As Si transistor technology advances beyond the 10-nm node, it has been more difficult to further enhance the performance of MOSFETs on the Si platform owing to the inherent limitations of the Si material itself. The device research community is increasingly paying close attention to the replacement of Si in the transistor channel with novel and high-mobility materials.^[1,2] Among several possible candidate materials, Ge is a promising material that can offer increases in the electron and hole mobilities by factors of approximately two and four, respectively, compared with those of Si and can provide compatibility with conventional Si integration technologies.^[1,3,4] However, several issues still need to be resolved before Ge can be completely applied to field-effect transistors (FETs) as a proven technology to attain a high performance. One of the critical issues is to provide a high-quality interfacial layer, which can avoid causing a substantial degradation in the drive current in both the low equivalent oxide thickness (EOT) and short-channel regimes.^[3]

Recently, wide ranges of materials and processes have been researched to achieve superior Ge surface passivation, including nitride,^[5–11] Si,^[12–15] S,^[16–20] and Ge dioxide or suboxide (GeO_x)^[20–38] passivation layers. High-quality GeO₂ has recently been reconsidered as a promising passivation layer owing to its extremely low D_it (∼ 6 × 10¹⁰ eV⁻¹·cm⁻²)^[39] and its potential to enable high-performance Ge n-MOSFETs.^[40] GeO₂ or GeO_x can be grown by thermal oxidation at atmospheric pressure^[21,22] and high pressure (∼ 70 atm; 1 atm = 1.01325 × 10⁵ Pa),^[23,24] respectively, as well as plasma post-oxidation (PPO)^{[33–38,41–46]} or ozone oxidation.^[29–32] Lee et al.^[24] proposed a two-step oxidation process with high-pressure (70 atm) oxidation and low-temperature oxygen annealing to form an ideal GeO₂/Ge stack based on thermodynamic and kinetic control. A GeO₂ thickness of about 20 nm and D_it < 10¹¹ eV⁻¹ · cm⁻² near the midgap were obtained with a peak electron mobility of 1100 cm² · V⁻¹·s⁻¹. Kuzum et al.^[30] used ozone oxidation to grow GeO_x and achieved a mobility that is ∼ 1.5 times higher than the universal Si mobility. Zhang et al.^[37] demonstrated that by using the electron cyclotron resonance oxygen PPO method, the EOT of the HfO₂/Al₂O₃/GeO_x/Ge gate stack can be scaled down to 0.7 nm–0.8 nm while maintaining D_it at a level of 10¹¹ eV⁻¹·cm⁻². Further, the peak electron and hole mobilities were 689 cm²·V⁻¹·s⁻¹ and 546 cm²·V⁻¹·s⁻¹, respectively.

In order to further improve the interfacial properties using PPO, it is necessary to understand the distributions of various Ge oxidation states in the GeO_x interfacial layer. In this paper, the areal intensity ratios of these oxidation states in a GeO_x interfacial layer formed by PPO technology are investigated using angle-resolved x-ray photoelectron spectroscopy (AR-XPS). Moreover, the distributions of the Ge oxidation states are obtained by these areal intensity ratios.

In order to fabricate the GeO_x/Ge interface, an HF-last process (100:1 H₂O:HF for 2 min) was carried out to remove the native Ge oxide on a Ge (100) wafer. Subsequently, the wafers were immediately capped with a 3-nm-thick Al₂O₃ layer by atomic layer deposition (ALD) at a substrate temperature of 300 °C with Al(CH₃)₃ (trimethylaluminum, TMA) and H₂O as precursors. Finally, the Al₂O₃/GeO_x/Ge structure was fabricated at room temperature (RT; ∼ 20 °C) by PPO. The inductively coupled plasma (ICP) was made up of an Ar (650 sccm) and O₂ (50 sccm) gas mixture, and the microwave power was 96 W. The bias voltage between the plasma source and the samples was less than 1 V. AR-XPS (Thermo Scientific ESCALAB 250xi) was performed with a monochromatic Al Kα radiation source. All high-resolution spectra were obtained with a pass energy of 15 eV. The takeoff angle was 90° when the photoelectron emission direction was normal to the sample surface.

