Influence of Different Annealing Ambients on the Properties of Zinc Sulfide Prepared by Atomic Layer Deposition

Abstract

The effects of different post annealing ambients (vacuum, O2, and H2S gases) on the chemical, structural, and optical properties of zinc sulfide (ZnS) thin films prepared by atomic layer deposition (ALD) were investigated. Diethylzinc [DEZ, Zn(C2H5)2] and H2S gas were used as precursor and reactant gas, respectively. Compared to as-deposited 50-nm-thick ZnS film, the optical energy band gap (Eg) of ZnS annealed under vacuum and H2S conditions increased from 3.73 to 3.85 eV, while it decreased down to 3.23 eV for the O2 annealing case. The change in the Eg of the thicker ZnS is similar to that of the thinner ZnS case. This behavior is related to the change in the Zn to S ratio. The vacuum and H2S anneals increases the Zn/S ratio, leading to higher Zn interstitial defects or S vacancy sites in the films. X-ray diffraction analysis reveals that ZnS thin film has a preferred orientation of hexagonal wurtizte (002) and cubic zinc blend (111) at ∼28.2°, and its grain size changes in a range from 18.79 to 28.14 nm after annealing. However, for O2 annealing, the patterns of both the newly formed ZnO phase and the reduced ZnS phase appear at 34.04°. This result suggests that change in the composition and crystal structure during the process significantly affects the optical properties of ZnS thin film, which should be taken into consideration in searching for an alternative buffer layer for Cu2InGaSe(S)4 (CIGS) thin film solar cell systems.