Ge(100) Stack and Its Effect on the Electronic

Water growth on GeO2 films on a Ge(100) substrate and their effect on the electronic properties of GeO2 films are investigated using ambient-pressure ...
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Water Growth on GeO2/Ge(100) Stack and Its Effect on the Electronic Properties of GeO2 Atsushi Mura,†,‡ Iori Hideshima,‡,‡ Zhi Liu,⊥ Takuji Hosoi,‡ Heiji Watanabe,‡ and Kenta Arima*,† †

Department of Precision Science and Technology and ‡Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ⊥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94709, United States ABSTRACT: Water growth on GeO2 films on a Ge(100) substrate and their effect on the electronic properties of GeO2 films are investigated using ambient-pressure X-ray photoelectron spectroscopy at relative humidity (RH) values from 0% to approximately 45%. Water adsorbs at low RHs and continues to grow gradually up to ∼1% RH, probably forming hydroxyls. Water grows rapidly above 1% RH, indicative of the formation of a molecular water film. The molecular water film formed reaches more than one monolayer in thickness at 10% RH. We show that the energy separation between Ge4+ and Ge0+ signals in Ge3d spectra increases with RH until it reaches 1%. In addition, the collapse of an initial abrupt GeO2/Ge interface is demonstrated in this humidity range, indicating that water molecules in the gas phase infiltrate into the permeable GeO2 film, and water-related species accumulate at the GeO2/ Ge interface. We propose that water-related species emit electrons to the Ge bulk and positive charges are created in GeO2 close to the GeO2/Ge interface, which is the origin of the specific features of Ge3d spectra. These positive charges are likely to be the cause of the reported negative shift of the flatband voltage in metal-oxide-semiconductor (Ge) capacitors with air-exposed GeO2.

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INTRODUCTION

© 2012 American Chemical Society

EXPERIMENTAL SECTION

Sample Preparation. We used p-type Ge (100) wafers with a resistivity in the 0.1−0.5 Ω cm range. We formed sacrificial oxides by dry oxidation at 450 °C for 30 min in a conventional furnace and cleaned them by cyclic treatment using diluted HF (5%) and ultrapure water. Then, thin GeO2 dielectric films were fabricated by dry oxidation at 550 °C for 1 min. The samples were stored immediately in a vacuum desiccator pumped by a diaphragm pump and were transferred to an AP-XPS chamber. We treated the samples in two different ways. One was annealed at 300 °C for 30 min in ultrahigh vacuum prior to exposing the samples to water vapor. The other was exposed to water vapor without being annealed. We hereafter call them annealed and as-prepared samples, respectively. XPS Experiments. XPS measurements were performed at beamline 9.3.2 of the Advanced Light Source (ALS), of the Lawrence Berkeley National Laboratory. The base pressure in the XPS chamber prior to the introduction of water vapor was 7 × 10−9 Torr. An approximate X-ray flux is 5 × 1010 photons/s.8 A differentially pumped electrostatic lens system separates the analysis chamber from a hemispherical photoelectron spectrometer.6−8 We used water (Aristar Plus HPLC, low TOC grade) from BDH, with a total organic carbon content of less than 20 ppb. The water was degassed in freeze−pump−thaw cycles in advance. Water vapor was introduced up to a pressure

