Latest developments in the p-on-n HgCdTe architecture at DEFIR

We report recent developments at Commissariat a l’Energie Atomique-Laboratoire d’Electronique des Technologies de l’Information Infrared Laboratory on the processing and characterization of p-on-n HgCdTe (MCT) planar infrared focal plane arrays (FPAs) operating in the long-wavelength infrared (LWIR) and very-long-wavelength infrared (VLWIR) spectral bands. The active layers in these FPAs were grown by liquid phase epitaxy (LPE) on a lattice-matched CdZnTe substrate. This technological process results in lower dark current and lower serial resistance than for n-on-p vacancy-doped architecture and thus is better adapted for lower flux detection or higher operating temperature. This architecture was evaluated for space applications in the LWIR and VLWIR spectral bands with cutoff wavelengths from 10 to 17 μm at 78 K. Innovations have been introduced to the technological process to form a heterojunction by use of an LPE growth technique. The initial objective was to reduce the dark current at low temperatures, by reducing the transition temperature from diffusion-limited to depletion-limited dark current. Another advantage is that the wider bandgap obtained in the vicinity of the junction ensures less sensitivity to the defects present at the interface between MCT and passivation layers. Electro-optical characterization of p-on-n photodiodes was performed on quarter video graphics array format FPAs with 25 and 30 μm pixel pitches. The results revealed excellent operabilities in current and responsivity, with low dispersion, and noise limited by current shot noise. Studies performed on the dark current show that dark current densities at 78 K are consistent with the heuristic prediction law “Rule 07”. Below this temperature, dark current varies as a pure diffusion current for a variety of devices from different manufacturers, introducing a temperature range limitation in the description of the “Rule 07” law.

One of the main challenges of modern astronomical instruments like adaptive optics (AO) systems or fringe trackers is to deal with the very low photons flux detection scenarios. The typical timescale of atmospheric turbulences being in the range of some tens of milliseconds, infrared wavefront sensors for AO systems needs frame rates higher than 1 KHz leading to integration times lower than 1 ms. This integration time associated with a low irradiance results in a few number of integrated photons per frame per pixel. To preserve the information coming from this weak signal, the focal plane array (FPA) has to present a low read out noise, a high quantum efficiency and a low dark current. Up to now, the output noise of high speed near infrared sensors is limited by the silicon read out circuit noise. The use of HgCdTe avalanche photodiodes with high gain at moderate reverse bias and low excess noise seems then a logical way to reduce the impact of the read noise on images signal to noise ratio. These low irradiance passive imaging applications with integration times in the millisecond range needs low photodiode dark current and low background current. These requirements lead to the choice of the photodiode cut off wavelength. The short wave infrared (SWIR) around 3 μm is a good compromise between the gain that can be obtain for a given APD bias and the background and dark current. The CEA LETI HgCdTe APD technology, and a fine analysis of the gain curve characteristic are presented in [1] and won’t be detailed here. The response time of the APD is also a key factor for a high frame rate FPA. This parameter has been evaluated in [2] and the results shows cut off frequencies in the GHz range. In this communication we report the performances of a SWIR APD FPA designed and fabricated by CEA LETI and SOFRADIR for astrophysical applications. This development was made in the frame of RAPID, a 4 years R&D project funded by the French FUI (Fond Unique Interministeriel). This project involves industrial and academic partners from the field of advanced infrared focal plane arrays fabrication (SOFRADIR and CEA LETI) and of astronomical/defense institutes (IPAG, LAM, ONERA). The goal of this program is to develop a fast and low noise SWIR camera for astronomical fast applications like adaptive optics wavefront sensing and fringe tracking for astronomical interferometers [3]. The first batch of FPA’s was based on liquid-phase epitaxy (LPE) grown photodiode arrays with 3 μm cut off wavelength. In order to get higher avalanche gain for a given photodiode reverse bias voltage, we have made a second batch with a cadmium composition leading to 3.3 μm cut off wavelength (λ c ). This paper described the read out circuit in the next section. The aim section III is to find the critical parameter that has to be measured to evaluate the signal to noise ratio (SNR) of an APD FPA. The main electro optical characteristics of an FPA based on 3.3μm cut off wavelength APDs are reported in “Rapid FPAs characterisation” section. The dark current evolution with temperature of a 3 μm FPA high and low APD bias is also detailed in this section.

