High C content Si/Si_1 yC_yheterostructures for n-type metal oxide semiconductor transistors

Abstract

We have grown by reduced pressure-chemical vapour deposition Si/Si 1−y C y /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 Si 1−y C y 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 SiH 6 flows, all the incoming carbon atoms are incorporated into substitutional sites. At 550 • C, when the SiCH 6 flow increases, the substitutional carbon concentration saturates at 1.44%. Meanwhile, the total carbon concentration C T still increases, following a simple law: C T /(1 − C T ) = (0.88-0.92)(F(SiCH 6 )/(F(SiH 4 )). 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 SiO 2 -masked wafers, but does not generate any adverse loading effect. We have integrated Si/Si 1−y C y /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 Si 1−y C y layer into the Si cap and the SiO 2 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 Si 1−y C y layers, with a good resulting structural quality.

Reduced pressure–chemical vapor deposition of high Ge content Si1−xGex and high C content Si1−yCy layers for advanced metal oxide semiconductor transistors

“…This way, two times higher F(SiH 4 )/F(H 2 ) and F(SiCH 6 )/F(H 2 ) mass flow ratios than in Ref. [6] have been obtained, hopefully with some gains both in terms of growth rate and substitutional C atoms incorporation.…”

“…550 1C) and high SiH 4 and SiCH 6 partial pressures were very beneficial for obtaining high substitutional C concentration Si 1Ày C y layers [6]. Having reached the maximum mass flows that could be delivered by our SiH 4 and SiCH 6 mass-flow controllers, we have thus divided the incoming flow of the H 2 carrier gas (a few tens of standard liters per minute) by a factor of two.…”

“…Because of the Ge (or C) content and of the compressive (or tensile strain), a type II band alignment occurs between Si and pseudomorphic Si 1Àx Ge x (Si 1Ày C y ), with a valence band offset (conduction band offset) of the order of 0.84x (6y) eV [1]. When capped with thin Si caps, high Ge content Si 1Àx Ge x layers (high C content Si 1Ày C y layers) thus act as efficient buried channels for the holes (for the electrons) in ptype MOSFETs [2][3][4] (n-type MOSFETs [5,6]). …”

“…dichlorosilane+germane, enable to achieve full selectivity versus SiO 2 on patterned wafers. Meanwhile, the silane+mono-methylsilane growth chemistry, which is mandatory in order to obtain high substitutional C content Si 1Ày C y layers [6,7], will not provide any degree of selectivity on dielectric masked wafers. Using lowgrowth temperatures (i.e.…”

“…Adopting low growth temperatures enables, when dealing with the selective epitaxial growth of very high Ge content (25-50%) Si 1Àx Ge x layers inside the Si windows of SiO 2 -masked wafers, to get rid of any elastic strain relaxation through the formation of an undulating growth front [8,9]. Reducing the growth temperature as much as possible (while still keeping growth rates which are not insignificant) permits to have most of the incorporated C atoms in very high C content Si 1Ày C y layers (y41.5%, typically) occupying substitutional sites instead of electrically harmful interstitial sites [5,6].…”