Multicolor detection capabilities, which bring information on the thermal and chemical composition of the scene, are desirable for advanced infrared (IR) imaging systems. This communication reviews intra and multiband solutions developed at CEA-Leti, from dual-band molecular beam epitaxy grown Mercury Cadmium Telluride (MCT) photodiodes to plasmon-enhanced multicolor IR detectors and backside pixelated filters. Spectral responses, quantum efficiency and detector noise performances, pros and cons regarding global system are discussed in regards to technology maturity, pixel pitch reduction, and affordability. From MWIR-LWIR large band to intra MWIR or LWIR bands peaked detection, results underline the full possibility developed at CEA-Leti.
The development of DB (Dual-Band) infrared detectors has been the core of research and technological
improvements for the last ten years at CEA-LETI and Sofradir: the semi planar structure uses a proven standard
process with robust reproducibility, leading to low-risk and a facilitated ramp-up to production. This makes it the
natural choice for the third generation detectors proposed by Sofradir. The fabrication of DB MCT detectors is
reaching maturity: ALTAIR with 24μm-pixel pitch arrays in TV format are available, showing median NETD
around 18mK with operability over 99.5%.
A second structure, based on two back to back diodes, with a single contact per pixel translates the DB pixel into
smaller cell therefore being more efficient in terms of pitch reduction.
These new technologies widen perspectives and open new horizons of applications such as large DB FPA, dual
mode capability providing both SAL (Semi Active Laser) and IR operations for more robust target engagement
or compact dual color detection with wide-angle integrated optics for missile warning system.
The optimization of HgCdTe avalanche-photo-diodes (e-APD) and focal plane arrays (FPA) are reported. The gain
performances was measured in planar homo-junction APDs with Cd compositions between x<sub>Cd</sub>=0.3 to 0.41.
Exponentially increasing gain, synonym of exclusive electron multiplication, was observed in all the devices up to
M>100. The high gain at high x<sub>Cd</sub> opens the path to thermo-electric cooled active imaging at T=200K. This perspective is
corroborated by the demonstration of high gains and low excess noise factor F=1.2 in extrinsically doped MW-APDs,
which enables reduced dark-current at high temperatures. The equivalent input dark current (Ieq_in) decreased from 200
fA to 1 fA, when the cut-off wavelength was reduced from λ
<sub>c</sub>=5.2 μm to 4.1 μm at M=100 and T=80 K. This shows that
sensitivity can be optimized by increasing xCd, at the cost of increased reverse bias. A new horizontal-gain-well (HGW)
hetero-structure was processed to optimize the sensitivity at high gain and low bias. The first HGW-APDs had gains
comparables with MW e-APDs and 50 times lower dark-current at T=200 K. They did also display surprisingly high
quantum efficiency in the MWIR range, η<sub>peak</sub>=30%, which enables thermal imaging at high operating temperature. The
high performance of MW-APDs was confirmed by the characterizations of a first 320x256 30μm pitch APD-FPA,
yielding a 99.8 % operability, low gain dispersion (<10%) and low noise equivalent photons (NEPh=3 at t<sub>int</sub>=1 μs) for
gains up to M=70. The maturity of the DEFIR HgCdTe e-APD-FPA technology was highlighted by the first
demonstration of passive amplified imaging.
This paper presents recent development made at CEA-LETI on manufacturing and characterization of planar p-on-n
HgCdTe photodiodes on long-, mid- and short-wavelength. HgCdTe (MCT) layer was grown both by liquid-phase
epitaxy (LPE) and by molecular beam epitaxy (MBE) on lattice matched CdZnTe (CZT). The n-type MCT base layer
was obtained by indium doping. Planar p-on-n photodiodes were manufactured by arsenic doping, which has been
activated by post-implanted annealing in Hg overpressure. As incorporation is achieved either by implantation or by
incorporation (during MBE growth). Electro-optical characterizations on these p-on-n photodiodes were made on FPAs.
Results show excellent operabilities (99.95% with ±0.5×mean value criterion) in responsivity and NETD and
background limited photodetectors. For long-wavelength FPAs, dark current is very low, leading to a R0A product
comparable to the state of the art at cut-off wavelength of λc = 9.2 μm. MBE mid-wavelength FPAs present very low
responsivity dispersion, reaching 1.1%. Comparisons are made between implantation and growth incorporation As
In this paper, we report the fabrication and electro-optical characterization of both long-wavelength (LWIR) and middle-wavelength
(MWIR) p-on-n infrared photodiodes in HgCdTe. LWIR and MWIR HgCdTe epitaxial layers were grown
by liquid phase and molecular beam epitaxy respectively. p-type doping was obtained by arsenic implantation and n-type
doping by indium incorporation during growth.
