Temperature-Dependent HfO2/Si Interface Structural Evolution and its Mechanism
- 193 Downloads
In this work, hafnium oxide (HfO2) thin films are deposited on p-type Si substrates by remote plasma atomic layer deposition on p-type Si at 250 °C, followed by a rapid thermal annealing in nitrogen. Effect of post-annealing temperature on the crystallization of HfO2 films and HfO2/Si interfaces is investigated. The crystallization of the HfO2 films and HfO2/Si interface is studied by field emission transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and atomic force microscopy. The experimental results show that during annealing, the oxygen diffuse from HfO2 to Si interface. For annealing temperature below 400 °C, the HfO2 film and interfacial layer are amorphous, and the latter consists of HfO2 and silicon dioxide (SiO2). At annealing temperature of 450-550 °C, the HfO2 film become multiphase polycrystalline, and a crystalline SiO2 is found at the interface. Finally, at annealing temperature beyond 550 °C, the HfO2 film is dominated by single-phase polycrystalline, and the interfacial layer is completely transformed to crystalline SiO2.
KeywordsHafnium oxide Atomic layer deposition Interface Annealing Crystallization
Atomic force microscopy
Amorphous hafnium oxide
Atomic layer deposition
Amorphous silicon dioxide
Crystalline silicon dioxide
Grazing incident X-ray diffraction
High-resolution transmission electron microscopy
Remote plasma atomic layer deposition
Rapid thermal annealing
Tetrakis (ethylmethylamino) hafnium
X-ray photoelectron spectroscopy
Hafnium oxide (HfO2) thin film is an interesting material for a variety of applications. It can be used in multilayer optical coating , protective coating , gate dielectric , passivating layer [4, 5, 6], and so on due to its excellent properties, such as high density, high refractive index, wide band gap, and relatively high thermal stability. Many methods have been used to prepare HfO2 thin film, such as electron beam evaporation , chemical solution deposition , reactive sputtering , metal organic chemical vapor deposition , molecular beam epitaxy , and atomic layer deposition (ALD). ALD is a promising method for obtaining thin films with both high-precision thickness control and high accuracy uniformity. Post-annealing is found to have significant influences on ALD HfO2 films [12, 13, 14, 15]. According to the research, HfO2 thin films can crystalize for an annealing temperature higher than 500 °C [16, 17, 18]. The crystalline structure of HfO2 strongly affects optical and electrical properties. For example, the structural change of HfO2 from amorphous to monoclinic crystalline phase could lead to changes of refractive index from 1.7 to 2.09, optical gap from 5.75 to 6.13 eV, and dielectric constant from 24.5 to 14.49 [19, 20]. For ALD HfO2 deposited on silicon substrates, an oxide layer is usually observed at HfO2/Si interface [21, 22]. The presence of this interfacial layer is reported to decrease the dielectric constant . In addition, Kopani et al.  presented the structural properties of 5-nm HfO2 films after nitric acid oxidation of n-doped Si substrates. They found that high annealing temperature increases the growth rate of crystalline nuclei. However, their crystallization properties particularly HfO2/substrate interface have scantly been studied. Therefore, the annealing temperature affecting the crystallization properties of HfO2 thin films prepared by ALD was worth for further investigation.
In this work, the HfO2 thin films were fabricated by a remote plasma atomic layer deposition (RP-ALD) on p-type silicon substrates. Post-annealing was performed by a rapid thermal annealing (RTA) system at different temperatures. The structural changes and crystallization properties of HfO2 thin films by RTA were characterized by atomic force microscopy (AFM), grazing incident X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM). The temperature-dependent HfO2/Si interface structural evolution and its mechanism are also investigated.
