Improvement of electrical properties of ZnS/CdTe-HgCdTe interface by (NH4)2S treatment

Abstract

Effect of 20% aqueous ammonium sulphide treatment on electrical properties of ZnS/CdTe-HgCdTe interface was studied by using high frequency capacitance-voltage (C-V) measurements showing significant improvement in surface conditions. MIS structures were fabricated on passivated HgCdTe wafers and C-V measurements were carried out. C-V curves were analysed to evaluate fast interface state distribution with energy, slow state density and fixed charged density. For the sake of comparison two HgCdTe samples were prepared under similar conditions, one was given (NH4)2S treatment after standard surface preparation by I-KI polishing and the other was untreated to act as the base sample. CdTe and ZnS layers were deposited on both the samples, A and B. It was observed that C-V characteristics of (NH4)2S treated sample show significant improvement in comparison to that of untreated sample. Fixed charge density and fast interface charge (fast and slow) density were estimated for both the samples. Average fixed charge density for sample treated with (NH4)2S (4.2 × 1010 cm−2) is reduced by a factor of ~6 compared to untreated sample (2.53 × 1011 cm−2), Average fast interface trap density for sample exposed to (NH4)2S treatment (~3.5 × 1011 cm−2eV−1) improved by a factor of ~3.5compared to untreated sample (~1.3 × 1012 cm−2eV−1) while average slow interface traps estimated for sample treated with (NH4)2S (~3.09 × 1010 cm−2) reduced by ~1.3times than that for untreated sample (~4.1 × 1010 cm−2). Surface and interface stability of the grown sulphide is also studied by repeated ellipsometry and C-V measurements respectively. It is found that it is important to deposit CdTe/ZnS passivation layers immediately after 20% aqueous ammonium sulphide treatment to achieve stable performance. Stability of (NH4)2S treatment with time was also investigated. C-V measurements after approximately 2 months showed improvement in interface properties.