Composition and bonding structure of boron nitride B1-xNx thin films grown by ion-beam assisted evaporation

We present a detailed study of B1-xNx (01000oC) chemical vapor deposition (CVD) techniques have been used for the deposition of boron subnitride layers. For instance, coatings with B58N to B23N compositions and Itetragonal structure were obtained through pyrolysis of BBr3-H2-N2 [14,19]. Moreover, Saitoh et al. proposed the presence of a B4N structure analogous to B4C in polycrystalline B3N films, which were grown at 1200 oC by a tungsten hot-filament assisted BCl3-NH3-H2 reaction [21]. On the other hand, with respect to physical vapor deposition (PVD) techniques, dual ion beam deposition of B22N to B1.5N coatings consisting of different ratios of a supposed B4N/h-BN mixed-phase structure was reported [20]. Besides, Möller et al. combined Monte Carlo collision computations and ion beam-assisted deposition (IBAD) experiments in a detailed study on the growing process of boron nitride films with B4N to B0.5N compositions [26,27]. For these samples, changes in the density suggested a transition from B/c-BN segregated phases to h-BN for decreasing B content. Like in HP-HT produced crystals, B-rich boron nitride compounds were mostly present as segregated phases in the films and not as pure materials. According to the above exposed, it is clear that the available publications show spread results on the composition and corresponding structure of non-stoichimetric B1xNx boron nitrides. In some cases, the structural determination was not conclusive and remains, to date, a matter of debate. The complexity of the system is especially noticeable in B1-xNx subnitride thin films, for which a systematic study relating the growing conditions with their final composition and structure is lacking. In this letter, we present a thorough study of the relationship between composition and bonding structure for a wide range of B1-xNx thin films from pure B to BN1.5. The samples were grown on p-type Si (100) wafers by IBAD through B evaporation and concurrent N2 + ion bombardment. In this way we have been able to determine and control the correlation between the atom-ion ratio and the final composition and structure of the coatings. Synthesis of a new heterogeneous thin film consisting of a 1:1 ratio of B13N2/h-BN phases is proved by X-ray Absorption Near-Edge Structure (XANES) analysis. Boron evaporation was performed with an AP&T; electron gun evaporator (model EV1-8) from crystalline lumps of 99.5% purity and 0.2 cm 3 average volume, which were fed and premelted in a graphite crucible placed ~28 cm below the deposition point. The EV1-8 evaporator works at a constant voltage of 7kV and a maximum electron beam intensity of 500mA. The atomic B flux (B) was determined following the procedure described in Ref. [28], wherein the B calibration curve as a function of the electron beam intensity can be found. No intentional heating was applied to the substrates, which were cleaned ex situ in consecutive ultrasonic baths of acetone and ethanol before deposition. Nevertheless, the substrate reached a temperature of approximately 100oC due to ion bombardment and radiation heat from the evaporated material. The working pressure was ~110 -4 mbar using 8 sccm of N2 gas flow. In this conditions, we could neglect pressure changes in the order of ~110 -5 mbar due to the different B evaporation rates. Besides, since the base pressure of the vacuum chamber was <110 -6 mbar, beam contamination due to residual gas was reduced to ~1% maximum. The assisting N2 + ion beam was generated with a 3cm Kaufman ion gun (Commonwealth Scientific, mod IBS) fed with 100% N2 gas. The energy of ion bombardment was fixed at 500eV for all samples, with an angle of incidence of 50o. The nitrogen ion flux could be tuned by varying the discharge current from 0.1A to 2.5A, approximately. The flux of N atoms (N) impinging the substrate during deposition was estimated from the N2 + ion current density – measured with a Faraday cup – as follows: