NIR/SWIR FPAs and Applications

A number of trends were emphasized in near infrared and shortwave infrared in presentations at this conference:

In order to address the power consumption problem, one manufacturer developed a camera without a Thermo-Electric Cooler (TEC-less) 640 × 512 InGaAs camera with temperature-based non uniformity corrections that allow the power consumption to be 1.2 to 1.3 W over a -30 to 60 °C temperature range—see Fig. 1.

Fig. 1

Power vs. temperature comparing the TECless camera and the TEC variant.

A 1280 × 1024 InGaAs FPA with 10 μm pixels and an asynchronous laser pulse detection mode has been fabricated, thus providing both passive and active imaging in low-light-level conditions. An image from this FPA is shown in Figure 2.

Fig. 2

SWIR image with an existing 10 μm ROIC..

The latest results on an innovative InGaAs/GaAsSb T2SL SWIR FPA with 2.35 μm cutoff were reported. Dark currents were less than for SWIR HgCdTe FPAs. With thicker InGaAs the cutoff can be extended to 2.55 μm.

In other SWIR superlattice work, InAs/InAs_1-xSb_x/AlAs_-x Sb_x T2SL SWIR detectors on GaSb substrates, had a dark current density of 1.3×10^-8 A/cm² at 200 K, with 36% quantum efficiency.

In HgCdTe, SWIR FPAs with 2.5 μm cutoff, 640 × 512 formats with 15 μm pixels operating in both passive and active modes were demonstrated. These are currently made by means of LPE but for production transfer to MBE on GaAs substrates is expected.

For passive imaging, the dark current in Fig.3 is seen to be less than one order of magnitude higher than that for the empirical Rule 07. Work is ongoing to reduce the dark current further.

Fig. 3

Dark current density vs. 1/T for 1.8 μm and 2.5 μm cut-off FPAs compared with Rule 07.

For active imaging with APDs (Avalanche PhotoDiodes) special ROICs were designed for Gated Viewing (GV) with a 1.57 μm Nd:YAG laser illuminator

Type II superlattice FPAs

There were a total of ten papers in the two sessions devoted to Type II Superlattice and Barrier detectors, and several more on this subject in the Poster session. This reflects the continued strong interest in the potential performance advantages that this technology has been predicted to have theoretically—long carrier lifetimes and a high optical absorption coefficient. Experimentally, lifetimes as long as those predicted have not yet been achieved. Lifetimes are still shorter than for HgCdTe with comparable bandgaps. This year continued a larger focus on LWIR devices.

Passivation of Type II structures using the Gibbs free energy was the focus of the first paper in the topical area. The authors compared ALD-deposited Al₂O₃, HfO₂, TiO₂, ZnO, PECVD deposited SiO₂, Si₃N₄ and sulphur containing octadecanethiol (ODT) self-assembled monolayers (SAM) passivation layers on InAs/GaSb pin superlattice photodetectors with cutoff wavelength at 5.1 μm. Fig. 4 shows how the dark current varies with temperature for a variety of passivation choices. Best results were reported for Al₂O₃.

Fig. 4

Temperature dependent dark current density for unpassivated and passivated type-II InAs/GaSb superlattice 400 × 400 μm photodiodes at -0.5 V bias voltage.

GaSb oxide was investigated with respect to its affect on sidewall leakage for Type II mesa structures. After annealing at temperatures above 300 °C, free Sb was detected on the surfaces that can cause sidewall leakage.

LWIR Type II superlattice development was updated in an important paper that showed R₀A values for test structures and test devices clustered around 10 × and 20 × the HgCdTe benchmark Rule 07 as shown in Fig. 5. This development has been achieved with standard InAs/GaSb material that includes a majority-carrier barrier and that has a short lifetime ~30 nsec. Nevertheless, good imagery was shown for a 15 μm pitch 512 × 640 FPA at 77 K having > 50 % quantum efficiency with a 9.3 μm cutoff at f/2.7 as illustrated in Fig. 6.

