Photoluminescence studies of polycrystalline diamond films
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J. E. Butler and U. Strom Naval Research Laboratory, Washington, DC 20375-5000 (Received 10 April 1990; accepted 17 July 1990)
The photoluminescence spectra of polycrystalline diamond films prepared by filament assisted chemical vapor deposition are dominated by a defect band with a strong zero phonon line near 1.68 eV and weak phonon replicas at lower energies. The 1.68 eV line is blueshifted from the 1.675 eV zero phonon line associated with the neutral vacancy in bulk diamond. The line shape and position of the 1.68 eV line are shown to depend on substrate material (Si, Mo, Ni). The 1.68 eV emission for Ni and Mo substrates is interpreted in terms of the stress shifted and broadened neutral vacancy emission. The broader 1.68 eV line observed for Si substrates may indicate the additional effects of Si absorption by the diamond films. Films prepared by an oxygen-acetylene flame technique exhibit two additional luminescence bands with zero phonon lines at 1.95 and 2.16 eV. These lines have been tentatively assigned to nitrogen-vacancy complexes. The temperature dependence (6 K-300 K) of the luminescence of a free-standing diamond film, which had been deposited on a molybdenum substrate, is comparable to similar observations reported for bulk diamond. We have also observed a strong dependence of the PL spectra radially across a given combustion film and associated this with details of the flame chemistry.
I. INTRODUCTION
It has recently been possible to grow relatively thick, large area polycrystalline diamond films. In order to realize potential opto-electronic applications of these films it is essential to understand the nature of the extrinsic and intrinsic defects in such films. One important aspect of such studies is to relate the defect structures to the film preparation technique and substrate type. A recent report1 described defect signatures using cathodoluminescence (CL) measurements on diamond films prepared by hot-filament chemical vapor deposition (FACVD) onto Si substrates. We report here preliminary photoluminescence (PL) studies of diamond films prepared by FACVD and an oxygenacetylene torch (combustion) process using Ni, Mo, as well as Si substrates. Whenever possible, the defects in the films are identified by comparing their luminescence spectra with well-known defect spectra in natural diamond.2"5 II. EXPERIMENTAL DETAILS A. Film deposition techniques
The FACVD deposition of diamond was performed in a vertical 4.45 cm i.d. fused silica tube. A tungsten or rhenium filament was suspended 5 mm above the substrate, which was supported on a 2.54 cm o.d. molybdenum encased heater. The filament temperature was maintained at 2200 ± 50 °C and the substrate at 2502
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J. Mater. Res., Vol. 5, No. 11, Nov 1990
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875 °C. The gas flow (ca. 100 seem of 0.5% methane in hydrogen) was introduced at the top of the vessel and the pressure was maintained at 40 Torr during the course of the deposition. The chamber was pumped by a two-stage mechanical pump and th
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