XPS and SEM studies of oxide reduction of germanium nanowires

Journal of Non-Crystalline Solids 356 (2010) 1988–1993

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

XPS and SEM studies of oxide reduction of germanium nanowires V. Grossi ⁎, L. Ottaviano, S. Santucci, M. Passacantando Dipartimento di Fisica, Università degli Studi dell'Aquila, Via Vetoio 10, I-67010 Coppito (L'Aquila), Italy

a r t i c l e

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Article history: Received 16 May 2010 Available online 17 June 2010 Keywords: Germanium; Oxidation; Nanocrystalline materials; Scanning electron microscopy; X-ray photoelectron spectroscopy

a b s t r a c t Single-crystal germanium nanowires (GeNWs) were grown by vapour–liquid–solid deposition onto silicon oxide substrates with Au catalyst nanoparticles. The GeNW surface and morphology were studied by means of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. Complete oxidation of the GeNW outer shells was evidenced by an XPS analysis. Most of the surface oxide could be removed by aqueous HF, aqueous HCl, and by pure H2O. A complete reduction of the oxide was evidenced after chemical treatment by XPS measurements. Furthermore, we could observe by a SEM analysis that the chemical etching produced a reduction of the diameter of the GeNWs, in agreement with effective surface oxide removal. In addition, the GeNW oxidation process was controlled after various exposure times in ambient atmosphere. © 2010 Elsevier B.V. All rights reserved.

1. Introduction One-dimensional (1D) nanomaterials have been proposed for use in numerous applications due to their unique optical, mechanical, and electrical properties [1–3]. Germanium nanowires (GeNWs) are a material arousing large interest for future computing owing to their higher carrier mobility than silicon nanowires (SiNWs) [4] and to an exciton radius of 24.3 nm, larger than Si (4.9 nm) with prominent quantum confinement effects [5]. Thus, they offer an appealing potential as a candidate of choice for future nanoelectronics. Nevertheless, a detailed understanding of the surface chemistry is required to meet the technological expectations. The chemical and electronic stability of nanowire surfaces is particularly important for such applications as building blocks for nanoscale 3D integrated circuits (field-effect transistors based on p-type GeNWs) [6] and chemical and biological sensors which require direct interfacing with their surrounding environment [7–10]. Few recent works report a chemical modification of Si [11] and Ge [12–15] semiconductor nanowire surfaces. It is known that the water solubility of germanium oxide (GeOx) [16] and the lack of a stable oxide have prevented Ge from being utilized as an electronic material in the past. The surface oxidation is a crucial problem for the GeNWs that have considerably higher surface areas than their bulk counterparts. GeOxfree surfaces can be obtained by developing chemical passivation of Ge, i.e. by exposure to dilute HCl or HF acid solutions or even pure water, to prevent oxidation [12–15,17]. Several methods for a pure GeNW synthesis have been reported: vapour transport [18,19], low-temperature chemical vapour deposi-

⁎ Corresponding author. Tel.: +39 3471927499. E-mail address: [email protected] (V. Grossi). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.05.042

tion (CVD) [20,21], laser ablation [22], liquid solution synthesis [23], and supercritical fluid–liquid–solid (SFLS) synthesis [24,25]. In this paper we present a chemical and morphological study of GeNWs grown by vapour–liquid–solid (VLS) deposition. In order to remove the GeNW surface oxide, we exposed them to dilute HCl and HF acid solutions and to pure water. The oxidation and possible passivation effects of the GeNW surface were studied by means of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). 2. Experimental GeNWs were synthesised by thermal evaporation of Ge powder onto SiO2/Si(100) substrates with Au catalyst nanoparticles. This mechanism is known as vapour–liquid–solid (VLS) growth. Au thin films (about 5 nm) were deposited by sputtering onto SiO2-terminated (a few nanometers of a native SiO2 layer) silicon (100) substrates, and then the samples were annealed at 500 °C for 5 min in a reactor for GeNW growth, obtaining a direct formation of gold nanoclusters [26,27]. The VLS growth was performed in a horizontal quartz tube mounted inside a furnace where the gradient of temperature was monitored by a thermocouple. Ge powder (Aldrich, 99.99%) was put in an alumina crucible placed in a high temperature zone of the furnace (middle of the tube). The sample was put on a graphite holder at 20 cm from the crucible position on the downstream side of the tube. For the GeNW growth the quartz tube was heated by radiation in vacuum (2 × 10− 5 Torr) up to a temperature of 700 °C. Then, a flux of 30 sccm of argon (used as carrier gas) was introduced into the quartz tube. The furnace temperature was increased to 950 °C and kept at

