Abstract
The microstructure of ferroelectric hafnium oxide plays a vital role for its application, e.g., non-volatile memories. In this study, transmission Kikuchi diffraction and scanning transmission electron microscopy STEM techniques are used to compare the crystallographic phase and orientation of Si and Zr doped HfO2 thin films as well as integrated in a 22 nm fully-depleted silicon-on-insulator (FDSOI) ferroelectric field effect transistor (FeFET). Both HfO2 films showed a predominately orthorhombic phase in accordance with electrical measurements and X-ray diffraction XRD data. Furthermore, a stronger texture is found for the microstructure of the Si doped HfO2 (HSO) thin film, which is attributed to stress conditions inside the film stack during crystallization. For the HSO thin film fabricated in a metal-oxide-semiconductor (MOS) like structure, a different microstructure, with no apparent texture as well as a different fraction of orthorhombic phase is observed. The 22 nm FDSOI FeFET showed an orthorhombic phase for the HSO layer, as well as an out-of-plane texture of the [111]-axis, which is preferable for the application as non-volatile memory.
Highlights
Ferroelectric properties in HfO2, which have been reported to originate from the orthorhombic phase of the space group Pca21 [6], have already been demonstrated in polycrystalline films doped with various elements such as Y, Sr, Al, Si, or Zr [5] as well as in undoped films [7]
We investigate Si:HfO2 (HSO) and Hf0.5Zr0.5O2 (HZO) films utilizing transmission Kikuchi diffraction (TKD), which allows for analyzing the microstructure as well as the local crystallographic phase and orientation of the HfO2 film [15], X-ray diffraction (XRD), and electrical characterization
After wake-up, both samples show a increased remanent polarization (Pr), but the HSO layer (2Pr = 40.68 μC/cm2) has a 20% lower 2Pr than the HZO layer (2Pr = 51.03 μC/cm2), while, in case of HZO, the peaks in the corresponding current-voltage (I–V) loops merged to a symmetric peak, HSO shows a very sharp peak in addition to a rather small broad peak
Summary
Ferroelectric properties in HfO2, which have been reported to originate from the orthorhombic phase of the space group Pca21 [6], have already been demonstrated in polycrystalline films doped with various elements such as Y, Sr, Al, Si, or Zr [5] as well as in undoped films [7]. Since the ferroelectric orthorhombic phase of HfO2 is only a metastable phase [10], polycrystalline films can contain a multitude of phases. It should be mentioned here that, except the phase of space group Pca, two other orthorhombic phases, which are of space group Pbca and Pnma respectively, have been reported for HfO2 [10]. As the stabilization of these phases requires high pressure and since both phases do not exhibit ferroelectricity, they are not discussed further in this article
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