| 1. |
- D Acunto, Giulio
(author)
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Reaction Mechanisms and Dynamics in the Early Stage of High-κ Oxide Atomic Layer Deposition : Investigations by In Situ and Operando X-ray Photoemission Spectroscopy
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Atomic layer deposition (ALD) is an outstanding deposition technique to deposit highly conformal and uniform thin films with atomic precision. In particular, ALD of transition metal oxide layers from metal amido complexes and water finds its way in several technological fields, including green energy devices and in the semiconductor industry. These ALD reactions are believed to follow a reaction scheme based on the ligand exchange mechanism, in which the surface on which deposition takes place plays a largely static role and the ligands of the used precursor are chemically unchanged during the reaction. To address the correctness of the model, time-resolved in situ and operando ambient pressure x-ray photoelectron spectroscopy (APXPS) technique was employed during the ALD of HfO2 on InAs covered by a thermal or native oxide, TiO2(101) and oxidised as well as clean Si(111).The classic ligand exchange reaction mechanism does not adequately describe the reaction path in any of the investigated sample systems. In particular, ALD of HfO2 on SiO2 follows a bimolecular reaction mechanism based on the insertion of an hydrogen atom of one of the ligands in an amido complex dimer. As a result of its bimolecular nature, this reaction can take place only on a SiO2 surface of a sufficiently high coverage of physisorbed complexes. Similarly, on TiO2 the early stage of the reaction is based on dissociative adsorption, followed by an intra- and inter- molecular reaction path, leading to the formation of new sets of surface species never before identified in any of the previous ALD models.For easily reducible surfaces, such as InAs oxide and TiO2, evidence is found for HfOx formation already during the first ALD half-cycle, due to the transfer of O atoms from the surface to the metal complex. Clearly, this contradicts the static role of the surface in standard ALD models. Interestingly, in the case of InAs covered by a thermal or native oxide, this phenomenon, which lies behind the so-called self cleaning effect, guarantees a sharp interface between the III-V material and HfO2, which is a prerequisite for next generation MOSFETs.These results open new doors for improving devices based on ALD. Time-resolved in situ and operando APXPS allows to follow the kinetics and mechanisms involved in ALD, in real time at second time resolution with significant benefit for the further improvement of general understanding of ALD reactions.
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| 2. |
- Liu, Yen-Po, et al.
(author)
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Hydrogen plasma enhanced oxide removal on GaSb planar and nanowire surfaces
- 2022
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In: Applied Surface Science. - : Elsevier BV. - 0169-4332.
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Journal article (peer-reviewed)abstract
- Due to its high hole-mobility, GaSb is a highly promising candidate for high-speed p-channels in electronic devices. However, GaSb exhibits a comparably thick native oxide causing detrimental interface defects, which has been proven difficult to remove. Here we present full oxide removal from GaSb surfaces using effective hydrogen plasma cleaning, studied in-situ by synchrotron-based X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV). GaSb nanowires turn out to be cleaned faster and more efficiently than planar substrates. Since the UHV conditions are not scalable for industrial sample processing, H-plasma cleaning is furthermore used as pre-treatment prior to atomic layer deposition (ALD) of a protective high-k layer to demonstrate the use of the cleaning step in a more realistic fabrication situation. We observe a cleaning effect of the H-plasma even in the ALD environment, but we also find residual Ga- and Sb-oxides at the GaSb-high-k interface, which we attribute to re-oxidation of the cleaned surface. Our results indicate that an improved control of the ALD reactor vacuum environment could realize oxide- and defect-free interfaces in GaSb-based electronics.
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| 3. |
- Yong, Zhihua, et al.
(author)
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Tuning oxygen vacancies and resistive switching properties in ultra-thin HfO2 RRAM via TiN bottom electrode and interface engineering
- 2021
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In: Applied Surface Science. - : Elsevier BV. - 1873-5584 .- 0169-4332. ; 551
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Journal article (peer-reviewed)abstract
- Resistive random access memory (RRAM) technologies based on non-volatile resistive filament redox switching oxides have the potential of drastically improving the performance of future mass-storage solutions. However, the physico-chemical properties of the TiN bottom metal electrode (BME) can significantly alter the resistive switching (RS) behavior of the oxygen-vacancy RRAM devices, yet the correlation between RS and the physico-chemical properties of TiN and HfOx/TiN interface remains unclear. Here, we establish this particular correlation via detailed material and electrical characterization for the purpose of achieving further performance enhancement of the stack integration. Two types of RRAM stacks were fabricated where the TiN BME was fabricated by physical vapor deposition (PVD) and atomic layer deposition (ALD), respectively. The HfOx layer in HfOx/PVD-TiN is more oxygen deficient than that of the HfOx/ALD-TiN because of more defective PVD-TiN and probably because pristine ALD-TiN has a thicker TiO2 overlayer. Higher concentration of oxygen vacancies induces a larger magnitude of band bending at the HfOx/PVD-TiN interface and leads to the formation of a higher Schottky barrier. Pulsed endurance measurements of up to 106 switches, with 10 μA ± 1.0 V pulses, demonstrate the potential of the studied ultra-thin-HfOx/TiN device stack for dense, large scale, and low-power memory integration.
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