Nitrogen Defect Concentration in Chemical Vapor Deposited Homoepitaxial Diamond at High TemperatureS
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impurities in concentrations of only a few ppm. This is particularly true for use of diamond crystal as an anvil in high pressure research. It is known in high pressure diamond anvils that nitrogen in the ppm range can make them unsuitable for high pressure spectroscopy experiments. In this paper, we present an electron paramagnetic resonance study of the substitutional nitrogen impurity, the PI center[l]. EXPERIMENT Diamond growth was performed using a microwave plasma CVD system with a cylindrical stainless steel vacuum chamber that has a water-cooled outer sleeve. The chamber was mechanically pumped to a base pressure of 0.1 Torr. The sample stage consists of a 1 inch long, 0.5 inch diameter molybdenum rod which is threaded into a water-cooled copper cylinder. The temperature of the water circulated through the copper cylinder was held constant at 10 C via a Neslab refrigerator/bath circulator. The {100 }oriented natural type Ila cylindrical diamond seed crystal (Harris Diamond, NJ) was placed on the polished top face of molybdenum rod. The initial crystal had a diameter of 1.5 mm and a thickness of 0.25 mm and the [100] axis of diamond plate was within 2 degrees of the cylindrical axis. At the beginning of the experiment, the diamond seed crystal was exposed to a pure hydrogen plasma for 15 minutes prior to the introduction of methane. During grovth of synthetic diamond by MPCVD, three main processing parameters are substrate temperature between 1000 to 239
Mat. Res. Soc. Symp. Proc. Vol. 555 0 1999 Materials Research Society
2000 C, reactor pressure between 60 to 150 Torr, and concentration of precursor gases (CH 4 / H 2 ) between 1 to 3% as listed in table 1. Substrate temperatures were measured using a two-color, model M77LS pyrometer from Mikron, with a 2.0 mm sampling diameter that is nearly filled by the 1.5 mm (diameter) substrate. There was some temperature drift of approximate 100 C during the typical 12 hours period for each run due to low thermal conductivity of the molybdenum rod. Therefore, Table 1 shows the average temperature during each experiment. At the end of deposition the grown diamond samples were exposed to a pure hydrogen plasma for 15 minutes to remove any surface graphite. EPR was used as a way to confirm the epitaxial nature of the grown diamond film as well as to quantitatively measure the nitrogen impurity in the grown film. The EPR data were taken at room temperature with a Bruker X-band spectrometer using 800 mG modulation intensity, 100 kHz modulation frequency, and 20 micro-watt microwave power. Every spectrum takes at least 5 hours (200 times) to do averaging. EPR analysis of the seed plate before deposition showed that
nitrogen level below the limit of 50 ppb[2]. Figure l. shows the EPR spectrum observed with the magnetic field H0 aligned along the [1001, [111] and [011] directions of the grown diamond crystal #DPlI in Table 1.
Average Samplem#sratue ( Temperature (C)
Pressure (Torr)
Substitutional Nitrogen Concentration (ppm)
CH 4 /H2 (%)
DPI
1496
60
1
19
DP5
1
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