In Situ Investigations of MIM Diode Behavior in the Presence of an Electromagnet

The wireless transfer of electrical energy using a directed electromagnetic beam is a process called power beaming. These free-space laser power networks transmit electrical power using aircraft or satellites as receivers and relays of optical energy. A growing interest in power beaming is motivated by an increasing number of satellites. Power beaming can be enabled via a process called rectification, which can be implemented by integrating a metal–insulator–metal (MIM) diode with a resonating metasurface. The design of the metasurface depends on the target frequency and materials properties. MIM diodes are comprised of layered materials; for example, our previous work used niobium as the base metal later and niobium oxide as the dielectric. A thin (100 nm) layer of ferromagnetic material (Co capped with Au) was incorporated as the top patterned layer. Electron transport between the Nb and Co layers can result from tunneling through the dielectric layer or thermionic emission. Electrical characterization was performed to assess device performance. Considering that <i>I-V</i> curve characteristics are dependent on dielectric layer thicknesses, barrier heights, and the work function difference between Nb and Co, fabrication of a high-quality device is essential to the repeatability of MIM diode performance. Functionality of the dielectric layer separating the two metals requires homogeneity with very low roughness and no pinholes. We found that replacing magnetron sputter deposition with atomic layer deposition provided more control over layer growth and improved quality. This has been evidenced by minimal electrical shorting and a linear dependance between pad surface area and current. Another consideration is the quality of the substrate. Low quality wafers contain defects and surface roughness, which can contribute to shorting between metal layers.<br/>The effect of a magnetic field on the electrical characteristics was studied by placing a small permanent magnet under the MIM diode.^[1]Magnetic characterization of 100 nm Co films on polyimide substrate indicated that saturation magnetization could be reach with a relatively low magnetic field (250 to 350 Oe). This preliminary result indicated that <i>in situ</i> electrical changes in the presence of a small permanent magnet were feasible. We anticipate that placing a small permanent magnet near the rectenna could be a low power strategy to control rectenna performance. Additional investigations are required to fully understand the mechanism behind the magnetically induced changes in electrical performance. For example, placing the diodes between the poles of an electromagnet requires a unique experimental test fixture. Wafers were coated in layers of Nb and Nb₂O₅, respectively, and then strip lines were patterned across the length of the wafer with a pad for electrical connection on one side. The strip lines can be placed between the poles to maximize the exposure to the magnetic field. This experimental configuration enables the study of changes in electrical characteristics in the presence of a range of magnetic field strengths. We anticipate that this knowledge could be used to maximize device control or reconfigurability.<br/><br/><br/>[1] Strack, G., Kim, J.H., Giardini, S. Akyurtlu, A, and Osgood, R. III Application of a magnetic field to ferromagnetic diodes. <i>MRS Advances</i> (2023). https://doi.org/10.1557/s43580-023-00520-6