Electrochemical surface conditioning of n-Si(111)

surface science ELSEVIER

Surface Science 335 (1995) 160-165

Electrochemical surface conditioning of n-Si(111) J. Rappich a,* M. Aggour b, S. Rauscher b, H.J. Lewerenz b, H. Jungblut b a Hahn-Meitner-lnstitut, Bereich Angewandte Physik - Abteilung Photovoltaik, Rudower Chaussee 5, D-12489 Berlin, Germany b Hahn-Meitner-lnstitut, Bereich Physikalische Chemie - Abteilung Grenzfliichen, Glienicker Strasse 100, D-14109 Berlin, Germany

Received 12 September 1994; acceptedfor publication 31 October 1994

Abstract The surface condition of n-Si(lll) exposed to an acidic fluoride solution under white light illumination is strongly influenced by the applied electrode potential. The microtopography of the surface is changed from rough and porous to oxidized or oxidized and porous, respectively, if oscillations occur. These surfaces were investigated by in situ Fourier-transform infrared spectroscopy (FTIR) using internal reflection techniques, ex situ scanning tunneling, scanning and transmission electron microscopy (STM, SEM and TEM) and X-ray photoelectron spectroscopy (XPS). The photocurrent oscillations followed by the dark current transient in the same solution lead to the best chemical and electronic hydrogen-passivated surfaces. Keywords: Semiconductor-electrolyteinterfaces; Silicon; Single crystal surfaces; Surface structure

1. Introduction Electrochemical investigations on the silicon/electrolyte interface have led to several surprising observations such as the occurrence of the hitherto unexplained dark current in fluoride containing solutions during and after oxide removal [1,2] which can be used for hydrogen termination of nSi(ll 1). H-passivated and unreconstructed Si(111)(1 × 1) surfaces have been prepared by chemical [3] as well as by electrochemical treatments [4-6] and result in a similar surface condition [5,7-9]. The advantage of the electrochemical method is the control of the surface preparation by electrode potential, current and illumination [4]. In addition to the dark current "observations", the physico-chemical origin of the photocurrrent oscillations at high anodic po-

* Corresponding author.

tentials and the formation of porous silicon [10-14] are still obscure [15-18]. Evidence that the oscillatory behavior leads to changes in surface morphology was recently obtained with in situ ellipsometry, scanning tunneling microscopy (STM), in situ FTIR and combined ex situ electrochemistry/surface analysis experiments [19-24]. In the present work we investigate the changes in surface microtopography due to the applied electrode potential in acidic fluoride containing solutions by SEM, TEM, XPS and in situ FTIR, respectively.

2. Experimental Infrared measurements were performed with an n-Si prism (doping level 1015 cm -3, [111] oriented and 4° mismatch) mounted in a multiple internal attenuated total reflectance (MI-ATR) configuration

0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95 )00416- 5

J. Rappich et al. / Surface Science 335 (1995) 160-165 8O0

oxide formation

II oscillations

600 < 7I.

400

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200 0-1

J I

I

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i

i

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I

0

1

2

3

4

5

6

7

Potential / VNHE

Fig. l. I / U characteristic of an n-Si(lll) electrode in 0.1M NH4F (pH 4.0) under illumination with white light ( ~ 20 mW cm-2).

as described in Ref. [25] or with an n-Si semicylinder ([111], 10 × 10 × 5 mm 3, doping level ~ 1015 c m - 3 ) by use of the single internal reflection. Si(111) samples used in the other experiments had a resistivity of 6 ~ . cm without any miscut. The electrolyte solutions were prepared from analytical grade purity reagents in triply distilled water. The electrochemical measurements were made in a teflon or plastic cell using a platinum counter electrode, a saturated calomel electrode or a 0.1M KC1/AgCI/Ag elec-

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trode (FTIR) as the reference electrode. The electrode potential was controlled by a potentiostatgalvanostat (Heka). Fig. 1 shows a typical current-voltage characteristic of an n-Si electrode in 0.1M NH4F (pH 4.0) solution under illumination (light intensity ~ 20 mW cm -2). At the shoulder of the first current peak porous silicon is formed [10-14]. After the first current peak the oxidation of the Si surface starts and the siliconoxide is simultaneously etched back by the HF/HF~- containing electrolyte (electropolishing). No Sill exists on the silicon surface at this potential region ( U > 0.2 VNHE) as measured by XPS and UPS. At higher anodic potentials photocurrent oscillations occur.

3. Results and discussion

Applying a potential of about - 4 0 mVNH E under white light illumination ( ~ 20 mW cm 2) for some minutes at the Si sample in this electrolyte leads to a rough surface as can be seen from the SEM image in Fig. 2. The Si surface is H-terminated during the roughening process (e.g. formation of steps with two dangling bonds on the [111] surface) as was recently

Fig. 2. SEM image of a Si sample after roughening at +0.15 VNHE in 0.1M NH4F (pH 4.0) under illumination for some minutes.

J. Rappich et aL / Surface Science 335 (1995) 160-165

162

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Fig. 3. Time dependence of the dark current density at different potentials: (a) +0.7 VNHE, (b) - 0 . 4 VNHE, (C) --0.7 VNrtE in 0.2M NaF (pH 4.5).

