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 NETD=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 (M=100 at Vb=-7V), low excess noise factor (=1.2) and low equivalent input current
(Ieq_in<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 M>100, associated with an operability of 99%. The operability at Ieq_in<1pA at M=30 was 95%. A record low value of Ieq_in=1fA was estimated in the MBE e-APDs at M=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 f/8 with a cold band pass filter at λ=1.55μm was 2 electrons rms for an integration time
A Monte Carlo model is developed for understanding the multiplication process in HgCdTe infrared avalanche
photodiodes (APD). A good agreement is achieved between simulations and experimental measurements of gain and
excess noise factor on midwave infrared electron injected Hg0.7 Cd0.3Te APD manufactured at CEA/LETI. In both cases,
an exponential gain and a low excess noise factor - close to unity out to gains greater than 1000 - were observed on 5.1-μm cut-off devices at 77K. These properties are indicative of a single ionizing carrier multiplication process that is to say
in our case the electron. Simulations also predict that holes do not achieve enough energy to impact ionize and to
contribute to the gain, which confirms the previous observation. A comparison study is presented to explain the effect of
different combinations of scattering processes on the avalanche phenomenon in HgCdTe. We find that alloy scattering
with random scattering angle increases multiplication gain and reduces excess noise factor compared to the case
including impact ionization only. It also appears that, in the more complete scattering environment, optical phonon
scattering delays significantly the onset of avalanche.
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
The characteristics of a multifunctional two-colour-avalanche gain Focal Plane Array, FPA, in which one of the bands can be used in avalanche mode to produce current gain with low excess noise and low dark current, are reported.
The multifunctional FPA is based on a bi-colour pseudo-planar MCT detector structure, developed at the CEA-LETI, which superposes two planar type diodes with different composition of Cd. The electro-optic characteristics of the multifunctional LW-MW-avalanche gain detectors are reported for 256x256 30μm pitch arrays hybridised on a bi-colour read-out circuit, and for direct measurements on 30μm pitch test arrays. An avalanche gain of M=5300 at an inverse bias of Vpol=-12.5V and a noise factor close to F=1, is reported for MW wavelength diode, characterised by a cut-off wavelength of λC=5.03μm. A new measure of the sensitivity limit of the APD, the equivalent shot noise limited dark current, ieq_in, was defined and estimated from dark current noise measurements ieq_in=3.0 10-10A at gain M<300 and ieq_in=1.0 10-10A at M=5300. The results will be discussed in view of the wide scope of applications which are enabled by the multifunctional FPAs.
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.