NbO2 is a promising candidate for resistive switching devices due to an insulator-metal transition above room temperature, which is related to a phase transition from a distorted rutile structure to undistorted one. However, the electrical resistivity of the NbO2 thin-films produced so far has been too low to achieve high on-off switching ratios. Here we report on the structural, electrical and optical characterization of single-crystalline NbO2 (001) thin-films grown by pulsed laser deposition on MgF2 (001) substrates. An annealing step reduced the full width at half maximum of the NbO2 (004) X-ray Bragg reflection by one order of magnitude, while the electrical resistivity of the films increased by two orders of magnitude to about 1 kΩcm at room-temperature. Temperature dependent resistivity measurements of an annealed sample revealed that below 650 K two deep-level defects with activation energies of 0.25 eV and 0.37 eV dominate the conduction, while above 650 K intrinsic conduction prevails. Optical characterization by spectroscopic ellipsometry and by absorption measurements with the electric field vector of the incident light perpendicular to the c-axis of the distorted rutile structure indicates the onset of fundamental absorption at about 0.76 eV at room temperature, while at 4 K the onset shifts to 0.85 eV. These optical transitions are interpreted to take place across the theoretically predicted indirect band gap of distorted rutile NbO2.

Homoepitaxial growth of SrTiO3 thin films on 0.5 wt% niobium doped SrTiO3 (100) substrates with high structural perfection was developed using liquid-delivery spin metal–organic vapor phase epitaxy (MOVPE). Exploiting the advantage of adjusting the partial pressures of the individual constituents independently, we tuned the Sr/Ti ratio of the gas phase for realizing, stoichiometric, as well as Sr deficient layers. Quantitative energy dispersive X-ray spectroscopy in a scanning transmission electron microscope confirm Sr deficiency of up to 20% in nominally off-stoichiometrically grown films. Our MOVPE process allows to grow such layers in phase pure state and without extended defect formation. Indications for oxygen deficiency could not be identified. Sr deficient layers exhibit an increased permittivity of ɛr = 202 and a larger vertical lattice parameter. Current–voltage characteristics (IVCs) of metal–oxide–semiconductor (Pt/SrTiO3/SrTiO3:Nb) structures reveal that Sr deficient SrTiO3 films show an intrinsic resistive switching with on–off ratios of three orders of magnitude at RT and seven orders of magnitude at 10 K. There is strong evidence that a large deviation from stoichiometry pronounces the resistive switching behavior. IVCs conducted at 10 K indicate a defect-based mechanism instead of mass transport by ion diffusion. This is supported by in-situ STEM investigations that show filaments to form at significant higher voltages than those were resistive switching is observed in our samples.

In this work, a vertical n-channel MISFET homoepitaxially grown on ammonothermal n-type GaN substrates by MOVPE is demonstrated. The MIS gate module consists of plasma enhanced atomic layer deposition of Al 2 O 3 combined with in-situ NH 3 plasma surface pre-treatment. An annealing step performed at 350 C in N 2 ambient drastically improves device performance. It reduces the ON-state resistance from %12.0 k Ω mm to %150 Ω mm, increases the ON-OFF ratio from 10 6 to 10 8 , and lifts up I ds_max from % 0.25 to % 40 mA mm À1 . The threshold voltage is above þ5 V and the median vertical off-state blocking voltage strength is %68 V μm À1 . Accumulated C-V characterization of planar MIS-capacitors on n-GaN gives insight to mechanisms boosting device performance after annealing.

No abstract

No abstract