Germanium (Ge) is regarded as an advanced substrate and channel material because it has higher mobilities for both holes and electrons than those of Si. Germanium oxide (GeO2) is one of the key materials in Ge-based transistors, because the GeO2/ Ge interface exhibits excellent electronic properties.1 However, GeO2 is permeable and soluble in water, unlike the more familiar silicon oxide (SiO2). This implies that GeO2 films will react with water vapor in air. Only a few attempts have so far been made at examining the wetting property of soluble GeO2 to water vapor.2,3 Recently, Hosoi et al. revealed the electrical characteristics of a GeO2/Ge structure, and detected the negative shift of the flatband voltage (VFB) as well as anomalous hysteresis and minority career response as exposure to air increases.4 As the negative VFB shift indicates positive charges in a GeO2 film, Ogawa et al. have investigated adsorbed species in GeO2 from air to identify the cause of the positive charges by ex situ methods, such as infrared spectroscopy and secondary ion mass spectrometry (SIMS).5 Despite these previous works, the interaction of water vapor with a GeO2 surface and its influence on the quality of the GeO2 film are still not clear and need to be understood in more detail. Ambient-pressure x-ray photoelectron spectroscopy (APXPS) gives photoelectron spectra in the presence of gases up to several Torr,6−8 and is used in various in situ observations such as catalytic reactions, oxidation/reduction processes, and wetting phenomena. This study aims to examine the dependence of water uptake of a GeO2 surface on relative humidity (RH) and its influence on the electronic properties of GeO2 films by using AP-XPS.

Received: May 4, 2012 Revised: November 22, 2012 Published: December 17, 2012 165

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higher binding energy than the one on Ge bulk (HOGe Ge) due to the difference in electronegativity between O and Ge atoms. This narrows the energy separation between adsorbed H2O and OH signals on GeO2, which prevents us from distinguishing adsorbed water and OH groups (OH GeO) in O1s spectra.

of 1.0 Torr, while the sample temperature was controlled from room temperature to −9.6 °C by a chiller. In this manner, we could obtain XPS spectra in the humidity range of 0−45%. The Ge substrate was connected to the ground. Spectra of the Ge3d and O1s core levels were taken for incident photon energies of 350 and 855 eV, respectively. These photon energies were chosen so that photoelectrons from these levels would have similar kinetic energies (∼320 eV), thus ensuring the probing depth to be similar in photoelectron spectra. The sample position was changed at each humidity condition in order to avoid X-ray beam damage to GeO2 and adsorbed water molecules. The GeO2 films on the Ge substrate ranged from 2.1 to 2.4 nm thick, as calculated from the Ge3d signal attenuation in vacuum using the formula by Himpsel et al. with a SiO2/Si system.9 We used the NIST database to calibrate binding energy scales10 and to obtain the inelastic mean free path (IMFP) of electrons in Ge, GeO2, and H2O.11 Several groups have pointed out that carbon contamination on an oxide film affects the wetting property of the oxide. For example, Verdaguer et al. used two SiO2 surfaces with ∼0.5 and ∼0.1 monolayers of carbon contaminations.12 They demonstrated that the sample with the higher contamination levels showed a lower water film thickness at RHs above 20% although the difference was less than one water monolayer. In our experiment, the amount of carbon contamination on GeO2 was estimated during experiments by taking C1s spectra at an incident photon energy of 610 eV. It was approximately 0.5 monolayers even on the annealed sample and reached 0.7−0.8 monolayers on the as-prepared sample. If all the contaminants reside on the surface of GeO2, the contamination levels are relatively high compared with those in the literature on the wetting properties of other oxides determined by AP-XPS. However, a previous SIMS study indicates that GeO2 absorbs organic molecules.5 Therefore, we expect that the carbon contaminants diffuse into GeO2, and do not have a great effect on the interaction between the GeO2 surface and water vapor. An O1s spectrum after a Shirley background subtraction includes contributions from oxygen in GeO2, in adsorbed species including molecular water and hydroxyls (OH), and in the gas phase. Because these peaks overlap with each other in a small energy range, deconvolution is necessary to separate them.12−14 First, the ratio of the O1s peak from GeO2 to a Ge4+ component in Ge3d peak (R = IO1s(oxide,dry)/IGe3d(Ge4+,dry)) as well as the shape (full width at half-maximum and Lorentzian− Gaussian mixing ratio) of the O1s peak were obtained in a vacuum with the annealed sample. Second, in the presence of water vapor, the signal from GeO2 in an O1s spectrum (IO1s(oxide)) was estimated using the Ge4+ peak in a Ge3d spectrum (IGe3d(Ge4+)) and the ratio R. We used R in analyzing all the spectra with/without water vapor on both the annealed and as-prepared samples. Then, the O1s spectra were fitted with three peaks. One is the contribution from GeO2 (IO1s(oxide)) using the line shape determined under the dry condition. Another is from gas-phase water (IO1s(gas)). The remaining contribution is assigned to molecular water and hydroxyls (IO1s(water)). This peak’s position and intensity and the line shape are the fitting parameters used to fit the measured spectra. According to the literature,15 the binding energy of O1s from adsorbed H2O molecules and that of surface hydroxyls on Ge bulk (HOGeGe) are 533.8 and 532.1 eV, respectively, which leads to an energy separation of 1.7 eV. We expect an O1s component from OH on GeO2 (HOGeO) to have

