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Biography
Yannick Wimmer was born in Steyr, Austria, in 1985. He studied physics at the Technische Universität Wien, where he received the degree of Diplomingenieur in 2012. He joined the Institute for Microelectronics in August 2012, where he is currently working on his doctoral degree. His current scientific interests include channel hot-carrier effects and NBTI.
Defect Properties of the Hydrogen Bridge in Amorphous Silica
During the last years, much effort has been put into determining the defects responsible for the Negative Bias Temperature Instability (NBTI), a key reliability issue in Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). Recent findings suggest that hydrogen participates in these processes. One probable defect-candidate is the hydrogen-bridge, where one oxygen atom in the material is basically replaced by a hydrogen atom. Motivated by the promising results obtained with the hydrogen bridge in an alpha-quartz lattice, we started to investigate this defect in an amorphous environment, resembling the structure of the gate-oxide of a MOSFET in real devices.
Amorphous silicon-dioxide structures were created using a bond-switching Metropolis Monte Carlo method. The subsequent atomistic simulations were carried out using the CP2K package, a Fortran-based Density Functional Theory (DFT) code using mixed Gaussian and plane wave basis sets.
In order to check if the defect is applicable to our four-state NBTI model (see Fig. 1), all four states of the model have to be constructed using DFT methods. For the full picture the energy levels of all the states and the transition barriers between them are calculated. From this, charge capture- and emission-times can be calculated and compared to the measurement results.
In alpha-quartz, due to the periodicity and symmetry, potentials are the same for every oxygen atom, therefore, one obtains the same result when introducing the defect at any oxygen-atom position. In an amorphous structure, however, conditions are different for every oxygen atom. Since all are possible defect sites, this means that in amorphous structures every defect is different. Like in measurement data, the outcome has to be a statistical distribution of several defect properties. Comparing the statistics of the simulation results to the measurement data is an ongoing effort and gives evidence of whether or not the Hydrogen-bridge is a good candidate for an NBTI defect.
Fig. 1: Schematics of the four-state model.
Fig. 2: Hydrogen bridge and electron wave function in primary neutral configuration (state 1).
Fig. 3. Hydrogen bridge and electron wave function in secondary, positive configuration (state 2).
