Determination of the specific surface area of porous silicon from its etch rate in HF solutions

surface science letters ELSEVIER

Surface

Science

Letters 306 (1994) L550-L554

Surface Science Letters

Determination

of the specific surface area of porous silicon from its etch rate in HF solutions A. Halimaoui * France Telecom, CNET, BP 98, 38243 Meylan Cedex, France

(Received

11 August

1993; accepted

for publication

6 January

1994)

Abstract

The chemical dissolution of porous silicon (PSI layers hydrofluoric (HF) acid solutions of various compositions has been investigated. It is shown that it is possible to determine the specific surface area of PS from comparison of bulk Si and PS etch rates. We have used this new technique to characterize PS layers obtained from lightly doped p-type Si. It is shown, for the first time, that the specific surface area significanfly decreases when the porosity of the material increases. This surface area decrease is discussed in relation to the photoluminescence (PL) properties of the porous layer. Furthermore, the etch rate of PS, which increases with decreasing HF concentration, is found to correlate more with the pH of the solution rather than the HF concentration.

Electrochemical anodization of lightly doped p-type silicon results in a porous layer which can contain a specific surface area as high as 600 m2/cm3 [l], pore diameters smaller than 5 nm 111

and silicon crystallite sizes in the range of 3 nm [2]. Since the discovery that porous silicon (PS) layers can emit visible light at room temperature [3], the material is more intensively studied. However, many of its properties and the mechanism involved in both the electrochemical and chemical etching processes are still poorly understood. In this Letter, a detailed study of the chemical dissolution process is presented and a model is tentatively proposed. In a previous paper [4], we have demonstrated that purely aqueous solutions of HF, do not infil-

* Corresponding

author.

Fax: + 33 76903443.

0039-6028/94/$07.00 0 1994 Elsevier SSDI 0167-2584(94)00262-7

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trate the tiny pores which constitute PS. In this case, only the top surface of the layer is etched. However, when some ethanol is added to the HF solutions, the wettability of the material is strongly improved. As a result, the ethanoic HF solutions completely infiltrate the pores and the entire volume of the layer is etched [4]. For these reasons, the present study is performed using only ethanoic HF solutions. The PS layers were obtained by anodization of standard 100 mm-diameter, lightly doped (10 R cm) p-type, (lOO)-oriented Si wafers, in HF solutions. The chemical dissolution experiments were carried-out in the dark, at room temperature (N 23°C) and the mass of dissolved silicon is determined by weight measurements. For both the anodization and chemical dissolution experiments, we used the same single-tank Teflon cell which provides a silicon electrode surface area of

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A. Halimaoui / Surface Science Letters 306 (1994) L550-L554

5 0

10

20 Time

30

40

50 Time

(minutes)

Fig. 1. Chemical disolution of PS layers as a immersion time in ethanoic solutions of different trations. (a): 5%, (b): 15% and (c): 25%. The thickness of the starting layer were 65% and 1 tively.

function of HF concenporosiy and Fm, respec-

58 cm’. This surface area is large enough for accurate weight measurements. Fig. 1 shows the mass of dissolved Si from PS layers as a function of immersion time in ethanoic solutions of different HF concentrations. The composition of these solutions is given in Table 1. More details on the pH measurements are given in Ref. [5]. During the chemical dissolution process the gravimetrically determined porosity of the PS layers increases as shown in Fig. 2. For comparison, we also investigated the chemical dissolution of bulk silicon of the same resistivity (10 fi cm> and where only the polished side of the wafer is etched. The results of these measurements are shown in Fig. 3. Since the dissolution times involved are very long (several hours), the dissolution of bulk silicon is achieved by stirring the solution in order to avoid the decrease in HF concentration at the surface. Furthermore, the cell was closed with a cover to

(minutes)

Fig. 2. The porosity as a function of immersion time in ethanoic solution of 5% HF. The porosiy and thickness of the starting layer were 51% and 1 pm, respectively.

