detectors, indicating that a detection model only considering quasiparticle diffusion is unsuitable for the meander-type NbN-based SNSPDs. Our work provides additional experimental data on SNSPD detection mechanism and may serve as an interesting reference for further investigation.

Mid-wavelength infrared InAsSb/InAs nBn detectors and FPAs with very low dark current density

There was a significant improvement in the performance of infrared nBn detectors utilizing InAs/InAsSb absorbers culminating in the development of infrared focal plane arrays (FPAs) with excellent operability (99.7%) and operating at temperature significantly higher than InSb FPAs. In this work, we demonstrated that these detectors can operate with very low dark current densities enabling their use in applications with a low-to-medium level of background illumination. We showed that single pixel photodetectors with a cut-off wavelength of 4.8 μm and a quantum efficiency of QEmax = 35% under backside-illumination have a dark current density of 1 × 10−10 A/cm2 at an operating bias of −0.1 V and temperature T = 100 K. Additionally, we compared the single pixel dark current results with measurements of the dark current in FPAs. The FPA showed excellent performance with an operability of 99.7% and was able to reach a mode dark current density of 5 × 10−11 A/cm2 at 80 K.

Resonant cavity enhanced photodiodes on GaSb for the mid-wave infrared

We report the design, growth, processing, and characterization of resonant cavity enhanced photodiodes for the midwave infrared at ∼3.72 μm on GaSb. Using AlAsSb/GaSb mirrors, AlAsSb barrier and spacer layers and a thin 96 nm InAsSb absorber, we observed dark current and detectivity behavior superior to common InAsSb nBn detectors in the literature, with peak specific detectivity values of 8×1010 and 1×1010 cm Hz1/2 W−1 measured at 250 K and 300 K, respectively. In the same temperature range, the linewidth of the detector response was <44 nm and the quality factor ∼80. The peak quantum efficiency was >60% where the enhancement due to the resonant cavity was ∼20x. We estimate that the devices can operate close to, or slightly above, the background-limited infrared performance limit imposed on broadband detectors for a 300 K scene.

Spatial dependence of carrier localization in InAsSb/InSb digital alloy nBn detector

Recently we have demonstrated a novel method of extending the cut-off wavelength of InAsSb nBn detectors, by incorporating a series of monolayers of InSb, which we have termed a digital alloy. In recent work we have shown that while increasing temperature from 15 K to 40 K we observe a 14 meV blue shift of the photoluminescence peak energy and a decrease in minority carrier lifetime. We have hypothesized this effect is due to carrier localization caused by macro-scale fluctuations in layer thickness or alloy composition. Here we study the spatial variation of this effect over the surface of a sample, demonstrating significant variation with position. We discuss implications of this for the carrier localization hypothesis.

InGaAs nBn SWIR detector design with lattice-matched InAlGaAs barrier

Dark current optimization with band gap engineering has been numerically studied for InGaAs nBn type infrared photodetectors. Undoped InAlGaAs grading layers are utilized in constructing the barrier and dipole delta-doped layers are placed in both sides of the graded layers for eliminating valence band offset. As a result, the high band gap barrier layer blocks the majority carriers and allows minority carrier flow while minimizing various dark current components, as expected from an nBn detector. Substantial improvement has been shown in the dark current level without compromising any photoresponse compared to the conventional pn junction and recently proposed all InGaAs nBn type photodetectors.