Chemical etching of porous silicon in diluted hydrofluoric acid

PERGAMON

Solid State Communications 109 (1999) 195±199

Chemical etching of porous silicon in diluted hydro¯uoric acid Takashi Tsuboi*, Tetsuo Sakka, Yukio H. Ogata Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan Received 14 August 1998; accepted 24 September 1998 by H. Akai

Abstract Chemical etching of n-type porous silicon in diluted hydro¯uoric acid was studied by Fourier transform infrared spectroscopy. A new peak was observed at 616 cm 21 after the etching, which is assigned to Si±H bending of H±Si[SiH(Si)2]2[Si(Si)3] con®guration. The absorption at 627 cm 21, owing to Si±H bending of H±Si[Si(Si)3]3 structure, was narrowed by the etching. The etching produces {011} microfacets, which are absent in as-prepared porous silicon. These features can be interpreted in terms of the dissolution chemistry and the pore morphology of n-type porous silicon. q 1998 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures; A. Semiconductors; A. Surfaces and interfaces

1. Introduction The recent discovery of ef®cient visible photoluminescence from porous silicon (PS) at ambient temperature [1] has stimulated many researchers to study this material, because light-emitting siliconbased devices could extend the functionality of silicon technology to optoelectronics. It has also been proposed that PS has potential application as a ``host matrix'' for catalysts and metals. For example, the PS electrode with platinum coating has improved solar cell character [2]. To realize these applications, the structure of PS should be studied in detail. PS surface passivation is achieved by hydrogen termination of silicon dangling bonds. We have studied the structure of PS surface by Fourier transform infrared (FTIR) spectroscopy [3±5]. In a previous paper [5], the changes in hydrogen environments with thermal annealing were reported to make an exact assignments around 2100 cm 21, correspond* Corresponding author.

ing to Si±Hx stretching region, and to clarify the ®ne structure of PS. The detailed assignments below 1000 cm 21, however, were not achieved because of broad absorption peaks and dif®culty of the comparison between the calculated and experimental results. FTIR spectra around 2100 cm 21 of Si wafer surface are strongly affected by the concentration of HF in which the wafer is dipped [6]. The vibrational lines are well-de®ned (narrow t 10 cm 21) when diluted HF (1±10% in H2O) is used. Narrower lines could be observed for PS etched chemically in diluted HF and the detailed discussion in lower frequencies may be permitted. In this communication, we report the FTIR spectra of PS etched chemically in diluted HF aqueous solution. The linewidths of absorption bands were analysed and the assignments in the low frequency region are discussed in detail.

2. Experimental A PS layer was prepared by anodization of n-type

0038-1098/99/$ - see front matter q 1998 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(98)00508-0

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Fig. 1. Transmission FTIR spectra of as-prepared and chemically etched PS samples. Silicon wafer was used as reference to remove the bulk absorption.

Si (1 0 0) wafer (3±5 V cm in resistivity) at constant current density under illumination. The electrolyte used was 20 wt% HF (47 wt% HF: ethanol ˆ 1 : 1.35 weight). Before anodization, the Si wafer was rinsed in acetone and dipped for a few minutes in the HF solution. The anodization current density was 5 mAcm 22 and the anodization time was 10 min. The PS was dried under argon gas ¯ow to avoid oxidation. The PS was then immersed in 1 wt% HF aqueous solution in the dark. Prior to the immersion, the PS was moistened for several seconds with the HF solution used for PS preparation to facilitate the contact to the electrolyte. After the immersion, the PS was dried by blowing argon gas. Transmission FTIR spectra was measured by means of a NICOLET AVATAR 360 FTIR spectrometer. The resolution used was 4 cm 21 and the data were averaged over 64 scans. A silicon wafer was used as reference to remove Si bulk absorptions. 3. Results Fig. 1 shows transmission FTIR spectra of PS samples. As-prepared PS was lustrous yellow in

