Measurements of phase changes in crystals using Ptychographic X-ray imaging Maria Civita October 2016 Declaration I, Maria Civita, confirm that the work presented in this Thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the Thesis. Abstract In a typical X-ray diffraction experiment we are only able to directly retrieve part of the information which characterizes the propagating wave transmitted through the sample: while its intensity can be recorded with the use of appropriate detectors, the phase is lost. Because the phase term which is accumulated when an X-ray beam is transmitted through a slab of material is due to refraction [1, 2], and hence it contains relevant information about the structure of the sample, finding a solution to the “phase problem” has been a central theme over the years. Many authors successfully developed a number of techniques which were able to solve the problem in the past [3, 4, 5, 6], but the interest around this subject also continues nowadays [7, 8]. With this Thesis work, we aim to give a valid contribution to the phase problem solution by illustrating the first application of the ptychographic imaging technique [9, 10, 11, 12, 13, 14] to measure the effect of Bragg diffraction on the transmitted phase, collected in the forward direction. In particular, we will discuss the experimental methodology which allowed to detect the small phase variations in the transmitted wave when changing the X-ray’s incidence angle around the Bragg condition. Furthermore, we will provide an overview of the theoretical frameworks which can allow to interpret the experimental results obtained. More specifically, we will also discuss a new quasi-kinematic approximation which was recently developed by Gorobtsov and Vartanyants [2] in order to highlight the potential for future applications of the methodology described in this work. In particular, this new theory, used in conjunction with the experimental technique here presented, will permit to investigate further the effects related to the phase of the transmitted beam, thus allowing to study the structure of strained crystals as well as to fully determine the phase of the structure factor. i Contents 1 The phase problem 1 1.1 Theoreticalbackground .................................... 2 1.1.1 A possible reformulation of the phase problem . 4 1.1.2 A new experimental technique developed with the use of ptychography . 7 2 Kinematical diffraction theory 10 2.1 Scattering by one electron . 10 2.2 Scattering by a single atom . 14 2.3 Scattering by one molecule . 16 2.4 Scattering by a crystal . 16 2.5 X-ray reflection and transmission by one atomic layer . 21 2.6 Reflection from a set of atomic layers . 25 3 Dynamical theory of X-ray diffraction 27 3.1 Dynamical diffraction regime . 27 3.1.1 Theoreticalbackground . 30 3.1.2 Dynamical diffraction in the Laue geometry . 34 3.1.3 Dynamical diffraction in the Bragg geometry . 43 3.1.4 Dynamical diffraction and sample’s thickness . 47 3.2 Different approaches to solve the phase problem . 49 3.2.1 Structure factors measurements through the analysis of Pendellösung fringes . 49 3.2.2 Phase problem solution with the use of X-ray standing wave fields . 53 ii 3.2.3 Phase shift investigation in X-ray forward diffraction . 55 4 Quasi-kinematical diffraction approximation 58 4.1 Preliminary considerations . 59 4.1.1 Phase shift in non periodic media . 59 4.1.2 Phase shift in periodic media . 60 4.2 Takagi-Taupin equations . 61 4.3 Kinematical & dynamical solutions . 65 4.4 Quasi-kinematical approximation . 67 5Ptychography 71 5.1 Theoreticalprinciplesofptychography . 72 5.2 PtychographicIterativeEngine(PIE) . 72 5.3 Extended Ptychographic Iterative Engine (ePIE) . 75 5.4 Difference Map method . 76 5.5 Artifacts introduced in the reconstructed phase . 78 5.5.1 Phasewrapping..................................... 79 5.5.2 Phase ramps . 84 6 First experimental results: gold nanocrystals 88 6.1 Ptychography on gold nanocrystals . 88 6.1.1 Experimental setup . 88 6.1.2 Data analysis . 92 6.1.3 Data fitting . 96 7 Design and preparation of new samples 102 7.1 Sample’s design . 104 7.2 Cleanroomproduction.....................................106 8 Si and InP: experimental results 112 8.1 Si samples . 113 8.1.1 Si pillar: 4x4 microns . 117 iii 8.1.2 Si pillar 4x8 micron . 124 8.2 InPsamples...........................................126 8.2.1 InP:{111}reflection ..................................127 8.2.2 InP:{220}reflection ..................................128 8.2.3 InP:{200}reflection ..................................132 9 Conclusions 135 Bibliography 139 iv List of Tables 3.1 Table summarizing the Pendellösung lengths calculated for the samples used in the experimental work analyzed in this Thesis. The calculation was performed by following Eq.3.39withtheuseof[15]. ................................. 