Implementation of an electrically modifiable artificial synapse based on ferroelectric field-effect transistors using Al-doped HfO₂thin films

Abstract

<p>Biological synaptic behaviors such as short-/long-term plasticity were implemented by ferroelectric polarization switching dynamics of Al-doped HfO₂ ferroelectric gate insulators.</p>

Device Feasibility of Ferroelectric Field‐Effect Transistors Using Al‐Doped HfO2 Gate Insulator Deposited with H2O Oxygen Precursor during Atomic Layer Deposition Process

“…3) For the Al:HfO 2 thin films prepared with PMA process (Figure 3a), there were no marked interfacial transition layers at Al:HfO 2 /TiN bottom interfaces. As mentioned earlier, considering that the oxygen‐related defects were easily generated during the high‐temperature PMA process due to the catalytic effect of Pt, the generated oxygen vacancies can drift toward Al:HfO 2 /TiN bottom interfaces from top interfaces under externally stressed situations [1e,17] . As a result, the amount of oxygen vacancies can be extrinsically saturated at the Al:HfO 2 /TiN bottom interfaces, and hence, the surface oxidation of TiN BE and the additional formation of the oxygen‐deficit layer within the Al:HfO 2 film may be suppressed.…”

“…In other words, as the polarization charge of the MFM capacitor is typically larger than the maximum charge available to be stored in the MIS capacitor, only a fraction of total polarization (corresponding to minor loop) can be utilized; otherwise, the conventional MFIS gate stacks are specially designed. As a result, the data retention time can be improved by enhancing the immunity against the depolarization field for the FeFETs implemented with the MFMIS gate stack [1f,2i,17] …”

“…In other words, 1) the absolute levels of the on‐ and off‐programmed currents, and their variations with changes in on‐/off‐programming and read‐out conditions practically reflect the program performance of the transistor‐type memory device. 2) I ON / I OFF s obtained at given read‐out conditions can be a practical design parameter in the memory applications [1d] . Figure 6c shows the variations in on‐ and off‐programmed I DS s with time evolution during the retention period, in which each program state was assigned by applying +5 and −5 V pulses with a duration of 10 μs, respectively.…”

“…Ferroelectric field‐effect transistors (FeFETs) using doped HfO 2 ‐based ferroelectric gate insulators have been considered as emerging devices for the memory applications with higher functionality because of their benefits such as excellent scalability in the nanoscale regime, complementary metal–oxide–semiconductor (CMOS) process compatibility, and robust device reliability. [ 1 ] As the device performance of the FeFETs can be primarily dominated by ferroelectric natures of the HfO 2 ferroelectric thin films, it is important to investigate the process parameters for controlling and improving the ferroelectric properties of the doped HfO 2 thin films, such as types of dopants, deposition methods, and crystallization annealing conditions. [ 2 ] The use of cation dopants with a smaller atomic radius than Hf, such as Al and Si, has been suggested to reduce the difference in volume‐free energies between the tetragonal (t)/cubic (c) phase and orthorhombic (o) phase, facilitating the phase transition from monoclinic (m) phase to t/c phase and contributing to stabilization of o phase.…”

“…[ 3 ] Therefore, the suitable choice of dopant has been commonly considered as one of the most effective methods to ensure the superior ferroelectricity of the HfO 2 thin films. In particular, we conducted various works characterizing the ferroelectric capacitors [2j,2k] and FeFETs using Al‐doped HfO 2 (Al:HfO 2 ) ferroelectric thin films for the nonvolatile memory [1f] and neuromorphic synapse device [1a] applications, in which the Al:HfO 2 thin films were prepared by atomic layer deposition (ALD) process with ozone (O 3 ) oxygen precursor.…”