Paulius Pobedinskas

Separation of intra- and intergranular magnetotransport properties in nanocrystalline diamond films on the metallic side of the metal-insulator transition

A systematic study on the morphology and electronic properties of thin heavily boron-doped nanocrystalline diamond (NCD)films is presented. The films have nominally the same thickness ( 150nm) and are grown with a fixed B/C ratio (5000ppm) but with different C/H ratios (0.5-5%) in the gas phase. The morphology of the films is investigated by x-ray diffraction and atomic force microscopy measurements, which confirm that lower C/H ratios lead to a larger average grain size. Magnetotransport measurements reveal a decrease in resistivity and a large increase in mobility, approaching the values obtained for single-crystal diamond as the average grain size of the films increases. In all films, the temperature dependence of resistivity decreases with larger grains and the charge carrier density and mobility are thermally activated. It is possible

Surface plasma pretreatment for enhanced diamond nucleation on AlN

The surface of polycrystalline aluminum nitride (AlN) thin films is exposed to different gas discharge plasmas (N2, O2, and CF4) followed by a water-based colloidal seeding of ultra-dispersed nanodiamond particles. Fluorination of the AlN surface enhances the seeding density, whereas the oxidized surface does not yield any nucleation sites. In the former case, the seeding density improves by almost three orders of magnitude as compared to the untreated and N2 pretreated samples, and allows to grow pinhole-free nanocrystalline diamond film on AlN. Finally, we demonstrate a route towards selective diamond growth on AlN thin films by employing CF4 plasma pretreatment together with photolithography.

Thin conductive diamond films as beam intensity monitors for soft x-ray beamlines

Quantitative analysis of X-ray absorption and dichroism data requires knowledge of the beamline photon flux during the measurements. We show that thin conductive (B-doped) diamond thin films can be an alternative to the widely used gold meshes for monitoring the beam intensity of soft X-ray beamlines in situ. Limited by the carbon extended x-ray absorption fine structure oscillations, the diamond films become applicable beginning from about 600 eV photon energy, where the important transition metal edges and the rare-earth edges are found. The 100 nm and 250 nm thick free-standing diamond films were grown and tested against standard gold meshes in real-life dichroism experiments performed at beamline ID08 of the European Synchrotron Radiation Facility, Grenoble, France. Quantitative agreement was found between the two experimental data sets. The films feature an extremely high transmission of about 90% and, at the same time, yield a sufficiently strong and clean reference signal. Furthermore, the thin films do not affect the shape of the transmitted beam. X-rays passing mesh-type monitors are subject to diffraction effects, which widen the beam and become particularly disturbing for small beamsizes in the micrometer range.

Optical phonon lifetimes in sputtered AlN thin films

We study the vibrational properties of AlN thin films deposited on silicon (100) substrates by the reactive DC-pulsed magnetron sputtering. The frequencies and lifetimes of the E1(TO) and A1(LO) optical phonons are calculated from Fourier transform infrared spectra using the factorized model of a damped oscillator. We analyze the structural properties by the x-ray diffraction technique to correlate the elongation of phonon lifetimes with increasing film thickness. The lifetimes of the phonon modes in AlN thin films are compared to the values in a single crystal.

Enhanced optoelectronic performances of vertically aligned hexagonal boron nitride nanowalls-nanocrystalline diamond heterostructures

Field electron emission (FEE) properties of vertically aligned hexagonal boron nitride nanowalls (hBNNWs) grown on Si have been markedly enhanced through the use of nitrogen doped nanocrystalline diamond (nNCD) films as an interlayer. The FEE properties of hBNNWs-nNCD heterostructures show a low turn-on field of 15.2 V/μm, a high FEE current density of 1.48 mA/cm2 and life-time up to a period of 248 min. These values are far superior to those for hBNNWs grown on Si substrates without the nNCD interlayer, which have a turn-on field of 46.6 V/μm with 0.21 mA/cm2 FEE current density and life-time of 27 min. Cross-sectional TEM investigation reveals that the utilization of the diamond interlayer circumvented the formation of amorphous boron nitride prior to the growth of hexagonal boron nitride. Moreover, incorporation of carbon in hBNNWs improves the conductivity of hBNNWs. Such a unique combination of materials results in efficient electron transport crossing nNCD-to-hBNNWs interface and inside the hBNNWs that results in enhanced field emission of electrons. The prospective application of these materials is manifested by plasma illumination measurements with lower threshold voltage (370 V) and longer life-time, authorizing the role of hBNNWs-nNCD heterostructures in the enhancement of electron emission.

