Altering Resistivity in Diamond Films Without Impurity Addition
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EXPERIMENTAL The diamond films in this study were grown in a 5 kW hydrogen/methane microwave plasma. The methane concentrations were varied over a range from 0.5% to 3% of total inlet gas feed. The total reactant flow-rate was maintained at 1000 sccm, and the pressure in the reactor was held at 120 torr. Freestanding films were obtained by growth on polished molybdenum substrates maintained at 800 +/-20 'C during each run. Growth rates varied from 2-8 micrometers per hour. The films were cleaned following the technique of Mori et al.3 by boiling in chromic acid, with a hydrogen peroxide/ammonium hydroxide rinse to remove the surface conductive layer that has been observed to form after growth. The films were characterized with Raman spectroscopy to determine their sp 2 and sp 3 bonding character. The characteristic diamond peak is found at 1332 cml and the broad graphite peak is found at 1580 cm'. The area under each curve was calculated to give a qualitative ratio of the graphitic to diamond bond character. Scanning electron microscopy (SEM) and optical microscopy were used to examine surface morphology and grain boundaries. A four-point probe sample holder was designed to measure volume resistivity of the films in conjunction with a Keithley model 6517A electrometer. Films were metallized with a stacking of 100 Angstroms of titanium, 500 Angstroms of platinum, and 3000 Angstroms of gold respectively. For temperature dependant measurements, the resistivity apparatus is placed in a Dewar under the flow of cold dry nitrogen gas. RESULTS AND DISCUSSION Figure 1 shows SEM images of the 0.5% and 3% methane diamond films. It is observed that the film morphology changes as the methane content in the feed stream is varied. It is also observed that an increase in methane content gives a decrease in grain size and sporadic secondary nucleation. Angus et al.4 postulated that this behavior is the consequence of a competition between growth and etching of diamond and non-diamond carbon. Much of the non-diamond carbon may be graphitic as supported by Raman spectroscopy (figure 2). Here, the Raman spectrum of the highest methane feed, 3% is shown, with the characteristic sp 2
3
diamond
peak at 1332 cm', and the broad sp graphitic peak at 1580 cm'. The integrated area under each curve was calculated to qualitatively determine the graphite to diamond character for each film. Note that although sp 2 bonding has been introduced into the film, the sp3 peak remains much more intense, indicating only moderate changes to the bulk structure of the film.
Figure 1. SEM images showing decreased grain size as methane concentration is increased. The film on the left was grown with 0.5% methane, and the one on the right with 3% methane.
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Figure 3 shows a plot of the ratio of sp 2 to sp 3 bonding character as a function of methane content in the feed. This plot shows that an increase in the amount of methane in the feed can increase the sp 2 bonding character in the film. It is believed that this sp 2 bonding is found primarily in gra
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