(Invited) Photoemission-Based Characterization of Gate Dielectrics and Stack Interfaces
Authors: Miyazaki Seiichi , Ohta Akio 
Published: February 2020

In the development of Metal-Insulator-Semiconductor (MIS) devices, gate dielectric technologies are of great importance and include major scientific and technological issues to be solved for required device performance and reliability. To gain a better understanding of physics of gate dielectric stacks and insights into interface control, characterization of electronic structures of gate dielectrics and quantification of electronic defects in dielectric stacks are imperative.
In this work, we have demonstrated how energy dependent dielectric responses of gate dielectrics are evaluated by photoelectron energy loss spectroscopy (PEELS) [1, 2] and how energy distributions of defect states in wide gap materials and their interfaces are quantified by total photoelectron electron yield spectroscopy (PYS) [3-5].

Determination of Complex Dielectric Function

The analyses of core-line photoemission spectra have been extensively conducted to characterize chemical bonding features in materials and heterointerfaces of interest. On the other hand, electron energy loss spectroscopy is an effectively used method for study the excitation spectra of atoms, molecules, and solids. Thus, we have measured and analyzed energy loss signals (ELS) incidental to core-line spectra not only to determine energy bandgap values of dielectrics but also to characterize dielectric functions of dielectrics since, inherently, photoexcited electrons passing through dielectrics can suffer inelastic losses due to electronic excitations such as plasmons and band-to-band transitions.
As the first step, we examined a complex dielectric function of 50nm-thick SiO2 grown at 1000 ºC in dry O2 from the ELS analyses of both Si2p3/2 and O1s lines. In each analysis, based on photoelectron take-off angle dependence of ELS, the pure bulk component being proportional to the negative imaginary part of inverse dielectric function was extracted carefully, and then the real part of inverse dielectric function can be calculated by Kramers-Kronig transformation of the extracted bulk component. Almost the same results were obtained from the ELS analyses of both Si2p3/2 and O1s photoelectrons, and are very consistent with the dielectric function calculated from reported optical constants of SiO2 glass [6]. Notice that obtained dielectric function enables us to confirm the validity of the energy band gap value determined simply from the onset energy of ELS. We have extended this PEELS to characterization of dielectric functions of CVD SiO2 formed by reaction between SiH4 and remote O2 plasma, Hf- and Al-silicates prepared by plasma-assisted ALD, and confirmed that spectral broadening and tailing become significant with an increase in structural fluctuation and incorporation of Hf and Al atoms.

Quantification of Energy Distribution of Defect States

Since photoelectron yield is obtained by counting the total number of photoelectrons emitting from the sample as a function of incident photon energy and normalized by the incident photon flux, the spectrum is related to an integral over the occupied density of states involving photoemission near the sample surface. Changes in photoelectron yield spectrum with wet-chemical thinning of 5.2nm SiO2 formed 500ºC by remote plasma enhanced CVD on n-type GaN(0001) show presence of occupied defects in SiO2, near the interface and in GaN but a relatively large number of defects are located within ~1.5nm from the SiO2/GaN interface. From the 1st derivative of the measured yield spectrum with respect to incident photon energy which leads us to the energy distribution of occupied defect state densities in consideration of density of states of the GaN valence band (VB), we found that occupied states are reduced down to ~1x1011cm-2eV-1 at the energy corresponding to the midgap of GaN near the SiO2/GaN interface with N2 anneal at 800ºC. The defect state density near the conduction band edge, which was crudely estimated in consideration of electron occupation probability based on the Fermi-Dirac distribution, are in good agreements with the result obtained from the capacitance-voltage (C-V) analysis using the Terman method. We also found that, for Hf-silicate with a compositional ratio of Hf/(Si+Hf) = ~75% and a relative dielectric constant of ~16 formed on GaN by plasma-assisted ALD and followed by 800ºC anneal in N2 ambience, a similar energy distribution of defect state density to the 800ºC-annealed SiO2/GaN was obtained in the energy range from ~1eV above the GaN valence band edge toward the conduction band edge.
References
[1] T. Yamamoto, et al., ECS Trans. 75(8) (2016) 777.
[2] S. Miyazaki, et al., ECS Trans. 79(1) (2017) 119
[3] A. Ohta, et al., Microelectro. Eng. 178 (2017) 85.
[4] A. Ohta, et al., Jpn. J. Appl. Phys. 57 (2018) 06KA08.
[5] N. X. Truyen et al., Jpn. J. Appl. Phys. 57 (2018) 01AD02.
[6] Handbook of Optical Constants of Solids I, Ed. E. D. Palik (Academic Press, Orand, 1985).