Dynamics at the nanoscale

However fascinating structures may be at the nanoscale, time-dependent behaviour at the nanoscale has far greater importance. Some of the dynamics is random, with fluctuations controlling rate processes and making thermal ratchets possible. Some of the dynamics causes the transfer of energy, of signals, or of charge. Such transfers are especially efficiently controlled in biological systems. Other dynamical processes occur when we wish to control the nanoscale, e.g., to avoid local failures of gate dielectrics, or to manipulate structures by electronic excitation, to use spin manipulation in quantum information processing. Our prime purpose is to make clear the enormous range and variety of time-dependent nanoscale phenomena. (C) 2006 Elsevier B.V. All rights reserved

The oxide gate dielectric: do we know all we should?

Silicon's importance as a semiconductor owes much to its oxide. This oxide passivates, enables key process steps, and has been the gate dielectric of choice for MOSFETs for many years. Experience and know-how in using this oxide makes it hard for radical alternatives to be accepted. Yet it may not be possible for silicon dioxide to meet the stringent demands of the Semiconductor Industry Roadmap as a gate dielectric. This leads to clear technological questions. Is the oxide the best that can be grown? Could a better oxide be obtained,by some modification of the growth process? What are the performance criteria that define the 'best' oxide? Are evolutionary changes, like incorporating nitrogen, to be preferred to introducing new oxides, such as those of Hf or Zr? Would there be long term problems with the new oxides? As so often in microelectronics, the technology demands new materials and new ideas from condensed matter physics. The critical role of the gate dielectric points to challenges for basic condensed matter theory, and this paper attempts to define these issues. It is certainly not sufficient to predict an equilibrium structure for oxide on silicon. The front and back regions of the oxide differ in measurable ways. Dynamical events like breakdown behaviour are certainly partly controlled by defects. Many experiments show the standard view of growth processes, 'bulk' diffusion and some interface reaction, is incomplete at best. How defects evolve as the oxide grows, and how impurities like H affect what happens could be crucial. Any analysis, even if only to create a framework of understanding, should address the various experiments exploiting different oxygen isotopes, the systematics of oxidation kinetics, and those electron microscope observations that show apparent layer-by-layer growth on terraces. The present paper aims to define an appropriate context for other detailed studies in the hope that progress in theory can contribute effectively to microelectronics futures

Atomistic modelling of radiation effects: Towards dynamics of exciton relaxation

This brief review is focused on recent results of atomistic modelling and simulation of exciton related processes in ionic materials. We present an analysis of thermal fluctuations of the electrostatic potential in cubic ionic crystals and their relation to formation of a tail in the electron density of states and localisation of electronic states. Then the possible 'fast' mechanism of formation of F-H pairs in KBr as a result of decomposition of relaxing excitons is discussed. We briefly describe some ideas related to the possibility of coherent control of exciton decomposition into Frenkel defects in alkali halides. Next we turn to the results of modelling of exciton excitation and localisation at the low-coordinated surface sites of MgO. And finally, we present the results of quantum-mechanical simulation of peroxy linkages and their reaction which creates oxygen molecules in silica. (C) 2000 Elsevier Science B.V. All rights reserved

Oxygen vacancies in cubic ZrO<inf>2</inf> nanocrystals studied by an ab initio embedded cluster method

We have developed an embedded cluster method for the calculation of the electronic structure and properties of point defects in cubic ZrO2 nanocrystallites. The accuracy of the method is tested through a detailed comparison of the atomic and electronic structures of the perfect lattice and defect properties with the results of periodic calculations. The optical absorption and magnetic properties of oxygen vacancies with charge states ranging from +2 |e| to -2 |e| are calculated. Furthermore, the method can be used to study the magnetic, optical, photoluminescence, chemical, and other properties of pure and doped ZrO2 powders and their mixtures with other materials. © 2008 The American Physical Society

