D. Cáceres

Mechanical properties of sputtered silicon nitride thin films

Silicon nitride thin films were prepared by reactive sputtering from different sputtering targets and using a range of Ar/N2 sputtering gas mixtures. The hardness and the Young’s modulus of the samples were determined by nanoindentation measurements. Depending on the preparation parameters, the obtained values were in the ranges 8–23 and 100–210 GPa, respectively. Additionally, Fourier-transform infrared spectroscopy, Rutherford backscattering spectroscopy, and x-ray diffraction were used to characterize samples with respect to different types of bonding, atomic concentrations, and structure of the films to explain the variation of mechanical properties. The hardness and Young’s modulus were determined as a function of film composition and structure and conditions giving the hardest film were found. Additionally, a model that assumes a series coupling of the elastic components, corresponding to the Si–O and Si–N bonds present in the sample has been proposed to explain the observed variations of hardness a...

Nanoindentation on AlGaN thin films

Hardness and Young’s modulus were measured in AlGaN thin films with different Al content, using a nanoindentation technique. Hardness slightly decreases with increasing Al content, ranging from 20.2 to 19.5 GPa for Al content from 0.09 to 0.27, respectively. No significant variations of Young’s modulus were observed. The resulting value of Young’s modulus is 375 GPa. Discontinuities in load–displacement curves were found, which are associated with dislocation nucleation. The threshold load for this discontinuity depends on the conditions of the nanoindentation test. Below the threshold load, the sample surface flexes elastically in response to the indenter contact and the displacements recover completely when the sample is unloaded.

Bonding and hardness in nonhydrogenated carbon films with moderate sp3 content

Amorphous carbon films with an sp3 content up to 25% and a negligible amount of hydrogen have been grown by evaporation of graphite with concurrent Ar+ ion bombardment. The sp3 content is maximized for Ar+ energies between 200 and 300 eV following a subplantation mechanism. Higher ion energies deteriorate the film due to sputtering and heating processes. The hardness of the films increases in the optimal assisting range from 8 to 18 GPa, and is explained by crosslinking of graphitic planes through sp3 connecting sites.

Microstructural characterization of MgO thin films grown by radio-frequency sputtering. Target and substrate-temperature effect

The microstructure of thin films deposited by radio-frequency (rf)-sputtering on a silicon substrate at several temperatures and with two different targets was investigated by x-ray diffractometry (XRD) and scanning electron microscopy. XRD spectra reveal that films deposited at room temperature from either an MgO or an Mg target contain small (∼5 nm) periclase MgO crystallites. Thermal treatments in air followed by a fast cooling improved the degree of crystallinity and increased the grain size. The films grown from an Mg target at high temperatures are polycrystalline with a rock-salt structure. However, in thin films deposited from a sintered MgO target at T⩾873 K, the cubic spinel structure due to a mix of periclase (MgO) and brucite [Mg(OH)2] is observed; hydrogen comes from the target contamination. Thermal treatments in air at high temperatures improved the degree of crystallinity and texturing. The film structure depends on the cooling rate from elevated temperatures. Nanoindentation measurements ...

Hardness and elastic modulus from nanoindentations in nominally pure and doped MgO single crystals

Abstract Using a nanoindentation technique, the hardness and Young′s modulus were determined for nominally pure MgO single crystals and for MgO crystals doped with H, Li, Ni or Co impurities, subjected to different thermal treatments. The resulting defects were monitored by optical absorption spectroscopy. Undoped crystals have a hardness of 9.2 ± 0.2 GPa. After thermochemical reduction up to about 2300 K in Mg vapour at 7 atm, resulting in a deficiency of anions, a hardness value of 10.4±0.1 GPa was obtained. The enhancement is attributed to O vacancies. For doped crystals, hardening was observed and was attributed to impurities, point defects, cavities and metallic precipitates. A constant value of 300 GPa for Young′s modulus was obtained in all cases, indicating that the elastic properties are not influenced by either impurities or intrinsic defects.

Optical and mechanical properties of MgO crystals implanted with lithium ions

Defect profile induced by implantation of Li+ ions with an energy of 175 keV and a fluence of 1×1017 ions/cm2 in MgO single crystals was characterized by Rutherford backscattering and optical absorption measurements. Several absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, identical to those found in neutron irradiated crystals, were observed and have been previously associated with oxygen vacancies and higher-order point defects involving oxygen vacancies. Despite the high fluence of Li+ ions, no evidence was found for the formation of Li nanocolloids during implantation. Nanoindentation experiments demonstrated that both the hardness and Young’s modulus were higher in the implanted layer than in the sample before implantation. The maximum values were H=(17.4±0.4) and E=(358±9) GPa, respectively, at a contact depth of ≈165 nm. Thermal annealings in flowing argon at increasing temperatures improved the crystalline quality of the implanted layer. After annealing at 500 K, two extinction bands at ≈2.75 and...

Nanoindentation on neutron irradiated MgO crystals

Using a nanoindentation technique, hardness and Youngs modulus were determined in as-grown MgO single crystals and after neutron irradiation with different doses up to 6.9×1018 n/cm2. The resulting defects were monitored by optical absorption spectroscopy. As-grown crystals have a hardness of (9.1±0.2) GPa. After neutron irradiation, hardness increases with dose, whereas Youngs modulus remains practically constant, indicating that the elastic properties are not influenced by the irradiation. For the highest neutron dose of 6.9×1018 n/cm2, the resulting value for hardness was (12.3±0.2) GPa. The recovery of radiation damage following thermal treatments is also investigated. The hardening observed in n-irradiated crystals is attributed to interstitials.

Nanoindentation on MgO crystals implanted with lithium ions

Abstract As-grown MgO single crystals, both nominally pure and lithium-doped, were implanted with Li ions with an energy of 175 KeV and a dose of 10 17 ions/cm 2 . MgO:Li crystals were also implanted after oxidation at 1550 K for 30 min. TRIM calculations yield a range of 610 nm for the implanted ions. Hardness and Youngs modulus were measured in all the samples before and after implantation using a nanoindentation technique. As-grown MgO and MgO:Li crystals show the same hardness value of (9.1±0.2) GPa and Youngs modulus of (290±15) GPa. After oxidation of MgO:Li crystals the hardness is (10.1±0.2) GPa. Implantation of Li ions hardens the near-surface region of all three samples: MgO, MgO:Li and oxidized MgO:Li. Implantation in MgO and MgO:Li showed the same behavior: hardness reaches a maximum value of (16.3±0.2) GPa at a penetration depth of ≈175 nm, and slowly diminishes with depth. In oxidized MgO:Li crystals the maximum hardness is (17.7±0.2) GPa at a penetration depth of 175 nm. The considerable hardening observed in the implanted regions is attributed to the extraordinarily large concentration of interstitials in this region.

Nanoindentation on nominally pure and doped MgO crystals

Abstract Hardness and Youngs modulus were determined in MgO crystals using a nanoindentation technique. Both nominally pure and hydrogen-doped crystals have a hardness of 9.1 GPa. After thermochemical reduction at 2200 K a hardness value of 10.2 GPa was obtained. Hardness enhancement is associated with oxygen vacancies induced by the thermal treatment. In lithium-doped crystals, the increase to 10.1 GPa after oxidation, and 9.9 GPa after reduction at 1873 K, was attributed to optically active defects absorbing at 1.8 eV and 5.3 eV, respectively. Doping with nickel increases the hardness to 10.1 GPa. Oxidation or reduction of MgO: Ni crystals do not further harden the crystals. In all these crystals, the Youngs modulus was 290 GPa, indicating that the elastic properties are not influenced either by impurities or intrinsic defects.