Nucleation and Growth Mechanisms of Polycrystalline Sige-Based Materials By Chemical Vapor Deposition at Reduced Pressure
Authors: Vives Jeremy , Chaussard Julie , Deprat Fabien , Frauenrath Marvin , Aybeke Ece , Dabertrand Karen , Chaussende Didier 
Published: December 2024

Polycrystalline Si and SiGe-based materials deposited on dielectrics have a wide range of key applications, like as gate materials for complementary metal oxide semiconductor (CMOS) devices [1-2], as active layers in thin film transistors (TFTs) [3] and as dopant diffusion sources to form ultra-shallow elevated junctions [4]. The control of the Si or SiGe nucleation on dielectric surfaces is also a key towards selective epitaxy at low growth temperature. At these low growth temperatures, the standard selective process, where the precursors and the etching gas are injected together, was replaced by Cyclic Deposition Etch (CDE) processes [5]. It generally consists of the repetition of two steps: A non-selective deposition with epitaxial growth on Si regions and amorphous/polycrystalline deposition on dielectric regions. This is followed by an in-situ Cl-based etching, usually with HCl. Such CDE processes would benefit from having the lowest deposition rate on the dielectrics with respect to the epilayer. The determination of key parameters that characterize the nucleation process on dielectric surfaces, such as incubation time, nucleation rate, coalescence point, and the final resulting microstructure is therefore beneficial to optimize the deposition/etch cycles. Pure Si deposition on dielectric surfaces was widely studied in various CVD reactors, but the nucleation and growth mechanisms of SiGe-based materials has not been studied as extensively.
The SiGe nucleation was investigated in a Reduced-pressure-Chemical Vapor Deposition (RP-CVD) reactor at 550 °C, 10 Torr, on blanket Si wafers covered with 10 nm of thermally grown SiO2 using either a SiH4 or Si2H6-based chemistry with GeH4. Using Si2H6, SiGe polycrystalline films were obtained even for low deposition times, in contrast with the use of SiH4, which led to a progressive increase in both density and size of the SiGe nuclei. This may be due to the higher reactivity of the Si2H6 precursor compared to the SiH4. The weaker Si-Si bonds than Si-H bonds resulted in easier chemical decomposition of the silicon precursor and the formation of hydrogenated Si sub-species (intermediates) with Si2H6 than with SiH4. This may lead to a higher frequency of effective collisions at the surface when using the Si2H6 chemistry, producing an extremely fast saturation of the nuclei density and a favorable growth of the nuclei. Consequently, two different film morphologies were obtained, depending on the type of silicon precursor used. The polycrystalline SiGe deposited using a Si2H6-based chemistry (Fig.1a) exhibited a smoother film (RMS = 1.33 nm) compared to a SiH4-based chemistry (RMS = 21.06 nm) (Fig.1b) and a large grain microstructure with SiH4 (Fig.1d), whereas with Si2H6 (Fig.1c), the microstructure was columnar with small grains.
The SiGe nucleation, using SiH4, was compared between 10 nm SiO2 and 10 nm Si3N4 surfaces. Due to the stronger O-H bond than the N-H bond, the Si3N4 surface was found to be more reactive than a SiO2 surface, resulting in significantly faster nucleation kinetics on Si3N4. This led to different microstructures for thicker films (Fig2). AFM and SEM images showed smoother film with a small grain microstructure for films deposited on Si3N4 (RMS = 2.96 nm) (Fig2.b).
Finally, the nucleation mechanisms on thermally grown SiO2 were investigated using a SiH4-based chemistry with various GeH4, SiH3CH3 and PH3 flows. The incubation time was not affected by the GeH4 flow with a common inflexion point around 20 s (Fig.3). Increasing the GeH4 flow reduced the nucleation rate (from 7.5x10-2 to 1.1x10-2 cm-2. s-1) and decreased the density of nuclei at saturation (from 4.40x109 to 0.74x109 cm-2). These results highlighted the advantage of high GeH4 flow ratios to achieve selectivity towards SiO2 surfaces.
The introduction of SiH3CH3 into the gas mixture, either at small (F(SiH3CH3/F(H2)=8.3x10-6) or large flows (F(SiH3CH3/F(H2)=6.7x10-5), did not affect the nucleation mechanisms. In contrast, the addition of a small amount of PH3 (F(PH3/F(H2)=1.7x10-7) greatly reduced the density of nuclei at saturation and increased the incubation time (from 20 to 41 s) which might be due to the poisoning effect of P adatoms on the surface.
This study will help to further understand the SiGe-based polycrystalline growth and optimize the deposition steps for cyclic deposition etch (CDE) approaches, with a detailed investigation of the nucleation and growth mechanisms as a function of precursors, the nature of the dielectric layer and the incorporation of impurities.
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[3] T..J.King, IEEE Trans. Electron Devices, ED-41, 1581 (1994)
[4] D.T.Grider, J. Electron. Mater., 24, 1369 (1995)
[5] M.Bauer, US Patent 0234504 A1 (2006)

Figure 1