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RESULTS AND DISCUSSION Water Growth on GeO2/Ge(100). Figure 1 shows O1s spectra with fitted peaks. On the annealed sample in Figure 1a,

Figure 1. Deconvoluted O1s spectra. Spectra for the annealed (upper) and as-prepared (lower) GeO2/Ge samples. In each spectrum, photoelectron intensity was normalized by the largest count. The binding energy of the O peak from GeO2 was fixed to 531.9 eV, as indicated by the dotted lines. The broken lines represent the peak positions from adsorbed water at 45% RH.

the measured spectrum is composed of one peak derived from GeO2. At 45% RH in Figure 1b, the measured data are composed of three peaks. When the water vapor was evacuated, the O1s spectrum changes to Figure 1c, for which the sample temperature and vacuum were 21.1 °C and 3.1 × 10−7 Torr, respectively. We can see that the gas-phase water peak completely disappears. However, we need to fit the experimental data with an additional small peak, as shown in Figure 1c. In Figure 1b, the peak position of the fitted curve from adsorbed water situates at 0.70 eV higher than the GeO2 peak, which is the same as the one in Figure 1c. A similar trend is seen on the as-prepared sample as shown in Figure 1e,f. However, the adsorbed species exists even in the initial vacuum condition (2.1 × 10−7 Torr) at room temperature (22.1 °C) as shown in Figure 1d. The additional small peaks in Figure 1c,d,f, which are detected in vacuum and situate at approximately 0.70 eV higher than the GeO2 peak, probably represent hydroxyls. They will be discussed in the next paragraph. The water layer thickness on GeO2 was estimated on the basis of the attenuation of peak intensities (IO1s(water) and 166

