This work demonstrates the low temperature thin-film deposition of silicon nitride (SiNx) for III-nitride-based high electron mobility transistors using inductively coupled plasma chemical vapor deposition. It is observed that the nonlinear dependency of the deposition temperature and gas flow rates have a profound impact on the film quality. The process parameter space is scanned and the optimum film quality is achieved, which is verified with physical and electrical characterizations. The best quality film is achieved at a deposition temperature of 380 °C demonstrating near ideal stoichiometry with negligible hydrogen (<5%) and oxygen (<3%) concentrations. In addition, the optimized film is found to have zero pinholes even at a thickness of 10 nm and is uniform over a large area with an rms roughness of 0.58 nm. The deposited films are characterized by atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The dielectric strength and dielectric constant of these films are determined from current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the metal-insulator-metal structure, respectively. For the best quality film, the values of dielectric strength and dielectric constant are measured to be ∼8 MV/cm and ∼7.5, respectively. A metal-insulator-semiconductor-heterostructure (metal/SiNx/AlGaN/GaN) capacitor is fabricated with the optimized recipe for interface characterization. The density of slow traps is determined from the hysteresis in the C-V curve and found to be 7.38×1010 cm−2. The frequency dependent conductance method is also used to investigate the trap density. The trap state density is found to be 1.67×1012 cm−2 eV−1 at 0.29 eV below conduction band.
Water dispersible citrate-capped LaF3:Eu(5%) nanocrystals (NCs) have been partially surface-functionalized by 1,10-phenanthroline (phen) via a ligand exchange method to produce novel water dispersed citrate/phen-capped LaF3:Eu(5%) NCs in which citrate ligands preserve the water dispersibility of the NCs and phen ligands act as sensitizers of surface Eu(3+)-dopant sites. The partial ligand exchange and the formation of water dispersed NCs have been monitored by (1)H NMR spectroscopy, as well as luminescence measurements at different time intervals during the reaction. These NCs display a distinct phen-sensitized Eu(3+)-emission profile with enhanced intensity in water as compared to the emission profile and intensity obtained upon direct excitation. Time-resolved (or time-gated) emission spectroscopy (TRES) has been used to probe PL dynamics of Eu(3+)-sites of LaF3:Eu(5%) NCs by taking advantage of selectively sensitizing surface Eu(3+)-dopant sites by phen ligands as well as by exciting all the Eu(3+)-sites in the NCs upon direct excitation. TRES upon direct excitation of the citrate-capped LaF3:Eu(5%) NCs reveals that Eu(3+)-dopants occupy at least three different sites, each with a different emission profile and lifetime, and emission from purely interior Eu(3+)-sites has been resolved due to their long lifetime as compared to the lifetime of purely surface and near surface Eu(3+)-sites. In contrast, the phen-sensitized emission from citrate/phen-capped LaF3:Eu(5%) NCs displays similar emission profiles and lifetimes in TRES measurements, which reveal that phen truly sensitizes purely surface dopant sites of the NCs in water, all of which have nearly the same local environment. The phen-sensitized Eu(3+)-emission of the NCs in water remains stable even upon addition of various buffer solutions at physiological pH, as well as upon addition of water-miscible organic solvents. Furthermore, the two-photon excitation (λex. = 720 nm) of these water-soluble phen-capped NCs produces bright red Eu(3+) emission, which reveals that these NCs are promising for potential applications in biological imaging.
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