Mladen Petravic

Large-scale mechanical peeling of boron nitride nanosheets by low-energy ball milling

Mechanical cleavage by Scotch tape was the first method to produce graphene and is still widely used in laboratories. However, a critical problem of this method is the extremely low yield. We have tailored ball milling conditions to produce gentle shear forces that produce high quality boron nitride (BN) nanosheets in high yield and efficiency. The in-plane structure of the BN nanosheets has not been damaged as shown by near edge X-ray absorption fine structure measurements. The benzyl benzoate acts as the milling agent to reduce the ball impacts and milling contamination. This method is applicable to any layered materials for producing nanosheets.

Gettering of copper to hydrogen‐induced cavities in silicon

Hydrogen implantation and subsequent thermal annealing is found to result in a well‐defined band of cavities in Si. This band is an extremely efficient gettering layer for Cu which is also introduced into the near surface of Si by ion implantation. Profiling of implanted Cu indicates that ∼95% of an initial 3×1015 cm−2 Cu implant is redistributed following annealing at a temperature of 780 °C from a near‐surface damaged layer to a narrow band of cavities of width ∼1000 A at a depth of ∼1 μm. Furthermore, the Si between the surface and the cavity band is essentially defect‐free and that some cavities contain the bulk Cu3Si phase.

Using doping superlattices to study transient‐enhanced diffusion of boron in regrown silicon

A boron‐doped silicon superlattice consisting of three boron spikes separated by 1700 A of undoped silicon has been grown by molecular beam epitaxy and used to study the evolution of point defects following an amorphizing implant of Si+. After MBE growth, the wafer was implanted at 77 K with either 146 or 292 keV Si+ at a dose of 5×1015/cm2. These implants produced amorphous layer depths that coincided with the depths of either the middle B peak or just below the deepest B peak. The samples were then annealed at 800 °C in an Ar ambient. Secondary‐ion‐mass spectrometry and transmission electron microscopy were used to monitor the diffusion of the boron spikes upon annealing and the evolution of the extended defects upon annealing, respectively. For the lower‐energy sample, an enhancement in the B diffusivity of over 500× was observed for both the surface B spike and the deepest B spike. The higher‐energy implant shows conclusively that the back flow of interstitials into the regrown region is coming from t...

Oxidation of silicon by low energy oxygen bombardment

High resolution Rutherford backscattering and channeling has been used to study the formation of surface oxides during room temperature bombardment of silicon with oxygen in a secondary ion mass spectrometry system. Stoichiometric SiO2 is formed at angles of incidence (to the surface normal)≤25° and the angular dependence is adequately modeled using the profile code. A linear dependence of oxide thickness on energy is obtained in the energy range 3–40 keV (per oxygen ion) and this is consistent with trim code calculations. The suboxide damage has also been measured and studied during annealing. Our data are consistent with a simple model of oxygen build up and formation of strong Si–O bonds during room temperature bombardment. Once a buried SiO2 layer is reached and Si bonds are saturated, oxygen can migrate in SiO2 to extend the oxide towards the surface.

Mechanically activated catalyst mixing for high-yield boron nitride nanotube growth

Boron nitride nanotubes (BNNTs) have many fascinating properties and a wide range of applications. An improved ball milling method has been developed for high-yield BNNT synthesis, in which metal nitrate, such as Fe(NO3)3, and amorphous boron powder are milled together to prepare a more effective precursor. The heating of the precursor in nitrogen-containing gas produces a high density of BNNTs with controlled structures. The chemical bonding and structure of the synthesized BNNTs are precisely probed by near-edge X-ray absorption fine structure spectroscopy. The higher efficiency of the precursor containing milling-activated catalyst is revealed by thermogravimetric analyses. Detailed X-ray diffraction and X-ray photoelectron spectroscopy investigations disclose that during ball milling the Fe(NO3)3 decomposes to Fe which greatly accelerates the nitriding reaction and therefore increases the yield of BNNTs. This improved synthesis method brings the large-scale production and application of BNNTs one step closer.

