SAXS study of the influence of the porous silicon morphology on the photoluminescence efficiency

LUMINESCENCE

Journal of Luminescence 57 (1993) 141—145

JOURNALOF

SAXS study of the influence of the porous silicon morphology on the photoluminescence efficiency Ph. Goudeaua, A. Naudona, A. Halimaoui~’and G. Bomchil” Laboratoire de Métallurgie Physique (URA 131 du CNRS) Université de Poitiers 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France b France Télécom-CNET, BP 98, 38243 Meylan Cedex, France

In this study, we applied small-angle X-ray scattering to investigate the microstructure and the morphology of photoluminescent porous silicon samples prepared by different ways, i.e. highly porous silicon layers produced by electrochemical dissolution of bulk Si in HF, and also lower-porosity layers subsequently oxidized by electrochemical anodisation. The modification which occurs in the scattering pattern after an oxidation treatment indicates the formation of a oxide monolayer at the interface pore—matter. The microstructure obtained after dissolution of this oxide layer in HF is compared with the one of a porous silicon having the same porosity but prepared by electrochemical dissolution. Differences between the scattering profiles appear in the region close to the angular origin which corresponds to large particles (voids or Si skeleton), although the part of the curve corresponding to the pore surface remains similar.

1. Introduction Highly porous silicon (PS) obtained by electrochemical dissolution of bulk Si is a efficient visible light emitter [1]. A great diversity of explanations for the photoluminescence (PL) of PS have been proposed [2]. The most likely hypothesis for the “normal” visible light emission is the quantum size effect of an inhomogeneous size distribution of silicon crystallites [3]. In such nanocrystalline structure, the number of surface atoms which are all located on the outer surface is greater than the one of the core atoms. Then the passivation state of the internal surface of pores certainly determines the photoluminescence efficiency [4]. Thus, even if the porous silicon is a three-dimensional nanocrystalline structure, its main characteristic is the surface. The models now developed, which recognize the quantum characteristic of the optical absorption, Correspondence to: Dr. Ph. Goudeau, Laboratoire de Metallurgie Physique (URA 131 du CNRS), Université de Poitiers, 40 Av. du Recteur Pineau, 86022 Poitiers Cedex, France. 0022-2313/93/$06.00 © 1993 SSDI 0022-2313(93)E0094-E

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account for the luminescence in terms of electronic states confined to the surface of the quantum particles. Recently, it has been shown that an anodic oxidation transforms the layer in such a manner that it becomes a bright emitter in the blue [5]. It has been demonstrated that the anodic oxidation process directly controls the non-radiative processes via the passivation of the silicon nanocrystallite. Furthermore, this oxidation treatment provides good mechanical properties to the porous layer. Structural evidence for the existence of quantum size has been previously obtained by small-angle X-ray scattering (SAXS) from light-emitting porous silicon [6—8].In highly porous silicon produced by electrochemical dissolution of bulk Si in HF [6], a large increase of the pore size is evidenced when the porosity is increased from 55% to 85%, while at the same time the silicon skeleton becomes thinner. In the case of 70% porosity layers subsequently oxidized by electrochemical anodisation SAXS results [8] showed that the pore size decreases due to the formation of an oxide layer on the inner porous

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/ Morphology

silicon surface, leading to a blueshift in the photoluminescence spectra. In both cases, the morphology of the layers is found isotropic and the microstructure can be described by a 3D network of very small Si crystallites leading to a local one- or two-dimensional quantum size effect in a disordered structure. However, the PS structure in oxidized PS consists of roughly spherical Si particles embedded in silicon dioxide while highly PS is believed to have a “wool-ball” structure. This difference may lead to a different photoluminescence efficiency, but the passivation of the Si nanocrystallites is certainly different and must be taken into account. In this study, we tried to explain the contribution of the PS morphology to the photoluminescence efficiency. We compared different samples of the same porosity but obtained by different preparation processes.

