Materials science communication
Studies on large area (∼50 cm2) MoS2 thin films deposited using successive ionic layer adsorption and reaction (SILAR) method

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Abstract

A simple method, successive ionic layer adsorption and reaction (SILAR), has been used to deposit MoS2 thin films onto various substrates using ammonium molybdate and sodium sulfide as cation and anion precursor solutions, respectively. Preparative parameters such as concentration, pH and temperature of cation and anion precursor solutions and adsorption, reaction and rinsing time durations were optimized to get good quality films. The films were deposited onto microsolide glass, fluorine doped tin oxide (FTO) coated glass and Si (1 1 1) wafer substrates. The versatility of the method was checked by depositing the films onto (∼50 cm2) glass substrates. X-ray diffraction, optical absorption, electrical resistivity and thermoemf measurement techniques were used for characterization of the films.

Introduction

The optical and electrical properties of transition metal chalcogenides make these semiconductors promising candidate for efficient solar energy conversion [1]. Polycrystalline thin films of metal chalcogenides and oxides have found wide spread industrial applications, mainly in the field of optical devices and electronics [2], [3]. The above range of prospective applications, particularly in the area of solar energy conversion and has promoted renewed interest in semiconductor metal chalcogenide thin films [4].

Pramanik and Bhattacharya [5] have deposited MoS2 thin films by chemical bath deposition technique using ammonium molybdate and thioacetamide as molybdenum and sulfide ions sources, respectively. Sodium dithionite was used as a reducing agent. The as-deposited MoS2 thin films were amorphous. After crystallization of precipitate at 300°C in a high-pressure autoclave under inert atmosphere, it showed sharp peaks of MoS2. The indirect band gap was found to be 1.17 eV. The films were found to be n-type semiconductor.

Patil [6] has carried out electrosynthesis of the molybdenum disulfide (MoS2) thin films using molybdenum trioxide and sodium thiosulfate as initial ingredients. The films were polycrystalline having 1.7 eV direct optical band gap. From scanning electron microscopy (SEM) studies, the films were continuous and uniform.

Nicolau [7] has introduced the successive ionic layer adsorption and reaction (SILAR) method to grow polycrystalline or epitaxial thin films of water insoluble ionic or ioncovalent of the CmAn type by heterogeneous chemical reactions at the solid-solution interface between adsorbed cations (CLp)n+ and anions (AL′q)m following the reaction:m(CLp)n++n(ALq)m−CmAn↓+mpL+nqLwhere Lp and L′q should be different ligands, but this is not a necessary condition.

The method involves an alternative immersion of the substrate in a solution containing a soluble salt of the cation and anion of the compounds to be grown. The substrate supporting the growing film is rinsed in high purity deionized water after each immersion in order to avoid homogeneous precipitation. The immersion and rinsing time periods can be experimentally determined. In principle, SILAR is a deposition method in which thickness of the layer has determined by the number of deposition cycles. The SILAR method has been employed for the deposition of CdS, ZnS, CdZnS, PbS, CuxS, CoS, etc. thin films on different substrates [8], [9], [10], [11], [12].

In this paper, we report for the first time, the deposition of MoS2 thin films by SILAR method. The X-ray diffraction, optical absorption, electrical resistivity and thermoemf measurements are carried out to study the properties of MoS2 thin films.

Section snippets

Sample preparation

Analytical reagent ammonium molybdate [(NH4)6Mo7O24·4H2O] and sodium sulfide [Na2S·H2O] were used in the deposition of MoS2 thin films. The cation precursor was 0.001 M ammonium molybdate solution. The pH was adjusted to 3 by adding dilute sulfuric acid. The source of sulfide ions was 0.2 M sodium sulfide (pH∼13.5). Prepared solutions were taken into beakers and for rinsing purpose ample quantity of deionized distilled water (resistivity ∼18  cm) was used. After every five deposition cycles the

Film thickness

Thickness of MoS2 thin films for different number of SILAR deposition cycles on glass substrate was determined by gravemetric weight difference method. For this, a sensitive microbalance was used and film density was assumed to be as bulk of MoS2 (5.06 g cm−3). Fig. 1 shows the plot of film thickness against number of deposition cycles. The film thickness increases rapidly up to 60 deposition cycles, and slowly thereafter up to 100. However, the film peels off from the glass substrate after 100

Conclusions

In this paper, we have reported a simple SILAR method for the production of MoS2 thin films onto various substrates. The versatility of the method was tested by deposition of large area (∼50 cm2) MoS2 thin films onto amorphous glass substrate. The direct optical band gap was found to be 1.74 eV. The films are semiconducting, having room temperature resistivity of the order of l04 Ω cm. From thermoemf studies, the films were found to be of p-type in electrical conductivity.

Acknowledgements

Authors are thankful to UGC, New Delhi for the financial support under the project No. F:10-7/97 (SR-I).

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