Charge carrier traffic at interfaces in nanoeletronic structures

Doktorsavhandling, 2010

This thesis describes investigations in relation to the search for materials with high dielectric constant, k, for future CMOS transistors. The most elementary quantities to be considered are k-value and energy band offsets between the dielectric and the silicon crystal on which it is deposited. Empirical relations for these two quantities are presented demonstrating that only a few dielectrics investigated up to now have properties providing the basic demands in the development of CMOS technology. Process development was done to deposit HfO2, Pr2O3 and HfPrO on silicon by reactive sputtering in order to fabricate MOS capacitors. Electrical properties of these oxides were investigated by employing different techniques such as capacitance-voltage (C-V), current-voltage (I-V), capacitance frequency spectroscopy, stepped C-V, multiparameter admittance spectroscopy (MPAS) and thermally stimulated current (TSC). A capacitance frequency spectroscopy technique was developed to investigate electron capture cross sections of the interface states at high-k/Si interface from experimental results. It is found that capture cross sections of electron states at this interface are thermally activated and exponentially depend on energy depth of interface states in the silicon bandgap. These processes indicate that the capture mechanism is governed by multiphonon. MPAS a diagnostic tool developed from the conductance method to deliver more information regarding charge carrier states in semiconductor structures. Using this technique on HfO2/Si interface, two different types of interface states with different capture mechanisms were found. An interlayer of SiOx is found between the silicon crystal and HfO2 in TEM pictures. A transition region, which is expected to have strong concentration gradients, exists between SiOx and HfO2. This region shows an unstable atomic arrangement and contains charge carrier traps. These traps exchange electrons with the conduction band of the silicon crystal through a combined thermal-tunneling mechanism.