Depicting the Electronic Structure of HfO2 Films by Spectroscopic Techniques
Authors: Corrêa Silma Alberton , Brizzi Simone , Schmeisser Dieter 
Published: February 2020

Hafnium oxide (HfO2) is one of the most promising high-k replacement for the SiO2 gate dielectric in complementary metal–oxide–semiconductor (CMOS) devices because of its high dielectric constant and high thermal stability [1]. Besides the compatibility with CMOS technology, the recently observed ferroelectricity of HfO2-based thin films may enable its application in non-volatile memories like ferroelectric field effect transistors (FeFET) or the capacitor based ferroelectric random access memory (FRAM) [2]. Atomic layer deposition (ALD) has been widely applied in fabrication of ultrathin HfO2 films since it provides precise control of film thickness and uniformity at the atomic scale. These characteristics of the ALD allow the integration of complex architectures, such as 3D schemes in HfO2-based FRAM capacitors.
Different growth techniques produce films of varying quality with different phase compositions, which influences their chemical and physical properties, including, in particular, properties that derive from the electronic structure. For instance, the experimental band gap reported for HfO2 is highly scattered, being in the 5.0 eV – 6.5 eV range [3-5]. This wide scattering could be due to the charging properties of HfO2 that still remain elusive. The X-ray bombardment during photoemission experiments induces charges within the dielectric and modifies the energy of the emitted photoelectrons [6]. This kind of de-charging effect may influence the correct determination of the band structure of the HfO2/SiO2 system. Therefore, in order to exploit the capabilities of HfO2 films in electronic devices, the electronic properties of these films must be precisely determined.
In this contribution, spectroscopic techniques were employed in order to investigate the electronic structure of HfO2 thin films deposited by atomic layer deposition (ALD) on Si/SiO2. Tetrakis-di-methyl-amido-Hf (TDMAHf) was used as the Hf-precursor and three different oxygen sources were investigated: water (H2O), oxygen (O2), and ozone (O3). Samples were characterized by resonant photoelectron spectroscopy, which allowed the determination of the positions of valence band maximum and conduction band minimum, the excitation range for the polaronic states and the range of charge transfer band. Results obtained for HfO2 films investigated indicate that characteristic values were not influenced by the preparation conditions. Electronic charges within the ALD-HfO2 films were identified depending on the particular preparation techniques, pointing out that the amount of charges is essential to determine the accurate position of the surface potentials.
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