Sputtered Ni Films Research Articles

Magnetic and Electrical Properties of Sputtered Ni Films

Magnetic and Electrical Properties of Sputtered Ni Films

Effect of structural disorder on the physical properties and electronic structure of the DC magnetron sputtered Ni films

The effect of the structural disorder in Ni films on their magnetic, magneto-optical (MO), transport, optical properties, and the spin polarization degree of conduction electrons as well as the electronic structure has been investigated. Room temperature (RT) DC magnetron sputtered onto glass substrates Ni films have been found to be amorphous. It was experimentally shown that an amorphous Ni films (i) are not ferromagnetically ordered at least for T>78K temperature range, (ii) do not show any MO response at RT, (iii) have 10 times larger resistivity than crystalline phase which nearly does not depend on temperature, (iv) exhibit the optical conductivity (OC) spectrum without interband absorption peaks typical for crystalline Ni, (v) have spin polarization degree of conduction electrons close to zero. Annealing of amorphous Ni films at T = 600 K causes their crystallization with the formation of the face-centered-cubic phase and restores the physical properties typical for crystalline Ni. The experimental OC spectrum of crystalline Ni film exhibits an intense interband absorption peaks located at E = 1.4 and 4.55 eV; it is in reasonable correspondence with the calculated interband OC spectrum of Ni. Main absorption peaks in the experimental OC spectrum were put in correspondence with the electron excitations between definite electronic states of the E(k) structure of Ni. The experimentally determined spin polarization degree of conduction electrons of crystalline Ni films was found to be close to that for bulk Ni and consists of P≈40%.

Temperature dependent current transport behavior of improved low noise NiGe schottky diodes for low leakage Ge-MOSFET

In this letter, we report on improved low noise NiGe/n-Ge (100) Schottky barrier diodes (SBDs) fabricated by post annealing of sputtered Ni films on germanium substrates. The results of x-ray diffraction and Raman spectroscopy confirm the formation of NiGe phase at elevated temperatures. Electrical behaviour of the SBDs at different temperatures have been investigated and compared. Experimental results indicate the presence of barrier inhomogeneity and low temperature I–V measurements were performed and the effect of operating temperature on barrier height was studied. The Schottky barrier heights were calculated and compared using current-voltage and capacitance-voltage methods. A low frequency noise measurement on SBDs showed typical 1/f noise behaviour. Study shows that NiGe can be used as a contact material for low leakage Ge-MOSFET applications.

Encapsulation of La1.5Mg0.5Ni7 nanocrystalline hydrogen storage alloy with Ni coatings and its electrochemical characterization

Nanocrystalline La1.5Mg0.5Ni7 alloy has been synthesized by mechanical alloying and heat treatment (850 °C) and a 20–50 μm fraction of its particles has been encapsulated with three kinds of Ni-based coatings: (i) electroless NiP, 1 μm thick, (ii) magnetron sputtered Ni, 0.087 μm thick and (iii) magnetron sputtered Ni, 0.29 μm thick. The electrochemical charge/discharge multicycling of powder composite electrodes has been carried out in view of material potential usefulness for Ni/MHx batteries. The nanocrystalline La1.5Mg0.5Ni7 alloy exhibits four times lower capacity fade as compared to a microcrystalline LaNi5 reference compound. Modification of La1.5Mg0.5Ni7 particle surface with comparatively thick layer of electroless NiP coating deteriorates electrode corrosion behavior and worsens kinetics of hydrogen electrosorption due to unsatisfactory adhesion of the coating to the Mg-containing substrate. Corrosion protection of the La1.5Mg0.5Ni7 nanomaterial by magnetron sputtered Ni films depends on average film thickness. Relatively thick (0.29 μm) sputtered Ni film limits corrosion degradation and stabilizes exchange current density of H2O/H2 system. Particle modification by Ni encapsulation does not affect the hydrogen diffusivity. Effective diffusion coefficient of hydrogen for La1.5Mg0.5Ni7 nanocrystalline material, irrespectively of surface modification, is close to 2·10−10 cm2s−1.

