(395a) Nano-Crystalline P-Type Diamond for High Power & High Temperature Applications

Conductive polycrystalline
diamond films are desirable for a variety of high temperature and high power
applications due to their high thermal conductivities, approaching the 20 W cm^-1
K^-1 of natural diamond.  P-type diamond films may be produced by
introducing boron as a dopant during growth by chemical vapor deposition
(CVD).  These films have properties which make them ideal for both passivation
layers and ohmic contacts when a thin layer of p-type diamond is grown on top
of non-conductive diamond.  Applications in electronic devices that benefit
from these films include passives, electromechanical devices, switches,
integrated circuits, sensors and diamond heat spreaders.  Nano-crystalline diamond
(NCD) films were produced in a 1.5 kW ASTEX system by Microwave Plasma Enhanced
Chemical Vapor Deposition (MWCVD) using precursor gas carbon concentrations
from 0.5-4.0%, at 900 W power, 3.3 kPa pressure, and a substrate temperature of
680 ºC.  Due to its relatively benign nature, boric acid used as the source of
boron.  The vapor from the saturated solution of boric acid dissolved in
methanol was delivered by bubbling hydrogen through a system of two bubblers in
series.  Boron concentration of 1.2 ? 2.4 × 10¹⁹ cm^-3 was
measured by Secondary Ion Mass Spectroscopy (SIMS) for NCD grown with
approximately 0.19% boron in the precursor gas.  Film morphology was examined
by Scanning electron microscopy (SEM) and grain size was found to be less than
500 nm.  Sheet resistance of 260 Ω/sq. and sheet concentrations of 5.8 ?
7.5 × 10¹⁴ cm^-2 were measured by the Van der Pauw method. 
Boron concentrations of 3.8 ? 5.0 × 10¹⁷ cm^-3 were
calculated from the measured sheet concentrations and film thicknesses.  The
nearly two order of magnitude difference in boron concentration measured by
SIMS and calculated from Van der Pauw measurements indicates that not all the
boron in the film is electrically active.  Work is continuing to further
understand this effect and to optimize this doping technique.