Synthesis and Electrical Characterization of \textit{n}-Type Nanocrystalline Diamond Films by Microwave Plasma-Enhanced Chemical Vapor Deposition

Nanocrystalline diamond (nanodiamond) films are composed of three different carbon phases: the diamond phase in form of nano-sized grains, trans-polyacetylene segments, and amorphous carbon. They are typically formed under C$_{2}$-rich conditions by CVD in Ar-rich/CH$_{4}$ plasmas. $n$-type conductivity in nitrogen-incorporated nanodiamond films is attributed to the formation of electronic states associated with carbon and nitrogen in the grain boundary. However, the origin of the high $n$-type conductivity still remains unclear. The authors investigate structure and electrical properties of $n$-type nanodiamond films prepared from a microwave Ar-rich/N$_{2}$/CH$_{4}$ plasma. The authors also investigate the rectification properties of $p-n$ diodes using $n$-type nanodiamond films. The plasma was characterized by strong emission from C$_{2}$ radicals. The room-temperature resistivity of the films decreased exponentially by three orders of magnitude with deposition temperature and was saturated at $\sim $10$^{-2}\Omega$ cm. The electron concentration increased up to 10$^{20}$ cm$^{-3}$, while the mobility was between 1 and 10 cm$^{2}$V$^{-1}$s$^{-1}$. Arrhenius plots of the conductivity showed a transition from semiconducting to quasi-metallic conduction with deposition temperature. The amount and clustering of the sp$^{2}$ phase were found to affect strongly the electrical conduction properties.