prevent any solvent evaporation which might change the concentration of HF. When comparing the chemical dissolution of PS (Fig. 1) and bulk silicon (Fig. 3) it appears that (i) the dissolution of bulk silicon is very low compared to that of PS, (ii) the shape of the curve corresponding to bulk Si is linear while those of PS are not and exhibit a saturation regime, (iii> the dissolution of PS increases with decreasing HF concentration. The same trend is observed with bulk silicon. The etch rate of bulk silicon can be defined as: A = dm/dt, where m is the mass of dissolved silicon per unit area and t the immersion time. The value of the etch rate can be deduced from Fig. 3 and we find A = 1.16 x lop8 g/cmt/min. In term of thickness change, a value of 0.5 A/min can be deduced from this etch rate (A/r = 0.5 A/n@, where r is the silicon density). A value of 0.3 A/min has already been reported [6] for the

Table 1 Composition of the HF solutions used for the chemical dissolution (for example, the 15% HF solution contains 5 volumes of ethanol, 2 volumes of water and 3 volumes of 50 wt% aqueous HF solution) C,H,OH (o/o) 5 15 25

50 50 50

Ef. 45 35 25

[lo])

1.12 0.44 - 0.28

9 0

0+

0

I I

I

I

I

1 I

10

20

30

40

50

Time

(hours)

Fig. 3. Chemical dissolution of bulk silicon in ethanoic solution of 5% HF.

A. Halimaoui / Surface Science Letters 306 (1994) L550-L554

dissolution of (Ill)-oriented n-type silicon in a 48 wt% aqueous HF solution. We believe that the mass of dissolved Si from PS is much higher, as the material contains an enormous specific surface area. If we assume that the silicon crystallites which constitute PS are chemically dissolved at the same rate as bulk silicon (no size effect), the mass M of dissolved silicon per unit area for the PS layer can be expressed as:

-

"E ," "E I 2 t OI ?; c

I

1000 soo--

l

I

l0 00

400--

tJ b

200--

0

0 50

I

I

I

60

70

80

Porosity

dM=ASW

dt,

(1)

where S and W are the specific surface area and the thickness of the PS layer, respectively. We should notice that A4 is the mass of dissolved silicon per unit area: the experimentally measured mass loss is divided by 58 cm* which corresponds to the surface area of the electrode. Since the etch rate of silicon in HF is very low (A = 0.5 A/min) we can assume that the thickness, W, of the layer is constant during the whole dissolution process (N 30 min). To verify this assumption we measured the thickness of a 50%porosity layer before and after etching up to a porosity of u 80% and it was found that the thickness is almost constant. As a result, the reason for which the curves corresponding to PS dissolution are not linear and saturate is that the specific surface area decreases with increasing porosity, i.e. increasing immersion time. Chemical dissolution was also performed on two PS layers of the same porosity and different thicknesses. We found that when the thickness is increased from 0.5 to 1 pm, the mass of dissolved silicon is doubled. This result is in agreement with formula (1) and also means that for both cases (0.5 and 1 pm>, the HF solutions completely infiltrate the pores and reach the interface between PS and the Si substrate. The change in the specific surface area during the chemical dissolution process can be deduced from formula (1). The values of dM/dt can be obtained from the chemical dissolution curves M(t) (Fig. 1). To do this, the curve M(t) is first approximated with a polynomial of the 3rd degree using curve fitting software and the resulting polynomial is then differentiated to extract dM/dt for the desired value of t. The results

I

o a

600--

g z L m

I

b

0 co

--

I 90

100

(9%)

Fig. 4. Specific surface area as a function of PS layer porosity. Data from PS layer 1 pm-thick and starting porosity of 51% (0) and 65% (0). (A): Data from Refs. [l] and [6].

obtained for PS layers of different starting porosities of 50 and 65% and the same thickness of 1 pm are shown in Fig. 4. Also are displayed on this same figure, data from Refs. [l] and [7] which are consistent with our measurements. It appears that the specific surface area decreases with increasing porosity. Furthermore, for a given porosity the two layers used (50 and 65% starting porosities) led to the same specific surface: this means that for PS layers obtained from lightly doped p-type Si, the relevant parameter is the porosity, independently on the anodization conditions. Recently, Robinson et al. [8], have predicted from Fourier transform infrared measurements that the specific surface area of PS should decrease with increasing porosity. Many workers [3,9] have shown that when the porosity is increased up to N 85% the PL intensity is strongly increased. Since the surface area decreases with increasing porosity, the luminescence of surface species, as often suggested [lo], can be ruled out. It has been suggested recently [ll] that many parameters such as the low refractive index of PS, the carrier confinement and the remarkable passivation of the surface by hydrogen terminations, contribute significantly to the enhancement of the PL. Although these parameters are likely to be involved, we believe that the decrease in the specific surface area greatly contributes to the PL increase when the porosity of the layers is increased. In fact, when the surface area decreases, the number of nonradiative re-