Fig. 5

77 K dark current: comparison of Type II superlattice test devices and FPAs with MCT Rule 07 (solid blue line) over a similar range of cut-off wavelengths

Fig. 6

Image registered with a demonstration camera containing the 15μm pitch, 640 × 512 LW FPA, operating with F/2.7 optics at 77 K and a scene distance of about 5 km.

Asymmetrical MWIR InAs/GaSb superlattice pin photodiodes were found to have very short minority-carrier diffusion lengths due to short lifetime of 30-35 nsec, leading to low quantum efficiency. Results were improved to about 42 % quantum efficiency by reversing the side of the structure that was illuminated. Higher QE results were found with test structures having a p-type absorber rather than an n-type absorber, taking advantage of the improved mobility of electrons compared to holes.

Development of a 6 μm cutoff MWIR Type II superlattice FPA was described using InAs/GaSb combined with barrier layers. The pin structure is illustrated in Fig. 7. The dark current of the pBiBn structure was 4 × 10^-7 A/cm² at reverse bias of -20 mV, which is lower than that of the pin structure, 7 × 10^-7 A/cm². An FPA with 256 × 320 pixels and a pixel pitch of 30 μm was fabricated and imaged.

Fig. 7

Epitaxial wafer structure

An approach for fabricating an antimonide FPA without using indium bumps by employing a membrane-transfer process was described.

Recent results with LWIR Type II superlattice FPAs was presented, including a 640 × 512 LWIR heterojunction InAs/GaSb T2SL camera with 15 μm pixel pitch. Fig. 8 illustrates an image from this FPA operated at 55 K and having a cutoff of 10.3 μm.

Fig. 8

Image from a 640 × 512 LWIR heterojunction InAs/GaSb T2SL camera with 15 μm pixel pitch.

Ga-free InAs/InAs_1-xSb_x type-II superlattices were reported with LWIR cutoffs and an nBn structure that is illustrated in Fig. 9. With a 6 μm thick absorber, the quantum efficiency was said to be 54 % for a sample with a 50 % cutoff of 10 μm at 77 K. Fig. 10 shows the spectral D* of this LWIR device.

Fig. 9

Schematic diagram and working principle of the nBn photodetector. The barrier blocks the transport of majority electrons, while allowing the diffusion of minority holes and photogenerated carriers from the active region on the left.

Fig. 10

Detectivity spectrum of the device with a 6 μm-thick absorption region at -90 mV applied bias voltage in front-side illumination configuration without any anti-reflection coating. Inset: Detectivity of the device at 7.9 μm under front-side illumination as a function of applied bias voltage. Detectivity is calculated based on the equation in the inset, where R_i is the device responsivity, J is the dark current density, RA is the resistance-area product, k_b is the Boltzmann constant, and T is the operating temperature.

Results from production and development of SWIR, MWIR, and LWIR Type-II superlattice InAs/GaSb detectors was presented. A new large array – see Fig. 11—is being developed in a 1280 × 1024 format with 12 μm pitch and is planned to be available in both MWIR and LWIR bands. Fig. 12 shows the LWIR spectral response.

Fig. 11

Photo of a new Type II superlattice FPA with a resolution of 1280 × 1024 and having 12 μm pitch pixels.

Fig. 12

LWIR Type II spectral response at 77 K as a function of bias, showing that the response is fully turned on already at 25 mV.

Dual-band barrier detectors featuring two LWIR bands—9.2 and ~12 μm—was the subject of another presentation. Quantum efficiency for the two bands as a function of bias is shown in Fig. 13.

Fig. 13

Bias-dependent QE measurements performed with 6.5 μm and 9.75 μm filters at 78 K.

Large, encapsulant-free GaSb single crystals were grown using the modified Czochralski method, yielding more than seventy 150 mm wafers per crystal or several hundred 75 mm or 100 mm wafers per crystal. Fig. 14 shows one of the large boules.

