Characterization of supercritically dried porous silicon

Thin Solid Films 255 (1995) 115-

Characterization St. FrohnhofP,

I 18

of supercritically

R. Arens-Fischer”,

T. Heinrichb,

dried porous silicon

J. Frickeb,

M. Arntzen”,

W. Theiss”

~‘lnstitut $ir Schieht- und Ionentechnik (ISI), Forschungszentrum Jiilich GmhH. D-52425 Jiilich. Germanic “Physikalisches Institut der Universitiit Wiir;hurg, D-97074 Wiirzhurg, Germanic ‘1. PhJ*.sikalisches Institut, R WTH Aachen. D-5205(1 Aacherl. German)

Abstract Porous silicon (PS) has been formed by electrochemical etching of p-type silicon. During drying in air the sponge-like structure of the porous layers is exposed to capillary forces which will partially destroy the microstructure of highly PS. One possibility for avoiding the partial structural collapse is by supercritically drying the PS. This technique is already known from the formation of highly porous aerogels and has now been applied to PS. Porosities of 90%~can be achieved. These highly porous layers were investigated using photoluminescence, Raman, reflectance and X-ray photoemission spectroscopy. The porosity was determined by gravimetric measurements. The effect of the drying process on the properties of PS was also studied. Key,vor&:

Fourier

transform

infrared

spectroscopy;

Luminescence;

1. Introduction Porous silicon (PS) is formed during anodization in HF solution and has been known for more than 30 years [ 11. Nevertheless, the formation of as-prepared thick layers with porosities higher than 75% which have been dried in atmospheric pressure is not possible. During drying under normal conditions the sponge-like structure of the porous layers is exposed to capillary forces which will partially destroy the microstructure of the PS. One possibility for avoiding the partial structural collapse is by supercritically drying the porous layers [2]. The liquid-vapour-phase boundary will be avoided if the liquid is heated under constant pressure beyond its critical point and converted to a gas by pressure reduction at constant temperature. This technique has been known for more than 60 years from the formation of highly porous aerogels [ 31 and has been used successfully to prepare a variety of low-density aerogels [4]. Supercritical drying has now been applied to porous Si in our laboratories. Porosities higher than 90% may be achieved [2]. Since such low-density structures exhibit a relatively low mechanical stability, it is important to use non-destructive tools such as optical spectroscopy to characterize the PS. Using Raman spectroscopy, the microstructure of supercritically dried PS was studied by a detailed line shape analysis of the phonon lines [5]. 0040-6090/95/$9.50 j‘: 1995 ~ SSDI 0040-6090(94)05634-X

Elsevier Science S.A. All rights reserved

Silicon;

Structural

properties

Assuming localization of phonons in the crystallites, a distribution of crystallite sizes within PS can be obtained. In addition, the strain of PS films can be determined. In order to characterize the highly porous layers by their dielectric function, reflectance spectroscopy in the visible and IR range was performed.

2. Experimental PS was formed on p-Si (0.2 R cm) and p+-Si (0.01 R cm) substrates with layer thicknesses between 7 and 21 pm. Anodization was performed using a mixture of 50% HF and ethanol ( 1: 1). After formation, the samples were rinsed with ethanol and placed in an autoclave (Bio-Rad E3100 critical point drier, volume 0.2 1) and stored in ethanol. The ethanol was replaced by rinsing with liquid CO, at 15 “C and 60 bar for about 1 day (0.2 1 min-’ flux). Then the CO1 was converted to supercritical conditions by linear heating above the critical point to 40 “C within 2 h while the pressure was kept below 100 bar [6]. After a delay of 1 h to ensure steady-state conditions, the pressure was decreased to atmospheric pressure within 4 h and then cooled to room temperature within 0.5 h. Raman and photoluminescence (PL) measurements were performed at room temperature under ultra-high-

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vacuum (UHV) conditions to avoid a photostimulated oxidation process [7]. Spectra were recorded using a triple monochromator (DILOR XY) with an optical multichannel detector and a photomultiplier tube. The excitation wavelength was 457 nm. To avoid heating of the highly porous samples, the laser power was kept below 0.1 mW and the spot size was about 100 nm. Reflectance spectra in the IR spectral region were recorded with a Bruker IFS 45 Fourier transform spectrometer (angle of incidence 30”, s-polarized light). For the reflectivity in the visible and UV region a PerkinElmer h2 grating instrument was used (almost normal incidence of light).

