Background

With the continuous scaling down of metal-oxide-semiconductor field-effect transistors (MOSFETs), Si-based MOSFET is approaching its physical and technical limitation. Alternative channel materials such as germanium (Ge) [1, 2] and III-V materials [3–5] have recently attracted a great deal of interest for high-performance logic device applications. Among them, Ge has the potential to replace silicon as the channel material in MOSFET because of its intrinsic higher hole carrier mobility [6]. However, direct deposition of high-k gate dielectrics on Ge substrates often causes high interface trap density (Dit) and the unwanted formation of interfacial layer between Ge and high-k dielectrics layers [7]. Therefore, in order to achieve high-speed and low-power Ge-based MOSFETs, it is very important to achieve a high-quality high-k/Ge interface. Fortunately, a lot of methods have been reported to improve the quality of high-k/Ge interface [8], such as the introduction of SiO2 [9], Si [10], GeO2 [11], Al2O3 [12, 13], GeOxNy [14, 15], and rare earth oxides [16, 17] as the interfacial control layer between Ge substrate and high-k gate dielectrics. In particular, the GeO2/Ge structure has superior interface properties, an extremely low interface state density (Dit) of less than 1 × 1011 cm−2 eV−1 can be achieved [18]. However, GeO2 would decompose above 425 °C, and it is soluble in water. As a result, an unacceptable Dit is always revealed for the Ge-MOS capacitor (MOSCAP) [6]. Fortunately, Kita et al. reported that capping layer on GeO2 can suppress the GeO2 degradation; however, the selection of the material for the cap layer should be very crucial [19–21]. For example, Si or Y2O3 works more efficiently than HfO2 to retard the Ge-O desorption. These results indicate the importance of high-k materials or interfacial control layer selection to inhibit the GeO2 degradation. Nakashima et al. reported that a very thin SiO2/GeO2 bilayer by physical vapor deposition (PVD) is a promising interlayer layer for Ge passivation, a Dit of 4 × 1011 cm-2 eV−1 was achieved near the midgap [22, 23]. Li et al. introduced the SiO2 interlayer on Ge by metal-organic chemical vapor deposition (MOCVD), and SiO2 interlayer can effectively suppress Ge out-diffusion during HfO2 growth and subsequent post-deposition annealing process [9]. Therefore, SiO2 should be a wonderful interfacial control layer for Ge substrate. However, compared to PVD and MOCVD, PEALD can provide a much more uniform passivation layer, especially for ultrathin thickness. Hence, PEALD-formed SiO2 may be a promising interfacial control layer to achieve high-performance Ge-based transistor devices.

Herein, we introduced in situ PEALD-formed SiO2 into HfO2/Ge stacks as interfacial layer. The interfacial, electrical, and band alignment characteristics of ALD HfO2 films on n-type Ge substrates have been investigated carefully. The SiO2 was first deposited on the Ge substrates as interfacial control layer by PEALD. Then, HfO2 gate dielectric was in situ deposited by thermal ALD mode. Post-deposition annealing (PDA) at 500 °C for 60 s in N2 was performed for the HfO2/SiO2 high-k gate dielectric stacks on Ge. The X-ray photoelectron spectroscopy analyses reveal that Si-O-Ge interlayer and GeO2 layer is formed on the Ge surface during PEALD SiO2 deposition. This Si-O-Ge interlayer not only shows fantastic thermal stability, but also it can suppress the thermal decomposition of GeO2. Therefore, good electrical properties were achieved for the HfO2/Si-O-Ge/GeO2/Ge stacks. Compared to MOCVD SiO2 interlayer, in situ PEALD SiO2 exhibits much improved electrical properties. Therefore, PEALD is a much more powerful technology than MOCVD in the area of MOSFETs fabrication, especially for ultrathin interfacial control layer deposition.

