Kenichi Haga

Low-Temperature Growth of ZnO Films by Spray Pyrolysis

Nondoped ZnO film on a polyethylene terephthalate film was successfully grown by conventional spray pyrolysis at 100 °C using a diethylzinc-based solution under air atmosphere. The samples had an average optical transmittance of more than 80%, a flat surface, and an a-axis orientation from the optical transmittance, scanning electron microscopy, and X-ray diffraction measurements, respectively. It was assumed that the growth at low temperature was attributed to atmospheric oxygen and/or steam.

Low Sheet Resistivity of Transparent Ga-Doped ZnO Film Grown by Atmospheric Spray Pyrolysis

Ga-doped ZnO film on polyethylene terephthalate film was successfully grown at 150 °C by conventional atmospheric spray pyrolysis using diethylzinc-based solution. The samples had an average optical transmittance of more than 80% and were strongly a-axis orientated according to the result of optical transmittance and X-ray diffraction analyses, respectively. The n-type Ga-doped ZnO films had a low sheet resistivity of 250 Ω/ at an optimal Ga content of 1 at. % upon UV irradiation.

Fabrication and characterization of ferroelectric-gate thin-film transistors with an amorphous oxide semiconductor, amorphous In–Ga–Zn–O

We fabricated and characterized ferroelectric-gate thin-film transistors (FeTFTs) with an amorphous In–Ga–Zn–O (a-IGZO) channel and an oxide ferroelectric (Bi,La)4Ti3O12 (BLT) gate insulator, and examined the fabrication conditions beforehand for realizing ferroelectric hysteresis in their drain current versus gate voltage (ID–VG) characteristics. First, the optimal annealing conditions for BLT-thin-film formation by the sol–gel method were considered in terms of chemical phenomena and crystallization mechanism. As a result, the leakage current density of the film was decreased by more than two orders of magnitude. Second, sputtering deposition conditions and postdeposition annealing temperatures of a-IGZO thin films were investigated to prepare the a-IGZO channel with good electrical properties. A small charge-injection-type hysteresis width of 0.2 V was obsereved in ID–VG characteristics of a-IGZO/SiO2 TFTs. Finally, a-IGZO/BLT FeTFTs were fabricated using the above results, and the ferroelectric hysteresis with a width of 0.4 V was observed.

Characterization of Spin Coated Nondoped and In-Doped ZnO Films Using Novel Precursor Solution

A novel precursor for ZnO film deposition with Zn-O structure was synthesized by the reaction of diethylzinc and water in some ether solvents. The novel precursor was characterized by 1H-NMR spectroscopy and differential scanning calorimetry (DSC). Nondoped and In-doped ZnO films on a glass substrate have been successfully grown by conventional spin coating using nondoped and In added novel precursor solution. The samples have an optical transmittance of more than 85%, and a smooth surface determined from optical transmittance and scanning electron microscopy, respectively. The sheet resistivity of In-doped ZnO films is lower than that of nondoped ZnO film.

Relationship between source/drain-contact structures and switching characteristics in oxide-channel ferroelectric-gate thin-film transistors

We report on oxide-channel ferroelectric-gate thin-film transistors (FeTFTs) with the bottom-contact structure for source and drain electrodes. FeTFTs with the bottom-contact structure have faster switching characteristics than those with the top-contact structure, especially for the switching from ON to OFF. First, we confirmed that FeTFTs with the top- or bottom-contact structure exhibit similar ID–VG characteristics and retention properties to each other. Next, we measured the C–V characteristics and confirmed that the oxide semiconductor channel under the source and drain electrodes is depleted in the top-contact structure. This depleted channel impedes the switching from ON to OFF. Finally, we demonstrated that FeTFTs with the bottom-contact structure possess fast switching characteristics for the switching from both OFF to ON and ON to OFF.

Growth of Spin-Coated ZnO Films Using Diethylzinc Solution

Nondoped ZnO films on a glass substrate have been successfully grown by conventional spin coating at room temperature using a diethylzinc-based solution. The samples have an optical transmittance of more than 80%, and a smooth surface determined from optical transmittance and scanning electron microscopy, respectively.