3. Results and discussion 0

On pi- and p-doped Si substrates, porous layers were formed by supercritical drying with porosities up to 90%. The porosities were determined by gravimetric measurements. Fig. 1 shows the PL spectra for p-PS taken at room temperature. The layer thickness for all samples was 7 urn. For this layer thickness, porous films with a porosity higher than 73% collapsed during normal drying in air due to mechanical instability, whereas supercritical drying was possible. The PL intensity increased with the porosity and exhibited a slight blue shift in the peak maximum position (inset, Fig. 1). For p+-doped PS similar results were obtained whereas the PL intensity was about two orders of magnitude less than in the p-doped case. The depen-

p - PS (0.2&m)

POROSITY

I

(56)

700

600

WAVELENGTH

800

(mn)

Fig. 1. PL spectra (taken at room temperature) of PS formed on p-doped (0.2 R cm) Si substrate with different anodization current densities I,: 29, 89, 141, 179 and 216 mA cme2, corresponding to gravimetrically measured porosities of 65, 73, 80, 85 and 90X, respectively. All samples were supercritically dried and exhibit a layer thickness of about 7 urn. The PL spectra were corrected for the spectral response of the monochromator. The inset shows the PL peak maximum spectral position as a function of the porosity. The dashed line is plotted as a guide for the eye.

30

60

90

120

150

SIZE (A) Fig. 2. Crystallite size distribution for supercritically dried PS formed on differently doped substrates with various anodization current densities. On p-doped (0.2 R cm) Si substrate PS was formed with anodization current densities of 141 and 28.6 mA cm-‘, corresponding to gravimetrically measured porosities of 80 and 65%, respectively. On p +-doped (0.01 s2 cm) Si substrate PS was formed with anodization current densities of 296, 98 and 3.8 mA cm-‘, corresponding to gravimetrically measured porosities of 80, 50 and 31X, respectively. Crystallites smaller than 12 8, cannot be considered within the size distributions obtained from the Raman measurements. To account for such small structures a contribution from amorphous silicon is also fitted in the Raman spectra and plotted at 5 A.

dence of the PL on the doping type and anodization current might have two reasons: first, the microstructure and therefore the number of PL-emitting nanopartitles of the PS might be changed [S], and second, the optical properties of the layers might be different. The microstructure of PS was characterized by Raman spectroscopy. Fig. 2 shows the size distribution for supercritically dried p- and p +-PS formed with different anodization current densities. No continuous size distribution was found for all samples. This has been explained by an extended quantum model [9]. On p +-PS an increase of the current density and, therefore, an increase of the porosity result in a loss of nanocrystals with diameters above 80 A and an increase of crystallites smaller than about 35 A. On p-PS a loss of crystallites with diameters above 13 A is observed. Nevertheless, the size distributions of p- and p +-PS of 80% differ considerably. For p-PS the dominant contribution to the size distribution is given by crystallites with diameters of about 13 A whereas on p + -PS the main peak is centred at about 35 A. Fig. 3 shows the measured reflectance spectrum of a highly porous layer (90% porosity, p-PS) and a simulated spectrum. The theoretical data were obtained using an algorithm similar to that described in Ref. [lo]. In addition to interference structures, Fig. 3 shows

St. Frohnhoff ct ul. 1 Thin Solid Films -755 (1995) I/5-

3

2

w

117

I IX

4

5

(eV)

6

....-.-..-.. Model calculation

0.0

I

0

2000

1000 WAVENUMBER

3000

[cm-‘]

O&W

Fig. 3. Reflectance spectrum of supercritically dried p-PS formed with an anodization current density of 217 mA cm-’ (90% porosity) and a theoretical fit obtained with the dielectric function for the solid phase shown in Fig. 4.

C-H,

500

1000

1500

2000

WAVENUMBER

2500

3000

[cm-‘]

Fig. 4. Imaginary part of the dielectric function the porous system used for the fit in Fig. 3.

of the solid phase in

the appearance of pronounced absorption bands which are most clearly seen in the imaginary part of the dielectric function used for the solid phase of the porous system (see Fig. 4). As shown in the figure, there are mainly hydrogenand oxygen-related vibrational modes. The strong SipH bands indicate a large fraction of silicon surface atoms. The high level of oxidation ( Si -0 bands) is due to storage of the sample in air between supercritical drying and the IR experiment. Ethanol remaining in the pores is reflected by the C-H bands appearing around 2900 cm-‘. Both the oxygen and carbon contents were also demonstrated by X-ray photoemission spectroscopy. The effective dielectric function of the porous layer (not shown), which is obtained using an effective medium concept based on the Bergman theory [ 11, 121, has a real part being close to unity (ca. 1.2) which is characteristic of high-porosity materials. Reflectance measurements in the visible and UV region are shown in Fig. 5. Here, large differences between porous layers on p- and p+-doped substrates occur: whereas the bulk silicon gap structures in the range 25 000-50 000 cm- ’ (3-6 eV) are always found for p _ -PS ( negligible luminescence), the luminescent

30000 WAVENUMBER

0.004 I 0000

..

