Two-dimensional InSe Field-effect Transistors And Their Applications In Artificial Synapses

As a two-dimensional semiconductor,indium selenide(InSe)has a small electron effective mass,high-speed carriers transport,and n-type conductivity.Due to its excellent physical properties,it can be used as a channel material for field-effect transistors(FETs).The electrical properties of InSe FETs can be effectively improved by optimizing the substrate interface,encapsulating the channel,and improving the contact type between the electrode and the semiconductor.The optimized InSe FETs have stable electrical properties,which is beneficial to the simulation of artificial synapses.In addition,when the artificial synapse devices based on InSe FETs receive external electrical signals or optical signals,the output currents change.During the signal transmission processes,the weights of the artificial synapses continuously update,simulating the learning processes.This work mainly focuses on the preparation of high-performance InSe FETs,and their application in artificial synapses,including the following parts:(1)By using an indium interface layer,the surface charge doping of the InSechannel is realized and the contact type between the source/drain electrode and InSeis optimized.Since the work function of indium is smaller than that of semiconductor InSe,the indium interface layer can realize n-type doping to the semiconductor channel and protect InSe.Furthermore,the indium interface layer presents discontinuous island shapes on the surface of InSe,which will not cause a short circuit between the source and the drain electrodes.At room temperature,the field-effect mobility of InSe FET with indium interlayer increases from?13 cm~2 V^-1 s^-1 to?900 cm~2 V^-1 s^-1,the current on/off ratio increases from 10~5 to 10~7,and the threshold voltage decreases from 28.27 V to 1.67V.Moreover,the hysteresis window of the transfer characteristic curve of the InSe FET is significantly reduced,indicating that the indium interface layer can effectively protect the InSechannel,which prevents it from oxidizing and reduce the generation of surface defects.In addition,the output curves of InSe FETs of the indium interface layer show a linear relationship,suggesting that the existence of the indium interface layer causes ohmic contact between the gold electrode and InSe.(2)Based on high-performance InSe FETs,artificial optical synapse devices are realized.Under light pulses,the optical synapse devices can emulate a variety of synaptic functions and the curative effect after drug stimulation.The InSe FET-based artificial optical synapse device can emulate the synaptic behaviors,including short-term plasticity and long-term plasticity and paired pulse facilitation.In addition,the tri-mode synaptic weight update(ascending/flatting/descending),under the same illumination conditions,is realized by applying diverse gate voltages to modulate the capturing/releasing event,which successfully imitates the medicine-acting metaplasticity with effective/stable/ineffective feature triggered by repeated drug stimulus with adequate/low/ultra-low dosage.(3)Based on floating-gate InSe FETs,the FET-type artificial electrical synapse devices are realized.Under electrical pulses,the devices can simulate inhibitory and excitatory synaptic behaviors and synaptic transmission fatigue properties.Based on InSe FETs,floating gate and tunneling medium are added to construct floating gate InSe FETs.The floating-gate in InSe FETs can store charge,enabling a large storage window of 80 V.In addition,the floating-gate InSe FETs have excellent data retention characteristics and durability.Since the process of storing/releasing charges at the floating-gate can be regulated by the electric pulses of the control gate,the artificial electrical synapses can emulate the inhibitory behavior under the positive electrical pulses and excitatory behavior under negative electrical pulses.Moreover,the increase in the frequency of electrical pulse will enhance inhibitory and excitatory behaviors.In addition,the artificial electrical synapse devices can simulate the fatigue characteristic of synaptic transmission,caused by the high intensity and long duration electrical pulses.Furthermore,the synaptic responses would be attenuated when continuous and strong electrical pulses are applied to the synapse.