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Abstract In this work, TiO₂thin films deposited by the atomic layer deposition (ALD) method were treated with a special N₂O plasma surface treatment and used as the gate dielectric for AlGaN/GaN metal insulator semiconductor high electron mobility transistors (MISHEMTs). The N₂O plasma surface treatment effectively reduces defects in the oxide during low-temperature ALD growth. In addition, it allows oxygen atoms to diffuse into the device cap layer to increase the barrier height and thus reduce the gate leakage current. These TiO₂films exhibit a dielectric constant of 54.8 and a two-terminal current of 1.96 × 10⁻¹⁰A mm⁻¹in 2μm distance. When applied as the gate dielectric, the AlGaN/GaN MISHEMT with a 2μm-gate-length shows a high on/off ratio of 2.59 × 10⁸and a low subthreshold slope (SS) of 84 mV dec⁻¹among all GaN MISHEMTs using TiO₂as the gate dielectric. This work provides a feasible way to significantly improve the TiO₂film electrical property for gate dielectrics, and it suggests that the developed TiO₂dielectric is a promising high-κgate oxide and a potential passivation layer for GaN-based MISHEMTs, which can be further extended to other transistors. 

Two heteroleptic monocationic Ir( iii ) complexes bearing 6,6′-bis(7-benzothiazolylfluoren-2-yl)-2,2′-biquinoline as the diimine ligand with different degrees of π-conjugation were synthesized and their photophysics was investigated by spectroscopic techniques and first principles calculations. These complexes possessed two intense absorption bands at 300–380 nm and 380–520 nm in toluene that are predominantly ascribed to the diimine ligand-localized 1 π,π* transition and intraligand charge transfer ( 1 ILCT)/ 1 π,π* transitions, respectively, with the latter being mixed with minor 1 MLCT (metal-to-ligand charge transfer)/ 1 LLCT (ligand-to-ligand charge transfer) configurations. Both complexes also exhibited a spin-forbidden, very weak 3 MLCT/ 3 LLCT/ 3 π,π* absorption band at 520–650 nm. The emission of these complexes appeared in the red spectral region ( λ em : 640 nm for Ir-1 and 648 nm for Ir-2 in toluene) with a quantum yield of <10% and a lifetime of hundreds of ns, which emanated from the 3 ILCT/ 3 π,π* state. The 3 ILCT/ 3 π,π* state also gave rise to broad and moderately strong transient absorption (TA) at ca. 480–800 nm. Extending the π-conjugation of the diimine ligand via inserting CC triplet bonds between the 7-benzothiazolylfluoren-2-yl substituents and 2,2′-biquinoline slightly red-shifted the absorption bands, the emission bands, and the TA bands in Ir-2 compared to those in Ir-1 that lacks the connecting CC triplet bonds in the diimine ligand. The stronger excited-state absorption with respect to the ground-state absorption at 532 nm led to strong reverse saturable absorption (RSA) for ns laser pulses at this wavelength, with the RSA of Ir-2 being slightly stronger than that of Ir-1, which correlated well with their ratios of the excited-state to ground-state absorption cross sections ( σ ex / σ 0 ). These results suggest that extending the π-conjugation of the 2,2′-biquinoline ligand via incorporating the 7-benzothiazolylfluoren-2-yl substituents retained the broad but weak ground-state absorption at 500–650 nm, meanwhile increased the triplet excited-state lifetimes, which resulted in the much stronger triplet excited-state absorption in this spectral region and strong RSA at 532 nm. Thus, these complexes are promising candidates as broadband reverse saturable absorbers.