Masahiro Takahashi

Temperature Dependence of Transistor Characteristics and Electronic Structure for Amorphous In–Ga–Zn-Oxide Thin Film Transistor

We fabricated an inverted-staggered amorphous In–Ga–Zn-oxide (a-IGZO) thin film transistor (TFT) and measured the temperature dependence of its characteristics. A threshold voltage (Vth) shift between 120 and 180 °C was as large as 4 V. In an analysis with two-dimensional (2D) numerical simulation, we reproduced the measured result by assuming two types of donor-like states as carrier generation sources. Furthermore, by ab initio molecular dynamics (MD) simulation, we determined the electronic structures of three types of a-IGZO structures, namely, stoichiometric a-IGZO, oxygen deficiency, and hydrogen doping.

Origin of major donor states in In–Ga–Zn oxide

To clarify the origin of the major donor states in indium gallium zinc oxide (IGZO), we report measurement results and an analysis of several physical properties of IGZO thin films. Specifically, the concentration of H atoms and O vacancies (VO), carrier concentration, and conductivity are investigated by hard X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, thermal desorption spectroscopy, and Hall effect measurements. The results of these experiments suggest that the origin of major donor states is H occupancy of VO sites. Furthermore, we use first-principles calculations to investigate the influence of the coexistence of VO and H in crystalline InGaO3(ZnO)m (mu2009=u20091). The results indicate that when H is trapped in VO, a stable complex is created that serves as a shallow-level donor.

Single crystalline In?Ga?Zn oxide films grown from c-axis aligned crystalline materials and their transistor characteristics

In this study, we analyzed the crystallinity of c-axis aligned crystalline In?Ga?Zn oxide (CAAC-IGZO) and single crystalline (sc) IGZO films. CAAC-IGZO films were formed on (111)-oriented yttria-stabilized-zirconia substrates by magnetron sputtering using a target. Sc-IGZO films were obtained by annealing CAAC-IGZO films at 1200 ?C. The proportion of Zn in the composition changed during growth of the films, and as a result, sc-InGaO3(ZnO)3 films were obtained. By using CAAC-IGZO films as the starting material, sc-IGZO films were formed even without a ZnO layer. This is presumably because the CAAC-IGZO film originally exhibits c-axis orientation. In addition, the characteristics of transistors fabricated using sc-IGZO and CAAC-IGZO films were compared, and no significant difference in current drivability, i.e., field-effect mobility, was observed between the different transistors. In this sense, CAAC-IGZO films that require no high temperature annealing are favorable for industrialization.

Investigation of defects in In–Ga–Zn oxide thin film using electron spin resonance signals

In–Ga–Zn oxide (IGZO) is a next-generation semiconductor material seen as an alternative to silicon. Despite the importance of the controllability of characteristics and the reliability of devices, defects in IGZO have not been fully understood. We investigated defects in IGZO thin films using electron spin resonance (ESR) spectroscopy. In as-sputtered IGZO thin films, we observed an ESR signal which had a g-value of gu2009=u20092.010, and the signal was found to disappear under thermal treatment. Annealing in a reductive atmosphere, such as N2 atmosphere, generated an ESR signal with gu2009=u20091.932 in IGZO thin films. The temperature dependence of the latter signal suggests that the signal is induced by delocalized unpaired electrons (i.e., conduction electrons). In fact, a comparison between the conductivity and ESR signal intensity revealed that the signals intensity is related to the number of conduction electrons in the IGZO thin film. The signals intensity did not increase with oxygen vacancy alone but also wi...

Nanometer-scale crystallinity in In–Ga–Zn-oxide thin film deposited at room temperature observed by nanobeam electron diffraction

We report a detailed analysis of the nanoscopic structure of an In–Ga–Zn-oxide (IGZO) thin film deposited at room temperature. Using nanobeam electron diffraction (NBED) with a 1-nm-diameter probe, we analyzed a nanoscopic region of the IGZO thin film in which the diffraction spots of crystal were unobservable in a macroscopic region using selected-area electron diffraction (SAED) with about 300-nm-diameter probe. However, they were clearly recognized in a NBED pattern. They indicate a minute crystalline structure in the film. We further confirm the presence of crystals by observing a NBED pattern with spots distributed with six-fold symmetry in a specimen thickness thinned down to10 nm. To check the effect of the electron-beam irradiation during a transmission electron microscopy (TEM) and NBED measurements, we examine changes in crystal size as a function of the cumulative electron dose. No crystal growth during the electron beam irradiation was observed in the film. These results allow us to conclude that the IGZO thin film is not amorphous but a nanocrystalline (nc)-IGZO thin film, composed of nanometer-size crystals.

Influence of heat treatment on physical properties of In?Ga?Zn?O thin films

For unstable In?Ga?Zn?O (IGZO) thin films, electron-beam irradiation during transmission electron microscopy or heat treatment is reported to change the film structure and enhance its crystallization. Using IGZO films formed under two conditions, we study how the physical properties of IGZO films correlate with the electron-beam irradiation dose or heat-treatment temperature. IGZO films deposited under high deposition pressure contain many voids, have a relatively high impurity concentration, and are crystallized by electron-beam irradiation or heat treatment. In contrast, IGZO films formed under low deposition pressure are dense and the crystal size in the film does not change under electron-beam irradiation or heat treatment. In addition, heat treatment further increases the density of an originally dense film and reduces the defect density.