Energetics of neutral interstitials in germanium
type: CreativeWork
description: Significant obstacles remain for the fabrication of germanium n-MOSFETs, including enhanced diffusion of donor atoms which leads to electrical deactivation. The preferential generation of self-interstitials via proton irradiation reduces donor diffusion and offers a promising route for effective fabrication of n-type devices. Fairly little effort has been devoted to the theoretical study of germanium interstitials. In this work, neutral self-interstitial formation energies are calculated with the accurate Diffusion Monte Carlo (DMC) method. Taking care to minimize errors, calculations are performed for 216 atom systems. Formation energies for hexagonal, tetragonal, and split/dumbbell structures are found to be 5.3 eV, 6.1 eV, and 4.7 eV, respectively; a significant increase over predictions from Density Functional Theory (DFT). This indicates a large error on the part of DFT, DMC, or both. If the DMC energies are biased, the most likely source is the fixed node error. Further calculations should be performed with better wavefunctions to resolve this concern, perhaps using fast multideterminant expansions beginning to be applied to condensed systems. A novel form of the quantum energy density is also introduced to exclude statistical noise from irrelevant portions of defect systems to improve DMC efficiency. The best results for the hexagonal structure achieve a 3x speedup over standard total energy calculations. No speedup is obtained for the tetragonal and split interstitial systems, perhaps due to the increased pressure in the fixed volume cells or the presence of charge-energy fluctuations across the defect/bulk boundary. Applications to even larger systems or further refinements to the method may be required to routinely increase efficiency for DMC formation energy calculations. Also included in this work are two sub-studies in addition to the major effort regarding germanium. The first involves the calculation of the vibrational excitation spectrum of bcc heliu…
abstract: Significant obstacles remain for the fabrication of germanium n-MOSFETs, including enhanced diffusion of donor atoms which leads to electrical deactivation. The preferential generation of self-interstitials via proton irradiation reduces donor diffusion and offers a promising route for effective fabrication of n-type devices. Fairly little effort has been devoted to the theoretical study of germanium interstitials. In this work, neutral self-interstitial formation energies are calculated with the accurate Diffusion Monte Carlo (DMC) method. Taking care to minimize errors, calculations are performed for 216 atom systems. Formation energies for hexagonal, tetragonal, and split/dumbbell structures are found to be 5.3 eV, 6.1 eV, and 4.7 eV, respectively; a significant increase over predictions from Density Functional Theory (DFT). This indicates a large error on the part of DFT, DMC, or both. If the DMC energies are biased, the most likely source is the fixed node error. Further calculations should be performed with better wavefunctions to resolve this concern, perhaps using fast multideterminant expansions beginning to be applied to condensed systems. A novel form of the quantum energy density is also introduced to exclude statistical noise from irrelevant portions of defect systems to improve DMC efficiency. The best results for the hexagonal structure achieve a 3x speedup over standard total energy calculations. No speedup is obtained for the tetragonal and split interstitial systems, perhaps due to the increased pressure in the fixed volume cells or the presence of charge-energy fluctuations across the defect/bulk boundary. Applications to even larger systems or further refinements to the method may be required to routinely increase efficiency for DMC formation energy calculations. Also included in this work are two sub-studies in addition to the major effort regarding germanium. The first involves the calculation of the vibrational excitation spectrum of bcc heliu…
author: Krogel, Jaron
language: en
keywords: quantum monte carlo, diffusion monte carlo, correlation function monte carlo, germanium, interstitial, defect, formation energy, energy density, helium, population control bias
date published: 2013-05-24T22:15:23Z
date modified: 2026-07-22T22:25:34Z
provider: University of Illinois - Urbana-Champaign
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Illinois at Urbana-Champaign

PropertyValue
property ID: Handle
value: http://hdl.handle.net/2142/44417