This article describes novel composite thin films consisting of GaN, C, and Ga (termed “GaCN”, as an analogue to BCN and other carbonitrides) as a prospective material for future optical applications. This is due to their tunable refractive index that depends on the carbon content. The composites are prepared by introducing alternating pulses of trimethylgallium (TMG) and ammonia (NH3) on silicon substrates to mimic an atomic layer deposition process. Because the GaCN material is hardly reported to the best of our knowledge, a comprehensive characterization is performed to investigate into its chemical nature, primarily to determine whether or not it exists as a single-phase material. It is revealed that GaCN is a composite, consisting of phase-segregated, nanoscale clusters of wurtzitic GaN polycrystals, in addition to inclusions of carbon, nitrogen, and gallium, which are chemically bonded into several forms, but not belonging to the GaN crystals itself. By varying the deposition temperature between 400 and 600 °C and the NH3 partial pressure between 0.7 × 10–3 and 7.25 mbar, layers with a wide compositional range of Ga, C, and N are prepared. The role of carbon on the GaCN optical properties is significant: an increase of the refractive index from 2.19 at 1500 nm (for carbon-free polycrystalline GaN) to 2.46 (for GaCN) is achieved by merely 10 at. % of carbon addition. The presence of sp2-hybridized C=N clusters and carbon at the interface of the GaN polycrystals are proposed to determine their optical properties. Furthermore, the formation of the GaN polycrystals in the composite occurs through a TMG:NH3 surface-adduct assisted pathway, whereas the inclusions of carbon, nitrogen, and gallium are formed by the thermal decomposition of the chemisorbed TMG species.

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This work considers the possible occurrence of two distinct phases in thin films of overall composition B1−xNx (0.21 ≤ x ≤ 0.47) grown by chemical vapor deposition from sequential pulses of diborane (B2H6) and ammonia (NH3). Two distinct peaks are identified in B1s x-ray photoelectron spectroscopy (XPS), related to two populations of B atoms with different oxidation states. The data are most consistent with a model in which one population mainly bonds to B atoms, and the other population mainly bonds to N atoms, as expected for a composite of B and BN. Based on peak broadening, interfaces between the two types contribute significantly to the spectra. Furthermore, spectroscopic ellipsometry (SE) found that the samples displayed optical absorption consistent with that of pure-B. This work, thus, developed a fit model to characterize the films optically by SE. Describing the films as composites of pure-B and BN, and using optical constants of reference layers thereof, the relative fractions could be estimated in reasonable agreement with XPS. Differences between the models and data in both SE and XPS are consistent with the effects of hydrogenation and the contribution of atoms in interface states. Evidence from SE suggests that the films may consist of stacked lamellar phases, which would indeed have a large surface-to-volume ratio.

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This work studies the deposition of boron/boron nitride (B/BN) composite films at low substrate temperature (275-375 °C) by alternating pulses of diborane (B 2 H 6 ) and ammonia (NH 3 ) with argon purging in between to avoid gas-phase reactions of the precursors. This process is similar to atomic layer deposition in which the dominance of surface reactions simplifies the growth mechanism. However, non-self-limiting decomposition of B 2 H 6 and incomplete nitridation lead to the incorporation of pure boron (pure-B), causing deviation from the desired 1:1 B:N stoichiometry. Using the pure-B fraction as a measure of incomplete nitridation, this article describes consecutive experiments to control this effect and ultimately understand it in the context of a surface reaction model. First, it is demonstrated that, in a purely thermal mode, the growth of the layers and their composition strongly depend on the total gas pressure. The pure-B content (not to be confused with the total boron content) could thus be varied in the range of ∼6-70 vol. %. Next, enhancement of nitridation by the dissociation of NH 3 into reactive radicals using a hot-wire was found to be insufficient to produce stoichiometric BN. Finally, plasma-assisted deposition at 310 °C resulted in nearly stoichiometric polycrystalline BN with an interplane distance matching that of hexagonal BN; the material was stable in air for at least six months. The pressure dependence in the purely thermal mode is consistent with a growth model of BN from B 2 H 6 and NH 3 via the so-called surface-adduct mechanism. The effects of the radical-enhanced methods on nitridation are explained using this model.