Porous silicon fabrication by anodisation: Progress towards the realisation of layers and powders with high surface area and micropore content

Accepted Manuscript Porous silicon fabrication by anodisation: Progress towards the realisation of layers and powders with high surface area and micropore content A. Loni, T. Defforge, E. Caffull, G. Gautier, L. Canham PII:

S1387-1811(15)00152-3

DOI:

10.1016/j.micromeso.2015.03.006

Reference:

MICMAT 7037

To appear in:

Microporous and Mesoporous Materials

Received Date: 3 February 2015 Revised Date:

5 March 2015

Accepted Date: 7 March 2015

Please cite this article as: A. Loni, T. Defforge, E. Caffull, G. Gautier, L. Canham, Porous silicon fabrication by anodisation: Progress towards the realisation of layers and powders with high surface area and micropore content, Microporous and Mesoporous Materials (2015), doi: 10.1016/ j.micromeso.2015.03.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT POROUS SILICON FABRICATION BY ANODISATION: PROGRESS TOWARDS THE REALISATION OF LAYERS AND POWDERS WITH HIGH SURFACE AREA

A. Loni1, T. Defforge2, E. Caffull1, G. Gautier2, and L. Canham1

pSiMedica Ltd, Malvern Hills Science Park, Geraldine Road, Malvern, Worcestershire,

WR14 3SZ, UK 2

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AND MICROPORE CONTENT

Université François Rabelais de Tours, CNRS, CEA, INSA-CVL, GREMAN UMR 7347,

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16 rue P. et M. Curie, 37071 Tours cedex 2, France

* Corresponding author: [email protected]

Abstract

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Tel +33(0) 47 42 40 00

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With a view to producing thick and very high surface area microporous silicon layers (and

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subsequently powders) by electrochemical anodisation, the incorporation of various types of chemical additives has been investigated, these in combination with hydrofluoric acid electrolyte and high-resistivity p-type parent substrates. Comparison under constant charge conditions shows that anodisation using 50 wt% hydrofluoric acid, or inclusion of the additives hydrochloric acid, sulphuric acid, or ammonium dodecylsulfate with lower concentration hydrofluoric acid, can facilitate powders with internal surface areas of up to 864 m²/g, average pore sizes in the region of 2.8-3.2 nm, and pore volumes in excess of 0.8 cm3/g - all as determined using nitrogen gas adsorption and associated isotherm analysis.

ACCEPTED MANUSCRIPT Porous silicon powders with appreciable micropore content have thus been achieved, for the first time. Relevant application areas for such material are diverse, and potentially include energetics, impurity gettering, gas sensing microchips, orthopaedic implants, hydrogen storage, and Li-ion battery anodes.

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Keywords:

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Microporous silicon, mesoporous silicon, surface area, gas adsorption, isotherm analysis.

ACCEPTED MANUSCRIPT I. INTRODUCTION Nanoporous materials, typically with pore diameters in the range 1-100 nm [1], are receiving increasing attention by nanotechnologists. According to current definitions from the

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International Union of Pure and Applied Chemistry (IUPAC) [2], there are three classes of pore that fall into this size regime: micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm). Mesoporous and macroporous silicon, created via electrochemical

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etching techniques, are currently being actively researched for a variety of applications in microelectronics (RF devices, sensors, MEMS, etc…), energy micro-sources (Micro-fuel

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cells, micro-supercapacitors, Li-ion battery anodes, etc…) or biology (therapy, orthopedics, etc…) for instance [3]. Mesoporous silicon layers are mainly obtained in highly-doped silicon (either n- or p-type), whereas macropores are observed after lightly-doped silicon electrochemical etching in dilute hydrofluoric acid (HF) [4, 5]. As for micropores, these

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dimensions are only obtained in very specific conditions, i.e. lightly-doped p-type silicon etched in highly concentrated HF solution [6].