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molecules appear at ∼1% RH, producing a monolayer at 15% RH. In other reports on the MgO(100) surface,18,19 it was proposed that hydroxylation occurs only at defect sites on MgO at RHs lower than 0.01%. At approximately 0.01% RH, a sharp onset in surface hydroxylation due to water dissociation at MgO terrace sites is detected, and the surface becomes fully saturated with one monolayer of OH at ∼0.1% RH. This is accompanied by a gradual increase in molecular water coverage, reaching one monolayer (0.3 nm) at 20% RH. Molecular water grows easily on a hydroxylated surface because the attractive interaction between H2O and OH is known to be stronger than that between two intact H2O molecules.20 Even on the SiO212 and Al2O313 surfaces, on which the OH peak could not be resolved from that of molecular water in the O1s spectra, researchers assume that hydroxylation precedes the growth of molecular water films. From the previous studies mentioned above, it seems reasonable to suppose that the gradual water growth below 1% RH in Figure 2a mainly represents hydroxylation on GeO2 by the dissociative adsorption of water molecules. In the semilog graph in Figure 2a, a sharp onset in the thickness increase is detected at approximately 1% RH. A likely explanation for this is that OH formation saturates at ∼1% RH, beyond which a molecular water film starts to grow rapidly and reaches more than one monolayer in thickness at 10% RH. As shown in Figure 1c,f, the contribution of adsorbed species remains in the O1s spectra even when water vapor is evacuated. If we attribute all the adsorbed species to molecular water, its thickness approximates to one monolayer, as shown in Figure 2a. The remaining peaks in Figure 1c and f do not represent carbon contaminants such as carboxyl and ester groups because the positions of the remaining peaks are identical to those of adsorbed water species at 45% RH in Figure 1b,e. It has been reported that thermal treatment is necessary to remove hydroxyls on an oxide layer completely.21 This leads us to infer that the remaining peaks in Figure 1c,f originate from surface hydroxyls on GeO2. Added to these, we think that hydroxyls in GeO2 also contribute to the remaining signals, which is deduced from the permeability of GeO2 reported in the literature.5,22 For example, the result of temperature-programmed desorption mass spectrometry in ultrahigh vacuum showed that the amount of moisture from GeO2 was approximately 1 order of magnitude higher than that from SiO2 after both samples (GeO2 and SiO2) were exposed to ambient air.5 It is easy to imagine that GeO2 absorbs water molecules once it is exposed to water vapor and that infiltrated water molecules create hydroxyls in a GeO2 film up to ∼1% RH. It is also likely that some water molecules are captured in GeO2 owing to the attractive interaction between H2O and OH in GeO2. Although a molecular water film on GeO2 desorbs readily by the evacuation, other species (e.g., surface hydroxyls, hydroxyls, and captured H2O molecules in GeO2) remain to form the additional small peaks in Figure 1c,f. The above discussion is also applicable to interpreting the adsorbed species in the initial vacuum on the as-prepared sample in Figure 1d. The behavior of the infiltrating water molecules in GeO2 is again discussed in the next section. Impact of Adsorbed Water on Quality of GeO2 Film. Representative Ge3d spectra acquired on both the annealed and the as-prepared samples are shown in Figure 3. Raw data (open circles) are fitted mainly by Ge0+ and Ge4+. In addition to Ge0+ and Ge4+, a small amount of suboxide components23 (Ge1+, Ge2+, and Ge3+) was detected. The dependence of suboxide structures on RH will be examined later. The most

IO1s(oxide)) due to the adsorbed H2O.12−14 Note that a water layer includes both molecular water and surface hydroxyls, because it is difficult to distinguish between the two species in O1s spectra. We assumed a layer structure (water/GeO2/Ge) for simplicity and set the densities of O atoms in GeO2 and H2O to be 4.88 × 1022 /cm3 and 3.34 × 1022 /cm3, respectively. The IMFPs at the 320 eV kinetic energy are 9.7 × 10−1 nm and 1.5 nm for GeO2 and H2O, respectively.11 Figure 2 shows water

Figure 2. Water growth from the initial vacuum condition as a function of RH. Horizontal axes in (a) and (b) are logarithmic and linear scales, respectively. Circles and triangles represent the values on the annealed and the as-prepared GeO2/Ge samples, respectively. Open symbols are the data taken after the evacuation of water vapor from the chamber. On the as-prepared sample, a 0.12-nm-thick water layer existed in the initial vacuum (see text).

layer thicknesses as a function of RH. On the as-prepared sample, the O1s spectrum in Figure 1d taken under vacuum revealed adsorbed species corresponding to a 0.12-nm-thick water layer. They were formed during the transfer of the GeO2/ Ge sample outside the AP-XPS chamber. Because the plots in Figure 2 show an additional growth from the initial vacuum condition, the total thicknesses for this sample are the plots plus 0.12 nm. Figure 2 reveals that the water uptake of a GeO2 surface is quite similar on the two samples, irrespective of whether the sample is annealed before the vapor introduction. The plots in Figure 2a begin to show an increase at very low RHs, and reach 0.3 nm at approximately 1% RH. Then, the plots show a rapid increase up to 0.7 nm below 10% RH and plateau up to 45% RH. AP-XPS has so far been used to investigate the interaction of water vapor with the surfaces of various metal oxides such as Cu2O,13 TiO2,16 Fe2O3,17 MgO,18,19 SiO2,12 and Al2O3.13 On the surfaces of the former four oxides, the OH contributions can be resolved from those of molecular water in the O1s spectra, which shows that hydroxylation occurs at low RHs and precedes the adsorption of molecular water. For example, on a Cu2O surface,13 hydroxyls form readily at 5 × 10−7 % RH, reaching saturation at ∼1% RH. This is due to the dissociative adsorption of water molecules on oxygen vacancies producing hydroxyl groups in a stoichiometric reaction: H2O + vacancy + Olattice = 2OH. Water 167

