The electrons' journey in thick metal oxides

Originally introduced in electronic manufacturing to replace the SiO2 insulating layer, metal oxides are now extensively used in a multitude of electronic devices. Understanding charge transport mechanisms in metal oxides is of paramount importance for device optimization; however, a detailed and self-consistent discussion of electron conduction at all applied electric fields is lacking in the literature. In this work, we investigated the conduction mechanisms in three model systems, Al2O3, HfO2, and Al-doped HfO2 metal-insulator-metal capacitors, determining the path that the electrons travel within the metal oxide. Traps properties are extracted from experimental current-voltage characteristics using the Ginestra® simulation software. Furthermore, the analysis allowed to visualize the location of traps most involved in the conduction and the dominant transport mechanisms at each applied electric field. Despite the different oxide properties, a similar trend was recognized at low electric fields, the electron transport through the oxide is negligible, and the dominant contribution to the measured current is ascribed to the charge/discharge of traps located near the metal/oxide interfaces, leading to displacement currents. At high electric fields, the transport of electrons occurs through the defect rich oxides in the two following ways: if a large density of traps is energetically located near the electrodes Fermi level (as in HfO2), the electrons tunnel from trap to trap until they reach the anode; otherwise, when traps are closer to the conduction band (as in Al2O3 and AlHfO), the electrons tunnel from the cathode into one trap and then into the oxide conduction band, interacting only with traps near the cathode.