Using Zeolites as Substrates for Diamond Thin Film Deposition
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incorporate low molecular weight, singly charged cations such as sodium and magnesium. At the other end of the spectrum, materials such as silicalite are composed solely of Si and 0. Since zeolites become more thermally stable as their Si/Al ratio increases, silicalite, which is stable to 1200 "C was selected as a substrate for diamond deposition. Because of their crystallinity, zeolite surfaces display a distinct, regular structure composed of pore openings that range from 2-15 A separated by interpore spacings which range from 5-31 A. In particular, silicalite, which has a ZSM-5 crystal structure, has pores that are 5.5 A in diameter and are spaced 12 A to.20 A apart, depending on the particular direction and crystal face. When, compared with porous silicon substrates, the nucleation sites on a silicalite crystal are then potentially two orders of magnitude closer together In this study, the silicalite crystals used are uniformly 60 x 15 x15 pm in size. Examples are shown in Figure 1.
EXPERIMENTAL SECTION All of the experiments were performed with a hot filament chemical vapor.deposition (HFCVD) system using hydrogen and acetone as the feed gases. The gases were activated with a planar, S-shaped tantalum filament. The substrate temperature was monitored by a Ktype thermocouple contacting the back of the Figure 1: silicalite crystals silicon wafer. Typical deposition conditions were gas. feeds of 150 sccm of H2, 1.5 sccm of acetone, and a substrate temperature of 780 'C. The reactor pressure was 100 torr during the first - hour of deposition and was changed to 20 torr for the remaining 2 hours. When silicon wafers seeded with 0.5 gm diamond grit were used as substrates for this process, well faceted, -7 gm thick polycrystalline diamond films resulted. Runs without the initial high pressure period resulted in etching of the silicalite crystals, probably due to the flux of H-atoms. •3 Bonded Hydrocarbon Seed Matsumoto and Matsui have previously suggested that adamantane (Ci0HI6; see Figure 2) and similar hydrocarbon cage compounds may act as "embryos" for diamond nucleation 5 . Working from the hypothesis that some form of sp 3 -bonded carbon acts as the nuclei for diamond formation, we used adamantane as our model compound. In addition, adamantane has an Figure 2 diameter of approximately 4 A which should allow it to diffuse through the silicalite pores easily. Seeding was accomplished by placing the calcined zeolite powder into a solution of adamantane in methylene chloride for several days. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the post-deposition samples. SEM was performed via a ElectroScan Environment Scanning Electron Microscope and no special sample preparation was necessary. TEM films were prepared by subjecting the as-deposited samples to a hydrogen-only feed in the HFCVD reactor for half an hour, any remaining silicalite crystals or non-diamond carbon films were etched away. The residual material was then scraped onto a number of carbon film coa
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