Ferroelectricity in nanocrystalline Hf0.5Zr0.5O2 thin films

Abstract

Hafnium dioxide (HfO_2) based thin films doped with various dopants (Si, Ge, Al, Gd, Sr, Zr) have been found to exhibit ferroelectricity. These dopants were found to stabilize the III orthorhombic phase in the hafnium oxide based thin films. This characteristic enables and allows various applications ranging from Non-volatile memory, Ferroelectric Field-Effect-Transistors, to Negative Capacitance Field-Effect-Transistors. In this master thesis, ZrO_2 were used as a dopant to vary the crystallinity of HfO_2 during rapid thermal annealing at various temperatures in nitrogen ambient. The structure of Hf_0.5Zr_0.5O_2 (HZO) was fabricated using 50% ZrO_2 and 50% HfO_2 with precise control by the Atomic Layer Deposition technique. A planar metal-ferroelectric insulator-metal capacitor (Au/TiN/HZO/TiN/Si) was fabricated and analyzed using a probe station to investigate the electrical properties. Prior to the top metal deposition, the surface and interface properties such as thickness, roughness, and density, were observed using x-ray reflectivity measurement. Polycrystallinity was observed using grazing-incidence x-ray diffraction measurements in only a few samples. The potential issues related to the process steps are discussed. The electrical measurements showed large leakage. A reference capacitor, Au/HfO_2/TiN/Si structured capacitor was fabricated. This pure HfO_2 based capacitor did not show leakage behavior. Further paths to improvement is discussed.

Popular Abstract

Nowadays, the electronic devices are widely used and applied in various applications such as cell phones, personal computer, laptops and so on. The building unit of the electronic devices are transistors and the most common one is called the Metal-oxide-semiconductor field effect transistor (MOSFET) which comprises a capacitor placed between two contacts called source and drain. The source contact and drain contact are contacted by metal to connect to the power supply.
In the past, the insulator (dielectric) part of the MOSFET is silicon dioxide, which has low dielectric constant, about 3.9. With the development of the electronic technology, the applied voltage on MOSFET has been scaled down and reduced in each generation of electronic circuits with increasing efficiency and lower power consumption. However, this becomes tough when scaling down to subnanometer range as the thickness of the insulator part is limited. Traditionally, the subthreshold swing of the scaled MOSFET in room temperature is about 60 millivolts per decade (a decade corresponds to a 10 times increase of the current). Thus, there is a strong effort to fabricate a new device which can have steeper turn-off speed than the traditional MOSFET, the so-called steep-slope device, for the purpose for various applications in memory technology, new field effect transistors, faster switches, optical devices, energy-related devices, and sensors.
One application is the so-called negative capacitance field effect transistor (NCFET) which uses negative capacitance as the insulator. NCFETs are based on the inclusion of ferroelectric material as the insulator (high dielectric constant value, around 10 to 30 or even higher) instead of the traditional silicon dioxide. The ferroelectric material was first studied and researched by Valasek in the early 1920s. The basic feature of the ferroelectric material is the polarization against electric field curve, such that the polarization changes as a closed curve with respect to the electric field is followed. Ferroelectric materials have two polarizations value (called remnant polarization) when the electric field is removed (zero value), and the positive polarization value can be ''1'' or ''up'' state while the negative polarization value can be ''0'' or ''down'' state, this can be used for memory applications.
The new ferroelectric material, hafnium zirconium dioxide, Hf0.5Zr0.5O2, has been specially researched and is attractive because it is simple to be fabricated by the mature Atomic Layer Deposition (ALD) technique and is cost effective. This ferroelectric material is preferred compared to the traditional lead-based ferroelectric material due to the environmental issues. Using lead based metal is prevented and forbidden in many countries. This high dielectric constant value ferroelectric material shows promising features. For instance, the thickness of the insulator layer can be made very thin, around 10 nanometers or less. It has good thermal stability, compatible with the traditional silicon technology. Also, this material, which I am currently doing research, on can be applied in a future memory application called FeRAM, ferroelectric field-effect transistors, optical devices, sensors, and energy-related applications.

• author: Sang, Lun
• supervisor: Mattias Borg; Ivan Maximov
• organization: Solid State Physics; Department of Physics
• course: FYSM60 20171
• year: 2018
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Physics and Astronomy
• keywords: ferroelectricity, Hf0.5Zr0.5O2, GIXRD, XRR
• language: English
• id: 8936464
• date added to LUP: 2018-03-07 11:10:47
• date last changed: 2018-03-07 11:10:47