Luminescence Properties of CdTe and CdZnTe Materials When Used as Substrate for IR Detectors

Abstract

I -IntroductionCd1-xZnxTe is a ternary alloy composed of atoms from columns II and VI of the periodic table. The amount of zinc can be tuned depending on the targeted applications, and the electrical properties vary as the amount of zinc is modified. Its high Z number and its high density make the Cd1-xZnxTe semiconductor a suited material for high-performance X-ray and gamma ray detectors for astrophysical and medical applications [1]. Moreover, the CdTe material is widely used in solar cells thanks to its high absorption coefficient in the visible domain [2]. The Cd1-xZnxTe is also used as a substrate material in the fabrication process of infrared (IR) detectors based on HgCdTe technology. The amount of zinc in the alloy is tuned to match the lattice parameter of the epitaxial layer of HgCdTe reducing interfacial defects which may degrade the performance of IR detectors. Generally, the amount of zinc in the alloy when used as substrate for HgCdTe-based IR detectors is approximately equal to 4 % [3].Indeed, the state-of-the-art IR detectors use CdZnTe material as substrate. These IR detectors may then be embarked inside the instruments of satellites. However, space is a harsh environment. In orbit, the satellite and its instruments are impacted by energetic ions (mainly protons) coming from the sun or galactic cosmic rays. Proton irradiation test campaigns on IR detectors have shown an unexpected increase of the detector background under irradiation [4]- [6]. Bright diffuse spots around the protons impact were observed. This pollution of the detector image under irradiation was suspected to be linked to energy deposition inside the Cd1-xZnxTe substrate, where the carriers excited by the passage of the proton inside the substrate lose their energy by emitting photons which are in turn detected by the HgCdTe layer. Thus, a parasitic photonic signal adds up to the useful photonic signal and degrade the detector's sensitivity.It has been shown, that a complete removal of the substrate solves radically the problem [7]-[9], but it constitutes a costy step in the fabrication process. Other published results stated that partially-removed-substrate detectors do not show any increase in the measured background when submitted to proton irradiation [10]. The underlying physical phenomena responsible for the image pollution under irradiation is not fully understood. In our previous work, we adopted a modelling approach in order better understand the latter phenomena and estimate the effect of luminescence in the CdZnTe substrate on the response of IR HgCdTe-based detectors under irradiation [11]. In this model, the optical parameters of CdZnTe material are crucial (particularly the absorption coefficient and energy of the photon emitted by spontaneous emission). In astrophysics, IR detectors are operated at low temperatures (typically 100 K in the short-wave IR domain) to reduce the dark current and increase the sensitivity. Up to our knowledge, not all necessary data are available at these operating temperatures for CdZnTe...