Polysilsesquioxane (PSQ) is used to make gate dielectric layers with pentacene to create thin‐film transistors (TFTs) by incorporating 3‐mercaptopropyl functional groups for UV light‐initiated thiol–ene polymerization with vinyl‐containing cross‐linkers. The dielectric constant (ε) of the polymerized PSQ films is controlled by the ratio of 3‐mercaptopropyl groups. When the 3‐mercaptopropyl group comprises 20%, it yields a polymerized film with a ε of 4.2 ± 0.2; upon decreasing this proportion to 10%, the ε for the subsequent film became 3.5 ± 0.2. The hole mobility of the fabricated pentacene TFTs is observed to increase as ε decreases, reaching a maximum of 0.96 cm2 V−1 s−1.

We have investigated current-voltage characteristics of individual CdSe colloidal nanodots by conductive-tip atomic force microscopy (AFM). The colloidal nanodots were spun-coat and scattered on a self-assembled monolayer of thiophene molecules formed on Au (111) surfaces for single dot measurements. A thin SiO2 layer was deposited on the sample surface in order to prevent the dots being moved by the tip during measurement. We imaged the topography of isolated single dots by AFM operated in contact mode, and measured current-voltage characteristics with the conductive tip positioned on single dots; large conductivity changes which suggest resonant tunneling through,a quantized energy level in the dot was observed even at room temperature.

We fabricated pentacene memory transistors using a monolayer of ligand-exchanged and energy-level-controlled PbS colloidal nanodots (NDs) as charge-trapping layers, and it was demonstrated that the writing and retention times were reduced and increased, respectively. This result is explained with a model that the memory effect is due to the electrons tunneled from the pentacene layer into the PbS NDs when a writing voltage was applied. According to this model, the writing and retention characteristics were improved because of the lowered electron energy levels in the NDs.

α-Ga2O3 is a metastable phase of gallium oxide (Ga2O3) and is important for application in solar-blind region optoelectronic devices. High-quality α-Ga2O3 thin films can be grown by mist chemical vapor deposition (mist-CVD). We systematically investigate the growth mechanism of α-Ga2O3 by mist-CVD using acetylacetonated Ga source solutions. We propose a growth mechanism of α-Ga2O3 in mist-CVD in which acetylacetonate ligands anchor to surface hydroxyls and Ga–O bonds are formed by a ligand exchange mechanism. The origin of oxygen atoms and impurity concentration profiles in grown α-Ga2O3 thin films are examined by secondary ion mass spectroscopy.

We fabricated pentacene thin-film transistors (TFTs) with ultra-violet (UV) light cured polysilsesquioxane (PSQ) gate dielectric layers using different photo-initiators to reduce UV-curing time. When PSQ layers were cured using IrgacureTM 184 as a photo-initiator, it took 60 minutes to cure them completely. However, the curing time was reduced to be 10 minutes when IrgacureTM 907 was used with a sensitizer because the UV light was absorbed more efficiently. It was also demonstrated that the hole mobility of the pentacene TFTs was not affected by changing the photo-initiator from IrgacureTM 184 to IrgacureTM 907 with a sensitizer.

In GaAs and AlAs atomic-layer epitaxy (ALE), it was experimentally found that the AlAs layer is deposited in two monolayers (2 ML) per ALE cycle, while the GaAs layer is deposited in 1 ML. In order to elucidate this growth mechanism, we compared the stability of Ga and Al atoms on the GaAs(100) surface by the first-principles total energy calculation based on density functional theory. Comparing adsorption energies, we found that Ga and Al stably adsorbed on the As-terminated GaAs surface in 1 ML. It was also revealed that Al can adsorb on an As-terminated GaAs(100) surface in 2 ML because excess Al atoms can be adsorbed by forming a metallic Al(110) plane-like structure. These results well explain the experimental results for ALE-GaAs and ALE-AlAs.