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Fig. 5. XP spectrum of a Si sample after the dark current transient decayed in 0.2M NH4F (pH 4.5).

found by in situ FTIR. A strongly increased IR absorption appears due to the stretching mode of the s i n a species [21]. On the other hand the photoinduced electropolishing treatments with and without photocurrent oscillations lead to a smoothening of the surface as measured by in situ FTIR and ex situ STM [21]. Fig. 3 shows the behavior of the dark current during the etch back of an anodically formed SiO 2 layer (+ 3.2 VNHE) monitored at different potentials. At + 0.7 VNHE the well known current transient with the current peak is obtained (Fig. 3a). At a potential near the flatband at about - 0 . 4 V~H E the dark

0

dark current decreases rapidly to a constant value of

- 5 0 / x A / c m 2. Independent of the applied electrode potential the Si surface is mainly H-terminated as can be seen from the in situ FTIR spectra in Fig. 4 which were plotted at the region of s i n stretching mode (sin: ~ 2 0 8 0 cm-1; s i n 2 : ~ 2 1 1 0 cm -1 current decayed in reference to the s i n free and oxidized surface measured at + 3.2 VNHE under illu-

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shows no current peak (Fig. 3c). On the contrary, the

[26,27]). These spectra were recorded after the dark

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current becomes cathodic after a short chemical etching period and only a very small current peak can be seen (Fig. 3b) with a maximum of + 2 /.tA/cm 2. The dark current decreases to a constant value of - 9 / x A / c m 2 after about 70 s. Applying a potential cathodic from the flatband situation ( - 0 . 7 Vr~E)

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Fig. 4. Relative change of the PTIR spectra after the dark current transient at different potentials (cf. Fig. 3) with reference to the s i n free surface: (a) +0.7 VNHe, (b) - 0 . 4 VNAE, (c) --0.7 VNHz in 0.2M NaF (pH 4.5).

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Fig. 6. Time dependence of the photocurrent oscillations at nS i ( l l l ) in 0.1M NH4F at different pH values.

J. Rappich et al. / Surface Science 335 (1995) 160-165

mination. The IR spectrum obtained at +0.7 VNHE indicates a better H-terminated surface due to a slightly higher IR absorption (about 5-10%) and by a small peak shift to lower wavenumbers (e.g. higher IR absorption due to Sill which points to a smoother surface) in comparison to the spectra obtained for the other electrode potentials in the dark. Furthermore, the process of the H-termination could be well monitored by the occurrence of the dark current peak and

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its decay at this anodic potential which was always used for our in situ and ex situ investigations concerning the H-passivation of silicon. For example, Fig. 5 shows a XP spectrum of n-Si after the dark current transient has decayed (0.2M NHnF, pH 4.5). The amount of O and F and C is below a tenth of a monolayer. At higher anodic potentials photocurrent oscillations occur. Applying the H-termination process in

Fig. 7. TEM imageof a Si sampleafter interrupting the photocurrent oscillations.

J. Rappich et aL / Surface Science 335 (1995) 160-165

164

the dark after some oscillations in 0.1M NaF (pH 4.0) leads to well hydrogen and electronic passivated Si surfaces as recently inspected by in situ FTIR and ex situ pulsed surface photovoltage (SPV) experiments [21-24] with about 95% of a monolayer Sill and a density of interface states at midgap below 1011 eV -1 cm -2. This electrolyte composition was taken due to the fact that the oscillations appear very periodically as seen in Fig. 6, where the photocurrent oscillations are plotted as a function of time for different pH values. At lower pH the oscillations transform into noise. At higher pH the period increases strongly and the photocurrent maximum decreases. Their origin is yet not understood. So far, a model was developed to explain the strong photocurrent modulation despite comparably small changes in oxide thickness. It was based on a fluctuating formation of pores by the photoinduced oxidation and the simultaneous etch back by HF and HF2- species in the electrolyte, respectively [19]. Therefore, Fig. 7 shows a TEM image after interruption of the oscillation process where the pores are well seen as white circles. In situ measured FTIR spectra during the photocurrent oscillations at the asymmetric S i - O - S i stretching mode region are shown in Fig. 8 by using the single internal reflection mode (see lower inset Fig. 8). These spectra are

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w a v e n u m b e r / crn 1 Fig. 8. Relative change of the FTIR spectra with reference to the Sill free surface taken at (a) for different periods of the photocurrent oscillations in 0.2M NaF (pH 4.5) as seen in the upper inset; lower inset: scheme of the experimental setup for the single internal reflection mode.

plotted with respect to the SiO 2 free surface recorded at position (a) as seen in the upper inset of Fig. 8 after the dark current transient has occurred. The well known splitting of the IR absorption at the asymmetric S i - O - S i stretching mode in the parallel and perpendicular component can be seen [28]. Fig. 8 shows that the amount of SiO 2 also changes periodically during the oscillations. At the increase of the photocurrent the oxide is formed (Fig. 8b) and in the decreasing time region the same amount of oxide is etched back (Fig. 8d). The change of the amount of SiO 2 is considerably larger than the values deduced from XPS and in situ ellipsometry measurements [29,30]. It appears that the attenuation of the parallel mode is larger than that of the perpendicular one, which indicates a somewhat preferential process.

Acknowledgements The authors thank Ms. Sieber and Dr. M. Giersig for recording the SEM and TEM images.

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