Μm Cutoff HgCdTe Research Articles

New and Better Detectors for the JWST Near-Infrared Spectrograph

ESA and NASA recently selected two 5 μm cutoff Teledyne H2RG sensor chip assemblies (SCA) for flight on the James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec). These HgCdTe SCAs incorporate Teledyne’s “improved barrier layer” design that eliminates the degradation that affected earlier JWST H2RGs. The better indium barrier, together with other design changes that Teledyne phased in from other programs over the years, has improved the performance and reliability of JWST’s SCAs. In this article, we describe the measured performance characteristics that most directly affect scientific observations including read noise, total noise, dark current, quantum efficiency (QE), and image persistence. As part of measuring QE, we inferred the quantum yield over the full NIRSpec pass band of λ = 0.6–5 μm and found that it exceeds unity for photon energies Eγ > (2.65 ± .2)Eg, where Eg is the HgCdTe bandgap energy. This corresponds to λ ≲ 2 μm for NIRSpec’s 5 μm cutoff HgCdTe.

A Highly Sensitive Multi-element HgCdTe e-APD Detector for IPDA Lidar Applications

An HgCdTe electron avalanche photodiode (e-APD) detector has been developed for lidar receivers, one application of which is integrated path differential absorption lidar measurements of such atmospheric trace gases as CO2 and CH4. The HgCdTe APD has a wide, visible to mid-wave-infrared, spectral response, high dynamic range, substantially improved sensitivity, and an expected improvement in operational lifetime. A demonstration sensor-chip assembly consisting of a 4.3 μm cutoff HgCdTe 4 × 4 APD detector array with 80 μm pitch pixels and a custom complementary metal–oxide–semiconductor readout integrated circuit was developed. For one typical array the APD gain was 654 at 12 V with corresponding gain normalized dark currents ranging from 1.2 fA to 3.2 fA. The 4 × 4 detector system was characterized at 77 K with a 1.55 μm wavelength, 1 μs wide, laser pulse. The measured unit gain detector photon conversion efficiency was 91.1%. At 11 V bias the mean measured APD gain at 77 K was 307.8 with σ/mean uniformity of 1.23%. The average, noise-bandwidth normalized, system noise-equivalent power (NEP) was 1.04 fW/Hz1/2 with a σ/mean of 3.8%. The measured, electronics-limited, bandwidth of 6.8 MHz was more than adequate for 1 μs pulse detection. The system had an NEP (3 MHz) of 0.4 fW/Hz1/2 at 12 V APD bias and a linear dynamic range close to 1000. A gain-independent quantum-limited SNR of 80% of full theoretical was indicative of a gain-independent excess noise factor very close to 1.0 and the expected APD mode quantum efficiency.

Linear mode photon counting with the noiseless gain HgCdTe e-avalanche photodiode

A linear mode photon counting focal plane array using HgCdTe mid-wave infrared (MWIR) cutoff electron initiated avalanche photodiodes (e-APDs) has been designed, fabricated, and characterized. The broad spectral range (0.4 to 4.3 μm) is unique among photon counters, making this a “first of its kind” system spanning the visible to the MWIR. The low excess noise [F(M)≈1] of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500–1000. The readout integrated circuit (ROIC) design included a very high gain-bandwidth product resistive transimpedance amplifier (3×1013 Ω-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high-bandwidth analogs and 16 low-voltage differential signaling digital outputs. The 2×8 array was integrated into an LN2 Dewar with a custom leadless chip carrier and daughter board design that preserved high-bandwidth analog and digital signal integrity. The 2×8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured avalanche photodiode gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse signal-to-noise ratio of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a photon background limited false event rate of about 1 MHz. The measured jitter was in the range of 550–800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.

Reciprocity Failure in HgCdTe Detectors: Measurements and Mitigation

ABSTRACTA detailed study of reciprocity failure in four 1.7 μm cutoff HgCdTe near-infrared detectors is presented. The sensitivity to reciprocity failure is approximately 0.1% decade-1 over up to 5 orders of magnitude in illumination intensity. The four detectors, which represent three successive production runs with modified growth recipes, show large differences in amount and spatial structure of reciprocity failure. Reciprocity failure could be reduced to negligible levels by cooling the detectors to about 110 K. No wavelength dependence was observed. The observed spatial structure appears to be weakly correlated with image persistence.

The Dark Current and Hot Pixel Percentage of James Webb Space Telescope 5 μm Cutoff HgCdTe Detector Arrays as Functions of Temperature

ABSTRACTWe measured the median dark current and “hot pixel” percentage of λco ∼ 5 μm cutoff HgCdTe James Webb Space Telescope (JWST) Near Infrared Spectrograph HAWAII-2RG (H2RG) detector arrays as functions of temperature. Although these measurements were made in the context of JWST, we believe that they will interest others using H2RGs. We found that the median dark current depended only weakly on temperature throughout the 36.5 K ≤ T ≤ 45 K temperature range (although there were some exceptions). In contrast, the hot pixel percentage depended strongly on temperature, with the percentage of hot pixels approximately doubling for each 6 K temperature increase. Moreover, the data suggest that this trend becomes stronger at warmer temperatures. These new measurements broadly confirm our previous finding that T ∼ 38 K is close to optimal for current-generation λco ∼ 5 μm cutoff H2RGs, although somewhat warmer (T ≤ 45 K) temperatures may be used if lower pixel operability is acceptable.

Correlated Noise and Gain in Unfilled and Epoxy‐Underfilled Hybridized HgCdTe Detectors

ABSTRACTSpatially correlated noise in focal plane arrays (FPAs) affects the photometric signal‐to‐noise ratio and can lead to overestimation of conversion gain (detector nodal capacitance). Capacitive coupling leads to charge sharing between neighboring pixels and correlates the measured pixel signal. This correlation can lead to image degradation, in particular for undersampled large survey telescopes. We present new comparative measurements of the interpixel coupling of 1.7 and 2.5 μm cutoff HgCdTe FPAs. Rockwell Scientific FPAs with epoxy underfill exhibit correlations that result in a 16%–20% systematic overestimation of the conversion gain. Results for a Raytheon Vision Systems 1.7 μm cutoff HgCdTe detector tested both before and after epoxy underfill show increased coupling in the presence of the epoxy, but the coupling is not spatially symmetric. We speculate that the difference in the observed correlation for each vendor is related to multiplexer geometry and pixel nodal capacitance.