Stress effects in meso-porous silicon nanostructures

phys. stat. sol. (c) 2, No. 9, 3203–3207 (2005) / DOI 10.1002/pssc.200461114

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Stress effects in meso-porous silicon nanostructures

V. Lysenko*1, C. Populaire1, B. Remaki1, B. Champagnon2, H. Artmann3, T. Pannek3, and D. Barbier1 1 Laboratoire de Physique de la Matière, UMR-5511 CNRS, INSA de Lyon, 7, avenue Jean Capelle, Bat.

Blaise Pascal, 69621 Villeurbanne Cedex, France 2 Laboratoire de Physico-Chimie de Matèriaux Luminescents, UMR-5620 CNRS, Université Claude Ber-

nard Lyon I, 43 boulevard 11 novembre 1918, Bat. Lippmann, 69622 Villeurbanne, France 3 Robert Bosch GmbH, FV/FLD, Postfach 10 60 50, 70049 Stuttgart, Germany

Received 10 March 2004, revised 11 September 2004, accepted 27 January 2005 Published online 9 June 2005

PACS 68.35.Gy, 81.07.Bc

Mechanisms of strain and stress appearing in as-prepared and treated meso-porous silicon nanostructures are described. Stress effects induced by nano-pores filling with ethanol and capping of the porous nanos-tructures are observed by micro-Raman spectroscopy and discussed in details. Strong correlations be-tween macroscopic and nanoscale stresses as well as with earlier X-ray measurements are found. Influ-ence of stress on thermal properties of meso-porous silicon nanostructures is demonstrated.


1 Introduction

Stress effects are well known to accompany almost all interfaces and multi-layers formations. High stress values can provoke structural deformations and defects at the near-interface regions without remarkable changes of physical properties of the bulk Si substrates. However, if at least one of the dimensions of the Si substrates (for example, at least thickness, in the case of thin Si films) becomes comparable or less than that of the stress affected near-interface region, it can lead to significant modifications of all physi-cal properties of such low dimensional Si structures [1, 2]. Friedersdorf et al. remarked stress induced blue shift of photoluminescence spectra of Si nanocrystals constituting a low-dimensional porous net-work [3]. Indeed, physical properties of Si nanocrystallites constituting porous Si (PS) nanostructures with huge surface-volume ratio, S ≈ 200-800 m2/cm3 [4] should be strongly influenced by strains and stresses coming from the near-surface regions of the Si nanocrystallites.

In this work, we discuss the nature of the initial strain and stress in an as-prepared nanostructured meso-PS sample and describe the stress evolution when the chemical composition of the Si nanocrystal-lites surface and nano-pores inside the meso-PS layer is modified. In particular, stress induced variation of thermal properties is shown and stress impact on other meso-PS properties is suggested.

2 Experimental procedures

Nanostructured meso-PS layers were formed by means of well known anodic dissolution process [5] that is usually performed in HF-based electrolytes. In this work, monocrystalline (100)-oriented highly doped p+-type Si wafers with resulting electrical resistivity of about 0.02 Ω cm and a standard electrochemical cell with metallic back-side electrode [5] were used. The anodization was performed in galvanostatic regime under day light by using HF(50%) acid diluted in ethanol [5]. As a result, a network of nanoscale quasi-columnar pores forming the meso-PS samples was obtained [6]. To ensure different porosity va-

* Corresponding author: e-mail: [email protected], Phone: +33 472 437 002, Fax: +33 472 436 082

3204 V. Lysenko et al.: Stress effects in meso-porous silicon nanostructures


lues (estimated from gravimetrical measurements), the samples were anodized at different anodization current densities. After fabrication, the porous samples were rinsed and dried in a N2 flux.

In order to investigate stress in micro- and nanostructured Si, Raman scattering is often used [7-11]. In this work, using an Ar laser with a wavelength of 514 nm focused on the surface or on the cross-section (for depth resolved measurements) of the studied meso-PS samples, the micro-Raman backscat-tered spectra were recorded at room temperature by means of an Olympus BH2 microscope (objective ×50), coupled with a Dilor XY monochromator operating in high resolution mode (1.1 cm–1 for a 100 µm slit) and a CCD detector cooled by liquid nitrogen. Low power (0.1–0.5 mW) of incident 5µm diameter laser beam was chosen to avoid any laser induced local heating effects on the recorded Raman spectra. Since crystalline properties of the meso-PS are quite close to a perfect silicon single crystal [12, 13], symmetry properties of the Raman tensor are assumed to be similar to those of the bulk Si allowing only longitudinal optical (LO) phonons (point Γ) in the first-order Raman scattering at the (100) Si surface. Raman peak positions measured in the backscattering configuration on the meso-PS samples formed on (100) oriented Si substrates are influenced by the X-Y biaxial stress. The biaxial X-Y in-plane stress, σ||, was deduced from Raman spectra according to the following relation [9]:

