This article reviews the history, the present status and possible future developments of HgCdTe ternary alloy for infrared (IR) detector applications. HgCdTe IR detectors have been intensively developed since the first synthesis of this material in 1958. This article summarizes the fundamental properties of this versatile narrow gap semiconductor, and relates the material properties to its successful applications as an IR photoconductive and photovoltaic detector material. An emphasis is put on key developments in the crystal growth and their influence on device evolution. Competitive technologies to HgCdTe ternary alloy are also presented. Recent advances of backside illuminated HgCdTe heterojunction photodiodes have enabled a third generation of multispectral instruments for remote sensing applications and have led to the practicality of multiband IR focal plane array technology. Finally, evaluation of HgCdTe for room temperature long wavelength IR applications is presented. (Some figures in this article are in colour only in the electronic version)

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HgCdTe infrared detector material: history, status and outlook

Hitherto, two distinct families of multielement detector arrays have been used for infrared (IR) imaging system applications: linear arrays for scanning systems (first generation) and two-dimensional arrays for staring systems (second generation). Nowadays, third-generation IR systems are being developed which, in the common understanding, provide enhanced capabilities such as larger numbers of pixels, higher frame rates, better thermal resolution, multicolor functionality, and/or other on-chip signal-processing functions. In this paper, fundamental and technological issues associated with the development and exploitation of third-generation IR photon detectors are discussed. In this class of detectors the two main competitors, HgCdTe photodiodes and quantum-well photoconductors, are considered. This is followed by discussions focused on the most recently developed focal plane arrays based on type-II strained-layer superlattices and quantum dot IR photodetectors. The main challenges facing multicolor devi...

Third-generation infrared photodetector arrays

Recent progress in infrared detector technologies

This article presents a review on the current status, challenges, and potential future development opportunities for HgCdTe infrared materials and detectortechnology. A brief history of HgCdTe infrared technology is firstly summarized and discussed, leading to the conclusion that HgCdTe-based infrared detectors will continue to be a core infrared technology with expanded capabilities in the future due to a unique combination of its favourable properties. Recent progress and the current status of HgCdTe infrared technology are reviewed, including material growth,device architecture, device processing, surface passivation, and focal plane array applications. The further development of infrared applications requires that future infrared detectors have the features of lower cost, smaller pixel size, larger array format size, higher operating temperature, and multi-band detection, which presents a number of serious challenges to current HgCdTe-based infrared technology. The primary challenges include well controlled p-type doping, lower cost, larger array format size, higher operating temperature, multi-band detection, and advanced plasma dry etching. Various new concepts and technologies are proposed and discussed that have the potential to overcome the existing primary challenges that are inhibiting the development of next generation HgCdTeinfrared detectortechnology.

Progress, challenges, and opportunities for HgCdTe infrared materials and detectors

Progress in focal plane array technologies

HgCdTe midwave infrared pin avalanche photodiodes (APDs) have been studied as a function of temperature and bias, for two types of junction profiles with different nominal junction width and the same cut-off wavelength λc = 5.0 μm at T = 77 K. A gain of 5,300 at a reverse bias of 12.5 V was demonstrated in the nominally wide junction pin-APD at T = 77 K. The nominally narrow pin-APD showed a higher gain at low bias, but the maximum gain was lower due to an earlier onset of excess currents. The gain was measured for temperatures (T) between 30 K and 150 K and was found to decrease with increasing temperatures, in correlation with the increase in band gap. However, the useful gain was reduced at lower temperatures, due to increased excess current at high reverse bias, indicating a tunnel limited origin of the sensitivity limiting excess current. The noise factor, F, showed a nearly deterministic multiplication of the carriers, with F = 1–1.5 up to gains of 5,000.

Gain and Dark Current Characteristics of Planar HgCdTe Avalanche Photo Diodes

In this communication we report high performance gain characteristics measured at T=77K in electron injected MW
HgCdTe APDs. A full set of characterisations, including gain, excess noise, dark current and first measurement of the
impulse response, was performed on test arrays of backside illuminated pin type MW APDs, manufactured at CEA LETI
using an MBE grown HgCdTe absorption layer. A record high avalanche gain of M=5300 have been demonstrated in
these diodes, associated with a low noise factor, F=1.0-1.3, and low dark current. The sensistivity of the APD is
discussed in terms of the impact of the distribution of the gain in the structure for different applications and we have
estimated a shot noise equivalent input current, Ieq_in=2.0 10-13 to 1.0 10-12A, for continuous measurements, and a dark
count rate for photon counting applications DCR=2.7 106 s-1. The first measurements of the impulse response of the MW HgCdTe APDs showed that the band width was only weakly
dependent on the gain, in coherence with the dominant electron multiplication evidenced by the low value of the noise
factor. At the maximum gain, M=5000, we measured a risetime of t10-90=88ps and a fall time of t90-10=2.4ns, yielding a record high band width product of GBW=723GHz (BW=145MHz), mainly limited by the diffusion and life time of the
minority electrons.

