Current transport mechanisms in n-type ultrananocrystalline diamond/p-type Si heterojunctions

Nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films were deposited on p-type Si substrates by coaxial arc plasma deposition. The deposited films possessed n-type conduction, and evidently formed pn heterojunctions with p-type Si substrates. The heterojunction devices showed typical rectification properties similar to those observed for conventional abrupt pn heterojunctions. The conduction mechanisms that govern current transport in these devices were analyzed using dark current-voltage measurements in the temperature range from 300 K to 80 K. The results revealed that a trap-assisted multi-step tunneling process is a dominant mechanism at lower temperatures and low forward bias. At least two defect levels with activation energies of 42 and 24 meV appear to activate this process. At moderate forward bias, the current followed a power-law dependence, attributable to a space-charge-limited current. This junction behavior might be owing to a large number of grain boundaries in the UNCD/a-C:H film that provide active centers for carrier recombination-tunneling processes at the junction interface.