Satoshi Yasuno

Transient photoconductivity responses in amorphous In-Ga-Zn-O films

We studied the photoconductivity responses in amorphous In-Ga-Zn-O (a-IGZO) films using a time-resolved microwave photoconductivity decay (μ-PCD) technique. The a-IGZO film characteristics are correlated with three components in the photoconductivity response: the peak value and two decay constants. The peak value originated from the density of the photo-generated free carriers through carrier generation and recombination processes during laser pulse irradiation. Power law characteristics indicated that the peak values are attributed to recombination process related to the exponential distribution of the conduction band tail states. After the laser pulse was turned off, the reflectivity signal decreased rapidly, indicating fast recombination of the photo-generated carriers. This fast decay component is suggested to be related to the recombination processes through the deep level states. Following the fast decay, a slow decay with a decay constant on the order of microseconds appeared. This slow decay was ...

Physical Properties of Amorphous In–Ga–Zn–O Films Deposited at Different Sputtering Pressures

The physical properties of amorphous In–Ga–Zn–O (a-IGZO) films deposited by DC sputtering under various sputtering pressures were investigated. The sputtering pressure was found to influence various physical properties. Lower sputtering pressures resulted in film densification and decreased both surface roughness and hydrogen concentration. In addition, transistor performance characteristics such as saturation mobility and sub-threshold swing improved as the sputtering pressure decreased. These results yield insight into the correlation between thin film transistor (TFT) performance and deposition conditions.

Effect of H and OH desorption and diffusion on electronic structure in amorphous In-Ga-Zn-O metal-oxide-semiconductor diodes with various gate insulators

Metal-oxide-semiconductor (MOS) diodes with various gate insulators (G/Is) were characterized by capacitance–voltage characteristics and isothermal capacitance transient spectroscopy (ICTS) to evaluate the effect of H and OH desorption and diffusion on the electronic structures in amorphous In–Ga–Zn–O (a-IGZO) thin films. The density and the distribution of the space charge were found to be varied depending on the nature of the G/I. In the case of thermally grown SiO2 (thermal SiO2) G/Is, a high space-charge region was observed near the a-IGZO and G/I interface. After thermal annealing, the space-charge density in the deeper region of the film decreased, whereas remained unchanged near the interface region. The ICTS spectra obtained from the MOS diodes with the thermal SiO2 G/Is consisted of two broad peaks at around 5 × 10−4 and 3 × 10−2 s before annealing, while one broad peak was observed at around 1 × 10−4 s at the interface and at around 1 × 10−3 s in the bulk after annealing. Further, the trap densi...

Electron traps in amorphous In–Ga–Zn–O thin films studied by isothermal capacitance transient spectroscopy

Electron traps in amorphous In–Ga–Zn–O (a-IGZO) thin films were studied using isothermal capacitance transient spectroscopy (ICTS). Broad peaks that shifted toward a longer elapsed time with an increase in the filling pulse width were detected from the ICTS spectra for metal-oxide-semiconductor diodes consisting of a Mo/SiO2/a-IGZO structure. The time constant of the peak position at 180 K was found to be from ∼1 m to ∼100 ms, corresponding to a range of energy level from ∼170 to ∼230 meV below the conduction band edge. The total trap density around the peak was estimated to be ∼1 × 1016 cm−3·eV−1. Further, according to the biasing voltage dependence of the ICTS signal, the density of the trap states increases by about three orders of magnitude near the interface between the a-IGZO and the gate dielectric layers. The electron transport in electronic devices using an a-IGZO could be influenced by the trap states detected in the present study.

Characterization of interface of Al-Ni/a-Si for thin film transistor using high-resolution Rutherford backscattering spectrometry.

High-resolution Rutherford backscattering spectrometry (HRBS) in combination with grazing angle argon sputtering was carried out to characterize the interface of aluminum-nickel (Al-Ni) alloy and amorphous-silicon films in a thin film transistor (TFT) for liquid crystal display (LCD). After thinning the top Al-Ni layer by a 1-keV Ar sputtering, the sensitivity of the interface oxygen was improved to be twice higher than that before sputtering. The results revealed that the oxygen at the interface relates to the contact characteristics.

Formation of epitaxial Ti-Si-C Ohmic contact on 4H-SiC C face using pulsed-laser annealing

Epitaxial Ti-Si-C Ohmic contacts on the 4H-SiC C-face for SiC power devices by pulsed-laser annealing were demonstrated. Titanium (Ti) is one of the carbon-interstitial type metals. In a conventional nickel silicide (NiSi) electrode on SiC, carbon agglomeration at the silicide/SiC interface occurs, and the contact resistance between NiSi and SiC substrates becomes larger. For the carbon agglomeration suppression, a nanosecond non-equilibrium laser annealing was performed, and also Ti was deposited for the formation of both silicide and carbide. As a result, the lowest specific contact resistance of 4.0 × 10−4 Ω cm2 was obtained for an Al/Ti-Si-C/SiC structure at a laser power of 2.5 J/cm2. The Ti-Si-C Ohmic electrode showed TiC and Ti5Si3 crystal phases, and these crystals were epitaxially grown on 4H-SiC at the interface.

Effects of applying bias voltage on metal-coated pentacene films on SiO2 studied by hard X-ray photoelectron spectroscopy

The effects of bias voltage application on C 1s photoelectron kinetic energies in Au- and Ag-coated pentacene films on SiO2 were studied by hard X-ray photoelectron spectroscopy. It was observed that the kinetic energies of C 1s were smaller in shallow regions in contact with metals than in mid regions of the pentacene films. The differences in C 1s kinetic energy between the shallow and mid regions of the Ag-coated pentacene films were slightly larger than those of the Au-coated films. The differences in the kinetic energies were decreased by applying negative voltages. The larger effect of voltage application was observed in the Ag-coated film than in the Au-coated film. In addition, partially reduced Si atoms in SiO2 were found at the interface to the pentacene film.

Observation of electric potential in organic thin-film transistor by bias-applied hard X-ray photoemission spectroscopy

The effect of gate voltage on electric potential in a pentacene (PEN) layer was studied by hard X-ray photoelectron spectroscopy under a bias voltage. It was observed that applying a negative gate voltage substantially increases the width of a C 1s peak. This suggested that injected and accumulated carriers in an organic thin film transistor channel modified the potential depth profile in PEN. It was also observed that the C 1s kinetic energy tends to increase monotonically with threshold voltage.