The thickness of GeO_x interface layer was controlled by regulating the time of post plasma oxidation, and the thickness of GeO_x interface layer was calculated as follows. According to the theory of x-ray photoelectron spectroscopy that the intensity attenuation of photoelectron with characteristic energy obeys the exponential distribution, if each layer of atoms emits a photoelectron intensity I₀ toward the surface, then the measured intensity of electrons (I′) of a given layer arriving at the surface layer will be

Fig. 1.

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	Fig. 1. (color online) The gate-stack structure of the sample.

On the basis of the above discussion, the equations of measured intensity of Ge and GeO_x are shown as follows:

In order to investigate the oxidation states and their distributions in the GeO_x interfacial layer formed by PPO, the XPS spectra of the Ge 3d core level were recorded and analyzed. Figure 2 shows the Ge 3d XPS spectra for Al₂O₃/GeO_x/Ge structures with different GeO_x thicknesses at a takeoff angle of 90°. In order to quantitatively estimate the various states of the Ge components, the Ge 3d spectra were deconvoluted into five peaks related to the Ge substrate (Ge⁰), Ge suboxide (Ge¹⁺, Ge²⁺, and Ge³⁺), and GeO₂ (Ge⁴⁺), as shown in Fig. 2. Further, the chemical shifts of Ge¹⁺, Ge²⁺, Ge³⁺, and Ge⁴⁺ relative to Ge⁰ are taken as 0.8, 1.8, 2.75, and 3.4 eV, respectively.^[48ȓ52] A spin–orbit splitting of 0.585 eV and a branching ratio of 2:3 are used for the 3d_5/2 and Ge 3d_3/2 doublets of the Ge 3d line shape. The Shirley background is used. The full width at half maximum (FWHM) is 0.57 eV for Ge⁰ and Ge¹⁺, 0.9 eV for Ge²⁺, 1.12 eV for Ge³⁺, and 1.2 eV for Ge⁴⁺.^[52] In addition, the symmetric Lorentzian–Gaussian convolution function is used, and the optimized percentage of the Lorentzian–Gaussian function is found to be 20%.

Fig. 2.

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	Fig. 2. (color online) Ge 3d XPS spectra for Al2O3/GeOx/Ge structures with different GeOx thicknesses at a takeoff angle of 90°. The Ge 3d spectra consist of five peaks related to the Ge substrate (Ge0), Ge suboxide (Ge1+, Ge2+, Ge3+), and GeO2 (Ge4+). The spin–orbit splitting of the Ge 3d5/2 and Ge 3d3/2 doublets is 0.585 eV.

To obtain a better understanding of the oxidation process, it is necessary to clarify the depth distributions of the various oxidation states of Ge. Figure 3 shows the areal intensity ratios of the various states of Ge components as a function of the takeoff angle for a GeO_x thickness of 0.59 nm after deconvolution of the spectra in Fig. 1. Considering that the spectra obtained with smaller takeoff angle are more sensitive to the surface, the photoelectrons received at a takeoff angle of 35° more often originate from the Ge atoms in the surface layer compared to those received at 90°. Therefore, the results measured at 35° are more likely to reflect the surface elements, whereas those measured at 90° reflect the elements in the entire sample.

Fig. 3.

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	Fig. 3. (color online) Areal intensity ratios of various states of Ge components as a function of the takeoff angle for a GeOx thickness of 0.59 nm. According to these results, the depth distributions of the Ge oxidation states are obtained.

It can be clearly observed from Fig. 3(a) that Ge^x+/Ge⁰ (x = 1, 2, 3, 4) decreases with increasing takeoff angle, suggesting that all oxidation states are located above the Ge⁰ states. A similar phenomenon is observed in Figs. 3(b), 3(c), and 3(d), indicating that the Ge^x+ (x = 2, 3, 4) states are located above the Ge¹⁺ states, and the Ge⁴⁺ states appear above the Ge²⁺ and Ge³⁺ states. In contrast, the ratio of Ge³⁺/Ge²⁺ in Fig. 3(c) increases with increasing takeoff angle, demonstrating that the Ge²⁺ states are located above the Ge³⁺ states. On the basis of the above discussion, the distributions of the various oxidation states of Ge in 0.59-nm-thick GeO_x are obtained as follows. The Ge⁴⁺, Ge²⁺, Ge³⁺, and Ge¹⁺ states are localized from the GeO_x surface to the GeO_x/Ge interface but not in the order of Ge⁴⁺, Ge³⁺, Ge²⁺, and Ge¹⁺ predicted by the layer-by-layer growth mode.^[38]

For a more objective understanding of the distributions of the Ge oxidation states during the oxidation process, the areal intensity ratios of various Ge oxidation states as a function of the GeO_x thickness for takeoff angles of 35° and 90° have been investigated, as shown in Figs. 4, 5, and 6.