Interaction of defects and metals with nanocavities in silicon

Abstract Ion implantation of H or He into silicon, followed by annealing can create a band of nanocavities. Such nanocavities can exhibit a range of interesting and often non-equilibrium interactions with defects and metals during subsequent implantation and annealing. This paper gives an overview of such interactions, concentrating on cavities produced by H-implantation. The evolution of cavities during annealing is briefly treated, followed by illustrations of the very efficient gettering ability of cavities for fast diffusing metals. For low metal concentrations introduced into the near-surface by implantation, the metal atoms decorate the cavity walls during annealing but can be displaced by oxygen under certain conditions. At high metal concentrations, precipitation and second phase (silicide) formation can occur at cavities but silicide formation and dissolution are found to be controlled by the availability or removal of silicon interstitials, leading to non-equilibrium behaviour. When silicon that contains cavities is irradiated with silicon ions, irradiation-induced defects interact with cavities, leading to preferential amorphisation at certain temperatures. Continued irradiation leads to cavity shrinkage during bombardment, which is most efficient when the region around the cavities is amorphised.

Nanosize diamond formation promoted by direct current glow discharge process: Synchrotron radiation and high resolution electron microscopy studies

Diamond nucleation on Si(100) surfaces can be promoted by a dc-glow discharge process, using a CH4/H2 gas mixture. However, the phase composition and structure of the carbon film deposited during the dc-glow discharge pretreatment are still unclear. In the present work, we report on a combined study of near edge x-ray absorption fine structure (NEXAFS), and high resolution scanning electron microscopy (HR-SEM) of this film as a function of substrate temperature. NEXAFS measurements of the films deposited by the dc-glow discharge process render unambiguous evidence of diamond phase formation in the 880–900 °C substrate temperature range. It is determined from HR-SEM measurements that in this temperature range, nanosize diamond particles are formed. At lower and higher substrate temperatures the NEXAFS results indicate the predominant formation of graphitic carbon. The changes in the film composition as a function of substrate temperature during the dc-glow discharge process is expressed in terms of relativ...

On the estimation of depth resolution during sputter profiling

We have estimated broadening of sputter profiles for several elements, either buried or implanted in Si, under Ar+, Cs+, and O2+ bombardment using a semiempirical model developed recently by P. C. Zalm and C. J. Vriezema [Nucl. Instrum. Methods B 67, 495 (1992)]. Excellent agreement has been found between this model and experimental data for all three types of primary ions. In the case of oxygen bombardment, however, good agreement is only achieved by taking into account beam‐induced changes at the surface, including swelling and the formation of a surface oxide.

Core-level photoemission and near-edge x-ray absorption fine-structure studies of GaN surface under low-energy ion bombardment

We have investigated compositional changes on GaN surfaces under low-energy Ar ion bombardment using synchrotron-based high-resolution core-level photoemission measurements and near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy. The low-energy ion bombardment of GaN produces a Ga-rich surface layer which transforms into a metallic Ga layer at higher bombarding energies. At the same time, the photoemission spectra around the N 1s core level reveal the presence of both uncoordinated nitrogen and nitrogen interstitials, which we have analyzed in more detail by x-ray absorption measurements at the N K-edge. We have proposed a mechanism for the relocation and loss of nitrogen during ion bombardment, in agreement with some recent experimental and theoretical studies of defect formation in GaN. We have also demonstrated that photoemission spectroscopy and NEXAFS provide a powerful combination for studying compositional changes and the creation of point defects at GaN surface.

The role of oxygen on the stability of gettering of metals to cavities in silicon

The effect of oxygen implanted into epitaxial Si layers on the ability to getter Au to nanocavities, previously formed by H implantation and annealing, has been studied by Rutherford backscattering, transmission electron microscopy, and secondary ion mass spectrometry. We demonstrate that oxygen is gettered to cavities during extended annealing at 950 °C. Furthermore, the arrival of oxygen at cavities is not only shown to inhibit subsequent attempts to getter Au to cavities, but also to eject chemisorbed Au from the cavity walls. Similar behavior is observed in Czochralski Si, where the source of oxygen is within the Si itself.