2. Experimental 2.1 Sample preparation The PS layers studied here were formed by electrochemical etching in ethanoic HF of a 5 cm diameter p-type silicon wafer (100) oriented with a resistivity of 1 L~cm. Such PS layers are called “asprepared” samples. In order to minimize the X-ray absorption in the transmission mode, and thus to get a good signal to background ratio, the silicon substrate were chemically thinned down from 300 j.tm to a uniform thickness of 100 ~im prior to

of PL porous silicon samples

porous silicon formation. Pieces of 1 x 1 cm were cut in the wafer in order to fit the X-ray diffractometer holder. The PS layer thickness and porosity were determined on separate samples by gravimetric measurements. The thicknesses are of about 5 tim. Three PS samples having a same porosity (80%) have been prepared using different electrochemical treatments (see Table 1): (i) as-prepared, (ii) as-prepared 65% porosity, followed by an oxidation treatment and an oxide dissolution in HF, (iii) as-prepared 65% porosity followed by an HF dissolution. The anodic oxidation treatment was done in the same electrochemical set-up than the one used for the preparation of the “as-prepared” samples. More details on anodic oxidation of PS have been published elsewhere [9]. At the end of the oxidation process, the exchanged coulombic charge is C0. The layer then consists of silicon crystallites embedded in silicon dioxide and electrically isolated from the substrate. After the oxidation treatment, the volume expansion resulting from the transformation of the silicon to silicon dioxide induces a diminution of the starting porosity value. In our case, the oxidation level and the resulting porosity were equal to C0/2 and 48%, respectively.

2.2. SAXS experiments SAXS experiments have been performed at LURE, the French Synchrotron Radiation facility

Table 1 Comparison between the different structural parameters obtained by SAXS on three PS samples of the same porosity (80%) and prepared using different electrochemical processes Samples (80% porosity)

Interface

Qo (el/A6)

R5 (nm)

A: as-prepared

P = — 3.65 Roughness (a = 0.35)

0.093

2.25

B: as-prepared 65% + oxidation + HF dissolution

P = — 3.75 Roughness (a = 0.25)

0.073

1.85

C: as-prepared 65% + HF dissolution

P = — 3.65 Roughness (a = 0.35)

0.082

2.40

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/ Morphology .of PL porous silicon

at Orsay, on beam line D22. The experimental apparatus used in our study has been described elsewhere [10]. Two sets of slits are used to define a pinhole collimated beam (cross-section ~ 1 mm2). The incident X-ray intensity and the sample absorption are measured using two similar Nal scmtillators. A sample holder with six positions, maintamed in a vacuum of 1.3 Pa, allows the automatic measurement of six different samples including also the parasitic scattering (virgin silicon). A one-dimensional position-sensitive counter records the scattering intensities as a function of the scattering vector q = 4it sin 0/2, 20 being the scattering angle and 2 the X-ray wavelength (1.5418 A). Two sample—detector distances have been chosen in order to extend the investigated q range: = 255mm and D 2 = 740 mm, so the minimum q value is q~,= 8 x 1O~A~.A reference sample, measured under the same conditions, allows one to obtain the We scattering intensities an absolute scale (el/As). subtract a SAXSoncurve of the virgin silicon in order to remove the background scattering,

samples

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high q-limit, the scattering intensities are often proportional to a negative power of the scattering vector q: i(q)ccq” [11]. The exponent p of the power law is then deduced by plotting log [i(q)] versus log (q). From this exponent, one can obtain information about the pore—matter interface: (i) if the surface is smooth, 1(q) falls off as q4 (Porod’s law), (ii) for rough surfaces, the absolute value of p is smaller than 4. The scattering law is then expressed by q(3+~)with 0 < ~ < 1, ~ being equal to 0.5 most of the time [12], (iii) a finite width of density transition a at the interface may produce deviations from Porod’s law with p > 4 [13]. If we assume that the transition zone between the two phases corresponds to a Gaussian distribution, one can determine a when plotting log[q41(q)]versus q2. In the case of nonparticulate two-phase systems such as highly PS solids, Porod introduced the concept of chords: a line crossing through the and sys1m in matter tern cut[6]. out From alternating chords Porod law, one 1.. inwill voids the classical can deduce an average intersect length <1> and we can distinguish chords in voids and in matter , with <1,,> = /(1 c) and = <1>/c. This concept cannot be used in the case of rough surfaces. Nevertheless, an estimation of the particle size R (voids or matter) can be obtained using the Guinier approximation in the region qR <<1 [14]. This law is valid for dilute systems of randomly oriented particles which is not exactly the case in highly PS systems. Hence, the values of the particle size (pores or Si crystallites) deduced from this approximation are rough estimates. —