color. The PS had two absorptions at 627 and 662 cm 21, weak absorption at 812 cm 21, sharp absorption at 912 cm 21, and a distinctive triplet at 2090±2150 cm 21. The triplet is actually composed of at least seven peaks [5]. In this spectrum ®ve peaks were found at 2089, 2102, 2110, 2139, and 2147 cm 21. The wave numbers were slightly different from those previously reported [3,5], probably due to the different experimental conditions. The spectrum did not change by the immersion in 47 wt% HF for 4 h following the above moistening. Immersion in 1 wt% HF altered the color to whitish and the FTIR spectrum remarkably. As the immersion time increased, intensity diminished for all peaks observed in as-prepared PS. The absorption at 812 cm 21 disappeared for PS immersed for 30 min. Fine structure was observed around 2100 cm 21 in one-hour-immersed PS in spite of the reduced intensities. The spectrum seemed to have seven peaks at 2071, 2081, 2089, 2102, 2110, 2137, and 2147 cm 21. We found a new peak at 2081 cm 21, although we could not detect the peak at 2115 cm 21 [5]. The absorption at 627 cm 21 was narrowed by the immersion. A peak at 616 cm 21 was newly observed after the immersion. The peak intensities at 616 and 627 cm 21 were comparable after the immersion for 16 h. The peaks around 1100 and 2250 cm 21, owing to Si±O±Si stretching and OySi±Hx stretching, respectively, were absent for all spectra. Oxidized species was thus not produced during the immersion. To estimate integrated absorbance and full width at half maximum (FWHM), we deconvoluted the peaks at 616, 627, 662, and 912 cm 21, assuming that each peak has a Lorentzian pro®le. We could not analyse the PS immersed for longer than 4 h, as the signal-tonoise ratio of the spectra was too low. We could not analyse the peaks around 2100 cm 21 because the bands consisting of at least seven peaks [5] are too complicated. The linewidths at 616, 662, and 912 cm 21 were almost constant and were about 5, 40, and 18 cm 21, respectively. In contrast, the FWHM at 627 cm 21 was decreased with increasing immersion time. It was reduced from 21 to 10 cm 21 by 4 h immersion. The intensity at 616 cm 21 was nearly equal to zero for as-prepared PS. After the immersion over 30 min, however, the integrated absorbance was about 0.3 cm 21 and was not much affected by the immersion time. In contrast, other

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peaks diminished in integrated absorbance by the immersion.

4. Discussion The absorption bands around 2100 cm 21 consist of at least seven peaks [5]. These bands are caused by Si±Hx stretching [3]. The intensities were reduced by the immersion, as shown in Fig. 1, suggesting that hydrogen concentration decreased during etching. The bands varied in their shape with immersion time. It seems that etching rates of Si±Hx are different, although all the species dissolve in diluted hydro¯uoric acid. We do not discuss the Si±Hx stretching region in further detail, since the bands are too complex. The PS immersed in 47 wt% HF was not different from as-prepared PS in the FTIR spectrum and in color. Etching in the HF was thus negligible. In contrast, drastic etching was observed in diluted hydro¯uoric acid (Fig. 1). As a concentration of hydroxide ion increases with decreasing concentration of hydro¯uoric acid, it can be expected that the attack by hydroxide ions to PS surface is a key step in etching of PS. The effect of pH should be understood. We immersed PS for 30 min in an aqueous solution containing 3.5 M NH4F and 0.5 M HF. The pH of the solution is expected to be about 5 [7]. The FTIR spectrum had absorption bands around 1100 and 2250 cm 21, meaning that silicon oxides were present in the PS layer [4]. The increase of pH, i.e., the increase in OH 2 concentration accelerates PS oxidation. The oxidation is expected to be faster than the etching of the oxide by ¯uoride ion. Fluorine was present on as-prepared PS surface, since the absorption at 812 cm 21 because of Si±Fx stretching [3], was observed. The immersion in diluted hydro¯uoric acid, however, eliminated this band; ¯uorine disappeared from the surface. In contrast, the intensity of ¯uorides did not change in 47 wt% hydro¯uoric acid. SiFx is stable in the concentrated hydro¯uoric acid as SiHx; in diluted HF, SiFx dissolves faster than SiHx. This is probably because the dissolution of SiFx is also dominated by the attack of hydroxide, although the process is much faster than the dissolution of SiHx. SiFx is kinetically more

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unstable than SiHx, as predicted theoretically by Trucks et al [8]. The assignment in the Si±Hx bending region has been proposed as follows [3]: 628 cm 21 arising from Si±H bending, 667 cm 21 from Si±H2 wagging, and 916 cm 21 from Si±H2 scissors bending. There is no possibility of assigning 628 cm 21 to the bulk phonon, since we used silicon wafer as reference. According to the study of a stepped Si(1 1 1) surface [9], three peaks were observed between 750 and 500 cm 21. They peaked at 615, 623, and 656 cm 21, which were assigned to wagging of monohydride chain at step edge, bending of monohydride on terrace, and wagging of vertical dihydride at step edge, respectively. Prior to the discussion on the absorption bands, we should note the pore morphology of PS formed on ntype (100) silicon. The morphology has been reported in detail [7,10]. PS layers are characterized by pores of square cross section. The pores are 100 nm or less in size and are oriented perpendicular to the (100) plane. The pore walls are de®ned by {011} planes. In addition to the main pore, side pores of approximately 20 nm in size grow from corner of the primary pore in the k001l and k010l directions. These side pores are, in turn, decorated with pores which are smaller than 10 nm in size, giving rise to a dendritic structure. The morphology of PS in this study would be different in size, as the pore morphology is dependent upon the experimental conditions. Nevertheless, the PS is expected to have primary pores which are of square cross section, several tens of nanometer in size, oriented perpendicular to the (100) plane, characterized by pore walls of {011} planes, and with side pores grown in the k001l and k010l directions. It has generally been accepted that the band around 910 cm 21 is because of Si±H2 scissors bending [3]; we thus assign the absorption at 912 cm 21 to the Si± H2 scissors bending. The absorption at 662 cm 21 shows the similar behaviors as that at 912 cm 21: constant linewidth and decreasing intensity. This suggests that SiH2 is the origin of this band. Infrared studies on silicon wafer [9] and PS [3] have attributed the peaks around 660 cm 21 to Si±H2 wagging. Therefore assigning the peak at 662 cm 21 to Si±H2 wagging bending is reasonable. The rests are bands at 627 and 616 cm 21. These absorptions are as a result of SiH and their etching