Methods

N-type Sb-doped Ge (100) with a resistivity of 0.2–0.3 Ω∙cm were used as substrates. The substrates were firstly cleaned by sonication in acetone, ethanol, isopropanol, and deionized water for 5 min, respectively. Then, a dilute HBr solution (H2O/HBr = 3:1) was used to etch the surface native oxides for 5 min. After wet chemical cleaning, the substrates were rinsed with deionized water and blown dry in pure N2. Subsequently, the substrates were immediately transferred to the PEALD (Picosun SUNALETM R-200) chamber. Before the high-k HfO2 films deposition, 10 cycles SiO2 film was deposited at 250 °C by PEALD as interlayer, where one cycle consisted of 1 s Si source injection, 10 s N2 purging, 13.5 s oxidant injection, and 4 s N2 purging. Tris-(dimethylamino)-silane (TDMAS) and O2 plasma were used as Si precursor and oxidant for SiO2 deposition, respectively. TDMAS was kept at room temperature. Pure O2 gas (99.999%) was used as O2 plasma source. The plasma power and O2 gas flow rate were 2500 W and 160 sccm, respectively. The growth rate of PEALD SiO2 was determined to be ~0.7 Å/cycle by ex situ spectroscopy ellipsometry. Then ~4 nm-thick HfO2 film was in situ deposited at 250 °C for 40 cycles by thermal ALD, where one cycle consisted of 0.1 s Hf source dosing, 4 s N2 purging, 0.1 s H2O dosing, and 4 s N2 purging. Tetrakis-(ethylmethylamino)-hafnium (TEMAH) and H2O were used as Hf precursor and oxidant for HfO2 deposition, respectively. TEMAH was evaporated at 150 °C and H2O was kept at room temperature. Pure N2 (99.999%) was used as carrier gas and purge gas. PDA was performed in N2 ambient at 500 °C for 60 s under atmospheric pressure using rapid thermal annealing.

The interfacial structures and chemical bonding of the films were investigated by ex situ X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha) with standard Al Kα (1486.7 eV) X-ray source. XPS spectra were collected at a takeoff angle of 90°. The binding energy scale was calibrated using the Ge 3d5/2 peak at 29.4 eV. In addition, XPS spectra were fitted with Gaussian-Lorentzian (G-L) functions after smart-type background subtraction. Pt top electrodes of area 1.55 × 10−4 cm2 were deposited on the surface of HfO2 films using a shadow mask by sputtering method for electrical measurements. The capacitance-voltage (C-V) and leakage current density-voltage (J-V) characteristics were measured by a Keithley 4200 semiconductor analyzer system with a probe platform (Cascade summit 12000B-M).

Results and Discussion

For the thin PEALD SiO2 (~0.7 nm) on Ge, Si 2p exhibits a peak at 102.4 eV corresponding to Si-O bond (Fig. 1a), which is smaller than binding energy of ideal SiO2 [24]. Both silicon suboxide (SiOx) deposition and Si-O-Ge formation on Ge surface during PEALD process can cause the Si 2p shift to lower energy. Therefore, Si 2p spectrum of thick PEALD (~7 nm) on Ge was also performed. It can be found that it exhibits a main peak at 103.6 eV corresponding to ideal SiO2 bonding, as shown Fig. 1b. So, the silicon oxide deposited by PEALD here is ideal SiO2. However, besides the strong Si-O-Si peak, there is a weak peak located at ~102.4 eV, which should correspond to Si-O-Ge bonding on Ge surface. Therefore, it can be concluded that Si-O-Ge is formed on Ge surface in the initial PEALD SiO2 growth. After in situ 4 nm HfO2 deposition, the Si 2p peak intensity decreases without obvious chemical shift (102.3 eV), as shown in Fig. 1a. Furthermore, the Si 2p peak also exhibits no evident chemical shift (102.2 eV) after the 500 °C PDA in N2, suggesting the good thermal stability of the HfO2/SiO2 interface during HfO2 deposition and PDA process. In Hf 4f spectrum of as-deposited HfO2/SiO2 gate stacks (Fig. 1c), the doublet at 16.5 and 18.2 eV can be assigned to Hf 4f7/2 and Hf 4f5/2 peaks of HfO2 with the spin orbit splitting energy of 1.7 eV, consistent with the literature value of HfO2 [25]. After 500 °C PDA, the Hf 4f spectrum shows no obvious change with only 0.1 eV shift to higher energy. It implies that there are no evident Hf-silicates formed during PDA process. In Fig. 1d, the Ge 3d spectrum of as-deposited sample displays the doublet peaks at 29.4 and 30.0 eV, which can be assigned to the Ge 3d5/2 and Ge 3d3/2 peaks of Ge substrate with the spin orbit splitting energy of 0.6 eV. Except the signal of Ge substrate, there is a huge peak at 32.7 eV for Ge-O bonding. The Ge-O peak should be resulted from the formation of Ge-O-Si and GeO2. The GeO2 layer was formed by surface oxygen plasma oxidation during PEALD SiO2 deposition process. Therefore, the real fabricated structure here is HfO2/Si-O-Ge/GeO2/Ge stacks. Moreover, the Ge 3d spectrum shows no evident change after 500 °C PDA treatment, indicating the thermal stability of HfO2/Si-O-Ge/GeO2/Ge stacks without GeO2 degradation. It has been reported by Kita et al. that some capping layers on GeO2 could suppress the GeO2 decomposition, such as Si or La2O3 [19]. Therefore, the PEALD induced the Si-O-Ge interlayer here can also suppress the GeO2 decomposition. Based on above XPS analysis, it can be concluded that an ultrathin Si-O-Ge interlayer is formed on Ge surface. Moreover, this interlayer exhibits fantastic thermal stability without Hf-silicates formation, it can also inhibit the GeO2 degradation.