V. Grossi et al. / Journal of Non-Crystalline Solids 356 (2010) 1988–1993

1989

this temperature for 30 min, thus completing the GeNW growth. The temperature at the substrate position was 500 (±5) °C. Different strategies for chemically modifying the NW surfaces were explored outside the reactor. GeNWs were treated with aqueous 5% HF or aqueous 5% HCl or pure water for 5 min in order to remove the native oxide. The morphology of the samples was investigated by means of scanning electron microscopy (SEM) (ZEISS-GEMINI LEO 1530), while the chemical composition was determined by X-ray photoemission spectroscopy (XPS) (PHI 1257 spectroscope, Al X-ray source, hν = 1486.6 eV). C 1s, located at a binding energy (BE) of 284.5 eV, was used for the spectra calibration.

Table 1 Average diameter and width (Full-Width at Half-Maximum FWHM), calculated by a Gaussian Fit (for as-grown and H2O treated GeNWs) and by a Log-Normal Fit (for HCl and HF-treated GeNWs), of the diameter histogram for different samples. We report the diameter range of 8 nm where there are the most size values (%).

3. Results

during the growth process when the Au catalyst particles formed liquid Au–Ge droplets, highly susceptible to aggregation with other Au seed particles. Fig. 1(c) shows an SEM image of GeNWs after treatment in aqueous 5% HF for 5 min. The macroscopic investigation showed that the GeNW film was uniform on the substrate and the microscopic SEM inspection revealed that the density of NWs was similar to the as-grown GeNWs. The Au–Ge particles were visible but with greater difficulty with respect to the HCl-treated sample. Nevertheless, the average diameter (17.2 nm) (inset (c)) was the same as the HCl-treated NWs but it was smaller than those of the asgrown GeNWs. 80% of size values were in the range between 16 nm and 24 nm (Table 1). Fig. 1(d) shows the SEM image of GeNWs after immersion in pure H2O for 5 min. In this case the samples appeared covered with a light brown film. The SEM investigation showed that the density of the GeNWs was actually lower than the density of the as-grown and treated GeNWs; then, the Au–Ge particles on the substrate were more visible than the other samples. The average diameter was 21.9 nm with a lager diameter distribution (inset (d)) than the acid-treated GeNWs: 73% of the size values were in the range between 16 nm and 24 nm (Table 1).

The GeNWs were grown onto a large area (∼4 cm × 4 cm) of the Si substrate obtaining a uniform opaque brown film. The sample was cut into four parts. Fig. 1 shows SEM images of GeNWs as-grown (a) and after HCl (b), HF (c), and H2O (d) treatment. By this morphological investigation, we observed a dense and intricate net of nanowires for the asgrown GeNWs (Fig. 1(a)). The GeNWs showed a length of 10–20 µm and a uniform and narrow diameter distribution (inset (a)). In particular, the average diameter was estimated to be ∼ 35.9 nm, with the 90% of size values in the range between 32 nm and 40 nm (Table 1). Fig. 1(b) shows an SEM image of the sample after treatment in aqueous 5% HCl for 5 min. The sample is covered by the original brown film and the SEM inspection still revealed the presence of GeNWs. They showed a similar density and a smaller average diameter than those of the as-grown GeNWs (inset (b)). The average diameter was ∼17.2 nm, with the 83% of size values in the range between 16 nm and 24 nm (Table 1). The nanoparticles were visible under the GeNWs. They were the Au–Ge particles which formed

Treatment

Average diameter d (nm)

FWHM Δd (nm)

Range of diameter (nm)

%

As-grown HCl 5% HF 5% H2O pure

35.9 17.2 17.2 21.9

9.0 10.1 10.4 11.3

32.0–40.0 16.0–24.0 16.0–24.0 16.0–24.0

90 83 80 73

Fig. 1. SEM image of GeNWs: (a) as-grown and after (b) HCl 5% treatment for 5 min, (c) HF 5% treatment for 5 min, and (d) H2O pure treatment for 5 min. The insets are the diameter histograms with the corresponding Gaussian Fits (inset (a),(d)) and Log-Normal Fits (inset (b),(c)).

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Fig. 2. Ge 3d (29.4 eV) and GeOx 3d (31.1 eV and 32.4 eV for Ge3+ and Ge4+ oxidation states, respectively) XPS peaks of as-grown GeNWs, as-treated in HCl, as-treated in HF and as-treated in pure H2O.