Fig. 14

GaSb boule capable of yielding over 70 6-inch wafers.

Progress in the development of current substrate polishing techniques has been demonstrated to deliver a consistent, improved surface on GaSb wafers with a readily desorbed oxide for epitaxial growth according to a companion paper. Six wafer polishing variants were compared.

InSb crystal growth improvements were described in a paper with a goal of growing 6-inch diameter <111> ingots weighing as much as 30 kg. Reduction of micro-resistivity striations was reported by control of the growth interface.

QWIP and colloidal quantum dot (CQD) detectors

An update was given on the development of resonator QWIP detectors. A quantum efficiency of 37% and conversion efficiency of 15% in a 1.3 μm-thick active material and 35% QE and 21% CE in a 0.6 μm-thick active material. Both detectors have a cutoff at 10.5 μm with a 2 μm bandwidth. The temperature at which photocurrent equals dark current is about 65 K under F/2 optics. The thicker detector shows a large QE polarity asymmetry due to nonlinear potential drop in the QWIP material layers. For one design, an FPA was measured with NEΔT of 27.2 mK, with 99.5% oper-ability at 55 K under F/2.5 optics and 4.46 ms integration time. Fig. 15 shows an image taken with one of these resonator QWIP FPAs.

Fig. 15

Image from a QWIP FPA at V ~ -1.1 V bias and T = 61 K.

QWIP production of FPAs with 640 × 480 and 384 × 288 pixels with 25 μm pitch totaled nearly 200 units in 2015 and are projected to double in 2016. MOVPE was used for the growth of quantum wells on GaAs substrates. Fig. 16 shows a histogram of the NEΔT for production the larger format.

Fig. 16

Distribution of NET for more than 500 production QWIP FPAs in a 640 × 480 format.

MWIR detection with HgTe colloidal quantum dots was reported. Fig. 17 illustrates the detector concept. Imagery was shown with FPAs made from this material.

Fig. 17

Diagram of photoconduction in a CQD detector. A photon excites a CQD. The excited electron “hops” from dot-to-dot, generally drifting up the electric field to the anode. The “hole” is filled by electrons that hop up the electric field from the cathode.

High Operating Temperature (HOT) FPAs

The goal of increasing the operating temperature of FPAs without sacrificing performance is motivated by the reduction in cooler power, improved cooler efficiency, longer cooler lifetime, smaller imager size, and lighter weight sensor systems that this makes possible. This goal is being pursued using HgCdTe, Type II superlattices, and nBn materials and has relevance especially in the MWIR and LWIR spectral bands.

Planar LWIR and VLWIR HgCdTe detectors in both p-on-n and n-on-p polarities were discussed. Thermal dark currents have been significantly reduced as compared to ‘Tennant’s Rule 07’ in both diode polarities, with quantum efficiency ≥ 60% and for operating temperatures between 30 K and 100 K. Fig. 18 shows the dark current vs. 1/lT. The demonstrated detector performance paves the way for a new generation of higher operating temperature LWIR MCT FPAs with < 30 mK NETD up to a 110 K detector operating temperature and with good operability. This allows for the same dark current performance at a 20 K higher operating temperature than with previous technology.

Fig. 18

Thermal dark current density behavior versus 1/lT for p-on-n LWIR and VLWIR MCT detector devices with responsivity cut-off wavelengths at 80 K as stated in the inset. BLIP D* for 5-stage devices with absorption QE of 70%, both under 300 K background with 2π field of view (FOV).

MWIR XBn detector arrays with 10 μm pitch and a large 1920 × 1536 format were reported. Data for operation up to 150 K with a cutoff of 4.2 μm was shown. The dark current histogram was well behaved at 150 K as shown in Fig. 19.

Fig. 19

Histogram of the dark current at 150 K from all pixels of the 1920 × 1536 FPA.