Fig. 1
figure 1

XPS spectra of SiO2/Ge and HfO2/SiO2/Ge structures. a Si 2p spectra of SiO2, as-deposited and annealed HfO2/SiO2 on Ge. b Si 2p spectra of thick SiO2(7 nm) on Ge. c, d Hf 4f and Ge 3d spectra of as-deposited and annealed HfO2/SiO2/Ge structures

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Figure 2a plots the high-frequency (1 MHz) C-V curves of HfO2/SiO2 gate stacks on Ge before and after PDA. It can be found that flat band voltage (V fb) values of HfO2/SiO2/Ge before and after PDA are 0.42 and 0.27 V, respectively. The calculated ideal V fb value is 0.55 V. The slightly negative V fb shift indicates positive fixed charges, which may be induced by the oxygen vacancies in the dielectrics [26, 27]. During the inert atmosphere annealing process, more oxygen vacancies may be induced, resulting in a slightly negative V fb shift. It has been demonstrated in many reported literatures that the GeO2 degradation during the annealing will cause the positive V fb shift. The desorption process of Ge-O is believed to generate additional negative charges [28, 29]. Therefore, it can also be concluded that GeO2 decomposition is suppressed by Ge-O-Si interlayer from V fb shift. The accumulation capacitance evidently increases from the original 1.92 to 2.25 μF/cm2 after PDA. The corresponding capacitance equivalent thickness (CET) values of the MOS capacitors can be calculated from the accumulation capacitances of the C-V curves using ε0εrA/Cacc [30]. Therefore, a smaller CET of 1.53 nm is obtained after PDA compared to as-deposited sample of 1.80 nm. It can be ascribed to the fact that a denser and thinner high-k layer can be acquired after PDA process. Figure 2b shows the leakage current characteristics of HfO2/SiO2 films on Ge before and after PDA. At the bias voltage of V fb + 1 V, the leakage current density is 2.1 × 10−3 A/cm2 and 2.2 × 10−4 A/cm2 for the sample before and after PDA, respectively. The increased leakage current density after PDA can be also attributed to the decrease of the gate dielectrics thickness.

Fig. 2
figure 2

Electric characteristics of HfO2/SiO2 gate stacks on Ge substrates before and after 500 °C PDA. a High-frequency (1 M Hz) C-V curves. b J-V curves

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In order to examine the interface quality of HfO2/SiO2/Ge quantitatively, the interface state density (D it) was determined by the conductance method [31]. Figure 3 shows the distribution of D it below Ec in the band gap extracted by the conductance method at room temperature for Pt/HfO2/SiO2/Ge before and after 500 °C PDA. The D it can be roughly calculated from D it = 2.5 × (G p/w)max/Aq, where (G p/w)max is the peak value of conductance-voltage characteristics, f(=w/2π) is the frequency, A is the electrode area, and q is the elemental charge. Therefore, D it values of Pt/HfO2/SiO2/Ge structures without and with PDA are determined to be 4.05 × 1012 eV−1 cm-2 and 5.37 × 1012 eV−1 cm−2 at E-Ev = 0.38 eV, respectively. The lower D it values of 2.03 × 1012 cm−2eV−1 and 2.67 × 1012 cm−2eV-1 near the bottom of conduction band are observed for the samples without and with PDA, respectively.