The chemical XPS analysis of GeNWs provided information about the oxidation state of the GeNW surface after exposure to different chemical treatment and after various exposure times in air atmosphere. Fig. 2 shows the XPS Ge 3d spectra of air exposed GeNWs immediately after growth and after treatment with acids or pure water. The lines reported in Fig. 2 correspond to the BE positions of Ge 3d (29.4 eV) and GeOx 3d (32.1 eV for Ge3+, 32.6 eV for Ge4+), respectively. From Fig. 2 we can see a high intensity signal of the peak due to the GeOx for the as-grown sample. A quantitative analysis of the GeNW as-grown peaks indicated that the GeOx/(GeOx + Ge) intensity ratio was 0.85. Chemical processes were used to remove the GeOx. Fig. 2 shows a considerable reduction of the GeOx signal for different samples treated by various chemical processes as discussed before. The GeOx/(GeOx + Ge) intensity ratio decreased to 0.05 for the HCl- and HF-treated GeNWs, and to 0.09 for the H2O-treated GeNWs (Table 2). In addition, the oxidation process of the chemically treated GeNWs was controlled by XPS measurements after various exposure times in ambient atmosphere: 7, 14, 21, 28, and 60 days. The GeOx/(GeOx + Ge) intensity ratios of the Ge and GeOx peaks at different times are also reported in Table 2. All the XPS measurements were performed on the same area of the samples. Fig. 3 shows that GeNWs, treated with HCl 5%, remained partially passivated for more days after air exposure, in fact, a slow increase in the GeOx was reported. However, the Ge peak remained 5 times more intensive than the GeOx peak, also after 60 days. The inset of Fig. 3 shows the XPS survey spectra of as-grown and as-treated (HCl) GeNWs. After HCl treatment a Cl 2p signal appeared and the Ge peaks (3d, 3p, and 3s) shifted to lower BE, from Ge oxidation state to a Ge metallic state.

Fig. 3. Ge 3d (29.4 eV) and GeOx 3d XPS peaks of GeNWs treated in HCl (diluted 5% for 5 min) and exposed to ambient air for different days. The inset shows the XPS spectra of as-grown and HCl as-treated GeNWs.

The Ge 3d spectrum of the GeNWs treated with HF 5% (Fig. 4) showed a pronounced GeOx peak after 7 days; the peak intensity did not change noticeably after 28 days. It was only after 60 days that there was an increase in the GeOx intensity peak that remained 4 times lower than the Ge peak. The inset of Fig. 4 shows the XPS survey spectra of as-grown and as-treated (HF) GeNWs. At 685.9 eV the F 1s peak was not present after the acid treatment and the O 1s peak almost completely disappeared. The Ge 3d XPS results of the GeNWs immersed in H2O are reported Fig. 5. In this case we could evidence a faster oxidation process: after 60 days the GeOx:Ge 3d intensity ratio was equal to 1:3. 4. Discussion Previous high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) measurements have demonstrated that a high-quality single GeNWs crystal is grown but the surface is encapsulated by an oxide outer shell [28]. A layer of ∼15 nm

Table 2 Intensity ratio of XPS peaks of germanium oxide and germanium. The peak intensity ratio GeOx/(GeOx + Ge) was calculated for the GeNWs immediately after chemical treatment and after various periods of air exposure. Treatment

Intensity ratio GeOx/(GeOx + Ge) As-treated

7 days

14 days

21 days

28 days

60 days

HCl 5% HF 5% H2O pure

0.05 0.05 0.05

0.13 0.22 0.16

0.15 0.22 0.18

0.16 0.20 0.23

0.18 0.19 0.27

0.20 0.25 0.28

Fig. 4. Ge 3d (29.4 eV) and GeOx 3d XPS peaks of GeNWs treated in HF (diluted 5% for 5 min) and exposed to ambient air for different days. The inset shows the XPS survey scans of as-grown and HF as-treated GeNWs.

