【Domestic Papers】Optoelectronic Synaptic Devices Based on ε-Ga₂O₃/ZnO Heterojunctions Grown by an Optimized Process

      Researchers from the North China Electric Power University have published a dissertation titled "Optoelectronic Synaptic Devices Based on ε-Ga₂O₃/ZnO Heterojunctions Grown by an Optimized Process" in Crystal Growth & Design.

Project Support

      This work was supported by the National Natural Science Foundation of China (No. 61704054).

Background

      ZnO is widely employed in device applications such as UV detectors, light-emitting diodes, solar cells, and piezoelectric sensors owing to its favorable properties, including a wide bandgap, high electron mobility, and piezoelectric characteristics, among others. Despite substantial progress in ZnO thin film growth technologies, significant challenges persist in achieving the cost-effective and efficient deposition of ZnO films with superior crystalline quality and surface integrity. The hexagonal wurtzite structure intrinsic to ZnO generates polar surfaces with an elevated surface energy. This inherent energy predisposition results in predominant island growth patterns during vapor deposition processes, where subsequent island coalescence frequently induces dislocation formation, ultimately yielding columnar or granular microstructures. This phenomenon is particularly pronounced on substrates with a high lattice mismatch, such as sapphire. Although the nanostructured and microstructural characteristics of ZnO have been thoroughly investigated and exploited across diverse applications, comparatively less research attention has been directed toward ZnO thin films.

Abstract

      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.

Conclusion

      Through the incorporation of a ε-Ga₂O₃ induction layer, we effectively controlled the growth mode of ZnO from island-like to pyramidal morphology. Consequently, this transformation led to the conversion of the initially disordered microstructure into a planar film with an improved crystal quality and enhanced surface properties. The resultant ZnO films exhibit well-defined step structures on their surfaces and a unidirectional growth orientation. Furthermore, these films display remarkably narrow rocking curves with half-widths as low as 551 arcsec and minimal surface roughness values of 7.38 nm. Based on these significant observations, we propose a novel approach termed the “growth process-synchronized thermal injection method”, which is further supported by our comprehensive elucidation using EDS spectroscopy analysis. We hypothesize that, in the presence of a reducing atmosphere and high-temperature environment, Ga₂O₃ and ZnO undergo mutual thermal injection, predominantly involving Ga ions. This thermal injection facilitates the formation of a homogeneous alloy induction layer on the growth surface. Consequently, under the influence of this alloy induction layer, lattice mismatch is mitigated, inducing a transition from the island growth mode to the pyramidal growth mode. The “growth process-synchronized thermal injection method” provides a pathway for optimizing the growth of ZnO thin films and offers novel insights into modifying the growth behavior of other materials to achieve superior film quality. Subsequently, we fabricated optoelectronic artificial synapses using the optimized ε-Ga₂O₃/ZnO heterojunction, which successfully mimicked the synaptic behaviors of short-term memory, long-term memory, relearning, and environmental influences, providing a valuable reference for the research of optoelectronic devices with ε-Ga₂O₃/ZnO heterojunctions.