Progress in MOCVD growth of HgCdTe was described, enabling advances in HOT detector operation. Changing the growth orientation from <111> to <100> enabled higher performance operation for 13 μm cutoff photoconductors. In addition, barrier detectors were reported for both MWIR and LWIR bands. An n⁺p⁺BpπN⁺ 10 μm cutoff LWIR structure enabled operation with carrier extraction at 230 K. Fig. 20 illustrates this structure.

Fig. 20

LWIR n⁺p⁺BpN⁺ HgCdTe structure and schematic photodiode band diagram. x is the alloy composition, N_A is the acceptor concentration, N_D is the donor concentration and π denotes the absorber region with low p-type extrinsic concentration.

SWIR Type II superlattice detectors using Ga-free material were reported with operation in the 200 - 300 K range and spectral cutoffs—as shown in Fig. 21—of 1.7 - 1.8 μm. At 300 K, the device exhibited a specific detectivity of 6.45 × 10¹⁰ Jones with a 300 K 2π field of view.

Fig. 21

Saturated 200 and 300 K quantum efficiency spectrum of the device under zero-bias condition in frontside illumination configuration without any anti-reflection coating. Inset: The %50 cut-off wavelength variation of the device vs. temperature between 200 to 300 K.

SWIR interband cascade detectors having a 3 μm cutoff and operating between 280 and 340 K were pre-sented. D* was limited by Johnson noise, as shown in Fig. 22. A mid-wave device was reported to have response approaching 1 GHz.

Fig. 22

Johnson-noise limited detectivity for twoand three-stage interband cascade devices.

Recent developments achieved in terms of HOT MCT extrinsic p on n technology, blue MW band (4.2 μm at 150 K) and extended MW band (5.3 um at 130 K) were reviewed. Fig. 23 shows the dark current for both red and blue bands vs. temperature compared to Rule 07. This paper also discusses reduction of 1/f noise and the properties of random telegraph noise or signal (RTN or RTS). Fig. 24 shows how the activation energy of RTN noise varies with bandgap.

Fig. 23

Mean dark current vs. temperature for MWIR redand blue bands compared with Rule 07.

Fig. 24

Random telegraph noise activation energies as a function of the bandgap energy.

In_0.982Al_0.018Sb diodes were fabricated on InSb substrates, followed by mesa diode fabrication. The 4.8 μm cutoff devices were imaged at 80 and 110 K.

HgCdTe

The HgCdTe alloy detector—characterized by a high absorption coefficient and a long lifetime—continues to dominate the choice for a broad range of infrared applications. Aside from applications that are ideal for either InSb in the MWIR spectral band, or InGaAs in the 1.7 μm SWIR band, or those that can utilize uncooled FPAs, HgCdTe continues to be the most popular choice. Papers in this section update how HgCdTe is continuing to develop and evolve. Papers on this topic were presented in the session on HgCdTe detectors as well as in the SWIR and HOT sessions, the Applications sessions, and in the Poster session.

HgCdTe for a variety of space missions was reviewed. This included SWIR bands around 2 - 3 μm for astrophysics, MWIR and LWIR bands for exoplanet discovery, and VLWIR for atmospheric sounding. Fig. 25 shows an arrhenius plot for some MWIR and LWIR devices and for both diode polarities. Fig. 26 shows dark current at 78 K for both diode polarities as a function of the cutoff wavelength.

Fig. 25

Arrhenius plot with dark current data for MWIR-LWIR n/p and p/n data

Fig. 26

Summary of dark currents measured at 78K for both n/p and p/n diodes from LWIR up to VLWIR spectral ranges.

