X-ray photoelectron spectroscopy characterization of stain-etched luminescent porous silicon films

Journal of Luminescence 80 (1999) 159—162

X-ray photoelectron spectroscopy characterization of stain-etched luminescent porous silicon films R. Zanoni *, G. Righini
, G. Mattogno, L. Schirone, G. Sotgiu, F. Rallo Dipartimento di Chimica, Universita% di Roma **La Sapienza++, piazzale A. Moro, 5-Box 34, Roma 62, 00185 Roma, Italy
CNR, Area della Ricerca di Roma, P.O. 10, 00016 Monterotondo Scalo, Italy  CNR, ICMAT, P.O. 10, 00016 Monterotondo Scalo, Italy  Dipartimento di Ingegneria Elettronica, Universita% di Roma Tre, via della Vasca Navale, 84-00146 Roma, Italy

Abstract The surface and in-depth chemical nature of the photoluminescent stained Si layer obtained with a novel procedure based on HF/HNO is presented. Oxide-free porous Si surfaces result from controlled preparation, storing and handling  of samples, as revealed by parallel X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy measurements, coupled with Ar> ion sputtering. The present findings support the model for the porous layer of oxidized samples of Si grains embedded in a silica gel matrix.  1999 Published by Elsevier Science B.V. All rights reserved. PACS: 81.60.Cp; 82.80.Pv; 68.55.Nq; 78.55.Hv; 81.05.Rm Keywords: Silicon; Porous; Stain etch; Photoluminescence; XPS; Photoelectron spectroscopy

1. Introduction Different procedures have been reported in the literature to produce porous silicon (PS) layers. A modification of the classic recipe for Si stain etching [1] has been proposed [2] and recently optimized for application as anti-reflection coatings for high-efficiency solar cells [3]. The main advantage of this procedure is its suitability to large surface areas processing, a prerequisite for industrial production of photovoltaic (PV) devices and photodetectors. A PV conversion efficiency of

* Corresponding author. E-mail: [email protected].

12.8% of a polycrystalline 12.8;12.8 cm solar Si cell, under standard AM1.5G simulated sunlight was recently reported by some of us [4]. We report here a surface analysis of the PS films prepared using this recipe. We have followed the effect of different HF/HNO etching solutions, processing  times and air/vacuum exposure on the surface composition of the resulting films by means of photoemission spectroscopy. The experimental conditions which lead to oxide-free surfaces are also reported. Several previous reports on PS investigation by X-ray photoelectron spectroscopy (XPS) have been reported (see Ref. [5] and references therein). Our approach makes use of XPS, Ar> sputtering and Auger parameter values to better extract differences in Si environments in a PS layer.

0022-2313/99/$ — see front matter  1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 0 8 8 - X

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R. Zanoni et al. / Journal of Luminescence 80 (1999) 159—162

2. Experimental P-doped Si(100) wafers (o"0.01 ) cm) were chemically etched in aqueous HF/HNO solutions  as described in Ref. [4], at HNO concentrations  6 and 24 mM. The corresponding series are denoted as L1-3, and H1-2, HW. H1 and L1 samples were rinsed and kept in acetone for 18 h in air while HW was rinsed and kept in water for 18 h in air, before XPS measurements. The H2, L2 and L3 samples were rinsed and kept for few minutes in acetone in a dry-box, where they were attached to metal holders by conductive tape and transferred to the XPS chamber without air exposure. XPS measurements were conducted within minutes after transfer in the machine. L3 was measured again after 5 days in vacuum in the XPS machine, where it developed partial oxidation. Its depth profile was studied by Ar> ion sputtering at 2 kV, 1 lA. XPS and X-ray induced Auger spectra (XAES) were run on a Vacuum Generators ESCALAB spectrometer, equipped with a hemispherical analyzer operated in the fixed analyzer transmission (FAT) mode. Alka photons (hl"1486.6 eV) were used to ex cite photoemission. The accuracy of the reported binding energies (BEs) was $0.2 eV, and the reproducibility of the results was within these values. Due to the high doping of the substrate, the samples did not show any build-up of charge during the measurements. The values for the Auger parameter, a [6], are obtained as the sum of the Si 2p BE and the Si KLL electrons kinetic energy (KE). All samples show photoluminescence emission, consistent with previous literature reports [7,8].

3. Results and discussion Table 1 collects relevant XPS BEs and atomic ratios and Fig. 1 shows the XPS Si 2p ionization region for the investigated samples. A large dependence on the preparation conditions is evident, primarily in the Si 2p spectra. The largest effect is represented by air-exposure, series 1 and 2 dramatically differing in the oxide content. The porosity profile of the samples, which is strongly affected by HNO  concentration [9] is also likely to play a role. This aspect is presently being investigated in more detail.

Table 1 Si 2p binding energies and Si Auger parameter values for the reported samples Sample

Si (0)

SiO 

a Si(0)

a SiO 

L1

99.5 1.000 99.7 1.000 99.3 1.000 99.7 1.000 99.8 1.000 99.4 1.000 99.4

103.4 0.943 n.d. 0 n.d. 0 103.3 1.580 n.d. 0 103.1 0.486

1715.5

1711.5

L2 L3 (extensive sputtering) H1 H2 HW Silicon [15] SiO [15]  SiO [15]  

1715.7 1716.1 1715.6

1711.4

1714.8 1715.7

1712.6

1716.1 103.4 103.6

1712.2 1711.5

eV a.r. eV a.r. eV a.r. eV a.r. eV a.r. eV a.r. eV eV eV

Note: a.r. are atomic ratios of Si components, obtained from measurements of XPS relative areas.

