Mercury cadmium telluride (MCT) cooled infrared (IR) detectors remain a technological benchmark for the IR community as defined by Rule 7 (J. Electron. Mater. 37:1406). Their performance depends on the ability to resolve small temperature differences between two black bodies, particularly for defense. This thermal resolution is highly sensitive to even slight variations in device quality and is commonly evaluated using residual fixed-pattern noise (RFPN), a second-order performance metric for focal plane array (FPA) characterization. Recent research efforts are focused on developing detectors capable of operating at high operating temperatures (HOT). However, increasing the operating temperature typically leads to a higher number of anomalous pixel responses and elevated dark current and noise, resulting in degradation of detection capabilities. One suspected cause of this degradation is the quality of the metal/p-type semiconductor interface, particularly due to variations in the Schottky barrier height (SBH). The non-ohmic behavior frequently observed at this interface is commonly attributed to the presence of a Fermi-level pinning effect. This work aims to investigate that hypothesis with a detailed physicochemical analysis of the interface’s formation and composition including a comprehensive correlation with electrical measurements. Metal contacts were fabricated on p-type Hg0.7Cd0.3Te doped with arsenic, with a doping concentration of approximately 1018 cm−3. A consistent surface preparation and deposition method with different metals were applied to form contacts. Electrical properties were characterized using the transfer line method (TLM) at 300 K and 110 K to extract specific contact resistance. To probe the initial surface and buried interface, hard x-ray photoelectron spectroscopy (HAXPES) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were employed, providing insights into the chemical composition and oxidation phenomena involved in interface state formation. This study highlights the predicted dependence of SBH on the intrinsic nature of metal at 300 K. At low temperature, this dependency is no longer valid due to the predominance of tunneled and trap-assisted conduction. The presence of a buried tellurium oxide layer could explain this deviation, as it may govern the electrical properties of the contact and determine the SBH.

InAs/InAsSb type-II superlattices (T2SLs) are emerging as alternatives to HgCdTe for infrared detection, but their anisotropic hole transport could be a limitation in small-pitch focal plane array (FPA) detectors. To quantitatively assess this anisotropy in the device, modulated electron beam induced current (AC-EBIC) is performed on deep-etched mid-wave infrared (MWIR) T2SL photodiodes. Analysis of the EBIC amplitude and phase profiles reveals a transport anisotropy of 2.5 between lateral and vertical diffusion lengths. A numerical diffusion model fits the data, yielding diffusion lengths (3–4 μm vertically, 7–10 μm laterally), minority carrier lifetime (400–670 ns), and surface recombination velocity (< 800 cm/s). While AC-EBIC effectively quantifies transport anisotropy, current frequency constraints limit the decoupling of individual parameters. Nevertheless, beyond surface characterization, this study demonstrates AC-EBIC’s potential to resolve anisotropic diffusion parameters in photodiodes structures.

Random telegraph signal (RTS) noise is widely discussed in the literature as a primary noise limiting the operating temperature in cooled infrared detectors. Moreover, the understanding of the underlying phenomena remains limited. The aim of this paper is to study RTS noise on a mercury cadmium telluride (MCT) detector in order to identify ways of improving its performance. To this end, we have performed measurements under different bias conditions and studied their impact on the frequency and amplitude of RTS jumps. The results show that the RTS amplitude increases with the bias voltage, confirming that RTS is located within the space charge region (SCR). To establish whether this increase could be due to electric field enhancement (EFE), a model of the EFE factor was calculated. However, there is no significant variation in the frequency of the RTS jumps, questioning the validity of RTS models based on a switching defect.

Since their initial synthesis in 1958, HgCdTe compounds have been extensively developed as a highly effective material for infrared detection. The performance of these materials has improved continuously in terms of the first-order figures of merit, including dark current, quantum efficiency, and signal-to-noise ratio. Nowadays, the optimization of second-order performances is a key objective, with a particular focus on low-amplitude transients of photocurrents induced by scene illumination variation. In this study, we conducted measurements of photocurrent transients induced by optical excitation on HgCdTe mid-wave infrared (MWIR) focal plane arrays (FPAs). The HgCdTe detection circuit is flip-chipped onto a silicon direct injection readout integrated circuit (DI ROIC). The transients were measured at various temperatures, wavelengths, and levels of illumination. Furthermore, the bias of the photodiodes was varied in order to examine the impact of the space charge region extension. The FPA exhibits remarkably brief transients at operational temperatures (2.6 ms at 130 K) and low amplitudes relative to typical operational requirements. Two exponential regimes with two distinct time constants were identified in each transient. By plotting the aforementioned time constants with regard to temperature in Arrhenius plots, the activation energies and cross sections of two families of traps responsible for the transients were extracted. Both trap families exhibit energies that are in close proximity to the mid-gap, situated at 40% and 60% above the valence band edge. The energies and cross sections are in accordance with the established literature on deep-level transient spectroscopy (DLTS). The measurements conducted at various wavelengths and photodiode polarizations, in conjunction with the diverse photodiode designs available on the array, led to the conclusion that the traps responsible for the transients are situated within the space charge region (SCR) of the HgCdTe photodiodes. It is our contention that this methodology could be employed as a standard diagnostic tool for the characterization of trap parameters within the SCR of photodiodes, as well as for the assessment of material quality at the ultimate stage of fabrication for an FPA using a DI ROIC.

