A comparative study of charge trapping in HfO2/Al2O3 and ZrO2/Al2O3 based multilayered metal/high-k/oxide/Si structures

• Sputtered HfO2/Al2O3/HfO2 and ZrO2/Al2O3/ZrO2 charge-trapping layers were studied.
• HfO2/Al2O3/HfO2 stacks show memory window up to 6 V and good retention times.
• Negatively charged oxygen vacancies were identified as main defects in the stacks.
• Electrical breakdown compromise the charge-trapping properties of ZrO2-based films.

The electrical properties of multilayered HfO2/Al2O3/HfO2/SiO2 and ZrO2/Al2O3/ZrO2/SiO2 metal-oxide semiconductor capacitors were investigated in order to evaluate the possibility of their application in charge-trapping non-volatile memory devices. The stacks were deposited by reactive radiofrequency magnetron sputtering on Si substrates with thermal SiO2 with a thickness ranging from 2 to 5 nm. Both types of stacks show negative initial oxide charge and its density is higher for HfO2-based structures. Memory window up to 6V at sweeping voltage range of ± 16V was obtained for HfO2-based stacks. The hysteresis in these structures is mainly due to a trapping of electrons injected from the Si substrate. The charge-trapping properties of ZrO2-based samples are compromised by the high leakage currents and the dielectric breakdown. The conduction through the capacitors at low applied voltages results from hopping of thermally excited electrons from one isolated state to another. The energy depth of the traps participating in the hopping conduction was determined as ~ 0.7 eV for the HfO2-based layers and ~ 0.6 eV for ZrO2-based ones, originating from negatively charged oxygen vacancies. At high electric fields, the current voltage characteristics were interpreted in terms of space charge limited currents, Fowler–Nordheim tunneling, Schottky emission, and Poole–Frenkel mechanism. The charge retention characteristics do not depend on the thickness of the tunnel SiO2.