Johan Piscator

Interface Defects in HfO2, LaSiOx, and Gd2O3 High-k/Metal-Gate Structures on Silicon

In this work, we present experimental results examining the energy distribution of the relatively high (> 1 X 10(11) cm(-2)) electrically active interface defects which are commonly observed in high-dielectric-constant (high-k) metal-insulator-silicon systems during high-k process development. This paper extends previous studies on the Si(100)/SiOx/HfO2 system to include a comparative analysis of the density and energy distribution of interface defects for HfO2, lanthanum silicate (LaSiOx), and Gd2O3 thin films on (100) orientation silicon formed by a range of deposition techniques. The analysis of the interface defect density across the energy gap, for samples which experience no H-2/N-2 annealing following the gate stack formation, reveals a peak density (similar to 2 X 10(12) cm(-2) eV(-1) to similar to 1 X 10(13) cm(-2) eV(-1)) at 0.83-0.92 eV above the silicon valence bandedge for the HfO2, LaSiOx, and Gd2O3 thin films on Si (100). The characteristic peak in the interface state density (0.83-0.92 eV) is obtained for samples where no interface silicon oxide layer is observed from transmission electron microscopy. Analysis suggests silicon dangling bond (P-bo) centers as the common origin for the dominant interface defects for the various Si(100)/SiOx/high-k/metal gate systems. The results of forming gas (H-2/N-2) annealing over the temperature range 350-555 degrees C are presented and indicate interface state density reduction, as expected for silicon dangling bond centers. The technological relevance of the results is discussed. (c) 2007 The Electrochemical Society.

Electron capture cross sections of InAs/GaAs quantum dots

By measuring the thermal emission rates of electrons from InAs∕GaAs quantum dots, capture cross sections in the extremely high region of 10−11–10−10cm2 have been found. These data have been confirmed by using an additional method based on a static measurement at thermal equilibrium, where the Fermi level is positioned at the free energy level of the quantum dot s shell.

Gd silicate: A High-k Dielectric Compatible with High Temperature Annealing

The authors report on the investigation of amorphous Gd-based silicates as high- k dielectrics. Two different stacks of amorphous gadolinium oxide (Gd2 O3) and silicon oxide (Si O2) on silicon substrates are compared after annealing at temperatures up to 1000 °C. Subsequently formed metal oxide semiconductor capacitors show a significant reduction in the capacitance equivalent thicknesses after annealing. Transmission electron microscopy, medium energy ion scattering, and x-ray diffraction analysis reveal distinct structural changes such as consumption of the Si O2 layer and formation of amorphous Gd silicate. The controlled formation of Gd silicates in this work indicates a route toward high- k dielectrics compatible with conventional, gate first complementary metal-oxide semiconductor integration schemes.

Vibronic nature of hafnium oxide/silicon interface states investigated by capacitance frequency spectroscopy

Using a method based on the frequency dependence of capacitance, cross sections for electron capture into energy states at the interlayer/silicon interface have been investigated for HfO2 that is deposited on silicon by reactive sputtering. We find that the capture cross sections are thermally activated and steeply increase with increasing energy depth. Both features can be attributed to the same physical origin, indicating vibronic trap properties, where the capture mechanism is governed by multiphonon processes.

Multiparameter admittance spectroscopy for metal-oxide-semiconductor systems

Admittance spectroscopy is extended for measuring capacitance and conductance on metal-oxide-semiconductor (MOS) structures as a function of gate voltage, frequency, and temperature. An automatic setup has been designed for collecting data along these dimensions in one measurement cycle. The theory for admittance spectroscopy has been developed by starting from basic charge carrier statistics. Using numerical integration of energy dependent parameters instead of the commonly used analytical solution, conductance dispersion curves are obtained which do not need to be adjusted by assuming lateral surface potential variations at the oxide-semiconductor interface. Also, we find that interface state densities extracted by using traditional methods are four times lower than those obtained by using our theory. Experimental data presented in three-dimensional plots are compared with theoretical calculations, revealing the possibilities and limitations of the conductance method.

The conductance method in a bottom-up approach applied on hafnium oxide/silicon interfaces

Starting from basic statistical properties of interface states, we demonstrate the influence of energy dependent interface state distributions and thermal emission rates including their capture cross sections on measured differential conductance data for Al/HfO2/SiOx/Si structures. Theoretical plots calculated this way reproduce experimental conductance data without correction for lateral surface potential variations. Close to the silicon conduction band edge, we find an energy dependence of the capture cross sections revealing the existence of electron states with capture processes deviating from the multiphonon mechanisms found for the deeper lying states at interfaces between high-k dielectrics and silicon.

Schottky barrier modulation on silicon nanowires

Oxide charge on the sidewalls of SiO2 embedded silicon wires with 20×20nm2 cross section is shown to influence the Schottky barrier height for Pd2Si∕Si junctions positioned on the end surfaces of the wires. Compared with results on planar silicon surfaces, the electron barrier height is 0.3eV lower for wires investigated as fabricated. By increasing the oxide charge through irradiation by ultraviolet light, the electron barrier decreases by an additional 0.15eV and the hole barrier correspondingly increases by about the same amount. The phenomenon is explained by assuming an oxide charge density in the range of 1012cm−2.

High-k-oxide/silicon interfaces characterized by capacitance frequency spectroscopy

Electron capture into insulator/silicon interface states is investigated for high-k dielectrics of Gd2O3 prepared by MBE and ALD, and for HfO2 prepared by reactive sputtering, by measuring the frequency dependence of MOS capacitance. The capture cross sections are found to be thermally activated and to increase steeply with the energy depth of the interface electron states. The methodology adopted is considered useful for increasing the understanding of high-k-oxide/silicon interfaces.

Characterization of Traps in the Transition Region at the HfO2 ∕ SiOx Interface by Thermally Stimulated Currents

Thermally stimulated currents (TSCs) have been measured to investigate electron traps in HfO2 prepared by reactive sputtering on silicon. Broken planes of the silicon crystal, which may contribute to the occurrence of interface states, were identified between the silicon and SiOx interlayer by transmission electron microscopy (TEM). A second domain was found between SiOx and HfO2 constituting a gradual transition region between the two oxides. This interface region was found to be a source of unstable charge traps where captured electrons interact with the silicon energy states through a combined tunneling and thermal process.

Schottky barriers on silicon nanowires influenced by charge configuration

Due to the geometry offered by nanowires, it is possible to introduce electric fields directed from the wire wall toward a Schottky contact positioned on the end surface of a wire. In the present work a simple model demonstrating the effect of charge on the wire walls close to the metal semiconductor interface is presented. This is also compared to measurements on fabricated nanowire devices, showing that additional positive charge close to the interface will lower the effective Schottky barrier height.