Localized organic grafting on photosensitive semiconductors substrates

Journal of Electroanalytical Chemistry 622 (2008) 238–241

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Short Communication

Localized organic grafting on photosensitive semiconductors substrates J. Charlier a,*, E. Clolus a, C. Bureau b, S. Palacin a a b

CEA, IRAMIS, SPCSI Chemistry of Surfaces and Interfaces, F-91191 Gif sur Yvette, France Alchimer S.A., 15 rue du Buisson aux Fraises, Z.I. de la Bonde, F-91300 Orsay, France

a r t i c l e

i n f o

Article history: Received 11 April 2008 Received in revised form 19 May 2008 Accepted 3 June 2008 Available online 11 June 2008 Keywords: Electro-photoinitiated grafting Microelectronics Organic coating Patterning Surface functionalization

a b s t r a c t This work is devoted to the development of a process which allows, in only one step, the localization of organic coating chemically bonded to initially homogeneous substrates thanks to the coating process itself. Photoconductive properties of silicon are used to fulfil this goal. Light illumination through a mask, that activates the photoconductive properties of the substrate was used to promote the local electrografting of thin organic coatings. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction The aim of this work is to develop a process which allows, in only one step, the localization of organic coating chemically bonded to initially homogeneous substrates. The purpose of this subject is to obtain these properties only because of the coating process (electrografting) itself, and not because of any threedimensional topological structuring of the substrate nor – a priori – because of selective interactions between the deposited molecules and some areas of the substrate. Photoconductive properties of silicon (chosen here as a reference substrate for its importance in microelectronics devices) are used to fulfil this goal. As recently reviewed by Buriak for SiH surfaces, and Reinhoudt for silica surfaces, many routes exist for chemical grafting of molecular species on those surfaces [1,2]. Among the different reactive species derived from stable organic molecules that can give rise to the formation of grafted films, radicals are very often used because they can be easily generated in ambient conditions, both in solution and in gaseous phases. So, it is not surprising that most grafting reactions involve steps of radical formation. For example, densely packed alkyl monolayers covalently bound to Si surfaces can be obtained by the pyrolysis of diacylperoxides, which creates alkyl radicals that react with the H-terminated Si substrate [3,4]. Diazonium compounds, whose chemistry has been known for long due to their applications in dye chemistry and photochemistry, are also prone to easily generate radicals. As there is a large variety of benzenediazonium compounds that can be synthesized * Corresponding author. Tel.: +33 1 69 08 21 49; fax: +33 1 69 08 64 62. E-mail address: [email protected] (J. Charlier). 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2008.06.007

with different chemical groups linked to the benzene ring it is thus possible to engineer interfaces between silicon and organic layers with properties which are determined by these chemical groups. The originality and the specificity of our approach are thus to combine the diazonium electrochemistry and the photoconductive properties of silicon in order to carry out a ‘‘spatially resolved functionalization”, thanks to localized light irradiation. This method could then constitute an alternative to the usual ways of microand nano-electronics (lithography, pre-deposits, etc.) and selfassembled layers [5–9]. 2. Experimental P-type silicon wafers (h1 0 0i) with native oxide layer having different doping levels (labelled hereafter Si–p, Si–p+, Si–p++, for low, medium and high doping level, respectively) were used as cathodes for the electrografting process. Before electrografting, the wafers were used as received (thus with the native oxide layer) and then cleaned as usual (ultrasonic rinsing with water, acetone and ethanol successively) to discard organics from the silicon oxide surface. We evaporated a continuous gold layer (on a chromium underlayer) on the back side of the sample to set the electrical contact and to ensure an ohmic contact (InGa eutectic method). Thus the same potential was applied evenly through the whole silicon substrate, despite its intrinsic resistance. The electrolytic medium was made of acidic aqueous solution of nitrobenzene diazonium tetrafluoroborate (NBDA) 2  103 mol dm3 in H2SO4 0.2 mol dm3. The electrolysis was performed in a one-compartment polypropylene cell where one side consists of a glass port-hole transparent to light irradiation, at room temperature

J. Charlier et al. / Journal of Electroanalytical Chemistry 622 (2008) 238–241

with a standard three-electrode arrangement (Pt counter electrode and the Hg/HgSO4 reference electrode (MSE; 0.64 V vs. NHE)). The electro-initiated grafting technique consists in applying cyclic linear voltammetry or amperometric mode to the solution. The irradiation source used was a filtered polychromatic source with k > 590 nm operating at 3 mW cm2 (Fiber Optic Illuminator 77501, Oriel). In view to obtain a well focused beam for localization applications, an optical lens was added to the experimental setup so as to minimize reflective interferences in the cell. The localization of the organic grafts was studied by optical microscopy and X-ray photoelectron spectroscopy (Vacuum Generator Escalab 210 spectrometer, Al Ka monochromatic source).

