ATOMIC LAYER DEPOSITION OF CADMIUM TELLURIDE FOR THE PASSIVATION OF MERCURY CADMIUM TELLURIDE

Abstract

Mercury cadmium telluride (MCT) is an important infrared (IR) detector material due to high quantum efficiency and the ability to tune the bandgap, covering important IR wavelengths from near-infrared (~1 m) to very-long wavelength infrared (>12 m) detection. Focal Plane Arrays (FPAs) are used to image in the infrared and consist of photodiodes that absorb IR photons, generating charge carriers that create an electric signal used to form an image by combining the signals from all of the photodiodes. Decreasing photodiode size increases the resolution of optical systems incorporating MCT FPAs, but challenges current state-of-the-art passivation processes. Passivation is needed to increase the signal-to-noise of a system by rendering benign the charge carrier transport. Physical-vapor-deposited (PVD) CdTe is the incumbent passivation material for MCT, but fails when applied to the next generation of MCT photodiodes because of non-conformal deposition. Atomic layer deposition (ALD) is a superior deposition technique in this regard because the vapor-phase chemicals enable conformal exposure of the surface as opposed to line-of-sight deposition in PVD. ALD of CdTe requires deposition temperature lowering to suppress out-gassing of Hg at elevated temperature, which leads to mercury vacancy formation, reducing signal-to-noise of any eventual detector. Previous demonstration of CdTe ALD was spontaneous above ~200 °C for chemisorbed dimethylcadmium (DMCd) to react with diethyltellurium (DETe). However, this temperature is incompatible with MCT devices, because of the loss of Hg from the material. This dissertation attempts to overcome the low temperature requirement of current CdTe ALD using a novel approach in which argon plasma successfully decomposes the chemisorbed DMCd, replacing temperature induced thermal decomposition, and induced CdTe growth using either DETe or bis(trimethylsilyl)telluride as the tellurium precursors at low temperatures. Film deposition conditions were developed through deposition on silicon substrates, and the process was transferred to MCT samples, demonstrating low temperature deposition, conformal deposition, and passivation of the MCT surface. The films were characterized by in situ spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x ray diffraction (XRD), and transmission electron microscopy (TEM). Photoconductive decay (PCD) measurements were made of MCT material passivated by CdTe ALD, demonstrating effective passivation through enhanced minority carrier lifetime.