Fig. 3
figure 3

Distribution of Dit below Ec in the band gap at room temperature for Pt/HfO2/SiO2/Ge before and after 500 °C PDA

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Figure 4 illustrates the leakage current density (J g)-CET relationship of Ge-based MOS capacitor with different interfacial control layer [32, 33]. Compared to the S-passivated Ge without interlayer reported by our previous work [34], the HfO2/SiO2/Ge in this work exhibits much improved properties with smaller CET (1.53 vs 2.18 nm), leakage current density (2.1 × 10−3 vs 3.1 A/cm2), and D it (4.37 × 1012 vs 8.61 × 1012 eV−1 cm−2). It implies that in situ PEALD-formed SiO2 is a wonderful passivation layer for Ge. Moreover, compared to the ex situ-formed SiO2 interlayer by MOCVD [9], the sample with in situ PEALD-formed SiO2 interlayer in this work shows better electrical performance with both smaller CET (1.53 vs 1.75 nm) and leakage current density (2.1 vs 3.9 mA/cm2). It can be ascribed to the fact that SiO2 deposited by PEALD are more uniform than MOCVD especially for ultrathin thickness.

Fig. 4
figure 4

Leakage current density (Jg)-CET relationship for Ge-based MOS capacitors with different interfacial control layer

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The band alignment at HfO2/SiO2/Ge interface was also determined by measuring the valence band offset ∆E v (VBO) using XPS. The VBO values can be obtained based on the assumption that the energy difference between the core level and the valence band (VB) edge of the substrate remains constant with/without the deposition of dielectrics films [35]. Here, the Ge substrate was chosen as the reference to determine the VBO between gate dielectrics stack and Ge substrate. Figure 5a presents the VB spectra of the clean Ge substrate, as-deposited and annealed HfO2/SiO2/Ge stacks determined by linear extrapolation method, respectively. The VB edge of the clean Ge substrate has been determined to be 0.10 eV. And, the VB edges of as-deposited and annealed HfO2/SiO2 samples are found to be 2.55 and 2.79 eV, respectively. It can be noticed that there is a small tail in VB spectra for HfO2/SiO2/Ge stacks, which is corresponding to Ge substrate signal [36]. The leading edge of this weak tail is measured to be 0.10 eV and the same as the VB edge of Ge substrate. Therefore, the VBOs at the interface of HfO2/SiO2/Ge with and without PDA are estimated to be 2.69 and 2.45 eV, respectively. The conduction-band offset ∆E c (CBO) can be obtained by subtracting the VBO and the bandgap of the substrate from the bandgap of HfO2:

Fig. 5
figure 5

Band alignment of as-deposited and annealed HfO2/SiO2 film on Ge. aValence-band spectra of the Ge substrate, as-deposited and annealed HfO2/SiO2 films. b Schematic of band alignment of as-deposited and annealed HfO2/SiO2 film on Ge

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$$ \varDelta {E}_c={E}_g\left({\mathrm{HfO}}_2\right) - {E}_g\left(\mathrm{Ge}\right) - \varDelta {E}_v, $$

where E g (HfO2) and E g (Ge) are the bandgap of HfO2 and Ge, respectively. The bandgaps of Ge and HfO2 are 0.67 and 5.6 eV, respectively. Therefore, the CBO values at the interface of HfO2/SiO2/Ge with and without PDA are estimated to be 2.24 and 2.48 eV, respectively. The CBO values are consistent with the previously reported data of 1.8–2.6 eV [37]. Figure 5b illustrates the corresponding band alignment of as-deposited and annealed HfO2/SiO2/Ge structures. Evidently, the HfO2/SiO2 high-k gate dielectric stacks on Ge exhibit large VBO and CBO values with huge barrier heights to inhibit leakage current.

Conclusions

In summary, SiO2 interlayer was introduced into HfO2 gate dielectrics on n-Ge substrates successfully by in situ PEALD. We have investigated the interfacial, electrical properties, and band alignment of HfO2/SiO2/Ge MOS. It has been demonstrated that Ge-O-Si interlayer and GeO2 layer is formed on Ge surface during the in situ SiO2 deposition. This Ge-O-Si interlayer shows fantastic thermal stability during PDA without Hf-silicates formation. Moreover, Ge-O-Si interlayer can also inhibit the GeO2 degradation during annealing process. The HfO2/SiO2/Ge sample after PDA exhibits a CET value of 1.53 nm with low leakage current density of 2.1 × 10−3 A/cm2 at Vfb + 1 V. The VBO values at the HfO2/SiO2/Ge with and without PDA are determined to be 2.69 and 2.45 eV, and the CBO values to be 2.24 and 2.48 eV, respectively. Compared to the ex situ-formed SiO2 interlayer by MOCVD, the sample with in situ PEALD-formed SiO2 interlayer in this work shows improved electrical performance, ascribed to the fact that SiO2 deposited by PEALD are more uniform than MOCVD. Therefore, PEALD is a much more powerful technology for ultrathin interfacial control layer deposition than MOCVD.