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Reference: B. K. Tanner, (1971). X-ray diffraction topography : methods and applications. DPhil. University of Oxford.Citable link to this page:

 

X-ray diffraction topography : methods and applications

Abstract: This thesis describes the application of the wellestablished technique of X-ray diffraction topography toa variety of problems, and includes considerations of theoptimum conditions for taking rapid topographs.Chapter I contains a brief review of the subject togetherwith an indication of the range of applicability. Severalmodifications of X-ray topography exist and several arebriefly described to illustrate the principles and mechanismsof image formation. Contrast is formed in one or both oftwo ways. Regions of crystal may be so badly misorientedfrom the bulk, that no beams from the source can satisfy thereflection condition formulated by Bragg. This type ofcontrast is known as orientation contrast. The second typeof contrast arises from the point to point lattice displacementsgiving a different reflecting power around the defect.This second type of contrast, termed extinction contrast,is dependent on the perfect crystal diffraction. A descriptionof the different types of extinction contrast is given in termsof the dynamical theory of diffraction in a perfect crystaland the significance of the direct, intermediate, anddynamical images stressed. The chapter concludes by sketchingsome of the important fields of application.The second chapter is concerned with the relative meritsof scanning and wide beam X-ray topography. A descriptionof the technique of wide beam topography using the Kandbeta; lineis given and it is demonstrated that the resolution is asgood as that using Lang's scanning technique. A new widebeam method using the tungsten Landalpha;1 line is proposed anddemonstrated. From expressions derived for the exposuretimes, it is seen that the important parameter to maximiseis the power per unit horizontal length of the X-ray source.The speeds of the two techniques are compared for a varietyof generators and the advantage of using an Elliott GX6 ismade clear.A direct imaging system is clearly preferable in principleto film recording and Chapter III is devoted to a discussionof such systems. Following a review of previously developedimaging systems, a new method of directly displaying X-rayimages using a channel plate is demonstrated. The remainderof the chapter gives detailed calculations of the efficiencyof a channel plate to X-rays in the wavelength region 0.5andndash;2andnbsp;andAring;.Using published data, the efficiency is calculatedfor several cases and compared with previous experimental work.To better than an order of magnitude, these efficiencies maybe used to predict the intensity of the image produced onthe image converter screen.Chapter IV falls into two sections. The first describesan experiment to determine the minimum thickness at whichdislocations are visible in crystals using normal topographictechniques. It was found that the value was dependent onthe fraction of the material thickness taken up by misorientedmaterial. This explanation was found to be inqualitative agreement with experiment, the minimum thicknessfell from O.4 of an extinction distance for the low orderreflections to 0.2 of an extinction distance for high orderreflections. The second part consists of a comparisonbetween experimental image profiles and those simulated ona computer. Quite reasonable agreement was found and thesources of error in the method of computing and ways to avoidthem are discussed in terms of practical and basic limitations.The remainder of this thesis describes application ofX-ray topography to four different types of material. ChapterV is devoted to a study of defects in a silicon slicefollowing device fabrication. Interest in the effect ofcrystallographic defects on the performance of integratedcircuit devices has led to a large number of investigationsover the last decade and these are reviewed and their conclusionssummarised. An attempt is made to measure the stressat the junction edge, produced by the mismatch in the ionicradii between dopant and matrix. As a result of approximationsin the theory and measurements, the force per unit length ofjunction, measured to be 2 andtimes; 104 dynes/cm, must be taken asorder of magnitude only. Similarly, only an order of magnitudeestimate of the stress is permissible. There follows adescription of a combined Scanning Electron Microscope andX-ray topographic experiment to determine the cause ofbreakdown fingers, sometimes seen at the junction edges inthe beam induced conductivity mode in the S.E.M. It isconcluded that they are due to scratches in the oxide maskprior to diffusion. The rest of the chapter is given to adetailed description of dislocations and their contrastobserved in a silicon slice heavily doped with boron. Theslip behaviour is not analysed in detail, but seems to supportthe conclusion that this deformation, due to thermal shock,is independent of the doping. Interactions between 'emitteredge' dislocations are described and the anomalously narrowimage widths of such dislocations explained by consideringthe effects of overlapping strain fields. Burgers vectors ofdislocations inside the diffused regions are determined,and the contrast of these dislocations at high density isinterpreted as an effect due to overlapping images. Interactionsbetween the 'inside' and 'emitter edge' dislocations aredescribed. In regions where the dislocation density was low,the dislocations exhibited a reversal of contrast on reversalof the diffraction vector. With the diffraction vectorparallel to the dislocation line, the contrast was black-white,reversing with the diffraction vector. With the diffractionvector at 60° to the line, the contrast was either blackor white, reversing with the diffraction vector. This effectis interpreted in terms of surface relaxation and computationsperformed using the Penning-Polder theory are in qualitativeagreement with the experimental results.Chapter VI describes the application of X-ray topographyto the study of defects in natural fluorite. Dislocationsare identified with Burgers vectors parallel to andlang;101andrang;directions. These are nearly pure edge in character and theBurgers vector is presumed to be 1andfrasl;2 andlang;101andrang;. A direct correspondenceis observed between bundles of dislocations andregions of birefringence contrast. A uniform birefringencecontrast in the matrix, which can not be explained in termsof dislocation stress, is also observed. This is explainedin terms of impurity atoms, sited between {111} planesduring growth, distorting the crystal normal to the {111}faces. The boundaries between regions of birefringence liealong inclined andlang;110andrang; directions and on this model, no strainis associated with these boundaries, in agreement with theX-ray topographic evidence. Extensive planar faults, lyingon {111} planes and with fault vector a non integral valueof the lattice spacing, are postulated to be thin lamellae ofmaterial containing markedly different impurity concentration.In Chapter VII, some vapour grown layer compounds arestudied. Individual dislocations are resolved in SnSe2,TiSe2, SnS2, TiS2, ZrS2 and HfS2. The Burgers vectors ofthe dislocations in SnS2 were determined and their originsdiscussed. Large area stacking faults bounded by 1andfrasl;3 andlang;1100andrang;partial dislocations are also observed in these specimens.Dislocation and defect configurations in the other compoundsare described and a modified divergent beam method fortaking topographs of bent crystals is demonstrated.Some preliminary experiments on pure iron are describedin Chapter VIII. Contrast is observed suggestive of dislocationhelices and rows of loops in these strain-anneal growncrystals. Arrays of dislocations of pure edge type and Burgersvector parallel to [100] are interpreted as being containedin a low angle boundary wall, following Futagami. The tiltangle across such boundaries is measured to be a few secondsof arc.Finally, Chapter IX suggests a few directions forfurther investigation.

Type of Award:DPhil Level of Award:Doctoral Awarding Institution: University of Oxford Notes:This thesis was digitised thanks to the generosity of Dr Leonard Polonsky

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Dr. C.J. HumphreysMore by this contributor

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Issue Date: 1971Identifiers

Urn: uuid:fa8aa052-81f4-4a5b-918d-b70c0cc6b934

Source identifier: 601870697 Item Description

Type: Thesis;

Language: eng Tiny URL: td:601870697

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Author: B. K. Tanner - institutionUniversity of Oxford Oxford, Department of Metallurgy oxfordCollegeOriel College - - - - Contributors D

Source: https://ora.ox.ac.uk/objects/uuid:fa8aa052-81f4-4a5b-918d-b70c0cc6b934



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