Redox-based memristive devices (ReRAM) are considered for future high-density non-volatile data storage devices owing to superior characteristics in speed, power and scalability.
Due to several benefits such as perfect thickness control, conformal 3D-growth and low-temperature deposition atomic layer deposition (ALD) is perfectly suited for the resistive switching oxide layer (MOx) growth in future vertical three dimension (3D) ReRAM concepts. However, the lack of ALD processes for reactive metals causes the necessity to grow the ALD MOx film on the chemically active non-noble metal electrode. Unfortunately, the near surface oxidation of transition metals during ALD oxide layer growth has been rarely investigated so far.
In this study, dense Hf metal layers of about 25 nm thickness deposited by off-axis sputtering were transferred through an UHV tunnel into a high vacuum ALD system in order to prevent any contamination of the metallic surface before the ALD metal oxide growth. Plasma assisted or thermal ALD processes were used to grow ultrathin layers of Al2O3 and TiO2 on top of the Hf metal.
Angle dependent X-ray photoelectron spectroscopy with different take-off angles was utilized to determine the thickness of the interfacial hafnium oxide layer formed as well as to analyse the valence states of the involved Hf metal cations. A direct comparison between TiO2 and Al2O3 capping layers clearly verified that Al2O3 acts effectively as a diffusion barrier resulting in a thinner HfO2 layer saturating at about 4 nm compared to the TiO2 deposition. For the plasma assisted Al2O3 growth on Hf at 250°C transmission electron microscopic analysis reveals a polycrystalline structure of the HfO2 layer formed with a sharp transition towards the Hf metal.