Ultra-highly linear Ga2O3-based cascade heterojunctions optoelectronic synapse with thousands of conductance states for neuromorphic visual system

Ultrawide bandgap semiconductor optoelectronic synapses can perform high-parallel computing with a low false alarm rate, making them ideal for building deep-ultraviolet (DUV) neuromorphic visual system (NVS). However, the rapid carrier recombination in these optoelectronic synapses results in a poor number of conductance states and a low linear weight update protocol, consequently degrading the image recognition accuracy of DUV NVSs. This work proposes a type of cascade heterojunctions capable of finely tuning the dynamics of photogenerated carriers, utilizing aluminum interdigital electrodes sandwiched between tin-doped Ga₂O₃ and oxygen-deficient hafnium oxide (GTO/Al/HfO_x) films. The built-in fields at the GTO/HfO_x heterojunction and the Al/HfO_x hole Schottky junction interfaces facilitate the separation of photogenerated carriers and the subsequent trapping of holes by the oxygen defects in the HfO_x, respectively. The GTO/Al/HfO_x optoelectronic synapses exhibit an ultrahigh responsivity of over 10⁴ A/W and a large photo-to-dark current ratio of 6 × 10⁵, which results in exceptional synaptic plasticity with unprecedented 4096 conductance states and excellent linearity with a fitting coefficient of 0.992. These attributes enable the GTO/Al/HfO_x optoelectronic synapses to execute logical operations with fault-tolerance capability and to achieve high-accuracy fingerprint classification. The innovative cascade heterojunctions design, along with the elucidated carrier dynamics modulation mechanism, facilitates the development of DUV NVSs.