# A Very Thin MCT Film in HDVIP Achieves High Absorption

## TL;DR

This paper shows that a very thin MCT film in HDVIP technology can achieve over 95% infrared absorption while reducing dark current and manufacturing challenges.

## Contribution

The study introduces a novel structure using photonic crystal resonances and CPA to enable high absorption in ultra-thin MCT films.

## Key findings

- An absorption rate of over 95% for infrared radiation in the 8 µm–10 µm range is achieved with a 1.5 µm–3 µm MCT film.
- The thin MCT film reduces dark current and simplifies the etching and metallization processes.
- COMSOL simulations were used to optimize the loop-hole photodiode structure for high absorption.

## Abstract

Compared to the traditional flip-chip bonded focal plane array, in high-density vertically integrated photodiode (HDVIP) focal plane technology, the thickness of the mercury cadmium telluride (MCT or Hg1−xCdxTe) layer serves as a more critical parameter. This parameter not only influences the efficiency of photon energy absorption but also defines the pn junction area, thereby affecting the magnitude of the dark current. Furthermore, it significantly impacts the manufacturability of via-hole etching and formation processes. This paper investigated the photonic crystal resonances and coherent perfect absorption (CPA) effect of a thin MCT layer in HDVIP by using COMSOL Multiphysics® 4.3b and optimized the structure of the loop-hole photodiode device. The CPA, which is formed by this structure, achieves high absorption of illumination in a very thin MCT film. It is demonstrated that an absorption rate of infrared radiation of more than 95% with a wavelength during the 8 µm–10 µm range can be achieved in Hg1−xCdxTe (x = 0.225) with a thickness of only 1.5 µm–3 µm. The benefit of thinner MCT film is that it decreases the dark current of pn junction and reduces the technical difficulty of etching and metallization of the loop-hole photodiode.