A Fourfold Coordinated Point Defect in Silicon

Abstract

Due to their technological importance, point defects in silicon are among the best studied physical systems. The experimental examination of point defects buried in bulk is difficult and evidence for the various defects usually indirect. Simulations of defects in silicon have been performed at various levels of sophistication ranging from fast force fields to accurate density functional calculations. The generally accepted viewpoint from all these studies is that vacancies and self interstitials are the basic point defects in silicon. We challenge this point of view by presenting density functional calculations that show that there is a new fourfold coordinated point defect in silicon that is lower in energy.PACS number: 61.72.JiThe stability of crystalline silicon comes from the fact that each silicon atom can accommodate its four valence electrons in four covalent bonds with its four neighbors. The traditional point defects in silicon, the vacancy and the various interstitials, are obtained by taking out or adding atoms to the crystal and thus all destroy fourfold coordination. A relatively high defect formation energy for these defects is the consequence. In addition there is a point defect, that conserves the number of particles, the Frenkel pair, consisting of a vacancy and an interstitial. If the vacancy and the interstitial are close, the formation energy of a Frenkel pair is less than the sum of an isolated vacancy and interstitial (Table I). Nevertheless the formation energy is still considerable since again bonds are broken. Figure 1 shows a novel defect configuration that has, in contrast to all other point defects, perfect fourfold coordination and will therefore be called Four Fold Coordinated Defect (FFCD).Using state of the art plane wave density functional theory (DFT) calculations we will now present evidence that the formation energy of the FFCD of 2.4 eV is lower than the formation energy of all other known point defects both in intrinsic and doped silicon. Even though several calculations of this type were published for the traditional defects, we have decided to repeat them for several reasons. a) All these calculations take advantage of error cancellations to obtain energy differences that are more precise than the total energies themselves. This cancellation is obviously best if all the total energies that are compared are calculated with exactly the same method. b) Most DFT calculations used the basic Local Density Approximation (LDA) whereas we used a more precise General Gradient Approximations [1]. c) As was recently shown [2] cells of at least 216 atoms are required for a reasonable convergence. Most published calculations were done with smaller cells. d) We constructed very accurate pseudopotentials [3] based on atomic calculations with the same density functional as used in our target calculation and we afforded a very large plane wave basis set (25 Ry for the LDA and 35 Ry for the GGA calculations). e) The point defect formation energies in doped silicon were up to now obtained in ...

Sampling the diffusion paths of a neutral vacancy in silicon with quantum mechanical calculations

“…These spectator events all correspond basically to the bond-switching mechanisms proposed many years ago by Wooten, Winer and Weaire as mechanism for amorphization 58 . In recent years, the WWW mechanism has also been identified as the bond defect 59 and the four fold coordinated defect (FFCD) 16 .…”

“…For example, in silicon, antimony impurities are vacancy diffusers and phosphorus are interstitial diffusers 1 . Therefore, self-diffusion in Si has been extensively investigated by experiments 1,2,3,4,5 and simulations 6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33 .…”

The role of primary point defects in the degradation of silicon detectors due to hadron and lepton irradiation

“…In the following two tables we present a review of the present knowledge on energy levels in the band gap for isolated vacancies and interstitials, as experimental data and model calculation together with the corresponding charge states. Recently, using state of the art plane wave density functional theory, Goedecker and co-workers [7] predicted the existence of a new type of primary defect: Si FFCD (Fourfolded Coordinated silicon Defect) that is a pseudo-vacancy. It is obtained by moving atoms from the initial positions, but this displacement does not break the bonds with the neighbours.…”

“…Goedecker and co-workers [7] predicted theoretically the existence of this defect, with energy level(s) in the band gap. The energy level in the band gap and cross-section for minority carrier capture are indirectly determined in the present work, imposing the condition that the results of the model must reproduce available experimental data obtained after lepton and hadron irradiation.…”

Atomistic modeling of ion implantation technologies in silicon

“…This defect has been characterized using ab-initio, TB and classical MD techniques [9,10,12,13]. The bond defect formation energy is of the order of the corresponding to typical point defects in Si.…”