Native Oxidation of HgCdTe Compound Semiconductors and Its Impact on Infrared Detectors Performances
Authors: BEL Steven , Lobre Clément , Petit Sarah , Veillerot Marc , Badano Giacomo 
Published: December 2024

The development of high-operating temperature (HOT) infrared imaging systems is motivated by the growing demand to reduce the size, weight, power consumption, and total cost of infrared detectors. HgCdTe photodiodes have unique material properties, making them one of the most promising candidates to produce infrared detectors with background-limited performances at room temperature, as predicted by the "Law 19" photodiode performance metric [1]. However, in state-of-the-art devices, the fraction of anomalous pixel responses increases alongside the detectors' operating temperature and cut-off wavelength. Numerous studies have already identified surface states induced by manufacturing processes as one of the main sources of this issue. As a result, the assessment and understanding of HgCdTe surface chemistry will be critical for future technological advancements.
HgCdTe surface processing and its exposition to the environment promotes the formation of surface oxides reported to influence the interface electrical behavior and the global material electronic properties [2]. Nonetheless, the complexity of the HgCdTe surface chemistry makes it challenging to characterize the nature of the surface states caused by the presence of native oxide. As a result, there is no consensus in the literature on the effect of the alloy composition on the formed surface native oxide and its subsequent effects on the passivation quality.
The purpose of this work is to investigate the mechanisms underlying the formation of HgCdTe native oxides and discuss their impact on the devices’ electrical performances. Therefore, X-ray Photoelectron Spectroscopy (XPS) and Spectroscopic Ellipsometry (SE) studies were performed in an attempt to understand HgCdTe oxidation as a function of its nominal bulk composition.
The samples used in this study were epitaxial Hg(1-x)Cd(x)Te layers grown on CdZnTe substrate with multiple cadmium molar fractions (x ≃ 0.2 to 0.7). A technologically relevant standard wet etching procedure was used to achieve clean, oxide-free surfaces. Freshly etched samples were transported to the XPS in an inert atmosphere using a transfer vessel to representative surfaces and prevent alterations. The surfaces showed no oxides identifiable by XPS, thus showing the effectiveness of the transfer procedure. Furthermore, it was found that the HgCdTe components of these surfaces were not stoichiometric. Particularly, the surfaces were enriched in tellurium regardless of the alloy composition under study. These differences from the nominal composition are induced by the preferred etching of the alloy's components (Cd > Hg >> Te).
Dynamic SE measurements were used to monitor native oxide formation in real-time, with scans taken every few seconds for the first hour after exposure to a clean-room environment. Furthermore, the long-term evolution of the surfaces was assessed by taking further measurements over several days. SE trends revealed a two-stage formation of native HgCdTe oxides, with an initial fast oxide growth (<1 hour) followed by a slower oxidation. The findings were supported by XPS measurements on additional samples exposed to air and kept in an airtight plastic box to obtain surface oxides of different thicknesses. Regardless of the alloy composition, the excess surface tellurium is oxidized during early oxide formation. While lengthier environment exposures resulted in oxide thicknesses correlated to the alloy cadmium molar fraction. Therefore, indicating for the first time a clear experimental demonstration of the cadmium contribution to further oxidize the material.
Consequently, the combination of surface characterization techniques used in this work allowed us to investigate the formation mechanisms surrounding the native oxide of wet-etched HgCdTe surfaces and to discuss the effect of their formation on the device's performance.

Acknowledgements :
This work, carried out on the Platform for Nanocharacterisation (PFNC), was supported by the “Recherche Technologique de Base” and "France 2030 - ANR-22-PEEL-0014" programs of the French National Research Agency (ANR)

References :
[1] M. Kopytko and A. Rogalski, New Insights into the Ultimate Performance of HgCdTe Photodiodes, Sensors Actuators A Phys. 339, 113511 (2022).
[2] E. R. Zakirov, V. G. Kesler, G. Y. Sidorov, and A. P. Kovchavtsev, Effect of HgCdTe Native Oxide on the Electro-Physical Properties of Metal-Insulator-Semiconductor Structures with Atomic Layer Deposited Al2O3, Semicond. Sci. Technol. 35, 0 (2020).