High resolution, wide field-of-view, infrared (IR) imagers find use in defense and civilian applications. The most demanding of them desire uniform sensitivity across an image’s field of view, while maintaining a small and light-weight optical design. These attributes can be achieved by curving of the focal plane array to reduce the need for field curvature correction. Using experimental and numerical methods, we investigated the spherical curving of hybridized arrays to demonstrate mechanical feasibility and opto-electronic performance. Each hybridized array comprised a 4k x 4k, 10 μm pixel pitch, midwave IR (MWIR) detector hybridized to a 67 mm diagonal fanout chip. We curve an array to 139.2 mm radius of curvature, resulting in a pixel area coverage of 0.086 sr. Measurements across the curved array revealed minimal variation in bandgap (<0.1 μm) and no appreciable difference in dark current.
Although HgCdTe imagers are a well-established technology, photodetectors fabricated using the same process still yield a large variation in their performance characteristics, largely stemming from hard-to-control pecu- liarities at the interface between the surface passivation and the active region of each photodiode. This work investigates the dark current characteristics of long-wave IR (cutoff wavelength of 10um) Hg0.774Cd0.226Te mesa photodiodes, which have been passivated with a CdTe film. We use a 2-D model of a p-on-n device structure to study how interface states and Cadmium diffusion at the passivation interface can influence the photodiode dark current.
Understanding the failure mechanisms in high performance detector arrays is critical for meeting the demands for a given application. For SWIR sensing, the detector requires the highest sensitivity possible to operate under photon-starved conditions. Using a 3D drift-diffusion model, we simulate the influence of heterointerface traps on the dark current in In0:53Ga0:47As / InP p-on-n planar heterojunction photodiodes. We calculate how the dark current changes with junction area, and show that it increases linearly with junction radius when the device is limited by surface recombination. An analytical model is developed to understand the geometric dependence of both bulk and surface generation/recombination currents. Finally, insight on mitigation strategies and possible impact on quantum efficiency are both discussed.
HgCdTe has been the material of choice for MWIR, and LWIR infrared sensing due to its highly tunable band gap and favorable material properties. However, HgCdTe growth and processing for the ESWIR spectral region is less developed, so alternative materials are actively researched. It is important to compare the fundamental limitations of each material to determine which offers optimal device performance. In this article, we investigate the intrinsic recombination mechanisms of ESWIR materials—InGaAs, GeSn, and HgCdTe—with cutoff wavelength near 2.5μm, and MWIR with cutoff of 5μm. First, using an empirical pseudo-potential model, we calculate the full band structure of each alloy using the virtual crystal approximation, modified to include disorder effects and spin-orbit coupling. We then evaluate the Auger and radiative recombination rates using a Green’s function based model, applied to the full material band structure, yielding intrinsic carrier lifetimes for each given temperature, carrier injection, doping density, and cutoff wavelength. For example, we show that ESWIR HgCdTe has longer carrier lifetimes than InGaAs when strained or relaxed near room temperature, which is advantageous for high operating temperature photodetectors. We perform similar analyses for varying composition GeSn by comparing the calculated lifetimes with InGaAs and HgCdTe. Finally, we compare HgCdTe, InAsSb and GeSn with a cutoff in the MWIR spectral band.
As material growth and processing have improved, state of the art infrared detector arrays remain limited by material properties and not processing or growth quality. In particular, the dark current can be dominated by diffusion of minority carriers in the quasineutral regions. In this work, we present a unique detector architecture that allows for dark current suppression below the fundamental diffusion limit. We have extensively studied this effect, and report dark current, photocurrent, and quantum efficiency. Finally, we conclude by offering a path to implementing this architecture into existing FPAs.
Indium gallium arsenide (In1−xGaxAs) is an ideal material choice for short wave infrared (SWIR) imaging due to its low dark current and excellent collection efficiency. By increasing the indium composition from 53% to 83%, it is possible to decrease the energy gap from 0.74 eV to 0.47 eV and consequently increase the cutoff wavelength from 1.7 μm to 2.63 μm for extended short wavelength (ESWIR) sensing. In this work, we apply our well-established numerical modeling methodology to the ESWIR InGaAs system to determine the intrinsic performance of pixel detectors. Furthermore, we investigate the effects of different buffer/cap materials. To accomplish this, we have developed composition-dependent models for In1−xGaxAs, In1−xAlxAs, and InAs1−y Py. Using a Green’s function formalism, we calculate the intrinsic recombination coefficients (Auger, radiative) to model the diffusion-limited behavior of the absorbing layer under ideal conditions. Our simulations indicate that, for a given total thickness of the buffer and absorbing layer, structures utilizing a linearly graded small-gap InGaAs buffer will produce two orders of magnitude more dark current than those with a wide gap, such as InAlAs or InAsP. Furthermore, when compared with experimental results for ESWIR photodiodes and arrays, we estimate that there is still a 1.5x magnitude of reduction in dark current before reaching diffusion-limited behavior.
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