Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Ruei-San Chen; Chih-Che Tang; Wei-Chu Shen; Ying-Sheng Huang

doi:10.3791/53200

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Method Article

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

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DOI:

10.3791/53200

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December 5th, 2015

Ruei-San Chen¹ , Chih-Che Tang² , Wei-Chu Shen² , Ying-Sheng Huang²

¹Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, ²Department of Electronic Engineering, National Taiwan University of Science and Technology

Summary

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe₂) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe₂-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Abstract

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe₂) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal–semiconductor contact of the FIB-fabricated MoSe₂ devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe₂-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 Ω⁻¹cm⁻¹, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe₂ multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe₂ are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS₂ and WS₂.

Introduction

Transition metal dichalcogenides (TMDs), such as MoS₂, MoSe₂, WS₂, and WSe₂, have an interesting two-dimensional (2D) layer structure and semiconducting properties^1-3. Scientists have recently discovered that the monolayer structure of MoS₂shows a substantially enhanced light-emitting efficiency because of the quantum confinement effect. The finding of the new direct-bandgap semiconductor material has attracted substantial attention^4-7. In addition, the easily stripped layer structure of TMDs is an excellent platform for studying the fundamental properties of 2D materials. Unlike metallic gr....