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Figure 4. Energy separation between Ge4+ and Ge0+ contributions (ΔE) as a function of RH. The definitions of the symbols are the same as those in Figure 2. Figure 3. Deconvoluted Ge3 spectra. Spectra for the annealed (upper) and as-prepared (lower) GeO2/Ge samples. In each spectrum, photoelectron intensity was normalized by the largest count. The binding energy of Ge3d5/2 from the Ge bulk (Ge0+), as indicated by the dotted lines, was calibrated using the known value for elemental Ge (29.7 eV). The broken lines indicate the positions of Ge3d5/2 from GeO2 (Ge4+) at the initial vacuum condition.

striking feature in Figure 3 is how introducing water vapor increases energy separations between Ge4+ and Ge0+ (ΔE). Specifically, the energy separation on the annealed sample is 3.10 eV in vacuum (Figure 3a). It reaches 3.46 eV at 45% RH (Figure 3b). The value shows a slight decrease (3.42 eV) by evacuation (Figure 3c), but it is still much larger than the original value in Figure 3a. When we used the as-prepared sample, the initial separation (3.38 eV) in Figure 3d is larger than on the annealed one in a vacuum. A small increment is seen at 45% RH, and the separation becomes 3.53 eV (Figure 3e). This value is kept even after evacuation as shown in Figure 3f. The energy separations (ΔE) were measured at various RHs and are plotted in Figure 4. The drastic increase in the separation is clear until around 1% RH especially on the f resh annealed sample. Figure 5 shows the full widths at halfmaximum (fwhm) of both Ge4+ and Ge0+ peaks as a function of RH in the Ge3d spectra of the annealed sample. The fwhm of Ge0+ peaks is constant at 0.70 ± 0.01 eV in the entire RH range tested. The fwhm of Ge4+ peaks remains almost constant at 1.05 eV, with a slight fluctuation of 0.015 eV up to 1% RH. Then, it gradually increases to 1.17 eV at 10% RH, and saturates up to 45% RH. When the water vapor was evacuated, the fwhm decreases to be 1.09 eV, close to the initial value of approximately 1.05 eV. Let us discuss how water vapor on GeO2 increases energy separations (ΔE) in Figures 3 and 4. To date, many factors have been reported to influence the energy separation between oxide-related and bulk peaks in XPS, which include the changes in either chemical bonds of oxides24 or the extra atomic relaxation energy,25 charging effects due to photoemission by

Figure 5. Full widths at half-maximum (fwhm) of Ge4+ and Ge0+ peaks as a function of RH. The data are derived from the Ge3d XPS spectra of the annealed GeO2/Ge(100) stack. Open symbols are data taken after the evacuation of water vapor from the chamber.

X-rays,26 and potential changes across an oxide due to a charging layer on the top of the oxide.27 A simple explanation for the trend in Figures 3 and 4 is positive charging of a water film due to the emission of photoelectrons by X-rays. In the previous section, we have proposed that hydroxyls are formed at low RHs of up to ∼1%, and that a molecular water film starts to grow rapidly beyond it. We cannot explain the trend in Figure 4 by a positively charged water film, because molecular water grows beyond 1% RH, at which the increase in ΔE, starting at around 10−6 % RH, almost saturates. Then, let us 168

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Figure 6. (a) Two Ge3d spectra of the annealed sample taken in vacuum. In each spectrum, the vertical scale is normalized by the highest intensity. The binding energy scale is referenced by the energy at which the highest intensity is detected. (b,c) Peak fitting results of black (circles) and purple (triangles) curves in (a), respectively. Blue and red curves represent Ge2+ and Ge3+, respectively.

