LOW-TEMPERATURE ELECTRICAL CHARACTERIZATION OF METAL-NITRIDED OXIDE-SILICON FIELD-EFFECT TRANSISTORS

A detailed investigation of the electrical properties of metal-oxide-s emiconductor (MOS) transistors with gate oxides nitrided for long (3 h ) and short (40 min) times has been conducted as a function of tempera ture (4.2-300 K). The nitrided oxides (NO) and Re-oxidized-nitrided ox ides devices have been fabricated using a low pressure plasma enhanced nitridation in ammonia. The P- and N- channel MOS transistors paramet ers, such as the threshold voltage, maximum mobility, mobility attenua tion 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 i ndependent of temperature for N-channel devices, whereas it decreases at low temperature for P-channel devices. A more pronounced decrease o f the interface state density (measured from the subthreshold slope) a fter nitridation has been found for P-channel devices at low temperatu re. This feature corresponds to a reduction of the donorlike interface state density near valence band and is responsible for a partial comp ensation of the nitridation-induced positive charge in P-channel devic es. The mobility data of N-channel devices clearly show that the nitro gen incorporation close to the interface results mainly in a higher Co ulomb scattering rate, whose coefficient found around 3300 and 1200 V s/C, depending on nitridation dose, is practically independent of temp erature. The corresponding mobility attenuation factor theta is also f ound to decrease after nitridation. The N-channel crossing of the tran sconductance characteristics at high gate voltage, associated with the theta decrease after nitridation, cannot be completely explained by t he influence of the nitridation-induced fixed positive charge. It seem s rather that the nitridation-induced modification of the Si/SiO2 inte rface gives rise to a drastic reduction of the surface roughness relat ed scattering mechanism. This theta reduction due to the nitridation p rocess is shown to be maintained in the whole temperature range studie d for both lightly and strongly nitrided oxides. However, the reductio n of the maximum mobility after nitridation is rather weak for lightly nitrided oxides, even at low temperature. In the case of P-channel de vices, a very different behavior is found. For strongly nitrided oxide s, both peak and high gate voltage transconductance decrease with a mo re pronounced difference between nitrided and non-nitrided devices as the temperature is lowered. For lightly nitrided oxides, the low and h igh field transconductance have been found to remain very comparable t o those of non-nitrided devices. This preservation of hole transport p roperties may be related to the substantial reduction of interface tra p density close to the valence band observed after plasma nitridation, which partly compensates the excess of positive fixed charge. Further more, 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 threshol d voltage optimization at cryogenic temperatures.