Many of the applications of mesoporous silicon utilise the large internal surface area that is

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accessible, which, when created via wafer anodisation, typically lies in the range 200600 m²/g [7, 8]. In this study, our objective was to investigate the anodisation conditions that

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would create surface areas well in excess of 600 m²/g, assessing the potential upper limits for a process that could generate both nominally thick layers and powders. Gas adsorption analysis is an established technique for quantifying mesopore size, pore volume and surface area in silicon [7-10] so we have utilised this characterisation tool exclusively. . II. EXPERIMENTAL Porous silicon layers were prepared via electrochemical etching of p-type, (100)-oriented silicon substrates. Two different resistivities were utilised in the expectation of achieving

ACCEPTED MANUSCRIPT small pore diameter: 1-5 ohm.cm [8] and 30-50 ohm.cm [11, 12]. Anodisation was possible after rendering the rear sides of these wafers ohmic via boron diffusion at 1050°C for 1 hour. The surface exposed to the electrolyte occupied an area of 3.44 cm². Anodisation was performed under constant charge conditions by varying both current density and etching

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duration. Different etching parameters were tested, including varying the HF concentration in the electrolyte and incorporating, singly, the following additives at various concentrations: acetic acid (AA), hydrochloric acid (HCl), sulfuric acid (H2SO4), and ammonium

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dodecylsulfate (ADS) wetting agent. After anodisation, the samples were rinsed in de-ionised water and dried in air on a hotplate (80-100°C, 10 minutes), to minimise residual electrolyte

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within the pores. Each porous layer was then mechanically detached and the resulting powder dried in ambient air a second time at 200°C for 10 minutes.

Nitrogen gas adsorption/desorption [10] was used to determine surface area, pore volume and average pore diameter for each powder, using a Micromeritics TristaR 3000. The isotherms

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so-obtained indicated the degree of ‘microporosity’ present, primarily from the Type H1 isotherm shape [2, 10]. Computational analysis of the isotherms yielded absolute values for surface area (A), based on the Brunauer-Emmett-Teller (BET) method, pore volume (V),

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based on the Barrett-Joyner-Halenda (BJH) adsorption method, and single-point adsorption

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(4 V/A), for average pore diameter [10]. Table 1 summarises the various processing parameters for the key samples exhibiting microporosity (with a mesoporous control sample, PS0 included for comparison). III. RESULTS AND DISCUSSION Of the seventeen samples processed under various conditions, we concentrate only on those that subsequently exhibited a high degree of microporosity and very high surface area. With reference to Table 1, the smallest average pore diameter and lowest surface area was obtained using 50 wt% HF electrolyte for the lowest resistivity wafers (sample PS1); the isotherm

ACCEPTED MANUSCRIPT shape, which can be classified as Type HI [2, 11], is indicative of appreciable microporosity, as evinced by the lack of significant hysteresis in comparison with the Type H2 hysteresis seen for mesoporous silicon, Figure 1. The addition of HCl to the HF electrolyte had the effect of increasing surface area and pore

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volume, whilst maintaining a similar pore size distribution and isotherm shape. With respect to pore size distributions and gas adsorption/desorption data, there is no apparent trend with increasing HCl content, in the region 7.5 wt%-22 wt%; the associated isotherms are very

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similar to that in Figure 1(a), again indicating significant microporosity, with the highest

wt% HF and no additive (PS1).

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surface area being 796 m²/g (PS3) – significantly higher than the 661 m²/g achieved for 50

The incorporation of either ADS or H2SO4 to the HF electrolyte yielded samples with higher surface area and pore volume than for those anodised using 50 wt% HF, alone (e.g. PS1), but did not improve upon the data achieved for the HCl additive (PS2, PS3, PS4, Table 1).

degree of microporosity.