2.3 SAXS analysis

In PS solids, SAXS curves result from an electron density contrast between voids (pores) and matter (silicon). When the scattering intensity 1(q) is isotropic, one can determine the integrated intensity Qo. Its value is obtained from the experimental scattering curve using the following formula [h1] Qo

=

(1/27t2)Jq2i(q)dq

(1) 3. Results and discussion

and can be calculated, in the two-phase model, according to 1 2 2 Qo c( c) where c is the porosity, and p~the electronic density of the silicon crystallites (0.7 el/As). In order to extract structural parameters, two regions in the spectra have to be distinguished: the region corresponding to large objects such as the Si skeleton (small q range) and the one linked with small objects (pores or Si crystallites) along with interfaces between pores and matter (large q range). In the — —

—

.~

The characteristic log—log curves of as-prepared samples have a general trend that we have already obtained with other p-type samples [6]: two linear parts are separated by a shoulder (Fig. 1). In the large q range, related to the interface pore—matter, the Porod law is obeyed for the sample having a porosity of 65%. The average intersect length in voids and matter are equal to 4.1 and 2.2 nm, respectively (<1> = 1.4 nm). The experimental and theoretical Qo values are similar and equal to 0.115 el/A6. When increasing the porosity till 80%,

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/

Morphology of FL porous silicon samples

the slope becomes equal to 3.65 which indicates a roughness of the interface with ~ = 0.35. This feature was not previously observed for asprepared highly PS and can be attributed to different sample preparation conditions which are very critical in the case of highly PS. In this case, the concept of chords cannot be applied and we used the Guinier approximation to get information about the particle size evolution (pores and Si crystallites) with increasing porosity. The radius of gyration increases from 1.75 to 2.25 nm as the porosity increases from 65% to 80%. This effect is correlated with an increase of the average intersect length <1>. Recently, we showed [6] that this increase was more important for <1w> than for ~ Furthermore, the decrease of the slope of the log—log scattering profile at smallest q value was due to a dramatic thinning of the silicon skeleton. After an oxidation treatment of the as-prepared samples having a porosity of 65%, the shape of the SAXS curves is modified in the large q range (Fig. 2). The exponent of the power-law scattering is equal to —4.95 which indicates the existence of a diffuse interface (“fuzzy” surface). The width a of the transition zone between pores and matter is equal to 0.36 nm. The average chord dimension in matter decreases from 2.2 nm down to 1.7 nm after the oxidation treatment. These results clearly evid—

ence a reduction of the Si crystallite size which is in agreement with the model where the PL blue-shift is due to the thinning of Si crystallites [8]. The log—log scattering spectra measured on three different samples of the same porosity but prepared by different electrochemical processes (see Table 1) are shown in Fig. 3. The structural

4

(1) .~

(2) —

I

I

I

I I I I

I I I

I I

I

I I I

log (q) (Ac’) .

Fig. 2. Modification of the log—log SAXS spectra due to the anodic oxidation treatment in the as-prepared PS sample having a porosity of65%: (1) as-prepared sample; (2) oxidized sample.

4.

~

)

I

...

-I

-2

log (q)

(A-i)

Fig. 1. Evolution of the log—log SAXS profiles as a function of the porosity for the as-prepared PS samples: (1) 65% porosity; (2) 80% porosity.

o

I

I I

I I I I

I

I I

I I I

I I I I

Il

log (q) (Ac’) Fig. 3. Comparison between the log—log SAXS spectra obtained for three PS samples of a same porosity (80%) prepared by different electrochemical processes: (A) as-prepared; (B) as-prepared 65% + anodic oxidation + HF dissolution; (C) asprepared 65% + HF dissolution.

