Discussion around IR material and structure issues to go toward small pixel pitch IR HOT FPAs (Conference Presentation)

Abstract

In the last decade, IR imaging detector trend has gone for smaller pixels and larger formats. Most of the time, this scaling is carried out at given total sensitive area for a single focal plane array (FPA). As an example, QVGA 30µm pitch and VGA 15µm pitch exhibit the exact same sensitive area. SXGA 10µm pitch tends to be very similar as well. This increase in format is beneficial to image resolution. However, this scaling to even smaller pixels raises questions because the pixel size becomes similar to the IR wavelength but also to typical transport dimensions in the absorbing material. Hence, maintaining resolution for such small pixel pitches requires a good control of the modulation transfer function (MTF) and quantum efficiency (QE) of the array, while decreasing the pixel size. This might not be obtained just scaling the pixel dimensions. As an example, bulk planar structures suffer from excessive lateral diffusion length inducing pixel-to-pixel cross talk and thus degrading MTF. Non-bulk semiconductor materials such as colloidal quantum dots might exhibit much smaller cross talk due to weak transport properties, but it usually strongly degrades the accessible QE. On the other side, mesa structures might minimize cross-talk physically separating pixels, but also tends to degrade the QE due to non-negligible pixel fill factor shrinking down the pixel size. This paper intend to discuss those issues, taking into account different material systems and structures, in the perspective of expected future pixel pitch IR FPAs. This paper also introduces an important issue in this context: how to reliably measure the MTF of those small pitch detectors. As an answer to this question we will share our first investigations of MTF measurement using the electron beam of an SEM instead of a photon beam (EBIC measurement)