All but HW sample present a traceable amount of fluorine. Rinsing in water lowers the fluorine content under XPS detectability limit, &0.1%. Fluorine, which is apparently not bonded to Si (as inferred from the lack of high-BE components in the Si 2p region for H2, L2 and L3) could be directly bonded to C and/or O. No Si—O or Si—C component, in fact, could be observed in the Si 2p spectrum of samples H2, L2 (see Fig. 1) and L3, although carbon and oxygen are present. The H2 and L2 samples, handled in controlled atmosphere, present an oxide-free PS layer. Their Si 2p BE values (99.7 and 99.8 eV, respectively), are closely comparable with literature reports for PS as well as for amorphous Si [10,11]. Notice that the corresponding a values for H2 and L2 are significantly different: 1715.7, 1714.8 eV, which calls for a different zerovalent Si environment. The ratio of the silicon oxides to silicon has been followed by XPS measurements carried out on the air-exposed samples, by collecting the Si 2p photoelectrons emitted at different angles from the surface. A progressive enhancement in the silica to silicon ratio has been experimentally found at

R. Zanoni et al. / Journal of Luminescence 80 (1999) 159—162

Fig. 1. Si 2p core level spectra recorded at a photon energy of 1486.6 eV. Porous silicon samples are identified as in the experimental section.

increasing surface sensitivity, which confirms that the formation of the oxide layer is mainly driven by surface exposure. In the literature, the presence of Si—H, Si—H and  Si—H bonds is associated to Si 2p components  shifted from the bulk respectively by 0.335, 0.67 and 1.00 eV to higher BEs (See, e.g., Refs. [12—14]). In all cases where oxides are present on the PS surface layer, any attempt to fit the complex Si 2p envelope by introducing peak components separated by (1 eV from the bulk is largely arbitrary, because of the intrinsic larger FWHM of the oxide-related components. We performed a check for L2, which showed the least asymmetric overall Si 2p line shape. The complex envelope can be consistently fitted with a single set of ,  spin—orbit compo  nents. The nature of the subsurface Si layer has been investigated on sample L3, which developed an initial oxidation layer after 5 days in vacuum, by means of Ar>-ion sputtering associated with XPS and XAES. The resulting depth profile curve, shown in Fig. 2, is divided into three main regions: the outermost surface layer, where C, O and F contaminants are associated to the main porous Si surface; a second layer, where Si(0) increasingly dominates and a third layer, extending into the bulk of the unreacted substrate. In Fig. 2, the etch-

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Fig. 2. Depth profile curve by Ar> ion-sputtering of the L3 sample, which experienced partial oxidation. C1s and O1s complex curves are reported as integrated total areas. The procedure for conversion of time scale into sputtered depths is described in the experimental section.

ing time has been translated into sputtering depths by means of a calibration procedure performed on the XPS machine with bulk Si standards. Sputtering depths in the region of the porous layer are moderate underestimates of the true values because of the increased sputtering yield. Within these limits, the inferred depth of the stain-etched layer, 60 nm, is consistent with the independent experimental findings by HREM [9]. Silicon and silicon oxides present distinct Si Auger parameter values, as shown in Table 1. Si a values for zerovalent Si have been collected as a function of different sputtering depths and plotted in Fig. 3. The outer surface layer presents a large variation in a values and it is followed by a nearly flat region where a"1715.5$0.3 eV. Si a values show a constant increase through 40—60 nm, until the value reported in the literature for bulk Si (1716.1 eV) is reached, at about 60 nm. This estimated value for the depth of the stain-etched layer is consistent with the above-reported result. The intermediate layer is assigned to the porous Si layer. The high sensitivity of a to the Si environment and its smooth variation in the 40—60 nm region can be taken as an indication of an islandlike model for the porous layer, where zerovalent

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parallel XPS—XAES measurements, coupled with Ar> sputtering. The electronic states of Si in the porous layer can be monitored by measuring the Auger parameter, a, for Si. It is shown that differently prepared and stored samples present distinct environments for porous Si, which result in characteristic a values. The nature of the oxide layer associated with porous Si in air-exposed samples is shown to be close to silica gel. A model for the porous layer in terms of Si grains embedded in a silica gel matrix is then supported by the results obtained for the air-exposed samples prepared by this method.

Fig. 3. Variation in the modified Auger parameter values for the zerovalent Si component of sample L3, as a function of the sputtered time or depth, obtained in the same conditions as in Fig. 2.

Si grains are dispersed in a matrix with variable density. Even in the case of the extensively oxidized samples L1, H1 and HW, zerovalent Si can be found, associated to oxidized Si. The corresponding peak components in the Si 2p and Si KLL Auger spectra can be easily separated, as reported in Table 1, allowing two series of corresponding a values to be extracted. We found that the silica-related Si2p and a fall at the corresponding literature values for silica gel: 103.59, 1711.46 eV [15]. A model for the porous Si layer can then be proposed, for the airexposed samples, in which small ensemble of Si atoms are embedded in a gel-like silica matrix.

4. Conclusions Controlled preparation, storing and handling of stain-etched porous Si films result in oxide-free porous Si surfaces. The surface and in-depth chemical nature of the stained layer was investigated by

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