V. Grossi et al. / Journal of Non-Crystalline Solids 356 (2010) 1988–1993

Fig. 5. Ge 3d (29.4 eV) and GeOx 3d XPS peaks of GeNWs treated in pure H2O for 5 min and exposed to ambient air for different days.

of amorphous GeOx covers the NW surface [28]. By the TEM and SEM (Fig. 1(a)) analyses we estimated that ∼ 83% of NWs was GeOx. Due to the high GeOx solubility in aqueous solutions, it could be removed by exposure to dilute HCl or HF acid solutions or even in pure water. After the treatment the NWs still covered the sample surface uniformly (Fig. 1(b,c,d) but they presented a smaller average diameter than the as-grown GeNWs (Table 1). There was also a different diameter distribution between the as-grown and acid-treated NWs. The nanoparticles which were formed from a film rupture process had a Log-Normal size distribution. We expect that the NW diameters have the same distribution as the nanoparticles because they grow from Au nanoparticles. A morphological analysis (Fig. 1) has shown that we obtain a Log-Normal diameter distribution for the GeNWs treated with acids and a Gaussian diameter distribution for the asgrown and H2O-treated GeNWs. We can observe that a larger diameter distribution, which also corresponds to a bigger oxide layer (see Tables 1 and 2) results in obtaining a Gaussian distribution of the GeNW diameter. However, the size distribution, for all samples, is narrower than the one reported by Tuan et al. [29] and than the one of our previous work [28]. From the SEM images and from a statistical analysis of diameters we deduce that the acid treatment is much more effective at removing the oxide layer than submersion in pure water because it leaves residual oxide on the surface with non-uniform thickness [14]. However, pure water is more effective in order to mechanically remove the GeNWs from the substrate as the SEM image (Fig. 1(d)) shows. The oxide process can be attributed to the exposure in air of the sample after growth, but we cannot exclude that, during the GeNW growth, the presence of residual oxygen in the reactor can be responsible for the surface NW oxidation. The next step will be to prevent oxidation during the growth phase by performing the growth at a low pressure (in UHV environment) or in a H2 flow [12]. The XPS study confirms the effectiveness of treatment in order to remove the oxide (Fig. 2). We studied the oxidation process by XPS after various exposure times in ambient atmosphere. After 1 week of exposure to atmospheric oxygen, a GeOx peak appeared. For HCl and H2O treatment this GeOx peak increased monotonically during the time (Fig. 3 and 5), and the rate of growth was faster for H2O than for the HCl treatment (see Table 2). For the HF-treated GeNWs the GeOx peak had an intensity ratio of 0.22 (Table 2) and remained stable for 28 days (Fig. 4). After 60 days, the intensity ratio increased and became bigger than that of the HCl-treated GeNWs (Table 2). By comparing the trend of the intensity ratios with the time we deduced

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that the GeNWs immersed in aqueous HCl had the most air-stable surface also after 60 days. Moreover, we had the presence of a Cl 2p peak (inset Fig. 2) and the absence of an F 1s peak (inset Fig. 3) in the XPS survey scan after the aqueous HCl and HF treatment of the GeNWs, respectively. Therefore, there is a good probability that the GeNWs treated with HCl are chlorine-terminated, as demonstrated in [12–15,30], because we had a weak increase in the oxide also after a long time (60 days), with an intensity ratio of 0.20. The Ge 3d photoelectron spectroscopy of GeNWs provided also information about the oxidation states of the Ge surface, as-grown (Fig. 6), and after exposure to various chemical environments (Fig. 7). The XPS spectra were deconvoluted to determine the extent of oxidation and the Ge oxidation state. In order to fit the Ge 3d and GeOx peaks, Voigt line shapes (a convolution of a Gaussian and a Lorentzian) were used during the least-squares fitting, and the Shirley background was applied (Figs. 6 and 7). The Ge 3d doublet, with a 3d3/2 and 3d5/2 spin–orbit splitting of 0.585 eV with an intensity ratio of 0.58 [31], has not been resolved. Therefore, each oxidation state was fitted by a single Voigt function. The Ge 3d peak obtained from as-grown nanowires was fitted as a combination of the bulk Ge0+ substrate feature (29.4 eV) and two oxide peaks associated with the Ge3+ and Ge4+ oxidation states with chemical shifts of 2.7 and 3.2 eV, respectively (Fig. 6). The Lorentzian and Gaussian width was eV and 2.0 eV, respectively. The binding energies of these oxide spectral components were consistent with literature reports [32]. The formation of a 3+ oxidation state requires that more Ge bonds break, meaning that the oxidation process must be more extensive. Probably, a greater part of Ge3+ oxide grows during the nanowire formation and the 4+ oxidation state appears after the light exposure, and it is in particular the UV light component that induces the Ge4+ oxide growth [13]. After exposure to various chemical environments the oxide peak is nearly completely removed and, with the Ge0+ substrate feature, it is only the Ge1+ and Ge3+ oxidation states that are present with chemical shifts of 0.8 and 2.7 eV, respectively (Fig. 7(a,c,d)). The Ge1+ oxidation state appears immediately after exposure to air oxygen and the Ge3+ occurs readily in water [14] due to the bond between Ge and –OH groups [33]. The Lorentzian and Gaussian widths for these fits were 0.4 eV and 1.1 eV, respectively. A prolonged exposure to oxygen atmosphere (60 days) increased the extent of oxidation with the appearance of Ge2+ and Ge4+ components, an increase in Ge3+ species, and the disappearance of the Ge1+ oxide state (Fig. 7(b,d,f)). For the Ge2+, Ge3+, and Ge4+ the chemical shifts were 1.8, 2.7 and 3.2 eV, respectively. 60 days of air

