Carrier conduction mechanisms in MIS capacitors with ultra-thin Al2O3 at cryogenic temperatures

In this work, a study comprising the electrical characterization and analysis of the electrical response of metal-insulator-semiconductor Al/Al2O3/Si capacitors in a temperature range from ambient temperature down to 3.6 K is presented. An ultra-thin 6 nm Al2O3 film, deposited by atomic layer deposition, was used as an insulating layer. Current-voltage and electrical stress measurements were performed on the capacitors in the specified temperature range, and the experimental data obtained were analyzed using current transport equations to model the conduction mechanisms that allow charge transport through the Al2O3. Energetic parameters associated with trap levels within the Al2O3 bandgap corresponding to the Poole-Frenkel emission and trap-assisted tunneling mechanisms were obtained, and their temperature dependances were studied and associated with the presence of physical material defects. The analysis of the modeling results points to trap-assisted tunneling as the dominant mechanism at low temperatures for intermediate electric field values. Additional phenomena that limit charge transport were also observed, such as charge trapping in the bulk of the Al2O3 upon the application of electrical stress at ambient temperature and silicon freeze out at cryogenic temperatures. Our findings constitute an effort at understanding the physical phenomena that govern the electrical behavior of thin-film Al2O3-based capacitors, especially at cryogenic temperatures, given that these materials and devices are of considerable importance for applications in CMOS-based cryoelectronics and quantum technologies, among others.