表 1: 格子定数の計算値 a c Al2O3 4.758Å 12.96Å Ga2O3 5.015Å 13.45Å Ir2O3 5.235Å 13.88Å 【格子定数の評価】計算は CPUに Core i7を搭載した一 般的な PCを用い、第一原理計算パッケージにはVASP[5] を用いた。単位格子はコランダム構造の最小単位格子で ある菱面体晶を用い、構造評価には PBE、逆格子空間は 9×9×9のMonkhorst Packで分割して計算した。Alにつ いては 11個、Gaは 13個、Irについては 17個の電子を 価電子としたポテンシャルを用いた。計算結果を六方晶に変換したときの格子定数を表 1に示し た。α-Al2O3, α-Ga2O3については実験値として報告されている値と比較すると大きな値となってい るが、差異は 0.7%以下である。α-Ir2O3については単結晶の報告がなかったので、計算結果の妥当 性を検証するためにルチル構造の IrO2の計算を同じ条件で行ったところ、格子定数は a=4.474Å、 c=3.166Åと求まり、実験値として報告されている値に対して 0.8%以下の差異に収まっていた。

First principles studies were carried out for α-Ga 2 O 3 and α-Ir 2 O 3 to investigate their electronic band structures. The lattice parameters of α-Ga 2 O 3 and rutile-structured IrO 2 were in good agreement with experimental results. We presented calculated lattice parameters of α-Ir 2 O 3 . HSE06 hybrid functional was used for calculations of their electronic band structures. α-Ir 2 O 3 was found to have an indirect bandgap of 2.62 eV and the valence and conduction bands were formed by d-electrons. By leveling Γ point of oxygen 2p electron energy, the band alignment of α-Ga 2 O 3 / α-Ir 2 O 3 heterostructures is expected to be a type-II staggered structure.

Pentacene thin-film transistors (TFTs) were fabricated with ultraviolet-light (UV)-cured polysilsesquioxane (PSQ) gate dielectric layers using crosslinker molecules with or without ester groups. To polymerize PSQ without ester groups, thiol-ene reaction was adopted. The TFTs fabricated with PSQ layers comprising ester-free cross-linkers showed a higher carrier mobility than the TFTs with PSQ layers cross-linked with ester groups, which had large electric dipole moments that limited the carrier mobility. It was demonstrated that the thiol-ene reaction is more suitable than the conventional radical reaction for UV-cured PSQ with small dielectric constant. (C) 2018 The Japan Society of Applied Physics

We fabricated field-effect transistors (FETs) using ammonium sulfide-treated PbS colloidal nano-dot (CND) films. After the ligand molecules were removed from the CNDs by the ammonium sulfide treatment, many cracks were observed in the atomic force microscope image of the PbS ND monolayer that was formed by horizontal lifting method. Those cracks were filled and almost disappeared after we repeated the formation of the PbS CND monolayer and removal of the ligand molecules for three times. The FETs fabricated with the cracked PbS ND films exhibited serious bias stress effect and very low hole mobility of 5.6×10-5 cm2V-1s-1, which were attributed to the carrier traps existing on the NDs surface. On the other hand, the carrier mobility of the FETs with the crack-free PbS ND films was improved to be 1.1×10-3 cm2V-1s-1 because the cracks were filled and the trap density was significantly reduced.

We fabricated pentacene memory transistors of which floating gate or charge‐trapping layer consisted of monolayer of PbS colloidal nanodots (NDs), and investigated their writing and retention characteristics. Both of the writing and retention times of the pentacene memory transistors were significantly reduced when the ligand molecules (oleic acid) were removed. We also found that the writing time was further reduced and the retention time was extended by using larger NDs, in which the quantized energy level in the conduction band was lower, that is, closer to the highest occupied molecular orbital (HOMO) and farther from the lowest unoccupied molecular orbital (LUMO) of the pentacene. These results are explained with a model that the memory effect is due to the trapped electrons in the NDs after a positive writing voltage is applied on the control gate, and suggest that more suitable arrangement of the quantized energy level in the NDs and HOMO/LUMO levels in organic molecules would improve the characteristics of the memory transistors.