In situ study of the atomic layer deposition of HfO2 on Si

Abstract

The authors investigated in situ the initial stages of the atomic layer deposition (ALD) growth of HfO2 on Si(001)/SiO2 substrates by using tetrakis-di-methyl-amino-Hf and H2O as precursors. The surface morphology and the chemical and electronic properties of HfO2 ultrathin films were studied after each ALD cycle by surface-sensitive techniques. Atomic force microscopy image analysis was performed by analyzing the height–height correlation function (HHCF), the root mean square surface roughness, and the surface fractal dimension, as function of the number of ALD cycles. Parameters directly related to HHCF, e.g., surface width, correlation length, local slope, and roughness exponent, were calculated and used for determination of scaling exponents. A complex behavior of all parameters up to the eighth ALD cycle was evidenced. High-resolution synchrotron radiation photoemission spectroscopy was applied to characterize the chemical nature of Si/SiO2/HfO2 interface. Changes arising in the Si 2p, O 1 s, and Hf 4f core level lines after each ALD cycle up to the complete formation of two layers of HfO2 were observed. The thickness of the growing HfO2 layer was calculated to estimate the growth per cycle to approximately 0.1 nm/cycle. By means of ultraviolet photoemission spectroscopy, variations of valence band maximum and secondary electron cutoff after each ALD cycle were observed and the presence of an interfacial dipole was pointed out. Finally, the loss function onset of electron energy loss spectroscopy changed during ALD because of bandgap variations from SiO2 to HfO2. By combining all experimental results a new and fully comprehensive growth model of ALD during the initial stages was developed.