Spectroscopic properties of oxygen vacancies in monoclinic Hf O2 calculated with periodic and embedded cluster density functional theory

We have calculated the electronic structure and spectroscopic properties of the oxygen vacancy in different charge states in the monoclinic phase of Hf O2. Periodic and embedded cluster calculations using density functional theory and a hybrid density functional reproduce the band gap of this material with good accuracy and predict the positions of the one-electron energy levels corresponding to five charge states of the vacancy in the band gap. The optical transition energies as well as optical and thermal ionization energies into the conduction band for all vacancy charge states and the g tensor for electron spin resonance (ESR) active states are calculated. We discuss the relation of the calculated properties to metrology of vacancies using spectroscopic ellipsometry, ESR, and electrical stress measurements. © 2007 The American Physical Society

Polaron-like charge trapping in oxygen deficient and disordered HfO <inf>2</inf>: Theoretical insight

We suggest that the polaron-like trapping of electrons is possible in the high-k dielectrics alongside with the commonly discussed Poole-Frankel resonant tunneling process or recapture of the electrons from the conduction band onto pre-existed defect states. The main feature of this mechanism is the charge self-trapping, i.e. formation of a localized defect state as a result of the electron-lattice interaction. We consider charge trapping properties of single oxygen vacancies and di-vacancies in the m-HfO2: as well as various models for oxygen deficient and stoichiometric disordered hafnia. We find evidence of polaronic trapping in all these systems, which may indicate its universality in high-k materials. copyright The Electrochemical Society

Theoretical prediction of intrinsic self-trapping of electrons and holes in monoclinic HfO2.

We predict, by means of ab initio calculations, stable electron and hole polaron states in perfect monoclinic HfO2. Hole polarons are localized on oxygen atoms in the two oxygen sublattices. An electron polaron is localized on hafnium atoms. Small barriers for polaron hopping suggest relatively high mobility of trapped charges. The one-electron energy levels in the gap, optical transition energies and ESR g-tensor components are calculated

Negative oxygen vacancies in HfO <inf>2</inf> as charge traps in high-k stacks

The optical excitation and thermal ionization energies of oxygen vacancies in m-HfO 2 are calculated using a non-local density functional theory with atomic basis sets and periodic supercell. The thermal ionization energies of negatively charged V - and V 2- centers are consistent with values obtained by the electrical measurements. The results suggest that negative oxygen vacancies are essentially polaronic in origin. They are likely candidates for intrinsic shallow electron traps in the hafnium based gate stack devices. © 2006 American Institute of Physics

Oxide muonics: II. Modelling the electrical activity of hydrogen in wide-gap and high-permittivity dielectrics

Following the prediction and confirmation that interstitial hydrogen forms shallow donors in zinc oxide, inducing electronic conductivity, the question arises as to whether it could do so in other oxides, not least in those under consideration as thin-film insulators or high-permittivity gate dielectrics. We have screened a wide selection of binary oxides for this behaviour, therefore, using muonium as an accessible experimental model for hydrogen. New examples of the shallow-donor states that are required for n-type doping are inferred from hyperfine broadening or splitting of the muon spin rotation spectra. Electron effective masses are estimated (for several materials where they are not previously reported) although polaronic rather than hydrogenic models appear in some cases to be appropriate. Deep states are characterized by hyperfine decoupling methods, with new examples found of the neutral interstitial atom even in materials where hydrogen is predicted to have negative-U character, as well as a highly anisotropic deep-donor state assigned to a muonium–vacancy complex. Comprehensive data on the thermal stability of the various neutral states are given, with effective ionization temperatures ranging from 10 K for the shallow to over 1000 K for the deep states, and corresponding activation energies between tens of meV and several eV. A striking feature of the systematics, rationalized in a new model, is the preponderance of shallow states in materials with band-gaps less below 5 eV, atomic states above 7 eV, and their coexistence in the intervening threshold range, 5–7 eV