49 9.1 Here we present a summary of the phase shift values measured during our experiments. ∆Φ is the maximum amplitude of the phase shift, while "max.min represent the maximum and minimum sizes of the error bars calculated for each phase point. Furthermore here we also show the average error-bar value obtained across the entire set of angular positions defining the rocking scan for each phase shift profile. By comparing this value with ∆Φ we can have a measure of the errors in the measured phases. 136 v List of Figures 1.1 Experimental setup of Friedrich, Knipping and von Laue in 1912 as described in [16]. 1 1.2 Construction of a 2D crystal structure from the convolution of a lattice and a basis as described in [1]. In this sketch we show how a crystal can be described as a regular repetition in space of a basic structural motif, which is defined as the crystal unit cell. In this figure the unit cell is described by the basis and is identified by vectors a1anda2. 2 1.3 Young’s interference experimental setup as described in [7]. 6 1.4 Experimental setup as described in [8]. A quasi monochromatic X-ray beam hits a crystal generating a forward-diffracted beam traveling in the rs0 direction and a reflected beam traveling along s1. In his paper Wolf described a method to determine the phase of the spectral degree of coherence µS0 (rs0, rs1, !) at a pair of points Q1(rS0 ) and Q2(r1).7 1.5 Schematic representation of the setup used for the experiments presented in this Thesis work. The sample was mounted in the Laue geometry on a 3D moving stage which allowed for it to be rocked at different angles on and offthe Bragg condition. The corresponding rocking curves were measured with the use of the detector in the reflection geometry D1. Furthermore, in order to perform ptychographic measurements, which require the collection of multiple diffraction patterns at overlapping beams positions projected on the sample, at each rocking angle the sample was also moved respect to the beam so to perform a circular scan. The forward diffracted beams intensities where then measured with detector D2...............................8 vi 2.1 Schematic representation of an X-ray scattering experiment. The incoming beam is characterized by the flux Φ0 which is proportional to the amplitude of the electric field E 2 and to the speed of light c. The scattered intensity is proportional to the | in| amplitude of the scattered electric field E 2 collected by the detector as described in | rad| [1].Figurereplicatedfollowing[1]. 11 2.2 Scattering of an X-ray by an electron. In the classical description of this phenomenon, the incoming beam is a plane wave with an associated electric field which is responsible for the electron’s vibration (blue arrows). The incoming beam propagates along the zdirectionanditselectricfieldispolarizedalongthexaxis(ˆ✏).Ontheotherhand, the scattered wave at an observation point X is spherical with a polarization along ✏ˆ0. From geometric considerations one can write that sin = ✏ˆ ✏ˆ0 where the minus sign − · accounts for the 180◦ phase shift between the incoming and scattered waves as shown in [1]. Figure replicated following [1]. 12 2.3 Schematic representation of the scattering from an atom as discussed in [1]. In the case of elastic scattering, one can write that k = k0 =2⇡/λ.Furthermorethephase | | | | difference between the two volume elements, one at the centre of the atom and one at position r is given by the scalar product between the wavevector k and r . In addition to that, the phase difference between the two scattered waves is given by k r. This can − · be explained by following what was described in the case of the single electron, where the second volume element at position r is located at the observation point X, so that it has a wavevector k0 (in the plane wave approximation) and a 180◦ phase shift respect to the element at the centre of the atom. By combining the two phase shifts one can conclude that ∆φ =(k k0) r. Figure replicated following [1]. 14 − · 2.4 Schematic representation of the scattering by a molecule as discussed in [1]. Here the molecule is composed by three atoms and the scattering vector Q is described within the scattering triangle generated by the incoming and scattered beams. Figure replicated following[1]. .......................................... 16 vii 2.5 Schematic representation of a 2D molecular crystal as discussed in [1]. Here we see that at each point in the lattice corresponds a molecule. The lattice is defined by vector Rn and the distance between each lattice plane is given by d. In order to specify a particular family of planes, once can use the 3D Miller indices (h, k, l) which identify a set of points on the ai axes defined as (a1/h, a2/k, a3/l).Fordifferentfamiliesofplanes, d = a the spacing d is defined by ph2+k2+l2 , where a is the elementary lattice parameter.