Electrical Characterization of Metal/Al₂O₃/SiO₂/Oxidized-Si-Terminated (C–Si–O) Diamond Capacitors

Metal–oxide–semiconductor (MOS) capacitors with oxidized silicon-terminated (C–Si–O) diamond as semiconductors and a stack of SiO
₂
and Al
₂
O
₃
as gate insulators were successfully fabricated and electrically characterized for the first time. C–Si diamond was first formed by the molecular beam deposition of a Si film and subsequent
in situ
vacuum annealing. The diamond surface turned into C–Si–O when exposed to the air, accompanying a naturally grown SiO
₂
film on top. The MOS capacitors exhibited an excellent electrical insulation capability for gate voltages of depletion and accumulation conditions. A dip specific to deep dopants in the substrate was observed for the first time in high-frequency capacitance–voltage (
${C}$
–
${V}$
) characteristics of diamond MOS capacitors. In the high-frequency
${C}$
–
${V}$
curve, accurate estimation of flat-band voltage was realized by locating it at the observed dip. Additionally, the margin of error of the flat-band voltage estimated using a conventional method of fitting a Mott–Schottky plot to experimental
${C}$
–
${V}$
curves was assessed and attributed to the neglect of interface-state charge. The gate insulator stack is found to contain a positive charge of
$7.8\times10$
¹¹
cm
⁻²
in units of the electronic charge, which clearly rules out the presence of 2-D hole gas and supports the normally- OFF operation of reported C–Si–O diamond devices. By using the high–low-frequency method, interface-state density at the Al
₂
O
₃
/SiO
₂
/C–Si–O diamond interface was estimated to be in the range of
$1.5\times10$
¹¹
–
$2\times10$
¹²
eV
⁻¹
cm
⁻²
for interface-state energies of 0.4–0.82 eV above the valence band maximum of the diamond. These results form a basis for applying the silicon-terminated diamond to electronic devices.