σ|| (MPa) = –250×∆ω (cm–1) (1)

where σ|| is the in-plane stress and ∆ω = ωs– ω0. In this expression, ω0 is the LO phonon wave number of the stress free sample and ωs is the wave number of the stressed sample. Negative or positive stress val-ues correspond to compressive and tensile stress, respectively. A part of the Raman peak shift due to the nanometre dimension of the Si crystallites constituting the porous layers was deduced from the total peak shift by using a calibration curve calculated in [14]. In such a way, only stress induced Raman peak shift component was taken into account and analyzed.

Thermal conductivities of the porous layers were measured by micro-Raman spectroscopy using ap-proach described in details in [15].

3 Results and discussion

3.1 Stress and strain in as-prepared meso-PS layer

First of all, let us discuss the origins of initial strain and stress in as-prepared PS samples stored in air [7, 8, 12]. After fabrication and storage in ambient air, patterned meso-PS layers are found to be expanded in Z-direction perpendicular to the wafer surface (Fig. 1). For these patterned samples, a Z direction strain of about 2×10–3 is found in central part of the layer from a ratio

zd d∆ , where d is the porous layer

thickness (100 µm in our case) and ∆dz is the layer thickness increase (layer step height: 0.2 µm in our case) [16]. This result is in good agreement with numerous X-ray diffraction measurements [17] detecting relative increase of the lattice parameter

za a∆ in the aged meso-PS samples up to 2×10–3 compared to

bulk Si. Such proximity of the macroscopic and sub-nanoscale strains can be explained by an elastic nature of the inter-atomic distance increase induced by the nanoscale porosification of bulk Si and air storage of the formed porous nanostructures.

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Fig. 1 2D profile of strained patterned as-prepared meso-PS layer.

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The origin of the initial strain is shown to be related to the presence of hydrogen (fresh PS) and/or oxygen (aged PS) covering the Si nanocrystallites surfaces [12, 17–19]. Indeed, H-terminated Si atoms situated at the nanocrystallites surfaces relax toward the formed nano-pores because (i) of the decrease of the binding energy of the surface Si atoms and (ii) of the mutual attraction between the hydrogen covered surfaces due to van der Waals forces [18]. The relaxation is accompanied by increasing of Si–Si bond length mainly between the atoms situated at the nanocristallites surfaces which can provoke conse-quently a similar bond length increase between the atoms even inside the nanocrystallites resulting in a mean lattice parameter increase. A gradient of the Si–Si bond length can also take place along the nanoc-ristallites radius and the inter-atomic bond length mainly inside the large (> 10 nm) nanocrystallites can be the same as in monocrystalline Si. Such a relaxation induced inter-atomic bond length gradient is accompanied by a nanoscale stress appearing at the level of each nanocrystallite.

Two possibilities exist for the Si nanocristallites relaxation after the nano-pores formation: (i) in the direction (Z) perpendicular to the porous layer surface and (ii) in the plane (X-Y) parallel to the layer surface. Numerous X-ray diffraction measurements performed on (100) oriented highly doped Si sub-strates demonstrate an increase of the lattice parameter only in the Z-direction (

za a∆ ≈ 10–4–10–3) and

almost none in the X-Y plane ( x ya aD-

≈ 0±5×10–5) [12, 13]. This could be understood by taking into account that (i) the majority of the columnar Si nanocrystallites constituting a meso-PS layer are strongly interconnected by multiple nanoscale branches [6] and (ii) the whole porous layer is strongly squeezed by surrounding bulk Si rending much more difficult (and even impossible in the case of small PS re-gions) relaxation in the direction of X-Y plane parallel to the layer surface.

In general, such a relaxation of Si nanocrystallites results in the macroscopic relaxation of the whole porous layer. If the PS layer volume is much smaller than that of the whole Si wafer, uniaxial lattice strain in the Z-direction is transformed in meso-PS layer expansion toward the free space (Fig. 1). In opposite case, if the PS layer volume (several hundred microns of thickness and occupying almost all surface of the Si wafer) is lager than that of the remaining Si frame, the nanocrystallites relaxation can also be transmitted in the X-Y plane leading to a curvature of the whole PS/mono-Si substrate that can be easily visible even to the naked eye.

3.2 Stress effect induced by nano-pores filling

One of the easiest and the most effective way to vary the initial stress level of each Si nanocrystallite and therefore of the whole as-prepared porous nanostructure consist of the nano-pores filling with a sub-stance. We have chosen ethanol because of its excellent properties of the hydrophilic interaction with Si surface ensuring very efficient nano-pores filling. In order to avoid an intensive ethanol evaporation provoking a temperature decrease at the surface of the porous sample which could perturb significantly the Raman measurements, the ethanol-filled meso-PS layer was covered by a thin optically transparent (for visible light used in the Raman set-up) mica film.

