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Title: Challenges in Nanoelectronics - Gate Dielectrics and Device Modeling (invited)

Abstract

Microelectronics has been tracking Moore's law for several decades with the same choices of materials: Silicon has been the semiconductor of choice; its native oxide, SiO{sub 2} has been used both as a gate dielectric (where a high dielectric constant is in principle desirable) and as an insulator for device isolation and interconnects (where low dielectric constant is in principle desirable); aluminum has been the metal of choice for interconnects. For device modeling, standard approximations to the Boltzmann equation, calculations of mobilities, and Fowler-Nordheim tunneling, have been very adequate. As feature dimensions are entering the 'nano' regime, however, revolutionary changes are becoming inevitable. Copper has already replaced aluminum as the metal of choice for interconnects. Silicon itself is facing changes, such as the adoption of strained layers for mobility enhancement, and is facing challenges from germanium as an alternative. The biggest change, however, is in the dominance of SiO{sub 2} as the gate dielectric. Despite decades of efforts, the Si-SiO{sub 2} system is the only one that works for a Metal-Oxide-Semiconductor field effect transistor. The key reason is that no other semiconductor has a stable native oxide that can serve as gate dielectric. Deposition of other insulators on semiconductors doesmore » not yield interfaces with low enough defect densities suitable for devices. Nevertheless, SiO{sub 2} has reached the end of its reign because scaling laws have pushed the gate-oxide thickness to about 1 nm, where tunneling currents become intolerable. Efforts to develop crystalline dielectrics deposited on Si have not been very promising. The main problem is the band offsets that one gets from materials that have a reasonable lattice match to Si. The most promising candidates are oxides that are 'piggy backed' on a very thin SiO{sub 2} layer. This talk covers new unpusblished results obtained by atomic-resolution Z-contrast microscopy on the Si-SiO{sub 2}-HfO{sub 2} system and first-principles theory. Z-contrast images show a very sharp Si-SiO{sub 2} interface, a very thin {approx}0.5 nm amorphous SiO{sub 2} layer, followed by HfO{sub 2}; they also show individual Hf atoms scattered in the SiO{sub 2} layer, but never getting closer than about 0.3 nm to the interface. First-principles calculations also find that approaching the interface closer than 0.3 nm is not energetically favored. The isolated Hf atoms have energy levels in the gap region and may contribute both to mobility degradation and to leakage currents. The new era of nanoelectronics also has raised new challenges in device modeling. Mobilities in MOSFETs fabricated with strained Si layers and in double-gated MOSFETs with ultrathin channels fabricated using SOI technology do not obey the 'universal mobility curve' that has served the industry very well for a long time. Attempts to model mobilities in these structures have run into difficulties because the standard approximations fail (effective approximation, infinite potential barrier at the Si-SiO{sub 2} interface, phenomenological interfaces roughness). This talk covers a new formulation for the calculation of mobilities using a first-principles approach. An atomic-scale model of an abrupt 'ideal' interface is used, the self-consistent Hamiltonian is calculated, and then the calculation is repeated for an interface containing elementary deviations from abruptness (Si-Si bonds on the oxide side, Si-O-Si protrusions in the Si side, single impurity, etc.). The difference represents a scattering potential that is used to calculate mobilities. The wave functions of Si electrons naturally penetrate into the oxide. Initial unpublished results are very illuminating and promising. Finally, modeling Fowler-Nordheim tunneling currents using the classical triangular potential barrier, even with modifications, is running into difficulties. Failure of the effective mass approximation is again the main culprit. This talk covers unpublished results of calculations of Fowler-Nordheim currents using atomic-scale models of the interface and the Lippmann-Schwinger method, recently demonstrated in calculations of current-voltage characteristics of single molecules. The results elucidate the role of atomic-scale detail in determining the Fowler-Nordheim tunneling current.« less

Authors:
 [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1003381
DOE Contract Number:  
DE-AC05-00OR22725
Resource Type:
Journal Article
Journal Name:
Journal of Physics Conference Series
Additional Journal Information:
Journal Volume: 10; Journal Issue: 1; Journal ID: ISSN 1742--6588
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALUMINIUM; BOLTZMANN EQUATION; DIELECTRIC MATERIALS; EFFECTIVE MASS; ENERGY LEVELS; FIELD EFFECT TRANSISTORS; GERMANIUM; HAMILTONIANS; LEAKAGE CURRENT; MICROELECTRONICS; MICROSCOPY; OXIDES; SCALING LAWS; SILICON; TUNNELING; WAVE FUNCTIONS