Ge Fermi level even without gate bias application. In our experiments, the Ge substrate is at the ground potential. When water-related species in GeO2 emit electrons to the Ge bulk, GeO2 is positively biased with respect to the substrate at the ground potential, which accounts for the increase in energy separation between Ge4+ and Ge0+ (ΔE) below 1% RH in Figure 4. If such a positive charge exists on the GeO2 surface, it would create a potential gradient across the GeO2 layer to broaden Ge4+ peaks. However, the fwhm of the Ge4+ peaks in Figure 5 does not change much up to ∼1% RH in which range ΔE increases dramatically (Figure 4), and positive charges are expected to be generated. This discrepancy can be explained by positive charges not on the surface of GeO2 but in GeO2 close to the GeO2/Ge interface. One typical situation for this is the dipole formation29 of positive charges in GeO2 with emitted electrons in the Ge bulk at the GeO2/Ge interface. Now that we have proposed that positive charges, originating from water-related species, are generated in GeO2 close to the GeO2/Ge interface at low RHs of up to ∼1%, the next step is to provide evidence of water-related species existing at the interface. As discussed in the previous section, adsorbed water forms hydroxyls in this humidity range. The mechanism of creating hydroxyls is the dissociative adsorption of H2O molecules, of which rate will be enhanced by oxygen vacancies in GeO2.30−32 Because hydroxyls form chemical bonds with Ge atoms, they are expected to affect suboxide (GeOx, x < 2) structures at the interface. Figure 6a shows two Ge3d spectra of the annealed sample taken under vacuum. In Figure 6a, the energy separation between the oxide and the Ge bulk (ΔE) increases once the GeO2 sample is exposed to water vapor, as described in previous paragraphs. What should be noticed here is the peak shapes of the oxide signals in Figure 6a. The slopes of the oxide peaks overlap exactly with each other on the higher-binding-energy side, as indicated by the green arrow in Figure 6a. On the other hand, the oxide peak of the sample after exposure to water vapor (purple triangles) has a small shoulder on the lower-binding-energy side, as indicated by the red arrow. This is clearer when a dotted rectangle area is magnified as shown in Figure 6a. Figure 6b,c shows the peak fitting results of the two spectra in Figure 6a. In addition to Ge0+ and Ge4+, the suboxide components (Ge1+, Ge2+, and Ge3+) were used to fit the spectra, the binding energies of

examine the impact of GeO2 hydroxylation on the chemical shift of Ge4+ peaks. According to a report on MgO, a hydroxylated peak has a higher binding energy than an oxide peak in the Mg2p spectrum.18 From this study, one may expect that the increase in ΔE in Figure 4 represents the structural change from GeO2 under a dry condition to GeOH in the presence of water vapor. If the two components of the oxide and hydroxyls have such a clear difference in binding energies, Ge4+ peaks should broaden upon hydroxylation in the RH range from 10−6 % to 1% because they must include both components of GeO2 and GeOH. However, Figure 5 shows that the fwhm of Ge4+ peaks remains almost constant up to ∼1% RH. Thus, we can rule out this possibility. We propose that GeO2 films exposed to water vapor are positively charged, which is induced by the transfer of electrons, probably by tunneling, from water-related species in GeO2 to Ge bulk. Enikeev et al. determined the contact potential and the electric conductivity of a Ge surface coated with an oxide film in water vapor in 1975.2 They stated that water molecules, impacting the outer surface of the oxide film, are fixed at the centers of chemisorption sites of the film, which accompanies the ionization of H2O and the transfer of electrons from H2O to the Ge bulk. This phenomenon is investigated in more detail in some recent, more sophisticated works. First, Hosoi et al. reported a negative VFB shift in GeO2/p-type Ge metal-oxidesemiconductor (MOS) capacitors when the GeO2/Ge stack was not annealed prior to metal deposition.4 Because the VFB shift is caused by fixed charges especially at the GeO2/Ge interface, its negative shift indicates that infiltrated water molecules create positive charges in the GeO2 layer close to the GeO2/Ge interface. Second, Oniki et al. measured the capacitance−voltage characteristics of metal/GeO2/Ge structures elaborately and discussed the nature of anomalous positive charges present in GeO2 as well as its generation mechanism.22,28 They insisted that infiltrated water molecules create neutral states in the bandgap of GeO2, when GeO2 is exposed to ambient air. Those states above the Ge Fermi-level emit electrons to the Ge bulk, leading to the formation of donorlike hole-trap states located uniformly in an interfacial GeO2 layer. Although the number of water-related hole traps increases by stress application in the form of an electric field in GeO2, they found that positive charges are generated above the 169

