Electrical Characteristics of Silicon Nitride Films Prepared by Silane‐Ammonia Reaction

The d‐c conduction and complex dielectric constant of silicon nitride films prepared by silane‐ammonia reaction have been studied as a function of the composition of the gaseous ambient in which they were formed. The d‐c conductivity of the material increases sharply when the silane‐ammonia ratio increases above 0.1. The increased conductivity is characterized by a lowering of the activation energy to about 1.0 ev from 1.5 ev for high resistivity nitride. The current‐voltage characteristics display a In I ‐ V ½ dependence at high fields in the temperature range 200°–500°K. Measurement of the magnitude of the slope of this characteristic and its dependence on film thickness, index of refraction, temperature, and bias polarity suggests that conduction arises from field aided thermal ionization of trapping centers in the bulk of the film, the Poole‐Frenkel effect. It is noted that the magnitudes of the slopes of the experimental In I ‐ V ½ plots, which are twice as large as Schottky emission slopes, are derivable within the framework of the Poole‐Frenkel mechanism only under a special assumption regarding the statistics governing the occupancy of the traps. The relative dielectric constant, ε r , of the films has a constant value of approximately 7 for silane‐ammonia ratios below 0.1 and increases gradually to about 10 for a silane‐ammonia ratio of unity. Very small variations in ε r with frequency and temperature were noted in the range of the measurements, 5 × 10 2 – 10 6 Hz and 200°–500°K. Values of dielectric loss, ε r ″ , lower than 10 −3 have been observed for the low silane‐ammonia ratio films, but these are also dependent on substrate surface preparation. An observed dependence of ε r ” on sample area in some cases suggests the presence of localized film defects in the less carefully prepared samples. No structure in the loss‐frequency‐temperature characteristics of the material has been observed in the above‐mentioned ranges of temperature and frequency that cannot be related to the d‐c conductance of the silicon nitride or to the substrate resistance.