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However, according to the isotherm shapes, these additives did result in a similarly high

The use of substrates of lower resistivity was found to yield samples with increased values of

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the gas adsorption/desorption parameters, for identical anodisation conditions; this can be

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seen by comparing sample PS8 with PS3, where the highest surface area of 864 m²/g was achieved for porous silicon produced from the 1-5 ohm.cm wafers, with a concomitant level of pore volume (0.806 cm3/g) and small average pore diameter (3.2 nm). Figure 2 compares the average pore size distributions for samples PS0 (mesoporous reference) and PS8, the latter being shifted towards smaller pore size with a very small percentage of pores above 5 nm in diameter. If solid silicon (bulk density of 5x1022 Si/cm3) is rendered mesoporous (50%), with an idealised array of orthogonal cylindrical pores and a mean density of Si atoms on the surfaces

ACCEPTED MANUSCRIPT of 7x1014 cm-3, the theoretical surface area increases rapidly from 344 m²/g at 5 nm pore diameter to 859 m²/g at 2 nm pore diameter [13]. Wholly-microporous silicon of this morphology (pore size in range 1-2 nm and 50% porosity) would have theoretical surface areas in the range 860-1718 m²/g [13]. Prior to the present study, the highest surface areas

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reported for anodised silicon were 812 m²/g for a thin (6 µm) layer [11] and 783 m²/g for a 150 µm thick layer [9] , both prepared in highly concentrated (50/55 wt%) HF. The highest value reported herein, 864 m²/g, significantly exceeds those values and starts to approach the

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1000 m²/g level predicted to be achievable in wholly-microporous material. Our layers are predominantly mesoporous with APD between 2.8-3.2 nm but the microporosity level must

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be significant to account for high surface areas quantified. We anticipate further improvements in surface area can be achieved by combining the anodisation conditions employed herein with supercritical drying [14], rather than ambient air drying, and such work is in progress. Very high surface silicon layers and powders could find applications in

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orthopaedics [15], gas sensing [16], hydrogen storage [17], gettering [18], energetics [19], and energy storage [20, 21]. IV. CONCLUSION

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In this paper, we showed that very high surface area porous silicon layers with high

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micropore content can be achieved by electrochemical anodisation of high-resistivity p-type substrates. Various chemical additives such as hydrochloric acid, sulphuric acid, or ammonium dodecylsulfate in highly concentrated hydrofluoric acid electrolyte have been investigated. Using nitrogen gas adsorption and isotherm analysis, it was shown that powders with internal surface areas of up to 864 m²/g, average pore sizes in the region of 2.8-3.2 nm, and pore volumes in excess of 0.8 cm3/g can be achieved in HF – HCl mixture. To the best of our knowledge, it is the highest surface area reported to date for anodised porous silicon.

ACCEPTED MANUSCRIPT Post-anodisation treatments such as supercritical drying are expected to provide further

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improvements in both pore volume and surface area.

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REFERENCES [1] ‘NANOPOROUS MATERIALS: SCIENCE AND ENGINEERING’, Edited by G O Lu and X S Zhao (Imperial College Press, London, 2004) ISBN 1860942113 [2] K.S.W. Sing, Pure & Appl Chem. 34 (1982) 2201-2210.

2014) ISBN 978-3-319-05743-9.

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[3] L.T. Canham in HANDBOOK OF POROUS SILICON’, Edited by L Canham (Springer,

[4] V. Lehmann, R. Stengl, A. Luigart, Mater. Sci. Eng. B 69 (2000) 11-22. [5] X.G. Zhang, J. Electrochem. Soc. 151(2004) C69-C80.

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[6] L.T. Canham in HANDBOOK OF POROUS SILICON’, Edited by L Canham (Springer, 2014) ISBN 978-3-319-05743-9.

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[7] G. Bomchil, R. Herino, K. Barla, J.C. Pfister, J. Electrochem. Soc. 130 (1983) 1611-1614. [8] R. Herino, G. Bomchil, K. Barla, C. Bertrand, J.L. Ginoux, J. Electrochem. Soc. 134 (1987) 1994-2000.

