Analysis of the Electrical Properties of Different HgCdTe Passivations for Infrared Detectors

Abstract

Passivation of HgCdTe is known to be a key point in the making of high performance cooled infrared imagers. In this work, the electrical properties of the passivation layer of n-type mid-wave HgCdTe layers are investigated, using metal–insulator-semiconductor (MIS) structures. Several CdTe based passivation stacks are explored, deposited by two different techniques. Some stacks also include a graded bandgap zone between the semiconductor and the passivation layer. Capacitance versus voltage (CV) measurements are conducted on every sample, and the different passivation structures are then compared with regard to their electrical properties. CV measurements can be challenging to interpret in this type of material, thus additional experimental techniques and numerical simulation tools are often useful in supporting a given interpretation. Special attention is taken concerning doping values extracted from capacitance voltage curves, compared to other techniques such as the Hall Effect or secondary ion mass spectrometry (SIMS). An apparent over-doping is witnessed at the interface of some samples, which can be explained by a defective interface. It is shown that an additional annealing on these structures moves the interface closer to flat band conditions and reduces the amount of excess charge in the passivation layer and defects at the interface. Nevertheless, a Fermi level pinning phenomenon has been evidenced on some of the samples, even when an additional annealing was performed, highlighting the presence of a huge interface trap density on the band edges. In some cases, interface traps are identified and characterized accurately by the conductance method. Their density is found to be in the 1011 cm−2 eV−1 range. A strong dependence on photon flux is observed for some types of interface defects. Two-dimensional (2D) finite element simulation of MIS structures are developed in parallel to support the analysis of the measurements, with an emphasis on photon flux dependence and graded bandgap layer effect. Several models of the electronic affinity of HgCdTe versus compositions from literature are tested. One of them provides a good fit to measurements.