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Fig. 2 Stress effect on the Raman scattering spectrum of meso-PS nanostructure resulting from filling of nano-pores with ethanol at ambient temperature and pressure.

3206 V. Lysenko et al.: Stress effects in meso-porous silicon nanostructures


Figure 2 shows that ethanol filling induced Raman spectra evolution of meso-PS nanostructure. The Raman peak corresponding to the ethanol-filled sample (i) shifts toward higher phonon energies ap-proaching to the peak position of bulk Si substrate and (ii) its full width at half maximum (FWHM) be-comes narrower compared to the state without ethanol. From stress evolution point of view (according to Eq. (1)) and in comparison with the initial stress state (before the ethanol filling), one can conclude on appearance of a compressive stress of 250 MPa in the nanocristallites after the ethanol penetration inside the pores. This can be explained by change of surface stress of the Si nanocristallites induced by the ethanol confined inside the nano-pores and exercising compression of the nanocrystallites. The Raman peak narrowing reflects Si atoms ordering improvement. Such a stress can be partially relaxed by addi-tional porous layer expansion in the Z-direction. Indeed, the lattice strain za aD increase on 2×10–4 was detected experimentally [12, 20]. An important point should be remarked: after ethanol evaporation from the pores, the Raman peak returns back to its almost initial position corresponding to the stress state before the ethanol introduction inside the nano-pores. Thus, the effect seems to be quasi-reversible indi-cating its elastic nature. A small difference between the initial and the post-ethanol states can be ex-plained (i) by none complete ethanol evaporation from the nano-pores or (ii) by possible slight changes in the Si nanocristallites surface chemistry due to the ethanol presence inside the porous nanostructure as it was also earlier observed by X-ray diffraction measurements [20].

3.3 Stress effects induced by capping layers

Technological applications of meso-PS nanostructures often require their protection from external envi-ronment and functional elements. The capping layers can introduce an additional stress modifying physi-cal properties of the porous Si nanostructures.

Figure 3a shows impacts of the LPCVD Si3N4 and SiN capping layers deposition on the Z-direction relaxation of the partially oxidized meso-PS structure. In particular, both capping layers ensure smaller expansion of the porous samples compared to the case with no capping. In-depth resolved nanoscale stress measurement performed by Raman spectroscopy (Fig. 3b) indicates an important compressive stress appearing in the porous capped samples compared to uncapped state. A strong correlation between the layer expansion and compressive stress values is noted: the smaller expansion of the sample in the Z-direction is (Fig. 3a), the higher porous sample compression by the capping layer is (Fig. 3b). Of course, both capping layer nature and thermal budget of the capsulation procedure are responsible on such result-ing stress levels.

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Fig. 3 Stress effects induced by presence of layers capping a patterned oxidized (600 °C) meso-PS nanostructure of 70 % porosity: a) 2D profiles of the patterned meso-PS samples with and without capping layers; b) in-depth resolved nanoscale stress measurements; c) in-depth resolved thermal conductivity measurements.

In-depth resolved thermal conductivities of the meso-PS structures with and without capping layer are

shown in Fig. 3c. The thermal conductivities values are in direct correlation with stress levels ensured

phys. stat. sol. (c) 2, No. 9 (2005) / www.pss-c.com 3207


by the capping layers (Fig. 3b). Indeed, the higher meso-PS compression ensured by capping layer is, the higher its thermal conductivity is. Ensuring enhancement of the porous network percolation strength or maybe even the porosity reduction, the meso-PS layer compression leads therefore to the thermal con-ductivity increase according to the thermal conductivity model described in details earlier [15].

4 Conclusions Nanoscale porosification of monocrystalline Si leads to a new structural equilibrium state of the formed porous nanostructures. This mechanical state can be easily modified by the chemical environment around the Si nanocrystallites. The stress effects can provoke changes of all physical properties of the nanocrys-tallites that should be considered if any post-anodisation chemical treatment or capping of the porous nanostructure are used.

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(1999), and references therein. [18] E. Vázquez, J. Tagüeña-Martínez, L. E. Sansores, C. Wang, J. Appl. Phys. 91, 3085 (2002). [19] P. Carrier, L. J. Lewis, M. W. C. Dharma-wardana, Phys. Rev. B 65, 165339 (2002). [20] D. Bellet, G. Dolino, Phys. Rev. B 50, 17162 (1994).

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Stress effects in meso-porous silicon nanostructures