Reducing an array’s pixel pitch reduces the size and weight of the focal plane array (FPA) and its associated dewar, cooler and optics. Higher operating temperatures reduce cool-down time and cooler power, enabling reduced cooler size and weight. High operating temperature small pitch (≤15 um) infrared detectors are therefore highly desirable. We have characterized a large number of MWIR and LWIR FPAs as a function of temperature and cutoff wavelength to determine the impact of these parameters on the FPA’s dark current, 1/f noise and defects. The 77K cutoff wavelength range for the MWIR arrays was 5.0-5.6 um, and 8.5-11 um for the LWIR arrays. DRS’ HDVIP® FPAs are based on a front-side illuminated, via interconnected, cylindrical geometry, N+/N/P architecture. An FPA’s 1/f noise is manifested as a tail in the FPA’s rmsnoise distribution. We have found that the model-independent nonparametric skew [(mean–median)/standard deviation] of the rmsnoise distribution is a highly effective tool for quantifying the magnitude of an FPA’s 1/f noise tail. In this paper we show that a standard FPA’s 1/f noise varies as ni (the intrinsic carrier concentration), in agreement with models that treat dislocations as donor pipes located within the P-volume of the unit cell. Nonstandard FPAs have been observed with systemic 1/f noise which varies as ni2.

Rapid progress in quantum well IR photodetectors (QWIPs) technology has made it possible to develop large format, and high uniformity QWIP focal plane arrays (FPAs) for IR imaging sensor applications. So far, all QWIP FPAs are made of n-type GaAs/AlGaAs material systems grown on GaAs substrates due to the maturity of this material system and excellent detector performance. However, due to quantum selection rules, normal incidence absorption is forbidden in n-type QWIPs. As a result, n-type QWIP FPAs are required to use dielectric or metal gratings to efficiently couple the IR radiation into the quantum wells under normal incidence illumination. On the other hand, normal incidence absorption is allowed in p-type QWIPs without using the grating couplers. This property makes p-type QWIPs potentially more attractive for large format FPA applications. This paper will review the recent development in both n-type and p-type QWIPs for mid-wavelength, long-wavelength and very long wavelength IR detections. The basic device physics, structures, and performance parameters such as dark current, spectral responsivity, and detectivity for both n-type and p-type QWIPs will be depicted. Finally, a comparison of the advantages and drawbacks of n-type and p-type QWIPs will also be given.

TV/4 format (320×256) detectors are today the most produced. They have demonstrated high thermal performance and are cost effective at the mass production level. However, in response to system requirements of very high resolution, miniaturization and cost reduction, SOFRADIR is now offering full TV format (640x512) detectors with graduated levels of performances regarding pitch size and cryogenics. In particular a TV format with 15 μm pitch is offered allowing the integration of highly miniaturized cryogenics dedicated to compact systems or, depending of the applications, the upgrade of existing systems by using the same miniature cryogenics as the ones used for TV/4 format. Regarding TV format (640x512) with 15μm pitch, technological adaptations were validated at CEA-LETI/LIR (France) at the beginning of 2002. With respect to the 20μm pitch TV formats whose purpose is to offer high resolution performances in slightly modified cryogenics, this new 15μm pitch MCT TV format exhibits even higher performances in very small size optimized cryogenics in order to fit the system requirements of miniaturization and cost reduction and to achieve a cost effective production level. In this way, this 15μm pitch 640x512 MWIR MCT detectors will become, in the coming years, the most affordable large format at production level.

