Optoelectronic Synaptic Devices Based on ε-Ga ₂ O ₃ /ZnO Heterojunctions Grown by an Optimized Process

In this study, we introduce a novel methodology, termed the “Growth Process-Synchronized Thermal Injection Method”, aimed at modifying the growth characteristics of ZnO thin films for application in optoelectronic artificial synapses. By incorporation of an ε-Ga2O3 film as an induction layer, Ga ions are introduced in situ during the growth of ZnO thin films. This approach enables the formation of an alloyed induction layer at the growth interface, which significantly reduces lattice mismatch and transforms the growth pattern from island-like to pyramidal. Initially, the unoptimized ZnO films exhibit columnar microstructures composed of dual crystalline phases and lack a dominant crystallographic orientation on their surfaces. However, after the growth habit is modified, the resulting ZnO films display single-phase characteristics with a distinct preferred orientation on out-of-plane crystal orientation. Optoelectronic synapses were engineered using heterojunctions formed between ε-Ga2O3 films and the optimized ZnO films, demonstrating the ability to emulate various synaptic behaviors, including short-term memory, long-term memory, forgetting, recall, and environment-modulated memory, thereby underscoring the potential of this architecture for brain-inspired computing. In summary, the proposed “Growth Process-Synchronized Thermal Injection Method” not only offers an effective strategy for enhancing the crystalline quality of ZnO thin films but also provides valuable insights into the manipulation of growth mechanisms for improving the structural and functional properties of other thin film materials. Moreover, the optoelectronic artificial synapses based on oxygen vacancy defects within ε-Ga2O3/ZnO heterojunctions exhibit robust synaptic behaviors and serve as a reference for future development and practical applications of optoelectronic devices incorporating such heterojunctions.