Anna Chernikova

Ultrathin Hf0.5Zr0.5O2 Ferroelectric Films on Si

Because of their immense scalability and manufacturability potential, the HfO2-based ferroelectric films attract significant attention as strong candidates for application in ferroelectric memories and related electronic devices. Here, we report the ferroelectric behavior of ultrathin Hf0.5Zr0.5O2 films, with the thickness of just 2.5 nm, which makes them suitable for use in ferroelectric tunnel junctions, thereby further expanding the area of their practical application. Transmission electron microscopy and electron diffraction analysis of the films grown on highly doped Si substrates confirms formation of the fully crystalline non-centrosymmetric orthorhombic phase responsible for ferroelectricity in Hf0.5Zr0.5O2. Piezoresponse force microscopy and pulsed switching testing performed on the deposited top TiN electrodes provide further evidence of the ferroelectric behavior of the Hf0.5Zr0.5O2 films. The electronic band lineup at the top TiN/Hf0.5Zr0.5O2 interface and band bending at the adjacent n(+)-Si bottom layer attributed to the polarization charges in Hf0.5Zr0.5O2 have been determined using in situ X-ray photoelectron spectroscopy analysis. The obtained results represent a significant step toward the experimental implementation of Si-based ferroelectric tunnel junctions.

Fully ALD-grown TiN/Hf0.5Zr0.5O2/TiN stacks: Ferroelectric and structural properties

Since the discovery of ferroelectricity (FE) in HfO2-based thin films, they are gaining increasing attention as a viable alternative to conventional FE in the next generation of non-volatile memory devices. In order to further increase the density of elements in the integrated circuits, it is essential to adopt a three-dimensional design. Since atomic layer deposition (ALD) processes are extremely conformal, ALD is the favored approach in the production of 3D ferroelectric random access memory. Here, we report the fabrication of fully ALD-grown capacitors comprising a 10-nm-thick FE Hf0.5Zr0.5O2 layer sandwiched between TiN electrodes, which are subjected to a detailed investigation of the structural and functional properties. The robust FE properties of Hf0.5Zr0.5O2 films in capacitors are established by several alternative techniques. We demonstrate a good scalability of TiN/Hf0.5Zr0.5O2/TiN FE capacitors down to 100-nm size and the polarization retention in the test “one transistor-one capacitor” (1T-1...

Ferroelectric properties of full plasma-enhanced ALD TiN/La:HfO2/TiN stacks

We report the possibility of employment of low temperature (≤330u2009°C) plasma-enhanced atomic layer deposition for the formation of both electrodes and hafnium-oxide based ferroelectric in the metal-insulator-metal structures. The structural and ferroelectric properties of La doped HfO2-based layers and its evolution with the change of both La content (2.1, 3.7 and 5.8 at.u2009%) and the temperature of the rapid thermal processing (550–750u2009°C) were investigated in detail. Ferroelectric properties emerged only for 2.1 and 3.7 at.u2009% of La due to the structural changes caused by the given doping levels. Ferroelectric properties were also found to depend strongly on annealing temperature, with the most robust ferroelectric response for lowest La concentration and intermediate 650u2009°C annealing temperature. The long term wake-up effect and such promising endurance characteristics as 3u2009×u2009108 switches by bipolar voltage cycles with 30u2009μs duration and ±u20093 MV/cm amplitude without any decrease of remnant polarization valu...

Charge transport mechanism in thin films of amorphous and ferroelectric Hf0.5Zr0.5O2

The charge transport mechanism in thin amorphous and ferroelectric Hf0.5Zr0.5O2 films has been studied. It has been shown that the transport mechanism in studied materials does not depend on the crystal phase and is phonon-assisted tunneling between traps. The comparison of the experimental current–voltage characteristics of TiN/Hf0.5Zr0.5O2/Pt structures with the calculated ones provides the trap parameters: thermal energy of 1.25 eV and the optical energy of 2.5 eV. The trap concentration has been estimated as ~1019–1020 cm–3.

Improved Ferroelectric Switching Endurance of La-Doped Hf0.5Zr0.5O2 Thin Films

Hf0.5Zr0.5O2 thin films are one of the most appealing HfO2-based ferroelectric thin films, which have been researched extensively for their applications in ferroelectric memory devices. In this work, a 1 mol % La-doped Hf0.5Zr0.5O2 thin film was grown by plasma-assisted atomic layer deposition and annealed at temperatures of 450 and 500 °C to crystallize the film into the desired orthorhombic phase. Despite the use of a lower temperature than that used in previous reports, the film showed highly promising ferroelectric properties-a remnant polarization of ∼30 μC/cm2 and switching cycle endurance up to 4 × 1010. The performance was much better than that of undoped Hf0.5Zr0.5O2 thin films, demonstrating the positive influence of La doping. Such improvements were mainly attributed to the decreased coercive field (by ∼30% compared to the undoped film), which allowed for the use of a lower applied field to drive the cycling tests while maintaining a high polarization value. La doping also decreased the leakage current by ∼3 orders of magnitude compared to the undoped film, which also contributed to the strongly improved endurance. Nonetheless, the La-doped film required a larger number of wake-up cycles (∼106 cycles) to reach a saturated remnant polarization value. This behavior might be explained by the increased generation of oxygen vacancies and slower migration of these vacancies from the interface to the bulk region. However, the maximum number of wake-up cycles was less than 0.01% of the total possible cycles, and therefore, initializing the film to the maximum performance state would not be a serious burden.