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positions seem likely to be subject to the influence of the light irradiation, in particular for the Si–p++ and Si–p+ samples. When the surfaces of these samples are illuminated, E.S.1 and E.S.2 positions are shifted towards less negative potentials while the surface states positions of the Si–p samples appear not very affected by the illumination. All those parameters are summarized in Table 1 and in Fig. 3. According to literature, grafting of aryl species to Si surfaces is possible via the electrochemical formation of radicals at the surface [14–20]. To our knowledge, most of the works undertaken on the grafting of diazonium salts on silicon substrates are carried out on Si–H-terminated substrates [15,18–24]. However, success-

3. Results and discussion The chemical integrity of the NBDA under those illumination conditions was checked. Indeed, for wavelengths higher than 550 nm, the absorbance of the solution tends to zero (Fig. 1a). The N 1s core level XPS spectrum of the electrografted coating obtained on the Si–p++ substrate after electrochemistry under illumination displays the characteristic peaks at 406 and 400 eV that are attributed to the nitrogen of the nitro group and to the azo groups inside the grafted structure, respectively [10–12]. This result means that the diazonium salt kept its chemical integrity under illumination and grafting. This was also confirmed by the voltammograms recorded both on a fresh diazonium solution and after 2 h of exposition to the filtered light source, which did not show significant modification (Fig. 1c). The characteristics of the various samples (doping level, flatband potential, surface states, etc.) were determined by electrochemical impedance measurements (EIS) in darkness and under illumination. The presence of surface states located in the gap [13] was highlighted (Fig. 2). Two distinct energy positions (E.S.1 and E.S.2) could be allocated to these states (arrows in Fig. 2). Their

Fig. 2. Impedance imaginary part vs. potential at 500 Hz carried out in the dark (square) and under illumination (circle) for a Si–p++ sample.

Fig. 1. (a) UV spectrum of the nitrobenzene diazonium salt in aqueous acidic solution (H2SO4 0.2 mol dm3); (b) N 1s XPS spectrum of the grafted film obtained under illumination by cyclic voltammetry on Si–p++; (c) cyclic voltammogram recorded on Si–p++ under illumination for a fresh nitrobenzene diazonium salt solution (- - -) and after 2 h irradiation (—).

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Fig. 3. Energy diagram representing the characteristic parameters of the various samples determined by electrochemical impedance spectroscopy: VB: valence band, CB: conduction band, EF: Fermi level, black and white squares: surfaces states positions in dark and under illumination, respectively, dotted lines: diazonium reduction half-peak potential (Eredox).

ful grafting of diazonium salts on oxidized surfaces (oxidized metallic surfaces [25], TiO2 [26]) have been already reported. We have successfully electrografted vinylic polymers [27] and diazonium salts [28] on native silicon dioxide. The oxide layer is thin enough (lower than 2 nm thick as measured by XPS from the attenuation of the Si 2p signal according to Himpsel et al. [29]) to allow the tunnelling electron transfer from the underlying silicon to the electrolytic solution. As the native silicon dioxide layer does not prevent electrografting process and as these substrates are much more easily produced and manipulated than Si–H ones, we concentrate mostly our study on silicon substrates with their native oxide layer. The localization principle by photo-assisted electrochemistry was validated after checking first that electroless deposition, if any, was slow enough to be of no effect on the possible photoassisted electrochemical localization performances. As shown in Table 2, electroless process is negligible when compared to the electrochemical grafting. Second, we have demonstrated that the electrochemical deposition occurs on each sample whatever the doping, both in the dark and under illumination. But, as one could expect taking into account the photosensitive properties of silicon, the current intensity measured by voltammetry is much higher under illumination than in the dark, whatever the Si doping

level (Fig. 4). So electrografting process under illumination is much more efficient than in the dark (Table 2). These electrochemical results indicate also that the surfaces states highlighted by EIS do not block the electronic transfer whatever the experimental conditions (dark or illumination). If we look at the evolution with respect to the doping level of the substrate, we observe that the electrochemical process, under illumination, is a little bit more efficient on the lowly doped wafer (Table 2). One explanation could be that, for Si–p, the energy position of the surface states labelled E.S.2 is very close to the top of the reduction peak of the diazonium salt in our electrolytic medium, while for Si–p++ and Si–p+ substrates, the difference between E.S.2 position and the top of the diazonium reduction peak is larger, as illustrated in Fig. 3. We can then assume that these surface states E.S.2 act as more efficient mediators in the charge transfer process in the case of the Si–p substrate. Indeed, the electrochemical reactions on a semiconductor electrode involve charge transfer between the species in the solution and the charge carriers in the semiconductor. The basic assumption in the kinetics of charge transfer reactions is that the electron transfer is most probable when the energy levels of the initial and final states of the system coincide [13,30].Thus the efficiency in the redox reaction processes is primarily controlled by the energy