Controlling defects in fine-grained sputtered nickel catalyst for graphene growth

Sputter-prepared nickel (Ni) films can lose more than half their starting thickness due to evaporation in hydrogen (H2) annealing environments. The loss rate of the sputtered Ni films during the chemical vapor deposition growth of graphene has not been reported earlier. The evaporation rate of sputtered Ni film with the amorphous, mixed, preferred ⟨111⟩ texture was experimentally determined to be 20, 11, and 6 nm/m, respectively. An increase of argon mixture in H2 was found to reduce pitting defects in the films during annealing. The quality of grown graphene on top of the Ni improved when the growth temperature was raised from 900 to 1000 °C, as monitored by Raman spectroscopy. More importantly, loss in the starting Ni film thickness can inhibit the growth of graphene layers. By maintaining the growth of the graphene to two layers or less, a high optical transparency of 95% or better can be achieved.

Tailoring the magnetic anisotropy, magnetization reversal, and anisotropic magnetoresistance of Ni films by ion sputtering

We studied surface morphology induced changes of magnetic anisotropy, magnetization reversal, and symmetry of the anisotropic magnetoresistance (AMR) in ion sputtered Ni films grown on MgO (001). Grazing-incidence ion sputtering generally develops anisotropic surface roughness of the Ni films, i.e., nanometer wide ripples parallel to the ion beam direction, giving rise to uniaxial magnetic anisotropy with the easy axis along the ion beam direction. The formed ripples act as domain wall nucleation and pinning sites during magnetization reversal, while two-jump domain wall motion dominates in the as-grown Ni films. More importantly, the azimuthal angular dependence of the AMR indicates a superposition of twofold symmetry and fourfold symmetry. By relying on grazing-incidence ion sputtering along specific crystallographic directions, we are able to tailor the relative weight of twofold and fourfold symmetry of AMR. We demonstrate that in contrast to the bulk case, the symmetry of the AMR becomes also sensitive to the surface morphology in thin films, which is in particular relevant for the design of magnetotransport based sensors.

Nano-Morphological, Magnetic and Structural Properties of Ni Films Prepared by RF-Sputtering

Sputtered Ni Films with various deposited times (30, 60, 90,120 and 150 min.) were prepared on glass substrate by RF-sputtering in Argon gas to study effects of sputtering time on their morphological, magnetic and thermal properties. Surface morphological of Ni films were investigated by AFM. The AFM images show a small variation of surface roughness with sputtering time. Average surface roughness of Ni films over scan area of 1μm x 1μm, 5μm x 5μm and 10μm x 10μm are about 0.57, 1.64 and 2.49 nm, respectively. The AFM result infers that Ni films prepared by RF-sputtering exhibit surface roughness in order at nano-scale and have smoother surface than that prepared by DC-sputtering [1]. Structure of Ni films was characterized by XRD. The results display that Ni films exhibit a broad peak of Ni (FCC) phase in (111) plane with a hump at 2θ = 23 o [1, 2]. Intensity of Ni (111) peak is increased with increasing sputtering time. Magnetic property of Ni films was study by VSM. The VSM results confirm that Ni films deposited for 90-150 min have a ferromagnetic phase and saturation magnetization is increased whereas coercive field is practically kept constant with increasing deposited time [3, 4]. The DTA result of Ni films show an exothermic peak at 850 o C corresponding to decomposition of Ni atoms from the glass substrate. The results confirm that surface roughness, magnetic and structural properties Ni films prepared by RF sputtering can be improved by an appropriate deposited time.

Thickness-dependent orientation evolution in nickel thin films grown on yttria-stabilized zirconia single crystals

Microstructure evolution along with crystallographic orientation change as a function of film thickness was investigated in Ni thin films grown on (100) yttria-stabilized zirconia (YSZ) single crystal substrates. Texture development with two different orientation relationships, OR1: Ni {111}//YSZ {100} and OR2: Ni {100}//YSZ {100}, cube on cube orientation were identified by X-ray diffraction and transmission electron microscopy depending on the film thickness. The observed orientation transition reveals the existence of a critical thickness (∼320 nm) favoring two different orientations in sputtered Ni films on YSZ (100) substrate.