Effect of dielectric stoichiometry and interface chemical state on band alignment between tantalum oxide and platinum

The tantalum oxide–platinum interface electronic properties determined by X-ray photoelectron spectroscopy are found to depend on the dielectric stoichiometry and platinum chemical state. We demonstrate the slow charging of the tantalum oxide in cases of Ta2O5/Pt and Ta2O5−y/Pt interfaces under the X-ray irradiation. This behavior is proposed to be related to the charge accumulation at oxygen vacancies induced traps. Based on the proposed methodology, we define the intrinsic conductive band offset (CBO) ∼1.3u2009eV (both for Ta2O5/Pt and Ta2O5−y/Pt) and CBO after the full saturation of the traps charging ∼0.5u2009eV, while the last one defines the energy position of charged traps below the bottom of conduction band. We demonstrate also the pining at the both Ta2O5/Pt and Ta2O5−y/Pt interfaces even in the “intrinsic” state, apparently induced by the presence of additional interfacial states. No shifts of Ta4f line and band alignment in over stoichiometric Ta2O5+x/Pt structure during X-ray irradiation, as well as t...

Investigation of the properties and manufacturing features of nonvolatile FRAM memory based on atomic layer deposition

The structural and electrical properties of materials based on hafnium oxide grown by atomic layer deposition (ALD) are analyzed. The possibility and prospects of the use of their nanoscale films in nonvolatile memory are considered. The possibility of scaling Ferroelectric random access memory (FRAM) to subμm dimensions while preserving the ferroelectric properties is shown.

Ferroelectric properties of lightly doped La:HfO2 thin films grown by plasma-assisted atomic layer deposition

The structural and ferroelectric properties of lightly La-doped (1u2009mol. %) HfO2 thin films grown by plasma-assisted atomic layer deposition were examined. An annealing temperature as low as 400u2009°C crystallized the film into the desired orthorhombic phase, which resulted in it displaying promising ferroelectric performance. The remanent polarization (Pr) increased with annealing temperature, but the performance enhancement seemed to saturate at 500u2009°C. A slight decrease in the dielectric constant, which was associated with the preferential formation of a polar orthorhombic phase at higher temperatures, was also observed. The long-term wake-up effect, i.e., a marked rise in the 2Pr value during field cycling, was demonstrated for films processed at all annealing temperatures. The presence of domain groups with opposite internal electric biases was found in the pristine state, while the internal bias distribution became more uniform during wake-up. The endurance of up to 4u2009×u2009108 switching cycles without marked fatigue using bipolar pulses with a duration of 600u2009ns, and an amplitude of ±3u2009MV/cm was demonstrated.

Atomic layer deposition of Al2O3 and AlxTi1−xOy thin films on N2O plasma pretreated carbon materials

A mild N2O plasma treatment technique (low power and low substrate temperature) for carbon materials (including graphite and carbon nanotubes) functionalization followed by subsequent high-k dielectric atomic layer deposition (ALD) was developed. It was shown that N2O plasma carbon functionalization leads to the formation of epoxide and carboxylic groups on the carbon surface which act as active centers for ALD and, as a result, conformal and uniform Al2O3 and TixAl1−xOy films growth occurred on the carbon surfaces. It was shown that the electrical properties of multinary TixAl1−xOy oxides are more promising in comparison to single Al2O3 oxide. Some electrical properties of the TixAl1−xOy films observed were a high dielectric constant ∼19, low leakage current density (<3u2009×u200910−5 A/cm2 at 1 MV/cm), and high breakdown field (∼5.5u2009MV/cm).

Ferroelectricity in Hf0.5Zr0.5O2 Thin Films: A Microscopic Study of the Polarization Switching Phenomenon and Field-Induced Phase Transformations

Because of their full compatibility with the modern Si-based technology, the HfO2-based ferroelectric films have recently emerged as viable candidates for application in nonvolatile memory devices. However, despite significant efforts, the mechanism of the polarization switching in this material is still under debate. In this work, we elucidate the microscopic nature of the polarization switching process in functional Hf0.5Zr0.5O2-based ferroelectric capacitors during its operation. In particular, the static domain structure and its switching dynamics following the application of the external electric field have been monitored with the advanced piezoresponse force microscopy (PFM) technique providing a nm resolution. Separate domains with strong built-in electric field have been found. Piezoresponse mapping of pristine Hf0.5Zr0.5O2 films revealed the mixture of polar phase grains and regions with low piezoresponse as well as the continuum of polarization orientations in the grains of polar orthorhombic phase. PFM data combined with the structural analysis of pristine versus trained film by plan-view transmission electron microscopy both speak in support of a monoclinic-to-orthorhombic phase transition in ferroelectric Hf0.5Zr0.5O2 layer during the wake-up process under an electrical stress.