Analysis of the extended defects in 3C-SiC.pdf - Nelson Mandela ...

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62 with direction. Values published by Perlado et al. (2000) range from 42 – 112 eV for Si and 26 – 58 eV for C depending on the direction. Heinisch et al. (2004), in a study on displacement damage in SiC in fission reactors, stated that the displacement value for Si could be taken as 35 eV and for C as 20 eV since these are the average values taken over all directions in the SiC the also stated that there are actually four minimum recoil damage energies that should be considered to create displacements in SiC, depending on the projectile/target combinations: 41 eV (C/Si), 35 eV (Si/Si), 24 eV (Si/C) and 20 eV (C/C). Vladimirov et al. (1998) used values of 93 eV for Si and 16.3 eV for C in their calculations as well as Ryazanov et al. (2002). Thus it is obvious that there is still a lot of uncertainty about the correct values for TDE to be used in calculations for SiC and is as mentioned before, a result of the anisotropy within the material. Hence the method of computer simulation is used to determine the displacement mechanisms and TDE in the material. Gao et al. (2001) used molecular dynamics (MD) software to simulate Si displacement cascades in SiC. Their simulation showed that during a high energy Si primary knock on atom (PKA), multiple sub-cascades were generated. This prevented both a compact structure of displaced atoms to form during the collision phase and a high-energy density during the thermal spike. They attributed this to the light masses of the SiC system and small scattering cross-sections of the Si and C atoms. Furthermore the high melting temperature of the SiC also inhibits the development of a liquid like cascade which results in a very short lifetime of the thermal spike. This along with the formation energies of defects provide a physical basis for defect production and clustering in the material. They found that at the peak of the thermal spikes the number of displaced Si and C atoms are essentially the same but that the Si sublattice partially recovers as the energy of the cascades dissipate into the lattice, while the damage in the C sublattice does not. The number of Si interstitials surviving after the cascade was on average three times smaller than for C. This is possible since most silicon atoms are only displaced a short distance from their lattice positions and recombine during the thermal spike phase. They explain that as a consequence of the above facts, the formation of a dispersed cascade is seen with defects spread over the simulation box, and most interstitials remain as single defects. Only a small fraction of interstitials form clusters, the biggest being four atoms. Furthermore Perlado et al.
63 (2000) found that subsequent annealing of their simulated cascades yielded no considerable change in the defect configuration. Thus it may be concluded that almost all interstitials are generated during the early stages of the cascade and then directly quenched into their final arrangements. When considering vacancies the same results are seen as for interstitials. Gao et al. (2001) also simulated high energy cascades in the SiC created by Au ions. They found a disordered region with a very high defect concentration but also with a large anisotropy regarding the amount of interstitials of Si and C. It clearly demonstrated that a heavy energetic ion can create a large disordered cluster in SiC.
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Analysis of the Extended Defects in
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My sincere thanks to the following
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OPSOMMING Hierdie dissertasie hande
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4.2.5. Two-Beam Scattering Matrix 4
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Dedicated to my wife and parents
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SUMMARY The dissertation focuses on
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CONTENTS Chapter 1: INTRODUCTION 1
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1 CHAPTER ONE INTRODUCTION Silicon
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2.1. Introduction 3 CHAPTER TWO OVE
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2.2.2 The Zinc-Blende Structure 5 T
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2.3 Point Defects, Defect Migration
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v m Em vm0 exp( ) (2.2) kT 9 wher
Page 25 and 26: 11 explained and the concept of par
Page 27 and 28: 13 Straight dislocations furthermor
Page 29 and 30: 2.5 Partial Dislocations and Slip 1
Page 31 and 32: 17 Fig. 2.13. The geometrical setup
Page 33 and 34: 19 extrinsic stacking faults are th
Page 35 and 36: 2.8 Twinning 21 Twinning is a proce
Page 37 and 38: 23 Fig. 2.20. The process of vacanc
Page 39 and 40: S n Td (3.2) 25 and is also referr
Page 41 and 42: 27 where a = a0(Z1 2/3 + Z2 2/3 ) -
Page 43 and 44: 29 Using this expression, curves of
Page 45 and 46: 31 As stated previously hard-sphere
Page 47 and 48: 33 where A is the atomic weight. Th
Page 49 and 50: 35 a vacancy dislocation loop if th
Page 51 and 52: 37 Fig. 4.1. (a) A wave incident on
Page 53 and 54: 39 Consider Fig. 4.2 which illustra
Page 55 and 56: 41 V0 is the mean inner potential o
Page 57 and 58: 43 beam left-hand side g' = 0 g' =
Page 59 and 60: 45 In addition another set of indep
Page 61 and 62: 47 When this is incorporated into t
Page 63 and 64: S ( s , z ) e T i Se ( / 0 )
Page 65 and 66: z0 z2 . xn cos and (4.47) D 1 proj
Page 67 and 68: 5.1 Introduction 53 CHAPTER FIVE LI
Page 69 and 70: 55 different orientations. The inte
Page 71 and 72: 57 Furthermore Pirouz et al. (1987)
Page 73 and 74: 59 intersection of the fault, the f
Page 75: 61 each nuclei because of the low s
Page 79 and 80: 6.3.1 Ion Range and Defect Producti
Page 81 and 82: 67 Fig. 6.3. Total number of displa
Page 83 and 84: 7.1 Introduction 69 CHAPTER SEVEN R
Page 85 and 86: Fig. 7.2. Simulated diffraction pat
Page 87 and 88: 73 Fig. 7.3. A HRTEM image of the S
Page 89 and 90: 75 Fig. 7.5. The stacking fault sel
Page 91 and 92: (Si/ni) 2 0.000018 0.000016 0.00001
Page 93 and 94: 79 may be concluded that the initia
Page 95 and 96: 81 Fig. 7.11. Bright field TEM micr
Page 97 and 98: 83 Fig. 7.12 which was obtained by
Page 99 and 100: 85 Fig. 7.13. Bright-field TEM micr
Page 101 and 102: 87 (a) (b) Fig. 7.15. The (a) exper
Page 103 and 104: 89 In the following paragraphs the
Page 105 and 106: Fig. 7.18. CBED pattern taken of th
Page 107 and 108: 93 Table 7.5. The measured paramete
Page 109 and 110: 7.4.2 Results 95 Fig. 7.22. Bright-
Page 111 and 112: 97 Twin platelets on {111} planes
Page 113 and 114: 99 REFERENCES Blumenau, A.T., Jones
Page 115 and 116: 101 Hull, D., Bacon, D.J. : (1984)
Page 117: 103 Smith, D.J., Jepps, N.W. and Pa
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