Citation Formats

Pantelides, Sokrates T. Challenges in Nanoelectronics - Gate Dielectrics and Device Modeling (invited). United States: N. p., 2005. Web. doi:10.1088/1742-6596/10/1/011.
Pantelides, Sokrates T. Challenges in Nanoelectronics - Gate Dielectrics and Device Modeling (invited). United States. doi:10.1088/1742-6596/10/1/011.
Pantelides, Sokrates T. Sat . "Challenges in Nanoelectronics - Gate Dielectrics and Device Modeling (invited)". United States. doi:10.1088/1742-6596/10/1/011.
@article{osti_1003381,
title = {Challenges in Nanoelectronics - Gate Dielectrics and Device Modeling (invited)},
author = {Pantelides, Sokrates T},
abstractNote = {Microelectronics has been tracking Moore's law for several decades with the same choices of materials: Silicon has been the semiconductor of choice; its native oxide, SiO{sub 2} has been used both as a gate dielectric (where a high dielectric constant is in principle desirable) and as an insulator for device isolation and interconnects (where low dielectric constant is in principle desirable); aluminum has been the metal of choice for interconnects. For device modeling, standard approximations to the Boltzmann equation, calculations of mobilities, and Fowler-Nordheim tunneling, have been very adequate. As feature dimensions are entering the 'nano' regime, however, revolutionary changes are becoming inevitable. Copper has already replaced aluminum as the metal of choice for interconnects. Silicon itself is facing changes, such as the adoption of strained layers for mobility enhancement, and is facing challenges from germanium as an alternative. The biggest change, however, is in the dominance of SiO{sub 2} as the gate dielectric. Despite decades of efforts, the Si-SiO{sub 2} system is the only one that works for a Metal-Oxide-Semiconductor field effect transistor. The key reason is that no other semiconductor has a stable native oxide that can serve as gate dielectric. Deposition of other insulators on semiconductors does not yield interfaces with low enough defect densities suitable for devices. Nevertheless, SiO{sub 2} has reached the end of its reign because scaling laws have pushed the gate-oxide thickness to about 1 nm, where tunneling currents become intolerable. Efforts to develop crystalline dielectrics deposited on Si have not been very promising. The main problem is the band offsets that one gets from materials that have a reasonable lattice match to Si. The most promising candidates are oxides that are 'piggy backed' on a very thin SiO{sub 2} layer. This talk covers new unpusblished results obtained by atomic-resolution Z-contrast microscopy on the Si-SiO{sub 2}-HfO{sub 2} system and first-principles theory. Z-contrast images show a very sharp Si-SiO{sub 2} interface, a very thin {approx}0.5 nm amorphous SiO{sub 2} layer, followed by HfO{sub 2}; they also show individual Hf atoms scattered in the SiO{sub 2} layer, but never getting closer than about 0.3 nm to the interface. First-principles calculations also find that approaching the interface closer than 0.3 nm is not energetically favored. The isolated Hf atoms have energy levels in the gap region and may contribute both to mobility degradation and to leakage currents. The new era of nanoelectronics also has raised new challenges in device modeling. Mobilities in MOSFETs fabricated with strained Si layers and in double-gated MOSFETs with ultrathin channels fabricated using SOI technology do not obey the 'universal mobility curve' that has served the industry very well for a long time. Attempts to model mobilities in these structures have run into difficulties because the standard approximations fail (effective approximation, infinite potential barrier at the Si-SiO{sub 2} interface, phenomenological interfaces roughness). This talk covers a new formulation for the calculation of mobilities using a first-principles approach. An atomic-scale model of an abrupt 'ideal' interface is used, the self-consistent Hamiltonian is calculated, and then the calculation is repeated for an interface containing elementary deviations from abruptness (Si-Si bonds on the oxide side, Si-O-Si protrusions in the Si side, single impurity, etc.). The difference represents a scattering potential that is used to calculate mobilities. The wave functions of Si electrons naturally penetrate into the oxide. Initial unpublished results are very illuminating and promising. Finally, modeling Fowler-Nordheim tunneling currents using the classical triangular potential barrier, even with modifications, is running into difficulties. Failure of the effective mass approximation is again the main culprit. This talk covers unpublished results of calculations of Fowler-Nordheim currents using atomic-scale models of the interface and the Lippmann-Schwinger method, recently demonstrated in calculations of current-voltage characteristics of single molecules. The results elucidate the role of atomic-scale detail in determining the Fowler-Nordheim tunneling current.},
doi = {10.1088/1742-6596/10/1/011},
journal = {Journal of Physics Conference Series},
issn = {1742--6588},
number = 1,
volume = 10,
place = {United States},
year = {2005},
month = {1}
}