A. Halimaoui / Surface Science Letters 306 (1994) LS50-L554

combination centers at the surface decreases thus leading to an increase luminescence efficiency. The chemical dissolution of silicon in aqueous solutions of HF is most probably a two-step reaction [6]. In a first step, silicon is oxidized by hydroxyl ions (OH-) [6] or water and the resulting oxide is dissolved by HF during a second step. Since the etch rate of Si decreases with increasing HF concentration, the second step could not be the rate-controlling step. This means that the oxide is removed as fast as it is formed. Furthermore Hu et al. [6] have demonstrated that the etch rate of n-type bulk silicon in aqueous solutions of HF increased with increasing hydroxyl ions and they suggested that the oxidation of Si by OH- is the rate-controlling step. This model well accounts for the results we obtained with PS layers. In fact, Table 1 shows that when the HF concentration was increased by keeping the same ethanol concentration, both the water content and pH of the solutions decreased. This means that (i) the ethanol is not directly involved in the chemical dissolution process, its role is only to improve the wettability. (ii) The decrease in etch rate with increasing HF concentration is more likely to be due to the decrease in water content and (or) pH of the solutions. To determine whether the water concentration or the pH controls the etch rate, we investigated the dissolution of PS layers using two ethanoic solutions containing the same HF and water concentration but exhibiting different pHs. In the first solution, the HF and water concentration were 5 and 45%, respectively, and the pH was 1.12 (the exact composition is given in Table 1). The pH of the second solution is reduced to 0.45 by adding 37 wt% aqueous HCI while keeping the same HF and water concentration of 5 and 45%. When the etch rate of 65%-porosity and 1 pm-thick PS layers were measured, we found that for a period of 15 min the mass loss was 7 x lo-’ g/cm2 in the first solution and only 5.6 x 10m5 g/cm2 in the second. We also notice that the former mass loss (5.6 X 10m5 g/cm2) is similar to that obtained with the 15%-HF solution of Table 1, which exhibits a pH of 0.44 close to that of the second solution. One can therefore conclude that the etch rate of PS in HF is completely controlled

by the pH of the solution and increases with increasing pH (i.e. the hydroxyl ion concentration). These results demonstrate that for a given HF concentration, the etch rate of PS decreases with increasing porosity. This phenomenon is attributed to the decrease in the specific surface area. The surface area decrease is suggested to be at the origin of PL intensity increase with increasing porosity. Our results show that there is no direct correlation between the etch rate of Si and the HF concentrations examined. The etch rate seems to be more correlated with the pH of the solutions. These results are of great importance for material processing. For example, to reduce the in-depth gradient porosity, which results from the extra chemical dissolution of the already formed porous layer, it is preferable to perform the anodization of the samples using a high HF concentration or an HF solution of low PH. Determination of the specific surface area of PS from its chemical dissolution rate is a reliable and simple technique. We will continue to characterize PS layers of different microstructures. We have recently obtained preliminary results using heavily doped p-type (p + 1 and n-type (n + 1 silicon substrates, and the values of the specific surface area are in agreement with those obtained by the BET method.

This work was undertaken with partial funding by ESPRIT basic research project number 7228 Emission of Light in Silicon (EOLIS).

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121A.G. Cullis and L.T. Canham, Nature 353 (1991) 33.5. [31 L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [41 A. Halimaoui, Appl. Phys. Lett. 63 (1993) 1264.

151C. Bertrand, These de doctorat, Institut National Polytechnique de Grenoble (1986).

161S.M. Hu and D.R. Kerr, J. Electrochem. Sot. 114 (1967) 414.

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Science Letters 306 (I 994) L550-L554

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[lOI Z.Y. Dll

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