High-performance short-wavelength infrared (SWIR) photodetectors can be realized in the InGaAs material system, providing a typical cutoff wavelength of 1.7 μm, which covers a wide part of the nightglow spectrum as well as the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, both, active and passive SWIR detection systems are mostly required to provide high responsivities and very low dark currents in order to detect extremely low photon fluxes. This can be achieved with pin photodiodes with high responsivities and very low dark current characteristics or by utilizing the internal signal gain as provided by avalanche photodiodes (APDs). We develop SWIR photodetectors based on InGaAs/InP pin diodes and InGaAs/InAlAs/InP APDs as single-element detectors as well as focal plane arrays (FPAs). The planar processed InGaAs/InP pin photodiodes for low-light-level SWIR cameras exhibit dark current densities of 10^-7 A/cm² at room temperature for 15 μm pitch detector elements. For the APDs, emphasis is put on the vertical detector design. Within three design iterations, the operating voltage for useful gain values M ~ 10 could be reduced from 27 V down to 18 V, which was crucial for the operation with the voltage limitation of the read-out circuit. FPAs of such InGaAs-APD material have been successfully integrated into SWIR cameras with 640 × 512 pixels at 15 μm pixel pitch. The avalanche operation on camera level has been demonstrated for both kinds, the standard (passive) as well as gated-viewing operation modes.

The core of SCD’s present day focal plane array (FPA) technology is a two-dimensional array of InSb photodiodes. It is produced by ion implantation and is attached to a silicon focal plane processor by indium bumps. We have extended this technology with In1−xAlxSb (0<x<0.03) alloys grown by molecular beam epitaxy on InSb substrates, utilizing a p-i-n structure. The advantages of the grown structures over those fabricated by implantation are already demonstrated in 320×256 element FPAs based on epitaxial InSb mesa diodes. These FPAs have dark currents at 100K comparable to those of implanted FPAs at 77K, with an operability (fraction of working diodes) in excess of 99.5%, and with a residual non-uniformity (RNU) at 100K of less than 0.03% (standard deviation/dynamic range) after a two-point non-uniformity correction (NUC). The RNU remains very low (<0.1%) even when the FPA temperature is shifted by as much as 10K from the NUC temperature. The average dark current at 100K and also the width of its distribution are further reduced by increasing the Al concentration in the FPAs by up to 3%, in accordance with the increase in the alloy energy gap. This has enabled us to achieve high quality thermal images at even higher temperatures than in the binary InSb FPAs. A key issue in achieving highly uniform FPAs is the degree of control of the InAlSb composition, misfit strain relaxation, and defect density. A series of In1−xAlxSb layers with x up to 6% was grown in order to study the effect of the growth conditions on these features. We discuss our results in terms of the misfit strain relaxation mechanisms in the In1−xAlxSb structures including dislocations, tilt, microcracks, and morphological crosshatch patterns.

Chapter 1 - HgCdTe Infrared Detectors

Raytheon Vision Systems (RVS) has obtained the initial performance data on a 1280x1024 format short wave infrared (SWIR) sensor with a dark current of 1 nA/cm2 and low noise input circuit of five noise electrons (5 e-), which is 2× lower electronic read-out noise and 2× lower dark current than previous designs. A remarkable feature of the sensor is that a novel high dynamic range circuit is also designed into the 15 × 15 μm pixel unit cell, in a large format, high density 1280×1024 SWIR FPA. The integration of the conflicting design requirements of extremely low noise with high dynamic range allows recognition of low contrast targets, without saturation from bright sources within the same frame of information. This enables operation in urban environments at low levels of ambient illumination, and simultaneously with bright sources that saturate conventional sensors.

The GaAs/AlGaAs based quantum well IR photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors because the wavelength of the peak response and cutoff can be continuously tailored over any wavelength between 6-20 micrometers . The spectral band width of these detectors can be tuned from narrow to wide allowing various applications. Also, QWIP offers multi-color IR cameras which is capable of simultaneously acquiring images in different IR bands. Each pixel of such array consists of vertically stacked, independently readable, QWIP detectors sensitive in different narrow IR bands. In this article, we discuss the result of a 10-16 micrometers large format broadband QWIP focal plane array IR bands. In this article, we discuss the results of a 10-16 micrometers large format broadband QWIP focal plane array (FPA). The size of the FPA is 640 X 512 and its pixel pitch is 25 microns. The highest operating temperature of the FPA is 45K, and it was determined by the charge storage capacity and the other features of the particular readout multiplexer used in this demonstration. Excellent imagery, with a noise equivalent differential temperature of 55 mK has been achieved. In addition, we will discus the developments and results of the 640 X 512 dual-band QWIP FPA.

