In this communication, we report on electrical and electro-optical characterizations of InAs/InAsSb Type-II superlattice (T2SL) MWIR photodetector, showing a cut-off wavelength at 5 μm. The device, made of a barrier structure in XBn configuration, was grown by molecular beam epitaxy (MBE) on GaSb substrate. At 150K, dark current measurements shows a device in the Shockley-Read-Hall (SRH) regime but with an absolute value comparable to the state-of-the-art. A quantum efficiency of 50% at the wavelength of 3 μm for a 3 μm thick absorption layer is found in simple pass configuration and front-side illumination. Combined with lifetime measurements performed on dedicated samples through time resolved photoluminescence (TRPL) technique, mobility is extracted from these measurements by using a theoretical calculation of the quantum efficiency thanks to Hovel’s equations. Such an approach helps us to better understand the hole minority carrier transport in Ga-free T2SL MWIR XBn detector and therefore to improve its performance.
Type-II InAs/GaSb superlattice (T2SL) has recently matured into a commercially available technology addressing both MWIR and LWIR spectral domains. As the prerequisites such as Quantum Efficiency (QE) and dark current were met, more advanced figures of merits related to the ElectroOptic (EO) system as a whole can now be studied in order to position this technology. In this paper, we focus on modulation transfer function (MTF) measurements. Knowing the MTF of a detector is indeed of primary importance for the EO system designers, since spatial filtering affects the system range. We realized MTF measurements on a 320x256 MWIR T2SL FPA provided by IRnova, using a Continuously Self Imaging Grating (CSIG). The advantage of this experimental configuration is that no high performance projection optics is required. Indeed, the CSIG exploits the self-imaging property (known as Talbot effect) to project a pattern with known spatial frequencies on the photodetector. Such MTF measurements have never been done in Integrated Detector Dewar Cooler Assembly (IDDCA) configuration, so we had to study the effect of the vibrations induced by the cryocooler. Vibrations indeed affect the MTF measurement in the same way electrical diffusion would do. Using three accelerometers we optimized our experimental setup and extracted MTF measurements with reduced vibrations. The pixel size is 26μm for a pitch of 30μm.
KEYWORDS: Long wavelength infrared, Mercury cadmium telluride, Avalanche photodetectors, Monte Carlo methods, Ionization, Mid-IR, Avalanche photodiodes, Signal to noise ratio, Temperature metrology, Mercury
We report the performances of LWIR (λc = 9.0 μm at 80K) HgCdTe electron injected
avalanche photodiodes (e-APD). In these devices, the exponential gain curve, up to gains
equal to 23 at 100K, and the low excess noise factor close to unity (F ~ 1-1.25) are indicative
of a single carrier multiplication process, which is electron impact ionization. The dark
current is mainly due to a diffusion current at low reverse bias and tunneling currents at high
reverse bias. A Monte Carlo model has been developed for understanding the multiplication
process in Hg1-xCdxTe e-APDs. We find a good agreement between first simulation results
and experimental measurements of the gain and the excess noise factor in both MWIR (x =
0.3) and LWIR (x = 0.235) e-APDs at 80K. Furthermore, simulations do not show any heavy
hole impact ionization. This model which enables to perform phenomenological studies aims
at identifying the main physical and technological parameters that influence the gain and the
excess noise. In the present work, it is used to study the influence of the thickness of the ndoped
region on the gain and the excess noise factor. We found that F still decreases while the
thickness of the n- layer decreases. However, an optimum thickness of the n- layer exists
around 1μm in terms of gain-voltage characteristic.
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.
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