This work discusses sensing properties of a long-period grating (LPG) and microcavity in-line Mach–Zehnder interferometer (µIMZI) when both are induced in the same single-mode optical fiber. LPGs were either etched or nanocoated with aluminum oxide (Al2O3) to increase its refractive index (RI) sensitivity up to ≈2000 and 9000 nm/RIU, respectively. The µIMZI was machined using a femtosecond laser as a cylindrical cavity (d = 60 μm) in the center of the LPG. In transmission measurements for various RI in the cavity and around the LPG we observed two effects coming from the two independently working sensors. This dual operation had no significant impact on either of the devices in terms of their functional properties, especially in a lower RI range. Moreover, due to the properties of combined sensors two major effects can be distinguished—sensitivity to the RI of the volume and sensitivity to the RI at the surface. Considering also the negligible temperature sensitivity of the µIMZI, it makes the combination of LPG and µIMZI sensors a promising approach to limit cross-sensitivity or tackle simultaneous measurements of multiple effects with high efficiency and reliability.
Abstract. The paper presents the results of characterization of MOS structures with aluminum oxide layer deposited by ALD method on silicon carbide substrates. The effect of the application of thin SiO 2 buffer layer on the electrical properties of the MOS structures with Al 2 O 3 layer has been examined. Critical electric field values at the level of 7.5-8 MV/cm were obtained. The use of 5 nm thick SiO 2 buffer layer caused a decrease in the leakage current of the gate by more than two decade of magnitude. Evaluated density of trap states near the conduction band of silicon carbide in Al 2 O 3 /4H-SiC MOS is about of 1x10 13 eV -1 cm -2 . In contrast, the density of the trap states in the Al 2 O 3 /SiO 2 /4H-SiC structure is lower about of one decade of magnitude i.e. 1x1012 eV -1 cm -2 . A remarkable change in the MOS structure is also a decrease of density of electron traps located deeply in the 4H-SiC conduction band below detection limit due to using of the SiO 2 buffer layer.Key words: aluminum oxide, MOS, silicon carbide, 4H-SiC, high-κ dielectrics.
Characterization of Al
Experimental detailsMOS structures used in this study were fabricated on low-resistivity 4H-SiC substrates (n ~ 5×10 18 cm -3 ) with n-type (n ~ 5×10 ), 10 μm thick epitaxial layer. Prior to the MOS fabrication the substrates were cleaned in organic solvents and dipped in 10% HF solution to remove the native oxide. The nickel layer was sputtered on the backside and annealed (Ar, 1050°C) to form an ohmic contact. Two kinds of samples were fabricated. On one part of samples, 5 nm SiO 2 buffer layer was fabricated by PECVD using SiH 4 and N 2 O plasma. Afterwards, a 50 nm layer of Al 2 O 3 was deposited on all samples by ALD. Al 2 O 3 layers were deposited at 200°C using trimethylaluminium (TMA) and water vapor (H 2 O) as Al and O precursors, respectively. Finally, a top layer of Ti/Al gate electrode (φ = 200 µm) was deposited on top of Al 2 O 3 by sputtering and patterned by lift-off technique. A schematic sketch of fabricated Al 2 O 3 /4H-SiC and Al 2 O 3 /SiO 2 /4H-SiC structures is presented in Fig. 1.
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