Investigation of the field emission current from polycrystalline diamond films
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The existence of negative electron affinity (NEA) on the hydrogen terminated single crystal (111) and (100) [1,2] diamond surfaces has been a motivation for the practical exploitation of this phenomenum in polycrystalline diamond (PCD) films. NEA has been observed in PCD films [3], and electron emission at low values of the electric field has been measured [4]. The origin of the electrons observed in field emission from p-type diamond is currently a topic of active research. In order to investigate from what states electron emission was occurring measurements on the temperature variation of the field emission current were carried out on boron doped PCD diamond films The diamond films characterized in this work were grown using a hot filament chemical vapor deposition system with a methanehydrogen ratio of 1/120 at a process pressure of 15 torr. Diborane or nitrogen was used as the dopant source. The nominal filament temperature was kept at 2100C and the substrate temperature during growth was 900C. Tantalum was employed as the filament material. Prior to initiation of the growth cycle, several gas purges of the growth chamber were carried out in order to reduce any residual oxygen or nitrogen. Following completion of the growth procedure, the samples were allowed to cool to room temperature in the chamber before being exposed to the atmosphere. Sample A during growth was doped using a boron/carbon ratio of 200 ppm. Sample B during growth was doped with a boron/carbon ratio of 100 ppm. The resultant boron concentration for both films was = 10 18/cm The field emission measurements were performed under UHV conditions at a pressure of 2.6 x 10-6 Pa in a turbomolecular pumped chamber. The pressure was monitored using a residual gas analyzer. Low levels of oxygen were of particular concern because it has been shown in the literature that oxygen reacts with the diamond surface at a temperature of around 500C [5]. The measurement of the 765 Mat. Res. Soc. Symp. Proc. Vol. 42301996 Materials Research Society
emission current was carried out using two distinct configurations. The setup shown in figure la was used to measure the field emission current as a function of temperature. This particular arrangement was used in order to minimize the influence of temperature effects on the Ta probe-diamond film distance and limit any leakage current. The temperature was measured using a chromel-alumel thermocouple placed in physical contact with the diamond surface adjacent to the emitting area. A Keithley 2001 voltmeter was used to measure the thermocouple voltage. The physical contact of the thermocouple on the diamond film had no effect on the measured emission current. A typical measurement cycle was initiated by heating a sample up to 350C. Upon removal of power from the substrate heater, the field emission current was sampled every 2 seconds. Using this method, the maximum rate of sample cooling at any point on the current-temperature curve was limited to less than 1 K/s. Limiting the sample temperature to below 350C was done
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