Recently, all-group-IV (Si)GeSn alloys attracted great attention as materials for Infra-Red optoelectronics monolithically integrated on Si substrates. In this work, we present the fabrication and the electro-optical characterization of direct bandgap GeSn photodiodes with 15.4% of Sn grown on Ge Strain-Relaxed Buffers, themselves on 200 mm Si(001) wafers. The Ge0.846Sn0.154 photodetectors have a cutoff wavelength of 3.5 mu m, e.g., they are suitable for methane detection around 3.3 mu m. At this wavelength, their specific detectivity D* at room temperature is 3.76 x 10(7) cm.Hz(1/2).W-1. This detectivity is 60 times better than that of previously reported photodetectors with equivalent Sn content. When such Ge0.846Sn0.154 photodiodes are placed in a gas cell together with a commercial Light Emitting Diode emitting at 3.3 mu m, the system presents a limit of detection for methane of 1 600 parts per million with a noise density of 0.78%.Hz(-1/2).

This article investigates the effects of proton radiation on low-flux P-on-N short-wavelength infrared (SWIR) mercury-cadmium-telluride (HgCdTe or MCT) focal plane arrays (FPAs). The study is motivated by the need to investigate and characterize the degradation induced by proton irradiation on such devices. Three steps of irradiation were conducted using 63-MeV protons with a cumulative fluence up to 4.7 x 10(11) protons/cm(2). Dark-signal degradation was observed after irradiation. The degradation evolved linearly up to the final fluence, with the highest dark signal reaching 2.6 e-/s, and annealed completely after a thermal cycle at room temperature. A pixel-to-pixel count of incident protons during an irradiation step also revealed a linear degradation of the dark signal over a large statistic of more than 100 k pixels. A degradation index was estimated using the fluence obtained with the facility dosimeter and the incident proton count method developed in this study. The values obtained are similar using both methods, giving a robust method of calculation of the degradation index following 63-MeV proton irradiations in our sensor technology.

Direct bandgap Ge 0.86 Sn 0.14 photodiodes were fabricated on Si substrates using layer transfer. The layer transfer did not degrade the noise of the initial photodiode stacks, while the underlying metallic mirrors improved the responsivity at 300 K by a factor of 2.9 at 2.5 µm.

The field of infrared detectors is experiencing a strong movement towards smaller pixel pitches, and it is now common to see pitches close to the wavelength. This raises major problems in measuring their characteristics, especially their transfer function. The latter can be measured by optical pattern projection methods using a low F/# objective, such as the spot scan method. However, in order to obtain the quantitative transfer function measurement of detectors, the objective spatial response should be measured and deconvoluted precisely, since the size of the focused optical spot is close to the pixel pitch. Therefore, the purpose of this paper is to propose a protocol that performs an absolute measurement of a high-quality infrared objective. The main idea of the article is to link the measurement standard to a fundamental theory, the Huygens-Fresnel principle, using a particular wavefront sensor as a secondary measurement standard. This has various metrological advantages, that allow the uncertainties of the optical transfer function measurement to be controlled and evaluated. The protocol and the propagation of errors described in the article are both used to demonstrate the value of this approach.

Today, one of the main challenges in the quantum infrared detection field is to increase the operating temperature in order to reduce size, weight and power-cost (SWAP). However, this leads to two major issues: an increase in the number of random telegraph signal (RTS) pixels and a residual fixed pattern noise (RFPN) instability. To identify these noise sources and assess the performance of a component, numerous measurements at the operating temperature are necessary, which can be laborious. Nevertheless, there exists a noise quality indicator for detectors at high temperatures known as the Tobin factor. In this paper, our objective is to investigate and understand the relationship between random telegraph signal (RTS) pixels and the Tobin factor. The aim of this study is to evaluate the relevance of the Tobin factor extracted at high temperatures to forecast noise performance at operating temperatures.

