Impact of Charge Transitions at Atomic Defect Sites on Electronic Device Performance

Chapter 7 Summary, Conclusions and Outlook

7.1 Summary

This thesis presents first-principles investigations of atomic charge trapping sites in various materials and links them to the functionality and performance of electronic devices. The localization of electrons and holes at different sites in the atomic network in various materials was analyzed using quantum-mechanical methods, in particular density functional theory (DFT) combined with a hybrid functional. The investigated charge trapping sites include:

• Oxygen vacancies and hydrogen-related defects in amorphous silicon dioxide ( $a$ -SiO $_{2}$ )
• Tungsten vacancy and selenium antisite in two-dimensional monolayer tungsten diselenide (2D 1L-WSe $_{2}$ )
• Over- and undercoordinated atoms in amorphous silicon nitride ( $a$ -Si $_{3}$ N $_{4}$ )
• Polaron formation at fully coordinated sites in hydrogenated amorphous silicon nitride ( $a$ -Si $_{3}$ N $_{4}$ :H)
• Oxygen vacancy, aluminum vacancy and aluminum split vacancy in crystalline aluminum oxide ( $α$ -Al $_{2}$ O $_{3}$ )

The charge transition levels (CTLs) and structural relaxations upon charge trapping were calculated to model the corresponding potential energy surface (PES) of the defects in different charge states. Due to the strong electron-phonon coupling involved in the investigated charge transitions, the relaxation process could be reduced to an effective phonon mode. This allowed for the efficient evaluation of optical and non-radiative charge transfer mechanisms.

The charge trapping sites in amorphous structures were statistically investigated to account for inherent structural disorder. The amorphous sample structures for these studies were created by simulating a melt-and-quench procedure with molecular dynamics (MD). To accurately model experimental observations in amorphous silicon nitride thin films, a machine learning interatomic potential (MLIP) was developed specifically for this task with the Gaussian approximation potential (GAP) method, which was trained on data from DFT calculations. Non-radiative charge capture and emission events at defect sites were analyzed in the context of charge exchange with the electronic band edges of the respective substrate. This includes investigations of the trapping probability in Si/SiO $_{2}$ and SiC/SiO $_{2}$ systems as employed in metal-oxide-semiconductor field-effect transistors (MOSFETs) as well as Si/Si $_{3}$ N $_{4}$ :H systems, which are relevant for charge trap flash (CTF) devices. The stability of different atomic configurations of the same defect was studied by evaluating their formation energies and thermal transition barriers between two configurations. Furthermore, the vibrational broadening of optical transitions at vacancies in corundum was calculated to guide the identification of luminescence and absorption spectra with a distinct defect type. Defects in 2D monolayer WSe $_{2}$ were analyzed in terms of their hole trapping properties to explain random telegraph noise (RTN) signals in ultra-scaled FETs. The conclusions drawn for electronic devices from these evaluations were published in [4, 45, 208, 237, 54, 53, 55] and are outlined in the following.