石井 久夫

Illumination stress (IS) and negative bias under illumination stress (NBIS) cause considerable device instability in thin-film transistors based on amorphous In–Ga–Zn–O (a-IGZO). Models using in-gap states are suggested to explain device instability. Therefore, to provide reliably their density of states (DOS), this study investigated the valence band, conduction band, and in-gap states of an a-IGZO thin film. The DOS of in-gap states was directly determined in a dynamic range of six orders of magnitude through constant final state yield spectroscopy (CFS-YS) using low-energy and low-flux photons. Furthermore, light irradiation irreversibly induced extra in-gap states near the Fermi level and shifted the Fermi level to the vacuum level side, which should be related to the device instability due to IS and NBIS. Hard x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy using synchrotron radiation observed the large DOS of in-gap states near the Fermi level as in previous works. Here, we reveal that they are not intrinsic electronic states of undamaged a-IGZO, but induced by the intense measurement light of synchrotron radiation. This study demonstrates that CFS-YS is useful for determining the reliable DOS of the in-gap states for samples that are sensitive to light irradiation. The absorption spectrum measured through photothermal deflection spectroscopy is interpreted based on DOS directly determined via photoemission spectroscopies. This indicates that the line shape in the energy region below the region assigned to the Urbach tail in previous works actually roughly reflects the DOS of occupied in-gap states.

Abstract Photodiode‐type solar‐blind photodetectors (SBPDs) with the self‐powered feature hold great promise for applications in unattended secure communication, flame detection, and missile warning. However, the responsivity of SBPDs is usually limited due to the severe solar‐blind (SB) light extinction in substrates and charge transport layers. Herein, a spectrally selective hole extraction structure (SHE) is proposed for high‐efficiency perovskite SBPDs. The SHE consisting of a tandem Fabry–Perot cavity and energy‐level‐matched hole transport layer endows the device with narrowband absorption in the SB region and optimized charge extraction capability from the CsPbI₂Br perovskite. The optimized SHE exhibits a peak transmittance of 27% at 255 nm and a half maximum at full width of 28 nm. Under SB light illumination, the champion device achieves a responsivity of 56.20 mA W⁻¹ and a detectivity (D^*) of 2.86 × 10¹³ Jones, which are the record values among the reported results. The approach demonstrated here paves the way for the optical and electrical design of perovskite photodetectors with spectrally selective detection.

In this paper, we present the observation results of the surface potential of micropatterned thick (>1 & mu;m) self-assembled electrets (SAEs) for MEMS vibrational energy harvesters (VEHs). To evaluate the surface potential of micropatterned SAEs, we propose and develop test devices with removable through-hole substrates corresponding to the moving electrodes of SAE-MEMS VEHs. In this study, SAEs are deposited simultaneously on two test devices with different through-hole spacings and on a reference flat substrate using the same vacuum evaporation process. The surface potential of SAEs is proportional to the film thickness, and when the film thickness of the SAE deposited on the flat substrate is 4.48 mu m, the surface potential exceeds 200 V. At this time, in a test device where the average thickness of micropatterned SAEs is 3.08 mu m, the measured surface potential is 68 V. In addition, it is experimentally observed that when micropatterned SAEs are formed using the through-hole structures, the microfabrication process causes the SAE pattern to spread wider than the through-hole dimensions, and the surface profiles are not flat. These findings provide useful insights for the design of SAE-MEMS VEHs using micropatterned SAEs with through-hole structures.