A detailed investigation of the electrical properties of metal-oxide-semiconductor (MOS) transistors with gate oxides nitrided for long (3 h) and short (40 min) times has been conducted as a function of temperature (4.2–300 K). The nitrided oxides (NO) and Re-oxidized-nitrided oxides devices have been fabricated using a low pressure plasma enhanced nitridation in ammonia. The P- and N- channel MOS transistors parameters, such as the threshold voltage, maximum mobility, mobility attenuation factor, and subthreshold slope have been extracted from the ohmic transfer characteristics. The negative shift of the threshold voltage due to the nitridation-induced positive charge has been found to be independent of temperature for N-channel devices, whereas it decreases at low temperature for P-channel devices. A more pronounced decrease of the interface state density (measured from the subthreshold slope) after nitridation has been found for P-channel devices at low temperature. This feature corresponds to a reduction of the donorlike interface state density near valence band and is responsible for a partial compensation of the nitridation-induced positive charge in P-channel devices. The mobility data of N-channel devices clearly show that the nitrogen incorporation close to the interface results mainly in a higher Coulomb scattering rate, whose coefficient found around 3300 and 1200 V s/C, depending on nitridation dose, is practically independent of temperature.
The corresponding mobility attenuation factor θ is also found to decrease after nitridation. The N-channel crossing of the transconductance characteristics at high gate voltage, associated with the θ decrease after nitridation, cannot be completely explained by the influence of the nitridation-induced fixed positive charge. It seems rather that the nitridation-induced modification of the Si/SiO2 interface gives rise to a drastic reduction of the surface roughness related scattering mechanism. This θ reduction due to the nitridation process is shown to be maintained in the whole temperature range studied for both lightly and strongly nitrided oxides. However, the reduction of the maximum mobility after nitridation is rather weak for lightly nitrided oxides, even at low temperature. In the case of P-channel devices, a very different behavior is found. For strongly nitrided oxides, both peak and high gate voltage transconductance decrease with a more pronounced difference between nitrided and non-nitrided devices as the temperature is lowered. For lightly nitrided oxides, the low and high field transconductance have been found to remain very comparable to those of non-nitrided devices. This preservation of hole transport properties may be related to the substantial reduction of interface trap density close to the valence band observed after plasma nitridation, which partly compensates the excess of positive fixed charge. Furthermore, the overall reduction of the interface trap density after plasma nitridation, which results in smaller subthreshold swings for N- and P-type devices, is expected to be very promising for a better threshold voltage optimization at cryogenic temperatures.