Flexible top emission organic light-emitting devices using sputter-deposited Ni films on copy paper substrates

We have developed flexible top emission organic light-emitting devices by using sputtered Ni films on the parylene coated copy paper substrates. The Ni film grew through the coalescence of the Ni islands nucleated on the cellulose fiber patterns. The parylene, acting as a buffer layer for preventing water absorption, affected the morphological and electrical characteristics of the deposited Ni films. The flexible top emission diode device composed of Ni anode and Ca/Ag semitransparent cathode exhibited a luminance of ∼1 cd/m 2 at a driving voltage of 14.5 V.

Measurement of spin diffusion length in sputtered Ni films using a special exchange-biased spin valve geometry

The authors report the spin diffusion length at 4.2K in sputtered Ni of lsfNi=21±2nm, and spin-dependent scattering parameters in Ni and at Ni∕Cu interfaces. They have employed current perpendicular to plane giant magnetoresistance in both a traditional and an alternative exchange-biased spin valve geometry that inserts a Ni “spoiler” layer into a Py/Cu/Py spin valve. Fits to data of AΔR vs Ni thickness using Valet-Fert theory [Phys. Rev. B 48, 7099 (1993)] show good agreement between fit parameters for both sample geometries.

Correlated structural and magnetization reversal studies on epitaxial Ni films grown with molecular beam epitaxy and with sputtering

We have studied the correlation between film structure and the azimuthal dependence of the magnetization reversal in (001) and (111) epitaxial Ni films grown on MgO substrates using two different deposition techniques: molecular beam epitaxy (MBE) and dc magnetron sputtering. The films were grown and in situ annealed under identical conditions. The magnetization reversal was investigated using MOKE. The coercive field in the sputtered (001) Ni films exhibits fourfold azimuthal symmetry as expected for crystalline films of good epitaxial quality, while MBE (001) Ni grown films exhibit an additional uniaxial symmetry superimposed to the fourfold symmetry. We performed high-resolution XRD studies as well as cross sectional TEM studies in order to establish similarities and differences in the structure of the films. Both types of films exhibit epitaxial growth and very good crystalline quality with no indication of strain. The main difference between the films is the different magnetic anisotropy. We postulate that this difference may be due to different interfacial structure and/or morphology due to the possible formation of a NiO interfacial layer only present or highly ordered in the MBE grown films. Polarized neutron reflectivity measurements performed on some of the films are correlated with the interfacial structure and magnetic anisotropy.

Effect of sputtering pressure on residual stress in Ni films using energy-dispersive x-ray diffraction

A procedure for determining the residual stress in thin films using energy dispersive x-ray diffraction was investigated. The effect of the sputtering pressure on the residual stress in dc magnetron sputtered Ni films was studied in greater detail using this approach. The behavior reported suggested the possibility of controlling or influencing the sign and/or magnitude of the residual stress. In addition, the stress variation with increasing negative bias voltage is also presented. In the range studied, between −15 and −150 V, residual stress is always tensile.

A new method for tensile testing of thin films

A new method for tensile testing of thin films is presented. The strain is measured directly on the unsupported thin film from the displacement of laser spots diffracted from a thin grating applied to its surface by photolithography. The diffraction grating is two-dimensional, allowing strain measurement both along and transverse to the tensile direction. In principle, Young’s modulus, Poisson’s ratio, and the yield stress of a thin film can be determined. Cu, Ag, and Ni thin films with strong ⟨111⟩ texture were tested. The measured Young moduli agreed with those measured on bulk crystals, but the measured Poisson ratios were consistently low, most likely due to slight transverse folding of the film that developed during the test. The yield stresses of the evaporated Cu and Ag thin films agreed well with an extrapolation of the Hall-Petch relation found for bulk materials. Ni thin films are known to deviate from a bulk Ni Hall–Petch relation for submicron grain sizes, and sputtered Ni films show much higher yield stresses than electrodeposited or vapor-deposited films of similar grain size. Our sputtered Ni films had higher yield stresses than other sputtered films from the literature.

Bias Effect on the Structure of Sputtered Ni Films

Bias Effect on the Structure of Sputtered Ni Films

Gas content of sputtered Ni films using laser induced flash evaporation and mass spectrometry

Gas content of sputtered Ni films using laser induced flash evaporation and mass spectrometry