Field electron emission enhancement in lithium implanted and annealed nitrogen-incorporated nanocrystalline diamond films

The field electron emission (FEE) properties of nitrogen-incorporated nanocrystalline diamond films were enhanced due to Li-ion implantation/annealing processes. Li-ion implantation mainly induced the formation of electron trap centers inside diamond grains, whereas post-annealing healed the defects and converted the a-C phase into nanographite, forming conduction channels for effective transport of electrons. This resulted in a high electrical conductivity of 11.0 S/cm and enhanced FEE performance with a low turn-on field of 10.6 V/μm, a high current density of 25.5 mA/cm2 (at 23.2 V/μm), and a high lifetime stability of 1,090 min for nitrogen incorporated nanocrystalline diamond films.

The pressure sensitivity of wrinkled B-doped nanocrystalline diamond membranes

Nanocrystalline diamond (NCD) membranes are promising candidates for use as sensitive pressure sensors. NCD membranes are able to withstand harsh conditions and are easily fabricated on glass. In this study the sensitivity of heavily boron doped NCD (B:NCD) pressure sensors is evaluated with respect to different types of supporting glass substrates, doping levels and membrane sizes. Higher pressure sensing sensitivities are obtained for membranes on Corning Eagle 2000 glass, which have a better match in thermal expansion coefficient with diamond compared to those on Schott AF45 glass. In addition, it is shown that larger and more heavily doped membranes are more sensitive. After fabrication of the membranes, the stress in the B:NCD films is released by the emergence of wrinkles. A better match between the thermal expansion coefficient of the NCD layer and the underlying substrate results in less stress and a smaller amount of wrinkles as confirmed by Raman spectroscopy and 3D surface imaging.

Enhancement of plasma illumination characteristics of few-layer graphene-diamond nanorods hybrid

Few-layer graphene (FLG) was catalytically formed on vertically aligned diamond nanorods (DNRs) by a high temperature annealing process. The presence of 4-5 layers of FLG on DNRs was confirmed by transmission electron microscopic studies. It enhances the field electron emission (FEE) behavior of the DNRs. The FLG-DNRs show excellent FEE characteristics with a low turn-on field of 4.21 V μm-1 and a large field enhancement factor of 3480. Moreover, using FLG-DNRs as cathode markedly enhances the plasma illumination behavior of a microplasma device, viz not only the plasma current density is increased, but also the robustness of the devices is improved.

Growth, structural and plasma illumination properties of nanocrystalline diamond-decorated graphene nanoflakes

The improvement of the plasma illumination (PI) properties of a microplasma device due to the application of nanocrystalline diamond-decorated graphene nanoflakes (NCD-GNFs) as a cathode is investigated. The improved plasma illumination (PI) behavior is closely related to the enhanced field electron emission (FEE) properties of the NCD-GNFs. The NCD-GNFs possess better FEE characteristics with a low turn-on field of 9.36 V μm−1 to induce the field emission, a high FEE current density of 2.57 mA cm−2 and a large field enhancement factor of 2380. The plasma can be triggered at a low voltage of 380 V, attaining a large plasma current density of 3.8 mA cm−2 at an applied voltage of 570 V. In addition, the NCD-GNF cathode shows enhanced lifetime stability of more than 21 min at an applied voltage of 430 V without showing any sign of degradation, whereas the bare GNFs can last only 4 min. The superior FEE and PI properties of the NCD-GNFs are ascribed to the unique combination of diamond and graphene. Transmission electron microscopic studies reveal that the NCD-GNFs contain nano-sized diamond films evenly decorated on the GNFs. Nanographitic phases in the grain boundaries of the diamond grains form electron transport networks that lead to improvement in the FEE characteristics of the NCD-GNFs.

Influence of hydrogen and hydrogen/methane plasmas on AlN thin films

Polycrystalline aluminum nitride (AlN) thin films are exposed to hydrogen and hydrogen/methane plasmas at different conditions. The latter plays an indispensable role in the subsequent deposition of nanocrystalline diamond thin films on AlN. The changes of AlN properties are investigated by means of Fourier transform infrared (FTIR) and Raman spectroscopies as well as atomic force microscopy. The E1(TO) and E22 phonon mode frequencies blue-shift after the exposure to plasmas. The damping constant of E1(TO) phonon, calculated from FTIR transmission spectra using the factorized model of a damped oscillator, and the width of E22 peak in Raman spectra decrease with increasing substrate temperature till the decomposition of AlN thin film becomes notable. It is proven that these changes are driven by the plasmas as annealing in vacuum does not induce them.