- Nano Express
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Interfacial, Electrical, and Band Alignment Characteristics of HfO2/Ge Stacks with In Situ-Formed SiO2 Interlayer by Plasma-Enhanced Atomic Layer Deposition
© The Author(s). 2017
Received: 16 March 2017
Accepted: 13 April 2017
Published: 25 May 2017
In situ-formed SiO2 was introduced into HfO2 gate dielectrics on Ge substrate as interlayer by plasma-enhanced atomic layer deposition (PEALD). The interfacial, electrical, and band alignment characteristics of the HfO2/SiO2 high-k gate dielectric stacks on Ge have been well investigated. It has been demonstrated that Si-O-Ge interlayer is formed on Ge surface during the in situ PEALD SiO2 deposition process. This interlayer shows fantastic thermal stability during annealing without obvious Hf-silicates formation. In addition, it can also suppress the GeO2 degradation. The electrical measurements show that capacitance equivalent thickness of 1.53 nm and a leakage current density of 2.1 × 10−3 A/cm2 at gate bias of Vfb + 1 V was obtained for the annealed sample. The conduction (valence) band offsets at the HfO2/SiO2/Ge interface with and without PDA are found to be 2.24 (2.69) and 2.48 (2.45) eV, respectively. These results indicate that in situ PEALD SiO2 may be a promising interfacial control layer for the realization of high-quality Ge-based transistor devices. Moreover, it can be demonstrated that PEALD is a much more powerful technology for ultrathin interfacial control layer deposition than MOCVD.
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 . 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 . 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 , such as the introduction of SiO2 , Si , GeO2 , 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 . 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) . 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 . 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.
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
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 . 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.
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.
This project is supported by the Natural Science Foundation of China and Jiangsu Province (51571111 and BK2016230), a grant from the State Key Program for Basic Research of China (2015CB921203).
YQC and BW carried out the sample fabrication and device measurements. YQC and BW did the data analysis and interpreted the results. ADL and DW participated in the discussion of results. YQC drafted the manuscript. ADL supervised the whole work and revised the manuscript. All authors critically read and commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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