HgCdTe Epitaxial Layer Surface Treatment Using Ammonium Sulfide Before Passivation
In this work we report the surface analysis of polished p-type Hg1−xCdxTe (x ~ 0.3) epitaxial layer treated with ammonium sulfide solution (20%) for five minutes at room temperature using X-ray Photoelectron spectroscopy (XPS). XPS analysis shows sulphide signature in substantial quantity and is bonded to Cd or Hg. Quantitative estimation of sulphur by fractional coverage on the Mercury Cadmium Telluride (MCT) surface revealed 60% coverage of Hg1−xCdxTe surface by sulphur atoms. It is also observed that sulphur in elemental or oxide form is not present on the surface. The process of sulfidation results in the removal of native oxides from the Hg1−xCdxTe surface through the formation of Hg–S and Cd–S bonds as revealed during XPS studies. These bonds might act as barrier against native oxide formation when ammonium sulfide treated Hg1−xCdxTe surface is exposed to air. We further studied the effect of sulfidation on the MIS device structure fabricated using the ammonium sulfide treated Hg1−xCdxTe epitaxial layer surface with CdTe/ZnS as the passivant. The Hg1−xCdxTe—passivant interface was analyzed using C–V measurements at 77 K. To evaluate the Hg1−xCdxTe—passivant interface of the Hg1−xCdxTe surface treated with ammonium sulfide the interfacial charge were examined by fabricating a MIS structure. CdTe/ZnS layer was deposited by thermal evaporation on two polished Hg1−xCdxTe epitaxial layers with one of the layer surface treated with ammonium sulfide before CdTe/ZnS deposition. The capacitance voltage (C–V) measurement of MIS structures were carried out at 77 K and measured at a frequency of 1 MHz. A comparison of ammonium sulfide treated MIS structure shows near flat band conditions with Qss ~ 2.4 × 1010 cm−2. The density of interface traps observed is Dit ~ 1010 cm−2 eV−1 which is one order of magnitude lower than that observed for untreated MIS structures.
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