The well-known Rule07 is a simple thus efficient way to compare available technologies for IR imaging detectors in terms of dark current. The noise is then often estimated using a shot noise approximation on the dark current. Both II-VI and III-V communities use this rule of thumb as a reference for well-performing IR photodiodes. For HOT applications, a dark current close to this rule07 is considered a necessary condition but not a sufficient one to obtain a high-performance IR imager. Indeed, when limited by shot noise, rule07 describes well the noise behavior of the considered device. However, when considering low-frequency noise, it fails to describe the expected performances. In this paper, we focus on another figure-of-merit, dedicated to detector low-frequency noise rather than dark current. Systemic 1/f noise investigation in an IR detector was first reported by Tobin et al. in 1980. There is today a relative consensus on the fact that measured 1/f noise is proportional to the dark current. The ratio between the amplitude of the 1/f noise and the dark current of the same devices may therefore be used as a figure-of-merit for a given technology. This ratio (called the Tobin factor alpha(T)) therefore appears adequate to compare different technologies as a figure-of-merit qualifying 1/f noise properties. This dimensionless ratio can also be very useful for optimizing a particular technology or process. However, in order to be relevant, this figure-of-merit must be estimated carefully as it appears, for instance, pixel pitch-dependent. Different examples of Tobin coefficient extraction are presented in this paper. We show that, depending on the technologies, the values of the Tobin coefficient can spread over several orders of magnitude. However, only low values result in high-quality IR imagers. Today, the best results we obtained show that alpha(T)=10-5 is a state-of-art value to be compared with.

For several years, CEA-LETI has worked on the development of IR MCT detectors for low flux applications in scientific imaging. MCT has remarkable material properties that allow for highly performing detection, with high QEs and minimal dark currents. Currently, both Teledyne US and LETI-Lynred in France achieve dark currents in the range of a fraction of e/s/pixel for NIR imaging, utilizing the p-on-n extrinsic diode structure, for astronomy applications. As part of the NEOCAM project, Teledyne has demonstrated the ability to extend the cut-off wavelength to significantly longer wavelengths (10.3 mu m and recently 13 mu m) while keeping the dark current in the e/s range. Furthermore, LETI is presently enhancing its p-on-n technology to satisfy the demanding specifications of low-flux applications. Achieving very low dark current leakage and large diode polarisation plateaus is required to achieve SFD input stage ROIC operation and to reach such low dark current values. This entails the mitigation of tunnelling currents that appear in low-gap materials. In this report, we present the production of a 15 mu m pitch TV-size prototype array that operates in the LW range (8 mu m @ 35K). Dark current values as low as 0.5 e/s/pixel have been measured below 40K, and will be discussed.

In the past few years, CEA LETI demonstrated MCT P on N photodiodes arrays achieving high level of detection for very low flux astronomy in the short wave infrared (SWIR), with dark currents values as low as 3 10(-3) e-/s/pixel at 100K and high quantum efficiency. Persistence was also a key element to monitor for the development of this technology, and significant improvements were demonstrated in the frame of ALFA program. From this reference technology, LETI developed a brand new P on N process, focused on decreasing defectiveness and improving low frequency stability for MWIR high operating temperature (>130K) detectors for tactical application. In this spectral range, low noise stability is characterized by Random Telegraph Signal (RTS) and noise distribution tail. In the SWIR range, persistence would be the best signature to probe this low frequency stability. Declining this new generation process for the SWIR range, we present and discuss on dark current and persistence characterization on TV format 15 mu m pixel pitch study array designed for SWIR low flux application.

LYNRED is a leading global provider of high-quality II-VI, III-V and bolometers infrared detectors for the aerospace, defence and commercial markets. Our vision is to preserve and protect, and provide the right technology to customers' needs. To consolidate our position among infrared detector manufacturer leaders and to enable us to respond to growing market demand for next-generation infrared technologies, a new state of the art industrial facility is breaking ground. This new industrial site named Campus will double the current cleanroom footprint and increase production capacity with optimal cleanliness classification for new high-performance products. Among these next generations technologies, Campus will serve the ongoing developments of sub-10 mu m pitch cooled infrared detectors, MCT HOT technology, for extended MW band and III-V HOT MW blue band technology. We will discuss in this paper the true figures of merit that have to be addressed during technology development and optimization to meet field mission requirements. We will then review latest results on II-VI and III-V HOT IDDCA (Integrated Detector Dewar Cryocooler Assembly) with 7.5 mu m pitch SXGA format focal plane array in terms of low frequency noise defects, stability and reproducibility of residual fixed pattern noise (RFPN) and Modulation Transfer Function (MTF) optimizations while maintaining high quantum efficiency to keep highest possible range.

