Controlled two-step synthesis of nanostructured WS2 thin films for enhanced UV–visible photodetector applications

https://doi.org/10.1016/j.sna.2022.113780Get rights and content

Highlights

  • WS2 nanostructures are successfully grown by two-step synthesis process.

  • Fabricated the Ag/WS2/Ag UV-Vis photodetector with ohmic contact behavior.

  • Ultrafast response time of 100 ms was achieved through interdigitated Ag electrodes.

  • Superior figures of merit were achieved by the WS2 photodetector.

Abstract

Nowadays, development of transition metal dichalcogenides (TMDs) integrated ultra-fast optoelectronics devices under broad spectral range has attracted our research community. The fabricated devices should have high photoresponsivity, specific detectivity, fast switching, and low power consumption. Herein, we put the efforts to successfully synthesize a large area of WS2 nanostructures using sputtering and chemical vapour deposition (CVD) techniques for ultraviolet-visible (UV-Vis) photodetector applications. It is found that following the sulfurization of sputter deposited tungsten films at 800 °C for 60 min facilitated nanostructures with high surface-area to volume ratio. Promising metal-semiconductor-metal (MSM) featured photodetector test-devices based upon interdigitated symmetric Ag electrodes are fabricated. Finally, interdigitated electrode structure, ohmic contact between metal and photoactive material, and enhanced structural properties allowed the test-device accelerates the photodetector performances with 100 ms of response time under UV and visible illuminations even at a bias voltage of 2 V.

Introduction

The discovery of layered graphene with attractive electrical and mechanical properties has fascinated tremendous amount of research interest in wide range of applications [1], [2]. However, the zero intrinsic bandgap of graphene and difficult fabrication methods can limit its applications in optoelectronic and photonic devices [3], [4]. On the other hand, the discovery of two-dimensional (2D) layer structured materials especially transition metal dichalcogenides (TMDs) prevails similar features like graphene and significant bandgap found, which suitable for optoelectronic applications [5], [6], [7], [8]. Therefore, WS2, MoS2, and other TMDs as layer structured semiconducting materials paved promising path for creating next generation ultrathin optoelectronic and energy conversion devices [9], [10], [11]. Importantly, these layer structures feasible to held together by weak van der Waals forces, where each layer consists of a metal (M) atom sandwiched between two sulfur (S) atoms (S-M-S). Each van der Waals layer is completely bonded with no dangling bonds, which reduces the recombination occur in the semiconducting material [12]. Among the TMDs, WS2 is a versatile compound suitable for ultraviolet-visible (UV-Vis) photodetector applications owing to its tunable bandgap of ~2.0 eV [13], [14]. In addition, WS2 also has ultra-high mobility, high spin orbit coupling, higher photoluminescence, strong optical absorption, thermal stability, and operational over wide temperatures [15], [16]. These interesting properties of monolayer and few layer WS2 thin films created a remarkable interest in wide range of applications including photodetectors, tunneling transistors, LED elements, and energy storage devices [17], [18], [19], [20].

For this, there are various prominent methods have been adopted to grow the WS2 thin films, which include (i) chemical and mechanical exfoliation, (ii) sulfurization of tungsten or tungsten oxide films using chemical vapor deposition (CVD), (iii) magnetron sputtering, (iv) atomic layer deposition, and (v) metal-organic chemical vapor deposition [21]. Among these, sulfurization of tungsten using facile CVD process is the most appropriate and cost-effective process for the growth of large area WS2 thin films [22]. However, it is at most priority to synthesize WS2 nanostructures with good crystallinity, morphology, and well controlled size for potential photodetector applications. Sulfurization time during the growth process is the crucial factor to define the crystallinity and evolution of WS2 nanostructures, which in a turn enhance the electrical transport properties of the resultant WS2 thin films [22]. Independent of the deposition techniques, the device architecture is also very much essential for achieving superior photodetector performance. For ultra-fast optical switch applications, photodetectors with ohmic contact is required to achieve photoconduction in both polarities. Metal-semiconductor-metal (MSM) planar structure with interdigitated electrodes fulfill rapid collection of photogenerated charge carriers under shortened mean path and reduced recombination rate [23].

