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

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58 as the propagation of a number of partials with the width of the twin determined by the number n of the partials. Such a system can be considered having Burgers vector equal to that of a superdislocation with b = na/6 121] [ _ having strength n 2a / 4 . Thus a twin formed by glide of these partials is much more efficient in mismatch accommodation than the edge misfit and shear dislocations. It is necessary to consider the lattice mismatch in the 110] also. A set of pure edge [ _ _ dislocations with b = a/2 110] at a spacing of 5d {110} orthogonal to the first array would satisfy the mismatch strain along this direction also. The strength of the edge _ _ [ _ _ component of an a/6 [ 121] partial along the 110] [ _ _ direction is 2a / 12 and thus the twin needs to be six (111) layers thick to be able to accommodate the mismatch along [ _ _ the 110] direction as efficiently as an edge misfit dislocation having b = a/2 110] . In another publication Powell et al. (1987) presented high resolution TEM results of misfit dislocations present at the SiC/Si interface. They found the dislocations occur on average every four lattice fringes. 5.3.3. Mechanisms for the Formation of Stacking Faults The mechanism of formation of stacking faults in SiC is still widely debated. It is obvious that the driving force behind the process is the low stacking fault energy in SiC but the exact process of formation of the SFs during growth seems not to be as a result of one thing but rather a combination of different processes during growth. The possible mechanisms are discussed by Powell et al. (1987) and Pirouz et al. (1987). (a) Firstly if it is assumed that the process of growth of the material occurs through the formation of discrete nuclei on the substrate surface, it can be envisioned that some of the nuclei may be deposited on incorrect sites forming surfaces of mismatch boundary. When the correctly deposited islands meet the incorrectly deposited islands an inverted pyramid of stacking fault could result growing on the inclined {111} faces with its {001} base at the interface. Using a simple geometric consideration (depicted in (e)) it can be shown that depending on the nature of the dislocations at the [ _ _
59 intersection of the fault, the faces of the pyramid would either be all of the same nature or alternatively intrinsic and extrinsic faults. It can also be shown that the measurement of the width of the fault at the surface of the foil enables one to measure the point of nucleation. (b) It is possible for interfacial stacking faults to have formed during the cooling stages due to the lattice mismatch between the substrate and film. In this case, the stacking fault would nucleate at the interface during the cooling stages and subsequently propagate into the bulk by glide of partial dislocations. However the activation energy of dislocation motion in SiC is very high and it is unlikely that the stacking fault nucleated would propagate throughout the bulk of the layer. Thus the faults are limited to a region close to the interface with the bonding partial dislocations parallel to the interface. (c) The next possibility is the spontaneous generation of dislocations at the interface as soon as the epitaxial layer begins to grow. Conditions on the interface such as contamination, roughness, moderate stresses would enhance the generation at lower temperatures of growth. According to Booker et al. (1978) the dislocations always form in pairs and once the dislocations are generated they split into two Shockley partials bounding a ribbon of stacking fault between them. (d) The fourth possibility is that dislocation loops are generated within the bulk of the material presumably during the cooling stages by the stresses caused by the thermal mismatch between the substrate and film. Following this, the glide dislocations would split up into Shockley partials bounding wide ribbons of stacking fault between them since the stacking fault energy in SiC is very low. The interaction between partials gliding on different {111} planes would give rise to intersecting faults.
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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
Page 21 and 22: 2.3 Point Defects, Defect Migration
Page 23 and 24: 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: 57 Furthermore Pirouz et al. (1987)
Page 75 and 76: 61 each nuclei because of the low s
Page 77 and 78: 63 (2000) found that subsequent ann
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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