The incorporation of germanium and carbon by RP-CVD epitaxy in silicon brings lots of interest in silicon devices thanks to the variety of properties that can be addressed: band gap or lattice parameter engineering, dopant diffusion reduction, chemical properties, optical properties...

For all the applications requiring SiGeC materials, carbon atoms must be incorporated in fully substitutional site (C_s) despite its low bulk solubility into the Si and SiGe lattice (3x10¹⁷ at.cm^-3 at the Si melting point). Beyond a total carbon concentration (depending on process parameter), carbon atoms are also incorporated into interstitial sites (C_i). These C_i atoms usually form extended defects such as clusters or SiC precipitates which are harmful for devices performances.

Solubility limits of carbon can be extended into the metastable domain by optimizing the epitaxy process. Indeed, it is well established that low temperature and high growth rate are favorable to obtain high C_s atoms incorporation [1].

This work compares silane (SiH₄) and disilane (Si₂H₆) precursors coupled with germane (GeH₄) and methylsilane (SiH₃CH₃ or MS) to process low temperature thin film of SiGeC with the highest amount of C_s without C_i.

First, the SiGe growth kinetics using silane and disilane have been studied. The temperature range investigated varied from 500 °C to 600 °C at low pressure. Using similar atomic fluxes of Si, and with all the other parameters constant, a higher growth rate and lower germanium content has been observed with disilane compared to silane. For instance, there is a factor of 1.85 at 550°C. The germanium concentration was varying from 30.4 to 22.3 % and from 19.4 to 15.7 % for silane and disilane, respectively.

To have a better understanding about this kinetics difference, an impoverishment rate or theoretical yield, of the gas phase, in reactive species has been calculated. The hypothesis is that the impoverishment rate is linked to the reactivity of a molecule, and thus related to the sticking probability of a molecule onto a same surface [2].

The impoverishment rate is defined as the ratio between the number of moles of Si (or Ge) deposited per minute (n_Deposited) and the number of moles of silicon precursor (or germanium) injected per minute(n_Injected) in the epitaxy reactor: α_{Precursor =} n_Deposited/n_Injected.

The impoverishment rate of the disilane molecule increases much more rapidly than the silane molecule as the temperature increases (Fig.1a). Disilane is more consumed than silane which is relevant of the higher reactivity of disilane compared to silane.

Moreover, the impoverishment of germane increases more rapidly with disilane than with silane (Fig.1b). The GeH₄ contribution to epitaxy is more significant using disilane than silane. The impoverishment rate of precursors according to the germane flow will be presented later.

Then, silane and disilane precursors have been compared with respect to the C incorporation at 550 °C. Ge concentration was fixed to 20 % with both silicon precursors. This leads to a growth rate of SiGe equal to 2.9 and 14.1 nm.min^-1 using silane and disilane, respectively. XRD has been used to gain access to the “apparent” Ge concentration in Si_1-x-yGe_xC_y layers (Fig.2(a), (b)). Carbon atoms, much smaller than silicon and germanium, compensate the compressive strain induced by germanium in the SiGe layers. It then yields smaller Ge concentration in XRD than the real Ge concentration. The carbon in substitutional position can therefore be extracted using this coefficient: 1 % of carbon compensate 12 % of Ge [3].

Using disilane, the substitutional carbon concentration linearly increases with the MS flow over the whole studied range (Fig.3). However, using silane, a deviation from the linearity is observed (above 0.7 %). In the linearity part of the curves, all the carbon incorporated are in substitutional site (as the total amount of carbon atoms increasing linearly with the MS flow [4]). When a non-linearity occurs, C atoms started to be incorporated in interstitial position also. SIMS measurement will be performed to confirm that.

At 550 °C, the growth rate of SiGe is more important with disilane precursor due its higher reactivity than silane. Its leads to a better substitutional carbon incorporation. Indeed, using disilane, up to 1.15 % of fully substitutional carbon can be reached while using silane, only 0.7 % is reached.

[1] V. Loup et al., J. Vac. Sci. Technol. B 21(1), Jan/Feb 2003.

[2] D.J. Robbins et al., Journal of Applied Physics 69, 3729 (1991).

[3] D. De Salvador et al., Phys. Rev. B, vol. 61, pp. 13005, 2000.

[4] V. Loup et al., J. Vac. Sci. Technol. B 20(3), May/Jun 2002.