GeSn-based group-IV alloys are attracting great attention in the Si photonics community, as they are considered to be compatible with Complementary Metal Oxide Semiconductor (CMOS) technology. Alloying germanium with more than 8% of tin (Sn) results in direct bandgap semiconductors, with some optical gain in devices such as lasers. Recently, room temperature optically pumped lasing was achieved thanks to high Sn content stacks. GeSn alloys can be used for near, short and mid wavelength infra-red spectral range operation, notably for gas detection. Current efforts are focused on electro-optical GeSn IR devices such as light-emitting devices (LEDs), electrically pumped lasers and photodetectors. In this paper, we evaluate the impact of various types of in situ n-type doped carrier injection layers on top of GeSn direct bandgap LEDs. More precisely, we compare the performances of GeSn:P and Ge:P –capped mid IR LEDs. Using reduced pressure chemical vapor deposition and metastable growth conditions (e.g. fast growth rates at low temperature), high crystalline quality GeSn layers were grown on 200 mm diameter Ge-buffered Si(001) wafers (Figure a). In situ n-type doped Ge layers (sample A) and GeSn layers (sample B) were used to inject carriers into direct band gap Ge0.87Sn0.13 active layers beneath (Figure b). I(V) curves (Figure c) and Electro-Luminescence spectra (Figure d) were compared for sample B (GeSn:P) and sample A (Ge:P). Very similar I(V) behaviors were observed under forward bias for both samples. However, a higher dark current was measured under reverse bias for sample B than for sample A. This could be due to a higher number of defects in the thicker capping layer of sample B (240 nm) compared to that of sample A (50 nm). However, a 2-fold increase in the EL signal was obtained for sample B than sample A (Figure d). Temperature dependence electroluminescence measurements and atom probe tomography data will be used to explain the electroluminescence behavior differences between Samples A and B. Finally, even higher Sn content LEDs were fabricated to emit light at 3.3 µm (Figure e). A direct band gap Ge0.846Sn0.154LED (Sample C) with an in situ n-type doped Ge cap was placed in a gas cell filled with diluted methane. Its emission overlapped well with the absorption of methane (Figure f). The methane detection limit of our setup was evaluated (data not shown). To further increase the emitted power of GeSn LEDs, current spreading was studied for different top contact geometries. Emission maps collected by an InSb camera at room temperature showed a definite dependence of light emission on electrical contact geometry (Figure g). Investigations are underway to further increase light extraction from LEDs and improve their performances for use in environmental sensors. Acknowledgement: This work was supported by the European Union’s Horizon 2020 LASTSTEP Project under the grant agreement ID: 101070208, the EDEN Carnot project and the CEA DRF-DRT Phare project. We gratefully acknowledge the clean room staff from LETI and IRIG for their technical support. Figure 1

The electron beam-induced current technique (EBIC) developed at CEA-Leti has been a valuable tool for studying the transport properties of minority carriers in HdCdTe. Indeed, previous work has demonstrated the use of this technique for estimating diffusion length from the exponential decay of EBIC as a function of junction distance, as well as direct measurement of the modulation transfer function (MTF) of small pixel pitch diodes. In this work, a modulated electron beam and a lock-in amplifier are used to measure the variation in current and phase shift with the scan distance to the junction. This work is focused on the estimation of the minority carrier lifetime from the linear evolution of the phase shift as a function of junction distance.

Lynred is leading the development of infrared detectors for high performances applications. Two trends are identified in the infrared range, the increase of the operating temperature and the pixel pitch reduction. For 15 years, the III-V technologies present an increasing interest to address both challenges. At LYNRED, these technologies allow to address Short Wave InfraRed (SWIR) and Mid-Wave InfraRed (MWIR) for ground applications. Many challenges have to be addressed for the future focal plane arrays (FPAs). Electrical and optical crosstalks as well as image quality and stability, are one of the prime concern for detectors with pixel pitch down to 7.5 mu m. In order to reach an industrial production level of infrared FPAs, technological developments are required at each steps: the epitaxy, the detector array process, flip chip and back end processing. Another key element is the Read Out Integrated Circuit (ROIC) designed in-house to fulfil our customer needs. We review the latest developments at LYNRED on III-V technologies, in terms of operability, residual fixed pattern noise (RFPN) and Modulation Transfer Function (MTF) optimizations.