The arsenic concentration profile determined by Secondary Ion Mass Spectroscopy showed multi-component diffusion
after Hg post-implant annealing. The process yields an arsenic activation efficiency of around 50%, estimated from
MEMSA (Maximum Entropy Mobility Spectrum Analysis) measurements. The damage induced by arsenic implantation
into HgCdTe have been examined by transmission electron microscopy (TEM) and suggest the formation of an array of
dislocations loops after arsenic implantation. However, after annealing under Hg overpressure, the impact of
implantation falls below the sensitivity of the TEM, suggesting that annealing effectively suppresses most of the defects.
The p-on-n photodiodes showed low leakage currents (shunt resistance>100 MOhms) and typical RoA values
comparable to the state of the art (RoA>4000 Ω.cm<sup>2</sup> for λc=9.2 μm at 77K). Finally, first results on p-on-n focal plane
arrays realized at CEA-LETI will be presented.
We report the latest developments of MW HgCdTe electron initiated avalanche photo-diodes (e-APDs) focal plane
arrays (FPAs) at CEA-LETI. The MW e-APD FPAs are developed in view of ultra-sensitive high dynamic range
passive starring arrays, active 2D/3D and dual-mode passive-active imaging, which is why both the passive imaging
performance and the gain characteristics of the APDs are of interest. A passive mode responsivity operability of 99.9%
was measured in LPE and MBE e-APDs FPAs associated with an average <i>NETD</i>=12mK, demonstrating that dual mode
passive-active imaging can be achieved with LETI e-APDs without degradation in the passive imaging performance. The
gain and sensitivity performances were measured in test arrays and using a low voltage technology (3.3V) CTIA test
pixel designed for 3D active imaging. The CTIA and test arrays measurements yielded comparable results in terms of
bias gain dependence (<i>M</i>=100 at <i>V<sub>b</sub></i>=-7V), low excess noise factor (<i><F></i>=1.2) and low equivalent input current
(<i>I</i><sub>eq_in</sub><1pA). These results validated the low voltage CTIA approach for integrating the current from a HgCdTe e-APD
under high bias. The test array measurements demonstrated a relative dispersion below 2% in both MBE and LPE e-
APDs for gains higher than <i>M</i>>100, associated with an operability of 99%. The operability at <i>I</i><sub>eq_in</sub><1pA at <i>M</i>=30 was 95%. A record low value of Ieq_in=1fA was estimated in the MBE e-APDs at <i>M</i>=100, indicating the potential for using the MW e-APDs for very low flux applications. The high potential of the MW e-APDS for active imaging was
demonstrated by impulse response measurements which yielded a typical rise time lower than 100ps and diffusion
limited fall time of 900ps to 5ns, depending on the pixel pitch. This potential was confirmed by the demonstration of a
2ns time of flight (TOF) resolution in the CTIA e-APD 3D pixel. The combined photon and dark current induced
equivalent back ground noise at <i>f/</i>8 with a cold band pass filter at λ=1.55μm was 2 electrons rms for an integration time
The Molecular Beam Epitaxy (MBE) approach was under investigation for several years to prepare both the very large array fabrication and the 3rd generation developments. This large step in Infrared (IR) detector mass production is also necessary for producing third generation of IR detectors such as bicolor and dual band FPAs which use more complex multi hetero-junctions architectures.
These new advanced HgCdTe technologies necessary for third generation developments have been validated and their producibility have been improved.
As far as dual band IR detectors are concerned, the technologies are developed and a full TV format (24μm pixel pitch) is currently under development with a first application in bicolor within medium waveband.
Future improvements including avalanche photodiodes (APD), will lead to more compact systems as well as a low cost approach.
The possibility to grow HgCdTe by Molecular Beam Epitaxy (MBE) on large alternative substrates opens
the way of increasing the size and reducing the cost of infrared FPAs operating in the Medium Wave
InfraRed (MWIR) bands. Germanium was chosen several years ago at Leti because its 'in situ' and 'ex
situ' surface preparations are much easier to control compared to the more conventionally used silicon
alternative substrate. Moreover extremely high quality germanium "epiready" substrates are commercially
available at a reasonable cost for wafer sizes up to 8 inches. MWIR HgCdTe wafers grown today by Defir
(LETI/Sofradir joint laboratory) on germanium (up to 4 inches diameter) using MBE, exhibit electrical and
physical properties that enables the fabrication of FPAs with various sizes (320×256, 640×512, 1280×1024)
and pitches (from 30μm to 15μm) with electro-optical performances similar to the standard process based
on the more conventional epilayers of HgCdTe grown on CdZnTe by Liquid Phase Epitaxy (LPE). Due to
the low microscopic and macroscopic defect density that can be obtained on such wafers, operabilities
above 99.9% are reached today.
