Synaptic neuron-like structure based on HfO2/GeSn with ferroelectric field effect that simulates a three-terminal memristor
Project Director: Dr. Adrian SLAV
This project proposes developing a synaptic structure based on HfOx/GexSn1-x with conductance modulated by the ferroelectric field-effect. By improving the ferroelectric characteristics of HfO2 using a high-mobility channel material (epi/poly GeSn), we obtain a synaptic neuron-like structure that simulates a three-terminal memristor for neuromorphic computing. The HfOx/GexSn1-x structure is obtained by reactive/non-reactive magnetron sputtering followed by Rapid Thermal Annealing in active working gas (H2/N2) to avoid the local disorder by passivating the dangling bonds and by healing the trap states. The plasticity of synaptic neuron-like structure HfOx/GexSn1-x will be modulated and enhanced by controlling the interface between HfO2 FeCAP and GeSn high-mobility channel.
In this stage, preliminary technological parameters were obtained for the deposition by magnetron sputtering (MS) of HfO2 and GexSn1-x films to create HfO2/GexSn1-x neuronal-type synaptic structures. GeSn films were obtained by co-deposition in DC mode from separate Ge and Sn targets. GeSn nanocrystals (NC) and the crystallization of HfO2 layers are formed by RTA thermal treatments in a hydrogen atmosphere. After the thermal treatments of the GeSn layers, crystallization is initiated from 350 oC with a slight formation of β - Sn, and at 450 oC we obtain a clear separation of the two constituent phases, Ge, respectively β - Sn. In the case of the HfO2/GeSn structure, the amorphous state is maintained up to 250 oC; above this temperature, the formation of GeSn NCs begins, which is very clearly evident at 450 oC, without the formation of the β-Sn phase. Above 450 oC, the crystallization of the HfO2 layer begins, which at 550 oC shows a mixture of monoclinic and orthorhombic phases together with GeSn NCs. The absence of β-Sn phase formation up to temperatures above 500 oC in the case of HfO2/GeSn structures compared to GeSn films can be explained by the appearance of stress at the interface of GeSn with HfO2 during deposition. This stress can be due to the flow of atoms reaching the support and their energy (translated experimentally by the deposition rate and the power applied to the magnetron) and/or by the implantation of atomic hydrogen in the structure. Through XRR measurements, the density of 8.5 g/cm3 (ρHfO2= 9.68 g/cm3-bulk) and the roughness of 0.7 nm for the HfO2 layer and the density of 5.42g/cm3 (ρGe= 5.32 g/cm3-bulk, ρSn= 7.3 g/cm3-bulk) and the roughness of 1.2 nm for the GeSn layers. From the analysis of the Raman spectra obtained at various values of the incident laser power, the concentration of Sn in the GeSn NC was obtained between 6.3% and 8.2% Sn concentration in GeSn NCs. For the nanostructures treated by RTA at 250 oC, 550 oC and without thermal treatment, a residual polarization of ~1 μC/cm2 was obtained, and the shape of the hysteresis curve shows the capacitive character combined with ohmic conduction caused by the formation of β - Sn.
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