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Fig. 2. Microstructures corresponding to absorption bands.

behaviors are different. SiH should have at least two microstructures. Chemical etching of silicon is known to be anisotropic. The (111) planes usually have the lowest etch rate and act as etch stops for the dissolution process [7]. The con®guration of HSi[Si(Si)3]3 characterizes this surface. In contrast, the pore walls are de®ned by {011} planes, as mentioned above. The surface has the HSi[SiH(Si)2]2 [Si(Si)3] con®guration. These con®gurations may vary in the frequency due to the bending vibration. Stepped (111) silicon surface has shown higher frequency for silicon monohydride on terrace than at step edge [9]. Therefore, we assign the absorption bands at 616 and 627 cm 21 to Si±H bending of HSi[SiH(Si)2]2 [Si(Si)3] and HSi[Si(Si)3]3 con®gurations, respectively. Fig. 2 represents the microstructures corresponding to these vibrational lines. The peak at 616 cm 21 was not observed in asprepared PS. This means that (111) and other microfacets should cover {011} planes of pore walls. These facets are dissolved by the etching in diluted HF, resulting in the appearance of hydrogen-terminated {011} planes. The {011} planes are expected to have relatively large and well-de®ned facets, as the band is very narrow. However, the band at 627 cm 21 decreased in the linewidth by the etching. This means that the etching increases the homogeneity of {111}

microfacets: larger and better-de®ned facets may be produced as the etching proceeds. Finally, we refer to the etching of p-type PS. The as-prepared PS had FTIR spectrum similar to that of n-type PS. The FTIR spectrum decreased in the intensity as etching time increased. In contrast to n-type PS, the peak around 627 cm 21 was nearly constant in linewidth and the band at 616 cm 21 was not observed. The porous layer of p-type PS is characterized by pores and their spacings of less than 10 nm in size [7]. The absence of {011} planes gives rise to the disappearance of the peak at 616 cm 21 and smaller crystallites do not become better-de®ned even after the etching in diluted hydro¯uoric acid. 5. Conclusion We have investigated the chemical etching of ntype PS in diluted HF using FTIR spectroscopy. A new peak at 616 cm 21, which is absent for as-prepared PS, appears by the etching. In addition, the band at 627 cm 21 decreased in the linewidth. These behaviors were not observed in p-type PS and can be understood in terms of the dissolution chemistry and pore morphology of n-type PS. We assign the absorption peaks of Si±Hx bending vibration (below 1000 cm 21)

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as follows; 616 cm arising from Si±H bending of H±Si[SiH(Si)2]2[Si(Si)2]2, 627 cm 21 from H± Si[Si(Si)3]3, 662 cm 21 from Si±H2 wagging, and 912 cm 21 from to Si±H2 scissors bending. Acknowledgements The authors are grateful to Drs M. Iwasaki, M. Mabuchi, and T. Kobayashi for useful discussions. This work was partly supported by a grant-in-aid for scienti®c research on priority area of Electrochemistry of Ordered Interfaces from Ministry of Education, Science, Sports, and Culture, Japan. References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046.

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[2] K. Kawakami, T. Fujii, S. Yae, Y. Nakato, J. Phys. Chem. B101 (1997) 4509. [3] Y. Ogata, H. Niki, T. Sakka, M. Iwasaki, J. Electrochem. Soc. 142 (1995) 195. [4] Y. Ogata, H. Niki, T. Sakka, M. Iwasaki, J. Electrochem. Soc. 142 (1995) 1595. [5] Y.H. Ogata, F. Kato, T. Tsuboi, T. Sakka, J. Electrochem. Soc. 145 (1998) 2439. [6] G.S. Higashi, Y.J. Chabal, G.W. Trucks, K. Raghavachari, App. Phys. Lett. 56 (1990) 656. [7] P.C. Searson, Advances in Electrochemical Science and Engineering, H. Gerischer, C.W. Tobias (Eds.), Vol. 4, VCH, Weinheim, 1995. [8] G.W. Trucks, K. Raghavachari, G.S. Higashi, Y.J. Chabal, Phys. Rev. Lett. 65 (1990) 504. [9] S. Watanabe, Y. Sugita, Chem. Phys. Lett. 244 (1995) 105. [10] P.C. Searson, J.M. Macaulay, F.M. Ross, J. Appl. Phys. 72 (1992) 253.