Infrared detection is widely used in astrophysics and plays a key role in several space missions aiming for example at scanning the sky to discover new objects (coolest stars, dust-obscured galaxies, exo-planets …) or studying the evolution of the universe, where light is redshifted in the infrared range. In many cases the space telescope involves an HgCdTe infrared detector operating at low frame rate over long integration time. Due to the very low input signal, dark current and readout noise are essential figures that must be minimized to get the best detector sensitivity. This kind of application also requires very large focal plane array (FPA) often relying on a butting arrangement of large detectors. The trend is to increase the single detector format from 1Kx1K to 2Kx2K and 4Kx4K. For very large formats, material quality and detector process may affect the production yield and the global infrared FPA cost. As a result the detector format could result from a trade-off taking into account producibility.

In this paper, results from the development of InAs/GaSb superlattice focal plane arrays (FPAs) at IRnova will be presented. A versatile and robust detector design is used that allows for adjustment of the detection cut-off wavelength from 2.5 μm up to 14.5 μm with only minor changes in the detector design. Performance of the fabricated detectors has been reviewed in terms of external quantum efficiency (EQE), dark current and noise for three designs with cut-off wavelengths of 4, 5.5 and 11 μm at 80 K (referred to as DEEP BLUE, RED HOT and VLWIR, respectively). Measurements on the 15 μm sized photodiodes demonstrated 70% EQE for the MWIR designs, and almost 40% - for VLWIR. At the same time, the dark current stayed close to the Rule07 benchmark for all studied samples. Noise mechanisms have been discussed and their relation to the passivation was examined. Mature in-house processing and passivation technique of resulted in very high spatial uniformity of VGA focal plane arrays (FPAs), i.e. low relative deviations of EQE (< 6%) and of dark current density (< 12%) and narrow noise distributions for both RED HOT and DEEP BLUE FPAs. We show also that <99.5% of these arrays operate close to the fundamental noise limit.

The Corrugated Quantum Well Infrared Photodetector (C-QWIP) holds significant performance and other advantages over other infrared (IR) detectors. However, one disadvantage of the detector is the relatively low operating temperature needed to suppress the dark current. By coating two additional layers (thin insulator and high critical temperature (Tc) superconductor) on the top contact layer of a C-QWIP wafer, the top three layers of the detector form a high-Tc superconducting single electron tunneling junction. It could act as an electron filter because of the presence of an energy gap in superconductors. For QWIPs, the photo electrons and dark electrons are well separated in energy, most dark current is conducting below the quantum well (QW) barrier height and most photo current is conducting above the barrier height. Most dark electrons thus could be blocked by the junction while most photo electrons pass the junction by applying an appropriate voltage. Therefore, both the sensitivity and the operating temperature of the detector could be improved. Our calculation shows that the filter could provide 40% or 70% improvement in Noise Equivalent Temperature Difference (NETD) of detector focal plane arrays (FPAs) at normal operating temperature, depending on whether the detector emitter photocurrent to dark current ratio is = 1 (Emitter is background limited BLIP) or = 0.1 (Emitter is far from BLIP). For both cases, the filter could increase the detector FPAs operating temperatures up to 90K (30K improvement) with 15% to 25% NETD improvement respectively.

Low-temperature electrical characterizations of InAs 1− x− ySb yP x photodiodes fabricated by liquid-phase epitaxy

The mission success of the geostationary operational satellite system Meteosat Third Generation (MTG) will significantly depend on the instrument performance in the very long wavelength infrared (VLWIR) spectral range. As far as dark current behavior, homogeneity, and operability are concerned, the VLWIR constitutes a major challenge for sensor material improvement and device development. This paper reports on the latest results on HgCdTe (MCT) VLWIR photovoltaic sensor development and characterization for possible use with MTG. In order to achieve low enough dark currents, extrinsically p-doped MCT material with various cut-off wavelengths in the long wavelength infrared (LWIR)/VLWIR has been developed and manufactured. Compared to standard intrinsic MCT, a reduction in dark current by more than an order of magnitude is achieved, meeting the challenging MTG requirements. In a (256x256) VLWIR MCT focal plane array (FPA) with an ~14.7μm cut-off wavelength at a 55K operating temperature, a dark current density of about 1pA/μm 2 is demonstrated. For a 291K reference scene and at half-well integration capacity, we obtain a noise equivalent temperature difference of (24.0±3.0)mK and a photo response of 13mV/K.