Solution-processed zinc sulphide and magnesium oxide for thin film transistors applications

Mohamed, Ahmed and Adamopoulos, George (2025) Solution-processed zinc sulphide and magnesium oxide for thin film transistors applications. PhD thesis, Lancaster University.

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Abstract

Transparent thin-film transistors (TFTs) have been extensively studied for display and imaging applications, where they enhance switching speed and stability. Various material systems have been explored to meet the demand for high-performance, low-power, large-area electronics, including thin-film silicon, organic semiconductors, and metal oxides. However, thin-film silicon has well-documented electronic limitations, while organic semiconductors, despite recent advancements, continue to suffer from low mobility and instability, limiting their application to niche markets. This thesis investigates the development of solution-processed dielectrics and semiconductors for TFTs using the spray pyrolysis technique. It aims to address the challenges associated with improving the performance, stability, and scalability of TFT applications. The central research objective is to optimize the material properties of zinc sulphide (ZnS) and magnesium oxide (MgO) thin films to enhance overall device performance—including electron mobility, subthreshold swing, and device stability—while ensuring cost-effectiveness and suitability for large-area applications. The first part of the study focuses on ZnS-based TFTs, where thin films were deposited using zinc chloride and thiourea as precursors, with varying Zn:S ratios (1:1 to 1:3). Optical analysis revealed a band gap ranging from 3.25 eV to 3.7 eV and a decrease in Urbach energy with increasing sulphur content, indicating improved material crystallinity. X-ray diffraction (XRD) confirmed a cubic phase structure, with crystallite sizes ranging from 9.8 nm to 14.4 nm, while atomic force microscopy (AFM) showed smooth surfaces with roughness values between 2.2 nm and 2.7 nm. The resulting ZnS-based TFTs exhibited an electron mobility of 4 cm²/Vs, a current modulation ratio of 10⁴, and a subthreshold swing of 10 V/dec, with an interface trap density of 3.5 × 10¹³ cm⁻². These findings demonstrate the potential of ZnS as an effective semiconductor material for TFTs. The second phase of the research examined ZnS films deposited using zinc diethyldithiocarbamate as a single precursor at substrate temperatures ranging from 450°C to 650°C. These films exhibited a band gap between 3.24 eV and 3.5 eV, with decreasing Urbach energy as the temperature increased, suggesting improved film quality and crystallinity. TFTs fabricated at 650°C demonstrated superior performance, including an electron mobility of 3 cm²/Vs, a high current modulation ratio of 10⁸, a subthreshold swing of 3 V/dec, and a lower interface trap density of 3.3 × 10¹² cm⁻², highlighting the significant influence of deposition temperature on TFT performance. In the final phase of this research, MgO films were investigated for their potential as high-k dielectric materials in TFTs. MgO films were deposited at temperatures ranging from 350°C to 600°C. Optical analysis revealed a band gap exceeding 6.2 eV, with decreasing Urbach energy as the substrate temperature increased. XRD analysis confirmed the formation of polycrystalline cubic MgO, with grain sizes ranging from 19.5 nm to 22.8 nm, while AFM imaging showed smooth surfaces with roughness values between 1.5 nm and 4.92 nm. The MgO films exhibited low leakage current densities (10⁻⁷ to 10⁻⁸ A/cm²), a high dielectric constant of 12, and an impressive breakdown voltage of 8 MV/cm, making them highly suitable for TFT applications. The integration of MgO as a dielectric layer in ZnO-based TFTs led to enhanced device performance, achieving an electron mobility of 9 cm²/Vs, a subthreshold swing of 0.21 V/dec, and a current modulation ratio of 10⁷. These results highlight the successful combination of ZnO and MgO for high-performance TFTs. This thesis makes a significant contribution to the field of thin-film transistors by demonstrating that solution-processed ZnS and MgO thin films, fabricated via spray pyrolysis, can be optimized to achieve high-performance TFTs. The findings underscore the potential of these materials for future integration into a broad range of electronic applications, advancing the development of transparent and large-area electronics.