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which are taken from the literature.23 Figure 6b demonstrates that the f resh sample annealed contains Ge2+ as a suboxide and small amounts of Ge1+ and Ge3+ are detected, which means the formation of an abrupt GeO2/Ge interface. On the other hand, Figure 6c shows a distinct Ge3+ peak, which is the origin of the small shoulder in the purple curve in Figure 6a. Figure 7 shows

the bandgap of GeO2 induced by hydroxylation as well as the relationship of their energy levels with the band alignment of the Ge bulk. They require simulations such as first-principles calculations. At RHs higher than ∼1%, several things remain to be tested. First, the fwhm of the Ge4+ peaks increases in this RH range in Figure 5. One reason for this is the charging of a molecular water film on GeO2, in which RH range molecular water grows rapidly, as shown in Figure 2. Second, the relative intensity of the Ge0+ signal increases in Figure 7. This represents the thinning of the GeO2 film, which needs further investigation. Figure 7 reveals that the Ge3+ signal, or water-induced suboxide, remains after evacuation. This indicates that waterrelated positive charges also remain in GeO2, which is in agreement with ΔE after evacuation (3.42 eV) being much larger than the original value in the initial vacuum condition (3.10 eV) for the f resh sample annealed, as mentioned in Figure 3a. The remaining positive charges in GeO2 after exposure to water vapor are also confirmed in Figure 3d on the as-prepared sample in which ΔE is already initially large in vacuum (3.38 eV). A message of this study is that, when we use a permeable thin oxide, captured water-related species in the oxide can emit electrons to a semiconductor bulk. As resulting positive charges or captured cations, especially at the oxide/semiconductor interface, are the cause of a negative VFB shift in MOS capacitors, they should be carefully controlled to achieve ideal and high-quality MOS devices.

Figure 7. Intensities of Ge3d components (Ge0+, Ge2+, Ge3+) of the annealed sample as functions of RH. The intensity of each component was normalized by Ge4+ (GeO2) intensity. The open symbols represent data from Figure 6c, or of the sample exposed to water vapor followed by evacuation. 0+