Table 1 Commercial specifications, experimental parameters and surface states positions in darkness and under illumination of the Si h1 0 0i p-type wafers in acidic solution (H2SO4 0.2 mol dm3) Samples

Resistivity (X cm)

Theoretical doped level (cm3)

Experimental doped level

Flatband potential (V/MSE)

Surface states positions in the dark (V/MSE)

Surface states positions under illumination (V/MSE)

Si–p++ Si–p+ Si–p

0.01–0.035 0.1–0.2 5–15

2  1018–1019 1017–2.5  1017 1015–5  1015

1.5  1018 2.1  1017 1.5  1015

0.43 0.50 0.47

E.S.1: 0.65 E.S.2: 1.10 E.S.1: 0.62 E.S.2: 0.96 E.S.1: 0.45 E.S.2: 0.90

E.S.1: 0.53 E.S.2: 0.83 E.S.1: 0.45 E.S.2: 0.75 E.S.1: 0.47 E.S.2: 0.90

Table 2 XPS relative atomic percent of nitrogen measured on the grafted films obtained on p-type Si substrates as a function of the experimental conditions N (relative atomic percent)

Electroless process in the dark (11 h)

Electroless process under illumination (5 h)

Electrochemical process in the dark (cyclic voltammetry between 0.4 and 1.6 V/MSE, 20 mV s1, 30 scans, i.e. 1 h time experiment)

Electrochemical process under illumination (cyclic voltammetry between 0.4 and 1.6 V/MSE, 20 mV s1, 30 scans, i.e. 1 h time experiment)

p++ p+ p

0.3 ± 0.03 – –

0.1 ± 0.01 0.3 ± 0.03 0.6 ± 0.03

0.3 ± 0.03 0.9 ± 0.09 0.6 ± 0.06

6.7 ± 0.7 7.0 ± 0.7 9.6 ± 1.0

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Fig. 5. (a) Optical image of the cross stainless-mask used for photolocalization; (b) optical image of the photolocalized coating on Si–p substrate after photoelectrochemical process: chronoamperometry: 15 s at 0.85 V/MSE.

Fig. 4. Cyclic voltammograms of nitrobenzene diazonium salts (2  103 mol dm3) in acidic solution (H2SO4 0.2 mol dm3) recorded at 20 mV s1 on p-type Si substrates: Si–p (black), Si–p+ (red), Si–p++ (blue), in the dark (dotted line) and under illumination (solid line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

overlap between the quantum states in the energy bands of the semiconductor and the donor or acceptor levels of the reactants in the electrolyte. Often, the overlap between the electronic states in the semiconductor and the levels in the electrolyte are unfavourable, and radical surface states that are located within the band gap become a key intermediate for the charge transfer [31]. We assume this to happen in the case of diazonium reduction on Si–p under illumination. The electrochemical process was successfully applied to a Si–p sample exposed to illumination through a mask. Fig. 5 shows the optical images of the mask and of its local print on the substrate obtained by photoelectrochemical grafting. To improve the resolution of the localized grafting, the amperometric mode was preferred to cyclic voltammetry in order to reduce the duration of the experiment and thus the time during which electroless process may occur. On the basis of the attenuation of the XPS Si 2p signal, we can estimate the mean thickness of the layer as 3–4 nm. 4. Conclusion The localization results thus from the localized modification of the electronic properties of the substrate induced by the illumination. This modification is ‘‘revealed” in situ by the electrografting process. So, by combining local illumination through a mask, photoconductive properties of the semiconductor substrate and electrochemistry, a grafted organic coating representing the image of the mask, with acceptable lateral resolution, is printed on the initially homogeneous substrate. This process may be useful in microelectronics and opens the field to a wide range of applications. Works are in progress to improve the lateral resolution of the technique. Acknowledgment This work was funded in part by CEA/Alchimer S. A. Nanoseed Grant/C5515 and C5515-1.

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