Yong Hun Kwon

p‑Channel Oxide Thin Film Transistors Using Solution-Processed Copper Oxide

Cu2O thin films were synthesized on Si (100) substrate with thermally grown 200-nm SiO2 by sol-gel spin coating method and postannealing under different oxygen partial pressure (0.04, 0.2, and 0.9 Torr). The morphology of Cu2O thin films was improved through N2 postannealing before O2 annealing. Under relatively high oxygen partial pressure of 0.9 Torr, the roughness of synthesized films was increased with the formation of CuO phase. Bottom-gated copper oxide (CuxO) thin film transistors (TFTs) were fabricated via conventional photolithography, and the electrical properties of the fabricated TFTs were measured. The resulting Cu2O TFTs exhibited p-channel operation, and field effect mobility of 0.16 cm2/(V s) and on-to-off drain current ratio of ∼1×10(2) were observed in the TFT device annealed at PO2 of 0.04 Torr. This study presented the potential of the solution-based process of the Cu2O TFT with p-channel characteristics for the first time.

Correlation between electrical properties and point defects in NiO thin films

The wide band-gap semiconductor NiO has p-type characteristics and is an alternative to p-ZnO, due to conduction mechanisms coming from the Ni vacancies and O interstitials. The correlation between the electrical properties and point defects was studied with NiO thin films grown by sputtering at various temperatures and in pure Ar and O2 atmospheres. The p-type characteristics of the NiO thin films were double-checked by Hall-effect and Seebeck coefficient measurements. Below 300 °C, the electrical resistivity of NiO films grown in an O2 atmosphere was lower than those grown in an Ar atmosphere due to the suppressed point defects. On the other hand, the electrical resistivity over 300 °C became semi-insulating due to the relatively stoichiometric NiO. The optical transmittance of the NiO film deposited in an O2 atmosphere and 600 °C was around 80% in the visible region. Finally, the p-NiO/n-ZnO heterojunction diodes showed a well-rectifying current-voltage curve.

Transparent and flexible oxide thin-film-transistors using an aluminum oxide gate insulator grown at low temperature by atomic layer deposition

Al2O3 dielectric layers with a dense and atomically flat surface were grown at relatively low temperatures of 150 °C by atomic layer deposition (ALD) for use as the gate oxide of transparent and flexible oxide thin-film-transistors (TFTs). The ALD growth of the high quality Al2O3 with a less rough surface at 120 °C allowed us to use the liftoff process without wet chemical etching and made the fabrication method for flexible electronics simple. This also improved the electrical performance of the oxide TFTs, such as high field effect mobility, low subthreshold gate swing, and low hysteresis behavior, due to the low charge trap sites at the gate oxide and channel interface. Finally, we fabricated InGaZnO TFTs with good device performance on a flexible substrate with poly-4-vinylphenol coating at a maximum processing temperature of 120 °C.

A combinatorial approach to solution-processed InGaO3(ZnO)m superlattice films: growth mechanisms and their thermoelectric properties

Multicomponent amorphous InGaZnO thin films with several metal cations have been synthesized with flexible chemical composition control based on a sol–gel process, and a combinatorial approach through a sol–gel process enables us to perform a systematic survey to fluently find the best film properties. Contrary to amorphous films, crystalline InGaO3(ZnO)m requires a refined chemical composition ratio among metal cations. These ratios are expected to affect the growth evolution and thermoelectric properties of two-dimensional InGaO3(ZnO)m superlattice structures with various compositional combinations. Here, we explore a combinatorial approach to the ratio of metal cations using various mole fractions of metal precursors in InGaZnO sol for amorphous InGaZnO films fabricated on an epitaxial ZnO buffer layer, and then, they were crystallized with various chemical compositions. The crystallized InGaO3(ZnO)m films can be classified as strong single-phase InGaO3(ZnO)m, double-phase InGaO3(ZnO)m/InGaO3(ZnO)m+1, and weak single-phase InGaO3(ZnO)m with excess metal ions. Among them, the strong single-phase InGaO3(ZnO)m films with superlattice structures showed superior thermoelectric power factors. The detailed microstructural growth evolution of single- and double-phase InGaO3(ZnO)m films was investigated using transmission electron microscopy.

Semiconducting p-type MgNiO:Li epitaxial films fabricated by cosputtering method

Li-doped ternary MgxNi1−xO thin films were deposited on (0001) Al2O3 substrates by a radio frequency (RF) magnetron cosputtering method with MgO and NiO:Li targets. The Mg mole fraction and Li content were relatively controlled by changing RF power for the MgO target over a range of 0–300 W, while the NiO:Li target was kept at 150 W. As a result, all films were epitaxially grown on (0001) Al2O3 substrates with the relationship of [1¯1¯0]NiO||[111¯0]Al2O3, [1¯12¯]NiO||[21¯1¯0]Al2O3 (in-plane), and [1¯11]NiO||[0001¯]Al2O3 (out-of-plane), and showed p-type semiconducting properties. Furthermore, from x-ray diffraction patterns, the authors found that MgO was effectively mixed with NiO:Li without structural deformation due to low lattice mismatch (0.8%) between NiO and MgO. However, the excess Li contents degraded the crystallinity of the MgNiO films. The band-gap of films was continuously shifted from 3.66 eV (339 nm) to 4.15 eV (299 nm) by the RF power of the MgO target. A visible transmittance of more than...