High C content Si/Si1−yCy heterostructures for n-type metal oxide semiconductor transistors

Abstract

We have grown by reduced pressure–chemical vapour deposition Si/Si1−yCy/Si heterostructures for electrical purposes. The incorporation of substitutional carbon atoms into Si creates a carrier confinement in the channel region of metal oxide semiconductor (MOS) transistors. Indeed, tensile-strained Si1−yCy layers present a type II band alignment with Si, with a conduction band offset of the order of 60 meV per atomic% of substitutional carbon atoms. For small SiH6 flows, all the incoming carbon atoms are incorporated into substitutional sites. At 550 °C, when the SiCH6 flow increases, the substitutional carbon concentration saturates at 1.44%. Meanwhile, the total carbon concentration CT still increases, following a simple law: CT/(1 − CT) = (0.88–0.92)(F(SiCH6)/(F(SiH4)). This is a sign that a growing number of C atoms incorporates into interstitial sites. The hydrogenated chemistry adopted does not enable us to achieve selectivity over SiO2-masked wafers, but does not generate any adverse loading effect. We have integrated Si/Si1−yCy/Si stacks (which have been shown to be stable versus conventional gate oxidations and electrical activation anneals) into the channel region of ultra-short gate length (40 nm) nMOS transistors. Secondary ion mass spectrometry profiling has shown that C atoms segregate from the Si1−yCy layer into the Si cap and the SiO2 gate, but also that they block the diffusion paths of B coming from the anti-punch through layer towards the gate, generating very retrograde doping profiles. The addition of C leads to a slight degradation of the electron mobility which seems to be linked to the presence of C atoms into interstitial sites. Finally, we have shown that using higher silane and methylsilane mass flows enables us to obtain higher substitutional C concentrations (max: 1.98%) in our Si1−yCy layers, with a good resulting structural quality.