High performance characteristics in pin MW HgCdTe e-APDs

The Leti-Lir has studied II–VI compounds for infrared (IR) detection for more than 20 years. The need to reduce the production cost of IR focal plane arrays (FPAs) sparked the development of heteroepitaxy on large-area substrates. Germanium has been chosen as the heterosubstrate for the third generation of IR detectors. First, we report on the progress achieved in HgCdTe growth on 3-in. and 4-in. (211)B CdTe/Ge. Then, we discuss the choice of a new machine for larger size and better homogeneity. Finally, we present the latest results on third-generation IR multicolor and megapixel devices. First-time results regarding a middle wavelength infrared (MWIR) dual-band FPA, with a reduced pitch of 25 µm, and a MWIR 1,280×1,024 FPA will be shown. Both detectors are based on molecular beam epitaxy (MBE)-grown HgCdTe on Ge. The results shown validate the choice of Ge as the substrate for third-generation detectors.

Molecular beam epitaxy growth of HgCdTe on Ge for third-generation infrared detectors

The third generation of HgCdTe infrared-detector focal-plane arrays (FPAs) should be able to detect simultaneously in two spectral bands. The feasibility of this type of dual-band detectors has already been shown in our laboratory with a pixel size of 50 µm in the 3–5-µm wavelength range. To improve the detector resolution, it is necessary to decrease the pixel pitch. Dry etching is a key process technology to fulfill this goal because of the high aspect-ratio structures needed (typically 10–15-µm deep and 2–5-µm wide trenches). In this paper, we present results of a parametric study on HgCdTe dry etching, as well as results obtained on detector arrays made with the dry-etching technique. The etching study has been done in a microwave plasma reactor with the aim of controlling the surface roughness, the etch rate, and the slope of the trench side. We show how these parameters are influenced by the reactive gas-mixture composition (based on CH4, H2, and Ar) and the substrate self-bias. We show how polymer film deposition can prevent etching from occurring but can improve anisotropy. We show some examples of results obtained when manufacturing the trenches that separate the pixels, keeping a high fill factor, and anisotropic etching. We also show results of the material surface characterizations done with scanning electron microscopy (SEM) and Hall effect measurements. These studies allow us to evaluate and compare the damages done to the HgCdTe surface with different etching conditions. Our best process allows us to make a light electrical damage, confined to less than a micron deep in the material. Using the dry-etching process, we have developed detector arrays fabricated with a pixel pitch as low as 30 µm. We finally present the results of the first electrical characterizations made on these arrays, showing promising results for the development of high-resolution dual-band detectors.

Study of the pixel-pitch reduction for HgCdTe infrared dual-band detectors

In this paper we show the latest achievements of HgCdTe-based infrared bispectral focal plane arrays (FPAs) at LETI infrared laboratory. We present and compare the two different pixel architectures that are studied now in our laboratory, named “NPN” and “pseudo-planar”. With these two technologies, a wide range of system applications in dual-band detection can be covered. Advantages of both architectures will be pointed out. We also review performances obtained with these different architectures. The first one has been studied for several years in our laboratory, and we review results obtained on FPAs of size 256 × 256 pixels on a 25 μm pitch, in the MWIR/MWIR (3 μm/5 μm) range. Very high noise equivalent temperature difference (NETD) operability is obtained, at 99.8% for the λc = 3 μm band and 98.7% for the λc = 5 μm band. The second one has been developed more recently, to address other applications that need temporal coherence as well as spatial coherence. We show detailed performances measured on pseudo-planar type FPAs of size 256 × 256 pixels on a 30 μm pitch, in the MWIR/LWIR (5 μm/9 μm) range. The results are also very promising for these prototypes, with NETD as low as 15 mK for an integration time as short as 1 ms, and good operability. The main manufacturing issues are also presented and discussed for both pixel architectures. Challenging process steps are, firstly, molecular beam epitaxy (MBE) HgCdTe heterostructure growth, on large substrates (cadmium zinc telluride) and heterosubstrates (germanium), and, secondly, detector array fabrication on a nonplanar surface. In particular, trenches or hole etching steps, photolithography and hybridization are crucial to improve uniformity, number of defects and performances. Some results of surface, structural and electrical characterizations are shown to illustrate these issues. On the basis of these results, the short-term and long-term objectives and trends for our research and development are presented, in terms of pixel pitch reduction, wavelengths, and dual-band FPA size.

Jacques Baylet

Papers

Gain and Dark Current Characteristics of Planar HgCdTe Avalanche Photo Diodes

High performance characteristics in pin MW HgCdTe e-APDs

Molecular beam epitaxy growth of HgCdTe on Ge for third-generation infrared detectors

Study of the pixel-pitch reduction for HgCdTe infrared dual-band detectors

Status of HgCdTe Bicolor and Dual-Band Infrared Focal Arrays at LETI