Fig. 4.

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	Fig. 4. (color online) Areal intensity ratios of Gex+/Ge0 (x = 1, 2, 3, 4) as a function of the GeOx thickness for takeoff angles of 35° and 90°. The relationship between the distributions of the Gex+ and Ge0 states can be acquired from these results.

Fig. 5.

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	Fig. 5. (color online) Areal intensity ratios of Gex+/Ge1+ (x = 2, 3, 4) and Ge4+/Ge3+ as a function of the GeOx thickness for takeoff angles of 35° and 90°.

Fig. 6.

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	Fig. 6. (color online) Areal intensity ratios of Ge3+/Ge2+ and Ge4+/Ge2+ as a function of the GeOx thickness for takeoff angles of 35° and 90°.

It can be clearly observed from Fig. 4 that the values of Ge^x+/Ge⁰ (x = 1, 2, 3, 4) at a takeoff angle of 35° are higher than those at 90° for GeO_x thicknesses of 0.42–0.75 nm, suggesting that all Ge oxidation states are located above the Ge⁰ states. A similar phenomenon is observed for the results in Fig. 5, indicating that the Ge^x+ (x = 2, 3, 4) states are located above the Ge¹⁺ states, and the Ge⁴⁺ states are located above the Ge³⁺ states. Figure 6 shows Ge³⁺/Ge²⁺ and Ge⁴⁺/Ge²⁺ as a function of the GeO_x thickness. For Ge³⁺/Ge²⁺ in Fig. 6(a), Ge³⁺/Ge²⁺ for a takeoff angle of 90° is larger than that for 35° for some GeO_x thicknesses and smaller for other GeO_x thicknesses. These results indicate that the Ge²⁺ states appear above the Ge³⁺ states for some GeO_x thicknesses and below the Ge³⁺ states for other GeO_x thicknesses. Figure 6(b) shows Ge⁴⁺/Ge²⁺ as a function of the GeO_x thickness. The Ge⁴⁺ states are located above the Ge²⁺ states. According to the discussion above, the distribution of the Ge oxidation states of the GeO_x interfacial layer during the oxidation process are obtained as follows. The Ge⁴⁺ states, a mixture of Ge²⁺ and Ge³⁺ states, and the Ge¹⁺ states are localized from the GeO_x surface to the GeO_x/Ge interface.

Figure 7 shows the areal intensity ratios of Ge^x+/Ge¹⁺ (x = 3, 4) and (Ge³⁺ + Ge⁴⁺)/Ge¹⁺ as a function of the GeO_x thickness for a takeoff angle of 90°. It can be clearly observed that almost all areal intensity ratios (except for three) were less than 1.0, indicating that the number of Ge¹⁺ states existing in a GeO_x layer with a thickness of 1 nm or less is far greater than the numbers of Ge³⁺ and Ge⁴⁺ states and even greater than their sum. The Ge–Ge distance in GeO₂ is in the range of 3.05 Å–3.15 Å,^[53] which means that two layers of Ge should be oxidized for 0.7-nm-thick GeO_x. Consequently, the Ge¹⁺ states are predominant when the two outermost layers of Ge atoms are oxidized.

Fig. 7.

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	Fig. 7. (color online) Areal intensity ratios of Gex+/Ge1+ (x = 3, 4) and (Ge3+ + Ge4+)/Ge1+ as a function of the GeOx thickness for a takeoff angle of 90°.

In summary, the growth process of a GeO_x film formed by PPO at RT is investigated on the basis of AR-XPS. In addition, the distributions of various Ge oxidation states are obtained by the areal intensity ratios of these states. It is experimentally found that the distributions of the Ge⁴⁺ states, a mixture of the Ge²⁺ and Ge³⁺ states, and the Ge¹⁺ states are localized from the GeO_x surface to the GeO_x/Ge interface. Furthermore, the Ge¹⁺ states are predominant when the two outermost layers of Ge atoms are oxidized. These experimental results are helpful for optimizing Ge-based gate stacks for future CMOSFET devices.