RPALD HfO2 deposition parameters
RPALD- HfO2 thin film
Substrate temperature (°C)
TEMAH pulse time (s)
O2 plasma pulse time (s)
O2 plasma power (W)
RTA-post annealing process
AFM measurements were performed in tapping mode for investigating the surface morphology of the HfO2 thin films. The AFM images shown in this work are 2 μm × 2 μm scans with a resolution of 256 points × 256 lines. The structure of HfO2 films were characterized by grazing incident X-ray diffraction (GIXRD, Rigaku TTRAXIII, Japan) measurements with a Cu long-fine-focus X-ray tube. X-rays with a wavelength of 0.154 nm were produced at an operating voltage of 50 kV and a current of 300 mA. An incident angle of 0.5° was selected to obtain diffraction patterns over a 2θ range of 20–60°. X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-alpha) was also performed using monochromatic Al Kα X-ray radiation (hν = 1486.6 eV). For the XPS analysis, a 100-μm diameter spot was used, and photoelectrons were collected at a take-off angle of 45°. The cross sections of the HfO2 thin films were prepared by a focused ion beam lift-out technique in a Hitachi NX2OOO system. The cross-sectional images of the HfO2 thin films were examined by a field emission high-resolution transmission electron microscopy (HR-TEM, JEM-2100F, USA).
Results and Discussion
HfO2 films are prepared using RP-ALD, and effect of annealing temperature on crystalline structure of the HfO2 has been investigated. For as-deposited HfO2 and that annealed below 400 °C, the HfO2 and the interfacial layer are amorphous. With increasing annealing temperature, the d-spacing of orthorhombic reduces while that of the c-SiO2 interfacial layer increases, indicating the oxygen diffusion from HfO2 to Si interface. Annealing temperature higher than 550 °C shows a HfO2 layer with polycrystalline orthorhombic single-phase, and the interfacial layer completely transforms to c-SiO2. Although annealing is required for HfO2 in many applications such as achieving high passivation of Si wafers and high dielectric constant, the crystallization could be harmful to the film properties. The annealing temperature of 500 °C can have the best Si wafer passivation quality and dielectric constant.
This work is sponsored by the Ministry of Science and Technology of Taiwan (nos. 104-2632-E-212-002-, 104-2622-E-212-005-CC3, 104-2221-E-212-002-MY3). This work is also sponsored by the National Natural Science Foundation of China (nos. 61534005 and 61474081), the Science and Technology innovation Project of Xiamen (nos. 3502Z20183054 and 3502Z20173040), and the Science and Technology Program of the Educational Office of Fujian Province (JT180432).
Availability of Data and Materials
All data supporting the conclusions of this article are included within the article.
XYZ carried out the characterization of the HfO2 thin films and drafted the manuscript. CHH, WYW, SLO, and SYL led the experimental and analytical effort. SYC, WH, WZZ, FBX, and SZ contributed to the valuable discussion on experimental and theoretical results. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
- 2.Cianci E, Lamperti A, Tallarida G, Zanuccoli M, Fiegna C, Lamagna L, Losa S, Rossini S, Vercesi F, Gatti D, Wiemer C et al (2018) Advanced protective coatings for reflectivity enhancement by low temperature atomic layer deposition of HfO2 on Al surfaces for micromirror applications. Sensors Actuators A 282:124–131CrossRefGoogle Scholar
- 3.Stoklas R, Gregusova D, Hasenohrl S, Brytavskyi E, Tajna M, Frohlich K, Hascik S, Mgregor JK et al (2018) Characterization of interface states in AlGaN/GaN metal-oxide semiconductor heterostructure field-effect transistors with HfO2 gate dielectric grown by atomic layer deposition. Appl Surf Sci 461:255–259CrossRefGoogle Scholar
- 6.Polydorou E, Martha B, Drivas C, Seintis K, Sakellis I, Soultati A, Kaltzoglou A, Speliotis T, Fakis M et al (2018) Insights into the passivation effect of atomic layer deposited hafnium oxide for efficiency and stability enhancement in organic solar cells. J Mater Chem 6:8051–8059Google Scholar
- 14.García H, Castán H, Dueñas S, Bailón L, Campabadal F, Beldarrain O et al (2013) Electrical characterization of atomic-layer-deposited hafnium oxide films from hafnium tetrakis (dimethylamide) and water/ozone: effects of growth temperature, oxygen source, and postdeposition annealing. J Vac Sci Technol A 31(1):01A127 Available from: http://avs.scitation.org/doi/10.1116/1.4768167 CrossRefGoogle Scholar
- 15.Chaudhary P (2015) Characterization of hafnium oxide film deposited using atomic layer deposition system. Int J Sci Res Eng Technol 4(8):836–840Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.