Infrared Absorption and Electron Spin Resonance Studies of Nanocrystalline Cubic Boron Nitride/Amorphous Hydrogenated Bo
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nitrogen and 23 at. % hydrogen. From the optical absorption results, the optical bandgap increases from 4.0 to 5.6 eV as the substrate temperature increases from 60 C to 250 C. From the infrared absorption results, the dominant absorption feature around 1400 cm-' consists of three separate lines, two associated with small bandgap amorphous hydrogenated boron nitride and one with large bandgap amorphous hydrogenated boron [6]. The ESR results consist of a broad spectrum, associated with dangling bonds in the amorphous regions, superimposed on two less intense spectra showing hyperfine structures. The latter are associated with defects in the cubic boron nitride nanocrystals [7]. In this paper, we extend our measurements to samples grown at various substrate temperatures to learn how the structure of our films changes with substrate temperature. EXPERIMENT The boron nitride thin films were grown in a capacitively coupled rf plasma reactor with a feedstock of diborane (B2H6 ), ammonia (NHi3 ) and hydrogen (H2 ). The growth parameters are: diborane partial pressure of 4.0 mT; ammonia partial pressure of 40 mT; hydrogen partial pressure of 356 mT; rf power of 23 W; self-bias voltage of 430 V; and the films were grown on silicon or aluminum foil substrates mounted on an anode heated between 60 C and 300 C. The films grown on silicon substrates were inserted into a Perkin Elmer Model 1610 FTIR spectrophotometer for infrared absorption measurements. The films grown on aluminum substrates were immersed in dilute hydrochloric acid until the aluminum was dissolved. The remaining film was washed, dried and placed in a 5mm OD sample tube which was inserted into the dual mode microwave cavity of a Varian E-12 ESR spectrometer. The signal averaging, spectral substractions and integrations were accomplished with software obtained from Scientific Software, Bloomington, Illinois. Both the first and second derivative spectra of the ESR absorptions were always recorded. For the best resolution together with reasonable sensitivity, it was preferable to use first derivative spectra for the four-line and broad central line features, and the second derivative spectra for the ten-line features. RESULTS The infrared absorption spectrum of boron nitride thin films grown at a substrate temperature of 150 C is shown in Figure 1 as the open circles. This spectrum is fitted to a sum of three Lorentzian lines with peaks at 1263, 1371, and 1505 cm-1 (solid line in Figure 1). This fitting procedure was performed on four other infrared spectra of samples grown at various substrate temperatures ranging from 60 C to 300 C. As a function of increasing substrate temperature, the integrated intensity of the 1371 cm-1 line increases until it totally dominates at substrate temperatures at or above 250 C, whereas the integrated intensity of the 1263 and 1505 cm- 1 lines decrease until at or above 250 C, they have a negligible contribution [6]. The transverse optical mode of cubic boron nitride is a very sharp line at 1070 cm-', but it is unobserved in any
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