Fig. 6. XPS Ge 3d peak of the as-grown GeNWs. This peak has been fitted (bold line) as a combination of various oxidation states (thin lines) of the bulk Ge0+ substrate feature: Ge1+, Ge2+, Ge3+, and Ge4+ with chemical shifts of 0.8, 1.8, 2.7, and 3.2, respectively.

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Fig. 7. XPS spectra of Ge 3d peak of GeNWs: (a) HCl as-treated and (b) after 60 days of air exposure, (c) HF as-treated and (d) after 60 days of air exposure, (e) H2O as-treated and (f) after 60 days of air exposure. All peaks have been fitted (bold line) as a combination of the bulk Ge0+ substrate feature and four oxide peaks (thin lines) associated with the Ge1+, Ge2+, Ge3+, Ge4+ oxidation states (with chemical shifts of 0.8, 1.8, 2.7, and 3.2, respectively).

exposure of GeNWs led to the growth of oxide which had the predominant oxidation state of Ge4+ (light exposure) and a comparable quantity of Ge2+ and Ge3+ components (Fig. 7(b,d,f)). The Lorentzian and Gaussian width of the Voigt functions was 0.3 eV and 1.2 eV, respectively. 5. Conclusions In conclusion, single-crystal GeNWs grew by the VLS mechanism. We showed that the as-grown GeNWs had an average diameter of 35.9 nm and that they were completely oxidized. The acid treatment (diluted 5% HCl or HF) for 5 min was effective to remove the outer

oxide shell of GeNWs. After this treatment the average diameter of NWs was reduced to 17.2 nm; the XPS analysis showed that the GeOx was in a smaller quantity than Ge, confirming partial passivation of the nanowire surface, also after 60 days. The pure water treatment produced a reduction of the average diameter of NWs (21.9 nm), but it was less effective to remove the GeOx and to passivate the NWs with time. The best treatment results were achieved by immersion in aqueous HCl (5%) for 5 min. We performed an analysis of the Ge oxidation states by a deconvolution of the Ge 3d and GeOx 3d peaks. This analysis showed that, immediately after chemical treatment, the predominant oxidation state was the Ge1+, and after a long air and light exposure

V. Grossi et al. / Journal of Non-Crystalline Solids 356 (2010) 1988–1993

(60 days), the predominant component was Ge4+ with smaller Ge2+ and Ge3+ contributions. The next step is to prevent GeNWs oxidation during growth: 1) to grow GeNWs at low pressure (in UHV environment); and 2) to grow NWs in a H2 flow (hydrogen passivates the NW surface). References [1] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [2] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [3] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [4] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [5] Y. Maeda, N. Tsukamoto, Y. Yazawa, Y. Kanemitsu, Y. Masumoto, Appl. Phys. Lett. 59 (1991) 3168. [6] D.W. Wang, Q. Wang, A. Javey, R. Tu, H. Dai, H. Kim, P.C. McIntyre, T. Krishnamohan, K.C. Saraswat, Appl. Phys. Lett. 83 (2003) 2432. [7] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289. [8] J.-in Hahm, C.M. Lieber, Nanoletters 4 (2004) 51. [9] D. Zhang, C. Li, X. Liu, S. Han, T. Tang, C. Zhou, Appl. Phys. Lett. 83 (2003) 1845. [10] A. Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, Adv. Mater. 15 (2003) 997. [11] X.H. Sun, S.D. Wang, N.B. Wong, D.D.D. Ma, S.T. Lee, Inorg. Chem. 42 (2003) 2398. [12] D. Wang, Y.-L. Chang, Z. Liu, H. Dai, J. Am. Chem. Soc. 127 (2005) 11871.

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