Ph. Goudeau et a!. / Morphology of FL porous silicon samples

parameters obtained by SAXS are reported in Table 1. At large q values, the scattering profiles are similar and the exponent of the power-law scattering, which is larger than 4 (Table 1), indicates the existence of a rough surface for the three samples. The roughness coefficient at is smaller (0.25) for the PS layer prepared via an oxidation treatment (sample B) than the one (0.35) obtained for the two other samples (A and C). Also, the radius of gyra-

145

ity and prepared by different preparation methods, small differences in structure and morphology which are relevant to differences in PL response.

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tion is smaller (1.85 nm) for this sample (B) compared to the one determined for the two other samples (A: 2.25 and C: 2.40 nm). Thus, the Si crystallite size for the sample prepared via an oxidation treatment is smaller than the one obtained for non-oxidized samples. The experimental Qo value determined with eq. (1) for the samples noted B and C (Table 1) is close to the theoretical one calculated with eq. (2) (0.078 el/A 6). Let us notice that for the as-prepared sample A, this experimental value of Q~is slightly greater (0.093 el/A6) than the theoretical one. This can be due to an uncertainty on the thickness of the layer obtained by gravimetric measurements. The most important effect which can be seen in Fig. 3 is that the log—log scattering profiles are different at small q values: the slope of the powerlaw scattering of the samples B and C is greater than the one for sample A. As this small q range is related to the Si backbone, this means that Si skeleton is thinner for the as-prepared sample than the one for samples B and C [6]. Although the PL spectra are similar in wavelength [15], there are differences in their intensities: the luminescence intensity of the as-prepared sample (A) is greater than the one observed for the two other samples. If one assumed a same passivation surface for the three samples, then, this difference in luminescence intensity may be due to different PS morphologies. In conclusion, such a study shows that SAXS is able to display, for samples of the same final poros-

Acknowledgement We wish to thank the technical staff of LUREDCI for providing the synchrotron beam and for assistance during experiment. References [1] L.T. Canham, AppI. Phys. Lett. 57 (1990) 1046. [2] F. Koch, Materials Research Society Symp. Proc., Vol. 298, Spring Meeting, San Francisco, 1993. [3] 1. Sagnes, A. Halimaoui, G. Vincent and P. A. Badoz, AppI. Phys. Lett. 62 (1993) 1155. [4] I. Mihalcescu, R. Romestain, J. C. Vial, A. Bsiesy, S. Billat, F. Gaspard, R. Herino, M. Ligeon and F. Muller, Materials Research Society Symp. Proc. Vol. 298, Spring Meeting, San Francisco, 1993. [5] J.C. Vial, A. Bsiesy, G. Fishman, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain and R.M. Macfarlane, Materials Research Society Symp. Proc., Vol. 283, Fall Meeting, Boston, 1992. [6] V. Vezin, P. Goudeau, A. Naudon, A. Halimaoui and G. Bomchil, AppI. Phys. Lett. 60 (1992) 2625. [7] H. Franz, H. Metzger, V. Petrova-Koch and J. Peisl, J. de Phys. IV, to appear. [8] A. Naudon, P. Goudeau, A. Halimaoui, B Lambert and G. Bomchil, J. AppI. Phys., to be published. [9] A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller and J.C. Oberlin, J. Electrochem. Soc. 138 (1991) 3450. [10] Ph. Goudeau, A. Naudon and J.-M. Welter, J. AppI. Cryst. 23 (i990) 266. [ii] G. Porod, in: Small-Angle X-ray Scattering ed. H. Brumberger (Gorden and Breach, New York, 1967), p. 1. [12] P.Z. Wong and A.J. Bray, Phys. Rev. Lett. 59 (1987) 1057. [13] P.W. Schmidt, D. Avnir, D. Levy, A. Höhr, M. Steiner and A. Roll, J. Chem. Phys. 94 (1991) 1474. [14] A. Guinier and G. Fournet, Small-Angle Scattering of X-ray Wiley, New York (1955). [15] A. Naudon, P. Goudeau, A. Halimaoui, B. Lambert and G. Bomchil, in preparation.