I

40000

--

30000

40000

WAVENUMBER

[cm ‘]

20000

50000

[cm-‘]

5c IC100

Fig. 5. Reilectance of supercritically dried /I -doped (top) and pdoped (bottom) PS from the near-infrared to the UV region. Note the different scaling of the reflectance axis. On p ‘-doped Si substrate PS was formed with anodization current densities of 3.8. 98 and 296 mA cm ‘, corresponding to gravimetrically measured porosities of 31, 50 and 80%. respectively; the layer thickness is 7, I4 and 21 pm. respectively. On p-doped Si substrate PS was formed with anodizdtion current densities of 28.6. 89 and ?I 7 mA cm ‘. corresponding to gravimetrically measured porosities of 65, 73 and LX)“,,,.respectively; the layer thickness is 7 pm for all sample\.

p-doped samples show basically one very broad peak ranging from 2.5 000 to 40 000 cm ‘. The significant sharp drop of the reflectance with a frequency around 20 000 cm -’ (2.5 eV) in the case of highly porous pdoped samples can be interpreted as the onset of absorption processes. Below 2.5 eV the porous layer is transparent and the large reflectivity of the silicon substrate is seen. Above this threshold the layer becomes opaque and the reflectance is the same as that of the interface between air and the porous layer. It is, of course, very low due to the small difference in the complex refractive indices of the adjacent materials. Note the very low reflectivity of the sample with 90% porosity above 3 eV (24 000 cm ‘) below 1%. In order to study the possible effect of the drying process on the properties of PS. samples of both doping levels and low porosities (up to 75%) which had been supercritically dried (SD) were immersed in ethanol again. After conventionally drying in air (AD) under a nitrogen atmosphere the samples were immediately placed in the UHV chamber for PC and Raman mea-

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St. Frohnhoff et al. 1 Thin Solid Films 2.55 (1995) 115-l 18

surements. For all samples the PL spectra did not differ significantly for AD and SD, but samples of lower porosity showed a slight decrease in PL intensity. The crystallite size distributions obtained from Raman measurements were the same for AD- and SD-treated samples. Even the strain in the layers which can also be determined by Raman measurements [5] was not changed by AD treatment if the porous layer was not detached from the substrate. Otherwise, if the porous layer peeled off from the substrate, strain relaxation was observed. Therefore, if the PS remained undetached on the substrate, which is valid for low-porosity layers, no changes in the microstructure were observed on a nanometre scale. In addition, it has been reported using electron microscopy studies that conventionally dried highly porous Si is heavily crazed on a macroscopic scale, which might be avoided by SD [2].

4. Conclusion

In this work, p- and p +-PS with porosities up to 90% have been formed by supercritically drying techniques. This has opened a new field of investigation of highly porous layers by optical spectroscopy which has been not accessible so far. It has been shown that the PL intensity can be improved by the formation of supercritically dried highly porous layers. For the first time the crystallite size distributions of PS with porosities higher than 75% have been obtained by Raman spectroscopy. The size distributions show a reduction in the larger crystallites in favour of smaller crystallites and

this confirms the tendency which is already known from lower porosity layers. The microstructure and the reflectance of p- and p +-PS differ considerably for highly porous layers. Possible effects of the drying process on the properties of PS have been studied. For low-porosity layers no changes of the microstructure were observed due to conventional drying after supercritical drying processes.

References [l] A. Uhlir, BeN Syst. Tech. J., 35 (1956) 333. [2] L. T. Canham, A. G. Cullis, C. Pickering, 0. D. Dosser, T. I. Cox and T. P. Lynch, Nature (London), 368 (1994) 133. [3] S. S. Kistler, Nature (London), 127 (1931) 741. [4] J. Fricke and A. Emmerling, J. Am. Ceram. Sot., 75( 1992) 2027. [5] H. Mtinder, M. G. Berger, H. Liith, R. Herino and M. Ligeon, Thin Solid Films, 221 (1992) 27. [6] U. Klett, T. Heinrich, A. Emmerling and J. Fricke, Proc. Int. Symp. Advances in Sol-Gel Processing, Chicago, August 25-26, 1993. [7] G. Mauckner, T. Walter, T. Baier, K. Thonk and R. Sauer, Mater. Res. Sot. Symp. Proc., 283 (1993) 109. [8] H. Miinder, M. G. Berger, S. Frohnhoff, H. Liith, U. Rossow, U. Frotscher and W. Richter, Mater. Res. Sot. Symp. Proc., 283 (1993) 281. [9] St. Frohnhoff, M. Marso, M. G. Berger, M. Thiinissen, H. Ltith and H. Miinder, J. Electrochem. Sot., submitted. [IO] W. Theiss, Proc. Les Houches Int. Winter Schoolon Luminescence of Porous Silicon and Silicon Nanostructures, February 7712. 2994, Editions de Physique, Les Houches Winter Schools, 1994. [I I] W. Theiss, in R. Helbig (ed.), FestkorperproblemejAdvances in Solid State Physics, Vol. 33, Vieweg, Braunschweig, 1994, p. 149. [ 121 W. Theiss and S. Henkel, Thin Solid Films 255 (1995) 177.