Investigation on the WS₂ thin film growth and device fabrication

Abstract

Transition metal dichalcogenides (TMDC) layered materials with their remarkable layer-dependent electrical and optical properties have attracted a great deal of research interests recently. The weak van der Waals force dominated layer interactions can accommodate large lattice mismatch to reduce interface defects, because the bonds in a single layer are fully saturated. Additionally, lattice strain is almost absent between layers for such materials. These features make TMDC layered materials to be as potential materials for the fabrication of ultra-thin and flexible electronic and optoelectronic devices. Single-layer or few-layer TMDC materials can be easily acquired by mechanical exfoliation from a single crystal for basic investigations to fabricate proof-of-concept electronic devices. Nevertheless, the exfoliation method is limited to microscale, which is not appropriate for production on large scale. In this thesis, wafer-scale p-type WS2 thin film growth by molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) had been systematically investigated. In addition, WS₂/GaN p-n junctions with low leakage current were fabricated by the transfer of p-type WS₂ thin films onto n-type GaN, and ultra-thin WS₂-based FETs were fabricated by using 40 nm WS₂ thin films as the channel layer. Only the (002) family X-ray diffraction (XRD) peaks were detected for the WS₂ thin film grown by MBE method, indicating strong preferential growth along the [001] crystal orientation of WS₂. For the CVD growth of the WS₂ thin film, by employing a thin Ni layer as texture promoter, the crystal structure of the WS₂ thin film changed from randomly oriented crystallites to large layered crystals with their c-axis oriented perpendicular to the growth substrate with the carrier mobility of 63.3 cm²/Vs. The liquid NiSx phase in the sulfurization process served as the liquid crystallization seeds for van der Waals rheotaxy resulting in horizontal growth of WS₂ crystallites with enhanced crystal size. Taking advantage of the hydrophobicity of WS₂ and hydrophilicity of sapphire substrates, the wafer-scale etching-free transfer method was developed to transfer the as-grown WS₂ thin films onto other substrates without inducing cracks or wrinkles to fabricate heterojunctions. Compared to conventional epitaxial-grown heterojunctions, a wide range of semiconductors can be used for the fabrication of heterojunctions by this etching-free transfer method. Additionally, the etching-free approach does not require the use of any destructive etchants and thereby enables the reuse of substrates. The p-n junctions fabricated by transferring p-type WS₂ thin films onto n-type GaN layer had a quite low leakage current density of 29.6 μA/cm², whereas the direct grown WS₂/GaN p-n junction had a large leakage current density of 92.4 mA/cm². This demonstrates superior performance of the transferred device compared to the as-grown WS2/GaN p-n junctions. This etching-free transfer method is expected to enormously expand the applications of WS₂ thin films for optoelectronic and electronic devices. Layered WS₂ thin films, which have fewer dangling bonds, are attractive for use as channel layers in ultra-thin field effect transistors (FETs). In this thesis, uniform large-area ultra-thin WS2 thin films, down to 40 nm, were obtained by a chemical etching method to fabricate ultra-thin FETs. The WS₂-based FET employing a 40 nm WS₂ thin film as the channel layer had a field-effect mobility of 0.54 cm2V-1s-1 and on/off ratio of 2700.