Investigating Pulsed Laser Annealing of Hafnia-Zirconia Using a Dynamic Transmission Electron Microscope

Gaining insight into transient behavior, including chemical reactions, structural deformations, or phase transformations, is the key for comprehending fundamental phenomena in chemistry, biology, and materials science. The direct observation and characterization of these intricate events enable a profound understanding of properties such as stability, reactivity, and strength, that empowers us to develop models that contribute to the creation of innovative materials and devices. Phase transition studies in polymorphic materials are paramount for understanding the structural and thermodynamic changes that occur when a substance undergoes a transition from one crystalline form to another. Polymorphs exhibit different physical and chemical properties, for instance, strength and conductivity, which can significantly impact their performance and functionality in different applications. By investigating phase transitions, researchers can optimize the synthesis, formulation, and processing conditions of polymorphs, leading to improved product design and performance. Moreover, studying phase transitions aids in predicting and controlling the stability of polymorphs. Ultimately, phase transition studies play a vital role in unlocking the full potential of polymorph materials and advancing technological advancements across diverse industries. A noteworthy example is the study of binary oxides of HfO2 and ZrO2 [1-5] with recent focus on their phase transition from tetragonal to monoclinic structure as it has been linked to the emergence of novel electronic properties. For example, the discovery of ferroelectricity in HfO2 [1, 6] has initiated further investigations into HfO2-based ferroelectric thin films. This unique property makes these films applicable for advanced electronic devices such as ferroelectric memories [2] and field-effect transistors [7], driving the exploration of their potential applications. Due to such numerous advantages, HfO2-based systems, specifically HfO2-ZrO2 solid solution (HZO), with ferroelectric properties are considered promising candidates for persistent electronic devices in industrial applications [8]. Extensive research efforts have been dedicated to understand the origin of ferroelectricity in these HfO2-based films and stabilizing the ferroelectric phase for practical use [3]. Ferroelectricity in these films arises when a non-centrosymmetric orthorhombic phase (space group: Pca21) is present. Various factors have influenced its formation, specifically, rapid thermal annealing (RTA) [9-11]. While the heating rate during RTA is regulated by the applied power, the cooling process is limited to around 102°C/s due to radiation and experimental conditions [12]. Therefore, exploring alternative annealing methods, such as pulsed laser annealing (PLA), is appealing. PLA offers significantly higher heating and cooling rates (>106°C/s) at the surface layer of the sample [13, 14]. Moreover, laser annealing is a well-established area of research that focuses on crystallizing amorphous silicon, oxide thin films [15], and perovskites. This method involves controlling the laser pulse for rapid heating and cooling. Laser annealing's localized and controlled nature allows for precise customization of material properties, including crystallinity, dopant activation, and surface morphology. The nucleation and growth process of the oxide crystal phase through laser annealing relies on the duration of the radiation input and the material's absorbance factor. Using PLA for annealing HfO2-based thin films, the growth of the monoclinic phase can be suppressed due to the fast heating process, considering the thermodynamic stability at a small crystal growth scale. On the other hand, the fast cooling process enables the possibility of rapidly quenching non-equilibrium metastable phases. It facilitates the transformation of crystalline HfO2 from the tetragonal to the orthorhombic phase by impeding phase relaxation through the application of significant tensile stress. Particularly, the polar orthorhombic phase in HfO2-based materials, for which the annealing temperature required for the crystallization of the ferroelectric phase is as low as T= 400°C [16-20] thus, potentially compatible with back-end-of-line processes [21]. Gaining insight into the effects of PLA on thin films of HZO opens new possibilities for fabricating advanced devices. Only a few studies exist that investigate the impact of PLA on HfO2-based systems [13, 14], highlighting the need for a deeper understanding. In this study, we aim to experimentally investigate the crystallization of HZO thin film with composition of Hf0.5Zr0.5O2 using an in-situ PLA process. Our unique dynamic transmission electron microscope (DTEM) enables us to perform in-situ PLA and electron diffraction studies, which have not been reported previously for HZO. DTEM is a novel TEM instrument which is coupled with and a drive laser (pump laser) that interacts with the sample, initiating an irreversible transformation in the material which can be captured in situ. The objective is the in-situ examination of the quantitative ferroelectric phase formation due to PLA and the effect of laser energy density and the HZO film thickness on it, aiming to identify the optimized laser parameters for achieving the desired ferroelectric crystal structure. This study is being done through the analysis of selected area electron diffraction (SAED) patterns which is a potent characterization tool for studying the structural properties of crystalline materials. The PLA experiment was carried out in-situ within the DTEM (JEM-2100PLUS, 200 kV, manufactured by JEOL and IDES). The second harmonic of a Nd:YAG laser (λ=532 nm) with a pulse duration of 11 ns and repetition rate of 10 Hz was coupled onto the sample inside the DTEM to induce the annealing process. Silicon nitride TEM grids were used as substrates for HZO samples. All samples were deposited by magnetron sputtering method. The samples consisted of thin films in a TiN/HZO bilayer heterostructure intended for the PLA experiment. The TiN thin film, serving as the light absorber layer and heating source, was sputtered at 500°C. Subsequently, the HZO thin film, with a composition of Hf0.5Zr0.5O2 was sputtered at room temperature, giving an amorphous structure. PLA experiment was conducted in different laser energy densities to determine the laser energy density in which the crystallization was observed, and the orthorhombic phase formation started. Selected area electron diffraction patterns were collected before and after the PLA process and later analyzed to examine the presence of the ferroelectric orthorhombic phase and to quantitatively assess the orthorhombic phase fraction within the PLA region of the sample. Figure 1 illustrates the SAED pattern of the TiN/HZO sample before and after exposure to a single laser pulse with the crystallization energy density. Additionally, Figure 2 presents the corresponding radial intensity distribution extracted from the SAED patterns. The pattern of the nonannealed sample only has two obvious peaks attributed to (111), (220) of monoclinic HZO, and the (220) plane corresponding to the crystalline TiN. Figures 1 and 2 show the SAED patterns and the insets represent corresponding radial distribution profiles of the samples before and after in-situ PLA. As can be seen in Figure 2 and the inset, a clear crystallization is observed after PLA with laser energy density of 1.02 µJ/cm2 In the pattern, the structure exhibits the presence of a peak corresponding to the orthorhombic phase (111) or tetragonal phase (101), which is labeled as o(111)/t(101). Changing the laser energy density applied in PLA, along with varying the thickness of the HZO film, can influence the outcomes of the formation of orthorhombic phases within the samples. This presentation will focus on quantifying the orthorhombic phase fraction and examining the impact of laser energy density and sample thickness on achieving optimized orthorhombic phase formation. These results provide new insights into the crystallization of HZO thin films by laser annealing, which is a promising end-of-the-line processing step to fabricate ferroelectric devices with a low thermal-budget. SAED pattern of the TiN/HZO sample before PLA, and the inset is the corresponding radial distribution profile of the pattern. SAED pattern of the TiN/HZO sample after PLA with laser energy density of 1.02 µJ/cm2, and the inset is the corresponding radial distribution profile of the pattern.

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