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CONCLUSIONS We have examined water growth on thin GeO2 films on Ge(100) using AP-XPS. We demonstrated the hydrophilic nature of the GeO2 surface. Namely, water adsorbs at a low RH of approximately 10−6 % and continues to grow gradually up to ∼1% RH. We expect adsorbed water to produce hydroxyls on GeO2 in this RH range. Above ∼1% RH, water grows rapidly, probably indicating the growth of molecular water films by the attractive interaction between water molecules and hydroxyls. Moreover, a molecular water film reaches more than one monolayer in thickness at 10% RH and almost saturates up to 45% RH. These hydrophilic natures are difficult to reveal by the conventional contact-angle measurement of a water droplet on “soluble” GeO2. A striking feature of Ge3d spectra is that the energy separation between Ge4+ and Ge0+ (ΔE) continues to increase up to ∼1% RH. This trend is more significant for an annealed GeO2/Ge stack than for an as-prepared one. We propose that water-related species, such as products of water dissociation (hydroxyls and hydrogen atoms) and water molecules trapped on hydroxyls, are captured in GeO2 close to the GeO2/Ge interface. In addition, they emit electrons to the Ge bulk, which generate positive charges in GeO2. These captured cations in GeO2 give a good account of the variations in both ΔE and fwhm of Ge4+ peaks as a function of RH. The accumulation of water-related species at the interface is evidenced by the collapse of an initial abrupt GeO2/Ge interface, or the increase in the signal intensity of Ge3+ and the decrease in that of Ge2+, with RH up to 1%. A very important finding of this study is that water-related positive charges in GeO2, which can cause the anomalous electrical characteristics of MOS capacitors with a GeO2/Ge stack,4 are formed at very low RHs below 1%. They are difficult to remove completely without annealing in vacuum, and can even increase in number with the application of an external bias stress, as reported in the literature.22,28

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intensities of the GeO2 components (Ge , Ge , Ge ) in Ge3d spectra as functions of RH. Each component is normalized by Ge4+ (GeO2) intensity. Plots are not shown for Ge1+ because its intensity was low at around the detection limit below ∼5% RH. In Figure 7, the plots show a marked change at low RHs of up to 1%. Namely, the intensity of the Ge2+ signal decreases gradually and becomes approximately 80% at 1% RH from its initial value in vacuum. That of the Ge0+ signal shows a small fluctuation, but does not change much. On the other hand, that of the Ge3+ signal shows a monotonic increase up to 1% RH and becomes three times higher than its initial value in vacuum. Figure 7 shows the collapse of an initial abrupt GeO2/Ge interface at very low RHs of up to 1% by either roughening or suboxide growth at the GeO2/Ge interface. It is reasonable to suppose from Figure 7 that water molecules in the gas phase infiltrate into the GeO2 film, and hydroxyls are formed at the GeO2/Ge interface by the dissociative adsorption of water molecules up to ∼1% RH, resulting in suboxide formation. Ogawa et al. showed the depth profiles of hydrogen in GeO2 films determined by SIMS after air exposure. They detected a certain amount of hydrogen near the GeO2/Ge interface (0 to 10 nm away from the GeO2/Ge interface), which was irrelevant to the GeO2 thickness (20−30 nm).5 Their results also provide evidence of the accumulation of hydroxyls at the interface. When water-related species emit electrons to the Ge bulk and positive charges are created, the specific features shown in Figures 4 and 5 (an increase in ΔE while a constant fwhm of Ge4+ up to 1% RH) are well explained. Candidate water-related species are (1) hydroxyls mainly formed up to ∼1% RH, (2) hydrogen atoms as accompanying products of the dissociative adsorption of H2O molecules, and (3) water molecules captured by the hydroxyls. To fully elucidate the electron transfer from the water-related species to the Ge bulk, we have to determine defect states in 170

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

‡ These authors (A. Mura and I. Hideshima) contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr. Albert Verdaguer, Dr. Hendrik Bluhm, Prof. Kouji Inagaki, Dr. Katsuhiro Kutsuki, Dr. Fabiano Bernardi, Young Pyo Hong, and Naila Jabeen for valuable comments and discussion. This research was supported by a grant from the Global COE program, ‘Center of Excellence for Atomically Controlled Fabrication Technology’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The work was also supported by a Grant-in-Aid for Young Scientists (A) (Grant No. 24686020) from Japan Society for the Promotion of Science. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on December 31, 2012. Figure 6 has been modified. The correct version was published on January 10, 2013.

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dx.doi.org/10.1021/jp304331c | J. Phys. Chem. C 2013, 117, 165−171