New features of the layer-by-layer deposition of microcrystalline silicon films revealed by spectroscopic ellipsometry and high resolution transmission electron microscopy

Abstract

Spectroscopic ellipsometry and high resolution transmission electron microscopy have been used to characterize microcrystalline silicon films. We obtain an excellent agreement between the multilayer model used in the analysis of the optical data and the microscopy measurements. Moreover, thanks to the high resolution achieved in the microscopy measurements and to the improved optical models, two new features of the layer-by-layer deposition of microcrystalline silicon have been detected: ͑i͒ the microcrystalline films present large crystals extending from the a-Si:H substrate to the film surface, despite the sequential process in the layer-by-layer deposition; and ͑ii͒ a porous layer exists between the amorphous silicon substrate and the microcrystalline silicon film. © 1996 American Institute of Physics. ͓S0003-6951͑96͒02730-1͔In situ ellipsometry is a powerful nondestructive technique providing detailed information on the growth mechanisms and optical properties of thin films.1 In order to get information on the composition of a film, the real ͗⑀ 1 ͘ and imaginary ͗⑀ 2 ͘ parts of the pseudodielectric function of the system film plus substrate, deduced from the ellipsometric angles ⌬ and ⌿, are compared to those of an optical model based on Bruggeman's effective-medium theory. 2 The differences between the measured and calculated data are minimized by a linear regression analysis. In recent years we have applied this technique to study the layer-by-layer deposition of microcrystalline silicon films.3 In this method, microcrystalline silicon films are obtained by alternating the deposition of hydrogenated amorphous silicon ͑a-Si:H͒ during a time T Si with its exposure to a hydrogen plasma during a time T H . The major conclusions deduced from these studies are: ͑i͒ the nucleation of a crystalline phase within the a-Si:H network takes place once the initially dense a-Si:H film has been converted into porous a-Si:H by the hydrogen plasma exposure, 4 ͑ii͒ the crystallization of the a-Si:H film deposited during the time T Si is related to the diffusion of hydrogen, leading to nanovoid and broken bond formation processes, 5 and ͑iii͒ there is a substrate dependence of the long term evolution of the properties of the already deposited films. 6 Because most of our previous results were based on in situ ellipsometry and because of the increasing complexity of optical models used to fit the experimental data, we have performed an independent validation of the optical models by high resolution transmission electron microscopy ͑HRTEM͒ measurements. An excellent agreement between the results of the optical models and the HRTEM measurements has been achieved, in agreement with previous reports.7 Moreover, the HRTEM measurements have allowed us to improve the optical models and reveal new features of the layer-by-layer deposition of c-Si.Microcrystalline silicon films were codeposited on different substrates by the layer-by-layer technique. Figure 1 shows the experimental and calculated real and imaginary parts of the pseudo...

The role of hydrogen in the formation of microcrystalline silicon

“…The preparation of the samples for cross sectional HTREM measurements has been previously described 10 .…”

“…In our previous studies we have combined the LBL technique with in situ ellipsometry analysis of the film properties and validated the optical models with high-resolution transmission electron microscopy (HRTEM) 10 and secondary ion mass spectrometry measurements 11 . We showed that both surface and growth zone processes are involved in the growth of µc-Si:H [12][13] .…”

“…We showed that both surface and growth zone processes are involved in the growth of µc-Si:H [12][13] . Moreover, the optical models used to analyse the ellipsometry measurements indicate that the nucleation of crystallites takes place in a highly porous layer below the film surface 14 . In this study, we combine in situ ellipsometry and conductivity measurements with ex-situ HRTEM measurements to confirm that the nucleation takes place in this highly porous layer.…”

Ultra-thin crystalline silicon films produced by plasma assisted epitaxial growth on silicon wafers and their transfer to foreign substrates

“…It has been demonstrated that in-situ spectroscopic UV-Visible ellipsometry is an excellent tool to characterize the composition of thin films such as microcrystalline silicon [16][17][18][19]. An in-situ UV-Visible spectroscopic ellipsometer (Jobin Yvon -MWR UVISEL) was used to measure the imaginary part of the pseudo-dielectric function (Im[ε]) of the epitaxial films deposited on the c-Si wafer at various stages of the process.…”

“…We used this technique to optimize the plasma cleaning conditions and as well the epitaxial silicon film growth. The thickness of the layers and their composition were determined by modeling the ellipsometry data using the Bruggemann effective medium approximation [16][17][18][19].…”