[9] M. Ruike, M. Houzouji, A. Motohashi, N. Murase, A. Kinoshita, K. Kaneko, Langmuir 12 (1996) 4828-4831.

[10] A. Loni in HANDBOOK OF POROUS SILICON’, Edited by L Canham (Springer,

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2014) ISBN 978-3-319-05743-9.

[11] L.T. Canham, A.J. Groszek, J. Appl. Phys. 72 (1992) 1558-1565. [12] J. Semai, G. Gautier, L. Ventura, J. Nanosci. Nanotechnol. 9 (2009) 3652-3656. [13] V. Lehmann, Chapter 6.4 (Wiley/VCH, 2002) ISBN 3-527-29321-3.

(1994) 133-135.

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[14] L.T. Canham, A.G. Cullis, C. Pickering, O.D. Dosser, T.I. Cox, T.P. Lynch, Nature 368

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[15] L.T. Canham, Adv. Mater. 7 (1995) 1033-1037. [16] J. Gao, T. Gao, Y.Y. Li, M.J. Sailor, Langmuir 18 (2002) 2229-2233. [17] V. Lysenko, F. Bidault, S. Alekseev, V. Saitsev, D. Barbier, C. Turpin, F. Geobaldo, P. Rivolo, E. Garrone, J. Phys. Chem. B 109 (2005) 19711-19718. [18] I. Kuzma-Filipek in HANDBOOK OF POROUS SILICON’, Edited by L Canham (Springer, 2014) ISBN 978-3-319-05743-9. [19] M. du Plessis in HANDBOOK OF POROUS SILICON’, Edited by L Canham (Springer, 2014) ISBN 978-3-319-05743-9. [20] E. Luais, F. Ghamouss, J. Wolfman, S. Desplobain, G. Gautier, F. Tran-Van, J. Sakai, J. Power Sources 274 (2015) 693-700.

ACCEPTED MANUSCRIPT [21] X. Li, M. Gu, S. Hu, R. Kennard, P. Yan, X. Chen, C. Wang, M.J. Sailor, J.G. Zhang, J

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Liu, Nature Comm. 5 (2014) 4105.

ACCEPTED MANUSCRIPT Figure captions

Figure 1:Adsorption/desorption isotherm for sample produced using (a) 50 wt% HF electrolyte, sample PS1, and comparison with (b) a typical mesoporous sample, PS0.

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Figure 2: Pore size distribution for mesoporous (PS0) and silicon with a high concentration of

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micropores (PS8).

ACCEPTED MANUSCRIPT Tables

Table 1. Processing parameters and nitrogen gas adsorption data of porous silicon powders: AA – acetic acid, HCl – aqueous hydrochloric acid, ADS - ammonium dodecylsulfate,

Wafer

HF:H2O

Additive &

BET

Pore

Resistivi

Concentratio

Concentration

(m²/g)

Volume

ty ( cm)

ns (wt %)

(wt %)

PS0

1-5

30:45

AA (25)

437

PS1

30-50

50:50

none

PS2

30-50

40:52.5

HCl (7.5)

PS3

30-50

30:55

HCl (15)

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PS4

30-50

20:48

PS5

30-50

50:50

PS6

30-50

30:55

PS7

30-50

30:32

PS8

1-5

30:55

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APD (nm)

(cm3/g) 0.988

8.9

0.410

2.8

756

0.686

3.0

796

0.758

3.3

HCl (22)

745

0.741

3.5

ADS

662

0.580

2.9

H2SO4 (15)

708

0.782

4.1

H2SO4 (38)

640

0.642

3.6

HCl (15)

864

0.806

3.2

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Sample

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sulphuric acid (H2SO4); APD – average pore diameter (anodisation at 65mA/cm2 for 2hr).

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Investigation of silicon electrochemical anodization with various types of chemical additives in HF electrolyte Porous silicon internal surface areas of up to 864 m²/g in HF – HCl mixture Pore sizes in the range of 2.8-3.2 nm

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