In this work, sulfurization of sputter deposited tungsten films successfully achieved by CVD technique at different sulfurization periods (45 min and 60 min) to evaluate the UV-Vis photodetector applications. The as-grown WS2 nanostructured thin films are carried out to study the structural and morphological properties. The MSM planar structure of Ag/WS2/Ag test-device is fabricated using interdigitated hard mask to estimate the photodetector performance under UV (λ = 382 nm) and visible (λ = 512 nm) light conditions.

Section snippets

Experimental

Initially, metallic tungsten (W) thin film was deposited at room temperature onto SiO2/Si substrate using radio frequency (RF) sputtering technique. Here, tungsten (W) metal target (2″ diameter and 3 mm thickness) with purity of 99.99% was used as a source material. Prior to the deposition, the deposition chamber is evacuated to a base pressure < 4 × 10−7 Torr. The sputter deposition of W was carried out for the duration of 45 s at RF power of 100 W, which obtained the film thickness of 10 nm.

Structural properties

The structural information of the as-grown nanostructured WS2 thin films was provided by the GIXRD technique in the 2θ range from 10° to 65°. Fig. 2 displays the XRD patterns of WS2 films sulfurized at 800 °C for 45 min and 60 min. The multiple diffraction peaks appeared in both the samples suggests the polycrystalline nature of the films. All the obtained reflection planes are assigned with the respective orientations of (002), (004), (100), (101), (006), and (008) according to the JCPDS No:

Conclusions

In conclusion, we successfully fabricated the effective large area nanostructured WS2 thin films following the sulfurization of RF sputtered W films at 800 °C for 45 min and 60 min. From the XRD analysis, significant improvement in the crystalline nature of WS2 phase was achieved for 60 min sulfurized sample, which aided to increase of photocurrent. From FESEM analysis, elegant nanostructured WS2 films with high density and surface-area to volume ratio. The Ag/WS2/Ag test-device sulfurized for

CRediT authorship contribution statement

P.V. Karthik Yadav: Methodology, Formal analysis, Data curation, Writing - original draft. Y. Ashok Kumar Reddy: Conceptualization, Visualization, Writing - review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to express their gratitude to the Department of Science and Technology (DST), New Delhi, Government of India to carry out the above work under DST-INSPIRE Faculty (DST/INSPIRE//04/2017/002531) scheme.

P.V. Karthik Yadav: Mr. Karthik Yadav currently doing his doctoral degree at the Department of Physics, Indian Institute of Information Technology Design and Manufacturing (IIITDM) Kancheepuram, Chennai, India. He completed his Masters in Physics at the Central University of Tamil Nadu, Thiruvarur, India in 2019. His area of research is nanostructured-based materials for photodetectors.

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    P.V. Karthik Yadav: Mr. Karthik Yadav currently doing his doctoral degree at the Department of Physics, Indian Institute of Information Technology Design and Manufacturing (IIITDM) Kancheepuram, Chennai, India. He completed his Masters in Physics at the Central University of Tamil Nadu, Thiruvarur, India in 2019. His area of research is nanostructured-based materials for photodetectors.

    Y. Ashok Kumar Reddy: Dr. Reddy currently working as an Assistant Professor of Physics at the Indian Institute of Information Technology Design and Manufacturing (IIITDM) Kancheepuram, Chennai, India. In earlier, he worked as a Postdoctoral Fellow, Research Professor, and Senior Researcher at the Korea Advanced Institute of Science and Technology (KAIST), South Korea from 2013 to 2018. He is the DST-INSPIRE Faculty Awardee, and IEEE Senior Member. He published his research results in more than 50 journals. His current research areas are nanostructured-based materials for Electronic, Defence, and Energy Devices.

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