A status of this MBE growth technology is presented as well as the FPAs performances, including
conventional industrial products manufactured such as 320×256 (pitch 30μm), 640×512 (pitch 15μm) and
the largest 1280×1024 (pitch of 15μm) more recently available.
The purpose of this paper is to present the latest developments in Defir (LETI / Sofradir joint laboratory) in the field of bi-color and dual band infrared focal plane arrays (FPA) made with HgCdTe.
The npn structure is achieved using the Molecular Beam Epitaxy (MBE) technique, planar ion implantation, and both dry and wet etching steps. This back to back diode architecture that allows a perfect spatial coherence with a high field factor and large quantum efficiencies needs only one indium bump connection per pixel. This makes it possible to achieve small pitches (below 25μm) and opens the way to the fabrication of large FPAs (TV/4 to TV) with reasonable wafer sizes.
In this paper we present electro optical characterizations of 256x256 prototypes fabricated in Defir operating in two MWIR bands (3.1 and 5μm) with a pitch of 25μm that exhibit background limited performances together with a very high operability (above 99.9%) and NEDT below 22mK for integration time of only 0.5ms. In parallel an industrial product soon available from Sofradir has been developed with a 320x256 format and with a 30μm pitch operating in the same bands. This product exhibits the same operability and NETD as low as 15mK for an integration time as short as 1 ms. Finally, last results regarding 256x256 prototypes operating in MWIR/LWIR bands are presented, together with preliminary APD operating mode for the MWIR photodiodes of this last dual band detector.
We report on the progress achieved in the molecular beam epitaxy of 3" and 4" HgCdTe on CdTe(211)B/Ge composite substrates, and the subsequent fabrication of high performance focal plane arrays. We first describe the growth of the heterostructures, and their characterization. Then we examine the fabrication of a 1280x1024 small-pitch focal plane array, which shows operability in excess of 99% for both the responsivity and the noise-equivalent thermal difference.
The purpose of this paper is to present the electro-optical performances of dual-band detector working in a fully spatially coherent mode, with small pixel pitch. The successive steps of device fabrication are first exposed including molecular beam epitaxy, technological processing and readout circuit design. It is shown that very high quality multiple layer heterostructures of HgCdTe can be grown and processed into 256x256 arrays of 25μm pitch mesas, each mesa including two photodiodes with different cut-off wavelength ranging in the midwave infrared (MWIR). Characterization of these focal plane arrays (FPAs) shows very good homogeneity, low defect density and operabilities usually above 99% for both responsivity and noise equivalent thermal difference (NETD).
A sensor based on selective optical absorption allows monitoring of hazardous engine exhaust emissions such as gaseous hydrocarbons and carbon monoxide. The IR components presented here offer the potential to develop a compact, fast and selective sensor reaching the technical and cost requirements for on-board automotive applications. Optical gas monitoring requires light sources above 3 μm since most of the gas species have their fundamental absorption peaks between 3 and 6 μm. We report here on resonant microcavity light sources emitting at room temperature between 3 and 5 μm. The emitter combines a Cd<sub>x</sub>Hg<sub>1-x</sub>Te light emitting heterostructure and two dielectric multilayered mirrors. It is optically pumped by a commercial III-V laser diode. The principle of the resonant microcavity emitter allows tailoring of the emission wavelength and the line width to fit the absorption band of a specific gas, ensuring a very good selectivity between species. Moreover, this kind of emitter allows fast modulation enabling high detectivity and short response time. We report performances of light sources in the range 3 - 5 μm allowing the detection of hydrocarbons and carbon monoxide. Association of emitters peaking at different characteristic wavelengths with a single broad band detector allows designing of an optical sensor for several gas species. Sensitivity and time response issues have been characterized: detection of less than 50 ppm of CH<sub>4</sub> on a 15 cm path has been demonstrated on synthetic gas; analysis of exhaust gases from a vehicle has allowed the resolution of a cylinder time. This optical sensor offers the potential of various on-board automotive applications.