Progress has been made in the development of 8 μm pitch MOVPE-grown HgCdTe photodiodes in a 1280 × 1024 format. A major effort has gone into raising the operating temperature of these 5+ μm cutoff MWIR devices. Fig. 27 shows the median NEΔT as a function of operating temperature for these FPAs. An image of a container ship taken with this array at ~1.6 km distance using 75 mm f/2.8 optics is shown in Fig. 28. An LWIR FPA based upon this format is under consideration, as well as additional formats, including a full 1080p format array, 4 K × 4 K arrays for surveillance, and smaller formats for handheld applications.

Fig. 27

NEΔT for 1280 × 1024 FPAs having 8 μm pitch pixels at f/2.8 as a function of the operating temperature.

Fig. 28

Image of a container ship at ~ 1.6 km being unloaded taken by an MWIR HgCdTe array with 75 mm f/2.8 optics.

The development of prototype MWIR—5.4 μm cut-off—XGA format (1024 × 768) HgCdTe detector arrays with 10 μm pitch were reported. The associated readout provided 2.8 × 10⁶ charge storage in an integrate-while-read mode, and 4.9 × 106 for integrate-then-read (ITR) operation. Smaller pixel development for LWIR and 3^rd gen is using 12 μm pixels in a 1280 × 720 format. Fig. 29 shows the histogram of an LWIR array in this format. The ITR mode with this larger pixel has a charge storage capacity of 8.3 × 10⁶ electrons.

Fig. 29

NEΔT for a 9 μm cutoff array with 12 μm pixels in a 1280 × 720 format.

MBE growth of HgCdTe on GaSb substrates has been explored and compared with growth on GaAs and CdZnTe. Fig. 30 shows the lattice and coefficient of thermal expansion of these materials. GaSb was found to have a lower etch pit density compared to GaAs and comparable X-ray rocking curve value. Device development has concentrated on nBn detector structures. These were grown to compare a solid barrier layer with a superlattice barrier in order minimize the voltage needed to overcome the minority carrier barrier. Fig. 31 shows the structure for the case of a superlattice barrier.

Fig. 30

Lattice and CTE mismatch between HgCdTe and several potential alternative substrates.

Fig. 31

HgCdTe nBn structure having a superlattice barrier layer.

A presentation describing the transition to smaller pitch—15 to 10 μm—was given covering both n-on-p and p-on-n diode polarities. Dark currents were significantly lower for the p-on-n polarity, but the smaller diodes in this case showed excess dark current ~30 %. This may have been due to the test structure design interacting with the long diffusion lengths (25 -30 μm) in this case—see Fig. 32.

Fig. 32

Extracted minority carrier mobility and minority carrier diffusion lenght (inset) from a LWIR 15 μm pitch array.

VLWIR (12 - 15 μm) photodiodes for atmospheric sounding were studied, as well as electron avalanche photodiodes for high speed and low-flux applications. Excess currents were modeled and compared to the reverse-bias characteristics for VLWIR devices. Issues with passivation were found to be critical.

Cap-layer variations including thermally-evaporated (TE) ZnS, TE CdTe, electron beam evaporated (EBE) CdTe and in-situ CdTe/ZnTe grown by MBE were compared with respect to their effect on As-ion implantation. Channeling effects were observed for thin layers of evaporated CdTe. An optimized thickness of ZnS was found to obtain the deepest As indiffusion after high temperature annealing, and the end-of-range (EOR) depth is linearly proportional to the thickness ratio of a-MCT layer/damage layer.

Inductively-coupled plasma etching of HgCdTe FPAs at cryogenic temperatures—123 K—was described. Fig. 33 shows an example of the etch profile.

A paper was presented on the bulk growth of <111> seeded 75 mm diameter cadmium zinc telluride (CZT) using the traveling heater method (THM) which allows for growth at lower temperatures. The report summarized work on development of epitaxy-ready surface finishing of THM-grown Cd_0.96Zn_0.04Te sub-strates. Xray rocking curve values averaged 22 ± 3 arc sec. The crystals were reported to be free of twins and slip and GDMS measurements reported less than 2 ppba of copper impurities.

Fig. 33

The profile after an optimized plasma etch.