First principles investigation of defects at interfaces between silicon and amorphous high-kappa oxides

Experimental characterization is facing severe difficulties in elucidating the nature of defects at interfaces between silicon and amorphous high-kappa oxides. This motivates recourse to first principle simulation approaches based on density functional theory. A major issue for these approaches is the choice of the model structure. indeed, the most trivial choice of parent crystalline structures may fail in describing the amorphous nature of the oxide or the specific bonding arrangements occurring at the interface. A second critical issue relates to the determination of energy levels with respect to band edges of the semiconductor and the oxide, which often represent the only contact with experimental data. An improved description of the electronic structure is required going beyond standard density functional approaches that severely underestimate electronic band gaps. Here, these issues are illustrated by a comparative study of the oxygen vacancy in monoclinic HfO2, in amorphous HfO2, and at the Si-HfO2 interface.

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.

EPFL2019

First principles vibrational spectra of tetrahedrally-bonded glasses

Luigi Giacomazzi

Disordered oxides, such as vitreous silica (v-SiO2) and vitreous germania, (v-GeO2) and chalcogenide glasses, such as vitreous germanium diselenide (v-GeSe2) are currently key materials in many technological fields, ranging from Si-based microelectronic to optical fibers and information storage devices production. Thus an increasing interest is nowadays devoted to understanding the structural arrangements of atoms in these materials. Diffraction probes like x-ray and neutron diffraction applied to amorphous materials can only partially characterize their atomic structure, since these materials lack translational symmetry. A complementary class of experimental techniques is constituted by vibrational spectroscopies, since the latter are also sensitive to the underlying structure. Yet, extracting structural information from the associated experimental data is rather difficult and above all it requires an accurate theoretical modeling of the material under investigation. This thesis is dedicated to the first-principles investigation of vibrational spectra of the tetrahedrally bonded glasses v-SiO2, v-GeO2 and v-GeSe2. To this aim, we make use of methods based on density functional theory which allow us to calculate vibrational spectra for a set of selected model structures for each one of these materials. We take advantage of a recent development which consists in applying a finite electric field under the constraint of periodic boundary conditions to calculate coupling factors, and hence to derive the infrared and Raman spectra. Through a comparison with experimental data of several vibrational spectroscopies, and by taking into account the structural differences of our models, we interpret features of the considered vibrational spectra. The extracted structural information represents a refinement of our knowledge of the investigated materials.

EPFL2007

First-principles study of defects at the SiC/SiO2 interface through hybrid functionals

Fabien Devynck

The requirements of present high-performance power electronic systems are exceeding the power density, efficiency, and reliability of silicon-based devices. Silicon carbide (SiC) is a candidate of choice for high-temperature, high-speed, high-frequency, and high-power applications: it has a wide band gap, high thermal conductivity, high saturated electron drift velocity, and high breakdown electric field. SiC is also hard, chemically stable, and resistant to radiation damage. Despite these desirable electronic properties, SiC-based devices are still facing many performance challenges due to the high density of defect states at the SiC/SiO2 interface. This thesis is dedicated to the first-principles study of defects at the SiC/SiO2 interface through hybrid functionals. We first generate a model structure providing a realistic description of the majority bonding arrangements at the 4H(0001)-SiC/SiO2 interface. The description of the electronic structure of the model through hybrid functionals leads to band offsets in excellent agreement with their experimental counterparts. We then study several semiconductor and oxide defects using spin-polarized hybrid density functionals and identify candidate defects responsible for the high density of interface defects measured in the 4H-SiC band gap. Finally, the possibility of growing epitaxial silicate adlayers on SiC is investigated as an alternative approach to greatly reduce the high density of defect states at the SiC/SiO2 interface.

EPFL2008

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

EPFL2019