HRTEM and EELS investigations in superconductive N b N films modified under ion beam irradiation

Abstract

In this work we investigated superconductive NbN thin films. Interest in the latter is caused by demand for high quality new devices. Cryoelectronic devices is nanoscale functional elements, for instance SSPD, SQUID, THz HEB, based on ultra‐thin films with high limiting characteristics. One of the techniques of creating such devices was developed in Russia in the NRC “Kurchatov Institute”. The developed technique was named radiation‐induced method. This method allows to realize a control manner selectively changes in the atomic structure of thin‐film materials and modifications of the physical properties under irradiation with low‐energy beams with different composition [1, 2]. Samples containing niobium and nitrogen (NbN) were deposited on a single crystal silicon substrate coated with a 0,15 microns layer of amorphous oxide SiO 2 . The thickness of the initial film was 5 nm. The NbN ultrathin films were irradiated by ion beams extracted from a high‐frequency discharge plasma. Ion beams consist of protons and OH ions with energies (0,1‐1) keV in a dosage range (1,6‐4) d. p. a. for nitrogen. To study the chemical composition of the origin and irradiated samples were applied electron energy‐loss spectroscopy (EELS). Spectra were recorded with a Titan 80‐300 electron microscope equipped with a GIF‐2001 energy loss spectrometer. Data were transferred into TEM Imaging & Analysis Software (TIA). EELS spectra were collected in the STEM mode at 200 keV beam energy. The collection semi‐angle at the sample, as defined by the objective aperture and camera length, was 5,6 mrad. Typically acquisition times were ~200 s. Cross sections samples NbN/SiO 2 /Si were prepared by FIB Helios Nanolab 650 at 30 keV accelerating voltage of ion gun and 2,5 nA current and the final thinning was done at 2‐5 keV and 0,12 nA current. Quantitative analysis was carried out with equation (1): N A /N B =[I A (β,Δ)*σ B (β,Δ)]/[I B (β,Δ)*σ A (β,Δ)], where I A , I B ‐ integrated intensities of the peaks after background subtracting, and σ A and σ B ‐ ionization cross section [3]. Determination of the phase composition in the initial and irradiated samples was performed by the Fourier ‐ transform diffraction pattern obtained from the corresponding HRTEM image. Diffraction analysis showed that grains of non‐irradiated material correspond to NbN cubic crystal system (Fm‐3m) with lattice parameter a=0,4394 nm. Phase composition changes were observed when the irradiation dose was 2 d. p. a. for nitrogen. Figure 1 presents the diffraction pattern analysis from individual grains. The formation NbN 0,64 O 1,36 monoclinic system phase (P21/c(14)) with cell parameters a=0.49808 nm, b=0.50250 nm, c=0.52097 nm, α=γ=90°, β=100° was defined. These data were confirmed by the electron energy loss spectroscopy (figure 2). Quantitative analysis demonstrated that films irradiated at different doses consist of two regions: region with modified atomic composition and region of the initial NbN film. According to the results of the profile spectra, asymptotic functions were calculated for nitrogen, niobium and oxygen (fig.3, 4). Using asymptotic function, we concluded following. First, with increasing ion irradiated dose, nitrogen atomic concentration decreased to zero in the region close to the surface and the ratio of atomic concentrations for oxygen and niobium correspond to niobium oxide. Secondly, the thickness of the oxidized upper layer increased with increasing irradiation dose, due to the volume changes accompanying the niobium oxide formation from the niobium nitride phase. Thirdly, the nitrogen atomic concentration decreased in the region adjoined to the substrate, but wasn't decreased to zero, and oxygen atomic concentration was increased, i.e. there was a partial replacement of the nitrogen atoms to the oxygen atoms as a result of the selective displacement of nitrogen atoms by oxygen atoms.